Stainless steel material and method for manufacturing the same

Abstract

To provide a stainless steel that controls a produced amount of composite oxide inclusions that contain Mn, Mg, Al, Cr while adding Ti, and have an equivalent circle diameter of 3 μm or more and is excellent in mirror polishing property, and a producing method thereof.SOLUTION: A stainless steel having a chemical composition containing C, Si, Mn, P, S, Ni, Cr, Al, Ti, O, N, Ca, and Mg in mass%, with the balance consisting of Fe and inevitable impurities. The stainless steel has crystal grain size of 4 or more and contains composite oxide inclusions that contain Mn etc., and has an equivalent circle diameter of 3 μm or more, in the composite oxide inclusions, a total concentration of MnO and MgO is 10.0 mass% or more and 50.0 mass% or less, a total concentration of Al2O3, Cr2O3 and Ti2O3 is 30.0 mass% or more and 80.0 mass% or less, and the concentration of Ti2O3 is 0.01 mass% or more and 5.00 mass% or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a stainless steel material and a method for producing the same.

Background Art

[0002] Stainless steel materials are widely used in home appliances, kitchenware, building materials, etc. because of their excellent corrosion resistance, workability, surface appearance, etc. In recent years, in stainless steel materials produced by the continuous casting method introduced for productivity improvement, the solidification structure becomes coarse, and the variation in crystal grains of the stainless steel material tends to increase.

[0003] Therefore, as a means for suppressing the variation in crystal grains of stainless steel materials, for example, by adding a trace amount of Ti, TiC and TiN are precipitated in the stainless steel material, and the crystal grain boundaries are pinned by the precipitates, thereby achieving the refinement of the crystal grain size and the suppression of the variation in crystal grain size of the stainless steel material. However, in such a method, Ti oxide-based inclusions tend to be generated and coarsened due to Ti addition. There was a problem that stainless steel materials in which coarsened Ti oxide-based inclusions were generated were not suitable for applications that require mirror polishing or fatigue characteristics.

[0004] Therefore, in Patent Document 1, as a method for obtaining a slab, a weld metal having excellent low-temperature toughness and hot workability and a stainless steel sheet having excellent surface properties without particularly limiting the casting conditions, a method for controlling the slab structure by controlling the Al, Mg concentrations and the Ti×N (nitrogen) concentration product has been proposed. However, there is a problem that there are many inclusions of 3 μm or more due to the excessively high Al, Mg, and O concentrations, resulting in poor mirror polishing properties. In addition, in Patent Document 2, regarding the refining method of stainless steel, by controlling trace components such as slag basicity and Mg, Al, and Ca in molten steel, the generation of MgO·Al2O3, which is a harmful non-metallic inclusion in molten steel, is suppressed, and an inclusion reduction measure for producing stainless steel with excellent surface quality while preventing nozzle adhesion has been proposed. However, this operation method has a problem that the inclusion reduction effect is not sufficiently exerted for materials containing Ti. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2001-323335 [Patent Document 2] Japanese Patent Publication No. 2015-074807 [Overview of the project] [Problems that the invention aims to solve]

[0006] In this regard, the inventors have found that by optimizing the concentration of trace components (Ti) and Ti2O3 in the slag, it is possible to control the complex oxide inclusions that are detrimental to mirror polishability. Therefore, the present invention has been made in view of the above problems, and aims to provide a stainless steel material with excellent mirror polishability and a method for producing the same, by controlling the amount of composite oxide inclusions that contain Mn, Mg, Al, and Cr and have an equivalent circle diameter of 3 μm or more while adding Ti. [Means for solving the problem]

[0007] The features of the present invention are listed below. (1) In mass%, C: 0.001% or more and 0.150% or less, Si: 0.1% or more and 3.0% or less, Mn: 0.1% or more and 15.0% or less, P: 0.005% or more and 0.040% or less, S: 0.0001% or more and 0.0100% or less, Ni: 2.0% or more and 20.0% or less, Cr: 10.0% or more and 30.0% or less, Al: 0.0001% or more and 0.01% or less, Ti: 0.001% or more and 0.019% or less, O: 0.001% or more and 0.02% or less, N: 0.01% or more and 0.5% or less, Ca: 0.0001% or more and 0.005% or less, A stainless steel material having a chemical composition consisting of Mg: 0.0001% or more and 0.0030% or less, with the remainder being Fe and unavoidable impurities, The aforementioned stainless steel material has a grain size of 4 or greater. The steel material contains a composite oxide inclusion having Mn, Mg, Al, Cr, and Ti, with an equivalent circle diameter of 3 μm or more. A stainless steel material in which the total concentration of MnO and MgO in the complex oxide inclusions is 10.0% by mass or more and 50.0% by mass or less, the total concentration of Al2O3, Cr2O3, and Ti2O3 is 30.0% by mass or more and 80.0% by mass or less, and the concentration of Ti2O3 is 0.01% by mass or more and 5.00% by mass or less.

[0008] (2) The chemical composition is expressed in mass%, and further, Mo: 0.01% or more and 5.0% or less, Cu: 0.01% or more and 5.0% or less, B: 0.0001% or more and 0.0050% or less, Nb: 0.1% or more and 0.6% or less, W: 0.01% or more and 0.5% or less, Sn: 0.01% or more and 0.5% or less, V: 0.01%% or more and 0.5% or less, Co: 0.01% or more and 0.5% or less, Zr: 0.01% or more and 0.5% or less, Pb: 0.0001% to 0.5%, and, REM: Stainless steel containing between 0.0003% and 0.3%.

[0009] (3) A method for producing stainless steel according to (1) or (2), comprising a refining step in which the total amount of Ti in the molten steel and refining slag after the addition of refining raw materials is 0.10 kg / t or more and 2.00 kg / t or less.

[0010] (4) The method for manufacturing a stainless steel material according to (3), wherein the slag composition after performing the refining step satisfies the ranges of the following formulas (1) to (4). Formula (1): 0.9 ≦ [CaO] / [SiO2] ≦ 3.0 Formula (2): [Al2O3] ≦ 5.0 Formula (3): [Ti2O3] ≦ 3.0 Formula (4): [MgO] ≦ 15.0 However, [CaO], [SiO2], [Al2O3], [Ti2O3], and [MgO] in Formulas (1) to (4) are the contents (mass%) of CaO, SiO2, Al2O3, Ti2O3, and MgO contained in the slag composition, respectively.

Advantages of the Invention

[0011] The present invention aims to optimize specific components contained in a stainless steel material, increase the crystal grain size of the stainless steel material to 4 or more, contain Mn, Mg, Al, Cr, and Ti present in the stainless steel material, and control composite oxide-based inclusions having an equivalent circle diameter of 3 μm or more, thereby providing a stainless steel excellent in mirror-polishing property and a method for manufacturing the same.

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described. The following description is an example of the embodiments of the present invention and does not limit the scope of the claims.

[0013] In order to solve the problems, the present inventors adjusted the Ti amount of the raw material added for slag adjustment with respect to the stainless steel molten steel after refining by a converter, AOD, or VOD in the decarburization treatment, and adjusted the composition of the slag, thereby suppressing the coarsening of the crystal grain size when making a stainless steel material and reducing composite oxide-based inclusions having an equivalent circle diameter exceeding 3 μm. As a result, since composite oxide-based inclusions exceeding 3 μm can be suppressed, it has become possible to manufacture a stainless steel material excellent in mirror-polishing property with few surface defects. Hereinafter, the stainless steel material of the present invention will be specifically described.

[0014] The stainless steel material of the present invention has a chemical composition in mass%, C: 0.001% or more and 0.150% or less, Si: 0.1% or more and 3.0% or less, Mn: 0.1% or more and 15.0% or less, P: 0.005% or more and 0.040% or less, S: 0.0001% or more and 0.0100% or less, Ni: 2.0% or more and 20.0% or less, Cr: 10.0% or more and 30.0% or less, Al: 0.0001% or more and 0.01% or less, Ti: 0.001% or more and 0.019% or less, O: 0.001% or more and 0.02% or less, N: 0.01% or more and 0.5% or less, Ca: 0.0001% or more and 0.005% or less, It contains Mg: 0.0001% to 0.0030%, with the remainder consisting of Fe and unavoidable impurities.

[0015] (chemical composition) The reasons for the limitations on each essential additive element are explained below. Note that in the following descriptions of each component of the chemical composition, "mass%" is simply represented as "%". (C: 0.001% or more and 0.150% or less) Carbon (C) is an austenite-stabilizing element and also improves strength. It is good to have 0.001% or more, preferably 0.004% or more. However, if it is present in excess, it reduces corrosion resistance and significantly reduces workability by forming Cr carbides, so the upper limit is good to 0.150% or less, preferably 0.100% or less.

[0016] (Si: 0.1% or more and 3.0% or less,) Silicon (Si) is an element that contributes to deoxidation, and the more Si is added, the lower the basicity of the complex oxide inclusions becomes, resulting in a softening effect. Adding Si is not necessary, but it may be added as a preliminary deoxidation before adding Al. If added, the content should be 0.1% or more, preferably 0.4% or more, to exhibit its effect. On the other hand, excessive addition leads to a decrease in processability, so it should be kept below 3.0%, preferably below 1.0%.

[0017] (Mn: 0.1% or more and 15.0% or less,) Manganese (Mn) is an austenite-stabilizing element and, like silicon, contributes to deoxidation. The higher the amount added, the lower the basicity of the complex oxide inclusions, resulting in a softening effect. To achieve this effect, the content should be 0.1% or more, preferably 0.3% or more. On the other hand, excessive addition leads to a decrease in processability and corrosion resistance, so it should be 15.0% or less, preferably 5.0% or less.

[0018] (P: 0.005% or more and 0.040% or less) Phosphorus (P) is harmful to stainless steel because it reduces toughness, hot workability, and corrosion resistance. Therefore, the lower the amount, the better, ideally below 0.040%. However, excessive reduction increases the burden during refining or necessitates the use of expensive raw materials. In actual operation, a content of 0.005% or more is preferred, with a range of 0.010-0.030% being desirable considering manufacturing costs.

[0019] (S: 0.0001% or more and 0.0100% or less) Sulfur (S) is harmful to stainless steel because it reduces toughness, hot workability, and corrosion resistance, so the less the better, ideally it should be below 0.0100%. However, excessive reduction would increase the burden during refining or necessitate the use of expensive raw materials, so in actual operation, the content is usually 0.0001% or higher.

[0020] (Ni: 2.0% or more and 20.0% or less) Adding nickel (Ni) further enhances the high corrosion resistance of stainless steel. When included, it is desirable to include 2.0% or more, preferably 6.0% or more, to obtain this effect. On the other hand, since it is an expensive element, adding more than 20.0% does not yield an effect that justifies the increase in alloy cost, so it is desirable to keep it at 20.0% or less, preferably 16% or less.

[0021] (Cr: 10.0% or more and 30.0% or less) Cr (chromium) is an element that provides corrosion resistance to stainless steel, and it is good to have a content of 10.0% or more, preferably 15.0% or more. On the other hand, a large amount of chromium leads to a decrease in workability, so it is good to have a content of 30.0% or less, preferably 22.0% or less.

[0022] (Al: 0.0001% or more and 0.010% or less) Aluminum (Al) is an element that contributes to deoxidation, but it promotes the formation of MgO·Al2O3, a hard inclusion in the spinel structure, which degrades product quality. Therefore, it is desirable to keep it below 0.010%, preferably below 0.005%, and even more preferably below 0.002%. However, excessive reduction increases the burden during refining or necessitates the use of expensive raw materials, so in actual operation, it is contained at 0.0001% or more.

[0023] (Ti: 0.001% or more and 0.019% or less) Titanium (Ti) is an element that ensures corrosion resistance through the stabilizing effect of carbon and nitrogen, and is also an essential element for controlling the grain size or size of the base material in the product material through the refinement of the cast structure and the pinning effect of carbonitride Ti(C,N). Therefore, it is contained at a concentration of 0.001% or more to obtain this effect. However, in stainless steel where surface properties are required, if Ti is present in excess, Ti will dissolve into complex oxide inclusions, promoting the precipitation of complex oxide inclusions. To resolve this problem, it is best to keep the Ti content below 0.019%, preferably below 0.005%.

[0024] (O: 0.001% or more and 0.02% or less) Oxygen (O) is an element that promotes the softening of complex oxide inclusions by generating SiO2, and its content should be 0.0010% or more, preferably 0.0020% or more. If it is present in amounts exceeding 0.02%, it will generate coarse Cr2O3 and reduce corrosion resistance, so it is best to keep it below 0.02%, preferably below 0.01%.

[0025] (N: 0.01% or more and 0.5% or less) Nitrogen (N) is an austenite-stabilizing element and improves corrosion resistance, so its concentration should be 0.010% or more, preferably 0.030% or more. On the other hand, excessive addition can lead to a decrease in processability, so its concentration should be 0.5% or less, preferably 0.3% or less.

[0026] (Ca: 0.0001% or more and 0.005% or less) Calcium (Ca) content should be 0.005% or less, as exceeding 0.005% increases the amount of large-grained complex oxide inclusions during the molten steel stage. More preferably, it should be 0.0020% or less. While there is no particular lower limit, Ca is the main component of slag, and some inclusion is unavoidable. Furthermore, complete removal is difficult, and excessive reduction increases the load during refining; therefore, in actual operation, it is contained at 0.0001% or more.

[0027] (Mg: 0.00001% or more and 0.0030% or less) Magnesium (Mg) promotes the formation of MgO·Al2O3, a hard inclusion in the spinel structure, which degrades product quality. Therefore, it is desirable to keep it below 0.0030%, preferably below 0.0020%. Mg is the main component of slag, and some inclusion is unavoidable. Furthermore, it is difficult to remove it completely, and excessive reduction increases the load during refining, so in actual operation, it is contained at a level of 0.00001% or higher.

[0028] Furthermore, the stainless steel material of the present invention may contain the following optional additive elements as needed.

[0029] (Mo: 0.01% or more and 5.0% or less) Mo (molybdenum) promotes the re-passivation of the passive film, further enhancing the high corrosion resistance of stainless steel. When included, it is desirable to include 0.01% or more, preferably 0.5% or more, to obtain this effect. On the other hand, because it is very expensive, adding more than 5.0% does not yield an effect that justifies the increased alloy cost, and moreover, it forms a brittle sigma phase with high Cr, leading to embrittlement and a decrease in corrosion resistance. Therefore, it is desirable to keep it at 5.0% or less, preferably 1.5% or less.

[0030] (Cu: 0.01% or more and 5.0% or less) Adding copper (Cu) further enhances the high corrosion resistance of stainless steel and improves its workability. When included, it is desirable to include 0.01% or more, preferably 0.5% or more, to obtain this effect. On the other hand, excessive addition does not result in performance improvement commensurate with the manufacturing cost, so it is desirable to keep it at 5.0% or less, preferably 1.5% or less.

[0031] (B: 0.0001% or more and 0.0050% or less) Boron (B) is an element that increases the strength of grain boundaries and contributes to improved processability. When included, it is desirable to include 0.0001% or more, preferably 0.0005% or more, in order to exhibit this effect. On the other hand, excessive addition can lead to a decrease in processability due to a reduction in elongation, so it is desirable to keep the content at 0.0100% or less, preferably 0.0050% or less.

[0032] (Nb: 0.1% or more and 0.6% or less) Niobium (Nb) enhances moldability and corrosion resistance. When included, it should be present in a concentration of 0.1% or more to achieve this effect. On the other hand, adding more than 0.6% makes recrystallization difficult and results in a coarser structure, so it is best to keep it below 0.6%, preferably below 0.5%.

[0033] (W: 0.01% or more and 0.5% or less) Tungsten (W), like molybdenum (Mo), has the effect of improving corrosion resistance. When included, it is desirable to include 0.01% or more, preferably 0.02% or more, to obtain this effect. On the other hand, if the W content exceeds 0.5%, the strength increases excessively and the workability decreases, so it is desirable to keep it at 0.5% or less, preferably 0.3% or less.

[0034] (Sn: 0.01% or more and 0.5% or less) Adding tin (Sn) further enhances the high corrosion resistance of stainless steel. When included, it is desirable to include 0.01% or more, preferably 0.02% or more, to obtain this effect. On the other hand, excessive addition leads to a decrease in workability, so it is desirable to keep it to 0.5% or less, preferably 0.3% or less.

[0035] (V: 0.01%% or more and 0.50% or less) Vanadium (V) is an element that suppresses the reduction in corrosion resistance caused by the precipitation of Cr nitrides by forming nitrides (VN). This effect is obtained when the V content is 0.01% or higher. However, if the V content exceeds 0.50%, the processability decreases. Therefore, when V is included, the V content should be between 0.01% and 0.50%.

[0036] (Co: 0.01% or more and 0.50% or less) Co (cobalt) is an austenite-stabilizing element that improves the crevice corrosion resistance of stainless steel. When included, it is desirable to include 0.01% or more, preferably 0.02% or more, to obtain this effect. On the other hand, excessive addition leads to a decrease in workability, so it is desirable to keep it at 0.50% or less, preferably 0.30% or less.

[0037] (Zr: 0.01% or more and 0.50% or less) Zr (zirconium) is an element that, when bonded with carbon and nitrogen, suppresses the reduction in corrosion resistance caused by the precipitation of zr carbonitrides. This effect is obtained when the zr content is 0.01% or higher. However, if the zr content exceeds 0.50%, the processability decreases. Therefore, when zr is included, the zr content should be between 0.01% and 0.50%.

[0038] (Pb: 0.0001% or more and 0.5% or less) Lead (Pb) is dispersed in steel and has the effect of improving machinability. On the other hand, if the Pb content exceeds 0.50%, low-melting-point compounds are excessively generated, reducing hot workability. Therefore, it is best to keep it below 0.50%, preferably below 0.30%. Furthermore, it is difficult to completely remove it, and excessive reduction increases the load during refining, so in actual operation, the content is 0.0001% or more.

[0039] (REM: 0.0003% or more and 0.30% or less) Rare earth metals (REMs) are elements that react with sulfur (S) to improve corrosion resistance. This effect is obtained when the REM content is 0.0003% or higher. On the other hand, excessive addition can cause nozzle clogging and negatively affect manufacturability, so it is best to keep it below 0.30%, preferably below 0.003%. Rare earth metals (REMs) refer to rare earth metals of the lanthanide series, actinide series, etc., such as Ce, Pr, and Sm, and metals composed of these.

[0040] (The remainder is Fe and unavoidable impurities) The remainder consists of Fe and unavoidable impurities, such as As and Sb. Here, unavoidable impurities refer to components that are mixed in during the industrial production of stainless steel due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0041] (Grain size and complex oxide inclusions) Next, we will explain the reasons for the limitations on the grain size and composite oxide inclusions of the stainless steel material of the present invention. The stainless steel material of the present invention has a grain size of 4 or higher, contains composite oxide inclusions in the steel material that contain Mn, Mg, Al, Cr, and Ti and have an equivalent circle diameter of 3 μm or more, the total concentration of MnO and MgO in the composite oxide inclusions is 10.0% by mass or more and 50.0% by mass or less, the total concentration of Al2O3, Cr2O3, and Ti2O3 is 30.0% by mass or more and 80.0% by mass or less, and the concentration of Ti2O3 is 0.01% by mass or more and 5.00% by mass or less.

[0042] The stainless steel material of the present invention has a grain size of 4 or higher. Grain size is an index that represents the size of the crystal grains on the surface of the stainless steel material. By analyzing the grain size of the metal structure, it is possible to inspect and evaluate whether the metal material possesses the desired mechanical properties. The measurement involved preparing a sample of the metallographic structure and measuring the grain size using a metallurgical microscope in accordance with JIS G 0551 "Microscopic Test Method for Grain Size of Steel". Three test pieces were cut from the manufactured stainless steel material of the present invention, and the cross-section parallel to the wire drawing direction (L-section) and the cross-section parallel to the sheet rolling direction (L-section) of the test pieces were polished. After polishing, the grain size was revealed on the surface of the test pieces using an appropriate etching solution according to the composition of the test piece, and then evaluated by comparing it with the standard grain size chart according to JIS G 0551. As a result of investigating the correlation between grain size and surface roughness after processing, it was found that by making the grain size 4.0 or finer, a stainless steel material with fewer surface defects and excellent mirror polishability could be obtained. On the other hand, although there is no particular upper limit to the grain size, if the grain size exceeds 9.0, the productivity of surface polishing decreases, the surface roughness after processing increases, or the grain size becomes smaller, increasing the grain boundaries and reducing mirror polishability. Therefore, it is preferable to set the upper limit of the grain size to 9.0. Furthermore, from the viewpoint of tensile strength, ductility, and manufacturability, it is even more preferable for the grain size to be between 4.5 and 8.0.

[0043] Furthermore, the stainless steel material of the present invention contains composite oxide inclusions with an equivalent circle diameter of 3 μm or more, the total concentration of MnO and MgO in the composite oxide inclusions is 10.0% by mass or more and 50.0% by mass or less, the total concentration of Al2O3, Cr2O3, and Ti2O3 is 30.0% by mass or more and 80.0% by mass or less, and the concentration of Ti2O3 is 0.01% by mass or more and 5.00% by mass or less.

[0044] The inclusions in the molten steel after the oxidation refining of the stainless steel material of the present invention are mainly lower oxides such as Cr2O3 and MnO. However, when these are reduced by the addition of Si, trace amounts of Al, Ti, Ca, etc. contained in the metal Si source and Ti source, as well as Ca, Mg, Al, etc. reduced from slag and refractories, are mixed into the molten steel, and as time passes and the temperature decreases, complex oxide inclusions are formed. In this case, the composite oxide inclusions contain many oxides. For example, MnO, MgO, Al2O3, Cr2O3, Ti2O3, SiO2, CaO, etc. Among these, composite oxide inclusions containing MnO, MgO, Al2O3, Cr2O3, and Ti2O3 remain as deep foreign matter perpendicular to the polished surface when manufactured by rolling or polishing on the surface of stainless steel, because they are not stretched. When they fall off during mirror polishing, they form pits and cause problems with surface quality. In contrast, softer composite oxide inclusions containing SiO2 and CaO, other than Ti-2O3 and Cr-2O3, remain as shallow foreign matter perpendicular to the polished surface when stretched during the rolling or polishing process and do not cause problems. Therefore, the number density is specified for composite oxide inclusions containing MnO, MgO, Al2O3, Cr2O3, and Ti2O3, which are detrimental to mirror polishing performance.

[0045] Furthermore, composite oxide inclusions having the above oxide composition and an equivalent circle diameter of 3 μm or more reduce the mirror polishability, so the number of inclusions per unit area should be 0.01 / mm². 2 ~5 pieces / mm 2 It is preferable to control it within the range of 0.03 pieces / mm². More preferably, 0.03 pieces / mm². 2~3 pieces / mm 2 It is within the range of [the specified range]. As a result, the stainless steel material of the present invention has excellent abrasive properties, and a mirror-finish stainless steel material with excellent mirror polishability can be obtained by polishing. Furthermore, the valency of Ti may change depending on the O concentration in the molten steel during refining. However, regardless of the valency of Ti, it promotes the formation of complex oxide inclusions and has adverse effects, so Ti2O3 in complex oxide inclusions may exist in states with different valencies, such as TiO, TiO2, or Ti3O5. In addition, the concentrations of each component are calculated by converting MgO + Al2O3 + SiO2 + CaO + Ti2O3 + Cr2O3 + MnO to 100 mass%.

[0046] Here, we will explain the method for measuring the composition and number density of the above-mentioned complex oxide inclusions. After polishing the wire-drawn cross section of the manufactured stainless steel material of the present invention with emery paper and buffing to a mirror finish, a 200 mm cross section is measured using SEM-EDS (Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy). 2 The number of composite oxide inclusions present in the area was measured, and the inclusion composition was measured using EDS to determine contamination and the type of composite oxide inclusion. For the observed composite oxide inclusions, the number density was measured and the average composition was calculated for composite oxide inclusions where the total concentration of MnO and MgO was between 10.0% by mass and 50.0% by mass, the total concentration of Al2O3, Cr2O3, and Ti2O3 was between 30.0% by mass and 80.0% by mass, the concentration of Ti2O3 was between 0.01% by mass and 5.00% by mass, and the equivalent circle diameter was 3 μm or larger.

[0047] Furthermore, the equivalent circle diameter is measured as follows: 200 mm using SEM-EDS (Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy). 2 When measuring the number of complex oxide inclusions present in the area, the size (area) of the observation field n=3 is measured, and the diameter of the circle corresponding to the average size (average area) of the measured precipitates is calculated. This calculated diameter is then defined as the equivalent circle diameter.

[0048] The stainless steel material of the present invention, controlled to the component composition, grain size, and composite oxide inclusions described above, exhibits excellent mirror polishability, resulting in an arithmetic mean roughness Ra of the steel surface of 0.1 μm or less. Preferably, it is 0.05 μm or less. The stainless steel material of the present invention is suitable for various applications where aesthetic appeal after mirror polishing is important. In other words, the surface roughness of the material is important for aesthetic appeal after mirror polishing, and if pits occur due to the loss of large inclusions compared to the surface roughness of the material that is originally obtained through mirror polishing, the quality will be inferior in appearance. For surface roughness measurement, the arithmetic mean roughness (Ra) is measured on the surface of the steel plate in a direction parallel or perpendicular to the rolling direction, in accordance with JIS B 0601. Normally, if the arithmetic mean roughness Ra is 0.2 μm or less, it is put into practical use as a mirror finish. It has become clear that the stainless steel material of the present invention satisfies aesthetic appeal even with a higher level of mirror polishing if the Ra is 0.1 μm or less. Therefore, it is suitable for applications where aesthetic appeal after mirror polishing is important.

[0049] Furthermore, the manufacturing method for this stainless steel material is described below. The present invention provides a method for manufacturing stainless steel, which includes a refining step in which the total amount of Ti contained in the molten steel and refining slag after the addition of refining raw materials is 0.10 kg / t or more and 2.00 kg / t or less.

[0050] The manufacturing method for the stainless steel material of the present invention involves melting and refining raw materials to produce stainless steel with adjusted composition. In the refining process, a converter, AOD, VOD, and LF are used. In this embodiment, in order to suppress the formation of composite oxide inclusions containing Ti that occur during reduction in the refining process, the reducing agent is made highly pure, the amount added is controlled, and the slag composition is also controlled to control the number density and composition of composite oxide inclusions in the stainless steel. The presence of Ti2O3 makes it easier for complex oxide inclusions to form at higher temperatures and cause them to coarseen. Therefore, in this embodiment, the steel components, slag composition, and basicity (CaO / SiO2) are adjusted to create a state where the Ti2O3 concentration in the complex oxide inclusions is easily reduced.

[0051] In the manufacturing method for the stainless steel material of the present invention, during the refining process, Al, Ti, Al2O3, and Ti2O3 contained in the raw materials or refractories used in refining are adjusted to remove them to a degree that does not hinder refining. Furthermore, deoxidation is performed using a sufficient amount of Fe-Si alloy or metallic Si or metallic Al so that the O concentration in the steel is within the above range, and then CaO or SiO2 is added. In particular, since the Ti component in the raw materials added in the refining process (refining raw materials) increases the Ti concentration in the steel and the TiO2 concentration in the slag, the raw materials are regulated so that the Ti contained in refining raw materials such as Fe-Si alloy or metallic Si, metallic Al, FeCr alloy, FeNi alloy, and lime is between 0.10 kg / t and 2.00 kg / t.

[0052] This controls the slag composition after the refining process to satisfy the range of equations (1) to (4) below. (1) Formula: 0.9≦[CaO] / [SiO2]≦3.0 (2) Formula: [Al2O3]≦5.0 (3) Formula: [Ti2O3]≦3.0 (4) Equation: [MgO] ≤ 15.0 However, in formulas (1) to (4), [CaO], [SiO2], [Al2O3], [Ti2O3], and [MgO] represent the mass %) content of CaO, SiO2, Al2O3, Ti2O3, and MgO contained in the slag composition, respectively.

[0053] This slag composition shall be the value after refining in VOD, AOD, or LF (ladle heating furnace). However, it is difficult to identify the valence of Ti in the slag during the refining process under normal operation, and Ti may take on different valences during the slag cooling process, so Ti2O3 in the slag may exist as TiO2 or Ti3O5. (3) When the value of [Ti2O3] exceeds 3.0 mass%, Ti solid-solves in the composite oxide inclusions, promoting precipitation and increasing the amount of composite oxide inclusions larger than 3 μm. (2) When the value of [Al2O3] exceeds 3.0, and (4) when the value of [MgO] exceeds 15.0, the amount of composite oxide inclusions larger than 3 μm also increases. Furthermore, if the value of equation (1) exceeds 3.0, the reduction effect of Si increases the Ti concentration in the molten steel. Even if the value of equation (3) in the slag is 1.0 mass%, the molten steel will contain 0.01 mass% or more of Ti, which promotes the precipitation of complex oxide inclusions larger than 3 μm, and may result in an excessive amount of inclusions larger than 3 μm.

[0054] Then, after the refining process, billets of a predetermined angular size or slabs of a predetermined size are formed by continuous casting or ingot-splitting. Subsequently, in the case of wire rod manufacturing, a billet of a predetermined square size is rolled to a predetermined diameter through a rolling process and a pickling process. In the case of sheet metal manufacturing, a slab of a predetermined size is rolled to a predetermined thickness through a rolling process. After that, both wire rod and sheet metal may undergo an annealing process and / or a pickling process depending on the required dimensions.

[0055] Thus, according to this embodiment, by regulating the total amount of Ti contained in the metal raw material for slag adjustment and adjusting the composition of the floating slag in the molten stainless steel after the refining process, it is possible to reduce the amount of Ti in the stainless steel material compared to conventional refining. As a result, the formation of Ti-containing composite oxide inclusions that occur when a large amount of Ti is added can be stably suppressed. Therefore, it becomes possible to manufacture stainless steel products with a grain size of 4 or more in the base material regardless of the manufacturing method by utilizing Ti, with fewer defects such as pits and pinholes caused by inclusions during polishing, and with very high mirror finish, i.e., excellent mirror polishability. Accordingly, it can be suitably used as stainless steel for materials that are used after mirror polishing. [Examples]

[0056] The present invention will be described in detail based on the following embodiments. However, the present invention is not limited to the embodiments shown below.

[0057] Table 1 shows the content of essential additive elements and, in some cases, optional additive elements in Examples 1-20 and Comparative Examples 1-8.

[0058] [Table 1]

[0059] Sixty tons of stainless steel of each composition from Examples 1 to 10 and Comparative Examples 1 to 6, as shown in Table 1, were melted into 150 mm square billets via an electric furnace → AOD → LF (ladle heating furnace) → continuous casting process. Of the conditions shown in Table 1, 100 kg of stainless steel of each composition from Examples 11 to 20 and Comparative Examples 7 and 8 were melted into 20 mm square steel ingots via a vacuum melting process.

[0060] Next, the molten steel was reacted with the refined slag, varying the CaO / SiO2 ratio from 0.9 to 4.0%, the MgO concentration from 1.8 to 20%, and the Al2O3 and Ti2O3 concentrations from 0.0 to 10.5%, according to the manufacturing conditions, to produce the refined slag compositions shown in Table 2.

[0061] [Table 2]

[0062] Subsequently, each billet and slab was rolled, annealed, and pickled. Examples 1 to 10 and Comparative Examples 1 to 6 were made into φ10 mm wire rods, and Examples 11 to 20 and Comparative Examples 7 and 8 were made into 3 mm thick plates. Samples were taken from Examples 1 to 20 and Comparative Examples 1 to 8. The collected samples were then evaluated.

[0063] Three test pieces were cut from the collected samples, and the cross-sections parallel to the wire drawing direction (L-section) and the cross-sections parallel to the sheet rolling direction (L-section) of the test pieces were polished. After polishing, the crystal grains were exposed on the surface of the test pieces using an appropriate etching solution according to the test piece composition, and then the crystal grain size was evaluated by comparing it with the standard crystal grain size chart according to JIS G 0551.

[0064] The cross-section of the collected sample in the direction of wire drawing was polished with emery paper and buffed to a mirror finish. A 200mm image was taken using SEM-EDS. 2 The number of inclusions present in the area was measured, and the composition of the inclusions was measured using EDS to determine contamination and the type of inclusion. For the observed inclusions, the total concentration of MnO and MgO was between 10.0% and 50.0% by mass, the total concentration of Al2O3, Cr2O3, and Ti2O3 was between 30.0% and 80.0% by mass, and the concentration of Ti2O3 was between 0.01% and 5.00% by mass, and furthermore, the number density was measured and the average composition was calculated for inclusions with an equivalent circle diameter of 3 μm or more.

[0065] The drawn cross-sections of the collected samples were polished with emery paper and buffed to a mirror finish. Based on JIS B 0651, each sample was evaluated three times using a needle-type roughness meter (needle tip diameter 2 μm) at a length of 0.4 mm at the circumferential center. The arithmetic mean roughness Ra was calculated from the three obtained data points. If the arithmetic mean roughness Ra was 0.1 μm or less, the polished surface was judged to have excellent mirror polishability, indicating minimal dirt and spots.

[0066] [Table 3]

[0067] As shown in Tables 2 and 3, the stainless steel materials of Examples 1 to 20 had a grain size of 4 or higher, a number density of composite inclusions of 0.01 or higher, and the total concentrations of MnO and MgO, the total concentrations of Al2O3, Cr2O3, and Ti2O3, and the concentration of Ti2O3 were within the range of the present invention. As the Ti content and slag composition in the molten steel and refined slag after the addition of appropriate refining raw materials were controlled, the manufactured wire rods or plates all had excellent mirror polishability with an arithmetic mean roughness Ra of 0.1 μm or less.

[0068] Comparative Example 1 has a high Ti content as a chemical component, and the value of equation (1) for the refining slag exceeds 3.0. In addition, the number density of composite oxide inclusions exceeds 5, and the concentration of Ti2O3 exceeds 5.00 mass%. For these reasons, the arithmetic mean roughness Ra exceeds 0.1 μm, and it is judged to have poor mirror polishability.

[0069] Comparative Example 2 has a high Al content as a chemical component, and the value of equation (2) of the refining slag exceeds 5.0. In addition, the number density of composite oxide inclusions exceeds 5. For this reason, the arithmetic mean roughness Ra exceeds 0.1 μm, and it is judged to have poor mirror polishability.

[0070] Comparative Example 3 has a high Mg content as a chemical component, and the value of equation (4) for the refining slag exceeds 15.0. In addition, the number density of composite oxide inclusions exceeds 5. For this reason, the arithmetic mean roughness Ra exceeds 0.1 μm, and it is judged to have poor mirror polishability.

[0071] Comparative Example 4 has a high Ti content as a chemical component, and the value of equation (3) of the refining slag exceeds 3.0. Furthermore, the Ti content in the molten steel and refining slag after the addition of refining raw materials exceeds 2.00 kg / t. In addition, the number density of composite oxide inclusions exceeds 5, and the concentration of Ti2O3 exceeds 5.00 mass%. For these reasons, the arithmetic mean roughness Ra exceeds 0.1 μm, and the mirror polishability is judged to be poor.

[0072] Comparative Example 5 has a high Ti content as a chemical component, and the value of equation (3) of the refining slag exceeds 3.0. Furthermore, the Ti content in the molten steel and refining slag after the addition of refining raw materials exceeds 2.00 kg / t. In addition, the number density of composite oxide inclusions exceeds 5, and the concentration of Ti2O3 exceeds 5.00 mass%. For these reasons, the arithmetic mean roughness Ra exceeds 0.1 μm, and the mirror polishability is judged to be poor.

[0073] Comparative Example 6 has a reduced Ti content as a chemical component. Furthermore, the Ti content in the molten steel and refining slag after the addition of refining raw materials is less than 0.1 kg / t. As a result, the arithmetic mean roughness Ra is less than 0.1 μm, but the equiaxed crystallization of the steel ingot and the pinning effect during recrystallization in the cooling process after hot rolling are insufficient, resulting in low grain size values ​​of 3.5 and less than 4, which is considered problematic in practical use.

[0074] Comparative Example 7 has high Al and Mg content as chemical components, and the value of equation (3) of the refining slag exceeds 3.0. In addition, the number density of composite oxide inclusions exceeds 5. For this reason, the arithmetic mean roughness Ra exceeds 0.1 μm, and it is judged to have poor mirror polishability.

[0075] Comparative Example 8 has a high Ti content as a chemical component, and the value of equation (3) of the refining slag exceeds 3.0. Furthermore, the Ti content in the molten steel and refining slag after the addition of refining raw materials exceeds 2.00 kg / t. In addition, the number density of composite oxide inclusions exceeds 5, and the concentration of Ti2O3 exceeds 5.00 mass%. For these reasons, the arithmetic mean roughness Ra exceeds 0.1 μm, and the mirror polishability is judged to be poor.

[0076] From the results of Examples 1-20 and Comparative Examples 1-8, it was found that in order to satisfy the target grain size and arithmetic mean roughness Ra of the polished surface of the present invention, the following conditions must be met: the chemical composition must be within the range specified in the present invention; the grain size of the stainless steel material must be 4 or greater; there must be composite oxide inclusions in the steel material with an equivalent circle diameter of 3 μm or greater; the total concentration of MnO and MgO in the composite oxide inclusions must be between 10.0% by mass and 50.0% by mass; the total concentration of Al2O3, Cr2O3, and Ti2O3 must be between 30.0% by mass and 80.0% by mass; and the concentration of Ti2O3 must be between 0.01% by mass and 5.00% by mass.

Claims

1. In mass percent, C: 0.001% or more and 0.150% or less, Si: 0.1% or more and 3.0% or less, Mn: 0.1% or more and 15.0% or less, P: 0.005% or more and 0.040% or less, S: 0.0001% or more and 0.0100% or less, Ni: 2.0% or more and 20.0% or less, Cr: 10.0% or more and 30.0% or less, Al: 0.0001% or more and 0.01% or less, Ti: 0.001% or more and 0.019% or less, O: 0.001% or more and 0.02% or less, N: 0.01% or more and 0.5% or less, Ca: 0.0001% or more and 0.005% or less, A stainless steel material having a chemical composition consisting of Mg: 0.0001% or more and 0.0030% or less, with the remainder being Fe and unavoidable impurities, The aforementioned stainless steel material has a grain size of 4 or greater. The steel material contains a composite oxide inclusion having Mn, Mg, Al, Cr, and Ti, with an equivalent circle diameter of 3 μm or more. The number density of the aforementioned composite oxide inclusions is 0.01 particles / mm³. ２ 5 pieces / mm or more ２ The following: In the composite oxide-based inclusion, the total concentration of MnO and MgO is 10.0% by mass or more and 50.0% by mass or less, and Al ２ O ３ , Cr ２ O ３ and Ti ２ O ３ The total concentration of is 30.0% by mass or more and 80.0% by mass or less, and the concentration of Ti ２ O ３ Is 0.01% by mass or more and 5.00% by mass or less, stainless steel material.

2. The aforementioned chemical composition is expressed in mass%, and further, Mo: 0.01% or more and 5.0% or less, Cu: 0.01% or more and 5.0% or less, B: 0.0001% or more and 0.0050% or less, Nb: 0.1% or more and 0.6% or less, W: 0.01% or more and 0.5% or less, Sn: 0.01% or more and 0.5% or less, V: 0.01% or more and 0.5% or less, Co: 0.01% or more and 0.5% or less, Zr: 0.01% or more and 0.5% or less, Pb: 0.0001% or more and 0.5% or less, The stainless steel material according to claim 1, comprising REM: 0.0003% or more and 0.3% or less.

3. A method for manufacturing stainless steel according to claim 1 or 2, The process includes a refining step in which the total amount of Ti in the molten steel and refining slag after the addition of refining raw materials is refined to be between 0.10 kg / t and 2.00 kg / t. A method for manufacturing stainless steel, wherein the slag composition after the refining process satisfies the range of equations (1) to (4) below. (1) Formula: 0.9≦[CaO] / [SiO2]≦3.0 (2) Equation: [Al₂O₃] ≤ 5.0 (3) Formula: [Ti 2 O 3 ]≦3.0 (4) Equation: [MgO] ≤ 15.0 However, in formulas (1) to (4), [CaO], [SiO2], [Al2O3], [Ti2O3], and [MgO] represent the mass %) content of CaO, SiO2, Al2O3, Ti2O3, and MgO, respectively, in the slag composition.

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