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(11) | EP 4 800 146 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | MARTENSITIC STAINLESS STEEL AND MANUFACTURING METHOD THEREFOR |
| (57) The present disclosure relates to a martensitic stainless steel and a method for
manufacturing the same. According to an embodiment of the present invention, the martensitic
stainless steel comprises, in weight %, C: 0.40% to 0.55%, N: 0.01% to 0.05%, Si:
0.10% to 0.80%, Mn: 0.20% to 0.80%, Cr: 13.0% to 15.5%, Ni: 0.01% to 0.50%, and Ti:
0.001% to 0.020%, with the balance being Fe and unavoidable impurities, wherein the
following Formula (1) is satisfied, and the number of pores inside a cold-rolled annealed
material is in a range of 1 * 104 to 50 * 104 ea/mm2: (wherein [Cr], [Si], [C], and [Ti] represent the content of each element) |
[Technical Field]
[0001] The present disclosure relates to martensitic stainless steel and a method for manufacturing the same.
[Background Art]
[0002] The improvement of living standards drives increased application of martensitic stainless steel, having high hardness, high strength, and high corrosion resistance, in household knives and industrial band saws, among others. For these applications, high hardness, high strength properties and corrosion resistance are required. Securing high hardness and high strength properties involves the utilization of tempered martensite structure generated by strengthening heat treatment and tempering.
[0003] Traditionally, methods for heat treating materials in an equilibrium temperature range, preventing primary carbide formation, are primarily proposed to remove primary carbides generated in martensitic steel. That is, controlling coarse carbides aims for quality improvement from the perspective of mechanical properties, but a problem of inferior polishing efficiency exists.
[Disclosure]
[Technical Problem]
[0004] The purpose of the present disclosure is the solving of the aforementioned problem and the provision of a martensitic stainless steel having enhanced polishability by appropriately controlling the number of pores through an alloy composition and a manufacturing method, and the provision of a manufacturing method thereof.
[0005] The technical problems the present disclosure aims to solve are not limited to the technical problems mentioned above, and a person of ordinary skill in the art will clearly understand other unmentioned technical problems from the following description.
[Technical Solution]
[0006] A martensitic stainless steel of an example of the present disclosure comprises, in weight %, C: 0.40% to 0.55%, N: 0.01% to 0.05%, Si: 0.10% to 0.80%, Mn: 0.20% to 0.80%, Cr: 13.0% to 15.5%, Ni: 0.01% to 0.50%, and Ti: 0.001% to 0.020%, with the balance being Fe and unavoidable impurities, wherein the following Formula (1) is satisfied, and the number of pores inside a cold-rolled annealed material is in a range of 1 * 104 to 50 * 104 ea/mm2.
(wherein [Cr], [Si], [C], and [Ti] represent the content of each element)
[0007] Also, the martensitic stainless steel according to an example of the present disclosure may further comprise at least one of Mo: 0.01% to 0.80% and V: 0.01% to 0.20%.
[0008] Furthermore, cracks do not occur during polishing of the martensitic stainless steel according to an example of the present disclosure.
[0009] Further, in an example of the present disclosure, the polishing time of the martensitic stainless steel may be less than 60 seconds.
[0010] Furthermore, a thickness of the martensitic stainless steel according to an example of the present disclosure may be 0.5 mm to 4.0 mm.
[0011] A method for manufacturing a martensitic stainless steel according to an example of the present disclosure comprises the steps of: preparing a cast slab by casting and reducing a slab comprising, in weight %, C: 0.40% to 0.55%, N: 0.01% to 0.05%, Si: 0.10% to 0.80%, Mn: 0.20% to 0.80%, Cr: 13.0% to 15.5%, Ni: 0.01% to 0.50%, and Ti: 0.001% to 0.020%, with the balance being Fe and unavoidable impurities, and satisfying the following Formula (1); reheating the cast slab at 1200°C to 1300°C; hot-rolling the reheated cast slab at 1000°C to 1200°C; coiling the hot-rolled steel sheet at a temperature of 700°C or higher; performing hot-rolled annealing by maintaining the steel sheet at 800°C to 900°C for 3 hours to 10 hours and then maintaining it at 700°C to 790°C for 5 hours to 15 hours; and performing cold-rolled annealing at 800°C to 1000°C.
(wherein [Cr], [Si], [C], and [Ti] represent the content of each element)
[0012] Further, a method for manufacturing a martensitic stainless steel in an example of the present disclosure may further comprise the steps of hot-rolled pickling and cold-rolling after the hot-rolled annealing and before the cold-rolled annealing.
[0013] Further, in the method for manufacturing a martensitic stainless steel according to an example of the present disclosure, the cold-rolling is performed at a total reduction ratio of 40% to 80% for all passes.
[0014] Furthermore, in a manufacturing method for a martensitic stainless steel according to an example of the present disclosure, the cold-rolled annealing time may be 50 seconds to 80 seconds.
[0015] Furthermore, a method for manufacturing a martensitic stainless steel of an example of the present disclosure further comprises at least one of Mo: 0.01% to 0.80% and V: 0.01% to 0.20%.
[Advantageous Effects]
[0016] Control of the number of pores, generated inside a carbide and between the carbide and the matrix, within a cold-rolled annealed material of a martensitic stainless steel according to an example of the present disclosure, to a range of 1*104 to 50*104 ea/mm2, prevents cracks during polishing.
[Description of Drawings]
[0017] FIG. 1 is a photograph taken with a scanning electron microscope (SEM) showing the number of pores generated inside the carbide and between the carbide and the matrix of Invention Example 4, according to an example of the present disclosure.
[Modes of the Invention]
[0018] Hereinafter, preferred modes of the present disclosure are described. However, the modes of the present disclosure can be modified into various other forms, and the technical spirit of the present disclosure is not limited to the modes described hereinafter. Further, the modes of the present disclosure are provided for a more complete explanation of the present disclosure to those with ordinary knowledge in the pertinent technical field.
[0019] Terms used in the present disclosure are used solely for describing specific examples. Consequently, for example, a singular expression includes plural expressions unless the context unambiguously requires a singular meaning. Furthermore, it should be noted that terms such as "comprising" or "having" used in the present disclosure are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.
[0020] The absence of an alternative definition dictates that all terms used in the present disclosure are to be construed as having the same meaning as commonly understood by one of ordinary skill in the technical field of the present disclosure. Accordingly, absent a clear definition in the present disclosure, a specific term is not to be interpreted in an excessively idealized or formal sense. For example, a singular expression in the present disclosure includes a plural expression, absent a clear contextual exception.
[0021] Furthermore, in the present disclosure, terms such as 'about' and 'substantially' are used in the sense of the recited values or approximate values when manufacturing and material tolerances inherent to the recited meaning are presented, and are used to prevent unscrupulous infringers from unduly exploiting disclosures reciting exact or absolute numerical values to assist understanding of the present invention.
[0022] A martensitic stainless steel comprising, in weight %, C: 0.40% to 0.55%, N: 0.01% to 0.05%, Si: 0.10% to 0.80%, Mn: 0.20% to 0.80%, Cr: 13.0% to 15.5%, Ni: 0.01% to 0.50%, and Ti: 0.001% to 0.020%, with the balance being Fe and unavoidable impurities, wherein the following Formula (1) is satisfied, and the number of pores inside a cold-rolled annealed material is in a range of 1 * 104 to 50 * 104 ea/mm2.
(wherein [Cr], [Si], [C], and [Ti] represent the content of each element)
[0023] Hereinafter, the reasons for limiting the composition range of each alloying element are described. Hereinafter, the unit is wt% unless otherwise specified.
[0025] Carbon is an essential element for improving steel hardness, requiring appropriate addition to ensure hardness after quenching and tempering heat treatment. Considering this, to satisfy the uses of the present disclosure, the C content can be 0.40% or more. However, excessive content can reduce the toughness of the steel sheet. Considering this, the upper limit of the C content can be limited to 0.55%. Preferably, the C content can be 0.43% or more and 0.53% or less.
[0027] Similar to C, N is an element effective in improving the hardness of steel. Consideration of this allows N to be added at 0.01% or more. However, an excessive N content causes the formation of chromium nitrides, which are low-temperature precipitates, and induces residual γ-phase, thereby possibly leading to insufficient strength after strengthening heat treatment. Therefore, an excessive N content can degrade fatigue resistance. Consideration of this allows the upper limit of the N content to be restricted to 0.05%. Preferably, the N content can be 0.02% to 0.04%.
[0029] Si is added for deoxidation of the steel. Additionally, Si is an effective element for securing strength through solid solution strengthening. In consideration of this, Si can be added at 0.10% or more. However, excessive Si content can form scale on the steel surface during hot rolling, impairing surface quality. In consideration of this, an upper limit of the Si content can be limited to 0.80%. Preferably, the Si content can be 0.12% to 0.70% or less.
[0031] Manganese is a highly effective element for improving hardenability and achieving solid solution strengthening effects by forming substitutional solid solutions within the matrix structure. Furthermore, a low Mn content prevents sufficient combination with S(sulfur) introduced as an impurity into the steel, thereby potentially causing casting cracks. Considering this, Mn may be added at 0.20% or more. However, an excessive Mn content may degrade the toughness of the steel. Considering this, the upper limit of the Mn content may be limited to 0.80%. Preferably, the Mn content is 0.33% to 0.70%.
[0033] Cr is an effective element for improving corrosion resistance, forming chromium carbides, and improving hardness and wear resistance. Considering this, Cr can be added at 13.0% or more. However, an excessive Cr content can increase hardenability more than necessary and increase manufacturing cost. Considering this, the upper limit of the Cr content can be limited to 15.5%. Preferably, the Cr content can be 13.5% to 15.3% or less.
[0035] Ni is an essential element for martensitic stainless steel, with its addition transforming the metal structure into an austenitic structure in the hot working region. Additionally, Ni is an element having a role in improving corrosion resistance and hardenability upon trace addition. In consideration thereof, Ni can be added at 0.01% or more. However, an excessive Ni content can deteriorate formability. An excessive amount of retained austenite after strengthening heat treatment can make securing product hardness difficult, and manufacturing costs can also increase. In consideration thereof, the upper limit of the Ni content can be limited to 0.50%. Preferably, the Ni content can be 0.11% to 0.40%.
[0037] Traditionally, Ti is an element added in ferritic steels such as 409L and 439 steel. Even in the absence of a Ti component in the raw materials during martensitic steel manufacturing in the same steelmaking plant, Ti's introduction from ladles and molds enables its small presence. Ti can be included in an amount of 0.001% or more to prevent grain boundary corrosion. However, Ti's combination with C can form TiC precipitates, which can degrade the martensitic strength. Therefore, Ti is controlled to be 0.020% or less. Preferably, the Ti content can be 0.010% to less than 0.020%.
[0038] A martensitic stainless steel cast slab according to an example of the present disclosure may further comprise at least one of Mo: 0.01% to 0.8% and V: 0.01% to 0.2%.
[0040] Mo's effectiveness in improving corrosion resistance and hardenability permits its optional inclusion in the present disclosure. Additionally, Mo, in conjunction with V, functions as an element promoting carbide refinement and inhibiting carbide growth. Consideration of these factors allows for Mo addition at 0.01% or more. However, an excessive Mo content may increase manufacturing costs. Consideration of this factor limits the upper limit of the Mo content to 0.8%.
[0042] V, an element forming carbides and effectively suppressing chromium carbide coarsening, allows for optional further inclusion in the present disclosure. Furthermore, V is an effective element for preventing grain coarsening during heat treatment and improving wear resistance. However, an excessive V content can result in the formation of carbides beyond necessity, decreasing the toughness of the steel and increasing manufacturing costs. Consideration of this limits the upper limit of the V content to 0.2%.
[0044] The balance is Fe. However, the unavoidable incorporation of unintended impurities from raw materials or the surrounding environment during a conventional manufacturing process prevents exclusion of said impurities. The inherent knowledge of said impurities by those skilled in the art of a conventional manufacturing process obviates specific mention of all their contents in the present disclosure.
[0045] A martensitic stainless steel according to an example of the present disclosure satisfies the following Formula (1), and the number of pores inside a cold-rolled annealed material, the pores generated inside carbides and between the carbides and matrix, is in a range of 1*104 to 50*104 ea/mm2.
(wherein [Cr], [Si], [C], and [Ti] represent the content of each element)
[0046] Formula (1) is correlated with the number of pores inside a cold-rolled annealed material of martensitic steel. The number of pores refers to the number of pores occurring in carbides inside carbides and between the carbides and the matrix.
[0047] A Formula (1) value of less than 80 leads to the number of said pores being less than 1 * 104 ea, resulting in a significant decrease in polishability after hardening heat treatment. For Formula (1) exceeding 105, cracks are observed to occur during polishing after hardening heat treatment. Accordingly, the range of said Formula (1) is preferably 80 to 105. Cr and C facilitate carbide formation. Addition of Si hardens the matrix and can promote pore generation. Ti inhibits carbide formation composed of Cr and C, thereby consequently suppressing pore generation within the carbides or between the carbides and the matrix. Pores present within carbides or between carbides and the matrix facilitate polishing and can reduce polishing time. An excessive number of pores, exceeding 50 * 104 ea/mm2, can act as crack sources and generate cracks during polishing. The number of said pores can preferably be 2 * 104 to 48 * 104 ea/mm2.
[0048] Another example of the present disclosure provides a martensitic stainless steel wherein the value of Formula (1) is preferably 84 to 105, more preferably 90 to 105, and even more preferably 95 to 104. Within the said range, the non-occurrence of polishing cracks is ensured, and the effect of shortening polishing time is further enhanced. Such an approach enables the realization of further improved polishing performance along with enhanced effects such as improved durability.
[0049] A martensitic stainless steel of another example of the present disclosure may have the number of said pores preferably in a range of 5 * 104 to 50 * 104 ea/mm2, more preferably in a range of 10 * 104 to 50 * 104 ea/mm2, and even more preferably in a range of 20 * 104 to 50 * 104 ea/mm2. Within said range, the non-occurrence of polishing cracks is achieved, and the effect of reducing polishing time can be further enhanced. In this case, the achievement of further improved polishability while simultaneously enhancing durability, etc., is possible.
[0050] Further, cracks do not occur during polishing of the martensitic stainless steel according to an example of the present disclosure. That is, the martensitic stainless steel according to an example of the present disclosure has a crack non-occurrence characteristic.
[0051] Additionally, the martensitic stainless steel of an example of the present disclosure has a polishing time of less than 60 seconds. In such a case, the polishability is evaluated as excellent by conventional standards. Preferably, the polishing time is less than 50 seconds. More preferably, it is less than 30 seconds.
[0052] The said polishing conditions involve the cutting to a size of 100 mm x 100 mm, the performance of lapping polishing using a #240 roughness whetstone, and the measurement of time taken for polishing to a thickness of 0.5 mm. Non-occurrence of cracks and a polishing time of less than 60 seconds indicate excellent polishability, while a polishing time of 60 seconds or more indicates inferior polishability.
[0053] In an example of the present disclosure, the martensitic stainless steel has a thickness of 0.5 mm to 4.0 mm. The thickness represents a thickness measured after cold-rolled annealing. Hereinafter, a method for manufacturing martensitic stainless steel in an example of the present disclosure, wherein the steel has the alloy composition described above, is described. The description of the martensitic stainless steel at this time is as described above.
[0054] A method for manufacturing a martensitic stainless steel according to an example of the present disclosure, the method comprising the steps of: preparing a cast slab by casting and reducing a slab comprising, in weight %, C: 0.40% to 0.55%, N: 0.01% to 0.05%, Si: 0.10% to 0.80%, Mn: 0.20% to 0.80%, Cr: 13.0% to 15.5%, Ni: 0.01% to 0.50%, and Ti: 0.001% to 0.020%, with the balance being Fe and unavoidable impurities, and satisfying the following Formula (1); reheating the cast slab at 1200°C to 1300°C; hot-rolling the reheated cast slab at 1000°C to 1200°C; coiling the hot-rolled steel sheet at a temperature of 700°C or higher; performing hot-rolled annealing by maintaining the steel sheet at 800°C to 900°C for 3 hours to 10 hours and then maintaining it at 700°C to 790°C for 5 hours to 15 hours; and performing cold-rolled annealing at 800°C to 1000°C.
(wherein [Cr], [Si], [C], and [Ti] represent the content of each element)
[0055] According to an example of the method for manufacturing a martensitic stainless steel of the present disclosure, hot-rolling of a cast slab having a thickness of 250 mm to 320 mm at a reduction ratio of 90% to 98% secures a thickness of the martensitic stainless steel in a range of 0.5 mm to 4.0 mm.
[0056] A method for manufacturing a martensitic stainless steel of the present disclosure includes a coiling step after hot-rolling and a hot-rolled annealing step. Performance of the coiling step is at a temperature of 700°C or higher. The hot-rolled annealing step includes charging into a hot-rolled annealing furnace at a temperature of 600°C or higher, followed by maintaining at 800°C to 900°C for 3 hours to 10 hours, and then maintaining at 700°C to 790°C for 5 hours to 15 hours to perform hot-rolled annealing.
[0057] A cold-rolled annealing temperature less than 800°C or a cold-rolled annealing time less than 50 seconds results in a slow recrystallization growth rate, an excessively high hardness, and low mechanical properties. Additionally, the number of pores generated within carbides and between carbides and the matrix inside the annealed material may be greater than 50 * 104 ea/mm2. A cold-rolled annealing temperature greater than 1000°C or a cold-rolled annealing time greater than 80 seconds results in the formation of non-uniform grains due to an increase in grain size, difficulty in ensuring strength, and the number of pores may also be less than 1 * 104 ea/mm2.
[0058] Additionally, a method for manufacturing a martensitic stainless steel according to an example of the present disclosure may further comprise the steps of hot-rolled pickling and cold-rolling after the hot-rolled annealing and before the cold-rolled annealing.
[0059] In addition, in the method for manufacturing martensitic stainless steel according to an example of the present disclosure, the cold rolling may be performed at a total reduction ratio of 40 to 80% over all passes.
[0060] Furthermore, in the method for manufacturing a martensitic stainless steel according to an example of the present disclosure, the cold-rolled annealing time may be 50 seconds to 80 seconds.
[0061] Furthermore, a method for manufacturing a martensitic stainless steel according to an example of the present disclosure may further comprise at least one of Mo: 0.01% to 0.80% and V: 0.01% to 0.20%.
{example}
[0062] A 200 mm thick slab having an alloy composition according to the following Table 1 was subjected to a reduction of 0% to 6% using an inline roller before complete solidification of the cast slab. The cast slab was reheated at 1250°C for approximately 3 hours, hot-rolled to a final thickness of 5 mm at a temperature of 1050°C, and the manufactured hot-rolled steel sheet was coiled at 700°C. The coiled material was charged into a hot-rolled annealing furnace at 600°C for hot-rolled annealing, maintained at 850°C for 10 hours, then maintained at 750°C for 10 hours, to perform hot-rolled annealing. Thereafter, pickling was performed, and cold-rolling was performed from 5 mm to 2.5 mm with a total reduction ratio of 50%, and the sum of the reduction ratios of the first pass and the second pass was 15%. Thereafter, cold-rolled annealing heat treatment was performed at 900°C for 60 seconds to prepare specimens for comparative examples and inventive examples described below.
[TABLE 1]
| Classification | C | Si | Mn | Cr | Ni | Ti | N | Mo | V |
| Comparative Example 1 | 0.78 | 0.51 | 0.54 | 15.4 | 0.28 | 0.010 | 0.03 | - | - |
| Comparative Example 2 | 0.54 | 0.79 | 0.33 | 17.4 | 0.22 | 0.010 | 0.02 | - | - |
| Comparative Example 3 | 0.61 | 0.59 | 0.31 | 14.3 | 0.25 | 0.010 | 0.04 | - | - |
| Comparative Example 4 | 0.55 | 0.81 | 0.44 | 15.5 | 0.27 | 0.010 | 0.03 | - | - |
| Comparative Example 5 | 0.43 | 0.20 | 0.40 | 13.5 | 0.15 | 0.020 | 0.02 | - | - |
| Comparative Example 6 | 0.42 | 0.21 | 0.55 | 12.4 | 0.16 | 0.010 | 0.03 | - | - |
| Comparative Example 7 | 0.40 | 0.11 | 0.38 | 13.5 | 0.16 | 0.010 | 0.04 | - | - |
| Comparative Example 8 | 0.30 | 0.23 | 0.47 | 13.8 | 0.26 | 0.010 | 0.03 | - | - |
| Inventive Example 1 | 0.53 | 0.53 | 0.33 | 15.3 | 0.19 | 0.010 | 0.02 | - | - |
| Inventive Example 2 | 0.47 | 0.55 | 0.39 | 15.0 | 0.23 | 0.010 | 0.04 | - | - |
| Inventive Example 3 | 0.49 | 0.41 | 0.41 | 14.8 | 0.21 | 0.010 | 0.04 | 0.65 | 0.15 |
| Inventive Example 4 | 0.44 | 0.22 | 0.40 | 13.5 | 0.11 | 0.010 | 0.03 | - | - |
| Inventive Example 5 | 0.43 | 0.12 | 0.33 | 13.6 | 0.18 | 0.010 | 0.03 | - | - |
[Material Property Evaluation]
Measurement of the number of pores
[0063] For the previously manufactured specimens, Table 2 shows the results of measuring the number of pores generated inside the carbides and between the carbides and the matrix, by observing the cross-section of the cold-rolled annealed material with a scanning electron microscope (SEM).
Evaluation of polishability and crack occurrence
[0064] For the cold-rolled annealed materials among the previously manufactured specimens, a strengthening heat treatment was performed by maintaining at 1050 °C for 300 seconds and then rapidly cooling. The strengthening heat-treated stainless steel was cut into a size of 100 mm * 100 mm, and lapping polishing was performed using a grinding stone with #240 roughness, and the time for polishing to a thickness of 0.5 mm was measured.
[0065] Inferior polishability (X) was evaluated for a polishing time of 90 seconds or more, or for measurement failure due to crack occurrence. Good polishability (O) was evaluated for a polishing time of 60 seconds or more to less than 90 seconds. Excellent polishability (⊚) was evaluated for a polishing time of less than 60 seconds.
[0066] Evaluation was stopped upon crack occurrence during polishing. The crack occurrence is shown in Table 2 below.
[Table 2]
| Classification | Formula (1) | Number of Carbide Pores (104/mm2) | Polishing Time (sec) | Polishing Property Evaluation | Polishing Crack Occurrence Evaluation |
| Comparative Example 1 | 128.9 | 93 | Measurement Failed | × | Crack Occurred |
| Comparative Example 2 | 115.6 | 84 | Measurement Failed | × | Crack Occurred |
| Comparative Example 3 | 109.9 | 72 | Measurement Failed | × | Crack Occurred |
| Comparative Example 4 | 111.3 | 56 | Measurement Failed | × | Crack Occurred |
| Comparative Example 5 | 78.9 | 0.8 | 90 | × | No Crack |
| Comparative Example 6 | 78.8 | 0.6 | 100 | × | No Crack |
| Comparative Example 7 | 78.4 | 0.3 | 120 | × | No Crack |
| Comparative Example 8 | 71.3 | 0.1 | 160 | × | No Crack |
| Inventive Example 1 | 103.9 | 48 | 32 | ⊚ | No Crack |
| Inventive Example 2 | 97.4 | 34 | 37 | ⊚ | No Crack |
| Inventive Example 3 | 96.4 | 29 | 42 | ⊚ | No Crack |
| Inventive Example 4 | 84.2 | 6 | 47 | ⊚ | No Crack |
| Inventive Example 5 | 81.8 | 2 | 56 | ⊚ | No Crack |
[0067] Referring to said Table 2, Invention Examples 1 to 5 according to the present disclosure satisfy the following Formula (1): Formula (1): 80 ≤ 3[Cr] + 17[Si] + 100[C] - 400[Ti] ≤ 105
[0068] Through the satisfaction of the number of pores inside the cold-rolled annealed material within a range of 1 * 104 to 50 * 104 ea/mm2, a dramatic shortening of the polishing time to less than 90 seconds, particularly less than 60 seconds, and simultaneous suppression of crack occurrence during polishing were confirmed.
[0069] Specifically, FIG. 1 is a photograph taken by a scanning electron microscope (SEM) showing the number of pores formed within the carbides and between the carbides and the matrix of example 4 according to the present disclosure. The width is 24 µm and the height is 18 µm, resulting in an area of 432 µm2. Since the number of pores is 26, the number of pores per unit area is confirmed to be 6 * 104 ea/mm2. Accordingly, cracks do not occur during polishing, and the polishing time is also 47 seconds, indicating excellent polishability.
[0070] On the other hand, in Comparative Examples 1 to 4, Formula (1) being greater than 105 led to the number of carbide pores being greater than 50 * 104 ea/mm2, with the occurrence of cracks during polishing confirmed.
[0071] Furthermore, in Comparative Examples 5 to 8, the value of Formula (1) according to the present disclosure being less than 80, the number of carbide pores is controlled to be less than 1 * 104 ea/mm2. This control results in the carbide pore count being less than 1×104 ea/mm2, thereby greatly extending the polishing time, thereby confirming the inferiority of the polishability.
[0072] In particular, Comparative Examples 5 and 7, despite satisfying the range of alloy composition according to the present disclosure, demonstrated inferior polishability depending on the number of pores in the carbides due to not satisfying Formula (1).
[0073] Furthermore, the final stainless steel is manufactured based on the composition and manufacturing method of said example 1, applying the cold-rolled annealing temperature and time specified in the following Table 3. The number of pores (unit: 1 * 104 ea/mm2) in this final stainless steel, its polishing performance evaluation according to polishing time, and its crack occurrence during polishing are shown in the following Table 3.
[Table 3]
| Cold-Rolled Annealing Temperature (°C) | Cold-Rolled Annealing Time (sec) | Number of Carbide Pores (104/mm2) | Polishing Time (sec) | Polishing Property Evaluation | Polishing Crack Occurrence Evaluation | |
| Experimental Example 1 | 920 | 55 | 44 | 38 | ⊚ | No Crack |
| Experimental Example 2 | 950 | 62 | 46 | 36 | ⊚ | No Crack |
| Experimental Example 3 | 980 | 70 | 48 | 35 | ⊚ | No Crack |
| Experimental Example 4 | 780 | 55 | 62 | Measurement Failed | × | Crack Occurred |
| Experimental Example 5 | 760 | 62 | 65 | Measurement Failed | × | Crack Occurred |
| Experimental Example 6 | 740 | 58 | 72 | Measurement Failed | × | Crack Occurred |
| Experimental Example 7 | 1050 | 55 | 0.6 | 113 | × | No Crack |
| Experimental Example 8 | 1100 | 55 | 0.7 | 108 | × | No Crack |
| Experimental Example 9 | 950 | 45 | 65 | Measurement Failed | × | Crack Occurred |
| Experimental Example 10 | 940 | 38 | 70 | Measurement Failed | × | Crack Occurred |
| Experimental Example 11 | 980 | 92 | 0.4 | 130 | × | No Crack |
| Experimental Example 12 | 820 | 110 | 0.8 | 142 | × | No Crack |
[0074] Experiment Examples 1 to 3 satisfied a cold-rolled annealing temperature of 800°C to 1000°C and a cold-rolled annealing time of 50 seconds to 80 seconds in accordance with the present disclosure. It was confirmed that the number of pores generated inside carbides and between carbides and the matrix within the cold-rolled annealed material was in a range of 1 * 104 to 50 * 104 ea/mm2. On the other hand, a cold-rolled annealing temperature less than 800°C or a cold-rolled annealing time less than 50 seconds in Experiment Examples 4 to 6 or Experiment Examples 9 and 10 resulted in the number of carbide pores exceeding 50 * 104 ea/mm2, causing cracks during polishing.
[0075] Furthermore, experimental examples 7 and 8 or experimental examples 11 and 12 confirm inferior polishability, a cold-rolled annealing temperature greater than 1000°C or a cold-rolled annealing time greater than 80 seconds resulting in a carbide pore count less than 1 * 104 ea/mm2 and a polishing time greater than 100 seconds.
1. A martensitic stainless steel comprising, in weight %, C: 0.40% to 0.55%, N: 0.01%
to 0.05%, Si: 0.10% to 0.80%, Mn: 0.20% to 0.80%, Cr: 13.0% to 15.5%, Ni: 0.01% to
0.50%, and Ti: 0.001% to 0.020%, with the balance being Fe and unavoidable impurities,
wherein the following Formula (1) is satisfied, and
the number of pores inside a cold-rolled annealed material is in a range of 1 * 104 to 50 * 104 ea/mm2:
(wherein [Cr], [Si], [C], and [Ti] represent the content of each element)
2. The martensitic stainless steel of claim 1, further comprising at least one of Mo:
0.01% to 0.80% and V: 0.01% to 0.20%.
3. The martensitic stainless steel of claim 1, wherein cracks do not occur during polishing.
4. The martensitic stainless steel of Claim 3, wherein the polishing time is less than
60 seconds.
5. The martensitic stainless steel of Claim 3, wherein a thickness of the martensitic
stainless steel is 0.5 mm to 4.0 mm.
6. A method for manufacturing a martensitic stainless steel, the method comprising the
steps of:
preparing a cast slab by casting and reducing a slab comprising, in weight %, C: 0.40 to 0.55%, N: 0.01 to 0.05%, Si: 0.1 to 0.8%, Mn: 0.20 to 0.80%, Cr: 13.0 to 15.5%, Ni: 0.01 to 0.50%, and Ti: 0.001 to 0.020%, with the balance being Fe and unavoidable impurities, and satisfying the following Formula (1);
reheating the cast slab at 1200°C to 1300°C;
hot-rolling the reheated cast slab at 1000°C to 1200°C;
coiling the hot-rolled steel sheet at a temperature of 700°C or higher;
performing hot-rolled annealing by maintaining the steel sheet at 800°C to 900°C for 3 hours to 10 hours and then maintaining it at 700°C to 790°C for 5 hours to 15 hours; and
performing cold-rolled annealing at 800°C to 1000°C;
(wherein [Cr], [Si], [C], and [Ti] represent the content of each element)
7. The method of Claim 6, further comprising the steps of hot-rolled pickling and cold-rolling
after the hot-rolled annealing and before the cold-rolled annealing.
8. The method of Claim 7, wherein the cold-rolling is performed at a total reduction
ratio of 40% to 80% for all passes.
9. The method of Claim 6, wherein a cold-rolled annealing time is 50 seconds to 80 seconds.
10. The method of Claim 6, wherein the martensitic stainless steel further comprises at
least one of Mo: 0.01% to 0.80% and V: 0.01% to 0.20%.