EP4589043A1

EP4589043A1 - Martensitic stainless steel and method of manufacturing same

Info

Publication number: EP4589043A1
Application number: EP23880311.8A
Authority: EP
Inventors: Byoungjun SONG; Sanghoon Kim; Gyujin JO
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-10-21
Filing date: 2023-10-23
Publication date: 2025-07-23
Also published as: WO2024085736A1; CN120435582A; KR20240056258A; EP4589043A4

Abstract

A martensitic stainless steel cast according to one embodiment of the present invention may be a martensitic stainless steel cast comprising in weight%: C: 0.4% to 0.55%, N: 0.01% to 0.1%, Si: 0.2% to 0.6%, Mn: 0.4% to 0.9%, Cr: 13.6% to 15.0%, Ni: 0.01% to 0.3%, and the remainder including Fe and inevitable impurities, wherein the area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone is 2.5% or less.

Description

[Technical Field]

The present disclosure relates to a martensitic stainless steel cast including high-quality primary carbides, a stainless steel, and a method of manufacturing the same, and more particularly, to a martensitic stainless steel economical by omitting a forging process by controlling the number of primary carbides and applicable to edge tools and automotive parts having high hardness and high corrosion resistance, and a method of manufacturing the same.

[Background Art]

With the rise of standard of living, the application of martensitic stainless steels having high hardness, high strength, and high corrosion resistance is increasing in household kitchen knives, industrial band saws, and the like. For this use, high hardness, high strength, abrasion resistance, and corrosion resistance are required, and tempered martensite structures, formed by strengthening heat treatment and tempering, are used to obtain high hardness and high strength properties. Such a tempered martensite structure is a very hard structure obtained by forming an austenite phase stable at a high temperature by strengthening heat treating and tempering an annealed structure (ferrite+fine chromium carbides), and then quickly cooling the structure. As the content of a solid solution of C increases in a matrix, a martensite structure with a higher hardness may be obtained. Meanwhile, in order to obtain abrasion resistance, a certain fraction of carbides may be remained and precipitated after the strengthening heat treatment.
Because the residue of chromium carbides has an effect of reducing the Cr content in a matrix, compensation for corrosion resistance corresponding to the residue of carbides is required to obtain excellent corrosion resistance. In addition, kitchen knives that require corrosion resistance and abrasion resistance include 0.4 to 0.6% of C and 13.5 to 15.5% of Cr. The finer the residual carbides, the higher quality the edges of knives. However, common hot-annealed steel materials manufactured via casting, hot rolling, and batch annealing (batch annealing furnace (BAF) processes has a structure consisting of primary chromium carbides (M₇C₃) having a size of several tens to hundreds of micrometers (µm) formed by central segregation of Cr and C during a casting process, secondary chromium carbide (M₂₃C₆) precipitated first along crystal grain boundaries during batch annealing after the hot rolling, and ferrite. Particularly, the primary chromium carbides formed in the center of the steel material at a high temperature are not decomposed during the hot rolling and batch annealing but remain in a coarse form. In addition, as the C content increases, the primary chromium carbides coarsen and the number thereof increases.
It is difficult to fragment coarse carbides with a size of 3 µm or more into small pieces even by cold rolling by applying a certain level of pressure, so that coarse carbides with a size of 3 µm or more remain. These coarse carbides remaining in cold-rolled steel materials are difficult to decompose by strengthening heat treatment performed by continuous heat treatment, but remain. Also, the coarse carbides are concentrated in the C and Cr segregation zones of the thickness center, thereby leaving linear defects on surfaces of products after the steel materials are polished into knives and causing an problem of increasing manufacturing costs.
US Patent No. 6273973 B1 mainly discloses methods for heat treatment of a material in an equilibrium temperature range where primary carbides are not formed to remove primary carbides formed in a high-carbon martensitic steel, such as a method for heat treating a cast ingot for a long time at a high temperature to remove coarse M₇C₃ carbides. In addition, in order to remove such coarse primary carbides, a method of manufacturing a hot-rolled steel sheet including preparing a slab for hot rolling via repeated forging processes after heat treating an ingot at a high temperature for a long time, and hot rolling the slab by reheating at a high temperature is used.
However, such a method has limitations of high manufacturing costs to produce steel materials and low productivity because all the formed primary carbides solidify only by maintaining the high temperature, where the primary carbides completely solidify, for a long time and an actual yield decreases due to shrinkage cavities by solidification during casting the ingot.
(Patent Document 1) US Patent No. 6273973 B1 (December 2, 1999 ) [Disclosure]

[Technical Problem]

The present disclosure has been proposed to solve the above-described problems, and provided are a martensitic stainless steel in which the number of primary carbides in a hot-annealed steel material is controlled to 22 ea/mm² or less without a forging process and a method of manufacturing the same.
However, the technical problems to be solved by the present disclosure are not limited to the aforementioned problems, and any other technical problems not mentioned herein will be clearly understood from the following description by those skilled in the art to which the present disclosure pertains.

[Technical Solution]

A martensitic stainless steel cast according to an embodiment of the present disclosure includes, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities, wherein an area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone is 2.5 % or less.
The martensitic stainless steel cast according to an embodiment of the present disclosure may further include at least one of 0.01 to 0.8% of Mo and 0.05 to 0.2% of V.
The martensitic stainless steel cast according to an embodiment of the present disclosure may have a thickness of 250 to 320 mm.
A martensitic stainless steel according to an embodiment of the present disclosure includes, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities, wherein the number of primary carbides having a diameter of 3 µm or more is 22 (ea/mm²) or less.
The martensitic stainless steel according to an embodiment of the present disclosure may further include at least one of 0.01 to 0.80% of Mo and 0.05 to 0.20% of V.
The martensitic stainless steel according to an embodiment of the present disclosure may have a thickness of 4 to 8 mm.
A method of manufacturing a martensitic stainless steel cast according to an embodiment of the present disclosure includes reducing a slab including, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities, with reduction ratio of 2 to 4% during a continuous casting process, wherein an area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone is 2.5% or less.
In the method of manufacturing a martensitic stainless steel cast according to an embodiment of the present disclosure, the slab may further include at least one of 0.01 to 0.8% of Mo and 0.05 to 0.2% of V.
In the method of manufacturing a martensitic stainless steel cast according to an embodiment of the present disclosure, a thickness of the cast may be 250 to 320 mm.
In the method of manufacturing a martensitic stainless steel cast according to an embodiment of the present disclosure, the reducing may be performed using an in-line roller.
A method of manufacturing a martensitic stainless steel according to an embodiment of the present disclosure includes: reducing a slab including, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities with a reduction ratio of 2 to 4% into a cast; first reheating the cast; first hot rolling the first reheated cast; second reheating the first hot-rolled cast; second hot rolling the second reheated cast; coiling the cast; and batch annealing the cast, wherein the first reheating is performed at 1200 to 1300 °C for 1 to 4 hours, the first hot rolling is performed with a reduction ratio of 50 to 60%, and the second reheating is performed at 1200 to 1300 °C for 1 to 4 hours.
In the method of manufacturing a martensitic stainless steel according to an embodiment of the present disclosure, the slab may further include at least one of 0.01 to 0.8% of Mo and 0.05 to 0.2% of V.
In the method of manufacturing a martensitic stainless steel according to an embodiment of the present disclosure, the coiling may be performed at a temperature of 700°C or above.
In the method of manufacturing a martensitic stainless steel according to an embodiment of the present disclosure, the batch annealing may be hot annealing performed by placing the cast in a hot annealing furnace at a temperature of 600°C or above, and maintaining the cast at a temperature of 800 to 900°C for 3 to 10 hours and then at a temperature of 700 to 790°C for 5 hours to 15 hours.
In the method of manufacturing a martensitic stainless steel according to an embodiment of the present disclosure, a thickness of the cast is 250 to 320 mm, and a total reduction ratio of hot rolling is 96.8% or more.
In the method of manufacturing a martensitic stainless steel according to an embodiment of the present disclosure, the number of primary carbides having a diameter of 3 µm or more is 22 (ea/mm²) or less in the martensitic stainless steel.

[Advantageous Effects]

According to an embodiment of the present disclosure, provided are a martensitic stainless steel sheet economical and including high-quality primary carbides by omitting a forging process and controlling the number of primary carbides (M₇C₃) in a hot-annealed steel sheet to 22 ea/mm² or less, and a method of manufacturing the same.

[ Description of Drawings ]

FIG. 1 is a view showing primary carbides of a hot-annealed steel sheet obtained by applying hot rolling twice.
FIG. 2 is a view showing primary carbides of a hot-annealed steel sheet obtained by applying hot rolling once.
FIG. 3 is a view showing linear defects on a final product manufactured according to a comparative example.

[Modes of the Invention]

Hereinafter, embodiments of the present disclosure will be described. However, the present disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.
The terms used herein are merely used to describe particular embodiments. Therefore, an expression used in the singular encompasses the expression of the plural, unless it should be clearly singular in the context. In addition, it is to be understood that the terms such as "including" or "having" are intended to indicate the existence of features, steps, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that other features, steps, functions, components, or combinations thereof may exist or may be added.
Meanwhile, unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Thus, these terms should not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
Also, the terms "about", "substantially", etc. used throughout the specification means that when natural manufacturing and substance allowable errors are suggested, such allowable errors correspond to the value or is similar to the value, and such values are intended for the sake of clear understanding of the present disclosure or to prevent an unconscious infringer from illegally using the present disclosure of the present.
A martensitic stainless steel cast according to an embodiment of the present disclosure may include, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni i, and the remainder including Fe and inevitable impurities.
In addition, the martensitic stainless steel cast according to an embodiment of the present disclosure may further include, in wt%, at least one of 0.01 to 0.8% of Mo and 0.05 to 0.2% of V.
In addition, the martensitic stainless steel according to an embodiment of the present disclosure may have the same wt% as that of the cast.
Hereinafter, reasons for numerical limitations on the contents of alloying elements in the embodiment of the present disclosure will be described.
The content of C may be 0.4 to 0.55 wt%.
C, as an essential element for improving hardness of a steel, should be added appropriately to obtain hardness after quenching and tempering heat treatment. In consideration thereof, C may be added in an amount of 0.4% or more to satisfy the purpose of the present disclosure. However, an excess of C may impair toughness of a steel sheet. In consideration thereof, the upper limit of the C content may be controlled to 0.55%. Preferably, the C content may be at least 0.43% but not more than 0.53%.
The content of N may be 0.01% to 0.1 wt%.
Like C, N is an effective element for improving hardness of a steel. In consideration thereof, N may be added in an amount of 0.01% or more. However, an excess of N may cause formation of a chromium nitride that is a low-temperature precipitation phase and cause a residual γ phase, thereby impairing strength after strengthening heat treatment. Therefore, an excess of N may deteriorate fatigue resistance. In consideration thereof, the upper limit of the N content may be controlled to 0.1%.
The content of Si may be 0.2 to 0.6 wt%.
Si is added to deoxidize a steel. In addition, Si is an effective element for obtaining strength by solid solution strengthening. In consideration thereof, Si may be added in an amount of 0.2% or more. However, an excess of Si may cause formation of scales on the surface of a steel impairing the surface quality. In consideration thereof, the upper limit of the Si content may be controlled to 0.6%.
The content of Mn may be 0.4 to 0.9 wt%.
Mn is a very effective element for improving hardenability and obtaining a solid solution strengthening effect by forming a substitutional solid solution in a matrix. Also, at a low Mn content, Mn cannot sufficiently combine with S introduced into a steel as an impurity, thereby causing cracks during continuous casting. In consideration thereof, Mn may be added in an amount of 0.4% or more. However, an excess of Mn may deteriorate toughness of a steel. In consideration thereof, the upper limit of the Mn content may be controlled to 0.9%.
The content of Cr may be 13.6 to 15.0 wt%.
Cr is an effective element for improving corrosion resistance and improving hardness and abrasion resistance by forming a chromium carbide. In consideration thereof, Cr may be added in an amount of 13.6% or more. However, an excess of Cr may excessively increase hardenability and increase manufacturing costs. In consideration thereof, the upper limit of the Cr content may be controlled to 15.0%.
The content of Ni may be 0.01% to 0.3 wt%.
Ni is an essential element added to transform a metal structure into an austenite structure in a hot working area in a martensitic stainless steel. In addition, when added in a trace amount, Ni is an element serving to improve corrosion resistance and quenchability. In consideration thereof, Ni may be added in an amount of 0.01% or more. However, an excess of Ni may impair workability, make it difficult to obtain hardness of a product because austenite excessively remains after strengthening heat treatment, and increase manufacturing costs. In consideration thereof, the upper limit of the Ni content may be controlled to 0.3%.
The content of Mo may be 0.01 to 0.8 wt%.
Mo, as an effective element for improving corrosion resistance and hardenability, may be optionally added in the present disclosure. In addition, together with V, Mo is an element inhibiting refinement and growth of a carbide. In consideration thereof, Mo may be added in an amount of 0.01% or more. However, an excess of Mo may increase manufacturing costs. In consideration thereof, the upper limit of the Mo content may be controlled to 0.8%.
The content of V may be 0.05% to 0.2 wt%.
V, as an effective element for inhibiting a chromium carbide from coarsening by forming a carbide, may be optionally added in the present disclosure. In addition, V is an effective element for inhibiting crystal grains from coarsening during heat treatment and improving abrasion resistance. However, an excess of V may form a carbide more than necessary to impair toughness of a steel and increase manufacturing costs. In consideration thereof, the upper limit of the V content may be controlled to 0.2%.
Meanwhile, the Mo and V may be added in combination in the manufacture of a steel.
The remaining component of the composition of the present disclosure is iron (Fe). However, the composition may include unintended impurities inevitably incorporated from raw materials or surrounding environments. In the present disclosure, addition of other unintended alloying elements is not excluded. The impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art.
Hereinafter, a martensitic stainless steel cast according to an embodiment of the present disclosure having the above-described composition of alloying elements will be described.
The martensitic stainless steel cast according to the present disclosure may be a martensitic stainless steel cast, wherein an area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone is 2.5% or less. In this regard, the central segregation zone refers to a region within -15 mm to 15 mm from the center of the cast.
In addition, the martensitic stainless steel cast of the present disclosure may be a martensitic stainless steel cast having a thickness of 250 to 320 mm.
In the martensitic stainless steel cast of the present disclosure, the area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone may be controlled to 2.5% or less and the thickness of the martensitic stainless steel may be controlled to 250 to 320 mm. While the hot-annealed stainless steel having a thickness of 8 mm is manufactured by using the cast, the number of primary carbides having a diameter of 3 µm or more may be controlled to 22 or less. Therefore, no surface linear defects occur after knife processing, and thus a martensite stainless steel sheet with high quaintly may be produced.
Hereinafter, a martensitic stainless steel according to an embodiment of the present disclosure having the above-described composition of alloying elements will be described.
The martensitic stainless steel of the present disclosure may be a martensitic stainless steel in which the number of primary carbides having a diameter of 3 µm or more is 22 (ea/mm²) or less.
In addition, the martensitic stainless steel of the present disclosure may have a thickness of 4 to 8 mm. Preferably, the martensitic stainless steel may have a thickness of 5 mm.
According to the martensitic stainless steel of the present disclosure, high-quality products may be manufactured by inhibiting surface linear defects in a final product by controlling the number of coarse primary carbides having a diameter of 3 µm or more to 22 or less.
Hereinafter, a method of manufacturing the martensitic stainless steel cast according to an embodiment of the present disclosure having the above-described composition of alloying elements will be described.
A martensitic stainless steel cast of the present disclosure may include reducing a slab including, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities with a reduction ratio of 2 to 4% during continuous casting.
The slab having the above-described composition of alloying elements is supplied to a tundish using a molten steel transfer device for a continuous casting process. The molten steel of the tundish is supplied to a mold that is cooled by water and solidification begins. In this case, the cast, which solidifies only on the surface, is released from the continuous casting mold and the solidification proceeds by secondary cooling using cooling water. In this case, unlike the surface area in contact with the continuous casting mold, the center of the cast solidifies slowly, causing segregation of C, Cr, and the like in the center to cause precipitation of coarse primary carbides.
The method of manufacturing a martensitic stainless steel cast of the present disclosure may include reducing the case with a reduction ratio of 2 to 4% before the cast completely solidifies. The reducing of the present disclosure may be performed by 2 to 4% using an in-line roller to reduce the cast before the cast completely solidifies. By reducing the cast before the cast completely solidifies, shrinkage cavities in the center of the cast may be removed and the segregated molten steel is squeezed to minimize segregation, thereby reducing the area where primary carbides occur.
According to the method of manufacturing a martensitic stainless steel cast of the present disclosure, the area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone may be 2.5% or less by reducing the cast with a reduction ratio of 2 to 4% before the cast completely solidifies. In this regard, the central segregation zone refers to a region within -15 mm to 15 mm from the center of the cast.
The slab does not break in the case where the cast is reduced with a reduction ratio less than 2% before the cast completely solidifies. However, the area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone exceeds 2.5%. In the case where the reducing is performed with a reduction ratio greater than 4%, the slab may crack or break. Therefore, in the present disclosure, the reduction ratio before the cast completely solidifies may be 2 to 4%.
The method of manufacturing a martensitic stainless steel cast of the present disclosure may be a method of manufacturing a cast with a thickness of 250 to 320 mm. In the case where the thickness is less than 250 mm, the area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone of the cast exceeds 2.5%, even when the cast is reduced with a reduction ratio of 2 to 4% before the cast completely solidifies. In addition, the thickness of the cast may be 250 to 320 mm to obtain a total reduction ratio of hot rolling of 96.8% or more in the subsequent process.
Therefore, a 4 to 8 mm-thick hot-annealed stainless steel may be manufactured as a cast manufactured according to the method of manufacturing a martensitic stainless steel cast of the present disclosure.
Hereinafter, a method of manufacturing the martensitic stainless steel according to an embodiment of the present disclosure having the above-described composition of alloying elements will be described.
A method of manufacturing a martensitic stainless steel of the present disclosure may include: reducing a slab including, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities with a reduction ratio of 2 to 4% into a cast; first reheating the cast; first hot rolling the first reheated cast; second reheating the first hot-rolled cast; second hot rolling the second reheated cast; coiling the cast; and batch annealing the cast.
Although the area fraction of primary carbides having an area of 2000 µm² or more in the cast central segregation zone is controlled to 2.5% or less by reducing the cast with a reduction ratio of 2 to 4% before the cast completely solidifies, coarse carbides may remain during a process of manufacturing the hot-rolled steel sheet.
According to the method of manufacturing a martensitic stainless steel of the present disclosure, after the manufacturing of the cast, the first reheating may be performed at 1200 to 1300 °C for 1 to 4 hours, the first hot rolling may be performed with a reduction ratio of 50 to 60%, the second reheating may be performed at 1200 to 1300 °C for 1 to 4 hours, and the second reheated cast may be second hot-rolled, and then the cast is may be coiled and batch annealed.
According to the martensitic stainless steel of the present disclosure, thermal decomposition and mechanical crushing processes may be performed by including the reheating and hot rolling processes twice. After the first reheating and hot rolling, the cast may undergo second reheating and hot rolling without performing batch annealing.
In general, a cast prepared by ingot-forging or by continuous casting is reheated at a high temperature and then hot-rolled, coiled, and batch-annealed, thereby obtaining a hot-annealed steel sheet. However, in the case of performing continuous casting, reheating and hot rolling are performed once, so that thermal decomposition and mechanical crushing processes of the primary carbides are insufficient. Therefore, the quality of primary carbides is inferior to that including the ingot-forging process. However, in the case of including the ingot-forging process, the production time increases, and an actual yield decreases due to formation of shrinkage cavities by ingot casting, so that there are limitations in terms of productivity and cost.
According to the method of manufacturing a martensitic stainless steel of the present disclosure, higher quality of primary carbides, higher productivity, and lower cost may be obtained, compared to the ingot-forging process, by continuous casting by first reheating and hot rolling the cast and then second reheating and hot rolling the first hot-rolled cast after the first hot rolling without performing the coiling and batch annealing of the first hot-rolled cast.
According to the martensitic stainless steel of the present disclosure, the thickness of the cast may be controlled to 250 to 320 mm in order to control the thickness of the stainless steel to 4 to 8 mm.
The martensitic stainless steel cast of the present disclosure may have a total hot rolling reduction ratio of 96.8% or more via the first hot rolling with a reduction ratio 50 to 60% to a thickness of 250 to 320 mm and the second hot rolling. Thereby, the thickness of the hot-annealed stainless steel sheet may be 4 to 8 mm.
If the reheating and hot rolling of the cast are performed only once, the number of primary carbides having a diameter of 3 µm or more may exceed 22, although the area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone is 2.5% or less. A hot-annealed steel material having aggregated coarse carbides may have surface linear defects after knife processing. If the second reheating and hot rolling are performed immediately after the first reheating and hot rolling without performing the batch annealing, the number of primary carbides having a diameter of 3 µm or more may be 22 or less in the hot-annealed steel sheet, so that the primary carbides are refined and have high quality. Such a hot-annealed steel sheet may not have surface linear defects after knife processing.
The method of manufacturing a martensitic stainless steel of the present disclosure may include coiling; and batch annealing, after the second hot rolling. The coil process may be performed at a temperature of 700°C or above. The batch annealing process may be hot annealing performed by placing the steel material in a hot annealing furnace at a temperature of 600°C or above, and maintaining a temperature of 800 to 900°C for 3 to 10 hours and then a temperature of 700 to 790°C for 5 hours to 15 hours.
According to the method of manufacturing a martensitic stainless steel of the present disclosure, the number of primary carbides having a diameter of 3 µm or more may be 22 (ea/mm²) or less. In addition, by using the martensitic stainless steel of the present disclosure, high quality products may be manufactured by inhibiting surface linear defects in final products.
According to the method of manufacturing a martensitic stainless steel cast of the present disclosure, the martensitic stainless steel cast of the present disclosure may be manufactured. In addition, by using the martensitic stainless steel cast of the present disclosure, the martensitic stainless steel of the present disclosure may be manufactured. In addition, according to the method of manufacturing a martensitic stainless steel cast of the present disclosure, the martensitic stainless steel of the present disclosure may be manufactured.
The martensitic stainless steel of the present disclosure may be used for edge tools with high hardness and high corrosion resistance. In addition, the martensitic stainless steel of the present disclosure may be used for automotive parts. In addition, the properties of stainless steels obtained by the present disclosure may also be applied to other uses.
Hereinafter, the present disclosure will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by matters described in the claims and able to be reasonably inferred therefrom.

{Examples}

Slabs respectively having compositions of alloying elements as shown in Table 1 were reduced with a reduction ratio of 0 to 6% by using an in-line roller before casts completely solidified.
In Comparative Examples 1, 3, 5, 8, 10, 11, 12, 14, 15, the casts were reheated at about 1250°C for about 3 hours and hot rolled to a final thickness of 8 mm, so that the reheating and hot rolling were performed once.
In Invention Examples 1 to 4 and Comparative Examples 2, 4, 6, 9, and 13, after the casts were first reheated at about 1250°C for about 3 hours and first hot-rolled with a reduction ratio of about 55%, the first hot-rolled steel sheets were placed in a reheating furnace again and second-reheated at about 1250°C for about 3 hours and second hot rolled to a final thickness of 8 mm, so that the reheating and hot rolling were performed twice in total.
The manufactured hot-rolled steel sheets were coiled at about 700°C. The coils were placed in a hot annealing furnace at about 600°C for batch annealing, and the coils were maintained at 850°C about 10 hours and then at about 750°C for 10 hours to perform batch annealing.
Cast samples were obtained from the centers of the prepared casts in the width direction and fractions of coarse carbides having an area of 2000 µm² or more in the cast central segregation zone (within -15 mm to +15 mm from the center were analyzed by image analysis using an optical microscope. Specimens were prepared by cutting hot-rolled samples for analysis of the primary carbides and analyzed by image analysis using an optical microscope, the number (ea/mm²) of primary carbides having a diameter of 3 µm or more was obtained, breakage of the slabs was identified based on occurrence of cracks with a length of 5 mm or more, and occurrence of surface linear defects after knife processing were identified, and the results are shown in Table 2 below.
Table 1 shows alloying elements of the composition. Table 1

Composition of alloying elements C Si Mn Cr Ni Mo V N

0.51 0.4 0.45 14.6 0.15 0.55 0.11 0.03
Table 2 shows thicknesses of the casts, reduction ratios of casts during casting before the casts completely solidify, breakage of the slabs, area fractions of primary carbides, the number of reheating and hot rolling, the number of primary carbides, and occurrence of surface linear defects.

Also, Table 2 is based on hot-annealed steel sheets with a thickness of 8 mm.

Table 2

	Cast thickn ess (mm)	Reduction ratio (%) of slab during casting before cast completely solidifies	Breakag e of slab	Area fraction (%) of primary carbides ≥2000 µm² in cast central segregation zone (±15 mm)	No. of reheating and hot rolling	No. of primary carbides ≥ 3 µm in hot-annealed steel sheet (ea/mm²)	Surface linear defect after knife process ing
Invention Example 1	250	2	X	2.3	2	22	X
Invention Example 2	250	4	X	1.6	2	15	X
Invention Example 3	300	2	X	2.5	2	19	X
Invention Example 4	300	4	X	2.0	2	12	X
Comparative Example 1	200	0	X	6.6	1	71	O
Comparative Example 2	200	0	X	6.3	2	56	O
Comparative Example 3	200	2	X	5.3	1	53	O
Comparative Example 4	200	2	X	5.2	2	41	O
Comparative Example 5	200	4	X	2.6	1	36	O
Comparative Example 6	200	4	X	2.6	2	28	O
Comparative Example 7	200	6	O	-	-	-	-
Comparative Example 8	250	0	X	5.7	1	67	O
Comparative Example 9	250	0	X	5.3	2	49	O
Comparative Example 10	250	2	X	2.5	1	51	O
Comparative Example 11	250	4	X	1.7	1	29	O
Comparative Example 12	300	0	X	5.7	1	63	O
Comparative Example 13	300	0	X	6.1	2	27	O
Comparative Example 14	300	2	X	2.4	1	46	O
Comparative Example 15	300	4	X	1.8	1	27	O
Comparative Example 16	300	6	O	-	-	-

In Invention Examples 1 to 4, the slabs were reduced with a reduction ratio of 2 to 4% during casting before the casts completely solidified to a cast thickness of 250 to 320 mm. It may be confirmed that in Invention Examples 1 to 4, the slabs did not break, and the area fractions of primary carbides having an area of 2000 µm² or more in the cast central segregation zone were 2.5% or less. Based thereon, it may be confirmed that the area where coarse primary carbides are formed may be reduced by removing shrinkage cavities of the center of the cast and minimizing segregation by reducing the slab with reduction ratio of 2 to 4% during casting before the cast completely solidifies.
In addition, it may be confirmed that according to Invention Examples 1 to 4, in the hot-annealed steel materials, which are manufactured by reducing the slabs with a reduction ratio of 2 to 4% before the casts completely solidify, and repeating the reheating and hot rolling twice, the number of primary carbides having a diameter of 3 µm or more is 22 or less. Based thereon, it may be confirmed that the quality of primary carbides is high by refining the primary carbides in the case of performing second reheating and hot rolling, without performing batch annealing, immediately after first reheating and hot rolling during continuous casting, without performing forging, because the number of primary carbides having a diameter of 3 µm or more is 22 or less in the hot-annealed steel material. Also, it may be confirmed that surface linear defects do not occur after knife processing in such a hot-annealed steel material.
In Comparative Examples 1 and 2, the slabs were not reduced during casting before the casts completely solidified. Although the slabs of Comparative Examples 1 and 2 did not break, it may be formed that the area fractions of coarse primary carbides in the central segregation zone were excessive as 6.6% and 6.3%.
In addition, it may be confirmed that the quality of primary carbides of Comparative Example 1, in which only the first hot rolling was performed, is inferior, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 71. Surface linear defects were observed after processing products according to Comparative Example 1. It may be confirmed that the quality of primary carbides according to Comparative Example 2, in which the second hot rolling was performed, is higher than that of Comparative Example 1, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 56. However, surface linear defects were observed after processing products according to Comparative Examples 1 and 2. Based thereon, it can be seen that the primary carbides have high quality by performing the reheating and hot rolling processes at least twice in total in the method of manufacturing a martensitic stainless steel.
In Comparative Examples 3 and 4, the slabs were reduced with a reduction ratio of 2% during casting before the casts completely solidified. Although the area fractions of primary carbides are smaller than that of Comparative Examples 1 and 2, the area fractions of coarse primary carbides in the cast central segregation zone are still excessive as 5.3% and 5.2%. Based thereon, it may be confirmed that effect of reducing the area fraction of primary carbides in the cast central segregation zone cannot be obtained in that case where the thickness of the casts is 200 mm to manufacture the hot-annealed steel material having a thickness of 8 mm.
In addition, it may be confirmed that the quality of primary carbides of Comparative Example 3, in which only the first hot rolling was performed, is inferior, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 53. Surface linear defects were observed after processing products according to Comparative Example 3. It may be confirmed that the quality of primary carbides according to Comparative Example 4, in which the second hot rolling was performed, is higher than that of Comparative Example 3, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 41. However, surface linear defects were observed after processing products according to Comparative Examples 3 and 4. Based thereon, it can be seen that the primary carbides have high quality by performing the reheating and hot rolling processes at least twice in total in the method of manufacturing a martensitic stainless steel.
In Comparative Examples 5 and 6, the slabs were reduced with a reduction ratio of 4% during casting before the casts completely solidified. Although the area fractions of primary carbides are smaller than that of Comparative Examples 3 and 4, the area fractions of coarse primary carbides in the cast central segregation zone are still excessive as 2.6% and 2.6%. Based thereon, it may be confirmed that effect of reducing the area fraction of primary carbides in the cast central segregation zone cannot be obtained in that case where the thickness of the casts is 200 mm to manufacture the hot-annealed steel material having a thickness of 8 mm.
In addition, it may be confirmed that the quality of primary carbides of Comparative Example 5, in which only the first hot rolling was performed, is inferior, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 36. Surface linear defects were observed after processing products according to Comparative Example 5. It may be confirmed that the quality of primary carbides according to Comparative Example 6, in which the second hot rolling was performed, is higher than that of Comparative Example 3, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 28. However, surface linear defects were observed after processing products according to Comparative Examples 5 and 6. Based thereon, it can be seen that the primary carbides have high quality by performing the reheating and hot rolling processes at least twice in total in the method of manufacturing a martensitic stainless steel.
In Comparative Example 7, the slab was reduced with a reduction ratio of 6% during casting before the cast completely solidified. The slab of Comparative Example 7 broke. Based thereon, the reduction ratio of 2 to 4% is appropriate for reducing the slab during casting before the cast completely solidifies.
In Comparative Examples 8 and 9, the slabs were not reduced during casting before the casts completely solidified. Although the casts of Comparative Examples 8 and 9 did not break, it may be confirmed that the area fractions of coarse primary carbides in the central segregation zone were excessive as 5.7% and 5.3%. Upon comparison between Comparative Examples 8 and 9 and Invention Examples 1 and 2, it may be confirmed that the area fraction of coarse primary carbides in the cast central segregation zone may be reduced by reducing the slabs with a reduction ratio of 2 to 4% during casting before the casts completely solidify.
In addition, it may be confirmed that the quality of primary carbides of Comparative Example 8, in which only the first hot rolling was performed, is inferior, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 67. Surface linear defects were observed after processing products according to Comparative Example 8. It may be confirmed that the quality of primary carbides according to Comparative Example 9, in which the second hot rolling was performed, is higher than that of Comparative Example 8, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 49. However, surface linear defects were observed after processing products according to Comparative Examples 8 and 9. Based thereon, it can be seen that the primary carbides have high quality by performing the reheating and hot rolling processes at least twice in total in the method of manufacturing a martensitic stainless steel.
Also, although the reheating and hot rolling processes were performed twice in total in Comparative Example 9 in the same manner as in Invention Examples 1 and 2, the quality of primary carbides was different therefrom because the slab was not reduced during casting before the cast completely solidified as described above. Based thereon, it can be seen that the primary carbides have high quality by reducing a 250 mm-thick cast with a reduction ratio of 2 to 4% during casting before the cast completely solidifies and by performing the reheating and hot rolling processes at least twice in total to manufacture the hot-annealed steel material having a thickness of 8 mm. In addition, it may be confirmed that surface linear defects are not observed after processing final products using such a hot-annealed steel material.
In Comparative Examples 10 and 11, the slabs were reduced with reduction ratios of 2% and 4% during casting before the casts completely solidified. It may be confirmed that the area fractions of primary carbides are 2.5% or less because the area fractions of coarse primary carbides in the centers of the casts are 2.5% and 1.7%. However, it may be confirmed that the quality of primary carbides of Comparative Examples 10 and 11, in which only the first hot rolling was performed, is inferior, because the numbers of primary carbides having a diameter of 3 µm or more in the hot-annealed steel materials are 51 and 29, respectively. In addition, surface linear defects were observed after processing them. In addition, upon comparison between Comparative Examples 10 and 11 and Invention Examples 1 and 2, it may be confirmed that the quality of primary carbides is superior in the case of performing the reheating and hot rolling processes at least twice in total. Also, it may be confirmed that surface linear defects are not observed after processing final products using such a hot-annealed steel material.
In Comparative Examples 12 and 13, the slabs were not reduced during casting before the casts completely solidified. Although the slabs of Comparative Examples 12 and 13 did not break, it may be formed that the area fractions of coarse primary carbides in the central segregation zone are excessive as 5.7% and 6.1%.
In addition, it may be confirmed that the quality of primary carbides of Comparative Example 12, in which only the first hot rolling was performed, is inferior, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 63. Surface linear defects were observed after processing products according to Comparative Example 12. It may be confirmed that the quality of primary carbides according to Comparative Example 13, in which the second hot rolling was performed, is higher than that of Comparative Example 1, because the number of primary carbides having a diameter of 3 µm or more in the hot-annealed steel material is 27. However, surface linear defects were observed after processing products according to Comparative Examples 12 and 13. Based thereon, it can be seen that the primary carbides have high quality by performing the reheating and hot rolling processes at least twice in total in the method of manufacturing a martensitic stainless steel.
In Comparative Examples 14 and 15, the slabs were reduced with reduction ratios of 2% and 4% during casting before the casts completely solidified. The area fractions of coarse primary carbides at the centers of the casts were 2.4% and 1.8%, respectively, confirming that the area fractions of primary carbides were 2.5% or less. However, it may be confirmed that the quality of primary carbides of Comparative Examples 14 and 15, in which only the first hot rolling was performed, is inferior, because the numbers of primary carbides having a diameter of 3 µm or more in the hot-annealed steel materials are 46 and 27, respectively. In addition, surface linear defects were observed in processed products according to Comparative Examples 14 and 15. Based thereon, it may be confirmed that the quality of primary carbides is superior in the case of performing the reheating and hot rolling processes at least twice in total in the method of manufacturing a martensitic stainless steel. Also, it may be confirmed that surface linear defects are not observed after processing final products using such a hot-annealed steel material.
In Comparative Example 16, the slab was reduced with a reduction ratio of 6% during casting before the cast completely solidified. The slab of Comparative Example 16 broke. Based thereon, the reduction ratio of 2 to 4% is appropriate for reducing the slab during casting before the cast completely solidifies.
Referring to FIGS. 1 and 2, it may be confirmed that a martensitic stainless steel according to the present disclosure, which is manufactured by using the cast prepared by reducing the slab with a reduction ratio of 2 to 4% during continuous casting and performing the second hot rolling, has a smaller number of primary carbides having a diameter of 3 µm or more than that of a martensitic stainless steel manufactured by only performing first hot rolling without a reducing process during casting. In the present disclosure, in the cast prepared by reducing the slab with a reduction ratio of 2 to 4% during continuous casting, the area fraction of a carbides having an area of 2000 µm² or more in the cast central segregation zone may be 2.5% or less. In addition, in the case where a martensitic stainless steel is manufactured using the cast by performing the second hot rolling, the number of primary carbides having a diameter of 3 µm or more may be 22 (ea/mm²) or less. In addition, linear defects may not be caused in the case of manufacturing a final product by using the martensitic stainless steel.
Referring to FIG. 3, in the martensitic stainless steel according to a comparative example, the area fraction of primary carbides having an area of 2000 µm² or more in the cast central segregation zone exceeded 2.5% and/or the number of primary carbides having a diameter of 3 µm or more exceeds about 22, so that it may be confirmed that linear defects are caused in final products in the case where the quality of primary carbides is inferior.

Claims

A martensitic stainless steel cast, comprising, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities,
wherein an area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone is 2.5 % or less.
The martensitic stainless steel cast according to claim 1, further comprising at least one of 0.01 to 0.8% of Mo and 0.05 to 0.2% of V.
The martensitic stainless steel cast according to claim 1, wherein a thickness is 250 to 320 mm.
A martensitic stainless steel comprising, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities,
wherein the number of primary carbides having a diameter of 3 µm or more is 22 (ea/mm²) or less.
The martensitic stainless steel according to claim 4, further comprising at least one of 0.01 to 0.8% of Mo and 0.05 to 0.2% of V.
The martensitic stainless steel according to claim 4, wherein a thickness is 4 to 8 mm.
A method of manufacturing a martensitic stainless steel cast, the method comprising:
reducing a slab comprising, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities, with a reduction ratio of 2 to 4% during continuous casting,

wherein the area fraction of primary carbides having an area of 2000 µm² or more in the central segregation zone is 2.5% or less.
The method according to claim 7, further comprising at least one of 0.01 to 0.8% of Mo and 0.05 to 0.2% of V.
The method according to claim 7, wherein a thickness of the cast is 250 to 320 mm.
The method according to claim 7, wherein the reducing is performed using an in-line roller.
A method of manufacturing a martensitic stainless steel, the method comprising:
reducing a slab comprising, in wt%, 0.4 to 0.55% of C, 0.01 to 0.1% of N, 0.2 to 0.6% of Si, 0.4 to 0.9% of Mn, 13.6 to 15.0% of Cr, 0.01 to 0.3% of Ni, and the remainder including Fe and inevitable impurities, with a reduction ratio of 2 to 4% into a cast;

first reheating the cast;

first hot rolling the first reheated cast;

second reheating the first hot-rolled cast;

second hot rolling the second reheated cast;

coiling the cast; and

batch annealing the cast,

wherein the first reheating is performed at 1200 to 1300°C for 1 to 4 hours,

the first hot rolling is performed with a reduction ratio of 50 to 60%, and

the second reheating is performed at 1200 to 1300°C for 1 to 4 hours.
The method according to claim 11, wherein the slab further comprises at least one of 0.01 to 0.8% of Mo and 0.05 to 0.2% of V.
The method according to claim 11, wherein the coiling is performed at a temperature of 700°C or above.
The method according to claim 11, wherein the batch annealing is hot annealing performed by
placing the cast in a hot annealing furnace at a temperature of 600°C or above, and

maintaining a temperature of 800 to 900°C for 3 to 10 hours and then a temperature of 700 to 790°C for 5 hours to 15 hours.
The method according to claim 11, wherein a thickness of the cast is 250 to 320 mm, and
a total reduction ratio of hot rolling is 96.8% or more.
The method according to claim 11, wherein the number of primary carbides having a diameter of 3 µm or more is 22 (ea/mm²) or less in the martensitic stainless steel.