Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
  • B22D11/002—Stainless steels
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
  • B21C47/02—Winding-up or coiling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
  • B21C47/26—Special arrangements with regard to simultaneous or subsequent treatment of the material
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling

Definitions

• 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.
• BAF batch annealing furnace
• 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 M 7 C 3 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.
• Patent Document 1 US Patent No. 6273973 B1 (December 2, 1999 ) [Disclosure]
• a martensitic stainless steel 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 ⁇ m or more is 22 (ea/mm 2 ) or less.
• 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.
• 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.
• N is an effective element for improving hardness of a steel.
• N may be added in an amount of 0.01% or more.
• an excess of N may cause formation of a chromium nitride that is a low-temperature precipitation phase and cause a residual ⁇ phase, thereby impairing strength after strengthening heat treatment. Therefore, an excess of N may deteriorate fatigue resistance.
• the upper limit of the N content may be controlled to 0.1%.
• Si is added to deoxidize a steel.
• Si is an effective element for obtaining strength by solid solution strengthening.
• Si may be added in an amount of 0.2% or more.
• an excess of Si may cause formation of scales on the surface of a steel impairing the surface quality.
• 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.
• Cr may be added in an amount of 13.6% or more.
• an excess of Cr may excessively increase hardenability and increase manufacturing costs.
• the upper limit of the Cr content may be controlled to 15.0%.
• 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.
• Ni is an element serving to improve corrosion resistance and quenchability.
• Ni may be added in an amount of 0.01% or more.
• 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.
• 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%.
• the content of V may be 0.05% to 0.2 wt%.
• 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).
• 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.
• 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 ⁇ m 2 or more in the central segregation zone is 2.5% or less.
• the central segregation zone refers to a region within -15 mm to 15 mm from the center of the cast.
• the martensitic stainless steel cast of the present disclosure may be a martensitic stainless steel cast having a thickness of 250 to 320 mm.
• the area fraction of primary carbides having an area of 2000 ⁇ m 2 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 ⁇ m 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.
• 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 ⁇ m or more is 22 (ea/mm 2 ) or less.
• the martensitic stainless steel of the present disclosure may have a thickness of 4 to 8 mm.
• the martensitic stainless steel may have a thickness of 5 mm.
• 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 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.
• the thickness is less than 250 mm, the area fraction of primary carbides having an area of 2000 ⁇ m 2 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.
• 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.
• 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 ⁇ m 2 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/mm 2 ) of primary carbides having a diameter of 3 ⁇ m 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 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 ⁇ m 2 in cast central segregation zone ( ⁇ 15 mm) No. of reheating and hot rolling No.
• 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 ⁇ m 2 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.
• 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.
• 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 ⁇ m or more in the hot-annealed steel material is 53.
• Surface linear defects were observed after processing products according to Comparative Example 3.
• 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 ⁇ m or more in the hot-annealed steel material is 41.
• 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.
• 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.
• 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 ⁇ m or more in the hot-annealed steel material is 36.
• Surface linear defects were observed after processing products according to Comparative Example 5.
• 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 ⁇ m or more in the hot-annealed steel material is 28.
• 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.
• 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.
• 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.
• 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 ⁇ m 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 ⁇ m or more in the hot-annealed steel material is 49.
• 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.
• 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 ⁇ m or more in the hot-annealed steel materials are 51 and 29, respectively. In addition, surface linear defects were observed after processing them.
• 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.
• 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 ⁇ m or more in the hot-annealed steel materials are 46 and 27, respectively.
• surface linear defects were observed in processed products according to Comparative Examples 14 and 15.
• 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 ⁇ m or more than that of a martensitic stainless steel manufactured by only performing first hot rolling without a reducing process during casting.
• the area fraction of a carbides having an area of 2000 ⁇ m 2 or more in the cast central segregation zone may be 2.5% or less.

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