Martensitic stainless steel and manufacturing method therefor

Abstract

A method for manufacturing a high-carbon martensitic stainless steel, according to one embodiment of the present invention, comprises the steps of: preparing a steel comprising, by wt%, 0.35-0.55% of C, 0.15-0.50% of Si, 0.40-0.60% of Mn, 13.50-15.00% of Cr, 0.30-0.50% of Ni, 0.95-1.10% of Mo, 0.001-0.500% of Cu, 0.010-0.025% of N, and the balance of Fe and inevitable impurities; reheating the steel to 1,200-1,300 °C; hot rolling and hot annealing the reheated steel; cold rolling and cold annealing same so as to manufacture a cold-rolled material; austenitizing the cold-rolled material; quenching same at a temperature of 270-400 °C for 10 seconds or less; and, after the quenching, tempering same at a temperature of 360-465 °C for 0.5-1 minutes, wherein the value of formula (1) is 35,000-331,000.

Description

Martensitic stainless steel and method of manufacturing the same

[0001] The present invention relates to a high-strength, high-ductility martensitic stainless steel and a method for manufacturing the same.

[0002] Martensitic stainless steel is essentially steel composed primarily of iron (Fe), chromium (Cr), and carbon (C), containing approximately 12 to 18% chromium by weight and up to about 1% or more carbon. Therefore, while the martensitic structure provides very high strength, its elongation and impact toughness are very low, making it prone to fracture during processing or use. Consequently, the microstructure is controlled to consist of ferrite and carbides during intermediate manufacturing stages. For this reason, when producing hot-rolled or cold-rolled martensitic stainless steel products, a soft microstructure composed of ferrite and chromium carbides is secured through a hot annealing process, followed by pickling and cold rolling. Subsequently, the product is manufactured by transforming the soft material into a high-strength martensitic structure using a strengthening heat treatment process.

[0003] Martensitic stainless steel containing 0.35 to 0.55% carbon in weight percent as mentioned in the present invention is used for various purposes, including general knives such as utility knives, as well as for home appliances and industrial applications. In particular, it is used for shock absorber shims that absorb shocks in home appliances and industrial compressor valves, automobiles, etc., which require impact characteristics and wear resistance characteristics such as having a tensile strength of 1700 MPa or more and an elongation of 7% or more after heat treatment for strengthening, and doctor blades that remove foreign substances adhering to various roll surfaces.

[0004] This strengthening heat treatment proceeds in the order of an austenitization process, which is a heat treatment process that redissolves chromium carbides into the matrix structure and transforms the matrix structure from ferrite to austenite; a quenching process, which rapidly cools from a high temperature to room temperature or 450°C to transform the austenite structure into high-strength martensite; and a tempering process, which imparts toughness to the brittle martensite structure.

[0005] These martensitic stainless steels require high strength depending on the usage environment where a long lifespan is required; however, generally, metal materials exhibit a phenomenon where elongation decreases as strength increases.

[0006] In particular, in the case of high-carbon martensitic stainless steel, even if a tempering process is performed to increase elongation and toughness after an austenitization and quenching process to secure high strength, the material has low elongation and high brittleness due to the high carbon content in the material, which leads to a problem of shortened lifespan caused by sudden fracture during use. Therefore, it is necessary to secure material properties such as toughness and impact characteristics by increasing strength and simultaneously increasing elongation.

[0007] The objective of the present invention to solve the aforementioned problem is to provide a high-carbon martensitic stainless steel and a method for manufacturing the same, which has a high tensile strength of 1700 MPa or more and secures an elongation of 7.0% or more by controlling the strengthening heat treatment conditions of the martensitic stainless steel.

[0008] However, the problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art from the description below.

[0009] As a means to achieve the above-mentioned purpose, a high-carbon martensitic stainless steel according to one embodiment of the present invention comprises, in weight percent, C: 0.35 to 0.55%, Si: 0.15 to 0.50%, Mn: 0.40 to 0.60%, Cr: 13.50 to 15.00%, Ni: 0.30 to 0.50%, Mo: 0.95 to 1.10%, Cu: 0.001 to 0.500%, N: 0.010 to 0.025%, and the remainder being Fe and unavoidable impurities, and comprises, in a microstructure, martensite and retained austenite formed within the martensite, wherein the retained austenite may comprise 0.4 to 1.5% in area fraction.

[0010] In a high-carbon martensitic stainless steel according to one embodiment of the present invention, the tensile strength of the martensitic stainless steel may be 1700 MPa or higher.

[0011] In a high-carbon martensitic stainless steel according to one embodiment of the present invention, the elongation of the martensitic stainless steel may be 7.0% or more.

[0012] In a high-carbon martensitic stainless steel according to one embodiment of the present invention, the yield strength of the martensitic stainless steel may be 1390 MPa or higher.

[0013] A method for manufacturing high-carbon martensitic stainless steel according to one embodiment of the present invention comprises the steps of: preparing a steel material comprising, in weight percent, C: 0.35 to 0.55%, Si: 0.15 to 0.50%, Mn: 0.40 to 0.60%, Cr: 13.50 to 15.00%, Ni: 0.30 to 0.50%, Mo: 0.95 to 1.10%, Cu: 0.001 to 0.500%, N: 0.010 to 0.025%, and the remainder being Fe and unavoidable impurities; reheating the steel material to 1200 to 1300°C; hot rolling and hot rolling annealing the reheated steel material; subsequently cold rolling and cold rolling annealing to produce a cold-rolled material; and austenitizing the cold-rolled material. Subsequently, the method includes a step of quenching at a temperature of 270 to 400°C for 10 seconds or less; and a step of tempering at a temperature of 360 to 465°C for 0.5 to 1 minute after quenching, wherein Equation (1) may be 35,000 to 331,000.

[0014] Equation (1): ATTR = AT × T × RA

[0015] In the above equation (1), ATTR (Austenitizing Temperature, Time, Retained Austenite Factor) is a high strength and high ductility prediction parameter, AT is the austenitizing temperature (°C), T is the holding time of the austenitizing temperature (sec), and RA is the content of retained austenite (%) in the martensite structure formed through austenitizing and quenching.

[0016] In a method for manufacturing high-carbon martensitic stainless steel according to one embodiment of the present invention, the steps of hot rolling and hot rolling annealing may include: a step of coiling a hot-rolled steel sheet at a temperature of 700°C or higher; a step of maintaining at 800 to 900°C for 3 hours or more, followed by a second step of hot rolling annealing at 700 to 790°C for 5 to 15 hours; and a step of pickling to remove a surface oxide layer of the hot-rolled steel.

[0017] In a method for manufacturing high-carbon martensitic stainless steel according to one embodiment of the present invention, the thickness of the cold-rolled material may be 0.1 to 0.5 mm, and the cold-rolled material may be cold-rolled and cold-rolled annealed.

[0018] In a method for manufacturing high-carbon martensitic stainless steel according to one embodiment of the present invention, the cold-rolled material may include ferrite and carbides in its microstructure.

[0019] In a method for manufacturing high-carbon martensitic stainless steel according to one embodiment of the present invention, in the austenitizing step and the quenching step, the residual austenite microstructure can be controlled to an area fraction of 0.4 to 1.5%.

[0020] In the austenitizing step of the method for manufacturing high-carbon martensitic stainless steel according to one embodiment of the present invention, the austenitizing temperature may be 1000 to 1015℃, and the holding time of the austenitizing temperature may be 172 seconds or more.

[0021] In the austenitizing step of the method for manufacturing high-carbon martensitic stainless steel according to one embodiment of the present invention, the austenitizing temperature may be 1045 to 1055℃, and the holding time of the austenitizing temperature may be less than 250 seconds.

[0022] According to one embodiment of the present invention, by controlling the strengthening heat treatment conditions and Equation (1), a high-carbon martensitic stainless steel having a tensile strength of 1700 MPa or more and an elongation of 7.0% or more and a method for manufacturing the same can be provided.

[0023] The effects obtainable from this invention are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art to which this invention pertains from the description below.

[0024] Figure 1 is a graph showing the correlation between the ATTR factor of Equation (1) and the elongation rate.

[0025] Figure 2 is a photograph of the microstructure of Comparative Example 3 after heat treatment taken with an Electron Back Scatter Diffraction (EBSD) analyzer.

[0026] Figure 3 is a photograph of the microstructure of Example 12 after heat treatment taken with an Electron Back Scatter Diffraction (EBSD) analyzer.

[0027] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0028] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application 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 existence of other features, steps, functions, components, or combinations thereof.

[0029] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense.

[0030] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0031] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight. The reasons for limiting the compositional range of each alloying element are explained below.

[0032] A martensitic stainless steel according to one embodiment of the present invention may comprise, in weight percent, C: 0.35 to 0.55%, Si: 0.15 to 0.50%, Mn: 0.40 to 0.60%, Cr: 13.50 to 15.00%, Ni: 0.30 to 0.50%, Mo: 0.95 to 1.10%, Cu: 0.001 to 0.500%, N: 0.010 to 0.025%, and the remainder being Fe and unavoidable impurities.

[0033] The content of C (carbon) can be 0.35 to 0.55%.

[0034] C is an essential element for ensuring the hardness of martensitic stainless steel, and is added in an amount of 0.35% or more to ensure hardness after quenching / tempering heat treatment. However, if the content is excessive, chromium carbides are formed excessively, which not only lowers the corrosion resistance of the material itself but also raises concerns about a decrease in toughness due to the residue of coarse carbides; therefore, the upper limit may be restricted to 0.55%.

[0035] The content of Si (silicon) is 0.15 to 0.50%.

[0036] Si is an essential element added for deoxidation and plays a role in improving strength, so in the present invention, it is added in an amount of 0.15% or more. However, if the content is excessive, it forms scale on the surface of the steel sheet during hot rolling, which impairs surface quality, so the upper limit may be limited to 0.50%.

[0037] The content of Mn (manganese) is 0.40 to 0.60%.

[0038] Mn is an element added to improve strength and hardenability. It plays a role in suppressing cracks caused by sulfur (S) by combining with sulfur (S), which is inevitably contained during the manufacturing process, to form MnS. In the present invention, it is added at a level of 0.40% or more. However, if the content is excessive, it may impair the surface quality and toughness of the steel, so the upper limit may be limited to 0.60%.

[0039] The content of Cr (chromium) can be 13.50% to 15.00%.

[0040] Cr is a basic element for ensuring corrosion resistance and plays a role in improving hardness and wear resistance by forming chromium carbides; therefore, in the present invention, it is added in an amount of 13.50% or more. However, if the content is excessive, manufacturing costs increase and hinder the formation of residual austenite to ensure elongation, so the upper limit may be limited to 15.00%.

[0041] The content of Ni (nickel) is 0.30 to 0.50%.

[0042] Ni is an essential element added to secure an austenite structure in the hot working region of martensitic stainless steel, and plays a role in improving corrosion resistance and hardenability and controlling the residual austenite content; therefore, in the present invention, it is added in an amount of 0.30% or more. However, if the content is excessive, there are problems such as increased manufacturing costs, reduced workability, and decreased strength, so the upper limit may be limited to 0.50%.

[0043] The content of Mo (molybdenum) is 0.95 to 1.10%.

[0044] Mo is an element that improves corrosion resistance, suppresses decarburization, and improves hardenability. It plays a role in refining carbides by replacing Cr in chromium carbides, and in the present invention, it is added in an amount of 0.95% or more. However, if the content is excessive, manufacturing costs increase, and the upper limit may be limited to 1.10% to increase hardenability.

[0045] The content of Cu (copper) is 0.001 to 0.500%.

[0046] Cu is an austenite-forming element that improves strength, hardness, and corrosion resistance, so in the present invention, it is added in an amount of 0.001% or more. However, if the content is excessive, manufacturing costs increase, hot workability decreases, and there are problems such as forming precipitates like CuS that are harmful to corrosion resistance by reacting with S, so the upper limit may be limited to 0.500%.

[0047] The content of N (nitrogen) is 0.010 to 0.025%.

[0048] N is an element added to simultaneously improve corrosion resistance and hardness, and even when added instead of C, it does not cause local micro-segregation, which has the advantage of not forming coarse precipitates in the product. To achieve this effect, the present invention adds N at a level of 0.010% or more. However, if the content is excessive, there is a problem of forming low-temperature precipitates such as Cr nitride and excessive residual austenite phases, so the upper limit may be limited to 0.025% to ensure fatigue properties.

[0049] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0050] In addition, a martensitic stainless steel according to one embodiment of the present invention may include martensite and retained austenite formed within the martensite as a microstructure.

[0051] The above residual austenite may contain 0.4 to 1.5% in area fraction.

[0052] The tensile strength of the martensitic stainless steel according to one embodiment of the present invention may be 1700 MPa or higher.

[0053] The elongation of the martensitic stainless steel according to one embodiment of the present invention may be 7.0% or more.

[0054] The yield strength of the martensitic stainless steel according to one embodiment of the present invention may be 1390 MPa or higher.

[0055] Hereinafter, a method for manufacturing a high-carbon martensitic stainless steel according to one embodiment of the present invention having the alloy composition described above will be explained.

[0056] A method for manufacturing a martensitic stainless steel according to one embodiment of the present invention comprises the steps of: preparing a steel material comprising, in weight percent, C: 0.35 to 0.55%, Si: 0.15 to 0.50%, Mn: 0.40 to 0.60%, Cr: 13.50 to 15.00%, Ni: 0.30 to 0.50%, Mo: 0.95 to 1.10%, Cu: 0.001 to 0.500%, N: 0.010 to 0.025%, and the remainder being Fe and unavoidable impurities; reheating the steel material to 1200 to 1300°C; hot rolling and hot rolling annealing the reheated steel material; subsequently cold rolling and cold rolling annealing to produce a cold-rolled material; and austenitizing the cold-rolled material. Subsequently, a step of quenching at a temperature of 270 to 400°C for 10 seconds or less; and a step of tempering at a temperature of 360 to 465°C for 0.5 to 1 minute after quenching, wherein Equation (1) is 35,000 or more and 331,000 or less.

[0057] Equation (1): ATTR = AT × T × RA

[0058] In the above equation (1), ATTR (Austenitizing Temperature, Time, Retained austenite Factor) is a high strength and high ductility prediction parameter, AT is the austenitizing temperature (°C), T is the holding time of the austenitizing temperature (sec), and RA is the content of retained austenite (%) in the martensite structure formed through austenitizing and quenching.

[0059] The reason for limiting the numerical ranges of the components of each alloy composition above is as described above, and each manufacturing step is explained in more detail below.

[0060] First, after preparing a steel material that satisfies the above alloy composition, it may undergo a series of processes including reheating, hot rolling, hot rolling annealing, cold rolling, cold rolling annealing, austenitizing, quenching, and tempering.

[0061] If the reheating temperature is low, it may be difficult to decompose coarse precipitates generated during steel manufacturing. Considering this, the reheating temperature may be 1200°C or higher. However, if the reheating temperature is excessively high, the internal grains may become too coarse, and severe surface oxidation may occur, causing surface defects. Considering this, the upper limit of the reheating temperature may be restricted to 1300°C or lower.

[0062] Next, the reheated steel can be hot-rolled and hot-rolled annealed.

[0063] The above steps of hot rolling and hot rolling annealing may include: a step of coiling the hot-rolled steel sheet at a temperature of 700°C or higher; a step of maintaining at 800 to 900°C for 3 hours or more, followed by a second step of hot rolling annealing at 700 to 790°C for 5 to 15 hours; and a step of pickling to remove the surface oxide layer of the hot-rolled steel. After the hot rolling annealing, cold rolling and cold rolling annealing may be performed to produce a cold-rolled material with a thickness of 0.1 to 0.5 mm.

[0064] When the thickness of the cold-rolled annealed stainless steel is 0.1 mm to 0.5 mm, it is easy to control the degree of decomposition of carbides during the time held at the austenitizing temperature, and a martensite structure can be obtained to obtain a desired tensile strength of 1700 MPa or more by quenching.

[0065] On the other hand, when the thickness of cold-rolled annealed stainless steel is less than 0.1 mm, the thin thickness allows for a faster cooling rate, which can result in high strength after quenching, but it is difficult to secure impact toughness because the area fraction of the retained austenite structure is low. Also, when the thickness exceeds 0.5 mm, the cooling rate slows down during quenching, which can lead to the precipitation of fine carbides and a decrease in strength.

[0066] The above stainless steel can be austenitized at an austenitizing temperature and an austenitizing temperature holding time that satisfy 35,000 to 331,000 of the formula (1).

[0067] Equation (1): ATTR = AT × T × RA

[0068] In the above equation (1), ATTR (Austenitizing Temperature, Time, Retained Austenite Factor) is a high strength and high ductility prediction parameter, AT is the austenitizing temperature (°C), T is the holding time of the austenitizing temperature (sec), and RA is the content of retained austenite (%) in the martensite structure formed through austenitizing and quenching.

[0069] When Equation (1) satisfies 35,000 to 331,000, chromium carbides are sufficiently re-dissolved into the matrix structure in the form of chromium and carbon, thereby increasing the strength of the martensitic stainless steel after the subsequent quenching step. In addition, the residual austenite microstructure is controlled to an area fraction of 0.4 to 1.5% or less, thereby securing the desired elongation of the present invention. When Equation (1) is satisfied, high strength of 1700 MPa or more can be secured, and an elongation of 7.0% or more can be secured. If the elongation is 7.0% or more, impact resistance is improved, which can suppress brittle fracture of the material, thereby extending the life of the material.

[0070] After the above austenitization, it can be quenched at a temperature of 270 to 400°C for 10 seconds or less.

[0071] When the quenching temperature is 270 to 400°C, thermal shock can be prevented, thereby preventing damage to the material.

[0072] After the above quenching, it can be tempered at a temperature of 360 to 465°C for 0.5 to 1 minute.

[0073] The 0.1 to 0.5 mm stainless steel of the present invention is composed of a martensite structure, a retained austenite structure, and carbides that remain without decomposition after quenching. Since the quenched stainless steel having the above structure has high tensile strength but low elongation and is highly brittle, resulting in low impact toughness, a tempering heat treatment process is performed at a temperature of 360 to 465°C for 0.5 to 1 minute to secure this. When tempering is performed under the above conditions, some of the supersaturated carbon in the martensite structure formed after quenching is released to form fine carbides, which then diffuse into the retained austenite structure, thereby increasing the stabilization of the retained austenite structure and enabling the material to increase its elongation when deformed by external forces.

[0074] If the tempering temperature exceeds 465℃ or the tempering time exceeds 1 minute, the dissolved carbon in the martensite matrix formed during the austenitizing heat treatment is excessively released, forming a high fraction of carbides, which may result in increased elongation but inferior tensile strength.

[0075] In addition, if the tempering temperature is less than 360℃ or the tempering time is less than 0.5 minutes, the brittleness of the material may be inferior due to insufficient diffusion of carbon in the martensite structure mentioned above.

[0076] In the method for manufacturing martensitic stainless steel according to one embodiment of the present invention, the cold-rolled annealed stainless steel may include ferrite and carbides in its microstructure.

[0077] In the austenitizing step and the quenching step of the method for manufacturing martensitic stainless steel according to one embodiment of the present invention, the residual austenite microstructure can be controlled to an area fraction of 0.4 to 1.5%.

[0078] In the austenitizing step of the method for manufacturing martensitic stainless steel according to one embodiment of the present invention, the austenitizing temperature may be 1000 to 1015℃, and the holding time of the austenitizing temperature may be 172 seconds or more.

[0079] In the austenitizing step of the method for manufacturing martensitic stainless steel according to one embodiment of the present invention, the holding time of the austenitizing temperature of 1045 to 1055℃ may be less than 250 seconds.

[0080] Hereinafter, the structure and operation of the present invention will be explained in more detail through preferred embodiments of the present invention. However, these are presented as preferred examples of the present invention and should not be interpreted in any way as limiting the present invention.

[0081] {Example}

[0082] For the various alloy composition ranges shown in Table 1 below, slabs were manufactured through melting and casting to prepare cast materials.

[0083] After that, the above-mentioned casting material was reheated at a temperature of 1250°C and then hot-rolled to produce a hot-rolled material. Next, the above-mentioned hot-rolled material was subjected to hot-rolled annealing at a temperature of 850°C for 10 hours, followed by pickling to remove the oxide film, and then cold-rolled and cold-rolled annealing to produce a cold-rolled stainless steel.

[0084] CSiMnCrNiMoCuN Content (Weight%) 0.39 0.32 0.55 13.65 0.35 1.03 0.03 0.025

[0085] The above-mentioned stainless steel produced by cold rolling was shown in the strengthening heat treatment conditions of Table 2 below, including the austenitization temperature (AT: Austenitization Temperature, °C), the holding time at the austenitization temperature (T: Time, seconds), the retained austenite (RA: Retained Austenite, %) in the martensite structure formed through austenitization and quenching, and the ATTR factor value of Equation (1) expressed as the product of these. In addition, the values ​​of the yield strength, tensile strength, and elongation of the stainless steel according to the changes in these strengthening heat treatment conditions were shown. The austenitization process was performed at temperatures of 980°C, 1010°C, and 1050°C, and the quenching process was performed by rapid cooling to a temperature of 270 to 400°C for 10 seconds or less to prevent thermal shock of the stainless steel. The tempering process was performed at a temperature of 360 to 465°C for 0.5 to 1 minute.

[0086] After heat treatment, the residual austenite content in the microstructure was measured using an Electron Back Scatter Diffraction (EBSD) analyzer. Measurements were taken in a 30㎛×30㎛ area under conditions of an acceleration voltage of 20kV and a step size of 0.03㎛. The phase fraction (%) of the measured data was calculated using analysis software and converted into an area fraction (%), and the average value of three measurements was used.

[0087] Subsequently, yield strength, tensile strength, and elongation were measured using a Zwick Roell tensile testing machine, processed according to JIS 13B standards, and tested at room temperature at a crosshead speed ranging from 10 mm / min to 20 mm / min. Three measurements were taken per specimen, and the average value was calculated.

[0088] AT(°C) T(sec) RA(%) Formula (1) ATTR Factor Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Comparative Example 1980 3430.133,614 1,455 1,761 4.6 Comparative Example 2980 2290.122,442 1,407 1,698 7.6 Comparative Example 3980 1720.116,856 1,378 1,662 8.0 Comparative Example 4980 1370 .113,4261,3531,6217.1 Example 1 10101720.469,4881,4481,7427.0 Comparative Example 5 10101140.111,5141,4441,7226.8 Comparative Example 6 1010860.18,6861,3821,6696.9 Comparative Example 7 1010690.16,9691, 3921,6356.4 Comparative Example 8 1010570.15,7571,342 1,5986.3 Comparative Example 9 10502501.6420,0001,4251,69411.2 Example 2 10502101.5330,7501,4501,7539.1 Example 3 10501721.2216,7201,493 1,8338.7 Example 4 10501 141.2 143,6401 4441 8038.9 Example 5 10508 60.98 1,2701 4241 7778.4 Example 6 10506 90.857 9601 4171 7758.5 ​​Example 7 10505 70.635 9101 3961 7477.4

[0089] Referring to Table 2, Examples 1 to 7 satisfied the alloy composition, manufacturing process, and Equation (1) presented in the present invention, and accordingly, satisfied a tensile strength of 1700 MPa or more and an elongation of 7.0% or more. That is, Examples 1 to 7 of the present invention had excellent impact resistance based on excellent tensile strength and elongation. Figure 1 shows the correlation between the ATTR Factor of Equation (1) and the elongation. Through Figure 1 and Table 2, it can be confirmed that when Equation (1) satisfies the range of 35,000 to 331,000, the elongation satisfies 7.0% or more.

[0090] Austenitizing temperature (AT) 980℃

[0091] Referring to Table 2, the following trends can be derived when comparing Comparative Examples 1 to 4, in which the austenitizing temperature (AT) was set to 980°C and the austenitizing temperature holding time (T) was varied. As the austenitizing temperature holding time (T) increases, the solid solution content of chromium and carbon in the matrix increases due to the decomposition of chromium carbides, and thus the yield strength and tensile strength increase. However, it can be seen that as the austenitizing temperature holding time (T) increases, the brittleness of the material increases, and the elongation decreases.

[0092] Figure 2 is a photograph of the microstructure of Comparative Example 3 after heat treatment taken with an Electron Back Scatter Diffraction (EBSD) analyzer. The bright areas in Figure 2 represent retained austenite. Referring to Table 2 and Figure 2, the retained austenite content (RA) at an austenitizing temperature (AT) of 980°C is 0.1% in area fraction.

[0093] In addition, the value of Equation (1) in Comparative Examples 1 to 4 is less than 35,000 as defined in the present invention. Therefore, it is not possible to obtain stainless steel with a tensile strength of 1700 MPa or more and an elongation of 7.0% or more as intended in the present invention.

[0094] Austenitizing temperature (AT) 1010℃

[0095] Example 1 and Comparative Examples 5 to 8 had an austenitizing temperature (AT) of 1010°C and different holding times (T) for the austenitizing temperature. Referring to Table 2, it can be seen that if the austenitizing temperature (AT) is increased to 1010°C, tensile strength and elongation can be secured even if the holding time (T) for the austenitizing temperature is short. However, the only case that satisfied the tensile strength of 1700 MPa or more and the elongation of 7.0% or more, which are the objectives of the present invention, was Example 1, in which the holding time (T) for the austenitizing temperature was 172 seconds. At this time, the residual austenite content (RA) was 0.4% as an area fraction, and the value of Equation (1) was 69,488. Therefore, when the austenitizing temperature (AT) is 1010℃ and the holding time (T) of the austenitizing temperature is 172 seconds or more, the value of Equation (1) falls within the range of 35,000 to 331,000 as defined in the present invention, and a stainless steel satisfying a tensile strength of 1700 MPa or more and an elongation of 7% or more can be manufactured.

[0096] Austenitizing temperature (AT) 1050℃

[0097] Comparative Example 9 and Examples 2 to 7 compared the ATTR factor, tensile strength, yield strength, and elongation of Equation (1) while varying the holding time (T) of the austenitizing temperature (AT) at an austenitizing temperature (AT) of 1050°C.

[0098] When compared to the case where the austenitizing temperature (AT) is 1010℃ under the same austenitizing temperature holding time (T), that is, when comparing Example 1 with Example 3, Comparative Example 5 with Example 4, Comparative Example 7 with Example 6, and Comparative Example 8 with Example 7, respectively, as the austenitizing temperature (AT) increases to 1050℃, the tensile strength and elongation increase simultaneously.

[0099] Figure 3 is a photograph of the microstructure of Example 3 after heat treatment using an Electron Back Scatter Diffraction (EBSD) analyzer. The bright areas in Figure 3 represent retained austenite. Referring to Table 2 and Figure 3, it can be seen that the retained austenite content (RA) in Example 3 increased to 1.2% in terms of area fraction.

[0100] In Examples 2 to 7, where the austenitizing temperature (AT) was 1050°C and the holding time (T) was less than 250 seconds, the value of Equation (1) satisfied the range of 35,000 to 331,000. Accordingly, the tensile strength was 1700 MPa or higher and the elongation was 7.0% or higher.

[0101] However, in the case of Comparative Example 9, when an austenitizing temperature (AT) of 1050°C and an austenitizing temperature holding time (T) of 250 seconds were applied, the elongation increased to 11.2%, but the tensile strength decreased to less than 1700 MPa. At this time, the residual austenite content (RA) was found to be 1.6% in terms of area fraction. The value of Equation (1) was 420,000, which exceeded the scope of the present invention. When heat treatment is performed at an austenitizing temperature (AT) of 1050°C for more than an austenitizing temperature holding time (T) of 210 seconds, the decomposition of chromium carbides is promoted, leading to the formation of excessive residual austenite (RA). As a result, the formation of a martensite structure, which plays a role in increasing tensile strength, is hindered, causing the elongation to increase but the tensile strength to decrease.

[0102] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims set forth below.

Claims

1. In weight%, C: 0.35 to 0.55%, Si: 0.15 to 0.50%, Mn: 0.40 to 0.60%, Cr: 13.50 to 15.00%, Ni: 0.30 to 0.50%, Mo: 0.95 to 1.10%, Cu: 0.001 to 0.500%, N: 0.010 to 0.025%, the remainder being Fe and unavoidable impurities, and The microstructure comprises martensite and retained austenite formed within the martensite, The above-mentioned residual austenite comprises 0.4 to 1.5% in area fraction, high-carbon martensitic stainless steel.

2. In Claim 1, The above-mentioned martensitic stainless steel has a tensile strength of 1700 MPa or higher, and is a high-carbon martensitic stainless steel.

3. In Claim 1, High-carbon martensitic stainless steel having an elongation of 7.0% or more.

4. In Claim 1, High-carbon martensitic stainless steel having a yield strength of 1390 MPa or higher 5. A step of preparing a steel material comprising, in weight%, C: 0.35 to 0.55%, Si: 0.15 to 0.50%, Mn: 0.40 to 0.60%, Cr: 13.50 to 15.00%, Ni: 0.30 to 0.50%, Mo: 0.95 to 1.10%, Cu: 0.001 to 0.500%, N: 0.010 to 0.025%, and the remainder being Fe and unavoidable impurities; A step of reheating the above steel to 1200 to 1300℃; Steps for hot rolling and hot rolling annealing of the reheated steel; Subsequently, a step of manufacturing a cold-rolled material by cold rolling and cold rolling annealing; A step of austenitizing the above cold-rolled material; Subsequently, a step of quenching at a temperature of 270 to 400℃ for 10 seconds or less; A method for manufacturing high-carbon martensitic stainless steel, comprising the step of tempering at a temperature of 360 to 465°C for 0.5 to 1 minute after the above quenching, wherein Formula (1) is 35,000 to 331,000: Equation (1): ATTR = AT × T × RA (In the above equation (1), ATTR (Austenitizing Temperature, Time, Retained Austenite Factor) is a high strength and high ductility prediction parameter, AT is the austenitizing temperature (°C), T is the holding time at the austenitizing temperature (sec), and RA is the content of retained austenite (%) in the martensite structure formed through austenitizing and quenching).

6. In Claim 5, The above steps of hot rolling and hot rolling annealing include the step of coiling the hot-rolled steel sheet at a temperature of 700℃ or higher; A step of maintaining at 800 to 900 ℃ for at least 3 hours, followed by a second step of hot rolling annealing at 700 to 790 ℃ for 5 to 15 hours; and A method for manufacturing high-carbon martensitic stainless steel comprising a pickling step for removing a surface oxide layer of hot-rolled steel.

7. In Claim 5, A method for manufacturing high-carbon martensitic stainless steel, characterized by cold rolling and cold annealing the thickness of the above cold-rolled material to 0.1 to 0.5 mm.

8. In Claim 5, A method for manufacturing high-carbon martensitic stainless steel, wherein the above cold-rolled material contains ferrite and carbides in its microstructure.

9. In Claim 5, A method for manufacturing high-carbon martensitic stainless steel, wherein, in the austenitizing and quenching steps above, the residual austenite microstructure is controlled to an area fraction of 0.4 to 1.5%.

10. In Claim 5, A method for manufacturing high-carbon martensitic stainless steel, wherein, in the austenitizing step above, the austenitizing temperature is 1000 to 1015℃ and the holding time of the austenitizing temperature is 172 seconds or more.

11. In Claim 5, A method for manufacturing high-carbon martensitic stainless steel, wherein, in the austenitizing step above, the austenitizing temperature is 1045 to 1055℃ and the holding time at the austenitizing temperature is less than 250 seconds.

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