S-containing stainless steel having excellent surface properties and method for producing same

Abstract

[Problem] To provide an S-containing stainless steel having excellent surface properties by finely controlling the composition of oxide-based inclusions and making the oxide-based inclusions harmless. The present invention also relates to a method for producing stainless steel containing S steel, and proposes a refining method for controlling the S concentration with high precision while controlling the form of inclusions. [Solution] The S-containing stainless steel is characterized by comprising, in mass%, 0.30% or less of C, 0.2% to 1.0% of Si, 1.2% to 1.8% of Mn, 5% to 10% of Ni, 15% to 20% of Cr, 0.05% to 0.60% of Mo, 0.05% to 0.60% of Cu, 0.005% or less of Al, 0.15% to 0.25% of S, 0.0001% to 0.0010% of Ca, 0.0010% or less of Mg, 0.0020% to less than 0.0080% of O, and the remainder being Fe and unavoidable impurities, the oxide-based inclusion is composed of one or both of CaO-SiO2-MgO-Al2O3-MnO-based inclusion and MgO-Al2O3-MnO-based inclusion, and the MgO-Al2O3-MnO-based inclusion contains 1% to 15% of MnO in mass%.

Description

Technical Field

[0001] The present invention relates to sulfur-containing stainless steel having excellent surface properties, which controls the composition of oxide-based non-metallic inclusions in molten steel by controlling the deoxidation method and slag composition, thereby rendering the oxide-based inclusions harmless. In addition, in the present invention, regarding sulfur-containing stainless steel having excellent surface properties, a manufacturing method for highly precisely controlling the S concentration while controlling the inclusion morphology is also proposed. Background Art

[0002] Sulfur-containing stainless steel represented by SUS303 is a steel type that has improved machinability and cuttability during grinding with a tool by containing a high concentration of S of 0.15 mass% or more and forming MnS particles, and is widely used for precision parts for machining, etc. In semiconductor manufacturing equipment, if there are small defects on the surface, it cannot maintain airtightness, which becomes a major problem. That is, sulfur-containing stainless steel requires excellent surface properties.

[0003] The refining method of sulfur-containing steel is different from that of general stainless steel (e.g., SUS304), and it is necessary to suppress the transfer of S introduced into the molten steel to the slag. The reaction of S in the molten steel with the slag can be represented by the reaction formulas of Formulas 1 to 3. Formula 1 represents the reaction of S in the molten steel with CaO in the slag to form CaS, Formula 2 represents deoxidation using Si as a deoxidizing material, and Formula 3 is a formula obtained by combining Formulas 1 and 2. According to Formulas 1 to 3, in order to suppress the transfer of S to the slag and highly precisely control the S concentration, it is preferable to perform a refining method that does not increase the CaO concentration of the slag, reduces the (CaO) / (SiO2) ratio, and further does not decrease the oxygen concentration in the molten steel and does not increase the Si concentration in the molten steel.

[0004] However, such a refining method can easily control the S concentration and the S retention rate is improved. However, due to an increase in the number of non-metallic inclusions caused by insufficient deoxidation and the non-metallic inclusion composition becoming high-melting-point MnO-SiO2-based inclusions, scab defects caused by inclusions are generated on the surface of the product and are removed by surface grinding or cutting. Therefore, there is a problem that the product yield is deteriorated, and there is also a problem that it cannot be used for parts applications of precision machines with strict surface property requirements.

[0005] (CaO)+ S =(CaS)+ O …Formula 1

[0006] Si +2 O =(SiO2)…Formula 2

[0007] 2(CaO)+2 S + Si =2(CaS)+(SiO2)…Formula 3

[0008] In the above formula, the components in parentheses represent the components in the slag, and the underlined components represent the components in the molten steel.

[0009] In Patent Document 1, an austenitic stainless steel is disclosed which contains 0.15 to 0.50% of sulfur, and the O concentration is adjusted to 80 to 200 ppm, whereby the shape of sulfides after hot working or hot / cold working is controlled to a granular type, improving the machinability. This technology shows that it is important for the oxides precipitated in the sulfides to be Si-Mn based oxides. However, since the O concentration is controlled at a very high level, the number of oxide-based inclusions is very large, and there is a risk of adverse effects such as cracks or nozzle blockage caused by the oxide-based inclusions.

[0010] In addition, in Patent Document 2, a stainless steel is disclosed which controls the oxide composition by adding REM (Rare Earth Metal), thereby controlling the distribution of sulfides and having excellent hot workability and machinability. However, since REMs are all very expensive metals, the cost increases. In addition, since REMs are very reactive metals, if REMs are added to molten steel with a high oxygen concentration, a large amount of REM oxides are generated, resulting in deterioration of the surface properties. That is, in the technology disclosed in Patent Document 2, there are still problems with the surface properties of S-containing steel.

[0011] In addition, in Patent Document 3, regarding a method for manufacturing S-containing stainless steel, a refining method for precisely controlling the S concentration is also disclosed. That is, in this method, a CaO-SiO2 based slag with a basicity of CaO / SiO2 controlled to 1 to 1.5 is formed, and 0.3 mass% to 0.6 mass% of S is added, whereby the S concentration is finally controlled to 0.15 to 0.25 mass%, and a slab is manufactured by continuous casting. However, there are problems such as a large amount of S addition, increased cost, and an extended refining time. In addition, oxide-based inclusions are not considered.

[0012] Thus, although a large number of inventions regarding the chemical composition of austenitic stainless steel for improving machinability or cutting performance, inventions for controlling sulfides and improving hot workability by controlling oxides, etc. are disclosed, there are few inventions focusing on the influence caused by oxide-based inclusions themselves. That is, it can be said that there are still problems related to the surface properties caused by oxide-based inclusions in S-containing steel.

[0013] Prior art documents

[0014] Patent documents

[0015] Patent Document 1: Japanese Patent Laid-Open No. 8-260102

[0016] Patent Document 2: Japanese Patent Laid-Open No. 2014-28997

[0017] Patent Document 3: Japanese Patent Application Laid-Open No. 2014-234543. Summary of the Invention

[0018] Problems to be Solved by the Invention

[0019] In view of the above problems, an object of the present invention is to provide an S-containing stainless steel having excellent surface properties by making oxide-based inclusions harmless through precise control of the composition of oxide-based inclusions. In addition, regarding the manufacturing method of S-containing stainless steel, a refining method for precisely controlling the S concentration while controlling the inclusion morphology is also proposed.

[0020] Means for Solving the Problems

[0021] The inventors analyzed the various effects on surface defects related to S-containing stainless steel based on the operation data of S-containing stainless steel manufactured under various operating conditions. Specifically, the inclusion composition and the number of inclusions larger than 5 μm in size in samples collected from the tundish during the continuous casting of SUS303, and the appearance of the SUS303 plate with a plate width of 1000 mm and a plate thickness of 3.0 mm after annealing and pickling were evaluated, and the relationship between surface defects longer than 1 mm and the slag composition, metal composition, the relationship with the oxide-based inclusion composition, and the refining method were analyzed. In addition, the factors affecting the S retention rate and the control accuracy of the S concentration were also analyzed.

[0022] First, the inclusions in the samples collected from the tundish during continuous casting were investigated. As a result, it was clarified that the composition of the oxide-based inclusions changes according to the oxygen concentration. In the case of high oxygen concentration, they are MnO-SiO2-based inclusions; in the case of low oxygen concentration, they are MgO-Al2O3-based inclusions; and in the case of intermediate oxygen concentration, they are CaO-SiO2-Al2O3-MgO-based inclusions. In addition, it was also clarified that when the oxygen concentration is high and the oxide-based inclusions are MnO-SiO2-based inclusions, the number of inclusions is large.

[0023] Furthermore, the surface quality of the products was investigated. As a result, it was clarified that among the oxide-based inclusions, the more MgO-Al2O3-based inclusions and MnO-SiO2-based inclusions there are, the more surface defects there are. This indicates that these oxide-based inclusions have a high melting point, adhere to the surface of the refractory material of the submerged nozzle used during continuous casting, and are easily sintered and aggregated, and after coarsening, they fall off, and the generated large oxide-based inclusions become the starting points of surface defects during hot rolling. In addition, since the MnO-SiO2-based inclusions are numerous, they are the main cause of the increase in surface defects. In addition, it was also clarified that the CaO-SiO2-Al2O3-MgO-based inclusions become fine inclusions on the product surface and it is difficult to form surface defects on the product surface.

[0024] In addition, the inventors found that by containing MnO in CaO-SiO2-Al2O3-MgO inclusions, surface defects were reduced. This is because by containing MnO in CaO-SiO2-Al2O3-MgO inclusions, the melting point of oxide inclusions was lowered, preventing the formation of surface defects on the product surface due to elongation, cutting, and refinement during hot rolling.

[0025] In addition, the inventors found that by containing MnO in MgO-Al2O3 inclusions, surface defects were reduced. It is known that MgO-Al2O3 inclusions adhere to the surface of the submerged nozzle refractory used during continuous casting, are prone to sintering and coalescence, fall off after coarsening, and the generated large oxide inclusions are prone to becoming the starting point of surface defects during hot rolling. However, by containing MnO in MgO-Al2O3 inclusions, a low-melting-point liquid-phase oxide containing MnO is generated around the MgO-Al2O3 inclusions at a refining temperature of 1600 °C. After adhering to the surface of the submerged nozzle refractory, it does not sinter and coalesce, and has the effect of reducing coarsening.

[0026] From the above insights, in order to improve the surface properties of sulfur-containing steel, it is preferable to control the oxygen level in the molten steel and control the oxide inclusions to CaO-SiO2-Al2O3-MgO-MnO inclusions or MgO-Al2O3-MnO inclusions. CaO-SiO2-Al2O3-MgO-MnO inclusions are particularly preferred. The oxide inclusions that should not be controlled are MnO-SiO2 inclusions and MgO-Al2O3 inclusions.

[0027] However, in order to control to CaO-SiO2-Al2O3-MgO-MnO inclusions or Al2O3-MgO-MnO inclusions, it is necessary to lower the oxygen level. If the oxygen level is lowered, the added S transfers to the slag through the reactions of Formulas 1 to 3 in the molten steel. Therefore, the retention rate of S is poor, and it is necessary to repeatedly add S until S reaches the target composition, resulting in the problem of a longer refining time. The inventors conducted various studies on the equilibrium theory and rate theory of the reaction between molten steel and slag, and developed a refining method that takes into account controlling to the preferred composition of oxide inclusions and quickly controlling the S concentration in the molten steel. The developed refining method will be described below.

[0028] The MgO component in the slag plays the most important role in the developed refining method. After deoxidation with Si or Si+Al, lime is added to form a slag of the CaO-SiO2-MnO system, and further MgO is added to bring the MgO content to 25-45 mass%. By adding MgO, the slag becomes a solid phase composed of MgO phase, MgO-SiO2 phase and CaO-MgO phase, and a liquid phase mainly composed of CaO-SiO2-MnO.

[0029] It is known that adding MgO to this slag to produce a solid phase composed of MgO phase, MgO-SiO2 phase and CaO-MgO phase reduces the amount of CaO in the liquid slag that reacts with S in the molten steel, and has the effect that S added to the molten steel is likely to remain in the molten steel. That is, although the reaction shown in Equation 3 proceeds to the right, due to the small amount of CaO in the liquid slag, CaS in the slag becomes saturated and the reaction no longer proceeds. That is, by adding MgO, the added S is likely to remain in the molten steel, and even if the oxygen concentration is reduced, the S concentration can be controlled quickly and with high precision.

[0030] In addition, it is known that adding MgO to the slag to produce a solid phase composed of MgO phase, MgO-SiO2 phase and CaO-MgO phase causes a part of CaO and SiO2 to transfer to the solid phase, and the MnO concentration in the liquid slag relatively increases, having the effect of increasing the MnO concentration of the oxide inclusions present in the molten steel in equilibrium with the liquid slag.

[0031] In addition, the inventors of the present application found that the addition method of the deoxidizing material also has a great influence on the control of the composition of oxide inclusions. The deoxidizing materials for the S-containing stainless steel targeted in the present application are Si, Mn, and Al, but it was found that by first adding Mn to the molten steel after decarburization, it has the effect of containing MnO in the CaO-SiO2-Al2O3-MgO-MnO system inclusions or MgO-Al2O3-MnO system inclusions. The effect of first adding Mn will be described.

[0032] The reason is that by first adding Mn to the molten steel with a high oxygen concentration after decarburization, Mn is oxidized, thereby generating oxide inclusions mainly composed of MnO in the molten steel, and the MnO concentration in the slag also increases; even if MnO is reduced later by adding Si and Al, MnO in the oxide inclusions and MnO in the slag will remain. On the contrary, in the case of first adding Si and Al, since the deoxidizing ability of Si and Al is stronger than that of Mn, the Mn added later can only be slightly oxidized, and the MnO in the oxide inclusions decreases.

[0033] As described above, in order to improve the surface properties of sulfur-containing steel, a refining method has been developed that controls oxide inclusions to CaO-SiO2-Al2O3-MgO-MnO-based inclusions or Al2O3-MgO-MnO-based inclusions while controlling the oxygen level in the molten steel, and rapidly controls the sulfur concentration in the molten steel.

[0034] Next, as described above, the inventors further analyzed the appropriate range of oxygen concentration and various operating conditions for controlling it to an appropriate range, and as a result, the present invention was completed.

[0035] That is, a sulfur-containing stainless steel with excellent surface properties, characterized in that, by mass%, consists of C: 0.30% or less, Si: 0.2 - 1.0%, Mn: 1.2 - 1.8%, Ni: 5 - 10%, Cr: 15 - 20%, Mo: 0.05 - 0.60%, Cu: 0.05 - 0.60%, Al: 0.005% or less, S: 0.15 - 0.25%, Ca: 0.0001 - 0.0010%, Mg: 0.0010% or less, O: 0.0020 - less than 0.0080%, with the balance being Fe and inevitable impurities, and the oxide inclusions are composed of one or two of CaO-SiO2-MgO-Al2O3-MnO-based inclusions and MgO-Al2O3-MnO-based inclusions, and the MgO-Al2O3-MnO-based inclusions contain 1 - 15% of MnO by mass%.

[0036] Furthermore, among the oxide inclusions, the CaO-SiO2-MgO-Al2O3-MnO-based inclusions preferably contain 1 - 15% of MnO by mass%.

[0037] Furthermore, among the oxide inclusions, the number of MgO-Al2O3-MnO-based inclusions is preferably 50% or less by number.

[0038] In the present invention, it is preferably manufactured by the following method. A method for manufacturing a stainless steel with excellent surface properties, characterized in that, first, the raw materials are melted in an electric furnace, then decarburized by AOD or VOD, after adding Mn, Cr is reduced using Si or Si+Al, limestone is added, the basicity of the slag CaO / SiO2 is controlled at 0.75 - less than 1.00, and then a MgO source is added to form a CaO-SiO2-MgO-MnO-based slag with the MgO concentration of the slag controlled at 25 - 45% by mass. Detailed Description

[0039] Illustrates the reasons for limiting the chemical composition of the sulfur-containing stainless steel sheet of the present invention. It should be noted that in the following description, "%" refers to "mass%" ("mass%").

[0040] C: 0.30 mass% or less

[0041] C is a useful element for maintaining strength, but when it is too high, it causes sensitization and reduces corrosion resistance. Therefore, it is set to 0.30 mass% or less. Preferably 0.15 mass% or less, more preferably 0.07 mass% or less.

[0042] Si: 0.2 - 1.0 mass%

[0043] Si is an extremely important element in the present invention because it helps with deoxidation. However, when it exceeds 1.0 mass%, it reduces the oxygen concentration, thus shifting the reactions of Formulas 1 to 3 to the right. That is, it causes S in the molten steel to transfer to the slag. In addition, since the oxygen concentration in the molten steel decreases, an excessive supply of Mg to the molten steel easily forms MgO - Al2O3, deteriorating the surface properties. Conversely, if it is less than 0.2 mass%, the oxygen concentration increases and the number of inclusions increases, deteriorating the cleanliness. Therefore, it is specified to be 0.2 to 1.0 mass%. Preferably 0.4 to 0.9 mass%, more preferably 0.6 to 0.8 mass%.

[0044] Mn: 1.0 - 2.0 mass%

[0045] Mn combines with S to form MnS and is an important element for maintaining machinability. Additionally, it is important for MnO which lowers the melting point of inclusions. Its effect cannot be fully exerted when it is less than 1.0 mass%. However, if it is higher than 2.0 mass%, it reduces the hot workability, and in addition, the MnO content of oxide - based inclusions increases. Therefore, it is specified to be 1.0 to 2.0 mass%. Preferably 1.1 to 1.9 mass%, more preferably 1.2 to 1.8 mass%.

[0046] Ni: 5 - 10 mass%

[0047] It is an essential element for austenitic stainless steel and is an element that stabilizes the austenite phase. When it is low, the δ - ferrite increases sharply, damaging the hot workability and making the austenite phase unstable. Therefore, the lower limit is set to 5 mass%. However, since Ni is an expensive element, the upper limit is set to 10 mass%. Preferably 7 to 9.5 mass%, more preferably 8 to 9 mass%.

[0048] Cr: 15 - 20 mass%

[0049] Cr is an element necessary for obtaining the corrosion resistance of austenitic stainless steel. However, if it exceeds 20 mass%, it damages the balance of the δ / γ structure and reduces the hot workability. Therefore, it is specified to be 15 to 20 mass%. Preferably 17 to 19 mass%, more preferably 18 to 18.5 mass%.

[0050] Mo: 0.05 - 0.60 mass%

[0051] Mo is an element that improves corrosion resistance. However, since it is a very expensive element, excessive inclusion leads to an increase in cost. In addition, Mo forms a σ-phase, which is a hard intermetallic compound, with Cr and Fe, deteriorating the machinability of sulfur-containing stainless steel. Therefore, it is specified to be 0.05 to 0.60 mass%. Preferably, it is 0.10 to 0.58 mass%, and more preferably, it is 0.20 to 0.55 mass%.

[0052] Cu: 0.05 - 0.60 mass%

[0053] Cu is an element that improves acid resistance, and its effect is effectively exerted when Cu is 0.05 mass% or more. However, when contained in a large amount, it reduces hot workability. Therefore, the Cu content is specified to be 0.05 to 0.60 mass%. It should be noted that preferably, it is 0.08 to 0.55 mass%, and more preferably, it is 0.10 to 0.50 mass%.

[0054] Al: 0.005 mass% or less

[0055] Al is a strong deoxidizer and has the effect of reducing the oxygen concentration in molten steel and improving cleanliness. However, if the oxygen in the molten steel is excessively reduced, the reaction of Equation 3 proceeds to the right, and S in the molten steel transfers to the slag phase, so S needs to be repeatedly added. In addition, if Al exceeds 0.005 mass%, MgO-Al2O3 inclusions that have an adverse effect on the surface quality are likely to be generated. Therefore, the Al content is specified to be 0.005 mass% or less. Preferably, it is 0.004 mass% or less, and more preferably, it is 0.003 mass% or less.

[0056] S: 0.15 - 0.25 mass%

[0057] S is an element that combines with Mn in stainless steel to form MnS particles, improving the machinability using tools. Since the MnS particles are smaller than 1 μm and are generated during the solidification of sulfur-containing stainless steel, they have no influence on the cleanliness of the product plate surface. Its effect cannot be fully exerted when it is less than 0.15 mass%. On the other hand, if it is excessively added to exceed 0.25 mass%, the hot crack sensitivity deteriorates. Therefore, the S content is specified to be 0.15 to 0.25 mass%. Preferably, it is 0.155 to 0.20 mass%, and more preferably, it is 0.160 to 0.175 mass%. In addition, in order to accurately control the S concentration, the deoxidation reaction and the reaction formulas of Equations 1 to 3 arranged according to the slag composition are important.

[0058] Ca: 0.0001 - 0.0035 mass%

[0059] Ca is an effective element for controlling the composition of non-metallic inclusions in steel to CaO-SiO2-Al2O3-MgO-MnO system oxides with good surface quality. Its effect cannot be obtained when the content is less than 0.0001% by mass. On the contrary, if the steel contains more than 0.0035% by mass of Ca, edge cracks will occur during hot rolling. Therefore, the Ca content is specified to be 0.0001 - 0.0035% by mass, preferably 0.0002 - 0.0010% by mass.

[0060] Ca is a component incorporated into the molten steel through the slag / metal reaction as shown in Equation 4. The Ca concentration is related to the deoxidation level. By controlling O at 0.002 - less than 0.008% by mass and the Si concentration at 0.2 - 1.0% by mass, the Ca content can be controlled within 0.0001 - 0.0035% by mass.

[0061] 2(CaO)+ Si =2 Ca +(SiO2)…Equation 4

[0062] In the above equation, the parentheses represent the components in the slag, and the underlines represent the components in the molten steel.

[0063] Mg: 0.0010 mass% or less

[0064] Mg is an inevitably incorporated component through the wear of refractory materials and the slag / metal reaction as shown in Equation 5. Since it is the main component of MgO-Al2O3 system inclusions that have an adverse effect on the surface quality, it is desirable to minimize it. Therefore, it is specified to be 0.0010% by mass or less, preferably 0.0009% by mass or less, and more preferably 0.0008% by mass or less. The Mg concentration is related to the deoxidation level. By controlling O at less than 0.0080% by mass and the Si concentration at 1.0% by mass or less, the Mg content can be controlled within 0.0010% by mass or less.

[0065] 2(MgO)+ Si =2 Mg +(SiO2)…Equation 5

[0066] In the above equation, the parentheses represent the components in the slag or refractory materials, and the underlines represent the components in the molten steel.

[0067] O: 0.0020 - less than 0.0080 mass%

[0068] The O concentration is a very important component in the present invention. If the O concentration is excessively high, the number of oxide-based inclusions increases, causing surface defects. On the other hand, if the O concentration is excessively low, due to the reaction formulas of Formulas 1 to 3, S in the steel transfers to the slag phase, so the S retention rate deteriorates, and furthermore, the S concentration adjustment accuracy deteriorates. Therefore, the O concentration is specified to be 0.0020 to less than 0.0080% by mass. It is preferably 0.0025 to 0.0075% by mass. More preferably, it is 0.0030 to 0.0070% by mass.

[0069] Oxide-based inclusions

[0070] In the present invention, it is preferably composed of one or two of CaO-SiO2-MgO-Al2O3-MnO inclusions and MgO-Al2O3-MnO inclusions, and the CaO-SiO2-MgO-Al2O3-MnO inclusions contain 1 to 15% by mass of MnO by mass%. A further preferred embodiment is that the number of MgO-Al2O3-MnO inclusions is 50% or less. Hereinafter, the basis for limiting the composition range and number ratio of each oxide inclusion is shown.

[0071] CaO - SiO2 - MgO - Al2O3 - MnO-based inclusions

[0072] Basically, the melting point of CaO-SiO2-MgO-Al2O3-MnO inclusions is low. Through elongation, truncation, and refinement during hot rolling, scab defects caused by inclusions are less likely to occur on the surface of the product, and it is a preferred oxide-based inclusion to be controlled. In the present application, the composition range of CaO-SiO2-MgO-Al2O3-MnO inclusions is CaO: 15 to 40% by mass, SiO2: 15 to 50% by mass, Al2O3: 5 to 35% by mass, MgO: 5 to 3% by mass, and the composition range of the oxides in this composition range has a melting point of 1300 °C or lower.

[0073] In addition, since MnO has the effect of lowering the melting point, it is preferably contained in an amount of 1% by mass or more, preferably 2% by mass or more, and more preferably 3% by mass or more. On the other hand, if it contains 15% by mass or more of MnO, MnO-SiO2 inclusions are likely to crystallize in the CaO-SiO2-MgO-Al2O3-MnO inclusions. MnO-SiO2 inclusions are high-melting-point inclusions, which adhere to the surface of the immersion nozzle refractory used during continuous casting, are easily sintered and aggregated, and fall off after coarsening. The generated large oxide-based inclusions are likely to become the starting point of surface defects during hot rolling, so they are oxide inclusions to be avoided. Therefore, the upper limit is specified to be 15% by mass.

[0074] MgO - Al2O3 - MnO-based inclusions

[0075] MgO·Al2O3 is a compound with a relatively wide solid solution. It is defined in this way because a solid solution is formed in the range where MgO is 10 to 40% by mass and Al2O3 is 60 to 90% by mass. Inclusions in the MgO - Al2O3 system adhere to the surface of the immersion nozzle refractory used during continuous casting, and are prone to sintering and agglomerating, falling off after coarsening. The large oxide - based inclusions generated are likely to become the starting point of surface defects during hot rolling, so they are oxide - based inclusions that should be avoided. However, by containing 1% by mass or more of MnO in the MgO - Al2O3 system inclusions, a low - melting - point liquid oxide containing MnO is formed around the MgO - Al2O3 system inclusions at a refining temperature of 1600°C. Thus, after adhering to the surface of the immersion nozzle refractory, they will not sinter and agglomerate, and have the effect of reducing coarsening. However, if the content of MnO exceeds 15% by mass, MnO reacts with Al2O3 to form a compound of MnO - Al2O3, which crystallizes out within the MgO - Al2O3 - MnO system inclusions. The behavior of the MnO - Al2O3 - based oxide in molten steel is similar to that of MgO - Al2O3, adhering to the surface of the immersion nozzle refractory used during continuous casting, being prone to sintering and agglomerating, falling off after coarsening, and the large oxide - based inclusions generated become the starting point of surface defects during hot rolling. That is, in order to improve the surface quality of the product, the MnO - Al2O3 oxide is also an oxide - based non - metallic inclusion that should be avoided. For the above reasons, MnO is specified to be 1 to 15% by mass or less. In addition, even if the content of Cr2O3 is 10% by mass or less, it will not affect the characteristics of the above - mentioned MgO - Al2O3 - MnO system inclusions.

[0076] In addition, for the above reasons, the number of MgO - Al2O3 - MnO system inclusions is preferably 50% or less by number.

[0077] MnO - SiO2-based inclusions

[0078] Although the MnO - SiO2 system is not the object of the oxide - based inclusions of this application, at a refining temperature of 1600°C, it forms high - melting - point compounds such as MnSiO3 and Mn2SiO4, adheres to the surface of the immersion nozzle refractory used during continuous casting, is prone to sintering and agglomerating, falls off after coarsening, and the large oxide - based inclusions generated become the starting point of surface defects during hot rolling. In addition, the generation of MnO - SiO2 system inclusions occurs when deoxidation is not carried out sufficiently, and there are a large number of oxide inclusions in the molten steel. That is, in order to obtain good surface quality of the product, the MnO - SiO2 system is an oxide - based inclusion that should be avoided.

[0079] Manufacturing method

[0080] In the invention of the present application, in order to control the composition of oxide inclusions to a preferred composition while increasing the S retention rate and accurately controlling the S concentration, a manufacturing method is also proposed. A preferred embodiment is that, first, raw materials are melted in an electric furnace, decarburization is carried out by AOD or VOD, after adding Mn, Cr reduction is carried out using Si or Si + Al, limestone is added, the basicity CaO / SiO2 of the slag is controlled to be 0.75 to less than 1.00, and then a MgO source is added to form a CaO - SiO2 - MgO - MnO system slag with the MgO concentration of the slag controlled to be 25 to 45 mass%. The reasons for the limitations on the slag basicity, MgO concentration, and Mn addition timing are as described below.

[0081] Slag basicity: 0.75 - less than 1.00

[0082] The basicity of the slag has a great influence on the equilibrium S concentration shown in Formula 3. The greater the slag basicity, the more the reaction in Formula 3 proceeds to the right, that is, S in the molten steel transfers to the slag phase. Therefore, the lower it is, the more likely S is to remain in the molten steel. However, if it is too low, that is, if SiO2 increases and is lower than 0.75, deoxidation becomes insufficient and the amount of oxide inclusions increases. In addition, there is a risk that S remains in the molten steel in excess and increases to more than 0.25 mass%. On the other hand, if the slag basicity is 1.00 or more, S will not remain in the molten steel. In addition, it also has the effect of promoting deoxidation and reducing the oxygen concentration in the molten steel. By reducing the oxygen concentration, the number of inclusions decreases. However, if deoxidation is carried out excessively, the composition of the oxide inclusions becomes a MgO - Al2O3 system oxide, which instead deteriorates the cleanliness of the product surface. Therefore, in the present invention, it is set to be 0.75 to less than 1.00. Preferably it is 0.80 to 0.98, and more preferably it is 0.85 to 0.95.

[0083] MgO concentration in slag: 25 - 45 mass%

[0084] If the MgO concentration of the slag is controlled to be 25 mass% or more in a state where the slag basicity is 0.75 to less than 1.00, the slag reaches the saturated state of MgO, and a solid composed of MgO phase, MgO - SiO2 phase, and CaO - MgO phase is generated. Since a solid composed of MgO phase, MgO - SiO2 phase, and CaO - MgO phase is generated in the slag, the amount of CaO in the liquid slag reacting with S in the molten steel decreases, and it has the effect that S added to the molten steel is likely to remain in the molten steel. That is, although the reaction shown in Formula 3 proceeds to the right, since the amount of CaO in the liquid slag is small, CaS in the slag becomes saturated and the reaction no longer proceeds. That is, by adding MgO, the added S is likely to remain in the molten steel, and even if the oxygen concentration is reduced, the S concentration can be quickly and accurately controlled.

[0085] In addition, since the MgO, MgO-SiO2 phase, and CaO-MgO phase crystallize out as solid phases, the concentration of MnO in the liquid phase relatively increases. As a result, the activity of MnO in the slag also increases, and the inclusion composition can be controlled to be harmless CaO-SiO2-Al2O3-MgO-MnO system inclusions. However, if the MgO concentration exceeds 45% by mass, the MnO content in the CaO-SiO2-Al2O3-MgO-MnO system inclusions and the MgO-Al2O3-MnO system inclusions exceeds 15% by mass or more. In addition, the reaction of Equation 5 proceeds to the right, and the Mg concentration exceeds 0.001% by mass. Therefore, the MgO concentration is set to 25 to 45% by mass. Preferably, it is 26 to 40% by mass, and more preferably, it is 27 to 38% by mass. It should be noted that the MgO source is preferably introduced into the AOD or VOD. Additionally, although not particularly limited, as the MgO source, waste bricks containing MgO can be used. For example, MgO-C, magnesia-chrome bricks, etc. can be cited.

[0086] In addition, in order to make the oxide-based inclusions of the present invention contain an appropriate amount of MnO, the MnO concentration in the slag is preferably 0.5 to 3.0% by mass.

[0087] Mn addition timing

[0088] Although the deoxidizing materials for S-containing steel targeted by this application are Si, Mn, and Al, by first adding Mn to the molten steel after decarburization, it has the effect of making the CaO-SiO2-Al2O3-MgO-MnO system inclusions or the Al2O3-MgO-MnO system inclusions contain MnO. This is because by first adding Mn to the molten steel with a high oxygen concentration after decarburization, Mn is oxidized, thereby generating oxide-based inclusions mainly composed of MnO in the molten steel, and the MnO concentration in the slag also increases. Even if MnO is reduced later by adding Si or Si+Al, the MnO in the oxide-based inclusions and the MnO in the slag will remain. Therefore, it is preferable to use Si or Si+Al to reduce Cr after adding Mn. It should be noted that Fe-Mn alloy or metallic Mn is used for Mn, Fe-Si alloy or metallic Si is used for Si, and Al pellets or Al rods are used for Al.

[0089] Examples

[0090] Next, examples are shown to further clarify the constitution and effects of the present invention, but the present invention is not limited to the following examples. In AOD or VOD, oxygen blowing refining (oxidative refining) for removing C is carried out, and Cr reduction is carried out by adding Fe-Mn alloy and Fe-Si alloy. Then, limestone is added to adjust the slag composition. Here, while carrying out refining for the purpose of composition adjustment, S is added. In this operation, FeS with an S purity of 30% by mass is used as the S source. After the refining process in AOD or VOD, ladle refining is carried out, and the temperature is adjusted while stirring with Ar. Finally, a slab is manufactured by continuous casting. It should be noted that the weight of the molten steel is 50 to 70 tons, and the weight of the slag is 4 to 6 tons.

[0091] The manufactured slab is made into a size of 1000 mm wide × 154 mm thick × 6000 to 8000 mm long. The surface of the slab is ground, heated to 1200 °C and hot-rolled to manufacture a plate with a width of 1000 mm and a thickness of 3 mm. Then, annealing and pickling are carried out to remove the scale on the surface. Furthermore, the annealed plate is observed, and an appearance inspection instrument is used to evaluate surface defects with a length of 1 mm or more.

[0092] Table 1 lists the evaluation results of the chemical composition of the obtained stainless steel alloy, the slag composition at the end of AOD or VOD refining, the oxide-based inclusion composition, the number ratio of oxide-based metal inclusions, the surface quality, and the S concentration control accuracy. Each evaluation method is as described below. It should be noted that the values in parentheses in the table are outside the scope of the claims of this application. Although there are values in parentheses in the inventive examples, this means that they do not meet the scope of the dependent claims and meet the scope of the independent claims.

[0093] 1) Chemical composition of the alloy and slag composition: Quantitative analysis is carried out using a fluorescent X-ray analyzer, and the oxygen concentration of the alloy is quantitatively analyzed by the inert gas pulse melting infrared absorption method. In addition, when the slag forms a solid phase and a liquid phase, all the cast slag is crushed, samples are collected from the uniformly mixed slag, and the composition is analyzed.

[0094] 2) Oxide-based inclusion composition: Immediately after casting, the sample collected in the tundish is mirror-polished, and SEM / EDS is used to randomly measure 20 inclusions with a size of 5 μm or more. When the composition of one inclusion is uneven and is in a form composed of two or more phases, it is evaluated with the average composition.

[0095] 3) Number ratio of oxide-based metal inclusions: Based on the measurement results in 2) above, the number ratio of MgO-Al2O3-MnO-based oxides to the total number of oxide-based inclusions is evaluated.

[0096] 4) Surface quality: Observe the surface of the product plate, measure the surface defects with a length of 1 mm or more detected by an appearance inspection instrument, and score as follows based on the number of surface defects per 100 m.

[0097] ◎: The number of surface defects within 100 m is 1 or less.

[0098] ○: The number of surface defects within 100 m is more than 2 and 4 or less.

[0099] △: The number of surface defects within 100 m is more than 5 and 7 or less.

[0100] ×: The number of surface defects within 100 m is 8 or more.

[0101] 5) S concentration control accuracy: Regarding the adjustment control of the S concentration, evaluate as follows.

[0102] ◎: Add S once and control the S concentration within the target range.

[0103] ○: Add S twice and control the S concentration within the target range.

[0104] △: Add S three times and control the S concentration within the target range.

[0105] ×: Add S four or more times to control the S concentration within the target range, or cases where the S cannot be controlled within the target range.

[0106] [Table 1]

[0107]

[0108] [Table 2]

[0109]

[0110] Since Invention Examples 1 to 12 satisfy the scope of the present invention, there are few surface defects, and in addition, there is no problem with the control accuracy of the S concentration. In particular, since Invention Examples 1 to 7 are within the preferred range, the surface defect evaluation and the S concentration control accuracy are very good.

[0111] In Invention Example 8, since the C / S is as high as 1.15, it takes time to control the S concentration. In addition, the input amount of Al for the deoxidizing material is large, the Al concentration increases to 0.005% by mass, the oxygen concentration decreases to 0.0022% by mass, and the proportion of Al2O3-MgO-MnO inclusions increases, resulting in surface defects.

[0112] In Invention Example 9, the C / S is as low as 0.74, and the deoxidation using Si is ineffective, and the O concentration increases to 0.0078% by mass. As a result, the number of oxide-based inclusions is also large, resulting in surface defects.

[0113] In Invention Example 10, since the MgO concentration in the slag was as high as 45.7 mass%, the solid phase of the slag increased, so the MnO concentration in the liquid phase of the slag increased. As a result, the MnO concentration in the CaO-SiO2-MgO-Al2O3-MnO inclusion increased to 16.1 mass%. In addition, it was also observed that MnO-SiO2 inclusions crystallized in the CaO-SiO2-MgO-Al2O3-MnO inclusions. As a result, the determination of surface defects was △.

[0114] In Invention Example 11, the MgO concentration in the slag was as low as 23.5 mass%. During refining, the slag completely formed a liquid phase, the retention rate of S was poor, and the control accuracy of the S concentration deteriorated. In addition, since there was no solid phase in the slag, the concentration of MnO in the slag did not occur. In addition, since Mn was added after Si, which was used as a deoxidizing material, was first added, the MnO concentration in the CaO-SiO2-MgO-Al2O3-MnO inclusion decreased to 0.4 mass%. As a result, the determination of surface defects was △.

[0115] In Invention Example 12, the addition amount of Al in the deoxidizing material was large. In addition, the addition amount of MgO to the slag was also large, and the MgO concentration in the slag increased to 40.4 mass%. As a result, the proportion of MgO-Al2O3-MnO inclusions increased, and the determination of surface defects caused by oxide inclusions was △.

[0116] On the other hand, Comparative Examples 13 to 20 deviated from the scope of the invention of the present application. Each example will be described below.

[0117] In Comparative Example 13, the MgO concentration in the slag was as low as 18.5 mass%, and it completely formed a liquid phase. The MnO activity of the slag did not increase. In addition, since Mn was added after Si and Al, which were deoxidizing materials, were first added, the MnO concentration in the slag was as low as 0.1 mass%. As a result, the oxide inclusion became a MgO-Al2O3-MnO inclusion with an MnO concentration of 0.2 mass%, which coagulated, merged, and coarsened on the inner wall of the immersion nozzle during casting, causing a large number of surface defects. The evaluation of surface defects was ×.

[0118] In Comparative Example 14, a large amount of Al, which was a deoxidizing material, was added, and the Al concentration was as high as 0.009 mass%. In addition, Mg was added at the end of refining, so the Mg concentration increased to 0.0018 mass%. As a result, the oxide inclusion became a MgO-Al2O3-MnO inclusion with an MnO concentration of 0.0 mass%, which coagulated, merged, and coarsened on the inner wall of the immersion nozzle during casting, causing a large number of surface defects. The evaluation of surface defects was ×.

[0119] In Comparative Example 15, Mn was excessively added, and the Mn concentration was increased to 2.12% by mass. In addition, the MgO concentration in the slag was as high as 47.0% by mass, the solid phase increased, and the activity of MnO in the molten slag phase relatively increased. In addition, since the MnO concentration in the slag was also as high as 3.5% by mass, the MnO concentration in the CaO-SiO2-MgO-Al2O3-MnO system inclusions was increased to 16.4% by mass, and the MnO concentration in the MgO-Al2O3-MnO inclusions was increased to 15.5% by mass. As a result, MnO-SiO2 system inclusions were also formed in the CaO-SiO2-MgO-Al2O3-MnO system inclusions. In addition, MnO-Al2O3 system inclusions were also observed in the MgO-Al2O3-MnO inclusions. Therefore, the evaluation of surface defects was ×.

[0120] In Comparative Example 16, the input amount of Si was small, and the Si concentration was as low as 0.14% by mass. In addition, the C / S of the slag was as low as 0.72, and deoxidation could not be fully effective. The O concentration was as high as 0.0092% by mass, and Ca was not mixed into the molten steel due to the slag / metal reaction, so the Ca concentration was reduced to 0.0000% by mass. As a result, CaO-SiO2-MgO-Al2O3-MnO system inclusions were not formed, and it was not the object of the present invention, but oxide system inclusions MnO-SiO2 system were formed. As a result, a large number of surface defects caused by oxides were generated, and the determination of surface defects was ×.

[0121] In Comparative Example 17, since Ca was added at the end of refining, the Ca concentration was increased to 0.0042% by mass. By adding Ca, it was controlled to be CaO-SiO2-MgO-Al2O3-MnO system inclusions, but since Ca with strong deoxidation ability was added to the molten steel with a high oxygen concentration, the number of inclusions increased. In addition, due to the high Ca concentration, edge cracks occurred during hot rolling. As a result, a large number of surface defects caused by oxides were generated, and the determination of surface defects was ×.

[0122] In Comparative Example 18, the addition amounts of Si and Al are large, the Si concentration is as high as 1.10% by mass, the Al concentration is as high as 0.007% by mass, and the oxygen concentration is reduced to 0.0015% by mass. As a result, Ca and Mg are reduced from the slag, the Ca concentration increases to 0.0036% by mass, and the Mg concentration increases to 0.0012% by mass. In addition, since the C / S of the slag is also as high as 1.19, a large amount of S in the molten steel transfers to the slag. Although S is repeatedly added, the S concentration is reduced to 0.141% by mass. Regarding oxide-based inclusions, MgO-Al2O3-MnO-based inclusions with an MnO concentration of 0.0% by mass are formed, adhere to the surface of the immersion nozzle refractory used during continuous casting, coagulate and merge, fall off after coarsening, and a large number of surface defects caused by the generated large oxide-based inclusions are generated. In addition, due to the high Ca concentration, the hot workability also deteriorates, and edge cracks are also generated during hot working.

[0123] In Comparative Example 19, the input amount of Mn is small, and the Mn concentration is reduced to 0.93% by mass. In addition, MnO in the slag is also reduced to 0.1% by mass. As a result, MnO is not sufficiently contained in the oxide-based inclusions, the MnO concentration in the CaO-SiO2-MgO-Al2O3-MnO-based inclusions is 0.1% by mass, and the MnO concentration in the MgO-Al2O3-MnO-based inclusions is 0.2% by mass. They adhere to the surface of the immersion nozzle refractory used during continuous casting, coagulate and merge, fall off after coarsening, and a large number of surface defects caused by the generated large oxide-based inclusions are generated.

[0124] In Comparative Example 20, Si is excessively added, and the Si concentration increases to 1.25% by mass. As a result, deoxidation progresses, and the O concentration is reduced to 0.0018% by mass. Excessive Mg is supplied to the molten steel, the MnO concentration in the MgO-Al2O3-MnO-based inclusions is 0.3% by mass, adheres to the surface of the immersion nozzle refractory used during continuous casting, coagulates and merge, falls off after coarsening, and a large number of surface defects caused by the generated large oxide-based inclusions are generated.

[0125] Industrial applicability

[0126] The technology of the present invention can make oxide-based inclusions harmless by precisely controlling the composition of oxide-based inclusions, and can provide S-containing stainless steel with excellent surface properties.

Claims

1. S-containing stainless steel, characterized in that, By mass percentage, it consists of C: 0.30% or less, Si: 0.2 - 1.0%, Mn: 1.2 - 1.8%, Ni: 5 - 10%, Cr: 15 - 20%, Mo: 0.05 - 0.60%, Cu: 0.05 - 0.60%, Al: 0.005% or less, S: 0.15 - 0.25%, Ca: 0.0001 - 0.0010%, Mg: 0.0010% or less, O: 0.0020 - less than 0.0080%, with the balance being Fe and inevitable impurities. The oxide-based inclusions are composed of one or two of CaO - SiO2 - MgO - Al2O3 - MnO-based inclusions and MgO - Al2O3 - MnO-based inclusions. The MgO - Al2O3 - MnO-based inclusions contain 1 - 15% of MnO by mass percentage.

2. The S-containing stainless steel according to claim 1, wherein Among the oxide-based inclusions, the CaO - SiO2 - MgO - Al2O3 - MnO-based inclusions contain 1 - 15 mass% of MnO by mass percentage.

3. The S-containing stainless steel according to claim 1, wherein Among the oxide-based inclusions, the number of MgO - Al2O3 - MnO-based inclusions is 50 number% or less.

4. The S-containing stainless steel according to claim 2, wherein Among the oxide-based inclusions, the number of MgO - Al2O3 - MnO-based inclusions is 50 number% or less.

5. Method for manufacturing S-containing stainless steel, characterized in that, When refining the S-containing stainless steel according to any one of claims 1 to 4, first, the raw materials are melted in an electric furnace, then decarburization is carried out by AOD or VOD. After adding Mn, Si or Si + Al is used for Cr reduction. Limestone is added, and the basicity CaO / SiO2 of the slag is controlled at 0.75 - less than 1.

00. Further, a MgO source is added to form a CaO - SiO2 - MgO - MnO-based slag with the MgO concentration of the slag controlled at 25 - 45 mass%.

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