WO2025047169A1 - Ni-based alloy with excellent surface properties - Google Patents

Abstract

Provided are an Ni-based alloy having no defects on the surface and having sound surface properties and a method for producing the Ni-based alloy. The Ni-based alloy contains, in terms of mass%, 0.001-0.1% of C, 0.01-0.4% of Si, 0.1-1% of Mn, up to 0.03% of P, up to 0.002% of S, 13-35% of Cr, up to 18% of Mo, up to 5% of W, up to 1% of Cu, 0.001-0.35% of Al, 0.001-0.2% of Ti, 0.0001-0.02% of Ca, 0.0001-0.02% of Mg, 0.001-0.03% of N, 2-11% of Fe, up to 0.005% of O, and up to 4% of one or more of Co, Nb, and B, with the remainder being Ni and unavoidable impurities, the average composition of non-metallic inclusions comprising, in terms of mass%, 30-70% of MgO, 15-40% of Al₂O₃, up to 30% of SiO₂, 10-40% of CaO, and up to 1% of MnO.

Description

Ni-based alloy with excellent surface properties

The present invention relates to a Ni-Cr-Mo-W-Fe system Ni-based alloy with excellent surface properties and a method for producing the same.

Ni-based alloys have excellent corrosion and acid resistance, and contain Cr, Mo, and W in addition to the main component Ni. Since these metals are extremely expensive compared to iron, it is extremely important to improve yield and reduce manufacturing costs. However, if surface defects such as linear scratches occur on the surface of the Ni-based alloy, it will not exhibit the required corrosion resistance, and there are many other issues, such as impeding weldability. Furthermore, since the defective areas significantly reduce the yield, there was a demand for Ni-based alloys with excellent surface properties.

Patent Document 1 discloses a technique for improving surface properties by carefully controlling the composition of nonmetallic inclusions in Ni-based alloys. However, depending on the structure of the nonmetallic inclusions, defects are not necessarily eliminated, posing a technical problem.

Patent Document 2 discloses a method for refining Ni-based alloys. However, the structure of nonmetallic inclusions is not examined.

Patent Document 3 describes a technology for surface treatment of Ni-based alloys. It explains that applying an appropriate passive film improves corrosion resistance. However, no technology was developed regarding surface defects.

In fact, Ni-based alloys containing large amounts of Ni, Cr, Mo, W, etc., and with improved corrosion resistance and acid resistance, are widely used in flue gas desulfurization equipment used on ships and in thermal power plants, as these equipment is operated in a harsh sulfuric acid environment. In recent years, the demand for Ni-based alloys has been expanding as environmental regulations regarding ship exhaust gases have become stricter. Furthermore, the importance of nuclear power generation has been highlighted from the perspective of carbon neutrality, and it is expected that uses will continue to expand in the future.

Patent Document 4 discloses technology that improves stainless steel for use in nuclear power plants. In recent years, in harsh operating environments, Ni-based alloys are required inside nuclear power pressure vessels, depending on the location.

Patent documents 5 and 6 present technology related to Ni-based alloy products intended for use as reactor structural components for long-term use in high-temperature water environments. However, the issue of surface defects was not resolved.

Patent No. 6990337 JP 2009-114544 A JP 2015-183290 A Special Publication No. 35-16661 Patent No. 4340899 Patent No. 4433230

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. 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 must be prevented because they form in an environment with a high oxygen concentration, resulting in a large number of inclusions in total and also being prone to clustering.

In other words, the object of the present invention is to comprehensively consider the above and provide a Ni-based alloy with sound surface properties free of surface defects, and a method for producing the same.

In order to solve the problems with the above-mentioned conventional technology, the inventors conducted repeated experiments as described below.

First, 20 kg of Ni-16%Cr-16%Mo-3%W-6%Fe alloy was melted in a magnesia crucible in a laboratory. 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 electrolytic iron, ferrochrome, Mo, copper wire, and W. Finally, Si and Al were added to deoxidize the mixture, and 1 kg of artificial slag of CaO-Al ₂ O ₃ -MgO-SiO ₂ -F system was charged and sampled to observe the composition of the nonmetallic inclusions. The 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 at least 10 points of the inclusions need to be analyzed to find the compositional characteristics, 20 points is better, and 30 points or more is sufficient.

The Ni-based 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 the so-called refining 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.

With this in mind, for some alloy types, the raw materials were melted in a 70-ton electric furnace, decarburized using an AOD, quicklime was added from above while FeSi was added at the same time, Cr reduction was carried out, and then Al was added to proceed with deoxidization and desulfurization for refinement. Slabs were then produced using 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 thin plates 0.5 mm thick. The surfaces were then observed with the naked eye to study the presence or absence of surface defects.

In other words, the present invention was developed by starting with the above laboratory studies and then conducting actual machine tests to confirm the results, as shown below.

The chemical components are all in mass % below: C: 0.001-0.1%, Si: 0.01-0.4%, Mn: 0.1-1%, P: 0.03% or less, S: 0.002% or less, Cr: 13-35%, Mo: 18% or less, W: 5% or less, Cu: 1% or less, Al: 0.001-0.35%, Ti: 0.001-0.2%, Ca: 0.0001-0.02%, Mg: 0.0001-0.02%, N: 0.001-0.03%, Fe: 2-11%, O: 0.005% or less, and one or more of Co, Nb, and B are contained in an amount of 4% or less, with the balance being Ni and unavoidable impurities. The average composition of non-metallic inclusions is MgO: 30-70%, Al ₂ O It is a Ni-based alloy composed of _{: SiO 2 :} 15-40%, CaO: 10-40%, and MnO: 1% or less.

In the Ni-based alloy of the present invention, the structural form of the nonmetallic inclusions is more preferably one or more of the following five types of nonmetallic inclusions a to e, in which the nonmetallic inclusions account for 70% or more of the total nonmetallic inclusions in terms of number ratio:
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

In addition, the above-mentioned CaO-Al ₂ O ₃ -MgO-based oxide is contained, and in a preferred embodiment, 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 TiO ₂ : 0.5% or less.

Here, the above-mentioned MgO.Al ₂ O ₃ is contained, and it is preferable that the MgO.Al ₂ O ₃ contains 0.5% or less of MnO.

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

The present invention also provides a manufacturing method, which is a method for manufacturing a Ni-based alloy, in which raw materials are melted in an electric furnace, then decarburized and refined in an AOD, quicklime, magnesia-containing waste bricks, fluorite, ferrosilicon alloy, and one or more of Si and Al are added to perform Cr reduction, Al is further added to form CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag, deoxidized and desulfurized, a slab is produced in a continuous casting machine, the surface is ground, and then a hot rolling process is performed, followed by cold rolling.

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: 0.001-0.1%
C is an important element for maintaining the strength of Ni-based alloys and for improving wear resistance by forming carbides with Cr. Therefore, 0.001% is necessary. On the other hand, adding more than 0.1% has the drawback of also forming carbides of Ti, Nb, etc., causing embrittlement. Therefore, the limit is set at 0.001-0.1%. Addition is controlled by adding pig iron after desulfurization in the AOD. The preferred range is 0.002-0.09%, and more preferably 0.005-0.07%.

Si: 0.01~0.4%
In addition to being effective in deoxidation, silicon is an element that improves heat resistance and is very important in the present invention. Deoxidation can be carried out according to the following formula. In other words, in AOD, FeSi alloy is added to Cr reduction.
Si + 2O =( _SiO2 )...(1)
Here, the underlined parts are the components in the molten alloy, 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 0.4%, not only will the σ phase form, reducing corrosion resistance, but it will also become embrittled. Therefore, the range is specified as 0.01 to 0.4%. The range is preferably 0.02 to 0.38%, and more preferably 0.02 to 0.37%.

Mn: 0.1 to 1%
Mn is an effective element for deoxidization, and therefore, its content is specified as 0.1 to 1%, preferably 0.2 to 0.9%, and more preferably 0.3 to 0.8%.

P: 0.03% 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. Therefore, the content is set to 0.03% or less. It is preferably 0.028% or less, and more preferably 0.025% or less.

S: 0.002% 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 0.002% 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 0.002% or less, the above problems can be avoided, so the content is set within this range. The content is preferably 0.0018% or less, more preferably 0.0017% or less, and even more preferably 0.0015% or less.

Cr: 13 to 35%
Cr is a major element in the present invention, and is essential for forming a dense passive film on the surface of the Ni-based alloy to maintain corrosion resistance. Therefore, the content is specified as 13 to 35%. The content is preferably 14 to 34%, more preferably 14.5 to 33%, and even more preferably 15 to 32.5%.

Mo: 18% or less Mo is an important element in the present invention. It plays a role in retarding the progress of pitting corrosion and improving corrosion resistance. However, if the content is too high, it forms intermetallic compounds and causes embrittlement. Therefore, it is specified to be 18% or less. It is preferably 17% or less, and more preferably 16% or less. It is also desirable to contain 0.5% or more. Furthermore, it is more desirable to contain 2% or more.

W: 5% or less W is an important element in the present invention. It plays a role in retarding the progression of pitting corrosion and improving corrosion resistance. However, if the content is too high, it forms intermetallic compounds and causes embrittlement. Therefore, it is specified to be 5% or less. It is preferably 4.5% or less, and more preferably 4.2% or less. It is also desirable to contain 0.5% or more. Furthermore, it is more desirable to contain 2% or more.

Cu: 1% or less Cu is an important element for improving acid resistance and sulfuric acid resistance, and for maintaining corrosion resistance. It also has the effect of softening in cold working, making it an extremely important element for deep drawing applications. However, if the Cu content is too high, the Ni-based alloy will soften. Therefore, it is specified to be 1% or less. It is preferably 0.9% or less, and more preferably 0.6% or less. It is also desirable for the Cu content to be 0.01% or more.

Al: 0.001-0.35%
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 0.35%. The range is preferably 0.005 to 0.3%, more preferably 0.007 to 0.28%, and even more preferably 0.01 to 0.25%.

Ti: 0.001-0.2%
Ti has the effect of fixing N in the form of TiN, preventing blowholes caused by nitrogen gas release during solidification. However, if the content is too high, not only will there be too much TiN, but the formation of TiC can also cause surface defects. Therefore, the Ti content is specified to be in the range of 0.001 to 0.2%. The preferred range is 0.002 to 0.1%. The more preferred range is 0.002 to 0.05%.

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%. It is preferably 0.0003 to 0.018%, and more preferably 0.0005 to 0.015%. It is further preferably 0.0006 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%. The preferred range is 0.0005 to 0.018%, and the more preferred range is 0.001 to 0.015%.

Fe: 2 to 11%
Fe is an important element for maintaining strength by dissolving in Ni-based alloys. It also allows the use of ferroalloys as a raw material, which reduces costs. However, if the content is too high, it reduces corrosion resistance. Therefore, it is specified to be 2 to 11%. It is preferably 3 to 10.5%, and more preferably 4 to 10%.

N: 0.001-0.03%
N is effective in maintaining strength by dissolving in Ni-based alloys, and also plays a role in preventing pitting corrosion by providing NH ₄ Cl to pitting corrosion. Therefore, the range of N is set to 0.001 to 0.03%, preferably 0.002 to 0.028%, and more preferably 0.008 to 0.025%.

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 0.002% or less, and even more preferably 0.001% or less.

One or more of Co, Nb, and B, 4% or less Co is an effective element for behaving in the same way as Ni. It is also sometimes contained in pure Ni raw materials, and plays an important role in terms of using inexpensive raw materials. Nb is useful for maintaining strength by forming a solid solution. B improves hot workability. It is more preferable to include one or more of these elements at 4% or less. In other words, they may be added as necessary.

The balance of the Ni-based alloy of the present invention is 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 scraps that are the raw materials.

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 alloy 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 in Figure 1(b) does not appear in the following operation. In order to control these structural forms, it is sufficient to control the chemical components Si, Al, Ca, Mg, O, and Mn within the ranges of the present invention, but this is not all. a and c are also characteristic of this application. In order to achieve this structural form, the key point is after the decarburization process in AOD. During the Cr reduction period, quicklime is first added, and at the same time, fluorite and waste bricks containing MgO are added. After that, when ferrosilicon alloy and Al are added, Si and Al are added to the molten slag that has already been formed. This directly reacts with the CaO and MgO in the slag, effectively adding Ca and Mg.

Mg is supplied to the molten alloy 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 alloy 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 will behave as described above and will be 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 are set because the molten state of CaO-Al ₂ O ₃ -MgO oxides can be maintained at the refining temperature, i.e., near 1500°C. If the CaO content is too high, harmful CaO simple inclusions are formed, so CaO: 20-60% is specified. If the Al ₂ O ₃ content is too high, Al ₂ O ₃ simple substance is formed, so Al ₂ O ₃ : 30-60% is specified. MgO: 1-30% is specified to maintain the molten state. If the SiO ₂ content is too high, harmful MnO-Cr ₂ O ₃ inclusions are formed, so 20% or less is specified. This is specified because _TiO2 can remain molten if its content is 0.5% 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 decarburized and refined using an AOD. The raw materials can be stainless steel scrap, Ni, ferronickel alloy, ferrosilicon alloy, iron scrap, Mo, Mo trioxide, ferromolybdenum alloy, ferroniobium alloy, ferrochrome alloy, copper wire, etc., which can be blended according to the desired alloy type.

After the decarburization process is completed, the Cr in the molten alloy is oxidized to form _{Cr2O3 .} Therefore, quicklime and FeSi are added to carry out Cr reduction according to the following reaction:
2( _{Cr2O3 )} + 3Si =3( _SiO2 )+ 4Cr ...(13)

Furthermore, Al is added to form CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag, which is then deoxidized and desulfurized, and 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 preferable 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 : 30% 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 30%, it creates Al ₂ O ₃ simplex inclusions and promotes clustering. Therefore, it is desirable to keep it at 30% or less. It is preferably 25% or less. It is more preferably 22% 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 alloy 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 : 3-32%
_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 alloy will be high, and the nonmetallic inclusion composition will also be MnO- _Cr2O3 - _based . As a result, surface defects will occur. Therefore, 3-32% is preferable. 5-30% is more preferable. 5-20% is even more preferable.

F: 0.5 to 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 AOD and the ladle, shortening their lifespan. Therefore, 0.5 to 10% is preferable. 1 to 9% is more preferable.

Furthermore, it is desirable that the concentrations of Cr ₂ O ₃ 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 0.3% or more is desirable, and 0.4% or more is even more desirable.

The effectiveness of the present invention will be clarified by showing examples below.
The raw materials were melted in a 70-ton electric furnace and then decarburized and refined by AOD. The raw materials were special steel scrap, stainless steel scrap, pure Ni, ferrosilicon alloy, ferrosilicon alloy, iron scrap, Mo, Mo trioxide, ferromolybdenum alloy, ferroniobium alloy, ferrochrome alloy, tungsten, Ti, copper wire, etc., which were blended according to the target alloy type.

After the decarburization process was completed, the _Cr2O3 that had been transferred to the slag was reduced. That is, quicklime and waste bricks containing magnesia, as well as fluorite, were added, and then Al was added. This reduced the Cr _oxide in the slag, completing the Cr reduction process in which the valuable metal Cr was returned to the molten alloy.

Furthermore, in some alloy types, ferrosilicon alloy was added to finally form CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag. Even when Al was not added, alumina was supplied by oxidation of Al, which is an impurity in the FeSi alloy.

For some alloy types, alloying elements such as Nb, Mo, C, Co, W, Cr, B, and Ni were added here to precisely adjust the chemical composition. After deoxidization and desulfurization in this way, slabs of 200 mmt x 1200 mmw x 7 m length were produced in a continuous casting machine. The oscillation marks on the surface were then ground away, and the slabs were heated to 1100-1250°C depending on the alloy type, 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 alloy 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 C and S were analyzed by the combustion method. N and O were analyzed by the infrared absorption method. In some cases, H was also measured by a hydrogen analyzer.
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) Overall evaluation: An inspector visually evaluated a 1 mm thick Ni-based alloy plate when it was threaded. The evaluation results were determined as follows. In the following, the allowable range for not cutting the defective portion 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).

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 total chemical components of the examples are less than 100% due to unavoidable impurities. Inventive examples No. 1-14 had chemical analysis within the range, and the slag composition also met the desired range. As a result, the structural forms of the non-metallic inclusions also met the ranges 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, so the evaluation was ◯. In No. 8, one CaO inclusion was observed. Therefore, the evaluation was ◯. In No. 10, the MnO and Cr ₂ O ₃ concentrations in the slag were slightly high, and reduction was poor, so Al ₂ O ₃ inclusions and MnO-Cr ₂ O ₃ inclusions were confirmed. Therefore, the evaluation was ◯. 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, and at the same time, CaO inclusions, Al ₂ O ₃ inclusions, and MnO-Cr ₂ O ₃ inclusions were confirmed. As a result, the evaluation was △.

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 a high _SiO2 concentration in the slag. The F concentration was also low, resulting in a lack of slag fluidity. The Ti concentration was also 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 the 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 and Al was not included, which resulted in low Ca and Mg concentrations, and the oxygen concentration was high, resulting in a large number of non-metallic inclusions. In addition, the slag composition was also out of range, with high silica and low CaO, which adversely affected the inclusion composition. The average composition of the inclusions was out of 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 out of range in the CaO-Al ₂ O ₃ -MgO oxide, and MnO-Cr ₂ O ₃ inclusions were confirmed. As a result, surface defects occurred along the entire length, resulting in scrapping.

No. 18 had a high Al concentration, and the alumina concentration in the slag was high, 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. _As a result, surface defects occurred along the entire length, causing scrapping.

No. 19 had a low Al concentration, which resulted in low Ca and Mg concentrations and a high oxygen concentration, which was off the mark. The average composition of the nonmetallic inclusions was high in Cr ₂ O ₃ , and three types of structure, f, g, and h, were formed. In particular, a large number of h.MnO-Cr ₂ O ₃ inclusions were observed. Furthermore, MgO.Al ₂ O ₃ simple inclusions and components that form a solid solution in MgO simple inclusions were also off the mark. In addition, the Ti and N concentrations were high, and TiN clusters were also observed. As a result, surface defects occurred along the entire length, resulting in scrapping.

In No. 20, the Ti and N values were too high, which resulted in many TiN clusters and surface defects, leading to scrapping. In Table 2, all of the inclusion types are a and b, which at first glance seems to be good, but this is because TiN is not counted as an inclusion.

Claims (6)

The average composition of the nonmetallic inclusions is as follows: C: 0.001-0.1%, Si: 0.01-0.4%, Mn: 0.1-1%, P: 0.03% or less, S: 0.002% or less, Cr: 13-35%, Mo: 18% or less, W: 5% or less, Cu: 1% or less, Al: 0.001-0.35%, Ti: 0.001-0.2%, Ca: 0.0001-0.02%, Mg: 0.0001-0.02%, N: 0.001-0.03%, Fe: 2-11%, O: 0.005% or less, and one or more of Co, Nb, and B are 4% or less, with the _balance being Ni and unavoidable impurities _. : 15-40%, SiO ₂ : 30% or less, CaO: 10-40%, MnO: 1% or less, and a Ni-based alloy having excellent surface properties. The Ni-based alloy having excellent surface properties according to 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-based alloy having excellent surface 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-based alloy having excellent surface 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-based alloy having excellent surface 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. A method for producing a Ni-based alloy according to any one of claims 1 to 5, characterized in that the raw materials are melted in an electric furnace, then decarburized and refined in an AOD, and one or more of quicklime, magnesia-containing waste bricks, fluorite and ferrosilicon alloy, Si and Al are added to perform Cr reduction, and then Al is added to form a CaO-Al ₂ O ₃ -MgO-SiO ₂ -F slag, which is then deoxidized and desulfurized, and then a slab is produced in a continuous casting machine, and the surface is ground, followed by a hot rolling process and then a cold rolling process.

Images