WO2025047170A1 - Ni alloy excellent in surface property and mechanical property - Google Patents

Abstract

An Ni-based alloy and a method for manufacturing the same, the Ni-based alloy being provided with good surface and mechanical properties with defects being absent from the surface, the N-based alloy comprising, in mass %, 0.0010-0.03% of C, 0.01-0.5% of Si, 0.01-0.5% of Mn, 0.01% or less of P, 0.005% or less of S, 0.5% or less of Cr, 0.1% or less of Mo, 0.1% or less of Cu, 0.001-0.2% of Al, 0.05% or less of Ti, 0.01-0.5% of Fe, 0.0001-0.02% of Ca, 0.0001-0.02% of Mg, 0.005% or less of N, 0.005% or less of O, 1% or less of one or more of Co, W, Nb, and B, with the balance being Ni and unavoidable impurities, and the average composition of non-metal inclusions being MgO: 30-70% of MgO, 15-40% Al₂O₃, 30% or less of SiO₂, 10-40% of CaO, and 1% or less of MnO.

Description

Ni alloy with excellent surface properties and mechanical properties

The present invention relates to a Ni alloy with excellent surface properties and mechanical properties, and a method for producing the same.

Because pure nickel plate has excellent corrosion resistance, it is used in conductive parts for the internal wiring of rechargeable lithium-ion batteries, lithium polymer batteries, nickel-metal hydride batteries, and other batteries that serve as power sources for portable electronic devices (see, for example, Patent Documents 1 and 2).

Nickel plates become softer more easily than materials such as iron, and technology has been developed to maintain their strength (see, for example, Patent Document 3).

In addition, nickel rolled sheets for deep drawing are used in a wide range of applications, including as electrodes for cold cathode fluorescent lamps. Similarly, because of their excellent corrosion resistance, particularly in alkaline environments, they are also used in nickel electrodes for caustic soda production (see, for example, Patent Documents 4 and 5).

As mentioned above, because they are used in extremely precise locations, the occurrence of surface defects must be limited. Furthermore, because corrosion also progresses from the defective areas, there has been a demand for technology to prevent this (see, for example, Patent Document 6).

Since such electronic components require soldering, efforts have been made to improve the wettability of the nickel plate and solder (see, for example, Patent Documents 7 and 8).

Furthermore, nickel plate is more expensive than commonly used metals such as iron and aluminum, so not only is an increase in cost due to surface defects unacceptable, but hot workability must also be maintained at a high level (see, for example, Patent Documents 9 to 10).

In this context, nickel alloys with excellent surface properties have been disclosed. However, the structure of the nonmetallic inclusions is not controlled, and in some cases, surface defects are formed. Furthermore, there are cases where it is difficult to maintain the mechanical properties (see, for example, Patent Document 11).

Japanese Unexamined Patent Publication No. 168259/1983 JP 2014-43623 A JP 2010-90438 A JP 2011-74437 A JP 2003-89835 A WO2010/064642 publication JP 2005-82886 A JP 2010-132933 A Japanese Patent Application Publication No. 8-143996 JP 2010-132934 A Patent No. 7015410

As described above, the main aim of the present invention is to prevent the clustering of nonmetallic inclusions, which can cause large surface defects, by controlling not only the composition of the nonmetallic inclusions but also their structure. Furthermore, the steel must have good mechanical properties. At the same time, the formation of CaO inclusions and Al ₂ O ₃ inclusions must be prevented because they tend to cluster. Furthermore, MnO-Cr ₂ O ₃ inclusions are formed in an environment with a high oxygen concentration, so the total number of inclusions is large and they are also prone to clustering, so they must be prevented.

In other words, the object of the present invention is to comprehensively consider the above and provide a Ni alloy that has sound surface properties and mechanical properties without surface defects, and a method for producing the same.

In order to solve the problems of the above-mentioned conventional techniques, the inventors have repeatedly carried out intensive experiments as described below.
First, 20 kg of Ni alloy was melted in a magnesia crucible in a laboratory. In this case, a high-frequency induction furnace was used as the melting equipment, and argon gas was blown from above the crucible to isolate it from the atmosphere. The raw materials used were pure Ni, electrolytic iron, ferrochrome, Mo, copper wire, Ti, nitrided ferrochrome, Co, W, NiNb, and B. Finally, Si and Al were added to deoxidize the alloy, and 1 kg of artificial slag of CaO-Al ₂ O ₃ -MgO-SiO ₂ -F system was added, and the composition of the nonmetallic inclusions was carefully observed by sampling. In some experiments, deoxidization was performed only with Si without adding Al. Observation was performed using a SEM. The analysis was performed using EDS. In particular, element mapping was performed carefully, and the structural morphology was carefully observed. It was found that analysis of at least 10 inclusions is necessary to find compositional characteristics, 20 is better, and 30 or more is sufficient.

The alloy ingot cast into the mold was forged to a thickness of 20 mm, after which the surface was ground and cold rolled. The final plate thickness was 0.5 mm, and the surface was carefully observed to determine whether there were any defects.

As a result, it was found that the occurrence of defects is strongly related to the structural morphology of nonmetallic inclusions, and that a structural morphology in which CaO-Al ₂ O ₃ -MgO oxides contain MgO or MgO.Al ₂ O ₃ has a high ability to suppress clustering. A schematic diagram of this is shown in Figure 1. Figure 1(a) shows a CaO-Al ₂ O ₃ -MgO oxide that completely contains MgO or MgO.Al ₂ O ₃ within its approximately spherical surface, which is a good morphology that can suppress clustering. On the other hand, it was revealed that a morphology in which a part of the outer edge is CaO-Al ₂ O ₃ -MgO system and most of the inside is high-melting point MgO or MgO.Al ₂ O ₃ oxide that protrudes from the approximately spherical surface of the inclusion, as shown in Figure 1(b), promotes clustering. Although the mechanism is largely unknown, it is presumed that the CaO-Al ₂ O ₃ -MgO melts act as sintering aids at steelmaking temperatures (1400-1550°C), adhering to each other and sintering. It has been assumed from experience that the main sintering location is the inner wall of the submerged nozzle of the continuous casting machine in an actual machine.

On the other hand, it was also confirmed that no sintering behavior occurs when MgO.Al ₂ O ₃ or MgO is used alone. It became clear that the structural form of the surrounding CaO-Al ₂ O ₃ -MgO system oxide melt has a large effect on the final quality.

Furthermore, to evaluate the mechanical properties, appropriate test pieces were taken and the Vickers hardness was measured on a cross section horizontal to the rolling direction. It was found that in order to maintain a hardness of 90 Hv or more and an elongation of 25% or more, it was necessary to contain appropriate amounts of Cr, Mo, Cu, Ti, and N. It was also found that it is more preferable to contain some Co, W, and Nb. Furthermore, it was found that the addition of B is desirable to maintain hot workability.

With this in mind, for some steel types, the raw materials were melted in a 60-ton electric furnace, vacuum degassed in a VOD, and refined by adding quicklime from above and FeSi and/or Al at the same time to proceed with deoxidization and desulfurization. Slabs were then produced in a continuous casting machine, and after surface grinding, they were hot rolled, cold rolled, and finally passed through the annealing and pickling line to produce 0.5 mm thick thin plates.

That is, the present invention was developed based on the above laboratory studies and through confirmation by actual equipment testing, as described below.
The chemical compositions, all in mass %, are as follows:
C: 0.001-0.03%, Si: 0.01-0.5%, Mn: 0.01-0.5%, P: 0.01% or less, S: 0.005% or less, Cr: 0.5% or less, Mo: 0.1% or less, Cu: 0.1% or less, Al: 0.001-0.2%, Ti: 0.05% or less, Fe: 0.01-0.5%, Ca: 0.0001-0.02%, Mg: 0.0001-0.02%, N: 0.005% or less, O: 0.005% or less, and one or more of Co, W, Nb, and B are 1% or less, with the balance being Ni and unavoidable impurities. The average composition of non-metallic inclusions is MgO: 30-70%, Al ₂ O ₃ : 15-40%, SiO ₂ : 30% or less, CaO: 10 to 40%, MnO: 1% or less.

Furthermore, in a more preferred embodiment, the structural form of the nonmetallic inclusions is such that 70% or more of the nonmetallic inclusions in terms of number ratio to the total nonmetallic inclusions are any one or more of the following five types a to e:
a: CaO-Al ₂ O ₃ -MgO oxide completely contains MgO and Al ₂ O ₃ within the approximately spherical surface b: CaO-Al ₂ O ₃ -MgO oxide c: CaO-Al ₂ O ₃ -MgO oxide completely contains MgO within the approximately spherical surface d: MgO alone e: MgO and Al ₂ O ₃ alone

It is more preferable that the above-mentioned CaO-Al ₂ O ₃ -MgO-based oxide is contained, and the CaO-Al ₂ O ₃ -MgO-based oxide contains CaO: 20 to 60%, Al ₂ O ₃ : 30 to 60%, MgO: 1 to 30%, SiO ₂ : 20% or less, and TiO2: 0.5% or less.

The Ni alloy described above contains MgO.Al ₂ O ₃ , and it is desirable that MgO.Al ₂ O ₃ contains 0.5% or less of MnO.

The above-mentioned simple substance MgO is contained, and it is desirable that the simple substance MgO contains Al ₂ O ₃ : 3% or less, SiO ₂ : 1% or less, CaO: 10% or less, and MnO: 1% or less.

The Ni alloy described above preferably has a Vickers hardness of 90 Hv or more and an elongation of 25% or more.

The present invention also provides a manufacturing method, which involves melting raw materials in an electric furnace, vacuum degassing in a VOD, adding quicklime, fluorite and a magnesia source to form a CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag, and simultaneously adding a ferrosilicon alloy and/or Al to deoxidize and desulfurize the slag, producing a slab in a continuous casting machine, grinding the surface, and then performing a hot rolling process and a cold rolling process.

FIG. 2 is a comparison diagram of the structural morphology of nonmetallic inclusions of the present invention and non-invention. FIG. 1 is a schematic diagram showing classification of the structural morphology of nonmetallic inclusions according to the present invention and those outside the present invention.

The reasons for the numerical limitations of the present invention and the scientific viewpoints thereof will be explained below. The units below are % by mass.
C: 0.001-0.03%
C is an important element for maintaining the strength of Ni alloys. Therefore, 0.001% is necessary. On the other hand, adding more than 0.03% has the drawback of forming carbides of Ti and the like, which leads to embrittlement. Therefore, the addition of 0.001-0.03% is controlled by adding a carbon source after desulfurization in VOD. It is preferably 0.002-0.025%, more preferably 0.005-0.022%.

Si: 0.01~0.5%
Silicon is an effective element for deoxidation and is very important in the present invention. Deoxidation can be carried out according to the following formula. In other words, deoxidation is carried out by adding FeSi alloy or pure silicon during VOD.
Si + 2O =( _SiO2 )...(1)
Here, the underlined parts are the components in the molten steel, and the parentheses are the components in the slag. As will be described later, by lowering the activity coefficient of _SiO2 in the slag, deoxidation can be effectively promoted and the oxygen concentration can be controlled to the oxygen concentration of the present invention. Furthermore, desulfurization can be performed at the same time. 0.01% is necessary to achieve this effect, and if it exceeds 0.5%, not only will the corrosion resistance decrease, but the steel will become embrittled. Therefore, the range is specified as 0.01 to 0.5%. The range is preferably 0.03 to 0.3%, and more preferably 0.05 to 0.28%.

Mn: 0.01~0.5%
Mn is an extremely effective element because it is useful for deoxidization. Therefore, the Mn content is specified to be 0.01 to 0.5%, preferably 0.03 to 0.47%, and more preferably 0.03 to 0.4%.

P: 0.01% or less P is a harmful element that not only segregates at grain boundaries and reduces corrosion resistance, but also causes red brittleness during welding, leading to cracking. P can be removed as _P2O5 in the slag by performing oxygen blowing in an electric furnace. Therefore, _the content is set to 0.01% or less. It is preferably 0.008% or less, and more preferably 0.002% or less.

S: 0.005% or less S is a harmful element because it segregates at grain boundaries and forms MnS, which becomes the starting point of pitting corrosion. It also reduces hot workability and causes edge cracks. Therefore, the content is set at 0.005% or less. As will be described in detail later, desulfurization can be promoted by the following reaction formula.
3(CaO)+2Al+3S= _{( Al2O3} )+3(CaS)...(2)
2(CaO)+Si+2S=( _SiO2 )+2(CaS)...(3)
The desulfurization reaction can be promoted efficiently by reducing the activity coefficient of alumina and silica in the slag. If the content is 0.005% or less, the above problems can be avoided, so the content is set within this range. The content is preferably 0.003% or less, more preferably 0.0015% or less, and even more preferably 0.0008% or less.

Ni: 99% or more Ni is a major element in the present invention and is an important element for maintaining mechanical properties, corrosion resistance, and workability. In the present invention, it is treated as the balance. If it is less than 99%, it will not only deviate from the required corrosion resistance, but also will not satisfy the mechanical properties. It will also impair the properties required for electronic parts, such as electrical conductivity. Therefore, it is specified to be 99% or more. It is preferably 99.2% or more, more preferably 99.3% or more, and even more preferably 99.4% or more.

Cr: 0.5% or less In the present invention, Cr is an important element for maintaining strength through solid solution strengthening. However, if the content is too high, there is a risk of forming MnO-Cr ₂ O ₃ inclusions. Therefore, the content is specified as 0.5% or less. It is preferably 0.4% or less, and more preferably 0.3% or less. Although not limited, a content of 0.01% or more is desirable.

Mo: 0.1% or less Like Cr, Mo is an important element for maintaining strength through solid solution strengthening. However, if the content is too high, it will cause abnormal oxidation during heating in the manufacturing process. Therefore, it is specified to be 0.1% or less. It is preferably 0.08% or less. Although not limited, it is desirable to contain 0.01% or more.

Cu: 0.1% or less Like Cr, Cu is an important element for maintaining strength through solid solution strengthening. However, if the Cu content is too high, it can cause the Ni alloy plate to soften. Therefore, the Cu content is set to 0.1% or less. It is preferably 0.05% or less, and more preferably 0.03% or less.

Al: 0.001-0.2%
Al is a useful element for deoxidation. It also improves high-temperature oxidation resistance. On the other hand, if the Al content is too high, AlN is formed, impairing hot workability, or the Ca and Mg concentrations become too high, exceeding the upper limit of the range of the present invention. As a result, harmful CaO-based inclusions are formed, or a low-melting point compound called _NiMg2 is formed, reducing hot workability. The deoxidation reaction proceeds as follows.
2 Al + 3 O = (Al ₂ O ₃ )...(4)
As will be described in detail later, by lowering the alumina activity in the slag, deoxidation proceeds efficiently. Furthermore, desulfurization also becomes possible at the same time. Therefore, the range is specified as 0.001 to 0.2%. The range is preferably 0.002 to 0.18%, more preferably 0.005 to 0.15%, and even more preferably 0.008 to 0.13%.

Ti: 0.05% or less Ti fixes N to form TiN, which prevents the formation of blowholes due to the nitrogen exceeding its solubility during solidification. Therefore, the content is specified to be 0.05% or less. It is preferably 0.02% or less. It is more preferably 0.01% or less.

Fe: 0.01~0.5%
In the present invention, Fe is an important element for maintaining strength through solid solution strengthening. However, if the content is too high, the purity of the Ni alloy decreases and corrosion resistance is impaired. Therefore, the content is specified to be 0.01 to 0.5%. The content is preferably 0.03 to 0.4%, and more preferably 0.04 to 0.3%.

Ca: 0.0001-0.02%
Ca is an important element for controlling the structure of nonmetallic inclusions to the preferred CaO-Al ₂ O ₃ -MgO oxides. By controlling the structure in this way, it is possible to manufacture sound products without clustering or forming surface defects. Ca is effectively added by utilizing the following reaction:
3(CaO)+ 2Al = _{( Al2O3} )+ 3Ca ...(5)
2(CaO)+ Si =( _SiO2 )+ 2Ca ...(6)
To control this reaction, the activity of alumina and silica in the slag should be controlled within an appropriate range. This will be explained in detail in the manufacturing method. On the other hand, if it is too high, harmful CaO inclusions are formed, which promotes clustering and creates defects. Therefore, the content is specified to be 0.0001 to 0.02%. The preferred range is 0.0005 to 0.015%, and the more preferred range is 0.001 to 0.01%.

Mg: 0.0001-0.02%
Mg is an important element for controlling the structure of nonmetallic inclusions to the preferred CaO-Al ₂ O ₃ -MgO oxides, MgO, and MgO.Al ₂ O _3. By controlling the structure in this way, it is possible to manufacture sound products without clustering or forming surface defects. Mg is effectively added by utilizing the following reaction.
3(MgO)+ 2Al = _{( Al2O3} )+ 3Mg ...(7)
2(MgO)+ Si =( _SiO2 )+ 2Mg ...(8)
To control this reaction, the activity of alumina and silica in the slag should be controlled within an appropriate range. This will be explained in detail in the manufacturing method section. On the other hand, if it is too high, Mg vaporizes during solidification and forms blowholes, which in turn form surface defects. Therefore, the content is specified to be 0.0001 to 0.02%. It is preferably 0.0005 to 0.015%, and more preferably 0.001 to 0.01%.

N: 0.005% or less N is an important element for maintaining strength through solid solution strengthening. However, if the N content is too high, TiN clusters are formed, causing surface defects. Therefore, the N content is set to 0.005% or less. It is preferably 0.003% or less, and more preferably 0.002% or less.

O: 0.005% or less The oxygen content must be reduced because it increases the number of nonmetallic inclusions and forms surface defects. Therefore, the oxygen content is set to 0.005% or less. It is preferably 0.003% or less, more preferably 0.001% or less, and even more preferably 0.0008% or less.

One or more of Co, W, Nb, and B, 1% or less Co is an effective element because it behaves in the same way as Ni. It is also sometimes contained in pure Ni raw materials, and plays an important role in terms of utilizing inexpensive raw materials. W is a beneficial element for maintaining mechanical properties. Nb is useful for maintaining strength through solid solution strengthening. B improves hot workability. It is more preferable to contain one or more of these elements at 1% or less. In other words, they may be added as necessary.

The Ni alloy of the present invention consists of the remainder Ni. In addition, it may contain trace amounts of unavoidable impurities, such as Pb, Sn, Ta, Ag, Na, K, and Zr. All of these are mixed in from the raw materials, pure Ni and ferroalloys.

Next, the average composition of the nonmetallic inclusions will be described.
The average composition of the nonmetallic inclusions is as follows:
MgO: 30-70%
MgO is effective for forming harmless CaO-Al ₂ O ₃ -MgO oxides, and is also extremely effective for forming simple MgO and simple MgO.Al ₂ O ₃ , which are similarly harmless. Therefore, the content is specified as 30 to 70%, and preferably 35 to 65%.

_Al2O3 : _15-40 %
Al ₂ O ₃ is effective for forming harmless CaO-Al ₂ O ₃ -MgO oxides, and is also extremely effective for forming MgO.Al ₂ O ₃ , which is also harmless. However, if it is higher than 40%, it forms harmful Al ₂ O ₃ simple substance, which causes defects. Therefore, it is specified to be 15 to 40%. It is preferably 18 to 39%.

_SiO2 : 30% or less _SiO2 is a compound present in CaO- _Al2O3 - _MgO oxides, and is effective in lowering the melting point. However, if it exceeds 30%, it acts in the direction of increasing the oxygen concentration, which increases the number of nonmetallic inclusions and also forms harmful MnO- _{Cr2O3 oxides} , resulting in defects. Therefore, it is specified to be 30% or less. Preferably, it is 28% or less.

CaO: 10-40%
Although CaO is effective for forming harmless CaO-Al ₂ O ₃ -MgO oxides, if the content is too high, harmful CaO inclusions are formed, resulting in defects. Therefore, the content is specified as 10 to 40%. Preferably, it is 12 to 39%. More preferably, it is 13 to 38%.

MnO: 1% or less MnO is harmless because it dissolves in the MgO sites of simple MgO and simple MgO-Al ₂ O ₃ inclusions. However, if the content is high, it forms harmful MnO-Cr ₂ O ₃ oxides, which creates defects. Therefore, the content is set to 1% or less. Preferably, it is 0.9% or less. More preferably, it is 0.8% or less.

_Cr2O3 : 1.5% or less Although not particularly limited, if _{the Cr2O3 content} is high, MnO- _{Cr2O3 -} based inclusions are more likely to form. Furthermore, the oxygen concentration in the molten steel also increases, and the number of nonmetallic inclusions increases, causing defects. Therefore, 1.5% or less is preferable. More preferably, 1.3% or less. Even more preferably, 0.6% or less.

Here, there are several methods for determining the average composition. This can be determined by randomly analyzing 10 points using SEM/EDS. In this case, it is necessary to analyze two points on the periphery of the center, and the average can be determined by taking a mapping and measuring the area ratio using image analysis. It is preferable to analyze 20 points or more, and more preferably 30 points or more. The other is the so-called electrolytic method, which is widely known as the Speed method. In other words, the metal parts are dissolved by applying an appropriate potential in an appropriate solution. The non-metallic inclusions that remain as residue are then filtered and collected. The average composition can be determined all at once by subjecting this to chemical analysis. There are no limitations on the analysis method.

Furthermore, the structural form of the nonmetallic inclusions will be explained. It is more preferable that 70% or more of the nonmetallic inclusions in terms of number ratio with respect to the total nonmetallic inclusions are one or more of the following five types a to e. The structural forms corresponding to a to e are also shown in Figures 2(a) to (e).
a: CaO-Al ₂ O ₃ -MgO-based oxide completely encapsulates MgO and Al ₂ O ₃ within the approximately spherical surface b: CaO-Al ₂ O ₃ -MgO-based oxide c: CaO-Al ₂ O ₃ -MgO-based oxide completely encapsulates MgO within the approximately spherical surface d: MgO alone e: MgO and Al ₂ O ₃ alone Here, completely contained within an approximately spherical surface refers to a state in which the inner material is completely contained within the outer material as shown in Figure 1 (a), and does not include a state in which the inner material partially protrudes to the outside as shown in Figure 1 (b).

As explained in the means for solving the problem, the form shown in FIG. 1(b) does not appear in the following operation. To control the structure to these forms, it is sufficient to control the chemical components Si, Al, Ca, Mg, O, and Mn within the range of the present invention, but this is not all. a and c are also characteristic of the present application. To achieve this structure, the key is the vacuum mixing process in the VOD. Dolomite is preferable for the bricks in the VOD. During the deoxidation period, quicklime is first added, and at the same time, fluorite and waste bricks containing MgO are added. MgO waste bricks can be added as needed. After that, when ferrosilicon alloy or pure Si and Al are added, Si and Al are added to the molten slag that has already been formed. Deoxidation is possible with only Si. This directly reacts with CaO and MgO in the slag, effectively adding Ca and Mg.

Mg is supplied to the molten steel in advance according to the reactions of formulas (7) and (8). The reason for this is unclear in many respects, but the reaction of Ca is delayed. This dissolved Mg reacts first with the original non-metallic inclusions, silica and alumina, as shown below.
2 Mg + SiO ₂ (inclusions) = 2MgO (inclusions) + 2 Si ...(9)
3 Mg + Al ₂ O ₃ (inclusions) = 3MgO (inclusions) + 2 Al ...(10)
2 Mg + 4 Al + 4 SiO ₂ (inclusions) = 2MgO・Al ₂ O ₃ (inclusions) + 4 Si ...(11)
3 Mg +4Al ₂ O ₃ (inclusions) = MgO・Al ₂ O ₃ (inclusions) + 2 Al … (12)
As shown by the above formulas (9) to (12), simple MgO and simple MgO.Al ₂ O ₃ are initially formed. After that, Ca is supplied from the slag, and the Ca in the molten steel reacts with the above inclusions, resulting in the vigorous formation of CaO-Al ₂ O ₃ -MgO-based oxides on the surface. These molten oxides then cover the simple MgO and simple MgO.Al ₂ O _3, rendering them harmless. In other words, they are molten and behave as harmless CaO-Al ₂ O ₃ -MgO-based inclusions. Therefore, if the non-metallic inclusions are spherical, they behave as described above and are harmless.

Furthermore, the composition range of the above CaO-Al ₂ O ₃ -MgO based oxide will be explained.
CaO: 20-60%, Al ₂ O ₃ : 30-60%, MgO: 1-30%, SiO ₂ : 20% or less, TiO ₂ : 0.5% or less The above ranges were set because they allow the steel to maintain a molten state at steelmaking temperatures, i.e., near 1500°C. In particular, if the CaO content is too high, harmful CaO simple inclusions are formed, so CaO: 20-60% was specified. If _{the Al 2} O ₃ content is too high, simple Al ₂ O ₃ is formed, so Al ₂ O ₃ : 30-60% was specified. If the SiO ₂ content is too high, harmful MnO-Cr ₂ O ₃ inclusions are formed, so it was set to 20% or less.

MnO concentration in MgO.Al ₂ O ₃ : 0.5% or less Since MnO dissolves in the MgO sites of MgO.Al ₂ O ₃ , MnO can be made harmless. Therefore, the content is specified to be 0.5% or less.

In the MgO simple inclusions, _Al2O3 : 3% or less, _SiO2 : 1% or less, CaO: 10% or less, MnO: 1% or less Since the MgO simple inclusions are harmless, it is desirable for _them to be dissolved to some extent in the inclusions. Taking into account the solid solubility limit, the contents are set to _Al2O3 : ₃ % or less, _SiO2 : 1% or less, CaO: 10% or less, and MnO: 1% or less.

The present invention also provides a manufacturing method. That is, the raw materials are melted in an electric furnace, and then vacuum degassed in a VOD. The raw materials can be blended according to the desired components, such as pure Ni, ferrosilicon alloy, ferrochrome alloy, iron scrap, Mo, Mo trioxide, ferromolybdenum alloy, ferroniobium alloy, and copper wire.

After the decarburization and dephosphorization processes in the electric furnace are completed, the slag is removed. Then, either or both of a ferrosilicon alloy and Al are added to form a CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag, which is then deoxidized and desulfurized. A slab is produced in a continuous casting machine, the surface is ground, and the slab is subjected to a hot rolling process and then cold rolling. At this time, in order to control the structure of the nonmetallic inclusions to a desired form, it is necessary to control the slag composition within an appropriate range.

The CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag is preferably in the following range.
CaO: 35-70%
The CaO concentration can be adjusted by adding quicklime. CaO is extremely effective in deoxidation and desulfurization. In other words, as shown in formulas (2) and (3), desulfurization is possible by making CaO exist stably in the slag as CaS. However, if the CaO concentration is higher than 70%, CaO inclusions will form. Therefore, a concentration of 35 to 70% is preferable. A concentration of 40 to 65% is preferable. A concentration of 50 to 63% is more preferable.

Al ₂ O ₃ : 35% or less Al ₂ O ₃ is effective for making CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag into a molten state. On the other hand, if it exceeds 35%, it creates Al ₂ O ₃ simplex inclusions and promotes clustering. Therefore, it is desirable to keep it at 35% or less. Preferably, it is 30% or less. More preferably, it is 25% or less.

MgO: 3 to 25%
MgO is effective for molten CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag. On the other hand, if the MgO concentration is high, the Mg concentration in the molten steel will be high, and Mg gas will be released during solidification, forming blowholes. Therefore, 3 to 25% is preferable. 5 to 20% is more preferable, and 7 to 15% is even more preferable.

_SiO2 : 32% or less _SiO2 is effective for molten CaO- _Al2O3 - _MgO - _SiO2 -F slag. On the other hand, if the _SiO2 concentration is high, the oxygen concentration in the molten steel will be high and the nonmetallic inclusion composition will also be MnO- _Cr2O3 - _based . As a result, surface defects will occur. Therefore, 32% or less is preferable. More preferably, 30% or less. Even more preferably, 25% or less.

F: 1-10%
F is added as fluorite. F is effective in molten CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag. On the other hand, if the F concentration is high, it will melt the bricks of the VOD and the ladle, shortening their lifespan. Therefore, 1 to 10% is preferable. 2 to 9% is more preferable.

Furthermore, it is desirable that the concentrations of Cr ₂ O3 and FeO are low.
Cr ₂ O ₃ : 1% or less If the Cr ₂ O ₃ concentration is high, the oxygen concentration will increase and MnO--Cr ₂ O ₃ inclusions will form. Therefore, it is best to keep it at 1% or less. It is preferably 0.7% or less. It is more preferably 0.6% or less.

FeO: 1% or less If the FeO concentration is high, the oxygen concentration will increase and MnO-Cr ₂ O ₃ inclusions will form. Therefore, it is best to keep it at 1% or less. It is preferably 0.9% or less. It is more preferably 0.8% or less.

Furthermore, a high S concentration is desirable because it indicates that desulfurization has progressed normally. A concentration of 0.01% or more is desirable, and 0.02% or more is even more desirable.

In the present invention, it is also necessary to adjust the mechanical properties. That is, the Vickers hardness is specified as 90 Hv or more, and preferably 250 Hv or less. The elongation must be 25% or more, and preferably 35% or more.

The effectiveness of the present invention will be clarified by showing examples below.
The raw materials were melted in an electric furnace at 60 to 70 tons, and then decarburized and refined in a VOD. The raw materials consisted of mainly pure Ni, stainless steel scrap, ferrosilicon alloy, ferrosilicon alloy, iron scrap, Mo, Mo trioxide, ferromolybdenum alloy, ferroniobium alloy, ferrochrome alloy, copper wire, etc., which were blended according to the target steel type.

After the decarburization and dephosphorization processes in the electric furnace were completed, the slag was removed in a slag pot. Then, gas components were removed by vacuum degassing in the VOD. Then, quicklime and magnesia-containing waste bricks, and fluorite were added, followed by pure Si or ferrosilicon alloy. Furthermore, Al was added to finally form a CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag. Even when Al was not added, the Al impurity in the FeSi alloy was oxidized to supply alumina.

For some Ni alloys, alloying elements such as Nb, Mo, C, Co, Cr, B, and Ni were added here to precisely adjust the chemical composition. After deoxidization and desulfurization in this way, slabs measuring 200 mmt x 1200 mmw x 7 m length were produced using a continuous casting machine. The oscillation marks on the surface were then ground away, and the slabs were heated to 1000-1250°C depending on the type of steel, after which they were hot-rolled and finally cold-rolled before passing through an annealing and pickling line. In this way, 1 mm cold-rolled coils were produced for all steel types.

At this time, the evaluation was carried out by the following methods.
1) Chemical composition: The surface of a suction sample of φ30 mm × 10 mm height taken from the tundish of a continuous casting machine was ground with a grinder. Major elements were analyzed by fluorescent X-ray analysis. Some of C and S were analyzed by the combustion method. N and O were also analyzed by the infrared absorption method.
2) Slag components: Slag was collected with an iron bar and crushed. This was then pressed into a cylindrical powder sample. The values of this sample were determined using X-ray fluorescence analysis. F was determined by chemical analysis.
3) Average composition of nonmetallic inclusions: The above-mentioned suction sample was cut out, embedded in resin, and mirror-polished. This was placed in an SEM for observation and quantitative analysis. Thirty inclusions of 5 μm or more were randomly selected, and the center and periphery were analyzed. Each inclusion was mapped to determine the element distribution, and the ratio of each phase was calculated by image analysis. Taking into account the weighted average, a representative analysis value of each inclusion particle was obtained. This was obtained by calculating the average value of the 30 points.
4) Morphology of nonmetallic inclusions: The morphology was classified as described above during the observation and analysis. The morphology shown in Figure 1(b) was not confirmed.
5) Composition of each oxide: Calculated from the composition of each oxide phase described above.
6) Mechanical properties: Vickers hardness was measured by embedding and polishing a cross section parallel to the rolling direction with a micro Vickers hardness tester. A Vickers hardness of 90 Hv or more was considered a pass and marked with a "good" mark, and anything less than that was considered a fail and marked with a "bad" mark. Tensile test pieces were taken and the elongation was measured with an extensometer. An elongation of 25% or more was considered a pass and marked with a "good" mark, and anything less than that was considered a fail and marked with a "bad" mark.
7) Overall evaluation: An inspector visually evaluated a 1 mm thick Ni alloy plate when it was threaded. The evaluation results were determined as follows. In the following, the allowable range without cutting means that there are up to three linear defects with a length of 1 mm or more per 10 ^m2 of the steel plate surface.
Pass: No surface defects (100% non-defective product at time of shipment)
Pass: Some surface defects occurred, but within the required quality tolerance (95% pass rate)
Pass: △ Surface defects occurred in some areas, but the product can be shipped after partial cutting (80% pass rate)
Rejected: × Surface defects occurred along the entire length of the coil, and the coil was scrapped (0% defective product rate).
Additionally, mechanical properties were considered, but evaluation of surface defects was prioritized.

The structural forms of the examples are shown in Figures 2(a)-(e), and those of the comparative examples are shown in Figures 2(f)-(h). The reason the chemical components do not add up to 100% is due to unavoidable impurities. Inventive examples No. 1-14 had chemical analysis that fell within the desired range, and the slag composition also fell within the desired range. As a result, the structural forms of the non-metallic inclusions also fell within the range of a-e. Ultimately, all products were judged to pass.

However, in No. 5, one alumina inclusion and one MnO-Cr ₂ O ₃ inclusion were confirmed. In No. 8, one CaO inclusion was observed. Therefore, the evaluation was given a rating of ◯. In No. 10, one Al ₂ O ₃ inclusion and one MnO-Cr ₂ O ₃ inclusion were observed. Therefore, the evaluation was given a rating of ◯. Furthermore, in No. 11, the MnO concentration in MgO.Al ₂ O ₃ was high, and the SiO ₂ and MnO in the MgO inclusions were high, while CaO inclusions, Al ₂ O ₃ inclusions, and MnO-Cr ₂ O ₃ inclusions were confirmed. As a result, the evaluation was given a rating of △.

Next, a comparative example will be described.
No. 15 had low Si. Conversely, Al was high and the CaO concentration in the slag exceeded the desired range, resulting in high Ca and Mg concentrations. As a result, MgO and CaO were not included in the average composition of the inclusions. The structure also had more f-CaO simple substance, with the CaO in the CaO-Al ₂ O ₃ -MgO oxides being high. In other words, the CaO simple substance inclusions became the center, causing defects and resulting in scrapping.

In No. 16, the Si concentration was too high, which resulted in high Ca and Mg concentrations. The high Si concentration also resulted in high _SiO2 concentration in the slag. The F concentration was also low, resulting in poor slag fluidity. The Ti and N concentrations were both too high, resulting in the formation of TiN clusters. In terms of the average composition of the inclusions, MgO and CaO were out of range, and alumina, MgO, and _SiO2 in the CaO- _Al2O3 - _MgO oxide were out of range, and the structural form also showed the formation of many f-CaO simple inclusions. The formation of many TiN and CaO simple inclusions caused numerous surface defects, resulting in the coil becoming scrap.

In No. 17, the Si content was low, Mn yield was low, and Al was not added, which resulted in low Ca and Mg concentrations, and a high oxygen concentration, which led to a large number of non-metallic inclusions. In addition, the slag composition was high in silica and low in CaO, which was outside the range, which also had a negative effect on the inclusion composition. The average composition of the inclusions was outside the range for all oxides, and the structure morphology also showed the formation of many h-MnO-Cr ₂ O ₃ inclusions. Alumina and silica were also outside the range for CaO-Al ₂ O ₃ -MgO oxides, and MnO-Cr ₂ O ₃ inclusions were confirmed. As a result, surface defects occurred along the entire length, and the coil was eventually scrapped.

No. 18 had a high Al concentration, resulting in a high alumina concentration in the slag, while the CaO concentration was _low . The average composition of the nonmetallic inclusions was 100% alumina, and the structure consisted of only g- _Al2O3 inclusions. The elements that maintain strength, such as Cr, were low, and both the hardness and elongation were outside the range. As a result, surface defects occurred along the entire length, resulting in scrapping.

No. 19 had a low Al concentration, which resulted in low Ca and Mg concentrations, and a high oxygen concentration. The average composition of the nonmetallic inclusions was high in Cr ₂ O ₃ , and three types of structure were formed: f, g, and h. In particular, many h-MnO-Cr ₂ O ₃ inclusions were observed. Furthermore, the MgO-Al ₂ O ₃ simple inclusions and the components dissolved in the MgO simple inclusions were also outside the range. In addition, the Ti and N concentrations were high, and TiN clusters were also observed. Furthermore, the elements that maintain strength, such as Cr, were low, and both the hardness and elongation were outside the range. As a result, surface defects occurred along the entire length, resulting in scrapping.

No. 20 had high Ti and N levels, which caused severe TiN clusters to form surface defects. In Table 2, all of the inclusion morphologies are a and b, which may seem good at first glance, but this is because TiN is not counted as an inclusion. Furthermore, the elements that maintain strength, such as C and Cr, were low, and both hardness and elongation were outside the range. As a result, the product could not be shipped and was scrapped.

Claims (7)

The composition is, in mass%, C: 0.001-0.03%, Si: 0.01-0.5%, Mn: 0.01-0.5%, P: 0.01% or less, S: 0.005% or less, Cr: 0.5% or less, Mo: 0.1% or less, Cu: 0.1% or less, Al: 0.001-0.2%, Ti: 0.05% or less, Fe: 0.01-0.5%, Ca: 0.0001-0.02%, Mg: 0.0001-0.02%, N: 0.005% or less, O: 0.005% or less, and one or more of Co, W, Nb, and B are 1% or less, with the balance being Ni and unavoidable impurities. The average composition of non-metallic inclusions is MgO: 30-70%, Al ₂ O ₃ : 15-40%, SiO ₂ : 30% or less, CaO: 10-40%, MnO: 1% or less, and having excellent surface properties and mechanical properties. The Ni alloy having excellent surface properties and mechanical properties as described in claim 1, characterized in that the structural form of the non-metallic inclusions is such that 70% or more of the non-metallic inclusions in terms of number ratio to the total non-metallic inclusions are any one or more of the following five types a to e.
a: CaO-Al ₂ O ₃ -MgO oxide completely contains MgO and Al ₂ O ₃ within the approximately spherical surface b: CaO-Al ₂ O ₃ -MgO oxide c: CaO-Al ₂ O ₃ -MgO oxide completely contains MgO within the approximately spherical surface d: MgO alone e: MgO and Al ₂ O ₃ alone The Ni alloy having excellent surface properties and mechanical properties as described in claim 2, characterized in that the CaO-Al ₂ O ₃ -MgO system oxide is contained, and the CaO-Al ₂ O ₃ -MgO system oxide is composed of CaO: 20-60%, Al ₂ O ₃ : 30-60%, MgO: 1-30%, SiO ₂ : 20% or less, and TiO2: 0.5% or less. The Ni alloy having excellent surface properties and mechanical properties according to claim 2, characterized in that the MgO.Al ₂ O ₃ is contained, and the MgO.Al ₂ O ₃ contains 0.5% or less of MnO. The Ni alloy having excellent surface properties and mechanical properties according to claim 2, characterized in that the MgO simple substance is contained, and the MgO simple substance contains _Al2O3 : ₃ % or less, _SiO2 : 1% or less, CaO: 10% or less, and MnO: 1% or less. The Ni alloy described in 1 has excellent surface properties and mechanical properties, characterized in that it has a Vickers hardness of 90Hv or more and an elongation of 25% or more. A method for producing a Ni alloy according to any one of claims 1 to 6, characterized in that the method comprises melting raw materials such as pure Ni in an electric furnace, then performing a vacuum degassing process in a VOD, adding quicklime, fluorite and a magnesia source to form a CaO-Al ₂ O ₃ -MgO-SiO ₂ -F-based slag, and simultaneously adding a ferrosilicon alloy and/or Al to perform deoxidation and desulfurization, producing a slab in a continuous casting machine, grinding the surface, and then performing a hot rolling process and a cold rolling process, thereby producing a Ni alloy with excellent surface properties and mechanical properties.

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