Fe-ni alloy having excellent surface properties, method for producing the same, and mold for cfpr

Abstract

The present invention relates to an Fe-Ni alloy consisting of C: 0.001 to 0.2 mass%, Si: 0.001 to 0.2 mass%, Mn: 0.005 to 0.7 mass%, Ni: 30.0 to 45.0 mass%, Cr: 0.3 mass% or less, Al: 0.001 to 0.1 mass%, Ti: 0.001 to 0.020 mass%, O: 0.007 mass% or less, Mg: 0.0030 mass% or less, N: 0.010 mass% or less, Ca: 0.0015 mass% or less, Na: 0.00005 to 0.001 mass%, the balance being Fe and unavoidable impurities, wherein non-metallic inclusions containing CaO-SiO2-Al2O3-MgO-MnO-Na2O-based complex oxides as essential components, and further containing one or more non-metallic inclusions of CaO, MgO, MgO·Al2O3, MnO·SiO2, and Na2O as optional components, wherein the number ratio of CaO-SiO2-Al2O3-MgO-MnO-Na2O inclusions is 40% or more in all non-metallic inclusions, thereby producing an Fe-Ni alloy having excellent surface properties.

Description

TECHNICAL FIELD

[0001] The present application relates to an Fe-Ni alloy having excellent surface properties, a refining method for an Fe-Ni alloy, an Fe-Ni alloy having excellent surface properties in which CaO-SiO2-Al2O3-MgO-MnO-Na2O system non-metallic inclusions in molten steel are controlled to be harmless and the number of inclusions on the surface is reduced by controlling the composition of a molten slag and Mg, Al, Ca and Na in molten steel, and a manufacturing method therefor, particularly an Fe-Ni alloy suitable for a mold for CFRP. BACKGROUND

[0002] Carbon fiber reinforced plastic (CFRP) is a raw material having high strength and light weight, and is used as a raw material in a wide range of fields from sports applications such as golf clubs to automobile or aerospace industries. In particular, in the case of use in the aircraft industry or the automobile industry, since very high dimensional accuracy is required, an Invar alloy (Fe-36% Ni) having a small coefficient of thermal expansion is widely used as a mold (for example, refer to Patent Document 1).

[0003] Here, in the case where the Invar alloy is used as a mold for CFRP, since the surface of the mold is transferred to CFRP, the surface properties of the mold itself are one of the most important factors, and a surface as smooth as possible is required.

[0004] In Patent Document 2, a method is proposed in which the surface of an Invar alloy for a mold is subjected to plating treatment to keep the mold surface sufficiently smooth.

[0005] However, plating of the mold surface leads to an increase in cost, and it is difficult to plate fine shape portions. In addition, non-metallic inclusions can affect the surface properties of the mold base material, but no description is found regarding the amount or composition of inclusions.

[0006] Here, several techniques for seeking harmless inclusions in Invar alloys are disclosed.

[0007] In Patent Document 3, the number of inclusions of MgO-Al2O3 or MnO-MgO-SiO2 as non-stretching system inclusions is controlled to 20% or less.

[0008] In addition, in Patent Document 4, a method is proposed in which inclusions are controlled to be MnO-SiO2-Al2O3-CaO-MgO-Cr2O3-FeO-TiO2 system, thereby manufacturing an Fe-Ni alloy having excellent cleanliness.

[0009] However, Patent Literature 3 and Patent Literature 4 are directed to a shadow mask or a lead frame, and are not directed to the characteristics required for a mold for CFRP.

[0010] Prior Art Documents

[0011] Patent Literature

[0012] Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 8-269613,

[0013] Patent Literature 2: Japanese Patent Application Laid-Open (JP-A) No. 2014-205317,

[0014] Patent Literature 3: Japanese Patent Application Laid-Open (JP-A) No. 2010-159437,

[0015] Patent Literature 4: Japanese Patent Application Laid-Open (JP-A) No. 2003-073779. SUMMARY

[0016] Problems to be Solved by the Invention

[0017] In view of the above problems, an object of the present application is to control the composition of non-metallic inclusions or the number of inclusions on the surface, thereby providing an Fe-Ni alloy having excellent surface properties, and particularly an Fe-Ni alloy having excellent suitability for a mold for CFRP. Further, a method for producing the alloy is also provided.

[0018] Means for Solving the Problems

[0019] The inventors have conducted intensive studies in order to solve the above problems. First, test pieces of 10 cm x 10 cm were collected from various Fe-Ni alloys having a plate thickness of 30 mm. The surface of the test pieces was mirror-polished, and using an optical microscope, the number of inclusions in an area of 200 mm 2the number of non-metallic inclusions of 40 μm or more in length aligned continuously on the surface of the test piece. Further, the composition of the non-metallic inclusions was analyzed using SEM / EDS. Further, the test piece was left in an atmosphere of 60% humidity and 40 degrees C for 24 hours, and then the number of non-metallic inclusions of 40 μm or more in length aligned continuously was measured again, and the composition of the non-metallic inclusions was measured using SEM / EDS. Further, the surface of the test piece was washed with water, and then polishing and grinding were performed to a depth of about 1 μm, and then the number of pits of 40 μm or more in diameter and 10 μm or more in depth on the surface of the test piece of 10 cm x 10 cm was measured using a 3D laser microscope. These measurement results were analyzed in depth. As a result, it was found that the number of pits of 40 μm or more in diameter and 10 μm or more in depth increased in the case where CaO and Na2O inclusions were present in large amounts. It was found that this was because the CaO and Na2O inclusions present on the surface reacted with moisture in the atmosphere to form hydrates, and fell off from the surface.

[0020] CaO + H2O = Ca(OH)2... (Formula 1)

[0021] Na2O + H2O = 2NaOH... (Formula 2)

[0022] Further analysis was performed, and as a result, it was found that if the non-metallic inclusions were controlled to be in a form that did not react with moisture in the atmosphere, pits of 40 μm or more in diameter and 10 μm or more in depth did not occur. That is, it was found that if the non-metallic inclusions were controlled to be in a form of glassy inclusions of CaO-SiO2-Al2O3-MgO-MnO-Na2O, pits of 40 μm or more in diameter and 10 μm or more in depth did not occur. Further, with respect to Fe-Ni alloys having the surface properties described above, the relationship with the operating conditions was repeatedly analyzed in depth. Based on the insights obtained through this analysis, the present application was completed.

[0023] That is, the present application is an Fe-Ni alloy having excellent surface properties, characterized by containing, as essential components, non-metallic inclusions of CaO-SiO2-Al2O3-MgO-MnO-Na2O system, and further containing, as optional components, one or more non-metallic inclusions selected from the group consisting of CaO, MgO, MgO-Al2O3, MnO-SiO2, and Na2O, wherein the number ratio of CaO-SiO2-Al2O3-MgO-MnO-Na2O inclusions is 40% or more in all non-metallic inclusions.

[0024] In the alloy of the present application, it is further preferable to contain Nb: 0.01 to 1.00 mass%.

[0025] Further, in the non-metallic inclusions, the number ratio of CaO and Na2O inclusions is preferably 20% or less, the number ratio of MgO-Al2O3 inclusions is preferably 20% or less, and the number ratio of MnO-SiO2 inclusions is preferably 20% or less.

[0026] Further, in the non-metallic inclusions, the CaO-SiO2-Al2O3-MgO-MnO-Na2O system oxide is composed of CaO: 20 to 60 mass%, SiO2: 10 to 40 mass%, Al2O3: 30 mass% or less, MgO: 5 to 50 mass%, and Na2O: 0.001 to 1 mass%, with the balance being MnO, and the MgO-Al2O3 is composed of MgO: 10 to 40 mass% and Al2O3: 60 to 90 mass%.

[0027] Further, in these non-metallic inclusions, in an area of 200 mm 2 of the surface of the alloy in parallel with the rolling direction, non-metallic inclusions having a width of 5 μm or more and arranged continuously for 40 μm or more are preferably 10 or less.

[0028] Further, in the present application, a manufacturing method of the Fe-Ni alloy is provided. The manufacturing method is a manufacturing method of the Fe-Ni alloy having excellent surface properties, characterized by melting raw materials with an electric furnace, then, after decarburization in an AOD and / or a VOD, adding lime, fluorite, ferrosilicon, and / or Al, performing deoxidation and desulfurization using a CaO-Al2O3-MgO-SiO2-Na2O-F system slag composed of CaO: 50 to 70 mass%, SiO2: 3 to 30 mass%, MgO: 3 to 15 mass%, Al2O3: 5 mass% or less, Na2O: 0.001 to 1 mass%, and the balance being F, while performing stirring with a large amount of Ar, performing temperature and composition adjustment after promoting the floating of inclusions caused by the Ar stirring with an LF, and then, performing casting with a continuous casting machine or a general ingot casting method to manufacture an ingot, performing hot forging on the ingot to manufacture a slab, then, performing hot rolling and cold rolling. DETAILED DESCRIPTION

[0029] First, the reason for the chemical composition limitation of the Fe-Ni alloy of the present application is shown.

[0030] C: 0.001 to 0.2 mass% or less

[0031] C is an element necessary to maintain the strength of the alloy. If the amount of C is less than 0.001 mass%, sufficient strength cannot be obtained, on the other hand, if it exceeds 0.2 mass%, the coefficient of thermal expansion increases, and therefore the content of C is specified to be 0.001 to 0.2 mass%. It is preferable to be 0.002 to 0.1 mass%. It is more preferable to be 0.003 to 0.05 mass%.

[0032] Si: 0.001 to 0.2 mass%

[0033] Si is an element effective for deoxidation, and has the effect of controlling the composition of non-metallic inclusions to be CaO-SiO2-Al2O3-MgO-MnO-Na2O system. If the amount of Si is less than 0.001 mass%, sufficient deoxidation effect cannot be obtained, and in addition, the composition of non-metallic inclusions cannot be controlled to be CaO-SiO2-Al2O3-MgO-MnO-Na2O system. On the other hand, if the content of Si exceeds 0.2 mass%, the coefficient of thermal expansion becomes large, the characteristics required for the Fe-Ni alloy plate are not satisfied, and MgO in the slag is reduced, and Mg is supplied to the molten steel. It reacts with Al, and thus the non-metallic inclusions become MgO-Al2O3 spinel that easily clusters, causing surface defects. Therefore, in the present application, the content of Si is specified to be 0.001 to 0.2 mass%. It is preferable to be 0.002 to 0.19 mass% within this range. It is more preferable to be 0.003 to 0.18 mass%.

[0034] Mn: 0.005 to 0.7 mass%

[0035] Mn is an effective deoxidizing element, controlling the composition of non-metallic inclusions to a CaO-SiO2-Al2O3-MgO-MnO-Na2O system. However, it also increases the thermal expansion coefficient of Fe-Ni alloys, and from this perspective, a low concentration is desirable. That is, if the Mn content is below 0.005% by mass, a sufficient deoxidizing effect cannot be achieved, and the composition of non-metallic inclusions cannot be controlled to a low-melting-point CaO-SiO2-Al2O3-MgO-MnO-Na2O system. On the other hand, if it exceeds 0.7% by mass, the thermal expansion coefficient of the Fe-Ni alloy increases, failing to meet the required quality of Fe-Ni alloy sheets. Therefore, in this invention, the Mn content is specified as 0.005 to 0.7% by mass, preferably 0.01 to 0.65% by mass, and more preferably 0.02 to 0.6% by mass.

[0036] Ni: 30.0–45.0% by mass

[0037] Ni is an element that significantly affects the coefficient of thermal expansion. It is known that the coefficient of thermal expansion reaches its minimum at around 36% by mass at 200°C and around 42% by mass at 500°C. However, if the Ni content is below 30% by mass or above 45% by mass, the coefficient of thermal expansion increases, failing to meet the required characteristics. Therefore, the Ni content is set to 30.0–45.0% by mass, preferably 32.0–43.0% by mass, and more preferably 35.0–42.0% by mass.

[0038] Cr: less than 0.3% by mass

[0039] Cr is an element that increases the coefficient of thermal expansion, and from this point of view, it is desirable to have the lowest possible concentration. Therefore, the Cr content is specified to be 0.3% by mass or less. Preferably, it is 0.25% by mass or less. More preferably, it is 0.20% by mass or less.

[0040] Al: 0.001–0.1% by mass

[0041] Al is a deoxidizing element and plays a very important role in this invention. If the amount of Al is less than 0.001% by mass, deoxidation is insufficient, and the O concentration increases to more than 0.007% by mass, resulting in an increase in the number of oxide inclusions and contributing to surface defects. On the other hand, if the amount of Al exceeds 0.1% by mass, the ability to reduce MgO and CaO in the slag is too strong, and the Mg and Ca in the steel exceed 0.001% by mass. Therefore, the inclusion composition is mainly composed of CaO or MgO·Al₂O₃. For this reason, Al is specified as 0.001 to 0.1% by mass, preferably 0.0015 to 0.08% by mass, and more preferably 0.002 to 0.07% by mass.

[0042] Ti: 0.001 to 0.020 mass %

[0043] Ti is an element effective for deoxidation. If less than 0.001 mass %, deoxidation effect cannot be exerted, and if 0.020 mass % or more, TiN is generated, which has a risk of causing surface defects. For this reason, Ti is specified to be 0.001 to 0.020 mass %. It is preferable to be 0.0012 to 0.015 mass %. It is more preferable to be 0.0015 to 0.010 mass %.

[0044] O: 0.007 mass % or less

[0045] O combines with the constituent components in the alloy to generate inclusions. If these inclusions are coarse, surface properties are deteriorated, and thus it is necessary to be reduced as much as possible. If more than 0.007 mass %, coarse inclusions are generated, and thus Ca is specified to be 0.007 mass % or less. It is preferable to be 0.006 mass % or less, and it is more preferable to be 0.005 mass % or less.

[0046] Mg: 0.0030 mass % or less

[0047] If Mg is more than 0.0030 mass %, non-metallic inclusions easily become MgO, MgO-Al2O3. MgO does not cluster, and is a fine non-metallic inclusion, and does not affect surface quality, but MgO-Al2O3 easily coagulates and becomes a cluster of large inclusions in the refining process of Fe-Ni molten steel, and causes surface defects of products. Thus, it is specified to be 0.0030 mass % or less in the present application. It is preferable to be 0.0020 mass % or less, and it is more preferable to be 0.0010 mass % or less.

[0048] N: 0.010 mass % or less

[0049] N is an element that needs to be reduced as much as possible because it generates nitrides with various elements. Thus, it is specified to be 0.010 mass % or less in the present application. It is preferable to be 0.009 mass % or less, and it is more preferable to be 0.008 mass % or less.

[0050] Ca: 0.0015 mass % or less

[0051] Ca is an element useful for controlling non-metallic inclusions to be CaO-SiO2-Al2O3-MgO-MnO-Na2O system non-metallic inclusions. However, if this Ca is more than 0.0015 mass %, CaO concentration in the inclusions is increased, and becomes a hydrate, which has a risk of deteriorating surface properties. From this viewpoint, the content of Ca is specified to be 0.0015 mass % or less. It is preferable to be 0.0007 mass % or less, and it is more preferable to be 0.0005 mass % or less.

[0052] Na: 0.00005 to 0.001 mass%

[0053] Na is a very important component in controlling non-metallic inclusions to CaO-SiO2-Al2O3-MgO-MnO-Na2O-based glassy inclusions. If less than 0.00005 mass%, the effect cannot be exerted, and if 0.001 mass% or more, Na2O inclusions are generated, which has a risk of adversely affecting surface properties. For this reason, Na is specified to be 0.00005 mass% to 0.001 mass%. It is preferable to be 0.00008 mass% to 0.0005 mass%. It is more preferable to be 0.00010 mass% to 0.0003 mass%. Note that Na can be supplied to the molten steel by reducing Na2O in the slag.

[0054] Nb: 0.01 to 1.00 mass%

[0055] In the Fe-Ni alloy of the present application, in addition to the above-mentioned components, it is preferable to further add Nb as needed. If Nb is in a trace amount, it has an effect of reducing the coefficient of thermal expansion, and in addition, if it is in the range of 0.01 to 1.00 mass%, it is an element effective for improving the strength of the Fe-Ni alloy sheet. If the strength of the Fe-Ni alloy sheet is required to be improved for low thermal expansion properties, the thickness of the Fe-Ni alloy sheet can be made thinner, achieving lightweight of the material, which is suitable for a mold for CFRP. However, if it exceeds 1.00 mass%, the coefficient of thermal expansion increases. For this reason, when Nb is added, it is specified to be 0.01 to 1.00 mass%. It is preferable to be in the range of 0.02 to 0.50 mass%. It is more preferable to be 0.10 to 0.30 mass%.

[0056] Non-metallic inclusions

[0057] In the present application, CaO-SiO2-Al2O3-MgO-MnO-Na2O-based complex oxides are contained as essential non-metallic inclusions, and in addition, one or more of CaO, MgO, MgO-Al2O3, MnO-SiO2, and Na2O can be further contained as optional non-metallic inclusions. In this case, a preferable mode is that, in the total non-metallic inclusions obtained by combining the essential non-metallic inclusions and the optional non-metallic inclusions, the number ratio of CaO-SiO2-Al2O3-MgO-MnO-Na2O inclusions is 40% or more. Hereinafter, the basis for limiting the number ratio of non-metallic inclusions is shown.

[0058] Further, the number ratio of the CaO-SiO2-Al2O3-MgO-MnO-Na2O inclusions is 40% or more

[0059] The Fe-Ni alloy according to the present application contains non-metallic inclusions, as essential components, CaO-SiO2-Al2O3-MgO-MnO-Na2O system oxides, and, as optional components, one or more of CaO, MgO, MgO-Al2O3, MnO-SiO2, and Na2O. Of these, the CaO-SiO2-Al2O3-MgO-MnO-Na2O oxides are very stable oxides, and thus do not react with moisture in the atmosphere to form hydrates and cause pits on the surface. If the content of the CaO-SiO2-Al2O3-MgO-MnO-Na2O oxides is 40% or more in terms of number ratio, the number of surface pits is small, and thus is specified to be 40% or more in terms of number ratio. It is preferably 45% or more, and more preferably 50% or more.

[0060] The number ratio of the CaO and Na2O inclusions is 20% or less

[0061] Since CaO and Na2O are inclusions that react with moisture in the atmosphere to form hydrates and fall off from the surface, causing pits, it is preferable to be as small as possible. Thus, the number ratio of the CaO and Na2O inclusions is specified to be 20% or less. It is preferably 15% or less, and more preferably 10% or less. Note that, although MgO also forms a hydrate Mg(OH)2in an atmospheric environment, compared with CaO and Na2O, it takes time to change into a hydrate, and thus has little influence in the present application, and thus is not particularly specified.

[0062] The number ratio of the MgO-Al2O3 inclusions is 20% or less

[0063] MgO-Al2O3 inclusions agglomerate and become coarse, and thus are a major cause of deterioration of surface properties, and thus it is preferable to be as small as possible. Thus, the number ratio of the MgO-Al2O3 inclusions is specified to be 20% or less. It is preferably 15% or less, and more preferably 10% or less.

[0064] The number ratio of the MnO-SiO2 inclusions is 20% or less

[0065] MnO-SiO2 inclusions are coarse non-metallic inclusions that cause surface defects, and thus it is preferable to have as few as possible. Therefore, the number ratio of MnO-SiO2 inclusions is specified to be 20% or less. It is preferable to be 15% or less, and more preferable to be 10% or less.

[0066] The reasons for specifying each component contained in the CaO-SiO2-Al2O3-MgO-MnO-Na2O system oxide will be described.

[0067] CaO: 20 to 60 mass%, SiO2: 10 to 40 mass%, Al2O3: 30 mass% or less, MgO: 5 to 50 mass%, Na2O: 0.001 to 1 mass%, and the balance being MnO

[0068] Basically, the ranges are set so that the melting point of the CaO-SiO2-Al2O3-MgO-MnO-Na2O oxide reaches about 1300°C or less, and becomes a vitreous oxide. Note that if CaO is less than 20 mass%, the melting point increases, and if CaO exceeds 60 mass%, CaO inclusions coexist. If SiO2 is less than 10 mass% and exceeds 40 mass%, the melting point increases. If Al2O3 exceeds 30 mass%, MgO-Al2O3 inclusions coexist. If MgO is less than 5 mass% and exceeds 50 mass%, the melting point increases. In addition, if Na2O is 0.001 mass% or more, the melting point of the inclusions can be lowered, and the effect of controlling to vitreous inclusions is obtained, but if it exceeds 1 mass%, pure Na2O inclusions coexist. For the above reasons, CaO is set to 20 to 60 mass%, SiO2 is set to 10 to 40 mass%, Al2O3 is set to 30 mass% or less, MgO is set to 5 to 50 mass%, Na2O is set to 0.001 to 1 mass%, and the balance is MnO.

[0069] The reasons for specifying the constituent components of MgO-Al2O3 will be described.

[0070] MgO-Al2O3 is MgO: 10 to 40 mass%, Al2O3: 60 to 90 mass%

[0071] MgO-Al2O3 is a compound having a wide solid solution. Since it becomes a solid solution within the above range, it is thus specified.

[0072] In addition, the reasons for specifying the number and size of non-metallic inclusions on the surface will be described.

[0073] Non-metallic inclusions having a width of 5 μm or more and arranged continuously for 40 μm or more in an area of 200 mm 2 on the surface of the alloy are 10 or less

[0074] The inclusion present on the surface of the alloy has a great influence on the surface properties. In particular, non-metallic inclusions having a width of 5 μm or more and arranged continuously for 40 μm or more become the starting point of surface defects such as linear defects, and thus it is desirable to be as small as possible. However, if the number of non-metallic inclusions having a width of 5 μm or more and arranged continuously for 40 μm or more in an area of 200 mm x 200 mm on the surface of the alloy is 10 or less, it is difficult to generate surface defects, and thus the regulation is made as described above. It is preferable to be 8 or less, and more preferable to be 5 or less. Note that inclusions arranged with a spacing of 20 μm or less are counted as one block, and inclusions arranged with a spacing exceeding 20 μm are counted as other blocks. 2 2 The number of non-metallic inclusions having a width of 5 μm or more and arranged continuously for 40 μm or more in an area of 200 mm x 200 mm on the surface of the alloy is 10 or less, it is difficult to generate surface defects, and thus the regulation is made as described above. It is preferable to be 8 or less, and more preferable to be 5 or less. Note that inclusions arranged with a spacing of 20 μm or less are counted as one block, and inclusions arranged with a spacing exceeding 20 μm are counted as other blocks.

[0075] Fe-Ni alloy having excellent surface properties characterized by excellent adaptability to CFRP mold use

[0076] The present application is an Fe-Ni alloy having excellent surface properties without generating pits. Thus, it has excellent adaptability to CFRP mold use.

[0077] Manufacturing method

[0078] In the present application, a manufacturing method of the Fe-Ni alloy is also proposed. The method is, first, melting a raw material, melting the Fe-Ni having a prescribed composition, then, performing decarburization in AOD and / or VOD, further controlling the N concentration to be 0.010 mass% or less, after that, charging lime, fluorite, ferrosilicon, and / or Al, performing deoxidation and desulfurization using a CaO-SiO2-MgO-Al2O3-Na2O-F system slag composed of CaO: 50 to 70 mass%, SiO2: 3 to 30 mass%, MgO: 3 to 15 mass%, Al2O3: 5 mass% or less, Na2O: 0.001 to 1 mass%, and the balance being F, while performing stirring with a large amount of Ar, further adding Ti, after controlling to a prescribed Ti concentration, performing temperature and composition adjustment using LF while promoting the floating of inclusions caused by Ar stirring, then, manufacturing an ingot using a continuous casting machine or a general ingot casting; further performing hot forging from the ingot to manufacture a slab; performing grinding on the surface of the manufactured slab, heating at 1200°C to perform hot rolling until a prescribed thickness, performing annealing and pickling, removing the surface scale, and finally manufacturing a plate having a prescribed thickness. Thus, the non-metallic inclusions can be controlled to be one or two or more of CaO, MgO, MgO-Al2O3, MnO-SiO2, and Na2O in addition to CaO-SiO2-Al2O3-MgO-MnO-Na2O system complex oxides. As a result, an Fe-Ni alloy having a content of CaO-SiO2-Al2O3-MgO-MnO-Na2O oxides of 40% or more in terms of the number ratio can be obtained.

[0079] In the manufacturing method of the Fe-Ni alloy according to the present application, the composition of the slag is characterized as described above. Hereinafter, the basis for the composition of the slag as described above in the present application is described.

[0080] CaO: 50 to 70 mass%

[0081] The CaO concentration in the slag is an important element for effectively performing deoxidation and desulfurization, and for performing inclusion control. The concentration is adjusted by adding lime. If the CaO concentration exceeds 70 mass%, the activity of CaO in the slag increases, the concentration of reduced Ca in the molten steel increases to exceed 0.001 mass%, and independent non-metallic inclusions of CaO are generated, resulting in pits on the surface of the final product. Therefore, the upper limit is set to 70 mass%. On the other hand, if the CaO concentration is less than 50 mass%, deoxidation and desulfurization cannot be performed, and the S concentration and the O concentration in the present application cannot be controlled to the range. Therefore, the lower limit is set to 50 mass%. Thus, the CaO concentration is set to 50 to 70 mass%. It is preferable that the CaO concentration be 52 to 68 mass%. It is more preferable that the CaO concentration be 55 to 65 mass%.

[0082] SiO2: 3 to 30 mass%

[0083] SiO2 in the slag is an important element for ensuring optimum fluidity, and thus needs to be 3 mass%. However, if SiO2 is excessively high to exceed 30 mass%, the O concentration also increases to exceed 0.007 mass%. Note that the SiO2 concentration can be adjusted by the amount of ferrosilicon added. As described above, the SiO2 concentration is specified to be 3 to 30 mass%. It is preferable that the SiO2 concentration be 3 to 28 mass%. It is more preferable that the SiO2 concentration be 3 to 25 mass%.

[0084] MgO: 3 to 15 mass%

[0085] MgO in the slag is an important element for controlling the Mg concentration contained in the molten steel to the concentration range described in the claims, and is also an important element for controlling non-metallic inclusions to the preferable composition of the present application. Therefore, the lower limit is set to 3 mass%. On the other hand, if the MgO concentration exceeds 15 mass%, the Mg concentration in the molten steel increases, MgO-Al2O3 is generated, and becomes a starting point of surface defects such as linear defects. Therefore, the upper limit of the MgO concentration is set to 15 mass%. It is preferable that the MgO concentration be 4 to 14 mass%, and it is more preferable that the MgO concentration be 5 to 12 mass%. MgO in the slag reaches the specified range by being dissolved into the slag from dolomite bricks or magnesia-chrome bricks used when AOD refining or VOD refining is performed. Alternatively, in order to control to the specified range, waste bricks of dolomite bricks or magnesia-chrome bricks can also be added.

[0086] Al2O3: 5 mass% or less

[0087] If the Al2O3 in the slag is high, MgO-Al2O3 inclusions are generated, and the number ratio of MgO-Al2O3 inclusions exceeds 20%, so it is necessary to reduce the Al2O3 concentration in the slag as much as possible. Therefore, the upper limit is set to 5% by mass or less. It is preferable to be 4% by mass or less, and more preferable to be 3% by mass or less.

[0088] Na2O: 0.001 to 1% by mass

[0089] Na2O in the slag has an effect of controlling the inclusion composition to be CaO-SiO2-Al2O3-MgO-MnO-Na2O system, so it is preferable to be 0.001% by mass or more. However, if it exceeds 1% by mass, Na2O inclusions are generated, so the Na2O concentration is specified to be 0.001 to 1% by mass. It is preferable to be 0.002 to 0.9% by mass. It is more preferable to be 0.003 to 0.5% by mass. Note that the Na2O concentration can be controlled by adding sodium carbonate.

[0090] Example

[0091] The following examples are given to further clarify the constitution and the effect of the present application, but the present application is not limited to the following examples. An electric furnace having a capacity of 60 tons was used to melt raw materials such as ferronickel, pure nickel, iron chips, and Fe-Ni alloy chips. Then, decarburization was performed by oxygen blowing refining (oxidation refining) for removing C in AOD and / or VOD, and further, the N concentration was controlled to be 0.010% by mass or less, and then limestone, fluorite, and sodium carbonate were charged to generate CaO-SiO2-Al2O3-MgO-Na2O-F system slag, and further, FeSi and / or Al were charged to perform deoxidation, and then Ar stirring was performed to perform desulfurization, and then Ti was added to control the Ti concentration. Then, the molten steel was tapped into a ladle, temperature adjustment and composition adjustment were performed, and a continuous casting machine or a general ingot casting was used to manufacture an ingot. Further, hot forging was performed on the ingot to manufacture a slab.

[0092] The surface of the manufactured slab was ground, heated at 1200°C, and hot-rolled from a sheet thickness of 200 mm to a sheet thickness of 30 mm, and a thick plate was manufactured through an annealing pickling process. Then, annealing and pickling were performed to remove the surface scale. The chemical composition of the resulting Fe-Ni alloy, the steelmaking process (EF: electric furnace, AOD: argon oxygen decarburization device, VOD: vacuum oxygen decarburization device, LF: ladle refining device, CC: continuous casting machine, IC: general ingot casting method), and the slag composition at the end of AOD or VOD refining are shown in Table 1. The non-metallic inclusion composition, and the shape and quality evaluation of the inclusions are shown in Table 2.

[0093] (1) Chemical composition of the alloy and slag composition: quantitative analysis was performed using a fluorescent X-ray analysis device, and the oxygen concentration of the alloy was quantitatively analyzed using an inert gas pulse melting infrared absorption method.

[0094] (2) Non-metallic inclusion composition: In the case of continuous casting, a Fe-Ni alloy sample was collected with a tundish immediately after the start of casting, and in the case of a normal ingot, a Fe-Ni alloy sample was collected with a sprue connected to a mold, and the solidified sample was mirror-polished, and non-metallic inclusions of 5 μm or more in size were randomly measured at 20 points using SEM-EDS.

[0095] (3) Number ratio of each non-metallic inclusion: The number ratio was evaluated based on the measurement results of (2) above.

[0096] (4) Inclusion number distribution: The obtained slab was hot-rolled from a thickness of 200 mm to a plate thickness of 30 mm (reduction: 98.5%), and a test piece of 10 cm x 10 cm was collected from the Fe-Ni alloy plate having a plate thickness of 30 mm, and the surface of the test piece was polished and mirror-finished. The number of non-metallic inclusions of 40 μm or more in size dispersed and arranged in parallel with the rolling direction in an area of 10 mm x 20 mm was measured on the surface of the polished sample using an optical microscope at a magnification of 200 times.

[0097] (5) Pit evaluation: The test piece subjected to mirror finishing in (4) above was left in an atmosphere of 60% humidity and 40 degrees C for 24 hours, and then the surface of the test piece was washed with water, and further polished and ground to a depth of about 1 μm, and then the number of pits of 10 μm or more in depth and 40 μm or more in diameter on the surface of the test piece of 10 cm x 10 cm was measured using a 3D laser microscope. Here, if the number of pits was 0, it was evaluated as A, if it was 1 to 2, it was evaluated as B, if it was 3 to 5, it was evaluated as C, and if it was 6 or more, it was evaluated as D.

[0098] (6) Surface defect evaluation: The number of surface defects in a Fe-Ni alloy plate of 10 m 2 which had been pickled and annealed to remove the surface scale was measured. If the number of surface defects in 10 m 2 was 0, it was evaluated as A, if it was 1, it was evaluated as B, if it was 2, it was evaluated as C, and if it was 3 or more, it was evaluated as D.

[0099] (7) Comprehensive evaluation: The pit evaluation and the surface defect evaluation were scored as follows, and if the total score of the pit evaluation and the surface defect evaluation was 6, it was A, if it was 4 to 5, it was B, if it was 3, it was C, and if it was 2 or less, or the pit evaluation or the surface defect evaluation was D, it was D.

[0100] Pit evaluation: A 3 points B 2 points C 1 point D 0 point

[0101] Surface defect evaluation: A 3 points B 2 points C 1 point D 0 point

[0102] [Table 1]

[0103]

[0104] [Table 2]

[0105]

[0106] Examples 1 to 17 of the application have few surface defects, and the number of large depressions exceeding 10 μm in depth and 40 μm in diameter on the surface of the test sample is almost zero, and thus good quality can be obtained.

[0107] On the other hand, Comparative Examples deviate from the scope of the application. Hereinafter, each example will be described.

[0108] The Si concentration of Comparative Example 18 is as low as 0.0004 mass%, and the Al concentration is as low as 0.0003 mass%, and deoxidation cannot be performed, and the O concentration is as high as 0.0121 mass%. In addition, Na is not supplied from the slag to the molten steel, and the Na concentration is as low as 0.00001 mass%. As a result, a large number of inclusions are formed as large MnO-SiO2 inclusions, and more than 10 surface defects caused by inclusions are generated in 10 m 2 of the surface of the test sample.

[0109] The Si concentration of Comparative Example 19 is as high as 0.370 mass%, and the Al concentration is as high as 0.210 mass%, and deoxidation is performed excessively, and as a result, Ca, Mg and Na are excessively supplied from the slag layer to the molten steel, the Ca concentration is as high as 0.0022 mass%, the Mg concentration is as high as 0.0038 mass%, and the Na concentration is as high as 0.0017 mass%. As a result, a large number of independent non-metallic inclusions of CaO and independent non-metallic inclusions of Na2O are generated, and a large number of depressions exceeding 10 μm in depth and 40 μm in diameter are observed on the surface of the test sample after conditioning. In addition, surface defects caused by MgO-Al2O3 inclusions are also observed.

[0110] Comparative Example 20 supplies excessive Na2O to the slag, and as a result, the Na concentration in the molten steel is as high as 0.002 mass%. As a result, inclusions are mainly Na2O, and a large number of depressions exceeding 10 μm in depth and 40 μm in diameter are observed on the surface of the test sample after conditioning.

[0111] Comparative Example 21 did not supply Na2O to the slag, and the Na concentration in the molten steel was also as low as 0.00002 mass%, and the inclusions became CaO-SiO2-Al2O3-MgO-MnO-based not containing Na2O. As a result, the melting point of the inclusions increased, and at the time of hot rolling, they could not be finely dispersed, causing surface defects. In addition, since a part of the inclusions did not become glassy, a part of pits exceeding 10 μm in depth and 40 μm in diameter were also observed on the surface of the sample after adjustment. Note that although the item of "inclusion number ratio" of Comparative Example 21 was all 0, inclusions were present in the items of "inclusion composition (mass%) analyzed by EDS for 20 points" and "inclusion number distribution". This is because the inclusions of Comparative Example 21 existed as CaO-SiO2-Al2O3-MgO-MnO-based not containing Na2O, and in the item of "inclusion number ratio", the CaO-SiO2-Al2O3-MgO-Na2O-MnO-based of the present application, which is the object of counting, was not present, and thus counting was not performed.

[0112] Comparative Example 22 had a high Ti and N concentration of 0.036 mass% and 0.012 mass%, and a large number of surface defects caused by TiN were observed on the surface of the thin sheet of Fe-Ni alloy.

[0113] Comparative Example 23 directly supplied Mg to the molten steel, and the Mg was as high as 0.0033 mass%. As a result, it reacted with Al2O3 in the slag, and a large number of MgO-Al2O3 inclusions were generated. As a result, the ratio of MgO-Al2O3 increased, and a large number of surface defects were detected.

[0114] Comparative Example 24 did not retain the Mn that was added, and the Mn concentration was as low as 0.003%. As a result, deoxidation could not be sufficiently performed, and the oxygen concentration was as high as 0.008%. In addition, the inclusion composition also became CaO-SiO2-MgO-Al2O3-Na2O-based not containing MnO. As a result, the melting point of the inclusions increased, and the number of inclusions also increased, and a large number of surface defects caused by inclusions were detected.

[0115] Industrial Applicability

[0116] The technology of the present application controls the composition of non-metallic inclusions or the number of inclusions on the surface, and an Fe-Ni alloy having excellent surface properties, and particularly an Fe-Ni alloy having excellent suitability for CFRP mold applications can be obtained.

Claims

1. An Fe-Ni alloy, characterized in that, Composition: C: 0.001–0.2 wt%, Si: 0.001–0.2 wt%, Mn: 0.005–0.7 wt%, Ni: 30.0–45.0 wt%, Cr: less than 0.3 wt%, Al: 0.001–0.1 wt%, Ti: 0.001–0.020 wt%, O: less than 0.007 wt%, Mg: less than 0.0030 wt%, N: less than 0.010 wt%, Ca: less than 0.0015 wt%, Na: 0.00005–0.0 wt%. It is composed of 0.1% by mass, with the balance being Fe and unavoidable impurities, and contains non-metallic inclusions of a CaO-SiO2-Al2O3-MgO-MnO-Na2O composite oxide as an essential component, and further contains one or more non-metallic inclusions selected from CaO, MgO, MgO·Al2O3, MnO·SiO2, and Na2O as optional components. Among all non-metallic inclusions, the proportion of CaO-SiO2-Al2O3-MgO-MnO-Na2O inclusions is more than 40%.

2. The Fe-Ni alloy according to claim 1, characterized in that, Contains Nb: 0.01–1.00% by mass.

3. The Fe-Ni alloy according to claim 1 or 2, characterized in that, Of all the non-metallic inclusions, the proportions of CaO and Na2O inclusions are each less than 20%.

4. The Fe-Ni alloy according to claim 1 or 2, characterized in that, Of all the non-metallic inclusions, the proportion of MgO·Al2O3 inclusions is less than 20%.

5. The Fe-Ni alloy according to claim 3, characterized in that, Of all the non-metallic inclusions, the proportion of MgO·Al2O3 inclusions is less than 20%.

6. The Fe-Ni alloy according to claim 1 or 2, characterized in that, Of all the non-metallic inclusions, the proportion of MnO·SiO2 inclusions is less than 20%.

7. The Fe-Ni alloy according to claim 3, characterized in that, Of all the non-metallic inclusions, the proportion of MnO·SiO2 inclusions is less than 20%.

8. The Fe-Ni alloy according to claim 4, characterized in that, Of all the non-metallic inclusions, the proportion of MnO·SiO2 inclusions is less than 20%.

9. The Fe-Ni alloy according to claim 1 or 2, characterized in that, The CaO-SiO2-Al2O3-MgO-MnO-Na2O system oxide is composed of CaO: 20-60% by mass, SiO2: 10-40% by mass, Al2O3: less than 30% by mass, MgO: 5-50% by mass, Na2O: 0.001-1% by mass, and the balance being MnO. The MgO·Al2O3 is composed of MgO: 10-40% by mass and Al2O3: 60-90% by mass.

10. The Fe-Ni alloy according to claim 1 or 2, characterized in that, When a 200mm thick slab is hot-rolled to a 30mm thick slab, a 200mm thick alloy surface is formed. 2 Within the area, there are fewer than 10 non-metallic inclusions that are dispersed parallel to the rolling direction, have a width of more than 5 μm, and are continuously arranged for more than 40 μm.

11. A CFRP mold made of the Fe-Ni alloy as described in claim 1 or 2.

12. A method for manufacturing an Fe-Ni alloy with excellent surface properties, wherein the method for manufacturing the Fe-Ni alloy as described in claim 1 or 2 is characterized in that, The raw materials are melted in an electric furnace. Then, after decarburization in AOD and / or VOD, lime, fluorite, ferrosilicon alloy and / or Al are added. A CaO-Al2O3-MgO-SiO2-Na2O-F slag system is used, consisting of CaO: 50-70% by mass, SiO2: 3-30% by mass, MgO: 3-15% by mass, Al2O3: less than 5% by mass, Na2O: 0.001-1% by mass, with the balance being F. While stirring with Ar, deoxidation and desulfurization are carried out. After adjusting the temperature and composition with LF to promote the flotation of inclusions caused by Ar stirring, the ingots are cast using a continuous casting machine or ordinary ingot casting method. The ingots are hot forged to produce slabs, followed by hot rolling and cold rolling.

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