Martensitic stainless steel and method for producing same

Abstract

A martensitic stainless steel slab according to an example of the present invention may contain, in wt%, 0.4-0.55% of C, 0.01-0.1% of N, 0.2-0.6% of Si, 0.4-0.9% of Mn, 13.6-15.0% of Cr, 0.01-0.3% of Ni, and the balance Fe and unavoidable impurities, and the area fraction of primary carbides having an area of 2000 [mu] m2 or more in a center segregation portion may be 2.5% or less.

Description

Technical Field

[0001] The present invention relates to a martensitic stainless steel ingot having excellent primary carbide quality, stainless steel, and a method for manufacturing the same. More specifically, the present invention relates to a martensitic stainless steel and a method for manufacturing the same. The martensitic stainless steel is economical and can be used for high-hardness, high-corrosion-resistant cutting tools and automotive parts by controlling the amount of primary carbides and omitting the forging process. Background Art

[0002] With the improvement of living standards, the application of martensitic stainless steel with high hardness, high strength and high corrosion resistance in household knives, industrial band saws (Saw) and other aspects is increasing. This application requires high hardness, high strength, wear resistance and corrosion resistance. In order to ensure high hardness and high strength characteristics, a tempered martensite structure generated by intensive heat treatment and tempering is used. This tempered martensite structure is a very hard structure that can be ensured by rapidly cooling the annealed structure (ferrite + fine chromium carbide) after forming an austenite phase as a high-temperature stable phase through intensive heat treatment and tempering. The higher the content of solid-dissolved carbon in the matrix, the more high-hardness martensite structure can be ensured. In addition, in order to ensure wear resistance, a certain fraction of carbides can be left or precipitated after intensive heat treatment and used.

[0003] The residue of chromium carbide shows the effect of reducing the chromium content of the matrix, so for excellent corrosion resistance, compensation of corrosion resistance corresponding to the residue of carbide is required. In addition, the carbon content in kitchen knives and the like that need to be corrosion-resistant and wear-resistant is 0.4-0.6%, and the Cr content is 13.5-15.5%. In order to ensure the quality of the blade of the knife type, the finer the size of the residual carbide, the better. However, the hot-rolled annealed material produced by the conventional continuous casting, hot rolling, batch annealing (batch annealing furnace (BAF)) process shows that the chromium carbide (M7C3) is composed of primary chromium carbide (M7C3), chromium carbide (M7C3) which is precipitated as secondary carbide along the grain boundary during batch annealing after hot rolling. 23 The structure consists of chromium carbides (M7C3) and ferrite, wherein the primary chromium carbides (M7C3) are generated by the central segregation of Cr and C during the casting process and have a size ranging from tens to hundreds of μm. In particular, the primary chromium carbides generated at high temperatures in the center of the material remain coarse and larger than 3 μm during hot rolling and batch annealing without being decomposed. Furthermore, the higher the carbon content of the primary chromium carbides, the coarser they are and the greater their number.

[0004] Even with a certain level of cold rolling reduction, coarse carbides larger than 3 μm are difficult to separate, and remain as coarse carbides larger than 3 μm. These coarse carbides remaining in the cold-rolled material are difficult to decompose into the matrix during the strengthening heat treatment, primarily through continuous heat treatment, and remain. Furthermore, if these carbides remain concentrated in the C and Cr segregation area at the center of the thickness, linear defects remain on the surface of the tool after grinding, making it unusable as a product and increasing manufacturing costs.

[0005] U.S. Patent No. 6,273,973 B1 primarily proposes a method for removing primary carbides formed in high-carbon martensitic steel by heat treating the material in an equilibrium temperature range where primary carbides are not formed. For example, a method is proposed in which a cast steel ingot is heat treated at high temperature for a long period of time to remove coarse M7C3 carbides. Furthermore, to remove these coarse primary carbides, a method is used in which a steel ingot is heat treated at high temperature for a long period of time, followed by repeated forging to produce a hot-rolled slab, which is then reheated at high temperature for hot rolling to produce a hot-rolled steel sheet.

[0006] However, this method is limited in that it requires a long period of time at a high temperature for the primary carbides to be dissolved in order to dissolve all the generated primary carbides. The actual yield is reduced due to shrinkage cavities caused by solidification during the casting of steel ingots, so the material production requires a lot of cost and productivity is reduced.

[0007] (Patent Document 1) U.S. Patent No. 6273973B1 (December 2, 1999) Summary of the Invention

[0008] (1) Technical issues to be resolved

[0009] The object of the present invention to solve the above problems is to provide a martensitic stainless steel and a method for manufacturing the same, wherein no forging process is required and the number of primary carbides inside the hot-rolled annealed material is 22 (ea) / mm 2 the following.

[0010] The technical problems to be solved by the present invention are not limited to the above-mentioned technical problems, and those skilled in the art can clearly understand other technical solutions not involved from the following description.

[0011] (2) Technical solution

[0012] As a means for achieving the above object, a martensitic stainless steel ingot according to one embodiment of the present invention may comprise, in weight %, C: 0.4-0.55%, N: 0.01-0.1%, Si: 0.2-0.6%, Mn: 0.4-0.9%, Cr: 13.6-15.0%, Ni: 0.01-0.3%, the balance being Fe and unavoidable impurities, and having a 2000 μm thick layer in the central segregation portion. 2 The above-mentioned area fraction of primary carbides can be 2.5% or less for the martensitic stainless steel ingot.

[0013] The martensitic stainless steel ingot according to one example of the present invention may further contain at least one of 0.01-0.8% of Mo and 0.05-0.2% of V.

[0014] The martensitic stainless steel ingot according to one embodiment of the present invention may have a thickness of 250-320 mm.

[0015] According to an example of the present invention, the martensitic stainless steel may contain, in weight %, C: 0.4-0.55%, N: 0.01-0.1%, Si: 0.2-0.6%, Mn: 0.4-0.9%, Cr: 13.6-15.0%, Ni: 0.01-0.3%, the balance being Fe and unavoidable impurities, and the number of primary carbides with a diameter of 3 μm or more may be 22 (pieces / mm 2 ) or below martensitic stainless steel.

[0016] The martensitic stainless steel according to one example of the present invention may further include one or more of 0.01-0.8% of Mo and 0.05-0.2% of V.

[0017] The martensitic stainless steel according to an example of the present invention may be a martensitic stainless steel having a thickness of 4-8 mm.

[0018] According to an example of the present invention, a method for manufacturing a martensitic stainless steel slab may include the following steps: reducing a slab at a reduction rate of 2-4% during continuous casting, wherein the slab comprises, by weight%, C: 0.4-0.55%, N: 0.01-0.1%, Si: 0.2-0.6%, Mn: 0.4-0.9%, Cr: 13.6-15.0%, Ni: 0.01-0.3%, and the balance Fe and unavoidable impurities, wherein the central segregation portion has a thickness of 2000 μm. 2 A method for producing a martensitic stainless steel slab in which the area fraction of primary carbides is 2.5% or less.

[0019] The method for producing a martensitic stainless steel slab according to one example of the present invention may further include at least one of 0.01-0.8% of Mo and 0.05-0.2% of V.

[0020] The method for manufacturing a martensitic stainless steel ingot according to an embodiment of the present invention may be a method for manufacturing a martensitic stainless steel ingot having a thickness of 250-320 mm.

[0021] According to an example of the present invention, the method for producing a martensitic stainless steel slab may include a method for producing a martensitic stainless steel slab in which the reduction step is a step of performing reduction using inline rolls.

[0022] A method for manufacturing martensitic stainless steel according to an example of the present invention may include the following steps: reducing a slab at a reduction rate of 2-4% during casting to produce a cast ingot, wherein the cast ingot comprises, by weight%, C: 0.4-0.55%, N: 0.01-0.1%, Si: 0.2-0.6%, Mn: 0.4-0.9%, Cr: 13.6-15.0%, Ni: 0.01-0.3%, and the balance Fe and unavoidable impurities; The cast billet is reheated once; the cast billet that has been reheated once is hot rolled once; the cast billet that has been hot rolled once is reheated twice; the cast billet that has been reheated twice is hot rolled twice; coiling; and batch annealing, wherein the first reheating step is carried out at 1200-1300°C for 1-4 hours, the first hot rolling step is carried out at a reduction rate of 50-60%, and the second reheating step is carried out at 1200-1300°C for 1-4 hours.

[0023] The method for producing martensitic stainless steel according to one example of the present invention may further include at least one of 0.01-0.8% of Mo and 0.05-0.2% of V.

[0024] According to an example of the present invention, the method for manufacturing martensitic stainless steel may be a method for manufacturing martensitic stainless steel in which the coiling step is performed at a temperature of 700° C. or higher.

[0025] According to an example of the present invention, a method for manufacturing martensitic stainless steel may be a method for manufacturing martensitic stainless steel, wherein the batch annealing step comprises the steps of placing the steel into a hot rolling annealing furnace at a temperature above 600° C., maintaining the steel at 800-900° C. for 3-10 hours, and then maintaining the steel at 700-790° C. for 5 to 15 hours to perform hot rolling annealing.

[0026] According to an embodiment of the present invention, a method for manufacturing martensitic stainless steel may be a method for manufacturing martensitic stainless steel in which the thickness of the ingot is 250-320 mm and the total reduction ratio of hot rolling is 96.8% or more.

[0027] According to an embodiment of the present invention, a method for manufacturing martensitic stainless steel may be as follows: the number of primary carbides with a diameter of 3 μm or more in the martensitic stainless steel is 22 (pieces / mm 2 ) is a method for manufacturing the following martensitic stainless steel.

[0028] (3) Beneficial effects

[0029] According to one embodiment of the present invention, by omitting the forging process and controlling the number of primary carbides (M7C3) inside the hot-rolled annealed steel sheet to 22 per mm 2 Hereinafter, a martensitic stainless steel sheet having economical efficiency and excellent primary carbide quality and a method for producing the same can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a diagram showing primary carbides of a hot-rolled annealed material obtained under conditions in which two hot rolling steps are applied.

[0031] Figure 2 This is a diagram showing primary carbides of a hot-rolled annealed material obtained under conditions in which one hot rolling was applied.

[0032] Figure 3 This figure shows linear defects in a final product manufactured using a comparative example of the present invention. DETAILED DESCRIPTION

[0033] The preferred embodiments of the present invention are described below. However, the embodiments of the present invention can be modified into various forms, and the technical concept of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

[0034] The terms used in this invention are intended only to illustrate specific examples. Therefore, unless the context clearly indicates that the singular is required, expressions in the singular include expressions in the plural. Furthermore, it should be noted that the terms "comprising," "including," or "having" used in this invention are intended to clearly indicate the presence of the features, steps, functions, constituent elements, or combinations thereof described in the specification, and are not intended to preliminarily exclude the presence of other features, steps, functions, constituent elements, or combinations thereof.

[0035] In addition, unless otherwise defined, all terms used in this specification should be deemed to have the same meaning as commonly understood by those skilled in the art to which the present invention belongs. Therefore, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense.

[0036] In addition, when the allowable errors of manufacturing and materials inherent in the mentioned meanings are set forth, the words "about", "substantially", etc. in this specification are used in the sense of the value or close to the value, and in order to assist in understanding the present invention, the words "about", "substantially", etc. are used to prevent unscrupulous infringers from unfairly using the disclosure mentioned in the exact or absolute numerical values.

[0037] The martensitic stainless steel ingot according to one example of the present invention may contain, by weight%, C: 0.4-0.55%, N: 0.01-0.1%, Si: 0.2-0.6%, Mn: 0.4-0.9%, Cr: 13.6-15.0%, Ni: 0.01-0.3%, and the balance Fe and inevitable impurities.

[0038] Furthermore, the martensitic stainless steel slab according to one example of the present invention may further contain, in terms of weight %, one or more of 0.01-0.8% of Mo and 0.05-0.2% of V.

[0039] In addition, the martensitic stainless steel according to one example of the present invention may have the same weight % as that of the ingot.

[0040] The reasons for limiting the composition range of each alloy element will be described below.

[0041] The content of C may be 0.4-0.55 wt %.

[0042] C is an essential element for increasing the hardness of steel and should be added in appropriate amounts to ensure hardness after quenching and tempering. With this in mind, a C content of 0.4% or more may be added to meet the requirements of the present invention. However, excessively high C content may reduce the toughness of the steel plate. With this in mind, the upper limit of the C content may be set at 0.55%. Preferably, the C content may be between 0.43% and 0.53%.

[0043] The content of N may be 0.01-0.1 wt %.

[0044] Like C, N is an effective element for increasing the hardness of steel. With this in mind, N additions of 0.01% or more are acceptable. However, excessively high N contents can lead to the formation of chromium nitrides as low-temperature precipitation phases and the retention of γ phase, potentially resulting in insufficient strength after heat treatment. Consequently, excessively high N contents can degrade fatigue resistance. With this in mind, the upper limit of the N content can be limited to 0.1%.

[0045] The Si content may be 0.2-0.6 wt %.

[0046] Si is added to deoxidize steel. Furthermore, Si is an effective element for ensuring strength through solid solution strengthening. With this in mind, Si additions of 0.2% or more are acceptable. However, excessive Si content can lead to the formation of scale on the steel surface during hot rolling, potentially impairing surface quality. With this in mind, the upper limit of Si content can be limited to 0.6%.

[0047] The content of Mn may be 0.4-0.9 wt %.

[0048] Mn is a very effective element that improves hardenability and forms a substitutional solid solution in the matrix structure, thereby showing a solid solution strengthening effect. In addition, when the Mn content is low, it cannot be fully combined with sulfur (S) introduced as an impurity in the steel, which may cause continuous casting cracks, etc. In view of this, more than 0.4% Mn can be added. However, when the Mn content is too high, the toughness of the steel may deteriorate. In view of this, the upper limit of the Mn content can be limited to 0.9%.

[0049] The Cr content may be 13.6-15.0 wt %.

[0050] Cr is an effective element for improving corrosion resistance and forming chromium carbides to increase hardness and wear resistance. With this in mind, 13.6% or more of Cr can be added. However, excessively high Cr content may increase hardenability beyond necessity and potentially increase manufacturing costs. With this in mind, the upper limit of the Cr content can be limited to 15.0%.

[0051] The content of Ni may be 0.01-0.3 wt %.

[0052] Ni is an essential element added to martensitic stainless steel in order to transform the metal structure into austenite in the hot working area. In addition, Ni is an element that plays a role in improving corrosion resistance and hardenability when added in trace amounts. Taking this into account, more than 0.01% Ni can be added. However, when the Ni content is too high, the processability may deteriorate, and too much austenite may remain after strengthening heat treatment, which may make it difficult to ensure the hardness of the product and may increase manufacturing costs. Taking this into account, the upper limit of the Ni content can be limited to 0.3%.

[0053] The content of Mo may be 0.01-0.8 wt %.

[0054] Mo is an effective element for improving corrosion resistance and hardenability, and therefore can be optionally added in the present invention. Furthermore, Mo, along with V, inhibits the miniaturization and growth of carbides. With this in mind, an addition of 0.01% or more of Mo can be used. However, excessively high Mo content can increase manufacturing costs. With this in mind, the upper limit of the Mo content can be limited to 0.8%.

[0055] The content of V may be 0.05-0.2 wt %.

[0056] V is an element that effectively suppresses the coarsening of chromium carbides by forming carbides, and can be optionally added in the present invention. Furthermore, V is an effective element for preventing grain coarsening during heat treatment and improving wear resistance. However, if the V content is too high, more carbides than necessary may form, potentially reducing the toughness of the steel and increasing manufacturing costs. Considering this, the upper limit of the V content can be limited to 0.2%.

[0057] In addition, the Mo and the V may be added in combination during steelmaking.

[0058] The remaining component of the present invention is iron (Fe). However, during typical manufacturing processes, unwanted impurities inevitably enter from the raw materials or the surrounding environment, and therefore cannot be completely eliminated. These impurities are well known to those skilled in the art, and therefore, all of them are not specifically mentioned in this specification.

[0059] Hereinafter, a martensitic stainless steel slab according to one embodiment of the present invention having the above-mentioned alloy composition will be described.

[0060] The martensitic stainless steel slab of the present invention may have a central segregation portion having a thickness of 2000 μm. 2 The area fraction of primary carbides in the martensitic stainless steel slab is 2.5% or less. The central segregation area refers to the area from -15 mm to 15 mm from the center of the slab.

[0061] Furthermore, the martensitic stainless steel ingot of the present invention may be a martensitic stainless steel ingot having a thickness of 250-320 mm.

[0062] In the martensitic stainless steel slab of the present invention, the slab center segregation portion having a thickness of 2000 μm can be 2The area fraction of primary carbides in areas larger than this is controlled to less than 2.5%, and the thickness is controlled to 250-320 mm. This martensitic stainless steel ingot can be used to produce 8 mm hot-rolled annealed stainless steel while keeping the number of primary carbides with a diameter of 3 μm or greater to less than 22. As a result, after tool processing, the surface is free of linear defects, enabling the production of high-quality martensitic stainless steel plates.

[0063] Hereinafter, the martensitic stainless steel according to one embodiment of the present invention having the above-mentioned alloy composition will be described.

[0064] The martensitic stainless steel of the present invention may have a primary carbide with a diameter of 3 μm or more and a number of 22 (pieces / mm 2 ) or below martensitic stainless steel.

[0065] In addition, the thickness of the martensitic stainless steel of the present invention may be 4-8 mm. Preferably, the thickness of the martensitic stainless steel may be 5 mm.

[0066] In the martensitic stainless steel of the present invention, by controlling the number of coarse primary carbides having a diameter of 3 μm or more to 22 or less, surface linear defects in the final product can be prevented, thereby enabling production of a product of excellent quality.

[0067] Hereinafter, a method for producing a martensitic stainless steel slab having the above-mentioned alloy composition according to one embodiment of the present invention will be described.

[0068] The martensitic stainless steel casting slab of the present invention may include the following steps: reducing the slab at a reduction rate of 2-4% in continuous casting, and the slab contains, in weight%, C: 0.4-0.55%, N: 0.01-0.1%, Si: 0.2-0.6%, Mn: 0.4-0.9%, Cr: 13.6-15.0%, Ni: 0.01-0.3%, and the balance Fe and unavoidable impurities.

[0069] A slab with the aforementioned alloy composition is supplied to a tundish for continuous casting using a molten steel conveyor. The molten steel in the tundish is fed into a water-cooled mold and begins solidification. After the slab, which has only solidified on the surface, leaves the continuous casting mold, it undergoes secondary cooling with cooling water to further solidify. Unlike the surface portion in contact with the continuous casting mold, the center of the slab solidifies more slowly, leading to the segregation of carbon, chromium, and other materials in the center, which can lead to the precipitation of coarse primary carbides.

[0070] The method for producing a martensitic stainless steel slab of the present invention may include performing a reduction roll at a reduction rate of 2-4% before the slab is fully solidified. The reduction roll method of the present invention uses in-line rollers to reduce the slab before it is fully solidified by 2-4%. Performing the reduction roll before the slab is fully solidified removes shrinkage cavities in the center of the slab, squeezes segregated molten steel, and minimizes segregation, thereby reducing the area where primary carbides form.

[0071] In the method for producing a martensitic stainless steel slab of the present invention, by rolling down the slab at a rolling reduction rate of 2-4% before the slab is completely solidified, the slab has a thickness of 2000 μm based on the segregation portion in the center of the slab. 2 The area fraction of primary carbides in the above area can be 2.5% or less. The central segregation area refers to the area from -15 mm to +15 mm from the center of the slab.

[0072] When the slab is rolled down at a reduction rate of less than 2% before the slab is completely solidified, the slab does not break, but a 2000 μm thick layer is formed in the center of the slab. 2 The area fraction of primary carbides in the area above 4% exceeds 2.5%. When the reduction rate exceeds 4%, the slab will crack or break. Therefore, in the present invention, the reduction rate is set to 2-4% before the slab is completely solidified.

[0073] The method for manufacturing a martensitic stainless steel slab of the present invention can be a method for manufacturing a slab having a thickness of 250-320 mm. When the thickness is less than 250 mm, even if the slab is rolled down at a reduction rate of 2-4% before the slab is completely solidified, the slab center segregation portion has a thickness of 2000 μm. 2 The area fraction of primary carbides in the above area will also exceed 2.5%. In addition, in order to make the total hot rolling reduction ratio in the subsequent step 96.8% or more, the thickness of the slab is preferably 250-320 mm.

[0074] Therefore, a 4-8 mm hot-rolled annealed stainless steel can be obtained by using the slab produced by the method for producing a martensitic stainless steel slab of the present invention.

[0075] Hereinafter, a method for producing the martensitic stainless steel according to one embodiment of the present invention having the above-mentioned alloy composition will be described.

[0076] The method for producing martensitic stainless steel of the present invention may comprise the following steps: reducing a slab at a reduction rate of 2-4% during casting to produce a cast slab, wherein the slab contains, in weight %, C: 0.4-0.55%, N: 0.01-0.1%, Si: 0.2-0.6%, Mn: 0.4-0.9%, Cr: 13.6-15.0%, Ni: 0.01-0.3%, and the balance Fe and unavoidable impurities; performing a primary reheating on the cast slab; performing a primary hot rolling on the cast slab after the primary reheating; performing a secondary reheating on the cast slab after the secondary hot rolling; coiling; and batch annealing.

[0077] Even if the rolling reduction rate is 2-4% before the slab is completely solidified, the slab with a thickness of 2000 μm based on the central segregation part of the slab will be reduced. 2 The area fraction of primary carbides in the above area is controlled to be below 2.5%, and coarse carbides may remain during the process of manufacturing the hot-rolled steel sheet.

[0078] The method for manufacturing martensitic stainless steel of the present invention may include the following steps: after the step of manufacturing a cast slab, performing a primary reheating step at 1200-1300° C. for 1-4 hours, performing a primary hot rolling step at a reduction ratio of 50-60%, performing a secondary reheating step at 1200-1300° C. for 1-4 hours, and performing secondary hot rolling on the cast slab that has undergone the secondary reheating, coiling, and batch annealing.

[0079] The method for producing martensitic stainless steel of the present invention includes two reheating and hot rolling steps, thereby ensuring the thermal decomposition and mechanical pulverization of carbides. After the first reheating and hot rolling, a second reheating and hot rolling can be performed without batch annealing.

[0080] Typically, slabs produced by ingot forging or continuous casting are reheated at high temperatures and then subjected to hot rolling, coiling, and batch annealing to obtain annealed hot-rolled steel sheets. However, when continuous casting is performed, a single reheating and hot rolling step is included, and the pyrolysis and mechanical crushing of the primary carbides is insufficient. Consequently, the quality of the primary carbides is inferior compared to when the ingot forging step is included. However, when the ingot forging step is included, production time is prolonged, and shrinkage cavities formed during the casting of the ingots, etc., result in a decrease in the actual yield, thus limiting productivity and cost.

[0081] According to the method for producing martensitic stainless steel of the present invention, the ingot is subjected to primary reheating and hot rolling, and the primary hot-rolled material is not subjected to coiling and batch annealing. Instead, continuous casting is performed by performing the steps of secondary reheating and hot rolling after the primary hot rolling. Therefore, compared with the case of ingot forging, the quality of the primary carbide is excellent, while productivity and cost can be ensured.

[0082] According to the method for producing martensitic stainless steel of the present invention, in order to ensure that the thickness of the stainless steel is 4-8 mm, the thickness of the ingot can be 250-320 mm.

[0083] By using the martensitic stainless steel slab of the present invention with a thickness of 250-320 mm, and performing a primary hot rolling step and a secondary hot rolling step at a reduction ratio of 50-60%, the total hot rolling reduction ratio can be greater than 96.8%. Consequently, the thickness of the hot-rolled annealed stainless steel sheet can be maintained at 4-8 mm.

[0084] When the slab is subjected to only one reheating and hot rolling step, even if the slab has a thickness of 2000 μm in the center, 2 The area fraction of primary carbides with an area greater than 3 μm is less than 2.5%, and the number of primary carbides with a diameter of 3 μm or greater in the hot-rolled annealed material may exceed 22. Hot-rolled annealed material with coarse carbide aggregates produces surface linear defects after tool processing. When the primary reheating and hot rolling are performed directly without batch annealing, the number of primary carbides with a diameter of 3 μm or greater in the hot-rolled annealed material is less than 22, and the primary carbides are refined, so that the primary carbides can have excellent quality. This hot-rolled annealed material can be free of surface linear defects after tool processing.

[0085] The method for producing martensitic stainless steel of the present invention may include the following steps: secondary hot rolling followed by coiling; and batch annealing. The coiling step may be performed at a temperature above 700°C. The batch annealing step may be performed by placing the steel into a hot rolling annealing furnace at a temperature above 600°C, maintaining the steel at 800-900°C for 3-10 hours, and then maintaining the steel at 700-790°C for 5 to 15 hours for hot rolling annealing.

[0086] The method for producing martensitic stainless steel of the present invention can produce primary carbides with a diameter of 3 μm or more and a number of 22 (pieces / mm 2 ) or less. In addition, the martensitic stainless steel of the present invention can produce products of excellent quality by preventing surface linear defects of the final product.

[0087] The martensitic stainless steel ingot of the present invention can be produced by the method for producing the martensitic stainless steel ingot of the present invention. Furthermore, the martensitic stainless steel ingot of the present invention can be produced by using the martensitic stainless steel ingot of the present invention. Furthermore, the martensitic stainless steel ingot of the present invention can be produced by the method for producing the martensitic stainless steel of the present invention.

[0088] The martensitic stainless steel of the present invention can be used as high-hardness and high-corrosion-resistant cutting tools. Furthermore, the martensitic stainless steel of the present invention can be used as automotive parts. Furthermore, it can be applied to other applications that can utilize the physical properties of the stainless steel obtained by the present invention.

[0089] The present invention is described in more detail below using examples. However, these examples are intended only to illustrate the present invention and are not intended to limit the present invention. The scope of the present invention is determined by the claims and any reasonable derivation thereof.

[0090] {Example}

[0091] Slabs having alloy compositions according to Table 1 below were reduced by 0-6% using inline rolls before the strands were fully solidified.

[0092] In the cases of Comparative Examples 1, 3, 5, 8, 10, 11, 12, 14, and 15, the ingots were reheated at approximately 1250°C for approximately 3 hours and hot rolled to a final thickness of 8 mm, thereby performing one reheating and hot rolling.

[0093] In the cases of Inventive Examples 1 to 4 and Comparative Examples 2, 4, 6, 9 and 13, the cast ingots were reheated once at about 1250° C. for about 3 hours and hot rolled once at a reduction ratio of about 55%. Then, the steel plates that had been rolled once were charged into a reheating furnace again and reheated a second time at about 1250° C. for about 3 hours and hot rolled a second time to a final thickness of 8 mm, whereby reheating and hot rolling were performed a total of two times.

[0094] The hot-rolled steel sheets were coiled at about 700° C. The coiled coils were loaded into a hot rolling annealing furnace at about 600° C. for batch annealing and held at 850° C. for about 10 hours. As a second step, they were held at about 750° C. for about 10 hours for batch annealing.

[0095] A cast material test piece was prepared from the center of the width direction of the obtained ingot, and the image of the cast material was analyzed by optical microscope. The segregation part in the center of the ingot (-15 mm to +15 mm from the center) with a thickness of 2000 μm was analyzed. 2In order to analyze the primary carbides, the hot-rolled test piece was cut to prepare a test piece, and the sample was analyzed by image analysis using an optical microscope to obtain the number of primary carbides with a diameter of 3 μm or more (pieces / mm 2 ), based on the occurrence of cracks with a length of 5 mm or more, confirm whether the slab is broken, and after tool processing, visually confirm whether there are surface linear defects, and show them in Table 2.

[0096] Table 1 shows the component system of the alloy composition.

[0097] [Table 1]

[0098]

[0099] Table 2 shows the thickness of the ingot, the reduction rate of the ingot before it is completely solidified during casting, whether the slab is broken, the area fraction of primary carbides, the number of reheating and hot rolling times, the number of primary carbides, and whether there are linear defects on the surface.

[0100] In addition, Table 2 is based on a hot rolled annealed material thickness of 8 mm.

[0101] [Table 2]

[0102]

[0103] In Invention Examples 1 to 4, the slabs were cast with a thickness of 250-320 mm and the slabs were rolled down at a reduction rate of 2-4% before the slabs were completely solidified. It was confirmed that the slabs of Invention Examples 1 to 4 did not break, and the slabs had a 2000 μm thick segregation in the center of the slabs. 2 The area fraction of carbides in the area above 2.5% is less than 2.5%. This confirms that by reducing the slab at a reduction rate of 2-4% before the slab is completely solidified during casting, shrinkage cavities in the center of the slab are removed and segregation is minimized, thereby reducing the area of coarse primary carbides.

[0104] Furthermore, in Inventive Examples 1 to 4, it was confirmed that the number of primary carbides with diameters of 3 μm or greater in the hot-rolled annealed material, which was subjected to a reduction ratio of 2-4% before the cast slab was fully solidified and then reheated and hot rolled twice, was 22 or fewer. This confirmed that when continuous casting without forging is performed and batch annealing is not performed directly after the first reheating and hot rolling, but rather a second reheating and hot rolling, the number of primary carbides with diameters of 3 μm or greater in the hot-rolled annealed material is 22 or fewer, resulting in the refinement of the primary carbides and excellent primary carbide quality. Furthermore, it was confirmed that this hot-rolled annealed material does not produce surface linear defects after tool processing.

[0105] In Comparative Examples 1 and 2, the slabs were not reduced during casting until they were completely solidified. While the slabs in Comparative Examples 1 and 2 did not fracture, the area fractions of coarse primary carbides in the central segregation were excessively high, at 6.6% and 6.3%, respectively.

[0106] In addition, when Comparative Example 1 was subjected to one hot rolling, it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 71, and the quality of the primary carbides was poor. It was confirmed that surface linear defects were generated in the product processed using Comparative Example 1. When Comparative Example 2 was subjected to two hot rollings, it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 56, and the quality of the primary carbides of Comparative Example 2 was slightly better than that of Comparative Example 1. However, it was confirmed that surface linear defects were generated in Comparative Example 1 and Comparative Example 2 after product processing. Therefore, it can be seen that in the manufacturing method of martensitic stainless steel, when the reheating and hot rolling steps are performed a total of more than 2 times, the quality of the primary carbides is excellent.

[0107] In Comparative Examples 3 and 4, the slabs were rolled down at a 2% reduction rate before complete solidification. While the area fraction of coarse primary carbides in the central segregation portion of the slabs in Comparative Examples 3 and 4 was lower than that in Comparative Examples 1 and 2, the area fractions of coarse primary carbides in the central segregation portion of the slabs were still excessively high, at 5.3% and 5.2% respectively. Therefore, it was confirmed that the desired effect of reducing the area fraction of primary carbides in the central segregation portion of the slab, as used in the production of 8 mm thick hot-rolled annealed material, may not be achieved when the slab thickness reaches 200 mm.

[0108] In addition, Comparative Example 3 was subjected to one hot rolling, and it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 53, and the quality of the primary carbides was poor. It was confirmed that surface linear defects were generated in the product processed using Comparative Example 3. Comparative Example 4 was subjected to two hot rollings, and it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 41, and the quality of the primary carbides of Comparative Example 4 was slightly better than that of Comparative Example 3. However, it was confirmed that surface linear defects were generated in Comparative Example 3 and Comparative Example 4 after product processing. Therefore, it can be seen that in the manufacturing method of martensitic stainless steel, when the reheating and hot rolling steps are performed a total of more than 2 times, the quality of the primary carbides is excellent.

[0109] During casting, Comparative Examples 5 and 6 were rolled down at a 4% reduction rate before the slabs were fully solidified. While the area fraction of coarse primary carbides in Comparative Examples 5 and 6 was lower than that in Comparative Examples 3 and 4, the area fraction of coarse primary carbides in the central segregation portion of the slabs was still excessively high at 2.6% and 2.6%. Therefore, it was confirmed that the desired effect of reducing the area fraction of primary carbides in the central portion of the slab, as intended by the present invention, may not be achieved when the slab thickness reaches 200 mm for producing 8 mm hot-rolled annealed material.

[0110] In addition, Comparative Example 5 was subjected to one hot rolling, and it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 36, and the quality of the primary carbides was poor. It was confirmed that surface linear defects were generated in the product processed using Comparative Example 5. Comparative Example 6 was subjected to two hot rollings, and it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 28, and the quality of the primary carbides of Comparative Example 6 was slightly better than that of Comparative Example 5. However, it was confirmed that surface linear defects were generated in Comparative Example 5 and Comparative Example 6 after product processing. Therefore, it can be seen that in the manufacturing method of martensitic stainless steel, when the reheating and hot rolling steps are performed a total of more than 2 times, the quality of the primary carbides is excellent.

[0111] In Comparative Example 7, the slab was reduced at a reduction ratio of 6% before the slab was completely solidified. The slab in Comparative Example 7 fractured. Therefore, it can be seen that when reducing the slab before the slab is completely solidified during casting, the appropriate reduction ratio is 2-4%.

[0112] In Comparative Examples 8 and 9, the slabs were not rolled down before they were completely solidified. While the slabs in Comparative Examples 8 and 9 did not fracture, the area fractions of coarse primary carbides in the central segregation of the slabs were excessively high, at 5.7% and 5.3%, respectively. Comparing Comparative Examples 8 and 9 with Inventive Examples 1 and 2 confirms that rolling down the slab at a reduction rate of 2-4% before it is completely solidified can reduce the area fraction of coarse primary carbides in the central segregation of the slab.

[0113] In addition, when Comparative Example 8 was subjected to the first hot rolling, it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 67, and the quality of the primary carbides was poor. It was confirmed that surface linear defects were generated in the product processed using Comparative Example 8. When Comparative Example 9 was subjected to the second hot rolling, it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 49, and the quality of the primary carbides of Comparative Example 9 was slightly better than that of Comparative Example 8. However, it was confirmed that surface linear defects were generated in Comparative Example 8 and Comparative Example 9 after product processing. Therefore, it can be seen that in the manufacturing method of martensitic stainless steel, the quality of the primary carbides is excellent when the reheating and hot rolling steps are performed a total of more than 2 times.

[0114] Furthermore, although the reheating and hot rolling steps were performed twice in Comparative Example 9, as in Inventive Examples 1 and 2, the slab was not reduced during casting in Comparative Example 9 before the slab was completely solidified, as described above. Therefore, it was confirmed that the quality of the primary carbides differed when the slab was cast at a reduction rate of 2-4% before the slab was completely solidified, and the reheating and hot rolling steps were performed twice or more in order to obtain an 8 mm hot-rolled annealed material. Furthermore, it was confirmed that this hot-rolled annealed material did not produce surface linear defects after final product processing.

[0115] In the casting of Comparative Examples 10 and 11, the slabs were reduced at reduction rates of 2% and 4% before the slabs were completely solidified. It was confirmed that the area fraction of coarse primary carbides in the center of the slab was 2.5% and 1.7%, respectively, so the area fraction of primary carbides could be ensured to be less than 2.5%. However, when Comparative Examples 10 and 11 were subjected to one hot rolling, it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed materials was 51 and 29, respectively, indicating that the quality of the primary carbides was poor. In addition, it was confirmed that surface linear defects were generated in the products processed from them. In addition, when Comparative Examples 10 and 11 were compared with Inventive Examples 1 and 2, it was found that the quality of the primary carbides was excellent when the reheating and hot rolling steps were performed a total of more than two times. In addition, it was confirmed that such hot-rolled annealed materials did not generate surface linear defects after processing into the final product.

[0116] In Comparative Examples 12 and 13, the slabs were not rolled down before they were completely solidified. While the slabs in Comparative Examples 12 and 13 did not fracture, the area fractions of coarse primary carbides in the central segregation portion of the slabs were excessively high, at 5.7% and 6.1%, respectively.

[0117] In addition, when Comparative Example 12 was subjected to the first hot rolling, it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 63, and the quality of the primary carbides was poor. It was confirmed that surface linear defects were generated in the product processed by Comparative Example 12. When Comparative Example 13 was subjected to the second hot rolling, it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed material was 27, and the quality of the primary carbides of Comparative Example 13 was slightly better than that of Comparative Example 1. However, it was confirmed that surface linear defects were generated in Comparative Example 12 and Comparative Example 13 after product processing. Therefore, it can be seen that in the manufacturing method of martensitic stainless steel, when the reheating and hot rolling steps are performed a total of more than 2 times, the quality of the primary carbides is excellent.

[0118] In Comparative Examples 14 and 15, the slabs were pressed down at reduction rates of 2% and 4% before the slabs were completely solidified. It was confirmed that the area fraction of coarse primary carbides in the center of the slab was 2.4% and 1.8%, respectively, so the area fraction of primary carbides could be ensured to be 2.5% or less. However, when Comparative Examples 14 and 15 were subjected to one hot rolling, it was confirmed that the number of primary carbides with a diameter of 3 μm or more in the hot-rolled annealed materials was 46 and 27, respectively, indicating that the quality of the primary carbides was poor. In addition, it was confirmed that surface linear defects were generated in the products processed using Comparative Examples 14 and 15. Therefore, it can be seen that in the manufacturing method of martensitic stainless steel, the quality of primary carbides is excellent when the reheating and hot rolling steps are performed a total of more than two times. In addition, it was confirmed that such hot-rolled annealed materials did not generate surface linear defects after processing into the final product.

[0119] In Comparative Example 16, the slab was rolled down at a reduction rate of 6% before the cast piece completely solidified. The slab in Comparative Example 16 fractured. Therefore, it can be seen that during casting, it is appropriate to roll down the slab at a reduction rate of 2-4% before the cast piece completely solidifies.

[0120] Reference Figure 1 and Figure 2 It was confirmed that in the present invention, the number of primary carbides with a diameter of 3 μm or more in the martensitic stainless steel subjected to secondary hot rolling in a slab subjected to 2-4% slab reduction in continuous casting was smaller than that in a martensitic stainless steel subjected to only one hot rolling without separate reduction in casting. In the present invention, the number of primary carbides with a diameter of 3 μm or more in the slab center segregation portion of a slab subjected to 2-4% slab reduction in continuous casting was smaller. 2 The area fraction of carbides with an area of more than 3 μm can be less than 2.5%. In addition, when the cast billet is subjected to secondary hot rolling to produce martensitic stainless steel, the number of primary carbides with a diameter of more than 3 μm can be 22 (pieces / mm 2) or less. In addition, when the final product is manufactured using martensitic stainless steel, linear defects may not be caused.

[0121] Reference Figure 3 The comparative example of the present invention is a 2000 μm 2 In martensitic stainless steels having an area fraction of primary carbides exceeding 2.5% and / or having a diameter of 3 μm or greater exceeding 22, it has been confirmed that poor quality of primary carbides can cause linear defects in the final product.

Claims

1. A martensitic stainless steel ingot, comprising, by weight, 0.4-0.55% C, 0.01-0.1% N, 0.2-0.6% Si, 0.4-0.9% Mn, 13.6-15.0% Cr, 0.01-0.3% Ni, and the remainder Fe and unavoidable impurities. The central segregation area has a thickness of 2000 μm 2 The area fraction of primary carbides in the above area is 2.5% or less.

2. The martensitic stainless steel ingot according to claim 1, wherein: The martensitic stainless steel slab further contains one or more of 0.01-0.8% of Mo and 0.05-0.2% of V.

3. The martensitic stainless steel ingot according to claim 1, wherein: The thickness of the martensitic stainless steel ingot is 250-320 mm.

4. A martensitic stainless steel comprising, in weight %, 0.4-0.55% C, 0.01-0.1% N, 0.2-0.6% Si, 0.4-0.9% Mn, 13.6-15.0% Cr, 0.01-0.3% Ni, and the remainder Fe and unavoidable impurities. The number of primary carbides with a diameter of 3 μm or more is 22 (pieces / mm 2 )the following.

5. The martensitic stainless steel according to claim 4, wherein: The martensitic stainless steel further contains one or more of 0.01-0.8% of Mo and 0.05-0.2% of V.

6. The martensitic stainless steel according to claim 4, wherein: The thickness of the martensitic stainless steel is 4-8 mm.

7. A method for producing a martensitic stainless steel ingot, comprising the following steps: The slab is reduced at a reduction rate of 2-4% during continuous casting, and the slab comprises, by weight%, C: 0.4-0.55%, N: 0.01-0.1%, Si: 0.2-0.6%, Mn: 0.4-0.9%, Cr: 13.6-15.0%, Ni: 0.01-0.3%, and the balance Fe and unavoidable impurities. Among them, the central segregation part has 2000μm 2 The area fraction of primary carbides in the above area is 2.5% or less.

8. The method for producing a martensitic stainless steel ingot according to claim 7, wherein: It further contains one or more of 0.01-0.8% of Mo and 0.05-0.2% of V.

9. The method for producing a martensitic stainless steel ingot according to claim 7, wherein: The thickness of the ingot is 250-320 mm.

10. The method for producing a martensitic stainless steel ingot according to claim 7, wherein: The pressing step is a step of performing pressing using straight rollers.

11. A method for producing martensitic stainless steel, comprising the following steps: The slab is reduced at a reduction rate of 2-4% during casting to produce a cast ingot, wherein the slab comprises, in terms of weight %, C: 0.4-0.55%, N: 0.01-0.1%, Si: 0.2-0.6%, Mn: 0.4-0.9%, Cr: 13.6-15.0%, Ni: 0.01-0.3%, and the balance Fe and unavoidable impurities; reheating the cast billet once; hot rolling the reheated slab; reheating the cast slab after the primary hot rolling for a second time; performing secondary hot rolling on the cast slab that has undergone the secondary reheating; Rolling up; as well as Batch annealing, The first reheating step is carried out at 1200-1300°C for 1-4 hours. The first hot rolling step is carried out at a reduction rate of 50-60%, The secondary reheating step is performed at 1200-1300° C. for 1-4 hours.

12. The method for producing martensitic stainless steel according to claim 11, wherein: It further contains one or more of 0.01-0.8% of Mo and 0.05-0.2% of V.

13. The method for producing martensitic stainless steel according to claim 11, wherein: The winding step is performed at a temperature above 700°C.

14. The method for producing martensitic stainless steel according to claim 11, wherein: The batch annealing step is a step of placing the steel into a hot rolling annealing furnace at a temperature above 600° C., maintaining the temperature at 800-900° C. for 3-10 hours, and then maintaining the temperature at 700-790° C. for 5 to 15 hours to perform hot rolling annealing.

15. The method for producing martensitic stainless steel according to claim 11, wherein: The thickness of the cast slab is 250-320 mm, and the total reduction ratio of hot rolling is above 96.8%.

16. The method for producing martensitic stainless steel according to claim 11, wherein: The number of primary carbides with a diameter of 3 μm or more in the martensitic stainless steel is 22 (pieces / mm 2 )the following.

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