US20190136339A1 - High-strength cold rolled steel sheet having excellent shear processability, and manufacturing method therefor - Google Patents

High-strength cold rolled steel sheet having excellent shear processability, and manufacturing method therefor Download PDF

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US20190136339A1
US20190136339A1 US15/772,644 US201615772644A US2019136339A1 US 20190136339 A1 US20190136339 A1 US 20190136339A1 US 201615772644 A US201615772644 A US 201615772644A US 2019136339 A1 US2019136339 A1 US 2019136339A1
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steel sheet
rolled steel
cold rolled
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Sung-il Kim
Seok-Jong SEO
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Posco Holdings Inc
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Posco Co Ltd
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    • 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
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    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
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    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
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Definitions

  • the present disclosure relates to a high-strength cold rolled steel sheet having high shear processability, and a method for manufacturing the high-strength cold rolled steel sheet.
  • Parts such as the friction plates of automatic transmissions of automobiles, are required to have an ability of suppressing the propagation of cracks in under frictional heat conditions and to have high strength and hardness in addition to having crack resistance in a shearing process.
  • Patent Document 1 a technique, in which low carbon steel or steel having various alloying elements is annealed by a recovery annealing method after a cold rolling process, has been applied to high-strength cold rolled steel sheets used for friction plates or guaranteeing hardness.
  • Patent Document 2 has proposed a technique in which cold rolling is performed twice, in a process of cold rolling, annealing, and cold rolling.
  • alloying elements such as carbon (C), silicon (Si), manganese (Mn), molybdenum (Mo), or chromium (Cr), mainly used to manufacture high-strength steel sheets through cold rolling, are effective in improving the strength of steel sheets through solid-solution strengthening
  • the excessive addition of such elements causes segregation of the elements and formation of non-uniform microstructures.
  • ferrite transformation is markedly delayed because of an increase in the hardenability of steel during cooling, low-temperature phases (martensite and austenite) are formed, and grain boundaries become non-uniform.
  • cracks increase during a shearing process, and if frictional heat is generated during operations, cracks may easily propagate and form defects.
  • Patent Document 1 Application No: KR 1998-0059809
  • Patent Document 2 Application No: DE 2005-031462
  • aspects of the present disclosure may provide a high-strength cold rolled steel sheet having high shear processability and capable of suppressing the formation of cracks in a shearing process and under frictional heat conditions, and a method for manufacturing the high-strength cold rolled steel sheet.
  • a high-strength cold rolled steel sheet having high shear processability including, by wt %, carbon (C): 0.05% to 0.10%, silicon (Si): 0.01% to 0.5%, manganese (Mn): 1.2% to 2.0%, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.005% to 0.3%, boron (B): 0.0003% to 0.0010%, molybdenum (Mo): 0.005% to 0.2%, phosphorus (P): 0.001% to 0.05%, sulfur (S): 0.001% to 0.01%, nitrogen (N): 0.001% to 0.01%, niobium (Nb): 0.005% to 0.08%, titanium (Ti): 0.005% to 0.13%, vanadium (V): 0.005% to 0.2%, and a balance of iron (Fe) and inevitable
  • a method for manufacturing a high-strength cold rolled steel sheet having high shear processability may include: reheating a steel slab to a temperature of 1200° C. to 1350° C., the steel slab including, by wt %, carbon (C): 0.05% to 0.10%, silicon (Si): 0.01% to 0.5%, manganese (Mn): 1.2% to 2.0%, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.005% to 0.3%, boron (B): 0.0003% to 0.0010%, molybdenum (Mo): 0.005% to 0.2%, phosphorus (P): 0.001% to 0.05%, sulfur (S): 0.001% to 0.01%, nitrogen (N): 0.001% to 0.01%, niobium (Nb): 0.005% to 0.08%, titanium (Ti): 0.005% to 0.13%, vanadium (V):
  • the present disclosure provides a high-strength cold rolled steel sheet having high shear processability and capable of suppressing the formation of cracks in a shearing process and under frictional heat conditions in addition to having high strength and high hardness, and a method of manufacturing the high-strength cold rolled steel sheet.
  • FIG. 1 is a graph showing values of Formulas 1 and 2 of examples.
  • the high-strength cold rolled steel sheet having high shear processability of the present disclosure includes, by wt %, carbon (C): 0.05% to 0.10%, silicon (Si): 0.01% to 0.5%, manganese (Mn): 1.2% to 2.0%, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.005% to 0.3%, boron (B): 0.0003% to 0.0010%, molybdenum (Mo): 0.005% to 0.2%, phosphorus (P): 0.001% to 0.05%, sulfur (S): 0.001% to 0.01%, nitrogen (N): 0.001% to 0.01%, niobium (Nb): 0.005% to 0.08%, titanium (Ti): 0.005% to 0.13%, vanadium (V): 0.005% to 0.2%, and a balance of iron (Fe) and inevitable impurities, wherein the high-strength cold rolled steel sheet satisfie
  • Carbon (C) is the most economical and effective element in strengthening steel, and as the content of carbon (C) increases, tensile strength increases by the effect of precipitation strengthening or an increase in the fraction of bainite. However, if the content of carbon (C) is less than 0.05%, reactions with titanium (Ti), niobium (Nb), and vanadium (V) for forming precipitates are reduced, and thus, the effect of precipitation strengthening is low. Conversely, if the content of carbon (C) is greater than 0.10 wt %, coarse carbides are easily formed along grain boundaries, and shear processability is poor because fine cracks are formed along coarse carbide interfaces during a shearing process. Therefore, it may be preferable that the content of carbon (C) be within the range of 0.05 wt % to 0.10 wt %.
  • Silicon (Si) has a deoxidizing effect on molten steel and a solid-solution strengthening effect, and improves formability by delaying the formation of coarse carbides.
  • the content of silicon (Si) is less than 0.01%, the effect of delaying the formation of carbides is low, and thus it may be difficult to improve formability.
  • the content of silicon (Si) is greater than 0.5%, the surface quality of the steel sheet may be markedly worsened because red scale may be formed on the surface of the steel sheet during a hot rolling process, and the ductility and weldability of the steel sheet may also be worsened. Therefore, it may be preferable that the content of silicon (Si) be within the range of 0.01% to 0.5%.
  • manganese (Mn) is an effective element having a solid-solution strengthening effect on steel and facilitating the formation of bainite in a weld heat affected zone after a welding process by increasing the hardenability of the steel.
  • the content of manganese (Mn) is less than 1.2%, these effects may not be sufficiently obtained by the addition of manganese (Mn).
  • the content of manganese (Mn) is greater than 2.0%, hardenability markedly increases to result in a delay in ferrite transformation and a decrease in the effect of precipitation strengthening.
  • the content of manganese (Mn) be within the range of 1.2% to 2.0%.
  • Molybdenum (Mo) has a solid-solution strengthening effect and increases hardenability, thereby increasing the strength of steel. However, if the content of molybdenum (Mo) is less than 0.005%, these effects may not be obtained. If the content of molybdenum (Mo) is greater than 0.2%, hardenability excessively increases, thereby delaying ferrite transformation and decreasing precipitation strengthening. In addition, it has a negative effect on economical aspects and weldability. Therefore, it may be preferable that the content of molybdenum (Mo) be within the range of 0.01% to 0.2%.
  • Chromium (Cr) has a solid-solution strengthening effect and increases hardenability, thereby increasing the strength of steel. However, if the content of chromium (Cr) is less than 0.005%, these effects may not be obtained. Conversely, if the content of chromium (Cr) is greater than 0.3%, ferrite transformation is excessively delayed, and thus martensite is formed to result in poor elongation. In addition, the effect of precipitation strengthening is reduced. Furthermore, like in the case of manganese (Mn), segregation markedly increases in a thickness wise center portion, and a non-uniform microstructure is formed in a thickness direction to result in poor sheer processability. Therefore, it may be preferable that the content of chromium (Cr) be within the range of 0.005% to 0.3%.
  • boron (B) may segregate along austenite grain boundaries at high temperatures, thereby stabilizing grain boundaries and improving impact resistance. However, if the content of boron (B) is less than 0.0003%, these effects may not be sufficiently obtained. Conversely, if the content of boron (B) is greater than 0.0010%, elongated grains increase because recrystallization is delayed during a hot rolling process, and a non-uniform microstructure is formed because ferrite transformation is delayed during cooling.
  • the content of boron (B) be within the range of 0.0003% to 0.0010%.
  • phosphorus (P) has a solid-solution strengthening effect and an effect of facilitating ferrite transformation.
  • the content of phosphorus (P) is less than 0.001%, it is not economically favorable, because of high manufacturing costs, and sufficient strength may also not be obtained.
  • the content of phosphorus (P) is greater than 0.05%, embrittlement may occur because of grain boundary segregation, fine cracks may easily be formed during a shearing process, and ductility and impact resistance may be worsened. Therefore, it may be preferable that the content of phosphorus (P) be within the range of 0.001% to 0.05%.
  • Sulfur (S) is an impurity existing in steel. If the content of sulfur (S) is greater than 0.01%, sulfur (S) may combine with an element such as manganese (Mn) and form a non-metallic inclusion, thereby facilitating the formation of fine cracks during a cutting process of steel and markedly decreasing stretch flangeability and impact resistance. Conversely, if the content of sulfur (S) is lower than 0.001%, performing a steel making process may require an excessive amount of time, and thus the productivity of the steel making process may be lowered. Therefore, it may be preferable that the content of sulfur (S) be within the range of 0.001% to 0.01%.
  • aluminum (Al) is added for deoxidation. If the content of aluminum (Al) is less than 0.01%, this effect is insufficient. Conversely, if the content of aluminum (Al) is greater than 0.1%, aluminum (Al) may combine with nitrogen (N) and form AlN, thereby increasing the possibility of cracks in slab corners during a continuous casting process and the possibility of inclusion defects in edge portions of a hot rolled steel sheet. In addition, surface defects may be formed during a cold rolling process after a hot rolling process, thereby resulting in poor surface quality. Therefore, it may be preferable that the content of aluminum (Al) be within the range of 0.01% to 0.1%.
  • nitrogen (N) is a typical element having a solid-solution strengthening effect and forms coarse precipitates together with elements such as titanium (Ti) or aluminum (Al).
  • nitrogen (N) has a solid-solution strengthening effect greater than that of carbon (C)
  • the content of nitrogen (N) increases in steel, the toughness of the steel decreases.
  • titanium (Ti) is a typical element having a precipitation strengthening effect and has a strong affinity for nitrogen (N), forming coarse TiN in steel.
  • TiN has an effect of suppressing the growth of grains during a heating process for hot rolling.
  • titanium (Ti) remaining after reaction with nitrogen (N) dissolves in steel and combines with carbon (C), thereby forming a TiC precipitate and thus improving the strength of the steel.
  • the content of titanium (Ti) is less than 0.005%, these effects may not be obtained, and if the content of titanium (Ti) is greater than 0.13%, coarse TiN is formed to worsen shear processability during a shearing process. Therefore, according to the present disclosure, it may be preferable that the content of titanium (Ti) be within the range of 0.005% to 0.13%.
  • niobium (Nb) is a typical element having a precipitation strengthening effect.
  • niobium (Nb) precipitates and delays recrystallization, thereby having a grain refinement effect and thus improving the strength and impact toughness of steel.
  • the content of niobium (Nb) is less than 0.005%, these effects may not be sufficiently obtained.
  • the content of niobium (Nb) is greater than 0.08%, recrystallization is excessively delayed during a hot rolling process, and thus shear processability deteriorates due to the formation of elongated grains and coarse complex precipitates. Therefore, according to the present disclosure, it may be preferable that the content of titanium (Ti) be within the range of 0.005% to 0.08%.
  • V Vanadium (V): 0.005% to 0.2%
  • vanadium (V) is a typical element having a precipitation strengthening effect. Vanadium (V) forms precipitates after a coiling process, thereby effectively improving the strength of steel. However, if the content of vanadium (V) is less than 0.005%, this effect may not be sufficiently obtained, and if the content of vanadium (V) is greater than 0.2%, coarse complex precipitates are formed, thereby worsening shear processability and having an economically negative effect. Therefore, according to the present disclosure, it may be preferable that the content of vanadium (V) be within the range of 0.005% to 0.2%.
  • the other component of the high-strength cold rolled steel sheet of the present disclosure is iron (Fe).
  • Fe iron
  • impurities of raw materials or manufacturing environments may be inevitably included in the high-strength cold rolled steel sheet, and such impurities may not be removed from the high-strength cold rolled steel sheet.
  • impurities are well-known to those of ordinary skill in manufacturing industries, and thus specific descriptions of the impurities will not be given in the present disclosure.
  • the composition of the high-strength cold rolled steel sheet may satisfy Formulas 1 and 2 below. Then, the microstructure of the high-strength cold rolled steel sheet may be uniform after a hot rolling process, and the formation of cracks may be suppressed during a shearing process after a cold rolling process.
  • Formula 1 relates the hardenability and segregates of steel.
  • Formula 1 is proposed by considering a solid-solution strengthening effect on steel and a non-uniform microstructure of steel.
  • Formula 1 is less than 2.0, the solid-solution strengthening effect on steel is insufficient, and thus sufficiently high strength is not obtained. Conversely, if Formula 1 is greater than 2.5, the microstructure of steel is non-uniform in a thickness direction of the steel, and ferrite transformation is delayed, thereby decreasing the effect of precipitation strengthening.
  • Formula 1 be within the range of 2.0 to 2.5.
  • Formula 2 regulates elements relating to precipitates in steel. That is, since the formation of precipitates relates to the relationship between the contents of Ti, Nb, and V, and the contents of C and N, Formula 2 is proposed to adjust the contents of Ti, Nb, and V according to the contents of C and N.
  • Formula 2 is less than 0.2, precipitation strengthening is markedly lowered, and thus, intended levels of strength and hardness may not be obtained. Conversely, if Formula 2 is greater than 0.5, fine precipitates are formed in large amounts, thereby markedly increasing yield strength and thus worsening cold rollability. In addition, precipitates may be unevenly formed in the thickness direction of the steel sheet, increasing the formation of cracks in a shearing process after a cold rolling process.
  • Formula 2 be within the range of 0.2 to 0.5.
  • the high-strength cold rolled steel sheet may have high shear processability and be capable of suppressing the formation of cracks in a shearing process and under frictional heat conditions in addition to having high strength and high hardness.
  • the cold rolled steel sheet of the present disclosure is obtained by performing a cold rolling process on a hot rolled steel sheet having a microstructure of ferrite and fine pearlite, and thus the cold rolled steel sheet has a microstructure (full hard microstructure) markedly deformed in the rolling direction.
  • each phase in the microstructure of the cold rolled steel sheet may not be identified, but before the cold rolling process, the hot rolled steel sheet may include ferrite in an area fraction of 90% or greater, fine pearlite in an area fraction of less than 5%, and bainite as an inevitable remainder.
  • the high-strength cold rolled steel sheet having high shear processability includes at least one of carbides, nitrides, and carbonitrides.
  • the high-strength cold rolled steel sheet may include at least one of TiN, TiC, NbC, NbN, (Ti,Nb) (C,N), (Ti,Mo,Nb) (C,N), (Fe,Mn) 3 C, and (Fe,Mn,Mo)C.
  • the average size of the carbides, nitrides, and carbonitrides may be within the range of 10 nm to 50 nm.
  • the yield strength of the hot rolled steel sheet may be excessively high, thereby easily causing local work hardening deviations during a cold rolling process, and easily causing cracks during a shearing process and a heat treatment process of the cold rolled steel sheet.
  • the average size is greater than 50 nm, it is difficult to obtain intended tensile strength and hardness.
  • the cold rolled steel sheet may have a tensile strength of 1200 MPa or greater and a hardness (micro-Vickers) of 340 Hv or greater. If the cold rolled steel sheet has tensile strength and hardness in these ranges, the cold rolled steel sheet may be suitably used for applications such as fiction plates of automatic transmissions of automobiles.
  • the maximum length of cracks may preferably be 1 mm or less.
  • the maximum length of cracks is measured by punching the cold rolled steel sheet using a circular die having a diameter of 10 mm with a clearance of 6%, heat treating the cold rolled steel sheet at 200° C. for 1 hour, and measuring the maximum length of cracks in a cross-section.
  • cracks may be formed in large amounts and may easily propagate during a shearing process, and the propagation of cracks may be facilitated if temperature increases due to frictional heat.
  • Another aspect of the present disclosure provides a method for manufacturing an high-strength cold rolled steel sheet having high shear processability, the method including: heating a steel slab having the above-described alloying composition to a temperature of 1200° C. to 1350° c.; hot rolling the heated steel slab within a temperature range of 850° C. to 1150° C. to form a hot rolled steel sheet; cooling the hot rolled steel sheet to a temperature of 550° C. to 750° C. and coiling the hot rolled steel sheet; and pickling the coiled hot rolled steel sheet and cold rolling the pickled hot rolled steel sheet at a reduction ratio of 60% to 70%.
  • a steel slab having the above-described alloying composition is heated to a temperature of 1200° C. to 1350° C.
  • the heating temperature is lower than 1200° C., sufficient amounts of precipitates may not redissolved. In this case, after a hot rolling process, the formation of precipitates decreases, and coarse TiN remains. Conversely, if the heating temperature is higher than 1350° C., strength decreases because of abnormal growth of austenite grains. Thus, it may be preferable that the reheating temperature be adjusted to be within the range of 1200° C. to 1350° C.
  • the steel slab may be manufactured through a continuous casting process directly connected to a hot rolling process.
  • this manufacturing method may be used for the case in which a continuous casting process is directly connected to a hot rolling process as described above.
  • the heated steel slab is hot rolled within the temperature range of 850° C. to 1150° C.
  • the hot rolling starts at a temperature higher than 1150° C., a hot rolled steel sheet having coarse grains and poor surface quality may be manufactured because the hot rolling temperature is too high.
  • the hot rolling finishes at a temperature lower than 850° C. since recrystallization is excessively delayed, elongated grains are developed, and yield strength increases, thereby worsening cold rollability and shear processability.
  • the hot rolled steel sheet After the hot rolling, the hot rolled steel sheet is cooled to a temperature of 550° C. to 750° C. and is then coiled.
  • the hot rolled steel sheet is cooled to a temperature less than 550° C. and is then coiled, the material quality of the steel sheet may be degraded due to the formation of bainite and martensite. Conversely, if the hot rolled steel sheet is cooled to a temperature greater than 750° C. and is then coiled, coarse ferrite is formed, and coarse carbides and nitrides may easily be formed, thereby worsening the material quality of the steel sheet.
  • the average rate of the cooling may be within the range of 10° C./sec to 70° C./sec.
  • the average cooling rate is less than 10° C./sec, a non-uniform microstructure may be formed because of the formation of coarse ferrite. Conversely, if the average cooling rate is greater than 70° C./sec, bainite may easily be formed, and the microstructure of the steel sheet may become non-uniform in the thickness direction, thereby worsening the shear processability of the steel sheet.
  • the hot rolled steel sheet is cold rolled at a reduction ratio of 60% to 70% to manufacture a cold rolled steel sheet.
  • the reduction ratio of the cold rolling process is less than 60%, the effect of work hardening may not be sufficiently obtained, and thus it is difficult to guarantee the strength and hardness of the cold rolled steel sheet. Conversely, if the reduction ratio of the cold rolling process is greater than 70%, the quality of the cold rolled steel sheet may decrease at edge portions, and the shear processability of the cold rolled steel sheet may deteriorate.
  • the cold rolled steel sheet manufactured by the above-described method may have high strength and high hardness, and may not be easily cracked in a shearing process and under frictional heat conditions.
  • the cold rolled steel sheet manufactured by the above-described method may include at least one of carbides, nitrides, and carbonitrides, and the average size of the carbides, nitrides, and carbonitrides may be within the range of 10 nm to 50 nm.
  • the cold rolled steel sheet may have a tensile strength of 1200 MPa or greater and a hardness of 340 Hv or greater, and the maximum length of cracks formed in the cold rolled steel sheet during a shearing process may be 1 mm or less.
  • Table 3 below shows mechanical properties and results of microstructure observation of inventive examples and comparative examples.
  • TS and Hv refer to the tensile strength and micro-Vickers hardness of each cold rolled steel sheet, respectively
  • a maximum length of cracks refers to a value measured by punching each cold rolled steel sheet using a circular die having a diameter of 10 mm with a clearance of 6%, heat treating the cold rolled steel sheet at 200° C. for 1 hour, and measuring the length of the maximum crack in a cross-section. The length of cracks was measured from results of observation using an optical microscope at a magnification of 100 times.
  • the size of carbides, nitrides, and carbonitrides was analyzed using hot rolled steel sheets before cold rolling.
  • Carbides, nitrides, and carbonitrides having an average size of 10 nm to 50 nm do not vary in size and fraction after cold rolling, and it is difficult to accurately measure the sizes and fractions of carbides, nitrides, and carbonitrides from microstructures markedly deformed after cold rolling.
  • the size thereof was analyzed using the hot rolled steel sheets.
  • the average size of carbides, nitrides, and carbonitrides was determined from results of measurements obtained using a transmission electron microscope.
  • Results of measurement at a magnification of 50,000 times were used to measure carbides and nitrides having an average size of 100 nm or greater, and results of measurement at a magnification of 100,000 times were used to measure precipitates having an average size of 100 nm or less.
  • a tensile test was performed using specimens taken by JIS5 from rolled steel sheets in a 0-degree direction with respect to the rolling direction of the rolled steel sheets.
  • Comparative Examples 1 and 2 did not satisfy both Formulas 1 and 2, and Comparative Example 1 had a carbon (C) content outside of the range proposed in the present disclosure.
  • C carbon
  • Formula 2 exceeded the upper limit because of a relative low content of carbon (C) and relatively excessive contents of titanium (Ti), niobium (Nb), and vanadium (V). Therefore, although the size of carbides, nitrides, and precipitates in steel was small, strength was insufficient.
  • Comparative Example 2 was prepared at a cold rolling reduction ratio greater than the range proposed in the present disclosure, somewhat excessive cracks were formed in a sheared surface after punching, and the quality of the sheared surface was poor.
  • Comparative Examples 3 and 5 did not satisfy Formula 1.
  • the value of Formula 1 was greater than the range proposed in the present disclosure, and thus, segregation increased in a center portion of steel, resulting in poor quality in a sheared portion.
  • Comparative Example 5 had very good quality on a sheared surface because contents of elements such as manganese (Mn), chromium (Cr), and boron (B) were low, and thus, the occurrence of segregation was low. However, solid-solution strengthening was not sufficient, and thus intended strength and hardness were not obtained.
  • Comparative Examples 4 and 6 did not satisfy Formula 2.
  • surplus carbon (C) remained and formed coarse precipitates and carbides, and thus intended strength and hardness were not obtained because of insufficient precipitation strengthening.
  • Comparative Example 6 the value of Formula 2 was greater than the range proposed in the present disclosure, and thus fine precipitates were formed in large amounts, thereby guaranteeing high strength but resulting in excessive cracking in a sheared portion.
  • Comparative Examples 7, 8, 9, and 10 were steel sheets manufactured to have a tensile strength of 1350 MPa or greater and a hardness of 355 Hv or greater after a cold rolling process. Although Comparative Example 7 did not satisfy Formula 1, Comparative Example 7 had intended physical properties owing to a high cold rolling reduction ratio. However, Comparative Example 7 had somewhat excessive cracks in a sheared portion because of the high cold rolling reduction ratio.
  • Comparative Examples 8 and 9 satisfied both Formulas 1 and 2, but the cold rolling reduction ratios thereof were improper, thereby failing to obtain intended physical properties or resulting in poor quality in sheared portions. Comparative Example 10 did not satisfy both Formulas 1 and 2 and had poor quality in a sheared portion.
  • inventive examples satisfying the composition, manufacturing conditions, and Formulas 1 and 2 proposed in the present disclosure had intended material characteristics and good quality in sheared portions.
  • FIG. 1 illustrates the values of Formulas 1 and 2 and maximum crack lengths of sheared portions of inventive examples and comparative examples.
  • a hatched region corresponds to the ranges proposed in the present disclosure.

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