Martensitic stainless steel and method of manufacturing same

Abstract

A martensitic stainless steel according to one embodiment of the present invention can be a martensitic stainless steel comprising, by wt%, 0.500-0.800% of carbon (C), 0.200-1.000% of silicon (Si), 0.300-1.000% of manganese (Mn), 0.001-0.500% of copper (Cu), 0.007-1.000% of nickel (Ni), 12.000-15.000% of chromium (Cr), 0.050-1.000% of molybdenum (Mo), 0.030-0.500% of vanadium (V), 0.003-0.100% of titanium (Ti), 0.020-1.000% of nitrogen (N), and the balance of iron (Fe) and inevitable impurities, wherein the value of carbide index relation (1) 327*[C]+25*[Si]-0.9*[Mn]+18*[Cu]+8.6*[Ni]+13.8*[Cr]+2.3*[Mo]+74*[V]+35*[Ti]-191*[N] is 425 or less.

Description

Martensitic stainless steel and its manufacturing method

[0001] The present invention relates to martensitic stainless steel and a method for manufacturing the same, and more specifically, to high-carbon martensitic stainless steel with excellent carbide quality that can be used as a material for various types of metalwork or air tools, and a method for manufacturing the same.

[0002] Commonly used cutting materials, such as esophagus, scissors, razors, and scalpels, require high hardness to maintain machinability and wear resistance. Furthermore, excellent corrosion resistance is essential due to their frequent contact with moisture and storage in humid environments. Accordingly, high-hardness, high-carbon martensitic stainless steels are primarily used as cutting materials.

[0003] General martensitic stainless steel is made by continuously casting molten steel or manufacturing slabs from ingots, then reheating and hot rolling. In the hot-rolled state, the steel structure exists as a mixture of martensite phase, tempered martensite phase, ferrite phase, and retained austenite phase. These hot-rolled coils go through a batch annealing process for the purpose of hot-rolled sheet annealing, transforming them into ferrite and carbides and softening them. The soft material from hot-rolled annealing goes through a pickling process to remove the scale formed during the hot-rolled annealing. The soft material after pickling is transformed into martensitic steel through a heat treatment process after cold rolling or product processing when the final demand arises.

[0004] Meanwhile, hot-rolled and annealed steel produced through the general casting, hot rolling, and batch annealing furnace (BAF) processes has a ferrite matrix and contains chromium carbide. Chromium carbide is a primary carbide (M7C) with a size of tens to hundreds of μm that is generated by the chromium and carbon center segregation during the casting process. 3, M is Cr:Fe=62.3%:26.7%) and secondary carbides (M) that are preferentially precipitated along grain boundaries during annealing after hot rolling 23 C6, M contains Cr:Fe=60.3%:32.3%).

[0005] In particular, primary carbides formed at high temperatures in the core of steel do not decompose during hot rolling and annealing, but remain coarsely, with sizes exceeding 10㎛. Furthermore, the higher the fraction of these primary carbides, the coarser their size. These coarse carbides remaining in cold-rolled steel are difficult to decompose into the matrix during short-term strengthening heat treatment, which is mainly performed through continuous heat treatment, and thus, they remain, lowering corrosion resistance and degrading the quality of the cutting edge.

[0006] Therefore, in order to secure excellent edge quality and corrosion resistance, an alloy composition capable of appropriately controlling the fraction of primary carbides is required.

[0007] In order to solve the above-described problems, the present invention aims to provide a high-carbon martensitic stainless steel having excellent corrosion resistance and edge quality and a method for manufacturing the same by controlling the volume fraction of primary carbides expressed as (Cr,Fe)7C3 in a cast structure, a hot-rolled annealed material, and a final heat-treated structure.

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

[0009] As a means for achieving the above-described purpose, a martensitic stainless steel according to one embodiment of the present invention contains, in weight %, carbon (C): 0.500 to 0.800%, silicon (Si): 0.200 to 1.000%, manganese (Mn): 0.300 to 1.000%, copper (Cu): 0.001 to 0.500%, nickel (Ni): 0.007 to 1.000%, chromium (Cr): 12.000 to 15.000%, molybdenum (Mo): 0.050 to 1.000%, vanadium (V): 0.030 to 0.500%, titanium (Ti): 0.003 to 0.100%, nitrogen (N): 0.020 to 1.000%, the remainder being iron (Fe) and unavoidable impurities, and the value of the following formula (1) is 425 Below, it may be martensitic stainless steel.

[0010] Formula (1): 327*[C]+25*[Si]-0.9*[Mn]+18*[Cu]+8.6*[Ni]+13.8*[Cr]+2.3*[Mo]+74*[V]+35*[Ti]-191*[N]

[0011] (In the above formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti] and [N] represent the content (weight %) of each element.

[0012] A martensitic stainless steel according to one embodiment of the present invention may be a martensitic stainless steel containing a primary carbide expressed as (Cr,Fe)7C3 in a volume fraction of 8.0% or less.

[0013] A martensitic stainless steel according to one embodiment of the present invention may be a martensitic stainless steel having a pitting resistance equivalent number (PREN) value of 13 or more in the following formula (2).

[0014] Formula (2): [Cr]+3.3*[Mo]+16*[N]

[0015] (In the above formula (2), [Cr], [Mo] and [N] represent the content (weight %) of each element.)

[0016] A martensitic stainless steel according to one embodiment of the present invention may be a martensitic stainless steel including primary carbides expressed as (Cr,Fe)7C3 having an average size of 1 μm to 5 μm.

[0017] A martensitic stainless steel according to one embodiment of the present invention may be a martensitic stainless steel having a formal potential of 210 mV or more in a 3.5% NaCl aqueous solution at 30°C.

[0018] A method for manufacturing martensitic stainless steel according to one embodiment of the present invention comprises: a steel material including, in weight %, carbon (C): 0.500 to 0.800%, silicon (Si): 0.200 to 1.000%, manganese (Mn): 0.300 to 1.000%, copper (Cu): 0.001 to 0.500%, nickel (Ni): 0.007 to 1.000%, chromium (Cr): 12.000 to 15.000%, molybdenum (Mo): 0.050 to 1.000%, vanadium (V): 0.030 to 0.500%, titanium (Ti): 0.003 to 0.100%, nitrogen (N): 0.020 to 1.000%, the remainder being iron (Fe) and unavoidable impurities, and having a value of 425 or less in the following formula (1): A method for manufacturing martensitic stainless steel may include the steps of: a step of manufacturing; a step of heating the steel to 1100 to 1300°C; a step of rolling the heated steel at a temperature of 800°C or higher to manufacture a hot-rolled material; a step of hot-rolling and annealing the hot-rolled material at a temperature of 850°C or higher for 5 hours or longer; a step of cold-rolling the annealed hot-rolled material to manufacture a cold-rolled material; and a step of cold-rolling and annealing the cold-rolled material.

[0019] Formula (1): 327*[C]+25*[Si]-0.9*[Mn]+18*[Cu]+8.6*[Ni]+13.8*[Cr]+2.3*[Mo]+74*[V]+35*[Ti]-191*[N]

[0020] (In the above formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti] and [N] represent the content (weight %) of each element.

[0021] A method for manufacturing martensitic stainless steel according to one embodiment of the present invention may be a method for manufacturing martensitic stainless steel having a pitting resistance equivalent number (PREN) value of 13 or more in the following formula (2).

[0022] Formula (2): [Cr]+3.3*[Mo]+16*[N]

[0023] (In the above formula (2), [Cr], [Mo] and [N] represent the content (weight %) of each element.)

[0024] In a method for manufacturing martensitic stainless steel according to one embodiment of the present invention, the steel material that has undergone the casting step may contain primary carbides expressed as (Cr,Fe)7C3 in a volume fraction of 8.0% or less.

[0025] In a method for manufacturing martensitic stainless steel according to one embodiment of the present invention, the hot-rolled material that has undergone the hot-rolled annealing step may contain primary carbides expressed as (Cr,Fe)7C3 in a volume fraction of 8.0% or less.

[0026] In a method for manufacturing martensitic stainless steel according to one embodiment of the present invention, the step of rolling at a temperature of 800°C or higher may include re-pressurization.

[0027] The martensitic stainless steel according to the present invention can produce a martensitic stainless steel having a long lifespan in a fatigue environment and excellent corrosion resistance by controlling the volume fraction of primary carbides expressed as (Cr,Fe)7C3 to 8.0% or less and refining the carbides.

[0028] Figure 1 is a scanning electron microscope (SEM) photograph showing the effect of coarse carbide on the final product. (a) shows the good edge condition of Inventive Example 13, and (b) shows the poor edge quality due to the shedding of coarse carbide of Comparative Example 4.

[0029] Figure 2 is a photograph of the primary carbide expressed as (Cr,Fe)7C3 in a coarse form of the hot-rolled material of Comparative Example 4, taken using a scanning electron microscope (SEM).

[0030] Figure 3 is a graph showing the size distribution of primary carbide expressed as (Cr,Fe)7C3 of Invention Example 9.

[0031] Preferred embodiments of the present invention are described below. However, the embodiments of the present invention may be modified in various ways, 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 fully explain the present invention to those of ordinary skill in the art.

[0032] The terminology used in this application is solely for the purpose of describing specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Additionally, it should be noted that terms such as "comprise" or "have" 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 presence of other features, steps, functions, components, or combinations thereof.

[0033] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, specific terms should not be interpreted in an overly idealistic or formal sense.

[0034] In addition, the terms "about", "substantially", etc. in this specification are used in the sense of or close to the numerical value when manufacturing and material tolerances inherent to the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly using the disclosure in which exact or absolute numerical values ​​are mentioned to aid understanding of the present invention.

[0035] Unless otherwise specifically stated herein, percentages indicating the content of each element are based on weight.

[0036] Primary carbides, generally expressed as (Cr,Fe)7C3, are known to occur when the carbon content is high. However, the formation of primary carbides is complexly affected not only by the carbon concentration but also by the major elements that contribute to the formation of carbides. Therefore, if this relationship is clearly understood, a composition system that minimizes the formation of primary carbides within the material, from the casting process to the annealing process that precipitates carbides, can be derived. In the present invention, the composition of carbides formed in martensitic steel with a high carbon content is controlled by a combination of the addition amounts of carbon, chromium, molybdenum, vanadium, silicon, manganese, copper, nickel, titanium, and nitrogen, which are elements that affect the formation of carbides.

[0037] A martensitic stainless steel according to one embodiment of the present invention contains, in wt%, carbon (C): 0.500 to 0.800%, silicon (Si): 0.200 to 1.000%, manganese (Mn): 0.300 to 1.000%, copper (Cu): 0.001 to 0.500%, nickel (Ni): 0.007 to 1.000%, chromium (Cr): 12.000 to 15.000%, molybdenum (Mo): 0.050 to 1.000%, vanadium (V): 0.030 to 0.500%, titanium (Ti): 0.003 to 0.100%, nitrogen (N): 0.020 to 1.000%, the remainder being iron (Fe) and unavoidable impurities.

[0038] Hereinafter, the reasons for numerical limitation of the alloy component content in the embodiment of the present invention will be explained.

[0039] C (carbon) can be 0.500% or more and 0.800% or less.

[0040] C is an essential element for improving the hardness of martensitic stainless steel and must be added appropriately to secure hardness after quenching / tempering heat treatment. More specifically, if the C content is less than 0.500%, high hardness cannot be obtained. Considering this, C may be added in an amount of 0.500% or more. On the other hand, if the C content exceeds 0.800%, excessively high hardness may be exhibited, which may reduce the toughness of the steel sheet. Considering this, the upper limit of the C content may be limited to 0.800%. The C content may be preferably 0.551% or more and 0.742% or less, and more preferably 0.601% or more and 0.684% or less.

[0041] Si (silicon) can be 0.200% or more and 1.000% or less.

[0042] Silicon (Si) helps improve the strength of steel through solid solution strengthening and deoxidation of molten steel. Considering this, Si may be added in amounts of 0.200% or more. However, excessive addition may cause scale to form on the surface of the steel sheet during hot rolling, thereby degrading the surface quality of the steel sheet. Considering this, the upper limit of the Si content may be limited to 1.000%. The Si content is preferably 0.210% or more and 0.770% or less, and more preferably 0.221% or more and 0.540% or less.

[0043] Mn (manganese) can be 0.300% or more and 1.000% or less.

[0044] Mn can improve the strength and hardenability of steel, and it plays a role in suppressing cracking caused by S (sulfur) by forming MnS by combining with S (sulfur) which is inevitably contained during the steel manufacturing process. Considering this, Mn can be added in an amount of 0.300% or more. However, if it is added excessively, the toughness of the steel may be reduced. Considering this, the upper limit of the Mn content can be limited to 1.000%. The Mn content can be preferably 0.404% or more and 0.895% or less, and more preferably 0.508% or more and 0.790% or less.

[0045] The content of Cu (copper) may be 0.001% or more and 0.500% or less.

[0046] Cu can also improve the strength and hardenability of steel, and by combining with sulfur (S), which is inevitably contained during the steel manufacturing process, to form CuS, it plays a role in suppressing cracking caused by sulfur (S). Considering this, Cu can be added in an amount of 0.001% or more. If the content of Cu is excessive, the toughness of the steel can be reduced. Considering this, the upper limit of the Cu content can be limited to 0.500%. The Cu content can be preferably 0.001% or more and 0.298% or less, and more preferably 0.001% or more and 0.096% or less.

[0047] The content of Ni (nickel) may be 0.007% or more and 1.000% or less.

[0048] Ni is an essential element added to martensitic stainless steels to change the metal structure to austenite during hot working. Even a small amount can improve corrosion resistance and hardenability. Considering this, Ni may be added in an amount of 0.007% or more. Larger amounts may deteriorate processability and, as it is an expensive element, increase manufacturing costs. Considering this, the upper limit of the Ni content may be limited to 1.000%. The Ni content is preferably 0.007% or more and 0.738% or less, and more preferably 0.007% or more and 0.475% or less.

[0049] The content of Cr (chromium) may be 12.000% or more and 15.000% or less.

[0050] Cr improves corrosion resistance and also plays an effective role in improving hardness and wear resistance by forming chromium carbide. Considering this, Cr can be added in an amount of 12,000% or more. However, if added excessively, not only will the hardenability increase unnecessarily, but because it is an expensive element, the manufacturing cost may increase. Considering this, the upper limit of the Cr content can be limited to 15,000%. The Cr content can be preferably 12.357% or more and 14.646% or less, and more preferably 12.713% or more and 14.292% or less.

[0051] The content of Mo (molybdenum) may be 0.050% or more and 1.000% or less.

[0052] Mo plays a role in improving corrosion resistance, suppressing decarburization, and improving hardenability, and can play a role in suppressing carbide refinement and growth by replacing Cr together with V in chromium carbide. Considering this, Mo can be added in an amount of 0.050% or more. However, if added excessively, not only will the hardenability increase unnecessarily, but since it is an expensive element, the manufacturing cost may increase. Considering this, the upper limit of the Mo content can be limited to 1.000%. The Mo content can be preferably 0.052% or more and 0.744% or less, and more preferably 0.053% or more and 0.488% or less.

[0053] The content of V (vanadium) may be 0.030% or more and 0.500% or less.

[0054] V forms carbides, suppresses the coarsening of chromium carbides, and effectively plays a role in preventing coarsening of grains during heat treatment and improving wear resistance. Considering this, V may be added in an amount of 0.030% or more. However, if added excessively, it may form carbides more than necessary, lowering the toughness of the steel. In addition, since it is an expensive element, the manufacturing cost may increase. Considering this, the upper limit of the V content may be limited to 0.500%. The V content may be preferably 0.035% or more and 0.339% or less, and more preferably 0.040% or more and 0.177% or less.

[0055] The content of Ti (titanium) may be 0.003% or more and 0.100% or less.

[0056] Typically, Ti is an element added to ferritic general-purpose steels such as 409L and 439 steel, and even when there is no Ti component in the raw materials when manufacturing martensitic steel in the same steel mill, a small amount may be present as it flows in from the ladle and mold. Considering this, Ti may be added in an amount of 0.003% or more. However, Ti may combine with C to form TiC precipitates, which may lower the martensite strength. Considering this, the upper limit of the Ti content may be limited to 0.100%. The Ti content may be preferably 0.003% or more and 0.060% or less, and more preferably 0.003% or more and 0.019% or less.

[0057] N (nitrogen) can be 0.020% or more and 1.000% or less.

[0058] Nitrogen, like C, is an effective element for improving the hardness of steel. Considering this, N may be added in an amount of 0.020% or more. However, if the content of N is excessive, chromium nitride, which is a low-temperature precipitation phase, is formed, and the γ phase remains, which may result in insufficient strength after strengthening heat treatment. Therefore, if the content of N is excessive, fatigue resistance may be deteriorated. Considering this, the upper limit of the N content may be limited to 1.000%. The N content may be preferably 0.022% or more and 0.539% or less, and more preferably 0.024% or more and 0.078% or less.

[0059] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0060] In addition to limiting the content of each alloy element to the conditions described above, the relationship between them can be further limited as follows.

[0061] The martensitic stainless steel of the present invention has a carbide index value of the following formula (1) of 425 or less.

[0062] Formula (1): 327*[C]+25*[Si]-0.9*[Mn]+18*[Cu]+8.6*[Ni]+13.8*[Cr]+2.3*[Mo]+74*[V]+35*[Ti]-191*[N]

[0063] (In the above formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti] and [N] represent the content (weight %) of each element.)

[0064] In addition to limiting the content of alloy elements to the conditions described above, the volume fraction of primary chromium carbide (M7C3) can be controlled to 8.0% or less by controlling the content of each alloy element so that the value of Equation (1) is 425 or less. By controlling the volume fraction of primary chromium carbide (M7C3) to 8.0% or less, coarse carbides that remain in the cold-rolled material and are not easily decomposed into the matrix during strengthening heat treatment, thereby deteriorating corrosion resistance and edge quality, are reduced, so that a product with excellent edge quality can be obtained.

[0065] When the value of equation (1) exceeds 425, the volume fraction of primary chromium carbide increases, and the edge quality of the product deteriorates due to the shedding of coarse carbide.

[0066] The martensitic stainless steel of the present invention has a pitting resistance equivalent number (PREN) value of 13 or more in the following formula (2).

[0067] Formula (2): [Cr]+3.3*[Mo]+16*[N]

[0068] (In the above formula (2), [Cr], [Mo], and [N] represent the content (weight%) of each element.)

[0069] In general, the pitting resistance equivalent number (PREN) is expressed as Cr+3.3Mo+16N. The pitting resistance equivalent number is an indicator that predicts the localized corrosion resistance of stainless steel. A higher PREN value indicates a higher resistance to localized corrosion by chlorides. In the present invention, in addition to limiting the content of alloying elements to the conditions described above, the corrosion resistance of the material can be secured by controlling the content of each alloying element so that the value of Equation (2) is 13 or more.

[0070] Hereinafter, a martensitic stainless steel according to one embodiment of the present invention having the above-described alloy composition will be described.

[0071] The martensitic stainless steel of the present invention may contain primary chromium carbide in a volume fraction of 8.0% or less.

[0072] The martensitic stainless steel of the present invention may include primary chromium carbide having an average size of 1 μm to 5 μm.

[0073] The martensitic stainless steel of the present invention may have a formal potential of 210 mV or more in a 3.5% NaCl aqueous solution at 30°C.

[0074] Hereinafter, a method for manufacturing a martensitic stainless steel according to one embodiment of the present invention having the above-described alloy composition will be described.

[0075] A method for manufacturing a martensitic stainless steel according to an example of the present invention comprises, in weight %, carbon (C): 0.500 to 0.800%, silicon (Si): 0.200 to 1.000%, manganese (Mn): 0.300 to 1.000%, copper (Cu): 0.001 to 0.500%, nickel (Ni): 0.007 to 1.000%, chromium (Cr): 12.000 to 15.000%, molybdenum (Mo): 0.050 to 1.000%, vanadium (V): 0.030 to 0.500%, titanium (Ti): 0.003 to 0.100%, nitrogen (N): 0.020 to 1.000%, the remainder being iron (Fe) and unavoidable impurities, and the following formula (1) is 425 or less. A method for manufacturing a martensitic stainless steel may include the steps of: manufacturing a steel material; heating the steel material to 1100 to 1300°C; rolling the heated steel material at a temperature of 800°C or higher to manufacture a hot-rolled material; hot-rolling and annealing the hot-rolled material at a temperature of 850°C or higher for 5 hours or longer; cold-rolling the annealed hot-rolled material to manufacture a cold-rolled material; and cold-rolling and annealing the cold-rolled material.

[0076] Formula (1): 327*[C]+25*[Si]-0.9*[Mn]+18*[Cu]+8.6*[Ni]+13.8*[Cr]+2.3*[Mo]+74*[V]+35*[Ti]-191*[N]

[0077] (In the above formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti] and [N] represent the content (weight %) of each element.

[0078] The reason for the numerical limitation of the component range of each alloy composition and the value of formula (1) is as described above, and each manufacturing step is described in more detail below.

[0079] First, after preparing steel that satisfies the above alloy composition, a series of processes including heating, hot rolling, annealing, cold rolling, and cold annealing can be performed.

[0080] First, the above steel material can be heated to 1100℃ to 1300℃.

[0081] At low heating temperatures, it can be difficult to redissolve coarse precipitates formed during steel manufacturing. Considering this, the heating temperature can be 1100°C or higher. However, if the heating temperature is excessively high, the internal grains can become excessively coarse, and severe surface oxidation can occur, leading to surface defects. Considering this, the upper limit of the heating temperature can be limited to 1300°C.

[0082] The above-mentioned heated steel may be rolled at a temperature of 800°C or higher to produce a hot-rolled steel. The step of rolling at a temperature of 800°C or higher may include repressurization. Repressurization can reduce the grain size and improve the uniformity of the microstructure, thereby increasing the strength and ductility of the steel. It can also increase plasticity and create a more uniform surface.

[0083] Next, the hot-rolled material may be placed in a hot-rolled annealing furnace and hot-rolled annealing may be performed at a temperature of 850°C or higher for 5 hours or longer. If the hot-rolled annealing temperature is low or the hot-rolled annealing time is short, the residual martensite fraction may increase, which may reduce workability. However, if the hot-rolled annealing temperature is high or the hot-rolled annealing time is long, a decrease in strength may occur due to grain coarsening and carbide coarsening.

[0084] The above-mentioned hot-rolled material can be cold rolled at a cold reduction ratio of 50% and a number of passes of 5 to 7, and then cold-rolled and annealed.

[0085] A method for manufacturing a martensitic stainless steel according to an example of the present invention may be a method for manufacturing a martensitic stainless steel having a pitting resistance equivalent number (PREN) value of 13 or more in the following formula (2).

[0086] Formula (2): [Cr]+3.3*[Mo]+16*[N]

[0087] (In the above formula (2), [Cr], [Mo] and [N] represent the content (weight %) of each element.)

[0088] In a method for manufacturing a martensitic stainless steel according to an example of the present invention, the steel material that has undergone the casting step may be a method for manufacturing a martensitic stainless steel that contains a primary carbide expressed as (Cr,Fe)7C3 in a volume fraction of 8.0% or less.

[0089] In a method for manufacturing martensitic stainless steel according to an example of the present invention, the hot-rolled material that has undergone the hot-rolling annealing step may be a method for manufacturing martensitic stainless steel, which contains primary carbides expressed as (Cr,Fe)7C3 (Cr:Fe=62.3%:26.7%) in a volume fraction of 8.0% or less.

[0090] High-carbon martensitic stainless steel undergoes carbide precipitation and growth not only during the casting process but also during the heat treatment process of manufacturing the annealed material into the final thickness cold-rolled material. However, since the quality of the carbide in the hot-rolled material determines the quality of the final product, the primary carbide volume fraction expressed as (Cr,Fe)7C3 (Cr:Fe=62.3%:26.7%) in the hot-rolled material was minimized.

[0091] Hereinafter, the present invention will be described in more detail through preferred embodiments.

[0092] Example

[0093] Specimens having an alloy composition according to Table 1 below were manufactured into 50 kg ingots using a vacuum melting device, and then specimens were collected to observe the carbide form.

[0094] The remaining specimen was reheated at 1200℃ for 2 hours and rolled to a final thickness of 4 mm to produce a hot-rolled specimen. After reheating the specimen at a temperature of 1250℃, rolling was completed at a temperature of approximately 800℃ or higher using a roughing mill and a continuous finishing mill, and then it was placed in an annealing furnace and hot-rolled (batch annealing) was performed at a temperature of 850℃ or higher for 5 hours or more, and cold-rolled at a cold reduction ratio of 50% and a number of passes of 5 to 7, and cold-rolled annealing was performed at 820℃.

[0095] Classification CSiMnCuNiCrMoVTiN Invention Example 10.6400.3390.7460.0620.26814.0690.4300.0440.0110.078 Invention Example 20.6440.5280.5830.0810.47513.6740.1180.1560.0070 .031Invention Example 30.6330.4230.7720.0470.41613.8020.2080.1690.0030.040Invention Example 40.6010.4550.5080.0760.44213.4840.1250.1740.0170.036Invention Example 50.6720. 5400.7900.0960.41012.8110.4250.1610.0150.036Invention Example 60.6250.4280.5120.0730.00714.2920.2500.0400.0190.051Invention Example 70.6840.4780.5250.0110. 36012.7130.4760.0400.0080.031Invention Example 80.6260.2210.7220.0100.12513.2050.3210.1270.0110.063Invention Example 90.6250.4910.5460.0300.22913.0910.3140 .1210.0190.036 Invention Example 100.6620.2210.5660.0060.23013.7060.1860.1400.0180.038 Invention Example 110.6400.5290.6480.0360.07813.4050.4880.1070.0180.02 5 inventions120.6090.2220.5830.0010.29514.2260.2810.0570.0180.053 inventions130.6740.2780.6510.0130.37713.1690.0530.1770.0080.034 inventions140.6330. 2740.5600.0760.05513.8730.2860.1010.0080.024Comparative Example 10.7000.5000.7500.1000.25013.3000.0800.1200.0150.050Comparative Example 20.7050.5310.7570.0180. 07913.5080.0390.1750.0130.035Comparative Example 30.7090.5410.5630.0070.32913.0760.2670.1490.0140.029Comparative Example 40.7190.5520.5940.0050.21313.3830.3260.1820.0070.044Comparative Example50.6570.4550.5630.0360.24612.2060.0490.1320.0070.020.

[0096] Table 2 below shows the values ​​of Equation (1), Equation (2), the volume fraction of primary carbide expressed as (Cr,Fe)7C3 (Cr:Fe=62.3%:26.7%), the average size of primary chromium carbide, pitting potential, and the presence or absence of defects during hot rolling. The values ​​of Equation (1) were calculated using Equation (1) below.

[0097] Formula (1): 327*[C]+25*[Si]-0.9*[Mn]+18*[Cu]+8.6*[Ni]+13.8*[Cr]+2.3*[Mo]+74*[V]+35*[Ti]-191*[N]

[0098] (In the above formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti] and [N] represent the content (weight %) of each element.

[0099] The value of equation (2) was calculated by using equation (2) below.

[0100] Formula (2): [Cr]+3.3*[Mo]+16*[N]

[0101] (In the above formula (2), [Cr], [Mo] and [N] represent the content (weight %) of each element.)

[0102] The volume fraction of primary carbides, expressed as (Cr,Fe)7C3 (Cr:Fe=62.3%:26.7%), was analyzed using a scanning electron microscope (SEM) of the carbide volume fraction of specimens collected from hot-rolled steel sheets. In high-carbon martensitic steel, carbide precipitation and growth occur not only during the casting process but also during the heat treatment process of manufacturing the annealed steel into the final thickness cold-rolled steel. However, since the quality of carbides in the hot-rolled steel determines the quality of the final product, the volume fraction of primary carbides in the hot-rolled steel was measured.

[0103] The average size of primary carbides expressed as (Cr,Fe)7C3 (Cr:Fe=62.3%:26.7%) was measured by processing SEM (scanning electron microscope) images using an image analyzer. As shown in Fig. 3, the distribution of primary carbides expressed as (Cr,Fe)7C3 (Cr:Fe=62.3%:26.7%) of each example was measured to obtain the average size value. The average size of the primary carbides of each example is shown in Table 2.

[0104] The pitting potential was measured using a potentiostat. The value was obtained by immersing the steel in a NaCl solution and applying a voltage of 20 mV / min, at which the current reached 100 μA (pitting potential). The temperature of the NaCl solution was 30°C, and the concentration was set to 3.5%. A higher pitting potential value indicates better corrosion resistance.

[0105] The presence or absence of a defect was confirmed by using the HAPL defect measurement system.

[0106] Classification formula (1) Formula (2) Primary carbide volume fraction (%) Primary carbide average size (㎛) Official potential (mV) Defects during hot rolling Invention Honors 1404.416.76.22.3237 Invention Honors 2423.614.67.43.1221 Invention Honors 3417.215.16.82.9223 Invention Honors 4405.614.56.62.6220 Invention Honors 5421.114.87.92.8228 Invention Honors 6407.715.96.12.7232 Invention Honors 7412.314.87.82.8219 Invention Honors 8391.515.36.42.2229 Invention Honors 9402.814.76.92.5221 Invention Honors 10416.914. 97.12.3225No invention honor 11413.115.46.72.5231No invention honor 12398.416.05.62.1225No invention honor 13419.013.98.02.4216No invention honor 14410.515.26.32.1227No comparison example 1428.314.48.52.3224No comparison example 2437.414.28.72.6226No comparison example 3434.914.48.72.8228No comparison example 4441.015.29.03.2231No comparison example 5403.212.76.32.3209No

[0107] Referring to Table 2, Invention Examples 1 to 14 satisfied the alloy composition, component range, formula (1), and manufacturing process presented in the present invention. Therefore, Invention Examples 1 to 14 satisfied the formula (2) value of 13 or more, the primary carbide volume fraction of 8.0% or less, the primary chromium carbide average size of 1 to 5 μm, and the pitting potential of 210 mV or more, and no defects occurred during hot rolling. That is, Invention Examples 1 to 14 had excellent fatigue resistance and corrosion resistance by improving the quality of the primary chromium carbide. However, Comparative Examples 1 to 4 did not satisfy the formula (1) value presented in the present invention. Therefore, Comparative Examples 1 to 4 had the primary carbide volume fraction exceeding 8.0%, and defects occurred during hot rolling.

[0108] Through this, by controlling the alloy composition and carbide index formula (1), the volume fraction of primary chromium carbide can be reduced, and accordingly, the edge quality can be confirmed to be excellent, so that no defects occur during hot rolling.

[0109] Comparative Example 5 satisfied the value of Equation (1) presented in the present invention, but did not satisfy the value of Equation (2). Therefore, Comparative Example 5 did not satisfy the pitting potential of 210 mV or more. Through this, it can be confirmed that basic corrosion resistance can be secured by controlling the pitting resistance index Equation (2).

[0110] 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 following claims.

Claims

1. In weight %, carbon (C): 0.500~0.800%, silicon (Si): 0.200~1.000%, manganese (Mn): 0.300~1.000%, copper (Cu): 0.001~0.500%, nickel (Ni): 0.007~1.000%, chromium (Cr): 12.000~15.000%, molybdenum (Mo): 0.050~1.000%, vanadium (V): 0.030~0.500%, titanium (Ti): 0.003~0.100%, nitrogen (N): 0.020~1.000%, the remainder containing iron (Fe) and inevitable impurities. Martensitic stainless steel with a value of 425 or less in the following formula (1): Formula (1):327*[C]+25*[Si]-0.9*[Mn]+18*[Cu]+8.6*[Ni]+13.8*[Cr]+2.3*[Mo]+74*[V]+35*[Ti]-191*[N] (In the above formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti] and [N] represent the content (weight %) of each element.

2. In claim 1, Martensitic stainless steel containing primary carbides expressed as (Cr,Fe)7C3 in a volume fraction of 8.0% or less.

3. In claim 1, Martensitic stainless steel with a pitting resistance equivalent number (PREN) value of 13 or higher in the following equation (2): Formula (2): [Cr]+3.3*[Mo]+16*[N] (In the above formula (2), [Cr], [Mo] and [N] represent the content (weight %) of each element.) 4. In claim 1, Martensitic stainless steel containing primary carbides expressed as (Cr,Fe)7C3 having an average size of 1 μm to 5 μm.

5. In claim 1, Martensitic stainless steel with a formal potential of 210 mV or more in a 3.5% NaCl aqueous solution at 30°C.

6. A step for manufacturing a steel containing, by weight %, carbon (C): 0.500 to 0.800%, silicon (Si): 0.200 to 1.000%, manganese (Mn): 0.300 to 1.000%, copper (Cu): 0.001 to 0.500%, nickel (Ni): 0.007 to 1.000%, chromium (Cr): 12.000 to 15.000%, molybdenum (Mo): 0.050 to 1.000%, vanadium (V): 0.030 to 0.500%, titanium (Ti): 0.003 to 0.100%, nitrogen (N): 0.020 to 1.000%, the remainder being iron (Fe) and unavoidable impurities, and having a value of 425 or less in the following formula (1); A step of heating the above steel material to 1100 to 1300°C; A step of manufacturing a hot-rolled material by rolling the above-mentioned heated steel at a temperature of 800°C or higher; A step of hot-rolling and annealing the above hot-rolled material at a temperature of 850°C or higher for 5 hours or longer; A method for manufacturing martensitic stainless steel, comprising: a step of cold rolling the annealed hot-rolled material to manufacture a cold-rolled material; and a step of cold-rolling and annealing the cold-rolled material: Formula (1):327*[C]+25*[Si]-0.9*[Mn]+18*[Cu]+8.6*[Ni]+13.8*[Cr]+2.3*[Mo]+74*[V]+35*[Ti]-191*[N] (In the above formula (1), [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti] and [N] represent the content (weight %) of each element.

7. In claim 6, Method for manufacturing martensitic stainless steel having a pitting resistance equivalent number (PREN) of 13 or more in the following formula (2): Formula (2): [Cr]+3.3*[Mo]+16*[N] (In the above formula (2), [Cr], [Mo] and [N] represent the content (weight %) of each element.) 8. In claim 6, A method for manufacturing martensitic stainless steel, wherein the steel material that has undergone the above casting step contains primary carbides expressed as (Cr,Fe)7C3 in a volume fraction of 8.0% or less.

9. In claim 6, A method for manufacturing martensitic stainless steel, wherein the hot-rolled material that has undergone the above hot-rolled annealing step contains primary carbides expressed as (Cr,Fe)7C3 in a volume fraction of 8.0% or less.

10. In claim 6, A method for manufacturing martensitic stainless steel, wherein the step of rolling at a temperature of 800°C or higher includes re-pressurization.

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