Austenitic stainless steel and method for manufacturing the same

Abstract

To reduce generation of Mg-derived defects on a steel plate surface in manufacture of an austenitic stainless steel even in a case where a refining vessel (converter, ladle, or the like) having an inner lining of a MgO-based refractory is employed during refining, and to propose an austenitic stainless steel plate having no Mg-derived defects on the steel plate surface and a method for manufacturing the same.SOLUTION: It has been found that favorable results can be obtained when, in deoxidation and desulfurization reactions of molten steel, deoxidizing agents and desulfurizing agents of Al and Ca are added prior to addition of REM to suppress excessive dissolution of Mg, and control is performed so as to satisfy a relational expression between Al2O3 and CaO (see the following formula). 0.5≤CaO / Al2O3≤2.5.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel and a method for producing the same. [Background technology]

[0002] In the stainless steel manufacturing process, the composition of molten steel is adjusted by refining in a refractory-lined refining vessel. In order to remove the sulfur and oxygen impurities in stainless steel at the molten steel stage, the refractory lining of the refining vessel contains MgO as its main component to promote desulfurization and deoxidation reactions.

[0003] However, depending on the concentrations of elements in molten steel and the composition of the slag during reduction refining, MgO in the refractories or slag may dissolve into the molten steel through the reaction MgO → Mg + O. Bubbles resulting from Mg dissolved in the molten steel may be partially exposed by grinding the surface of the slab, and the exposed portions may oxidize in the heating furnace before hot rolling. The oxidized areas may then be elongated during rolling, resulting in defects on the steel sheet surface. Various countermeasures have been investigated to prevent MgO from dissolving from refractories during refining.

[0004] For example, Patent Document 1 proposes a method of lining the refractory of a refining vessel with an alumina-based refractory as a means of suppressing an increase in the Mg concentration in molten steel, thereby preventing the elution of MgO from the refractory.

[0005] For example, Patent Document 2 proposes a technology for suppressing the corrosion of MgO-based refractories. This technology involves adding MgO to the slag during ladle refining of high-Cr molten steel with a carbon content of 0.3% or less, thereby suppressing the corrosion of MgO bricks that come into contact with the slag in the ladle. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 7-116779 [Patent Document 2] Japanese Patent Application Publication No. 10-251737 Summary of the Invention [Problem to be solved by the invention]

[0007] The technology of Patent Document 1 requires high costs for lining the refractory with an alumina-based material. Furthermore, alumina-based refractories are neutral, which is thermodynamically unfavorable for promoting deoxidation and desulfurization reactions during reduction refining. Furthermore, the preparation of a special vessel leads to various operational constraints.

[0008] In the technology of Patent Document 2, MgO-based refractories often contain carbon as an aggregate, and when heated to high temperatures, these MgO-C-based refractories undergo a reaction such as MgO+C→Mg+CO (gas) and are sublimated. In addition, when the basicity of the slag is low, MgO in the MgO-C dissolves in the slag, which not only causes the bricks to melt and wear, but also may supply carbon to the molten steel, resulting in poor decarburization. Furthermore, MgO-based inclusions may be detected in the product and cause defects.

[0009] Furthermore, neither Patent Document 1 nor Patent Document 2 makes any mention of steel sheet surface defects caused by Mg bubbles. In particular, steel sheet surface defects caused by Mg bubbles are likely to become apparent when REM is contained.

[0010] In view of the above-mentioned conventional problems, an object of the present invention is to suppress the occurrence of Mg-induced defects on the steel sheet surface even when a refining vessel (such as a converter or ladle) lined with an MgO-based refractory is used during refining in the production of REM-containing austenitic stainless steel, and to provide an austenitic stainless steel sheet that is free from Mg-induced defects on the steel sheet surface, and a method for producing the same. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to achieve the above object and have obtained the following findings. (a) We investigated the mechanism by which Mg oxides cause surface defects in steel sheets and obtained the following findings. Specifically, during molten steel refining, basic slag comes into contact with the refractory material on the inner surface of the refining vessel, causing MgO in the refractory to melt and become mixed into the slag. The mixed MgO reacts with REM and Al in the slag, reducing the MgO and dissolving Mg into the molten steel via the slag (see the formula below). As the molten steel temperature decreases, the dissolved Mg becomes supersaturated and solidifies, forming Mg bubbles in the surface layer of the cast slab. When the slab surface is ground using a scarfing process or other method before hot rolling, Mg bubbles (pinholes) appear on the slab surface, and the interior of the pinholes oxidizes during heating before hot rolling. If the slab is hot rolled in this state, the Mg oxides formed inside the oxidized pinholes are also expanded, resulting in surface defects on the rolled steel sheet. (MgO) + 2[REM] → (REM2O3) + 3[Mg] (MgO) + 2[Al] → (Al2O3) + 3[Mg]

[0012] (stomach) Therefore, it was discovered that it is effective to add Al or Ca deoxidizers and desulfurizers before adding REM in the deoxidation and desulfurization reactions of molten steel to suppress excessive dissolution of Mg and to control the relationship between Al2O3 and CaO in the slag (see the formula below) so as to satisfy the formula. 0.5≦CaO / Al2O3≦2.5

[0013] The present invention has been made based on the above findings, and has the following gist.

[0014] [1] In mass%, C: 0.01~0.20%, Si: 0 to 2.0% Mn: 0 to 3.00% P: 0.050% or less, S: 0.0050% or less, Cr: 15.0~30.0%, Ni: 8.0 to 25.0% N: 0.010~0.200%, Al: 0.05 to 2.0%, Mg: 0 to 0.010% and REM: 0.010 to 0.200% The balance is Fe and impurities, Among the surface defects, the number of defects with an inner Mg concentration of 0.010% or more is 5 / m 2 An austenitic stainless steel sheet characterized by the following: [2] A method for producing an austenitic stainless steel sheet according to the above [1], This is a refining method in which the composition of the slag after the deoxidation process in the refining process is controlled so that the slag that comes into contact with molten steel is a CaO-MgO-Al2O3 system, and the slag composition after adjusting the molten steel components and before casting is controlled so that it is 0.5≦CaO / Al2O3≦2.5. That is, the composition of the slag after the deoxidation process is: CaO: 40~60% by mass, MgO: 5 to 10 mass %, and A method for producing an austenitic stainless steel sheet, characterized by satisfying the following formula 1: 0.5≦CaO / Al2O3≦2.5...Equation 1 However, CaO and Al2O3 in formula 1 indicate the contents (mass%) of CaO and Al2O3, respectively, in the slag, and 0 is substituted if no CaO or Al2O3 is contained. [Effects of the Invention]

[0015] According to the present invention, in the manufacturing process of an austenitic stainless steel sheet containing REM, even if a refining vessel (such as a converter or ladle) lined with an MgO-based refractory is used during refining, it is possible to suppress the occurrence of Mg-induced defects on the steel sheet surface. DETAILED DESCRIPTION OF THE INVENTION

[0016] An embodiment of the present invention (hereinafter simply referred to as the present invention) will be described below. Unless otherwise specified, "%" regarding the content of a component indicates % by mass. When no lower limit is specified or when the lower limit is 0%, this also includes the case where the component is not contained (0%).

[0017] <About steel composition> C: 0.01 to 0.20% C is an element effective in stabilizing the austenite phase, but if it is too much, Cr-based carbides precipitate at grain boundaries, reducing weather resistance, so it is best to keep it at 0.20% or less, and preferably 0.18% or less, 0.15% or less, or 0.10% or less. On the other hand, an excessive reduction in C content increases refining costs, so it is preferably 0.01% or more, 0.02% or more, or 0.04% or more.

[0018] Si: 0 to 2.0% Silicon is an effective deoxidizing element and contributes to improving strength through solid solution strengthening, but excessive silicon content promotes the formation of intermetallic compounds such as the σ phase, reducing cold workability, so the silicon content is preferably 2.0% or less, more preferably 1.8%, 1.6%, or 1.4%. There is no particular lower limit, but an excessive reduction leads to an increase in refining costs, so the silicon content is preferably 0.01%, 0.1%, or 0.2%.

[0019] Mn: 0 to 3.00% Mn is an austenite-forming element, but excessive content can significantly deteriorate manufacturability. Therefore, the Mn content is set to 3.0% or less, and preferably 2.5% or less. There is no particular lower limit for Mn, but because Mn has the effect of improving corrosion resistance and workability, it is preferably contained in an amount of 0.05% or more, 0.10% or more, 0.30% or more, or 0.50% or more.

[0020] P:0.050% or less Since P is harmful to stainless steel, reducing toughness, hot workability, and corrosion resistance, the less P there is, the better, and it should be 0.050% or less, and preferably 0.040% or less. However, an excessive reduction in P content increases the load during refining or requires the use of expensive raw materials, so in reality, a content of 0.001% or more is acceptable.

[0021] S: 0.0050% or less S segregates at grain boundaries and reduces grain boundary strength. It also forms sulfides such as CaS, which can cause early rusting and are harmful to stainless steel. Therefore, the less S there is, the better. The upper limit should be 0.0050% or less, and preferably 0.0030% or less. However, excessive reductions increase the load during refining or require the use of expensive raw materials, so in reality, a content of 0.0001% or more is acceptable.

[0022] Cr: 15.0 to 30.0% Cr is an important element that provides corrosion resistance to stainless steel, and its content should be 15.0% or more, preferably 15.5% or more, 16.0% or more, 17.0% or more, 18.0% or more, 19.0% or more, or 20.0% or more. On the other hand, Si is a σ-phase forming element, and a large content not only reduces workability but also destabilizes the austenite phase, so its content should be 30.0% or less, and preferably 29.0% or less, 28.0% or less, 27.0% or less, or 26.0% or less.

[0023] Ni: 8.0 to 25.0% Ni is an element that stabilizes the austenite phase and promotes the occurrence of deformation twins during cold working and the subsequent process, thereby increasing strength, and also improves corrosion resistance to various acids and low-temperature toughness. Therefore, Ni content should be 8.0% or more, and preferably 9.0% or more, 10.0% or more, 11.0% or more, 12.0% or more, or 13.0% or more. However, since Ni is an expensive element, even if it is added in large amounts, the effect does not justify the increase in alloy cost. Therefore, Ni content should be 25.0% or less, and preferably 23.0% or less, 21.0% or less, or 19.0% or less.

[0024] N: 0.010 to 0.200% Like C, N is an austenite-forming element and also a solid-solution strengthening element, so its content is set to 0.010% or more, preferably 0.030% or more. However, if N is added in large amounts, the 0.2% yield strength increases, which may harden the steel material and significantly deteriorate manufacturability. Therefore, the N content is set to 0.200% or less, preferably 0.150% or less.

[0025] Al: 0.05 to 2.00% Since Al is an effective deoxidizing element and has the effect of reducing excess free oxygen in steel before the addition of REM and improving the yield of REM in molten steel, the Al content should be 0.05% or more, and preferably 0.06% or more, 0.07% or more, 0.08% or more, or 0.09% or more. On the other hand, since an excessively high Al content may cause refractory corrosion, the Al content should be 2.00% or less, and preferably 1.90% or less, 1.80% or less, 1.70% or less, 1.60% or less, 1.50% or less, or 1.40% or less.

[0026] Mg: 0 to 0.010% Magnesium, together with aluminum, easily forms MgO·Al2O3 spinel inclusions. These spinel inclusions are very hard. Therefore, spinel inclusions near the surface of steel sheets cause scratches, so it is desirable to minimize their presence. Furthermore, it is also desirable to minimize their presence in steel sheets from the viewpoint of preventing surface defects caused by magnesium bubbles that are generated as a result of leaching from the refractory material on the inner surface of the refining vessel. Therefore, the magnesium content should be 0.010% or less, preferably 0.008% or less, 0.007% or less, 0.006% or less, or 0.005% or less.

[0027] REM: 0.010~0.200% REM is an element that improves oxidation resistance. Furthermore, because it has strong deoxidizing and desulfurizing properties, it is advisable to include it in an amount of 0.010% or more, preferably 0.020% or more, 0.040% or more, 0.060% or more, or 0.080% or more. However, REM is expensive, and even if it is included in a large amount, its effect saturates. Therefore, the REM content is set to 0.200% or less, preferably 0.180% or less, 0.160% or less, or 0.140% or less.

[0028] According to the general definition, REM (rare earth elements) refers to a total of 17 elements: two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). These REM elements may be contained alone or in combination. When multiple REM elements are contained, the total amount is taken as the REM content.

[0029] The balance of the above steel components is Fe and impurities. Here, the term "impurities" refers to components that are mixed in during industrial steel production due to various factors in the manufacturing process, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention.

[0030] <Manufacturing method> Next, a method for producing an austenitic stainless steel sheet according to the present invention will be described. As mentioned above, surface defects of steel sheets caused by magnesium oxides occur when molten steel and slag come into contact with the refractory material on the inner surface of the refining vessel during molten steel refining. The magnesium oxide in the refractory melts and is mixed into the slag. The magnesium reacts with the REM and aluminum in the slag, reducing the magnesium to magnesium, which then dissolves into the molten steel. As the temperature of the molten steel decreases, the magnesium dissolved in the molten steel becomes supersaturated and solidifies, forming magnesium bubbles in the surface layer of the cast slab. When the slab surface is ground by a scarfing process or other method before hot rolling, magnesium bubbles (pinholes) appear on the slab surface. The interior of the pinholes oxidizes during heating before hot rolling. If the slab is hot rolled in this state, the magnesium oxides formed inside the pinholes are also elongated, resulting in defects on the surface of the rolled steel sheet. Therefore, when Al2O3 and REM are added to the molten steel in the deoxidation and desulfurization reactions during the refining process, the amount of REM and Al2O3 added is controlled to prevent excessive dissolution of Mg. Therefore, the present invention is particularly effective for austenitic stainless steel sheets containing REM.

[0031] Well-known stainless steel refining processes include argon oxygen decarburization (AOD) and vacuum oxygen decarburization (VOD), and various technologies have been developed to enable economical mass production of various stainless steels. In these processes, basic refractories are generally used to line the refining vessels to promote desulfurization and deoxidation and improve steel quality. For example, refractories with MgO as the main component (MgO-based refractories) such as magnesia-chrome refractories (e.g., MgO: 60%, Al2O3: 10%, Cr2O3: 20%) and magnesia-dro refractories (e.g., MgO: 57%, CaO: 40%) are commonly used. These MgO-based refractories offer excellent resistance to slag erosion, spalling, and abrasion, making them ideal for general stainless steel refining.

[0032] However, when REM-added stainless steel is melted, MgO-based refractories are susceptible to chemical damage during the reduction refining process using deoxidizing slags such as CaO and Al2O3. The MgO in the refractory is reduced and dissolved into the molten steel as Mg. As the temperature of the molten steel drops during casting, the refractory becomes supersaturated, easily forming Mg bubbles (pinholes) inside the slab. It was also found that Mg bubbles (pinholes) tend to form in a cylindrical shape from below the surface, 2-3 mm deep, toward the interior of the ingot. Mg in molten steel is primarily formed by the reduction of MgO mixed in the slag by the REM and Al in the molten steel, as follows: 3MgO+2REM=REM2O3+3Mg 3MgO+2Al=Al2O3+3Mg

[0033] MgO in slag is often generated by chemical damage, spalling, or corrosion caused by contact of basic slag with MgO-based refractories during refining. Therefore, when using conventional refining processes using MgO-based refractory vessels, an increase in the Mg concentration in REM-containing molten steel is unavoidable to some extent. Because REMs have a very strong oxygen affinity, they oxidize excessively in molten steel, activating the reduction reaction of MgO. Therefore, we have found that for REM-containing steels, it is effective to add Al and Ca deoxidizers and desulfurizers to form the slag before adding REM. In other words, it is preferable to avoid adding REM during the deoxidation process during refining, for example, by adding REM immediately before casting after the completion of the entire refining process, including deoxidation. This is because the increase in Mg concentration due to REM addition can be suppressed.

[0034] <Relationship between Ca, Al, and Mg oxides in slag after deoxidation process during refining treatment> CaO: 40~60% by mass Ca is added as calcium oxide (CaO), a basic compound, to promote deoxidation and desulfurization reactions in molten steel along with deoxidizers such as Al and Si. Al and Si combine with oxygen in the molten steel to form oxides such as Al2O3 and SiO2, and CaO is added in a proportion appropriate for promoting the reduction reaction. If the amount of CaO added is too small, it does not contribute to promoting the reduction reaction and the S concentration cannot be reduced to 0.005% or less. Therefore, the amount of CaO added should be 40% or more of the total slag volume, and preferably 53% or more. On the other hand, if too much CaO is added, it will not dissolve in the molten steel and the reduction reaction will not proceed, so the amount of CaO added should be 60% or less of the total slag volume.

[0035] MgO: 5~10% by mass If the MgO content in the slag exceeds 10%, the Mg concentration in the molten steel increases, which may lead to the formation of Mg bubbles in the slab after casting. Therefore, it is recommended that the MgO content in the slag be 10% or less of the total slag volume. The lower the MgO content, the better, but the lower limit is 5%. However, as mentioned above, MgO is often caused by chemical damage, spalling, and corrosion caused by contact of basic slag with MgO-based refractories during refining. Therefore, as long as conventional refining methods using MgO-based refractory vessels are followed, the presence of MgO in the slag is unavoidable to some extent.

[0036] As a result of research and investigation by the inventors, it was found that it is desirable that the CaO / Al2O3 value (slag ratio) among CaO, MgO, and Al2O3 in the slag after the deoxidation process during refining satisfies the relationship of the following formula 1, and that the MgO concentration be 10 mass% or less. 0.5≦CaO / Al2O3≦2.5...Equation 1 However, CaO and Al2O3 in formula 1 indicate the contents (mass%) of CaO and Al2O3, respectively, in the slag, and 0 is substituted if no CaO or Al2O3 is contained.

[0037] It has been found that if the CaO / Al2O3 ratio shown in Equation 1 is too small, the desulfurization reaction does not proceed. Therefore, it is desirable for the CaO / Al2O3 value to be 0.5 or higher. On the other hand, it has been found that if the CaO / Al2O3 ratio is too large, the amount of Mg in the molten steel increases, promoting the formation of Mg bubbles (pinholes). Therefore, it is desirable for the CaO / Al2O3 value to be 2.5 or lower, preferably 1.5 or lower.

[0038] <Surface defects on austenitic stainless steel sheets> The above method suppresses the occurrence of defects caused by Mg bubbles (pinholes) on the surface of the austenitic stainless steel sheet obtained. Defects caused by Mg bubbles (pinholes) can be determined by whether or not Mg is present on the inner surface of the defect, and defects can be determined to be caused by Mg bubbles when the Mg concentration is 0.010% or more. The austenitic stainless steel sheet according to the present invention has a steel sheet surface with fewer than 5 defects caused by Mg bubbles per m2. 2 The result is as follows. [Example]

[0039] An embodiment of the present invention will be described. A refractory material made of dolomite (MgO 65%, CaO 35%) was placed as a crucible in a 1.100 kg vacuum high-frequency induction melting furnace, and a 100 kg cast iron mold was installed for casting. 2. The raw material for melting, steel having the composition shown in Table 1, was placed in a crucible, melted, and maintained at a molten steel temperature of 1550°C. 3. In order to deoxidize and desulfurize the molten steel held at 1550°C, a predetermined concentration of Al was added as a deoxidizer. 4. Five minutes after adding Al, the specified amounts of Si and CaO were added. 5. After the added CaO was melted, a predetermined amount of misch metal containing REM elements Y, Nd, or Ce and La was added to the molten steel. 6.The furnace was then tilted and the molten steel was poured into the mold. 7. Samples for hot and cold rolling were cut out from the obtained ingot. The cut sample dimensions were a rectangular parallelepiped with a thickness of 40 mm, a width of 50 mm, and a length of 70 mm. The cutting position from the ingot was adjusted so that the wide surface of the sample was directly below the surface of the ingot. 8. Furthermore, the surface of a sample cut from this ingot was ground 2 mm. This was in accordance with the fact that in actual operation, the surface quality of hot-rolled steel sheets is not good after casting, so the surface of the slab is ground to a thickness of about 2 to 3 mm. 9. After that, the sample was heated at 1230°C for 30 minutes, and then hot-rolled at 1200°C with the wide surface in contact with the hot-rolling roll to produce a hot-rolled sheet with a thickness of 5 mm. 10. The hot-rolled sheets were pickled to prepare samples suitable for cold rolling. Then, cold rolling was carried out to a thickness of 3 mm.

[0040] <How to identify pinhole defects> 1. Surface defects on cold-rolled sheets that were judged to be caused by pinholes were determined using a two-stage method. 1) Defects present on the surface of the cold-rolled sheet were visually confirmed. 2) The steel plate was cut perpendicular to the flaw and embedded in resin so that the flaw was the observation surface in the thickness direction to prepare a sample for observation. 3) The Mg concentration of these observation samples was measured using an EF-EPMA (field emission electron probe microanalyzer), and defects with an Mg concentration of 0.010% or more inside the defect were determined as pinhole defects, and their number density was calculated. The calculated number density of defects caused by Mg bubbles (pinholes) is shown in Table 1.

[0041] Samples Nos. 1 to 10 satisfied the above-described conditions, and therefore no defects caused by Mg bubbles (pinholes) were found on the surface of the austenitic stainless steel sheet.

[0042] Samples No. 6 and No. 7 had a CaO concentration exceeding 60%, and No. 6 did not meet the total REM concentration requirement. Heat No. 7 had a CaO concentration exceeding 60%, and the slag ratio also fell outside the optimum range. As a result, arrowhead-shaped defects were observed in the cold-rolled steel sheets of Samples No. 6 and No. 7. FE-EPMA analysis revealed a Mg concentration of 0.01-0.02%, confirming that the defects were caused by Mg bubbles.

[0043] In sample No. 8, the low CaO concentration resulted in a high S concentration in the metal of 0.009%, and although no pinholes were observed, S segregated to the grain boundaries during solidification, causing a decrease in grain boundary strength, and grain boundary oxidation defects and edge cracks were observed on the surface when the sample was hot rolled.

[0044] Sample No. 9 had a high total REM concentration and the slag composition ratio including CaO exceeded the range explained above, so defects caused by Mg bubbles were observed on the surface of the cold-rolled steel sheet.

[0045] Sample No. 10 also had a high slag ratio composed of CaO, and defects caused by Mg bubbles were observed on the surface of the cold-rolled steel sheet.

[0046] [Table 1] [Industrial Applicability]

[0047] The present invention can be used in the metal manufacturing industry, particularly in the steel and stainless steel manufacturing industries.

Claims

1. In mass%, C: 0.01~0.20%, Si: 0-2.0%, Mn: 0 to 3.00%, P: 0.050% or less, S: 0.0050% or less, Cr: 15.0-30.0%, Ni: 8.0 to 25.0%, N: 0.010-0.200%, Al: 0.05-2.0%, Mg: 0-0.010% and REM: 0.010 to 0.200%; The balance is Fe and impurities, Among the surface defects, the number of defects with an inner Mg concentration of 0.010% or more is 5 / m ２ An austenitic stainless steel sheet characterized by the following:

2. 2. The method for producing an austenitic stainless steel sheet according to claim 1, The composition of the slag after the deoxidation process during refining is: CaO: 40 to 60% by mass, MgO: 5 to 10 mass %, and A method for producing an austenitic stainless steel sheet, characterized in that the following formula 1 is satisfied: 0.5≦CaO / Al ２ O ３ ≦2.5 ...Formula 1 However, CaO and Al in formula 1 ２ O ３ are the CaO and Al in the slag, respectively. ２ O ３ If the component is not contained, enter 0.

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