Martensitic stainless steel and method for producing the same

Abstract

To provide a martensitic stainless steel having satisfactory fatigue resistance by improving its fatigue properties by simple operation control by applying an ordinarily used production apparatus, and to provide its production method.SOLUTION: ECDR is 25 μm or less, where A (mm2) is an area of an observation region where inclusions are observed, X is an integer obtained by rounding up A * 0.01 to the nearest whole number, and ECDR is a grain size (area circle equivalent size) of an X-th largest inclusion among inclusions observed in the observation region.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a martensitic stainless steel and a method for manufacturing a martensitic stainless steel. Unless otherwise specified, the term "martensitic stainless steel" in this specification includes a steel material (martensitic stainless steel material) made of the martensitic stainless steel, and the form of the martensitic stainless steel material is not particularly limited, and includes steel plate, steel bar, wire rod, steel section, steel pipe, etc. [Background technology]

[0002] Martensitic stainless steel is a Cr-based stainless steel that has a hard martensite phase formed by rapid cooling from the high-temperature austenite phase. Its hardness is used for cutting tools and shafts. Precipitation-hardened martensitic stainless steel is particularly popular as a structural material and various spring materials, and it is also required to have excellent fatigue resistance. As a means for improving fatigue resistance, it has been proposed to refine and disperse precipitates and inclusions (hereinafter collectively referred to as inclusions) to prevent them from becoming the starting point for crack generation (Patent Documents 1 to 4). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-11515 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-83940 [Patent Document 3] International Publication No. 2022 / 138194 [Patent Document 4] International Publication No. 2021 / 256145 Summary of the Invention [Problem to be solved by the invention]

[0004] The technology described in Patent Document 1 proposes that adding Zr finely disperses ZrN in molten steel, creating sites for TiN formation, and further reducing the amount of N that converts to TiN to suppress the formation of coarse TiN that can serve as fracture initiation points, thereby improving fatigue properties. However, the upper limit of the N concentration is low at 0.0020 mass%, making it difficult to control the N content in actual stainless steel manufacturing sites.

[0005] The technology described in Patent Document 2 proposes controlling the relationship between the Mg and O concentrations, and concentrating oxide-based inclusions in the steel mainly composed of Mg, thereby making them more easily crushable during rolling and reducing their size, thereby preventing them from becoming the starting point for cracks. However, while controlling the Mg content is important because it can cause unstable yields, a stable method for controlling it has not been clarified. In addition, the manufacturing process requires a vacuum melting furnace, and the upper limit of the N concentration is low at 0.0030 mass%, making it difficult to control the N content in actual stainless steel manufacturing sites, as in Patent Document 1.

[0006] The technology described in Patent Document 3 involves dispersing oxide inclusions, primarily MgO, in molten steel to act as nuclei for TiN precipitation, thereby finely dispersing TiN and suppressing the formation of coarse TiN grains, thereby preventing a decrease in fatigue strength. Patent Document 3 proposes that, after melting, primary refining is performed in a converter, and secondary refining is performed using AOD (argon oxygen decarburization) or VOD (vacuum oxygen decarburization), and the Mg content is adjusted during this secondary refining. However, because Mg readily reacts with slag, it is difficult to control the Mg concentration by incorporating Mg alloys into AOD or VOD, making it difficult to maintain stable production.

[0007] The technology described in Patent Document 4 has a lower Mg content than Patent Document 3, and non-metallic inclusions such as TiN and Al2O3 with a particle size of 10 μm or more are refined to the minimum (0.100 pieces / mm 2However, magnesium is inevitably mixed in from the refractory material, and in order to suppress the formation of oxide-based non-metallic inclusions such as aluminum, it is necessary to control the CaO / Al2O3 ratio in the slag according to the aluminum content, which makes it difficult to maintain stable production.

[0008] As described above, in conventional techniques, attempts to improve fatigue strength by suppressing the precipitation of coarse TiN require excessive restriction of the N concentration and difficult control of properties such as the Mg concentration and slag properties in secondary refining, making it difficult to maintain stable operation. Therefore, the present invention aims to improve the fatigue properties of martensitic stainless steels through simple operational control, and to provide such martensitic stainless steels and a method for producing the same. [Means for solving the problem]

[0009] In order to achieve the above object, the present inventors have conducted extensive research and have obtained the following findings.

[0010] (a) The relationship between TiN grain size and fatigue properties in martensitic stainless steel was investigated. As a result, a new index, equivalent representative circle diameter (ECDR), was derived, and it was found that there is a correlation between ECDR and fatigue strength.

[0011] When fatigue fracture occurs in steel, the fracture origin is often at the location of the largest inclusion, so we realized that it was important to evaluate the largest size of inclusions in the evaluation sample, and we proceeded with our research.Even if the largest inclusions are observed using an optical microscope or SEM (scanning electron microscope), the evaluation is two-dimensional, so the particle size of the inclusions is not accurately evaluated, and it is theoretically impossible to accurately evaluate and measure inclusion size as long as the sample cross-section is observed.

[0012] Therefore, extreme value statistics is a conventional method for estimating the maximum size of inclusions in evaluation samples from two-dimensional observations. However, the inventors have developed an evaluation method using ECDR (equivalent representative circle diameter) as a simpler method, and have found that it correlates with the fatigue strength of the evaluation sample. Specifically, ECDR is calculated by dividing the area of ​​the observation region where inclusion particles are observed (observation area (mm 2 )) by 0.01, the decimal point of the resulting value is rounded up to an integer X, and the size (area circle diameter (unit: μm)) of the largest Xth inclusion in the observed area is used as the representative value (ECDR (unit: μm)) of the inclusion size (particle size) in the evaluation sample.

[0013] From the correlation between the ECDR and fatigue properties of martensitic stainless steel, it was found that by keeping the ECDR at 25 μm or less, the fatigue limit stress can be set at 400 MPa or more. It was also found that the fatigue resistance properties (fatigue limit stress) of martensitic stainless steel can be easily evaluated using the ECDR.

[0014] (stomach) We investigated the formation behavior of TiN in martensitic stainless steel. As disclosed in Patent Document 3, TiN usually exists as a composite inclusion attached around MgO inclusions, or it may exist independently. Therefore, we considered TiN formation regardless of its form, and, from the perspective of the thermodynamic activity of Ti and N, we envisioned a relationship between the concentration product and the temperature of molten steel in the tundish immediately before solidification, and proceeded with development. As a result, we found that the relationship between the concentration product of Ti and N and the temperature of molten steel in the tundish is expressed by the following formula 1. T-TLL-315ln([Ti]·[N])-1726>0 ···Formula 1 Here, [Ti] and [N] in Equation 1 represent the Ti and N contents (mass%) in the stainless steel, respectively, T represents the temperature of the molten steel in the tundish (unit: °C), and TLL represents the liquidus temperature of the stainless steel (unit: °C).

[0015] The present invention was made based on the above findings, and the gist of the present invention is as follows.

[0016] [1] In mass%, C: 0.080% or less, Si: 0.50~3.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.008% or less, Cr: 10.0 to 17.0%, Ni: 3.50~10.00%, Mo: 0.03 to 3.00%, Cu: 0.10-2.00% Al: 0.005 to 0.200%, Ti: 0.150~0.500%, N: 0 to 0.0160%, O: 0 to 0.0075%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, B: 0 to 0.0040%, REM: 0~0.01%, Ta: 0 to 0.10%, W: 0~2.00%, V: 0~1.00%, Nb: 0 to 0.60% Sn: 0 to 0.10% Sb: 0 to 0.30% Co: 0 to 1.00%, Zr: 0 to 0.005%, Ga: 0 to 0.01%, A steel (steel material) with the balance being Fe and impurities, The area of ​​the observation region on the surface of the steel (steel material) where inclusions are observed is defined as A (mm 2) and X is the integer of A × 0.01 rounded up to the nearest integer, the martensitic stainless steel (steel material) characterized in that the diameter (area circle diameter) of the Xth inclusion counting from the largest diameter (area circle diameter) among the inclusions observed in the observation region is 25 μm or less. [2] The martensitic stainless steel (steel material) according to [1] above, wherein the martensitic stainless steel (steel material) has a fatigue limit stress of 400 MPa or more. [3] In mass%, C: 0.080% or less, Si: 0.50~3.00% or less, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.008% or less, Cr: 10.0 to 17.0%, Ni: 3.50~10.00%, Mo: 0.03 to 3.00%, Cu: 0.10-2.00% Al: 0.005 to 0.200%, Ti: 0.150~0.500%, N: 0 to 0.0160%, O: 0 to 0.0075%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, B: 0 to 0.0040%, REM: 0~0.01%, Ta: 0 to 0.10%, W: 0~2.00%, V: 0~1.00%, Nb: 0 to 0.60% Sn: 0 to 0.10% Sb: 0 to 0.30% Co: 0 to 1.00%, Zr: 0 to 0.005%, Ga: 0 to 0.01%, The area of ​​the observation region where inclusions are observed on the surface of martensitic stainless steel (steel material) consisting of the remainder Fe and impurities is defined as A (mm 2 ) and X is the integer of A×0.01 rounded up to the nearest whole number, the diameter (area circle diameter) of the Xth inclusion from the largest diameter (area circle diameter) among the inclusions observed in the observation region, ECDR, is set to 25 μm or less. [4] The method for producing the martensitic stainless steel (steel material) according to the above [3], wherein when molten steel containing the components of the martensitic stainless steel (steel material) is cast in a casting device having a tundish, the following formula 1 is satisfied: T-TLL-315ln([Ti]·[N])-1726>0 ...Formula 1 In Equation 1, [Ti] and [N] respectively represent the contents (mass%) of Ti and N in the molten steel, T represents the temperature (unit: °C) of the molten steel in the tundish, and TLL represents the liquidus temperature (unit: °C) of the molten steel. [5] The method for producing a martensitic stainless steel (steel material) according to [3] or [4], wherein the martensitic stainless steel (steel material) has a fatigue limit stress of 400 MPa or more. [Effects of the Invention]

[0017] According to the present invention, it is possible to suppress the formation of coarse TiN by applying commonly used manufacturing equipment and by simple operational control, and as a result, it is possible to obtain martensitic stainless steel with good fatigue resistance properties. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 shows the results of the example (Table 2). DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as "the present invention") will be described. As described above, unless otherwise specified, in this specification, martensitic stainless steel (hereinafter may be simply referred to as "steel") includes steel materials made of the martensitic stainless steel (martensitic stainless steel materials (hereinafter may be simply referred to as "steel materials")), and the form of the martensitic stainless steel materials is not particularly limited, including steel plates, bars, wire rods, sections, steel pipes, etc.

[0020] [Components of Martensitic Stainless Steel] Hereinafter, the components of the martensitic stainless steel according to the present invention will be described. Unless otherwise specified, "%" regarding components indicates mass % in the steel, and includes cases where no lower limit is particularly specified for the content of the component and cases where the lower limit is 0% and the component is not contained (0%).

[0021] <C: 0.080% or less> C (carbon) is an element effective in improving the strength of the steel and suppressing the δ-ferrite phase generated at high temperatures. However, when the C content increases, the hardness of the martensite phase generated by quenching increases, and the cold working deformation ability decreases. As a result, the forming workability becomes insufficient, and it becomes difficult to obtain a single-phase martensite structure by cooling after solution treatment. Furthermore, when the C content increases, the generation of TiC in the annealed state is promoted, and the toughness decreases. Therefore, the C content is set to 0.080% or less. Preferably it is 0.070% or less, 0.060% or less, 0.050% or less, or 0.040% or less. On the other hand, the lower limit of C is not particularly limited, but excessive reduction of the C content increases the operation load, so preferably it is 0.001% or more, 0.005% or more, 0.010% or more, or 0.015% or more.

[0022] <Si: 0.50 - 3.00%> Si (silicon) is an element with a large solid solution strengthening ability and has the effect of strengthening the matrix. Also, when Si is contained in combination with Ti and Ni, fine coherent precipitation of intermetallic compounds composed of elements such as Si, Ti, and Ni occurs during aging treatment, improving the strength of the steel. In addition, by acting as a deoxidizer, it has the effect of stabilizing the yield of Ti, which is an important element in the present invention. Therefore, the Si content is set to 0.50% or more. Preferably, it is 0.80% or more, 1.00% or more, 1.20% or more, or 1.30% or more. On the other hand, when the Si content increases, the formation of the δ-ferrite phase is promoted, and the strength and toughness decrease. Therefore, the Si content is set to 3.00% or less. Preferably, it is 2.50% or less, 2.20% or less, 2.00% or less, or 1.80% or less.

[0023] <Mn: 0.10 - 3.00%> Mn (manganese) is an element that has the effect of suppressing the formation of the δ-ferrite phase in the high-temperature range. Also, when Mn acts as a deoxidizer, the oxygen concentration in the steel is reduced. As a result, the outflow of Ti outside the steel as an oxide is suppressed, so it has the effect of stabilizing the residual yield of Ti in the steel. Therefore, the Mn content is set to 0.10% or more. Preferably, it is 0.13% or more, 0.15% or more, 0.18% or more, or 0.20% or more. On the other hand, when the Mn content increases, it is likely to cause a decrease in the toughness of the welded part and a decrease in welding workability. Therefore, the Mn content is set to 3.00% or less. Preferably, it is 2.50% or less, 2.00% or less, 1.50% or less, or 1.00% or less.

[0024] <P: 0.050% or less> P (phosphorus) is an impurity. In addition to promoting hot workability and solidification cracking during manufacturing, it hardens and reduces ductility. Therefore, the lower the P content, the better. Thus, the upper limit is set to 0.050%. Preferably, it is 0.020% or less, 0.010% or less, 0.008% or less, or 0.005% or less. The lower limit is not particularly limited, but even if it is 0.001% or more, excessive reduction is not preferred because the operation load is high.

[0025] <0.0080% or less> S (sulfur) exists in steel as non-metallic inclusions such as MnS and has an adverse effect on fatigue strength, toughness, corrosion resistance, etc. Therefore, the S content is preferably as low as possible, and its upper limit is set to 0.0080%. Preferably, it is 0.0050% or less, 0.0030% or less, or 0.0020% or less. The lower limit is not particularly limited, but excessive reduction is not preferred due to high operating load, so it may be 0.0001% or more.

[0026] <Ni: 3.50 - 10.00%> Ni (nickel) is an element that contributes to precipitation hardening and suppresses the formation of the δ-ferrite phase. In the martensitic stainless steel according to the present invention, since it has the effect of maintaining high strength and high toughness without reducing the age hardening ability, the Ni content is 3.50% or more. Preferably, it is 4.00% or more, 4.50% or more, 5.00% or more, 5.50% or more, or 6.00% or more. On the other hand, when the Ni content increases, the amount of retained austenite phase after quenching increases and strength cannot be obtained, so the Ni content is 10.00% or less. Preferably, it is 9.00% or less, 8.00% or less, or 7.50% or less.

[0027] <Cr: 10.00 - 17.00%> Cr (chromium) has the effect of enhancing the corrosion resistance of steel, so the Cr content is 10.00% or more. Preferably, it is 10.50% or more, 11.00% or more, 11.50% or more, 12.00% or more, 12.50% or more, or 13.00% or more. On the other hand, when the Cr content increases, the δ-ferrite phase and the retained austenite phase are formed, which causes a decrease in the strength of the welded part, so the Cr content is 17.00% or less. Preferably, it is 16.00% or less, 15.00% or less, or 14.00% or less.

[0028] <Mo: 0.03 - 3.00%> Mo (molybdenum) has the effect of improving strength, toughness, and corrosion resistance, so the Mo content is set to 0.03% or more. Preferably, it is set to 0.05% or more, 0.07% or more, 0.10% or more, 0.20% or more, 0.30% or more, 0.40% or more, 0.50% or more, 0.60% or more, or 0.70% or more. On the other hand, if the Mo content is too high, not only will the improvement in strength and toughness commensurate with the increase in Mo content not be achieved, but the formation of δ-ferrite phase will be promoted, which will likely reduce the strength of the weld. Therefore, the Mo content is set to 3.00% or less. Preferably, it is set to 2.50% or less, 2.00% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.90% or less, or 0.80% or less.

[0029] <Cu:0.10~2.00%> Since Cu (copper) is an effective element for ensuring corrosion resistance, the Cu content is set to 0.10% or more. Preferably, it is set to 0.20% or more, 0.30% or more, 0.40% or more, 0.50% or more, 0.60% or more, or 0.65% or more. On the other hand, if the Cu content is too high, hot workability deteriorates, defects such as cracks may occur on the processed material surface, and toughness tends to decrease when the strength is increased. Therefore, the Cu content is set to 2.00% or less. Preferably, it is set to 1.70% or less, 1.40% or less, 1.10% or less, 1.00% or less, 0.90% or less, or 0.85% or less.

[0030] <Al:0.005~0.200%> Al (aluminum) is an element that acts as a deoxidizer and has the effect of stabilizing the yield of Ti, so the Al content is set to 0.005% or more. It is preferably set to 0.010% or more, 0.015% or more, 0.020% or more, 0.025% or more, or 0.030% or more. On the other hand, since a high Al content deteriorates workability, the Al content is set to 0.200% or less. It is preferably set to 0.170% or less, 0.140% or less, 0.110% or less, 0.100% or less, 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0031] <Ti: 0.150 - 0.500%> Ti (titanium) is an element that contributes to precipitation hardening. From the perspective of ensuring strength, the Ti content should be 0.150% or more. Preferably, it should be 0.160% or more, 0.170% or more, 0.180% or more, 0.190% or more, 0.200% or more, 0.210% or more, 0.220% or more, 0.230% or more, or 0.240% or more. On the other hand, when the Ti content increases, toughness decreases due to excessive precipitation hardening reaction, so the Ti content should be 0.500% or less. Preferably, it should be 0.470% or less, 0.440% or less, 0.410% or less, 0.400% or less, 0.390% or less, 0.380% or less, 0.370% or less, or 0.360% or less.

[0032] <N: 0 - 0.0160%> N (nitrogen) has a high affinity with Ti and will consume a part of the Ti component acting as a precipitation hardening element by the formation of TiN. Also, as the N content increases, the TiN inclusions become larger, which causes a decrease in fatigue strength and toughness, so the N content should be 0.0160% or less. Preferably, it should be 0.0155% or less, 0.0150% or less, 0.0145% or less, 0.0140% or less, or 0.0135% or less. On the other hand, the lower the N content, the better, so the lower limit is not particularly limited. However, reducing the N content excessively increases the operation load, so the N content may also be 0.0001% or more. Preferably, it may be 0.0010% or more, or 0.0020% or more.

[0033] <O: 0 - 0.0075% or less> O (oxygen) is an impurity element contained in steel. A high O content not only deteriorates the cleanliness of the steel, but also consumes a portion of the Ti component, which functions as a precipitation hardening element, through the formation of Ti oxides. Therefore, the O content is set to 0.0075% or less. Preferably, it is set to 0.0070% or less, 0.0065% or less, 0.0060% or less, 0.0055% or less, 0.0050% or less, 0.0045% or less, or 0.0040% or less. While a lower O content is preferable, excessive reduction increases the operational load and leads to higher costs. Therefore, the O content may be set to 0.0001% or more. Preferably, it may be set to 0.0002% or more, or 0.0003% or more.

[0034] <Ca:0~0.0100%> Ca (calcium) is an element that contributes to improving hot workability and may be added as needed. However, a high Ca content can produce large oxide-based inclusions that affect fatigue resistance and sulfides that affect corrosion resistance. Therefore, the Ca content is preferably 0.0100% or less. More preferably, it is 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0030% or less.

[0035] <Mg:0~0.0100%> Magnesium (Mg) may be added as a deoxidizing element, but it also refines the slab structure and contributes to improving hot workability and formability, so it may be added as needed. However, if the Mg content is high, it can form large oxide-based inclusions that affect fatigue resistance and sulfides that affect corrosion resistance, so the Mg content is preferably 0.0100% or less. Preferably, it is 0.0080% or less, 0.0070% or less, 0.0060% or less, or 0.0050% or less.

[0036] <B:0~0.0040%> B has the effect of increasing the strength of grain boundaries and improving workability. Therefore, it may be added as needed. On the other hand, if B is added in excess, ductility decreases, and workability decreases instead, so the B content is preferably 0.0040% or less. The B content is more preferably 0.0025% or less, and even more preferably 0.0020% or less. On the other hand, although there is no particular lower limit, the B content may be 0.0001% or more to ensure the above effects.

[0037] <REM:0~0.0100%> REM (Rare-Earth Metal) is an element that acts as a powerful deoxidizer and contributes to improving hot workability, and may be added as needed. On the other hand, if the REM content is high, it can cause nozzle clogging during casting, reducing manufacturability and making surface defects more likely to occur. Therefore, the REM content is preferably 0.0100% or less. Preferably, it is 0.0090% or less, 0.0080% or less, 0.0070% or less, 0.0060% or less, or 0.0050% or less. REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content mentioned above means the total content of these elements. Industrially, REM is often contained in the form of misch metal.

[0038] <Ta:0~0.10%> Ta has a high affinity with O and acts as a deoxidizer, thereby stabilizing the yield of Ti, an important element in the present invention. Therefore, it may be added as needed. However, excessive Ta content reduces room-temperature ductility and toughness. Therefore, the Ta content is preferably 0.10% or less.

[0039] <W:0~2.00%> W has the effect of improving corrosion resistance. Therefore, it may be contained as necessary. However, W is an expensive element, and excessive W content increases production costs. Therefore, the W content is preferably 2.00% or less. The W content is more preferably 1.50% or less, and even more preferably 1.00% or less. There is no particular lower limit, but to reliably obtain the above effects, the W content is preferably 0.05% or more.

[0040] <V:0~1.00%> V has the effect of increasing corrosion resistance. Therefore, it may be contained as needed. However, if an excessive amount of V is contained, toughness decreases. Therefore, the V content is preferably 1.00% or less. The V content is more preferably 0.50% or less, and even more preferably 0.25% or less. On the other hand, in order to reliably obtain the above effects, the V content is preferably 0.05% or more.

[0041] <Nb:0~0.600%> Nb has the effect of improving formability and corrosion resistance. Therefore, it may be added as needed. However, if Nb is added in excess, toughness decreases. Therefore, the Nb content is preferably 0.600% or less. The Nb content is more preferably 0.100% or less, and even more preferably 0.080% or less, or 0.050% or less. On the other hand, in order to reliably obtain the above effects, the Nb content is preferably 0.001% or more.

[0042] <Sn:0~0.100%> Sn has the effect of enhancing corrosion resistance. Therefore, it may be contained as necessary. However, if Sn is contained in excess, workability decreases, so the Sn content is preferably 0.100% or less. The Sn content is more preferably 0.050% or less, and even more preferably 0.020% or less. On the other hand, in order to reliably obtain the above effects, the Sn content is preferably 0.010% or more.

[0043] <Sb:0~0.30%> Sb has the effect of enhancing corrosion resistance. Therefore, it may be contained as necessary. However, excessive Sb content may promote the formation of TiN, which may cause surface defects, so the Sb content is preferably 0.30% or less. The Sb content is more preferably 0.10% or less, and even more preferably 0.05% or less. On the other hand, in order to reliably obtain the above effects, the Sb content is preferably 0.005% or more.

[0044] <Co:0~1.00%> Co has the effect of enhancing corrosion resistance. Therefore, it may be contained as necessary. However, since Co is a very expensive element, excessive Co content increases the manufacturing cost, so the Co content is preferably 1.00% or less. The Co content is more preferably 0.70% or less, and even more preferably 0.40% or less. On the other hand, to reliably obtain the above effects, the Co content is preferably 0.03% or more.

[0045] <Zr:0~0.0050%> Zr has the effect of improving corrosion resistance by fixing S. Therefore, it may be contained as necessary. However, if Zr is contained in excess, coarse sulfides may be formed in the molten steel, which may actually reduce corrosion resistance. Therefore, the Zr content is preferably 0.0050% or less. The Zr content is more preferably 0.0030% or less, and even more preferably 0.0020% or less. On the other hand, in order to reliably obtain the above effects, the Zr content is preferably 0.0001% or more.

[0046] <Ga:0~0.010%> Since Ga has the effect of enhancing corrosion resistance, it may be contained as necessary. However, if Ga is contained in excess, the hot workability may decrease. Therefore, the Ga content is preferably 0.010% or less. The Ga content is more preferably 0.005% or less, and even more preferably 0.003% or less. On the other hand, in order to surely obtain the above effect, the Ga content is preferably 0.0001% or more.

[0047] The balance of the above steel components is Fe and impurities. Here, the impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when steel is industrially manufactured, and are those that are allowed within a range that does not adversely affect the present invention.

[0048] <ECDR (equivalent diameter of representative circle)> The relationship between the particle size of TiN and the fatigue characteristics in martensitic stainless steel was investigated. As a result, the equivalent diameter of the representative circle (ECDR) was derived as a new index, and it was found that there is a correlation between ECDR and fatigue strength.

[0049] When fatigue fracture occurs in a steel material, the fracture origin often occurs at the location where the inclusion with the largest size exists. Therefore, it was recalled that it is important to evaluate the maximum size of the inclusions in the evaluation sample, and the investigation was advanced. Even if the inclusion with the largest size is observed with an optical microscope or SEM (scanning electron microscope), as long as it is observed on the sample cross-section, the evaluation is two-dimensional due to various circumstances, and it is theoretically impossible to correctly evaluate and measure the particle size of the inclusions.

[0050] Therefore, there is extreme value statistics as a method for estimating the maximum size of the inclusions in the evaluation sample from the conventional two-dimensional observation. However, the inventors of the present invention have found an evaluation method using ECDR (equivalent diameter of representative circle) as a simpler method, and have found that there is a correlation with the fatigue strength of the evaluation sample.

[0051] The measuring method of ECDR is the area of the observation region for observing inclusion particles (observation area (mm2 )) by 0.01, and round up the decimal point to an integer X. The size (area equivalent circle diameter (unit: μm)) of the Xth largest inclusion in the observation area is taken as the representative value (ECDR (unit: μm)) of the inclusion size (grain size) in the evaluation sample. In other words, the area of ​​the observation area where inclusions are observed is defined as A (mm 2 ) and X is the integer obtained by rounding up A × 0.01 to the nearest whole number. The particle size (area circle diameter) of the Xth inclusion counting from the largest particle size (area circle diameter) among the inclusions observed in the observation region is defined as ECDR.

[0052] The size of the observation area is not particularly limited, but the observation area is 100 mm, which can be obtained by multiplying by 0.01 and rounding up to the nearest integer. 2 It is desirable that the distance is at least 500 mm. 2 Over 800mm 2 Over 1000mm 2 If the observation area is too large, not only will the observation time increase, but the particle detection accuracy will also decrease, so it should be determined appropriately in relation to the measurement device.

[0053] When observing the surface of a steel material, it is preferable to observe the surface of the steel material after removing the work-hardened layer on the surface. For example, the surface of the steel material may be polished with emery paper (e.g., #120 to #1000), then buffed (e.g., with diamond paste) to a mirror finish, and then observed.

[0054] From the correlation between the ECDR and fatigue properties of the martensitic stainless steel according to the present invention, it was confirmed that if the ECDR is 25 μm or less, the fatigue limit stress of the steel is 400 MPa or more.

[0055] It has been confirmed that the smaller the ECDR, the better the fatigue limit stress of the steel, and it is presumed that the smaller the ECDR, the more suppressed the formation of coarse TiN and the smaller the maximum diameter of TiN inclusions. Therefore, a smaller ECDR is preferable, for example, 23 μm or less, 21 μm or less, 20 μm or less, 18 μm or less, 16 μm or less, 14 μm or less, 12 μm or less, or 10 μm or less. The ECDR can be used to easily evaluate the fatigue resistance properties (fatigue limit stress) of martensitic stainless steel.

[0056] <Manufacturing method> Next, a method for producing a martensitic stainless steel according to the present invention will be described. The production method described below is one embodiment for obtaining a martensitic stainless steel (steel material) according to the present invention, and is not limited to this production method. As long as the steel according to the present invention can be obtained, the production method is not limited.

[0057] <Melting~Casting> First, steel having the above-mentioned composition is melted to obtain molten steel. The melting method is not particularly limited, and conventional melting methods can be applied. For example, an electric furnace may be used for the melting process, or molten iron may be obtained by the blast furnace method. Next, the steel is subjected to a refining process to adjust the steel composition to the above-mentioned composition. For example, the refining process may use a converter as the primary refining process and AOD (argon oxygen decarburization) or VOD (vacuum oxygen decarburization) as the secondary refining process to obtain molten steel with the adjusted composition in the steel.

[0058] The resulting molten steel is then passed through a casting process to be solidified and cast into a cast piece (such as a slab or billet) of a desired shape. Typically, the casting process is continuous casting. A casting apparatus (continuous casting apparatus) used for continuous casting has a tundish, into which the molten steel after secondary refining is poured, and from which the molten steel is poured into a mold. While solidifying in the mold, the molten steel is drawn vertically downward and continuously cast into a desired shape. According to the method for producing martensitic stainless steel of the present invention, the temperature control of the molten steel in the tundish can suppress the formation of coarse TiN, thereby reducing the ECDR to a predetermined value or less.

[0059] <Controlling the temperature of molten steel in the tundish> The present inventors investigated the formation behavior of TiN in martensitic stainless steels. TiN may exist as a composite inclusion attached around MgO inclusions or as a separate inclusion. Therefore, regardless of its form, they considered TiN formation and, from the perspective of the thermodynamic activity of Ti and N, assumed a relationship between the concentration product and the molten steel temperature in the tundish immediately before solidification. As a result, they found the relationship between the concentration product of Ti and N and the molten steel temperature in the tundish, as shown in Equation 1 below. Using multiple regression analysis of various test data, the present inventors found that the ECDR and fatigue properties (fatigue limit stress) can be calculated from the relationship between the molten steel temperature in the tundish, T, and ln([Ti]·[N]), which is the natural logarithm of the concentration product of Ti and N. Specifically, they found that the ECDR can be reduced to a predetermined value (e.g., 25 μm) if the relationship satisfies Equation 1 below. They found that a fatigue limit stress of 400 MPa or more can be achieved when the ECDR is 25 μm or less.

[0060] T-TLL-315ln([Ti]·[N])-1726>0 ···Formula 1 Here, [Ti] and [N] in Equation 1 represent the Ti and N contents (mass%) in the stainless steel, respectively, T represents the temperature of the molten steel in the tundish (unit: °C), and TLL represents the liquidus temperature of the stainless steel (unit: °C).

[0061] The temperature at which TiN begins to form in steel varies depending on the concentrations of Ti and N in the steel, and it is known that the higher the product of the concentrations of Ti and N, the higher the temperature at which TiN forms. It is also known that the amount of TiN formed at that temperature increases as the temperature decreases.

[0062] If TiN begins to form before the steel solidifies, it grows larger because the diffusion of Ti and N is faster than after solidification. The molten steel temperature, T, refers to the temperature of the molten steel in the tundish before pouring into the mold. After the molten steel is poured from the tundish into the mold, the steel surface immediately cools and solidifies, so the subsequent growth of the already formed TiN is almost negligible. In other words, the steel surface related to fatigue has already solidified directly below the mold, and the growth of TiN on the shell of the slab surface is almost negligible after the slab solidifies. Therefore, the ECDR can be reduced by controlling the temperature at which TiN does not form, or if it does form, only a small amount, based on the relationship between the molten steel temperature (tundish temperature) before casting and the concentration product of Ti and N, [Ti] × [N] (sometimes expressed as [Ti] × [N]). Reducing the ECDR ultimately increases the fatigue strength (fatigue limit stress) of the steel.

[0063] In summary, the greater the difference between the molten steel temperature in the tundish (the temperature of the molten steel before casting) T (unit: °C) and the liquidus temperature TLL (unit: °C) of the steel, and the smaller the solubility product [Ti] × [N], the more the growth of TiN can be suppressed.

[0064] There is no particular limitation on the method for measuring the molten steel temperature T (unit: °C) in the tundish (the temperature of the molten steel before casting). For example, it can be measured using a non-contact thermometer such as an infrared thermometer. Since measurements are often performed during normal operation, the molten steel temperature in the tundish can be determined without adding any new measuring equipment.

[0065] The liquidus temperature TLL (unit: °C) of steel can be calculated based on the molten steel composition. For example, it can be calculated using commercially available software such as ThermoCalc.

[0066] <Rolling and other processes> The cast piece (slab or billet) obtained by the above-mentioned casting method is heated to a predetermined temperature and hot-rolled to a predetermined thickness. After hot-rolling, the cast piece may be cold-rolled as needed. There are no particular limitations on the hot-rolling method or the cold-rolling method, and conventional methods can be used.

[0067] The conditions in the manufacturing process may be selected as appropriate. For example, the slab thickness, the hot-rolled sheet thickness, etc. may be set as appropriate. After coiling, the hot-rolled sheet may be immersed in a water-cooled pool. The pickling process after hot rolling or after hot-rolling annealing is not particularly limited, and the mechanical descaling method, such as shot blasting, bending, or brushing, may be selected as appropriate. The pickling solution after hot rolling is also not particularly limited, and existing conditions, such as sulfuric acid or nitric hydrofluoric acid, may be used. Furthermore, coil surface grinding may be performed thereafter. Annealing may be performed after hot rolling or cold rolling. These processes are also not particularly limited, and conventional methods can be applied.

[0068] <Fatigue limit stress> The fatigue resistance characteristics (fatigue limit stress) can be determined in accordance with JIS Z 2275-1978, Plane bending fatigue test method for metal plates. The test is terminated when the test piece breaks or when the number of repetitions reaches 10 million (10 7 The stress when the number of repetitions reaches 10 million times without fracture is defined as the fatigue limit stress (unit: MPa).

[0069] The inventors repeated experiments and confirmed that if the ECDR of the martensitic stainless steel according to the present invention is 25 μm or less, the fatigue limit stress can be ensured to be 400 MPa or more. It was confirmed that if the ECDR is 20 μm or less, 15 μm or less, 10 μm or less, or 5 μm or less, the fatigue limit stress can be ensured to be 450 MPa or more, 500 MPa or more, 550 MPa or more, and 600 MPa or more, respectively.

Example

[0070] Hereinafter, examples of the present invention will be described. Note that the present invention is not limited to the conditions used in the following examples.

[0071] For each steel shown in Table 1, molten steel having its components was poured into a tundish, and cast by a continuous casting machine having a tundish to obtain a slab-shaped stainless steel ingot. At that time, the temperature in the tundish (casting temperature) was controlled to the temperature shown in Table 2.

[0072] The obtained stainless steel ingot was heated to 1100 - 1250 °C and then hot-rolled, and then annealed by holding in the temperature range of 900 - 1100 °C for 30 - 150 seconds. Then, pickling and cold rolling were performed, and then aging treatment was performed by holding in the temperature range of 400 - 600 °C for 10 - 80 minutes to produce a martensitic stainless steel sheet (test cold-rolled sheet) with a thickness of 1.0 - 3.5 mm. The TTL of each steel was calculated using the TCFE10 database with Thermocalc, a commercially available software, considering C, Si, Mn, P, S, Ni, Cr, Mo, Cu, Al, Ti, Nb, V, W, N, and the balance Fe as the considered elements. The tundish temperature (casting temperature), TTL (unit: °C), the value on the left side of Equation 1, the ECDR measurement value, and the fatigue limit stress measurement value of each steel are shown in Table 2.

[0073] <Measurement of ECDR> A 40 mm square specimen for observation was taken from the widthwise center of the obtained martensitic stainless steel plate. The surface of the specimen was polished with emery paper of #120 to #1000 grit, and then buffed with diamond paste to obtain a mirror finish. The surface of the mirror-finished specimen was observed using SEM-EDS, and particles containing TiN with an equivalent circle diameter (equivalent area circle diameter) of 3 μm or more were counted. Here, "particles containing TiN" includes composite inclusions composed of TiN and other compounds, such as TiN particles attached to the periphery (including part of the outer periphery) of oxide-based inclusions.

[0074] The observation area must be at least 800 mm 2 The observation area (mm 2 ) × 0.01, rounded up to the nearest integer X (for example, if the observation area is 901 mm 2 In this case, 901 × 0.01 = 9.01, and the integer value 10 rounded up to the nearest whole number becomes X.) Among the particles obtained from observation of the observation surface, the ECDR was determined to be the equivalent circular diameter of the Xth particle in descending order of equivalent circular diameter.

[0075] <Measurement of fatigue limit stress> Fatigue test specimens of specified dimensions with the rolling direction as the longitudinal direction were cut out from each test cold-rolled sheet, and the surfaces and end faces were dry-polished with #600 emery paper. Subsequently, they were heat-treated at 480°C for 1 hour, and then air-cooled to room temperature.

[0076] The fatigue tests were conducted in accordance with JIS Z 2275-1978 using a plane bending fatigue testing machine in an atmospheric test environment at room temperature. The test was terminated when the specimen broke or when the number of repetitions reached 10 million, and the stress at which the specimen broke without breaking was taken as the fatigue limit stress (MPa). Five test specimens were manufactured from each cold-rolled plate and placed simultaneously in the fatigue testing machine. The fatigue limit stress for each test specimen was calculated as the simple arithmetic average (the total value for the five specimens divided by 5) to determine the fatigue limit stress for that steel.

[0077] The evaluation results of fatigue resistance properties are shown in Table 2 and Figure 1. A fatigue limit stress of 400 MPa or more can be evaluated as having good fatigue resistance properties. There is a correlation between ECDR and fatigue limit stress, and it was confirmed that a fatigue limit stress of 400 MPa or more can be ensured when the ECDR is 25 μm or less. It was confirmed that a fatigue limit stress of 450 MPa or more, 500 MPa or more, 550 MPa or more, and 600 MPa or more can be ensured when the ECDR is 20 μm or less, 15 μm or less, 10 μm or less, and 5 μm or less, respectively.

[0078] [Table 1]

[0079] [Table 2] [Industrial Applicability]

[0080] The present invention can be used not only in the steel industry that produces stainless steel, but also in a wide range of industries that use martensitic stainless steel materials, such as the civil engineering and construction industry related to social infrastructure such as general buildings and bridges, the chemical industry, the machinery industry, and the automobile industry.

Claims

1. In mass%, C: 0.080% or less, Si: 0.50-3.00%, Mn: 0.10-3.00%, P: 0.050% or less, S: 0.008% or less, Cr: 10.0-17.0%, Ni: 3.50-10.00%, Mo: 0.03-3.00%, Cu: 0.10-2.00%, Al: 0.005-0.200%, Ti: 0.150-0.500%, N: 0 to 0.0160%, O: 0 to 0.0075%, Ca: 0-0.01%, Mg: 0 to 0.01%, B: 0 to 0.0040%, REM: 0-0.01%, Ta: 0-0.10%, W: 0-2.00%, V: 0 to 1.00%, Nb: 0 to 0.60%, Sn: 0 to 0.10%, Sb: 0 to 0.30%, Co: 0-1.00%, Zr: 0 to 0.005%, and Ga: 0 to 0.01%; A steel comprising the balance Fe and impurities, The area of ​​the observation region on the surface of the steel where inclusions are observed is defined as A (mm ２ ) and X is the integer obtained by rounding up A×0.01 to the nearest whole number, the martensitic stainless steel characterized in that ECDR, which is the diameter (area circle diameter) of the Xth inclusion counting from the largest diameter (area circle diameter) among the inclusions observed in the observation region, is 25 μm or less.

2. 2. The martensitic stainless steel according to claim 1, wherein the martensitic stainless steel has a fatigue limit stress of 400 MPa or more.

3. In mass%, C: 0.080% or less, Si: 0.50 to 3.00% or less, Mn: 0.10-3.00%, P: 0.050% or less, S: 0.008% or less, Cr: 10.0-17.0%, Ni: 3.50-10.00%, Mo: 0.03-3.00%, Cu: 0.10-2.00%, Al: 0.005-0.200%, Ti: 0.150-0.500%, N: 0 to 0.0160%, O: 0 to 0.0075%, Ca: 0-0.01%, Mg: 0 to 0.01%, B: 0 to 0.0040%, REM: 0-0.01%, Ta: 0-0.10%, W: 0-2.00%, V: 0 to 1.00%, Nb: 0 to 0.60%, Sn: 0 to 0.10%, Sb: 0 to 0.30%, Co: 0-1.00%, Zr: 0 to 0.005%, Ga: 0 to 0.01%; The area of ​​the observation region where inclusions are observed on the surface of martensitic stainless steel, the balance of which is Fe and impurities, is defined as A (mm ２ ) and X is the integer obtained by rounding up the decimal point of A×0.01, the diameter (area circle diameter) of the Xth inclusion counting from the largest among the diameters (area circle diameter) of inclusions observed in the observation region is set to 25 μm or less.

4. 4. The method for producing a martensitic stainless steel according to claim 3, wherein the following formula 1 is satisfied when molten steel containing the components of the martensitic stainless steel is cast in a casting device having a tundish. T-TLL-315ln([Ti]・[N])-1726>0...Formula 1 In the formula 1, [Ti] and [N] respectively represent the contents (mass%) of Ti and N in the molten steel, T represents the temperature (unit: °C) of the molten steel in the tundish, and TLL represents the liquidus temperature (unit: °C) of the molten steel.

5. 5. The method for producing a martensitic stainless steel according to claim 3, wherein the martensitic stainless steel has a fatigue limit stress of 400 MPa or more.

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