WO2024085002A1

WO2024085002A1 - Fe-Cr-Ni-BASED ALLOY HAVING EXCELLENT SURFACE PROPERTIES, AND METHOD FOR MANUFACTURING SAME

Info

Publication number: WO2024085002A1
Application number: PCT/JP2023/036560
Authority: WO
Inventors: 大樹小笠原; 建次水野
Original assignee: 日本冶金工業株式会社
Priority date: 2022-10-21
Filing date: 2023-10-06
Publication date: 2024-04-25
Also published as: JP2024061288A; JP7369266B1

Abstract

An Fe-Cr-Ni-based alloy comprising 0.020-0.150% of C, 0.05-0.80% of Si, 0.10-1.50% of Mn, 0.035% or less of P, 0.0050% or less of S, 34.0-48.0% of Ni, 22.0-29.0% of Cr, 0.20-1.20% of Mo, 0.005-0.180% of Al, 0.0001-0.0100% of Mg, 0.0001-0.0100% of Ca, 0.20-0.80% of Nb, 0.050-0.500% of N, 0.0001-0.0060% of O, 0.80% or less of Cu, 0.100% or less of Ti, and 0.50% or less of Co, the remaining portion being Fe and unavoidable impurities, wherein the alloy includes one or more nonmetal inclusions among MgO, CaO, CaO-MgO-based oxides, CaO-Al₂O₃-MgO-based oxides, and MgO·Al₂O₃, and the count proportion of MgO·Al₂O₃ with respect to all oxide-based nonmetal inclusions is 50% or less.

Description

Fe-Cr-Ni alloy with excellent surface properties and manufacturing method thereof

The present invention relates to an Fe-Cr-Ni alloy with excellent surface properties and a method for producing the same, and in particular to an Fe-Cr-Ni alloy with excellent surface properties in which the non-metallic inclusions in the molten metal are controlled to a harmless composition by controlling the slag composition and the amounts of Si, Al, Mg, Ca, and O in the molten metal, and the number of inclusions on the surface is reduced, and to a method for producing the same, and relates to an Fe-Cr-Ni alloy that is required to have strict corrosion resistance and high-temperature strength as a structural material for reactors and the like.

In the manufacture of solar power generation equipment, Fe-Cr-Ni alloy plates are primarily used as the material for reactors used to refine polysilicon, the raw material for power generation elements. These reactors are operated at high temperatures and pressures, creating an extremely harsh environment in terms of high-temperature strength and corrosion resistance. If high-temperature strength and corrosion resistance are insufficient, the useful life of the reactor will be shortened, so there is a demand for Fe-Cr-Ni alloys that meet these characteristics.

Fe-Cr-Ni alloys have excellent high-temperature strength and corrosion resistance, and in addition to the main component Fe, they also contain Cr, Ni, and Mo. Because these metals are extremely expensive compared to iron, it is extremely important to improve yield and reduce manufacturing costs. Here, if surface defects such as linear scratches occur on the surface of an Fe-Cr-Ni alloy, they must be removed by grinding or cutting, which significantly reduces the yield, and therefore there is a demand for Fe-Cr-Ni alloys with excellent surface properties.

Patent Document 1 discloses a technology that suppresses the coarsening of crystal grains at the annealing temperature of a product by controlling the size and number of Nb and Ti nitrides or carbides in an Fe-Cr-Ni alloy with excellent high-temperature strength, thereby achieving high creep rupture properties. However, Nb and Ti nitrides or carbides do not cause defects on the surface of the product, and the invention of Patent Document 1 cannot be applied to the problem of surface properties caused by oxide-based non-metallic inclusions formed during the refining process, which is the subject of the present invention, and the problem of surface defects caused by oxide-based non-metallic inclusions remains.

Patent Document 2 discloses a technique for manufacturing a high-Ni alloy and a high-Ni alloy for high temperatures containing Al and Ti, in which the composition of the oxide-based inclusions is controlled to a CaO-Al ₂ O 3-based alloy with a low melting point by setting the Ca/Al mass ratio in the oxide-based inclusions in the range of 1.0 to _1.5 , thereby preventing clogging of the submerged nozzle of a continuous casting machine and preventing surface defects of the product. However, Patent Document 2 discloses that, since Ti is contained at 0.15 to 1.5%, CaO-Al ₂ O ₃ -TiO ₂ is generated as an oxide-based inclusion, causing nozzle clogging. Since the Ti content in the present invention is 0.10% or less, nozzle clogging due to CaO-Al ₂ O ₃ -TiO ₂ does not occur, so the contents described in Patent Document 2 cannot be said to sufficiently improve the surface properties of the Fe-Cr-Ni alloy of the present invention.

Patent Document 3 discloses a technique for reducing surface defects in high Ni alloys by controlling the composition of nonmetallic inclusions in the alloy and making them low-melting inclusions with good stretchability and separability during hot or cold rolling. However, the high Ni alloys in Patent Document 3 are intended for alloys containing 0.5% or less Cr or 3-10%, which differs from the Fe-Cr-Ni alloys of the present invention, which contain 22.0-29.0% Cr. The Cr content has a large effect on inclusion composition control, and even if the trace elements such as Ca, Mg, Al, Si, and O are the same, the composition of oxide-based nonmetallic inclusions is significantly different. In other words, the method for controlling the composition of nonmetallic inclusions described in Patent Document 3 cannot be said to sufficiently improve the surface properties of the Fe-Cr-Ni alloys of the present invention.

Patent Document 4 reports a technique for reducing surface defects by controlling inclusions in stainless steel sheets to harmless MgO and CaO-Al ₂ O ₃ -MgO oxides. Incidentally, Nb, which is contained in the Fe-Cr-Ni alloy of the present invention at 0.20-0.80%, has the same oxidation ability as Si and Mn. That is, Nb oxides are contained in the inclusions, even though in small amounts, and improve the elongation and detachability in hot or cold rolling by lowering the melting point of the inclusions, so Nb is an important element in controlling surface defects. However, the stainless steel sheet described in Patent Document 4 does not contain Nb, and is different from the technique for obtaining inclusion properties that are less likely to cause surface defects in the present invention.

Patent Document 5 reports a technology for preventing surface defects by reducing the number ratio of MgO.Al ₂ O ₃ among nonmetallic inclusions in stainless steel sheet to 50% or less. However, whereas Al is mainly used to deoxidize the molten metal in the present invention, Si is used in Patent Document 5. For this reason, the basicity of the slag in Patent Document 5 is 2 to 5, which is lower than that of the present invention, and since basicity is a factor that greatly affects the properties of inclusions, the technology described in Patent Document 5 is a technology different from the present invention.

Patent Document 6 reports a technique for adding Nb to Fe-Ni-Cr alloys with a high yield. However, in the present invention, Nb is added at the time of primary deoxidation of the molten metal, i.e., when the oxygen concentration is relatively high, 0.0070 to 0.0120%, to control the Nb concentration in nonmetallic inclusions, whereas Patent Document 6 states that Nb must be added after preliminary deoxidation with Si followed by deoxidation with Al to sufficiently lower the oxygen concentration. Furthermore, Patent Document 6 states that Nb ₂ O ₅ inclusions should be avoided because they cause surface defects, but in the present invention, Nb is added by the above method to cause a small amount of NbO to be contained in CaO-Al ₂ O ₃ -MgO inclusions, improving the elongation and separability during hot or cold rolling, thereby preventing surface defects. Therefore, the technique described in Patent Document 6 is different from the present invention.

JP 2017-057461 A JP 2021-70838 A Japanese Patent Application Laid-Open No. 11-315354 JP 2019-35124 A JP 2015-074807 A JP 2014-105341 A

In view of the above problems, the present invention aims to provide an Fe-Cr-Ni alloy with excellent surface properties by controlling the composition of nonmetallic inclusions that affect the surface properties. Furthermore, it also provides a manufacturing method for the Fe-Cr-Ni alloy to achieve this.

The inventors have conducted extensive research and investigations to solve the above problems, and have found that the cause of the surface defects in Fe-Cr-Ni alloy sheets is _nonmetallic inclusions of _MgO.Al2O3 , CaO, and CaO-MgO oxides by performing detailed analysis of the surface defects in the Fe-Cr-Ni alloy sheets using a scanning electron microscope (SEM) and an energy dispersive X-ray analyzer (EDS). These types of nonmetallic inclusions are prone to become large when attached to the inner wall of a submerged nozzle for pouring molten metal from a tundish into a mold in a continuous casting machine, and those that fall off are likely to be captured by the solidified shell and become the origin of surface defects. In addition, because of their high melting point, they are difficult to elongate during hot rolling, and are not dispersed into small pieces, becoming the origin of surface defects in Fe-Cr-Ni alloy sheets.

The inventors further conducted intensive research into the relationship between inclusion composition and metal components in Fe-Cr-Ni alloys. Specifically, during the manufacturing process of Fe-Cr-Ni alloys, metal samples of Fe-Cr-Ni alloys were taken from the tundish inside the continuous casting machine, and 20 inclusions larger than 5 μm were randomly selected from the samples, and the inclusion composition was measured using SEM/EDS. In addition, a submerged nozzle for supplying molten metal from the tundish inside the continuous casting machine to the mold was taken, and the composition of the deposits on the inner wall of the nozzle was analyzed using SEM/EDS. Based on the above, intensive research was conducted into the relationship between inclusion composition, metal components, and deposits on the inner wall of the submerged nozzle.

As a result, it was found that the nonmetallic inclusions in Fe-Cr-Ni alloys contain one or more of MgO, CaO, CaO-MgO oxides, CaO-Al ₂ O ₃ -MgO oxides, and MgO.Al ₂ O ₃ , and further, a guideline was obtained that allows the inclusion composition to basically be controlled to MgO or CaO-Al ₂ O 3 -MgO oxides by controlling the Si concentration to 0.05 to 0.80 mass% and the Al concentration to 0.005 to 0.180 mass%, while adjusting the Mg concentration to 0.0001 to 0.0100 mass%, the Ca concentration to 0.0001 to _0.0100 mass%, and the O concentration to O: 0.0001 to 0.0060 mass%. Furthermore, it _was found that when the ratio of _MgO.Al2O3 to all oxide-based nonmetallic inclusions is 50% or less and the combined ratio of CaO and CaO-MgO-based oxides is 50% or less, the nonmetallic inclusions are less likely to adhere to and accumulate on the inner wall of the submerged nozzle, i.e., are less likely to grow in size and become a cause of surface defects. It was also found that such nonmetallic inclusions have excellent cleanliness because they are finely divided by hot rolling and cold rolling.

The Fe-Cr-Ni alloy of the present invention has been developed based on the above findings, and contains, in mass %, C: 0.020-0.150%, Si: 0.05-0.80%, Mn: 0.10-1.50%, P: 0.035% or less, S: 0.0050% or less, Ni: 34.0-48.0%, Cr: 22.0-29.0%, Mo: 0.20-1.20%, Al: 0.005-0.1 ... An Fe-Cr-Ni based alloy comprising 0.0001-0.0100% Ca, 0.0001-0.0100% Nb, 0.20-0.80% N, 0.050-0.500%, 0.0001-0.0060%, 0.80% or less Cu, 0.100% or less Ti, 0.50% or less Co, and the balance being Fe and unavoidable impurities, wherein the nonmetallic inclusions are MgO and CaO-Al The composition is characterized in _that it contains either or both of CaO and CaO-MgO- _based oxides as essential components, and can contain any of CaO, CaO-MgO-based oxides, and MgO.Al ₂ O ₃ as optional components, the number ratio of MgO.Al ₂ O ₃ to all oxide-based non-metallic inclusions being 50% or less, and the combined number ratio of CaO and CaO-MgO-based oxides being 50% or less.

In the present invention, the CaO-Al ₂ O ₃ -MgO-based oxides in the non-metallic inclusions preferably contain 0.01 to 0.60 mass % of NbO.

In the present invention, it is preferable that the CaO-MgO-based oxides in the non-metallic inclusions are, in mass %, CaO: 20-80% and MgO: 20-80%, the CaO-Al ₂ O ₃ -MgO-based oxides are CaO: 10-60%, Al ₂ O ₃ : 5-60%, MgO: 10-80%, and SiO ₂ : 10% or less, and the MgO.Al ₂ O ₃ is MgO: 10-40%, and Al ₂ O ₃ : 60-90%.

Furthermore, the present invention also provides a manufacturing method, namely, melting the raw materials in an electric furnace, then decarburizing them with AOD or AOD followed by VOD, then adding lime and fluorite, then adding one or two of ferrosilicon alloy and pure silicon, and Al for primary deoxidization, and when the O concentration reaches 0.0070-0.0120%, adding Nb, and producing a CaO-SiO 2 -MgO-Al 2 O ₃ alloy consisting of CaO: 45-75%, SiO ₂ : 1-15%, Al ₂ O ₃ : 10-30%, MgO: _5-20 _% , and F: 1-15%. The method for producing an Fe-Cr-Ni based alloy having excellent surface properties is characterized in that a slag containing 1-F is used, then one or both of a ferrosilicon alloy and pure silicon, and Al are added, followed by Cr reduction, secondary deoxidization, and desulfurization, and then a slab or an ingot is produced by a continuous casting machine or a normal ingot making machine. In the case of an ingot, hot forging is performed, and subsequently hot rolling or hot rolling followed by cold rolling is performed.

First, the reasons for limiting the chemical components of the Fe-Cr-Ni alloy of the present invention will be described. In the following explanation, "%" means "mass% (mass %)".
(C: 0.020 to 0.150%)
C is an element that stabilizes the austenite phase, but if present in large quantities, it combines with Cr and Mo to form carbides, reducing the amount of solute Cr and Mo contained in the base material and deteriorating corrosion resistance. Therefore, the C content is set to 0.020 to 0.150%, preferably 0.030 to 0.100%, and more preferably 0.040 to 0.070%.

(Si: 0.05 to 0.80%)
Since Si is an effective element for deoxidization, it is an important element in the present invention. 0.05% is necessary to control the oxygen concentration to 0.0001-0.0060%. Furthermore, it also plays a role in reducing CaO and MgO in the CaO-SiO ₂ -MgO-Al ₂ O ₃ -F slag and adjusting Mg and Ca in the molten metal to 0.0001-0.0100% and 0.0001-0.0100%, respectively. This has the effect of maintaining the inclusions in harmless MgO and CaO-Al ₂ O ₃ -MgO system. From this viewpoint, 0.05% is necessary. On the other hand, if it is contained in excess of 0.80%, the CaO and MgO in the slag are reduced too much, and Mg and Ca are supplied in excess of 0.0100%. As a result, CaO and CaO-MgO oxides are generated in a total number ratio exceeding 50%, which causes many surface defects and pits in the product and deteriorates the surface properties. In addition, excessive Mg content in the alloy reduces hot workability and generates cracks during hot rolling, resulting in surface defects. Therefore, the Si content is specified to be 0.05-0.80%, preferably 0.08-0.60%, and more preferably 0.10-0.40%.

(Mn: 0.10 to 1.50%)
Mn is an austenite phase stabilizing element and also contributes to deoxidation, so it is necessary to add 0.10% or more. However, adding a large amount impairs oxidation resistance, so the upper limit is set at 1.50%. The preferred range is 0.30 to 1.00%, and the more preferred range is 0.50 to 0.80%.

(P: 0.035% or less)
Since P is a harmful element that segregates at grain boundaries and causes cracks during hot working, it is desirable to reduce the P content as much as possible and limit it to 0.035% or less, preferably 0.030% or less, and more preferably 0.025% or less.

(S: 0.0050% or less)
S is a harmful element that segregates at grain boundaries to form low melting point compounds and impair hot workability, so it is desirable to reduce it as much as possible and limit it to 0.0050% or less. To achieve this, the lower limit of the Al content is set to 0.005%, and deoxidation is promoted by controlling the O concentration to the range of 0.0001 to 0.0060%, thereby promoting desulfurization. It is preferably 0.0030% or less. More preferably, it is 0.0010% or less.

(Ni: 34.0 to 48.0%)
Ni is the main element in the Fe-Cr-Ni alloy of the present invention, and is an element that stabilizes the austenite phase, maintains high-temperature strength, and has high corrosion resistance. By containing 34.0% or more, pitting corrosion resistance and acid resistance that can withstand use in severe corrosive environments can be obtained. However, since Ni is a very expensive raw material compared to Fe, adding a large amount of Ni increases the manufacturing cost, which is not preferable. Therefore, the upper limit is specified as 48.0%. It is preferably 35.0 to 45.0%. More preferably, it is 37.0 to 40.0%.

(Cr: 22.0% to 29.0%)
Cr is an element that forms a passive film on the surface of an Fe-Cr-Ni alloy, and is the most important element as a component of a base material for improving acid resistance, pitting corrosion resistance, crevice corrosion resistance, and stress corrosion cracking resistance. However, if the Cr content is less than 22.0%, sufficient corrosion resistance cannot be obtained. Conversely, if the Cr content exceeds 29.0%, a σ phase is generated, resulting in embrittlement. For the above reasons, the Cr content is specified to be 22.0 to 29.0%. It is preferably 23.0 to 27.0%. More preferably, it is 24.0 to 26.0%.

(Mo: 0.20 to 1.20%)
Even a small amount of Mo significantly improves corrosion resistance in a wet environment and a high-temperature air environment where chlorides are present, and Mo has the effect of improving corrosion resistance in proportion to the amount added. Furthermore, while the upper limit of Si, which is effective for deoxidization, is 0.80%, Mo has the effect of increasing the activity coefficient of Si to compensate for the deoxidizing power, and is a useful element. Therefore, it is necessary to add 0.20% or more. On the other hand, in a material to which a large amount of Mo is added, when it is in a high-temperature air environment and the oxygen potential of the surface is low, Mo causes preferential oxidation, causing peeling of the oxide film, which may cause surface defects, so the upper limit is set to 1.20%. Preferably, it is 0.30 to 1.00%. More preferably, it is 0.40 to 0.80%.

(Al: 0.005 to 0.180%)
Al is a very effective element for deoxidization, and is a particularly important element in the present invention. It can control the oxygen concentration to the range of 0.0001 to 0.0060%, and also has the effect of reducing MgO and CaO in CaO-SiO ₂ -MgO-Al ₂ O ₃ -F slag, supplying 0.0001% or more of Mg and 0.0001% or more of Ca to the molten metal, and controlling inclusions to harmless MgO and CaO-Al ₂ O ₃ -MgO. These are due to the following reactions.
3(MgO)+ 2Al = 3Mg +( _Al2O3 ₎ ... (1)
3(CaO) + 2Al = 3Ca + ( _Al2O3 ₎ ... (2)
The numbers in parentheses indicate the components in the slag, and the underlines indicate the components in the molten metal.

If the Al concentration is less than 0.005%, deoxidation will not proceed sufficiently, and the oxygen concentration will exceed 0.0060%, resulting in a high oxygen concentration. Furthermore, because deoxidation does not proceed, desulfurization will be inhibited, and the S concentration will exceed 0.0050%. On the other hand, if the Al concentration is high, exceeding 0.180%, the Mg concentration will exceed 0.0100% due to the reaction of formula (1) above, and the Ca concentration will also exceed 0.0100% due to the reaction of formula (2) above. Therefore, the range of Al content is specified as 0.005 to 0.180%. Preferably, it is 0.010 to 0.120%. More preferably, it is 0.020 to 0.100%.

(Mg: 0.0001 to 0.0100%)
Mg is an element effective in controlling the composition of nonmetallic inclusions in the molten metal to MgO and CaO-Al ₂ O ₃ -MgO oxides that do not adversely affect the surface properties. This effect cannot be obtained if the content is less than 0.0001%, and conversely, if the content exceeds 0.0100%, the hot workability decreases, making it easier for cracks to occur in the hot rolling process, resulting in surface defects in the final product. For this reason, the Mg content is specified to be 0.0001 to 0.0100%. Preferably, it is 0.0002 to 0.0050%. More preferably, it is 0.0003 to 0.0020%.

To effectively add Mg to the molten metal, it is preferable to utilize the reaction shown in formula (1). To control Mg within the above range, the slag composition should be controlled to CaO: 45-75%, _SiO2 : 1-15%, _Al2O3 : 10-30%, MgO: 5-20%, _and F: 1-15%.

(Ca: 0.0001 to 0.0100%)
Ca is an element effective in controlling the composition of nonmetallic inclusions in the molten metal to CaO-Al ₂ O ₃ -MgO oxides that do not form clusters and have no adverse effect on surface quality. This effect cannot be obtained if the content is less than 0.0001%, and conversely, if the content exceeds 0.0100%, a large number of inclusions of simple CaO and/or CaO-MgO oxides are formed, causing surface defects and pits in the final product. Therefore, the Ca content is specified to be 0.0001 to 0.0100%, preferably 0.0002 to 0.0030%, and more preferably 0.0003 to 0.0020%.

To effectively supply Ca to the molten metal, it is preferable to utilize the reaction shown in formula (2). To control the Ca content within the above range, the slag composition should be controlled to CaO: 45-75%, _SiO2 : 1-15%, _Al2O3 : 10-30%, MgO: 5-20%, _and F: 1-15%.

(Nb: 0.20 to 0.80%)
Nb is an important element in the present invention, and in order to improve the strength of the Fe-Cr-Ni alloy, it is necessary to add at least 0.20% or more. However, if added in excess, it increases the thermal expansion coefficient and also increases the weld cracking susceptibility. Therefore, the upper limit is set at 0.80%. It is preferably 0.30 to 0.70%. More preferably, it is 0.40 to 0.60%.

In order to effectively retain Nb in the molten metal, since Nb is an element that easily oxidizes, it is necessary to add 0.005% or more of Al to promote deoxidation and control the O in the molten metal to 0.0060% or less.

(N: 0.050 to 0.500%)
N is a solid solution strengthening element and has the effect of increasing the strength of Fe-Cr-Ni alloys, so it is contained in an amount of at least 0.050%. However, excessive N content forms NbN, which is a nitride with Nb, and the amount of dissolved Nb that is effective in improving the strength of Fe-Cr-Ni alloys decreases. Therefore, the upper limit is set to 0.500%. The content is preferably 0.100 to 0.400%. More preferably, the content is 0.200 to 0.300%.

(O: 0.0001 to 0.0060%)
The oxygen concentration is very important in the present invention because it is closely related to inclusions. If the O content in the alloy exceeds 0.0060%, the number of inclusions increases, which leads to the generation of surface defects, and desulfurization is hindered, resulting in a high S concentration. However, if the O content is less than 0.0001%, the Al's ability to reduce CaO and MgO in the slag increases too much, so that the Mg concentration exceeds the upper limit of 0.0100%, and the Ca concentration exceeds the upper limit of 0.0100%. Therefore, the O content is specified to be 0.0001 to 0.0060%, preferably 0.0003 to 0.0050%, and more preferably 0.0005 to 0.0040%.

(Cu: 0.80% or less)
Cu is effective in improving sulfuric acid corrosion resistance, but excessive addition of Cu reduces hot workability and generates cracks, which can cause surface defects, so the Cu content is specified to be 0.80% or less, preferably 0.50% or less, and more preferably 0.30% or less.

(Co: 0.50% or less)
Co is one of the elements that stabilize austenite, but adding a large amount of Co increases the cost of raw materials, so the content is limited to 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

(Ti: 0.100% or less)
Ti may be added since it is an effective element for deoxidizing the molten metal. However, if added in excess, it will adhere as an oxide inside the submerged nozzle in the continuous casting machine, causing the nozzle to become clogged, and casting will have to be stopped. Therefore, the range of Ti content is specified to be 0.100% or less. Preferably, it is 0.050% or less. More preferably, it is 0.020% or less.

(Non-metallic inclusions)
In the present invention, the non-metallic inclusion composition preferably contains one or more of MgO, CaO, CaO-MgO-based oxides, CaO-Al ₂ O ₃ -MgO-based oxides, and MgO.Al ₂ O ₃ , with the number ratio of MgO.Al ₂ O ₃ being 50% or less and the total number ratio of CaO and CaO-MgO-based oxides being 50% or less.
In addition, it is a preferred embodiment that the CaO-Al ₂ O ₃ -MgO system oxide contains 0.01 to 0.60% of NbO.
The reasons for limiting the components and number ratios of nonmetallic inclusions are given below.

(The non-metallic inclusion composition includes one or more of MgO, CaO, CaO-MgO oxides, CaO-Al ₂ O ₃ -MgO oxides, and MgO.Al ₂ O ₃ ).
The Fe-Cr-Ni alloy according to the present invention contains one or more of MgO, CaO, CaO-MgO oxides, CaO-Al ₂ O ₃ -MgO oxides, and MgO.Al ₂ O ₃ , depending on the contents of Si, Al, Mg, and Ca in the Fe-Cr-Ni alloy. Among the methods of expressing the nonmetallic inclusion compositions, those connected with "-" indicate that the inclusion species are in a homogeneous molten state at 1600°C, the refining temperature of the Fe-Cr-Ni alloy, and those connected with "." indicate that the inclusion species form a solid intermediate compound at 1600°C, the refining temperature of the Fe-Cr-Ni alloy. Regarding CaO-MgO oxides, on the binary phase diagram of CaO and MgO, they have a eutectic composition of CaO and MgO at 1600°C, but because CaO and MgO are finely dispersed over a wide range of components in the CaO-MgO oxides, they are indicated by "-" which indicates a solid solution. Of the above non-metallic inclusions, there is no problem if MgO and CaO-Al ₂ O ₃ -MgO oxides are contained without restrictions on the number ratio, because MgO and CaO-Al ₂ O ₃ -MgO oxides do not adhere to the inner wall of the submerged nozzle for pouring molten metal from the tundish into the mold in a continuous casting machine, and therefore do not form large adhesion deposits or cause surface defects.

(The ratio _of _MgO.Al2O3 is 50% or less)
MgO.Al ₂ O ₃ can cause surface defects when large deposits of MgO.Al 2 O 3 adhere to the submerged nozzle inside the continuous casting machine, fall off, are carried into the mold together with the molten metal, and are captured by the solidified shell. However, it was found that if the number ratio of MgO.Al ₂ O ₃ is 50% or less, the tendency for adhesion is mild and the number of surface defects is suppressed. Therefore, the number ratio of MgO.Al ₂ O ₃ is specified to be 50% or less.

(NbO in CaO-Al ₂ O ₃ -MgO oxide is 0.01 to 0.60%)
NbO contained in CaO-Al ₂ O ₃ -MgO-based oxides has the effect of improving the stretchability and severability during hot or cold rolling by lowering the melting point of inclusions, so it is preferable that the content is 0.01% or more. However, if NbO is contained in excess, not only does it increase the melting point of the inclusions and deteriorate the stretchability and severability during hot or cold rolling, but it also makes it easier for inclusions of simple Nb ₂ O ₅ to occur, which causes surface defects due to the inclusions and deteriorates the surface properties. Furthermore, Nb is not effectively retained in the Fe-Cr-Ni-based alloy. Therefore, the upper limit is specified as 0.60%.

In order to make the CaO-Al ₂ O ₃ -MgO oxide contain 0.01-0.60% NbO, one or both of ferrosilicon alloy and pure silicon, and Al were added as primary deoxidation, and Nb was added when the O concentration reached 0.0070-0.0120%. Then, one or both of ferrosilicon alloy and pure silicon, and Al were added for secondary deoxidation, whereby the final O concentration was controlled to 0.0001-0.0060% while the Nb concentration in the inclusions could be precisely controlled.

(The composition ratio of CaO-MgO oxide is CaO: 20-80%, MgO: 20-80%)
The concentrations of CaO and MgO in the CaO-MgO oxide correspond to the phase ratio of CaO and MgO in the CaO-MgO oxide. If the CaO concentration is higher than 80%, the influence of the CaO phase is large, resulting in a behavior similar to that of CaO inclusions, whereas if the MgO concentration is higher than 80%, the influence of the MgO phase is large, resulting in a behavior similar to that of MgO inclusions. Therefore, the CaO concentration of the CaO-MgO oxide is specified to be 20-80%, and the MgO concentration is specified to be 20-80%.

(The composition ratio of CaO-Al ₂ O ₃ -MgO oxide is CaO: 10-60%, Al ₂ O ₃ : 5-60%, MgO: 10-80%, SiO ₂ : 10% or less)
It is more preferable that the composition of CaO, Al ₂ O ₃ , and MgO in the CaO-Al ₂ O ₃ -MgO oxides is within the above range, since it maintains a molten state at the temperature inside the submerged nozzle. Outside this range, it behaves as a solid, and tends to adhere to the inner wall of the submerged nozzle in the continuous casting machine, causing surface defects. Also, if the SiO ₂ content exceeds this range, the number of large coarsened inclusions increases, causing surface defects. Therefore, CaO is specified to be 10-60%, Al ₂ O ₃ is specified to be 5-60%, MgO is specified to be 10-80%, and SiO ₂ is specified to be 10% or less.

(The composition ratio of MgO.Al ₂ O ₃ is MgO: 10-40%, Al ₂ O ₃ : 60-90%)
MgO.Al ₂ O ₃ is a compound that has a relatively wide range of solid solutions, and is defined as such because it forms a solid solution within the above range.

(The total number ratio of CaO and CaO-MgO oxides is 50% or less)
CaO is an inclusion that reacts with moisture in the air on the surface of a product to become a hydrate, which causes it to fall off from the surface and cause pits. CaO-MgO oxides are inclusions that have a mixed phase of CaO and MgO phases in one inclusion. Compared to MgO, CaO-MgO oxides are more likely to become hydrates and fall off from the surface of a product to cause pits. If there are pits on the surface of a product, corrosion will easily progress from the pits in a corrosive environment, resulting in large hole-like defects on the surface of the product. In addition, CaO and CaO-MgO oxides adhere to the inner wall of a submerged nozzle for pouring molten metal from a tundish into a mold in a continuous casting machine, and the large attached deposits fall off and are carried into the mold together with the molten metal, and are captured by the solidified shell, which can cause surface defects. On the other hand, it was found that if the total number ratio of CaO and CaO-MgO oxides is 50% or less, the tendency for CaO to adhere to the nozzle is mild, suppressing the number of surface defects, and suppressing the occurrence of pits due to hydrates falling off the surface of the product. Therefore, the total number ratio of CaO and CaO-MgO oxides is specified to be 50% or less.

(Production method)
The present invention also proposes a method for producing an Fe-Cr-Ni alloy. First, the raw materials are melted in an electric furnace to produce a molten Fe-Cr-Ni alloy having a predetermined composition. Next, the molten metal is decarburized using AOD (Argon Oxygen Decarburization) or AOD followed by VOD (Vacuum Oxygen Decarburization). Then, lime and fluorite are added, and one or two of ferrosilicon alloy and pure silicon, and Al are added as the primary deoxidation. When the O concentration reaches 0.0070-0.0120%, Nb is added, and then one or two of ferrosilicon alloy and pure silicon, and Al are added for secondary deoxidation. The molten metal is refined using a CaO-SiO ₂ -MgO-Al ₂ O ₃ -F slag consisting of CaO: 45-75%, SiO ₂ : 1-15%, Al ₂ O ₃ : 10-30%, MgO: 5-20%, and F: 1-15%. The molten metal is then tapped into a ladle, and the temperature and composition are adjusted, followed by the production of a slab or ingot by a continuous casting machine or a normal ingot casting machine. The ingot is then hot forged to produce a slab. This allows the nonmetallic inclusions to be controlled to one or more of MgO, CaO, CaO-MgO oxides, CaO-Al ₂ O ₃ -MgO oxides, and MgO.Al ₂ O ₃ , and the total number ratio of CaO and CaO-MgO oxides is 50% or less, and the number ratio of MgO.Al ₂ O ₃ is suppressed to 50% or less, so that an Fe-Cr-Ni alloy with excellent surface properties can be obtained. The surface of the produced slab is ground and heated, and then hot-rolled or hot-rolled and then cold-rolled, annealed, pickled, surface scale is removed, and finally a plate is produced.

In the method for producing an Fe-Cr-Ni alloy according to the present invention, the slag composition is characterized as described above. The basis for the slag composition specified in the present invention will be explained below.
(CaO: 45 to 75%)
The CaO concentration and _SiO2 concentration in the slag are elements for efficient deoxidation and desulfurization and inclusion control. If the CaO concentration exceeds 75%, the activity of CaO in the slag becomes high, and the reaction of formula (2) proceeds too much. Therefore, the Ca concentration reduced in the molten metal exceeds 0.0100%, and nonmetallic inclusions of CaO alone and/or CaO-MgO oxides are generated. When these inclusions adhere to the inside of the immersion nozzle in the continuous casting machine, the adhering deposits fall off and are carried into the mold together with the molten metal, and are captured by the solidified shell, causing surface defects in the final product. In addition, CaO and CaO-MgO oxides react with moisture in the air to become hydrates, which fall off from the surface of the final product to cause pits, so that excessive presence of CaO and CaO-MgO oxides causes deterioration of the surface properties. Therefore, the upper limit is set at 75%. On the other hand, if the CaO concentration is less than 45%, deoxidation and desulfurization do not proceed, and it becomes impossible to control the S concentration and O concentration in the present invention within the specified range. Therefore, the lower limit is specified as 45%, preferably 50 to 70%, and more preferably 53 to 68%.

( _SiO2 : 1 to 15%)
_SiO2 in the slag is an element necessary for ensuring the proper fluidity of the slag, so at least 1% is necessary. However, if it exceeds 15%, the Al concentration, Mg concentration, and Ca concentration in the molten metal will fall below the specified range, so the upper limit is set to 15%. It is preferably 3 to 10%. More preferably, it is 5 to 8%.

( _Al2O3 : _10-30 %)
If the Al ₂ O ₃ content in the slag is high, deoxidation does not proceed sufficiently and the O concentration is not controlled within the specified range, resulting in the formation of non-metallic inclusions of MgO.Al ₂ O ₃ at a number ratio exceeding 50%. Furthermore, Al ₂ O ₃ inclusions that are prone to clustering are also formed. On the other hand, if the Al ₂ O ₃ content in the slag is low, the total number ratio of CaO and CaO-MgO oxides among the non-metallic inclusions exceeds 50%. Therefore, the Al ₂ O ₃ concentration is specified to be 10-30%, preferably 13-27%, and more preferably 15-25%.

(MgO: 5 to 20%)
MgO in the slag is an important element for controlling the Mg concentration in the molten metal to the concentration range described in the claims, and is also an important element for controlling nonmetallic inclusions to a composition preferable for the present invention. Therefore, the MgO content in the slag must be at least 5%. On the other hand, if the MgO content exceeds 20%, the reaction of formula (1) proceeds too much, the Mg content in the molten metal becomes high, and the hot workability decreases, resulting in surface defects in the final product. Therefore, the upper limit of the MgO content is set to 20%. The MgO content in the slag is within a predetermined range when the dolomite bricks or magnesia-chrome bricks used in AOD refining or VOD refining dissolve into the slag. Alternatively, in order to control the content within a predetermined range, one or both of the waste dolomite bricks and magnesia-chrome bricks may be added. It is preferably 6 to 18%. More preferably, it is 8 to 16%.

(F: 1-15%)
F has the role of keeping the slag molten during slag refining, so it is necessary to add at least 1%. If the F concentration falls below 1%, the slag will not melt and will have low fluidity. On the other hand, if the F concentration exceeds 15%, the fluidity of the slag will increase significantly, causing significant damage to the bricks. Therefore, the F concentration is specified as 1-15%.

Next, examples are presented to clarify the effects of the present invention. However, the present invention is not limited to the following examples. Ferronickel, pure nickel, ferrochrome, iron scrap, stainless steel scrap, Fe-Ni-based alloy scrap, Fe-Mo, and other raw materials were melted in a 60-ton electric furnace. Then, oxygen refining (oxidation refining) was performed to remove C in AOD or VOD following AOD, limestone and fluorite were added, CaO-SiO ₂ -Al ₂ O ₃ -MgO-F slag was generated, one or two of FeSi alloy and pure Si, and Al were added, Cr reduction was performed, and then deoxidization was performed. Then, Ar stirring was performed to further promote desulfurization. In AOD and VOD, the furnace body was lined with magnesia-chrome bricks. Then, the molten metal was tapped into a ladle, and the temperature and composition were adjusted, and slabs and ingots were produced by continuous casting or ordinary ingot casting. The ingots were hot forged to produce slabs.

The surface of the manufactured slab was ground, and then hot rolling was performed to produce a hot strip. After that, annealing and pickling were performed to remove surface scale, and a plate with a thickness of 20 mm was produced and its quality was evaluated. Next, cold rolling was performed to produce a cold strip, after which annealing and pickling were performed to remove surface scale, and a plate with a thickness of 1 mm was produced and its quality was evaluated.

Table 1 shows the chemical composition of the obtained Fe-Cr-Ni alloy and the slag composition at the end of AOD or VOD refining, while Table 2 shows the nonmetallic inclusion composition, inclusion morphology, and quality evaluation. Here, Example 5 was refined by VOD, Example 6 was refined by AOD followed by VOD, and the rest were refined by AOD. In addition, Example 3 was produced by normal ingot casting, and the others were produced by continuous casting. Numerical values in [ ] indicate that they are outside the scope of the claims of the present invention. Note that in Tables 1 and 2, there are examples of the invention that are marked with [ ], but these do not satisfy the dependent claims, but do satisfy the scope of the independent claims.

(1) Chemical components of the alloy and slag composition: Quantitative analysis was performed using an X-ray fluorescence analyzer, and the oxygen concentration of the alloy was quantitatively analyzed by an inert gas impulse fusion infrared absorption method.
(2) Composition of nonmetallic inclusions: Immediately after the start of casting, a sample taken from the tundish was mirror-polished, and inclusions having a size of 5 μm or more were measured at 20 random points using SEM/EDS.
(3) Number ratio of inclusions: From the results of the measurement in (2) above, the number ratio of MgO.Al ₂ O ₃ (spinel type in the table) to the total number of nonmetallic inclusions and the total number ratio of CaO and CaO-MgO type oxides were evaluated.
(4) Evaluation of surface defects of hot-rolled sheets: The surface of a sheet having a thickness of 20 mm produced by hot rolling was visually observed over the entire length, and the number of nonmetallic inclusions and surface defects caused by hot workability were counted within a width of 1 m and a length of 30 m. In the quality evaluation, if there were 2 or less surface defects, it was marked as ◎, if there were 3 to 5 defects, it was marked as ○, if there were 6 to 10 defects, it was marked as △, and if there were 11 or more defects, it was marked as ×.
(5) Evaluation of hot-rolled sheet pits: A test piece was taken from the 20 mm thick sheet (4) above, mirror-finished, and held for 24 hours in an atmosphere with a humidity of 60% and a temperature of 40° C. The surface of the test piece was washed with water and further buffed to a depth of about 1 μm. The number of pits exceeding 10 μm in depth and 40 μm in diameter was then measured on the surface of the 10 cm x 10 cm test piece using a 3D laser microscope. Here, the number of pits was evaluated as 0 (◎), 1 to 2 (◯), 3 to 5 (△), and 6 or more (×).
(6) Overall evaluation of hot-rolled sheet: The results of the evaluation of surface defects and the evaluation of pits on the hot-rolled sheet were scored as follows.
Hot-rolled sheet surface defect evaluation: ◎ 3 points, 〇 2 points, △ 1 point, × 0 point Hot-rolled sheet pit evaluation: ◎ 3 points, 〇 2 points, △ 1 point, × 0 point After that, for the overall evaluation, if the total score of the hot-rolled sheet surface defect evaluation and the hot-rolled sheet pit evaluation was 6 points, it was rated as ◎, if it was 4 to 5 points, it was rated as 〇, if it was 3 points, it was rated as △, and if it was 2 points or less or the hot-rolled sheet surface defect evaluation or the hot-rolled sheet pit evaluation was ×, it was rated as ×.
(7) Evaluation of surface defects of cold-rolled sheets: The surface of a sheet having a thickness of 1 mm produced by hot rolling followed by cold rolling was visually observed over the entire length, and the number of nonmetallic inclusions and surface defects caused by hot workability were counted within a width of 1 m and a length of 100 m. In the quality evaluation, if there were 2 or less surface defects, it was marked as ⊚, if there were 3 to 5 defects, it was marked as ◯, if there were 6 to 10 defects, it was marked as △, and if there were 11 or more defects, it was marked as ×.
(8) Cold-rolled sheet pit evaluation: A test piece was taken from the 1 mm thick sheet (8) above, mirror-finished, and held for 24 hours in an atmosphere with a humidity of 60% and a temperature of 40° C. The surface of the test piece was washed with water and further buffed to a depth of about 1 μm. The number of pits exceeding 10 μm in depth and 40 μm in diameter was then measured on the surface of a 10 cm × 10 cm test piece using a 3D laser microscope. Here, the number of pits was evaluated as 0 if there was no pit, as ⊚ if there were 1 to 2 pits, as ◯ if there were 3 to 5 pits, and as × if there were 6 or more pits.
(9) Overall evaluation of cold-rolled sheet: The results of the cold-rolled sheet surface defect evaluation and the cold-rolled sheet pit evaluation were scored as follows.
Cold-rolled sheet surface defect evaluation: ◎ 3 points 〇 2 points △ 1 point × 0 point Cold-rolled sheet pit evaluation: ◎ 3 points 〇 2 points △ 1 point × 0 point Then, for the overall evaluation, if the total score of the cold-rolled sheet surface defect evaluation and the cold-rolled sheet pit evaluation was 6 points, it was evaluated as ◎, if it was 4 to 5 points, it was evaluated as 〇, if it was 3 points, it was evaluated as △, and if it was 2 points or less or the cold-rolled sheet surface defect evaluation or the cold-rolled sheet pit evaluation was ×, it was evaluated as ×.

Inventive Examples 1 to 15 satisfied the range of the present invention, and therefore had few surface defects on the plate, with almost no coarse pits exceeding 10 μm in depth and 40 μm in diameter being observed, resulting in good surface properties.
In Example 6, the Si concentration was 0.66% and the Al concentration was 0.121%, both of which were within the prescribed range but high, so deoxidation was somewhat strong, resulting in a somewhat large supply of Mg and Ca from the slag, and a slightly higher total number ratio of CaO and CaO-MgO oxides. As a result, a few pits with a depth of 10 μm and a diameter of more than 40 μm were observed on the surface of the 10 cm x 10 cm test piece.
In Example 7, the Si concentration was 0.07% and the Al concentration was 0.008%, both of which were within the prescribed range but low, so deoxidation was somewhat insufficient, the supply of Mg and Ca from the slag was somewhat insufficient, and the number ratio of _MgO.Al2O3 was slightly high. As a result, it became easy for the _MgO.Al2O3 to adhere to the inner wall of the submerged nozzle, and the enlarged _MgO.Al2O3 _was captured in the alloy, causing slight surface defects.
In Example 8 of the invention, the oxygen potential before deoxidation was high, which resulted in increased oxidation of Si, which also acts as a deoxidizer, and the _SiO2 concentration in the slag was somewhat high at 9.0%, and as a result, the Si concentration was 0.08%, the Mg concentration was 0.0002%, and the Ca concentration was 0.0001%, all of which were low but within the specified range. As a result, the _SiO2 in the CaO- _Al2O3 - _MgO -based oxide was high at 15.9%, which made it easier for inclusions to become larger, and as a result, slight surface defects occurred.
Inventive Example 9, the Si concentration was 0.09% and the Mn concentration was 0.29%, both of which were somewhat low although within the specified range, resulting in somewhat insufficient deoxidation, as well as a small amount of added lime, a somewhat low CaO concentration in the slag, and a somewhat insufficient supply of Ca from the slag, resulting in a low CaO content of 9.2% and a high _Al2O3 _content of 60.9% in the CaO- _Al2O3 - _MgO _oxides , and the formation of _MgO.Al2O3 . As a result, these adhered to the inner wall of the submerged nozzle inside the continuous casting machine, making the inclusions more likely to grow in size, resulting in the occurrence of slight surface defects.
In Example 10, when Al was added just before the end of refining, the _Al2O3 concentration in the _slag increased slightly to 27.4%, and the Al concentration also increased slightly to 0.122%. As a result, _{the Al2O3} _{concentration} in _MgO.Al2O3 increased to 91.1%, which made the properties similar to those of _simple _Al2O3 and made it easier to generate _clusters . However, the number ratio of _MgO.Al2O3 generated was less than ₅₀ %, so only a few surface defects occurred.
In Example 11, Mg was added directly just before the end of refining, and the Mg concentration became slightly higher at 0.0067%. As a result, the MgO concentration in MgO.Al ₂ O ₃ became higher at 44.7%, and the melting point of MgO.Al ₂ O ₃ decreased, making it easier for clusters to form. However, the number ratio of the generated MgO.Al ₂ O ₃ was 50% or less, so only a few surface defects occurred.
In Example 12, the melting of the refractory of the furnace body was somewhat large, and the supply of Mg from the slag to the molten metal was increased, resulting in a somewhat high Mg concentration of 0.0068%. As a result, the MgO concentration in MgO.Al ₂ O ₃ was high at 42.8%, and the melting point of MgO.Al ₂ O ₃ was lowered, making it easier for clusters to form. However, the number ratio of the generated MgO.Al ₂ O ₃ was 50% or less, so only a few surface defects were generated.
In Example 13, the Al concentration was 0.136%, which was somewhat high but within the specified range, and as a result of the excessive deoxidation reaction, Mg and Ca were excessively supplied from the slag to the molten metal, resulting in high Mg and Ca concentrations. As a result, CaO-MgO oxides were generated at a number ratio slightly exceeding 50%, and the proportion of MgO in the CaO-MgO oxides was higher than the specified range, lowering the melting point and making it easier for clusters to form, and a few pits with a depth of 10 μm and a diameter of more than 40 μm were observed on the surface of the 10 cm x 10 cm test piece.
Inventive Example 14, the Si concentration was 0.61% and the Al concentration was 0.127%, both of which were somewhat high but within the specified range, and as a result of the excessive deoxidation reaction proceeding, Mg and Ca were excessively supplied from the slag to the molten metal, resulting in high Mg and Ca concentrations. As a result, CaO-MgO oxides were generated at a number ratio slightly exceeding 50%, and the MgO in the CaO-MgO oxides was higher than the specified range, lowering the melting point and making it easier for clusters to form, and a few pits exceeding 10 μm in depth and 40 μm in diameter were observed on the surface of the 10 cm x 10 cm test piece.
In Example 15, the amount of lime added during refining was slightly large, so the CaO concentration in the slag was slightly high at 70.8%. This resulted in a high CaO activity in the slag, and excess Ca was supplied to the molten metal, resulting in a slightly high Ca concentration of 0.0032%. As a result, CaO inclusions and CaO-MgO oxides were generated at a number ratio exceeding 50% in total, and the CaO in the CaO-MgO oxides was high beyond the specified range, making it easier for hydrates to form, and a few pits with a depth of 10 μm and a diameter of more than 40 μm were observed on the surface of a 10 cm x 10 cm test piece.

On the other hand, the comparative examples were outside the scope of the present invention, and thus many surface defects and/or pits were generated, resulting in poor surface properties. Each example will be described below.
In Comparative Example 16, the Al concentration was 0.182% and the Si concentration was 0.82%, both of which were higher than the prescribed range, and the deoxidation reaction proceeded excessively, resulting in a low O concentration of 0.00006%, below the prescribed range, and as a result, Mg and Ca were excessively supplied from the slag to the molten metal, and the Mg concentration and Ca concentration exceeded the prescribed range. As a result, a large number of nonmetallic inclusions of CaO and CaO-MgO oxides were generated, and many pits with a depth of 10 μm and a diameter of more than 40 μm were observed on the surface of the 10 cm x 10 cm test piece, deteriorating the surface properties. In addition, since the O concentration was lower than expected, Nb was hardly oxidized, and the NbO in the CaO-Al ₂ O ₃ -MgO oxide was low at 0.002%, and the elongation and detachability of the inclusions during hot and cold rolling was reduced, resulting in many surface defects.
In Comparative Example 17, the Si concentration was 0.03%, the Mn concentration was 0.080%, and the Al concentration was 0.004%, all of which were lower than the prescribed ranges, so deoxidation did not proceed sufficiently, and the O concentration was high at 0.0076%, and as a result, although the main component was CaO-Al ₂ O ₃ -MgO-based oxide, the high O concentration increased the number of nonmetallic inclusions, and many surface defects due to the inclusions occurred. In addition, because the O concentration was high, Nb was oxidized and was not sufficiently retained in the molten metal, decreasing to 0.15%, and the NbO in the CaO-Al ₂ O ₃ -MgO-based oxide increased to 0.74%, and the high melting point prevented the product from being stretched or divided during hot and cold rolling, resulting in surface defects in the product. In addition, as the oxidation of Nb in the molten metal progressed, surface defects due to inclusions of Nb ₂ O ₅ alone also occurred, and the surface properties deteriorated.
In Comparative Example 18, granular Al was added from above the slag, so that the added Al came into direct contact with the slag and did not remain in the molten metal, but turned into oxide, and the Al ₂ O ₃ concentration in the slag became high at 30.2%. Furthermore, as a result of insufficient Al in the molten metal and insufficient deoxidation, the supply of Mg and Ca from the slag was insufficient, and the Mg concentration and Ca concentration became lower than the specified concentration. As a result, MgO and Al ₂ O ₃ were generated at a number ratio of more than 50% and were clustered, and non-metallic inclusions of Al ₂ O ₃ alone were also generated and clustered, resulting in numerous surface defects in the final product. In addition, insufficient deoxidation caused the oxidation of Nb in the molten metal to progress, and surface defects due to inclusions of Nb ₂ O ₅ alone also occurred, resulting in deterioration of the surface properties.
In Comparative Example 19, the refractory was severely melted, so that the MgO concentration in the slag was 20.3%, which exceeded the prescribed range, and Mg was supplied to the molten metal in excess, so that the Mg concentration was 0.0136%, which exceeded the prescribed range. As a result, the hot workability was significantly deteriorated, and many surface defects due to the hot workability were generated in the final product, resulting in a deterioration in surface properties.
In Comparative Example 20, a large amount of Mg was added just before the end of refining to adjust the composition, and it reacted with Al ₂ O ₃ in the slag, generating a large number of MgO.Al ₂ O ₃ inclusions. As a result, MgO.Al ₂ O ₃ inclusions adhered to and accumulated on the immersion nozzle inside the continuous casting machine, and large ones fell off and were captured by the solidified shell, generating a large number of surface defects. In addition, as a result of the Mg concentration being 0.0150%, which was higher than the specified range, the hot workability was significantly deteriorated, and a large number of surface defects caused by hot workability were generated in the final product, deteriorating the surface properties.
In Comparative Example 21, because of the excessive addition of lime, the CaO concentration in the slag was 76.5%, which was higher than the specified range, and the _SiO2 concentration was 1.1%, and _the _Al2O3 concentration was 9.7%, both of which were lower than the specified range. This resulted in an increase in CaO activity in the slag, which led to an excessive supply of Ca to the molten metal, resulting in a high Ca concentration of 0.0158%. As a result, a large number of CaO inclusions were generated, and the CaO in the CaO-MgO oxides was higher than the specified range, which led to surface defects caused by the inclusions, and a large number of pits with a depth of 10 μm and a diameter of more than 40 μm were observed on the surface of the 10 cm x 10 cm test piece, deteriorating the surface properties.

The technology of the present invention can provide an Fe-Cr-Ni alloy with excellent surface properties suitable for use in reaction towers, which require corrosion resistance and high-temperature strength, by controlling the morphology of non-metallic inclusions.

Claims

In mass%, C: 0.020 to 0.150%, Si: 0.05 to 0.80%, Mn: 0.10 to 1.50%, P: 0.035% or less, S: 0.0050% or less, Ni: 34.0 to 48.0%, Cr: 22.0 to 29.0%, Mo: 0.20 to 1.20%, Al: 0.005 to 0.180%, Mg: 0.0001 to 0 An Fe-Cr-Ni based alloy comprising: 0.0100%, Ca: 0.0001-0.0100%, Nb: 0.20-0.80%, N: 0.050-0.500%, O: 0.0001-0.0060%, Cu: 0.80% or less, Ti: 0.100% or less, Co: 0.50% or less, and the balance being Fe and unavoidable impurities;
The nonmetallic inclusions contain either or both of MgO and CaO-Al 2 O 3 -MgO-based oxides as essential components, and may contain any of CaO, CaO-MgO-based oxides, and MgO.Al 2 O 3 as optional components, and the number ratio of MgO.Al 2 O 3 to all oxide-based nonmetallic inclusions is 50% or less, and the combined number ratio of CaO and CaO-MgO-based oxides is 50% or less in an Fe-Cr-Ni alloy with excellent surface properties.
2. The Fe--Cr--Ni alloy having excellent surface properties according to claim 1, wherein the CaO--Al 2 O 3 --MgO oxide contains 0.01 to 0.60 mass % of NbO.
The Fe- Cr -Ni alloy having excellent surface properties as described in claim 1 or 2, characterized in that the CaO-MgO oxide has, in mass %, CaO: 20-80% and MgO: 20-80%, the CaO-Al 2 O 3 -MgO oxide has CaO: 10-60%, Al 2 O 3 : 5-60%, MgO: 10-80%, and SiO 2 : 10% or less, and the MgO.Al 2 O 3 has MgO: 10-40% and Al 2 O 3 : 60-90%.
A method for producing an Fe-Cr-Ni alloy having excellent surface properties according to claim 1 or 2, comprising the steps of melting raw materials in an electric furnace, decarburizing the raw materials by AOD and/or VOD, adding lime and fluorite, and then adding one or two of a ferrosilicon alloy and pure silicon, and Al for primary deoxidization. When the O concentration reaches 0.0070-0.0120%, Nb is added, and a CaO-SiO 2 -MgO-Al 2 O 3 alloy consisting of CaO: 45-75%, SiO 2 : 1-15%, Al 2 O 3 : 10-30%, MgO: 5-20%, and F: 1-15% is obtained . The present invention relates to a method for producing an Fe-Cr-Ni based alloy having excellent surface properties, the method comprising the steps of: using an Fe-F based slag; then adding one or both of a ferrosilicon alloy and pure silicon, and Al; carrying out Cr reduction, secondary deoxidization, and desulfurization; producing a slab or an ingot using a continuous casting machine or ordinary ingot making; and in the case of an ingot, carrying out hot forging, followed by hot rolling or hot rolling and cold rolling.
A method for producing an Fe-Cr-Ni alloy having excellent surface properties according to claim 3, comprising the steps of melting raw materials in an electric furnace, decarburizing the raw materials by AOD and/or VOD, adding lime and fluorite, and then adding one or two of a ferrosilicon alloy and pure silicon, and Al for primary deoxidization. When the O concentration reaches 0.0070-0.0120%, Nb is added, and a CaO-SiO 2 -MgO-Al 2 O 3 alloy having CaO: 45-75%, SiO 2 : 1-15%, Al 2 O 3 : 10-30%, MgO: 5-20%, and F: 1-15% is obtained. The present invention relates to a method for producing an Fe-Cr-Ni based alloy having excellent surface properties, the method comprising the steps of: using an Fe-F based slag; then adding one or both of a ferrosilicon alloy and pure silicon, and Al; carrying out Cr reduction, secondary deoxidization, and desulfurization; producing a slab or an ingot using a continuous casting machine or ordinary ingot making; and in the case of an ingot, carrying out hot forging, followed by hot rolling or hot rolling and cold rolling.