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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a high hardness abrasion resistant steel and a manufacturing method therefor, and more particularly, to a high hardness abrasion resistant steel which may be used for construction machinery, and the like, and a manufacturing method therefor.
• abrasion resistance and hardness of a thick steel sheet may be correlated with each other, it may be necessary to increase hardness in the thick steel sheet concerned, to be worn down.
• it may be necessary to have uniform hardness from the surface of the thick steel sheet to the inside (around t/2, t thickness) of the sheet thickness (that is, having the same degree of hardness on the surface of the thick steel sheet and inside).
• a method of reheating to an Ac3 temperature or higher after rolling and quenching may be widely used.
• cited references 1 and 2 disclose a method of increasing a content of C and adding a large amount of hardenability enhancing elements such as Cr and Mo, thereby increasing surface hardness.
• An aspect of the present disclosure may be to provide a high hardness abrasion resistant steel which may have excellent abrasion resistance and also high strength and high impact toughness, and a manufacturing method therefor.
• An example embodiment of the present disclosure provides an abrasion resistant steel having excellent hardness and impact toughness including, by weight %, 0.33-0.42% of carbon (C), 0.1-0.7% of silicon (Si), 0.6-1.6% of manganese (Mn), 0.05% or less of phosphorus (P) (excluding 0), 0.02% or less of sulfur (S) (excluding 0), 0.07% or less of aluminum (Al) (excluding 0), 0.55-5.0% of nickel (Ni), 0.01-1.5% of copper (Cu), 0.01-0.8% of chromium (Cr), 0.01-0.8% of molybdenum (Mo), 50 ppm or less of boron (B) (excluding 0), and 0.02% or less of cobalt (Co) (excluding 0) and further comprising one or more selected from a group consisting of 0.02% or less of titanium (Ti) (excluding 0), 0.05% or less of niobium (Nb) (excluding 0), 0.05% or less of
• Another example embodiment of the present disclosure provides a method of manufacturing an abrasion resistant steel having excellent hardness and impact toughness including heating a steel slab including, by weight %, 0.33-0.42% of carbon (C), 0.1-0.7% of silicon (Si), 0.6-1.6% of manganese (Mn), 0.05% or less of phosphorus (P) (excluding 0), 0.02% or less of sulfur (S) (excluding 0), 0.07% or less of aluminum (Al) (excluding 0), 0.55-5.0% of nickel (Ni), 0.01-1.5% of copper (Cu), 0.01-0.8% of chromium (Cr), 0.01-0.8% of molybdenum (Mo), 50 ppm or less of boron (B) (excluding 0), and 0.02% or less of cobalt (Co) (excluding 0) and further comprising one or more selected from a group consisting of 0.02% or less of titanium (Ti) (excluding 0), 0.05% or less of niobium (Nb)
• an effect of providing an abrasion resistant steel having a thickness of 60 mm or less and having high hardness and excellent low-temperature toughness may be obtained.
• the alloy composition of the present disclosure will be described first.
• the content of the alloy composition described below may be represented by weight %.
• Carbon (C) may be effective in increasing strength and hardness in a steel having a martensite structure and may be effective for improving hardenability. To sufficiently secure the above-described effect, it may be preferable to add C by 0.33% or more. However, when the content thereof exceeds 0.42%, weldability and toughness may degrade, such that an additional heat treatment such as tempering may be inevitable. Therefore, in the present disclosure, it may be preferable to control the content of C to be 0.33-0.42%.
• a lower limit of the content of C may more preferably be 0.34%, even more preferably 0.35%, and most preferably 0.36%.
• An upper limit of the content of C may more preferably be 0.40%, even more preferably 0.39%, and most preferably 0.38%.
• Silicon (Si) may be effective in improving strength according to deoxidation and solid solution strengthening. To obtain the above effect, it may be preferable to add Si by 0.1% or more. However, when the content thereof exceeds 0.7%, weldability may deteriorate, which may not be preferable. Therefore, in the present disclosure, it may be preferable to control the content of Si to be 0.1-0.7%.
• a lower limit of the content of Si may more preferably be 0.12%, even more preferably 0.15%, and most preferably 0.2%.
• An upper limit of the Si content may more preferably be 0.5%, even more preferably 0.45%, and most preferably 0.4%.
• Manganese (Mn) may suppress ferrite formation and may effectively improve hardenability by decreasing Ar3 temperature, thereby improving strength and toughness of steel.
• Mn may be preferable to include Mn in an amount of 0.6% or more. When the content exceeds 1.6%, weldability may degrade. Therefore, in the present disclosure, it may be preferable to control the content of Mn to be 0.6-1.6%.
• a lower limit of the content of Mn may more preferably be 0.65%, even more preferably 0.70%, and most preferably 0.75%.
• An upper limit of the content of Mn may more preferably be 1.55%, even more preferably 1.50%, and most preferably 1.45%.
• Phosphorus (P) may be inevitably included in steel, and may degrade toughness of steel. Therefore, it may be preferable to control the content of P to be less than 0.05% by lowering the content as much as possible, but 0% may be excluded in consideration of the inevitably included amount.
• the content of P may more preferably be 0.03% or less, even more preferably 0.02% or less, and most preferably 0.01% or less.
• S Sulfur
• the content of S may more preferably be 0.01% or less, even more preferably 0.005% or less, and most preferably 0.003% or less.
• Aluminum (Al) may be effective in lowering an oxygen content in molten steel as a deoxidizing agent for steel.
• the content of Al exceeds 0.07%, cleanliness of the steel may be impaired, which may not be preferable. Therefore, in the present disclosure, it may be preferable to control the content of Al to be 0.07% or less, and 0% may be excluded in consideration of load and an increase in manufacturing costs in the steelmaking process.
• the content of Al may more preferably be 0.05% or less, even more preferably 0.04% or less, and most preferably 0.03% or less.
• Nickel (Ni) may be generally effective in improving toughness along with strength of steel. To obtain the above-described effect, it may be preferable to add Ni in an amount of 0.55% or more. However, when the content thereof exceeds 5.0%, the manufacturing costs may increase as Ni is an expensive element. Therefore, in the present disclosure, it may be preferable to control the content of Ni to be 0.55-5.0%. A lower limit of the content of Ni may more preferably be 0.6%, even more preferably 0.7%, and most preferably 0.8%. An upper limit of the content of Ni may more preferably be 4.5%, even more preferably 4.0%, and most preferably 3.5%.
• Copper (Cu) may simultaneously increase strength and toughness of steel along with Ni. To obtain the above effect, it may be preferable to add Cu in an amount of 0.01% or more. However, when the content of Cu exceeds 1.5%, possibility of surface defects may increase, and hot workability may be deteriorated. Therefore, in the present disclosure, it may be preferable to control the content of Cu to be 0.01-1.5%.
• a lower limit of the content of Cu may more preferably be 0.05%, even more preferably 0.10%, and most preferably 0.15%.
• An upper limit of the Cu content may more preferably be 1.2%, even more preferably 1.0%, and most preferably 0.8%.
• Chromium (Cr) may increase strength of steel by increasing hardenability, and may be advantageous in securing hardness. To obtain the above-described effect, it may be preferable to add Cr in an amount of 0.01% or more, but when the content thereof exceeds 0.8%, weldability may be deteriorated and the manufacturing costs may increase. Therefore, in the present disclosure, it may be preferable to control the content of Cr to be 0.01-0.8%.
• a lower limit of the Cr content may more preferably be 0.1%, even more preferably 0.15%, and most preferably 0.2%.
• An upper limit of the content of Cr may more preferably be 0.75%, even more preferably 0.70%, and most preferably 0.65%.
• Molybdenum (Mo) may increase hardenability of steel, and may be effective in improving hardness of a thick steel material. To sufficiently obtain the above-described effect, it may be preferable to add Mo in an amount of 0.01% or more. However, as Mo is also an expensive element, when the content thereof exceeds 0.8%, the manufacturing costs may increase, and weldability may degrade. Therefore, in the present disclosure, it may be preferable to control the content of Mo to be 0.01-0.8%. A lower limit of the content of Mo may more preferably be 0.1%, even more preferably 0.12%, and most preferably 0.15%. An upper limit of the Mo content may more preferably be 0.75%, even more preferably 0.72%, and most preferably 0.70%.
• Boron (B) may be effective in improving strength by effectively increasing hardenability of steel even by adding a small amount of B.
• the content thereof is excessive, however, toughness and weldability of steel may be deteriorated, and thus, it may be preferable to control the content to be 50 ppm or less.
• a lower limit of the content of B may more preferably be 2 ppm, even more preferably 3 ppm, and most preferably 5 ppm.
• An upper limit of the content of B may more preferably be 40 ppm, even more preferably 35 ppm, and most preferably 30 ppm.
• Co Co
• Co Co
• a lower limit of the Co content may more preferably be 0.001%, even more preferably 0.002% or less, and most preferably 0.003% or less.
• An upper limit of the Co content may more preferably be 0.018%, even more preferably 0.015%, and most preferably 0.013%.
• the abrasion-resistant steel in the present disclosure may further include elements advantageous for securing physical properties aimed in the present disclosure.
• the abrasion-resistant steel may further include one or more selected from a group consisting of 0.02% or less of titanium (Ti) (excluding 0), 0.05% or less of niobium (Nb) (excluding 0), 0.05% or less of vanadium (V) (excluding 0) and 2-100 ppm of calcium (Ca).
• Titanium (Ti) may maximize the effect of B, an element effective in improving hardenability of steel.
• Ti may form a TiN precipitate by being combined with nitrogen (N), such that formation of BN may be prevented. Accordingly, solid solute B may increase, such that improvement of hardenability may be maximized.
• N nitrogen
• solid solute B may increase, such that improvement of hardenability may be maximized.
• a lower limit of the content of Ti may more preferably be 0.005%, even more preferably 0.007%, and most preferably 0.010%.
• An upper limit of the content of Ti may more preferably be 0.019%, even more preferably 0.017%, and most preferably 0.015%.
• Niobium (Nb) may be solid-solute in austenite and may increase hardenability of austenite, and may form carbonitride such as Nb (C,N) such that Nb may be effective for increasing strength of steel and inhibiting austenite grain growth.
• Nb may be effective for increasing strength of steel and inhibiting austenite grain growth.
• a lower limit of the content of Nb may more preferably be 0.002%, even more preferably 0.003%, and most preferably 0.005%.
• An upper limit of the content of Nb may more preferably be 0.040%, even more preferably 0.035%, and most preferably 0.030%.
• V Vanadium (V): 0.05% or Less (Excluding 0)
• Vanadium (V) may form a VC carbide during reheating after hot-rolling, such that growth of austenite grains may be inhibited, and V may be advantageous to securing strength and toughness by improving hardenability of steel.
• V is an expensive element, when the content thereof exceeds 0.05%, V may become a factor increasing the manufacturing costs. Therefore, in the present disclosure, it may be preferable to control the content thereof to be 0.05% or less.
• a lower limit of the content of V may more preferably be 0.002%, even more preferably 0.003%, and most preferably 0.005%.
• An upper limit of the content of V may more preferably be 0.045%, even more preferably 0.042%, and most preferably 0.040%.
• Ca may have good bonding strength with S such that Ca may have an effect of inhibiting formation of MnS segregated in a center of a thickness of a steel material by generating CaS. Also, CaS created by adding Ca may have an effect of increasing corrosion resistance in a humid external environment. To obtain the above-described effect, it may be preferable to add Ca in an amount of 2 ppm or more. However, when the content thereof exceeds 100 ppm, it may not be preferable as Ca may cause clogging of a nozzle in steelmaking. Therefore, in the present disclosure, it may be preferable to control the content of Ca to be 2-100 ppm.
• a lower limit of the content of Ca may more preferably be 3 ppm, even more preferably 4 ppm, and most preferably 5 ppm.
• An upper limit of the content of Ca may more preferably be 80 ppm, even more preferably 60 ppm, and most preferably 40 ppm.
• the abrasion resistant steel in the present disclosure may further include one or more selected from a group consisting of 0.05% or less of arsenic (As) (excluding 0), 0.05% or less of tin (Sn) (excluding 0), and 0.05% or less of tungsten (W) (excluding 0).
• As arsenic
• Sn tin
• W tungsten
• each content of As, Sn, and W may be 0.05% or less.
• a lower limit of each content of As, Sn, and W may more preferably be 0.001%, even more preferably 0.002%, and most preferably 0.003%.
• An upper limit of each content of As, Sn and W may more preferably be 0.04%, even more preferably 0.03%, and most preferably 0.02%.
• a remainder of the present disclosure may be iron (Fe).
• Fe iron
• inevitable impurities may be inevitably added from raw materials or an ambient environment, and thus, impurities may not be excluded.
• a person skilled in the art of a general manufacturing process may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.
• C and Ni of the above-described alloy composition may satisfy relational expression 1 as below.
• relational expression 1 should be satisfied preferably.
• a value of [C] ⁇ [Ni] may preferably be 0.231 or more.
• a value of [C] ⁇ [Ni] may more preferably be 0.396 or more, even more preferably 0.792 or more, and most preferably 1 or more. The higher the value of [C] ⁇ [Ni], the more advantageous the effect may be implemented, and thus, an upper limit of the value of [C] ⁇ [Ni] may not be particularly limited in the present disclosure.
• a microstructure of the abrasion resistant steel in the present disclosure may include martensite as a matrix structure. More specifically, the abrasion resistant steel in the present disclosure may include 95% or more (including 100%) of martensite by an area fraction. When the fraction of martensite is less than 95%, it may be difficult to secure a target level of strength and hardness.
• the microstructure of the abrasion resistant steel in the present disclosure may further include bainite by 5 area % or less, and accordingly, low-temperature impact toughness may further improve.
• a fraction of martensite may more preferably be 96% or more, and even more preferably 97% or more.
• a fraction of bainite may more preferably be 4% or less, and even more preferably 3% or less.
• the abrasion resistant steel in the present disclosure provided as above may an effect of securing surface hardness of 550-650 HB and also having an impact absorption energy of 21 J or more at a low temperature of ⁇ 40° C.
• HB indicates surface hardness of the steel measured by the Brinell hardness tester.
• hardness (HB) and impact absorption energy (J) of the abrasion resistant steel in the present disclosure satisfy relational expression 2 as below.
• low-temperature toughness properties may improve in addition to high hardness, and to this end, it may be preferable to satisfy relational expression 2 as below.
• relational expression 2 when only surface hardness is high and impact toughness is degraded such that relational expression 2 is not satisfied, or when impact toughness is excellent but surface hardness does not reach a target value such that relational expression 2 is not satisfied, final target high hardness and low temperature toughness properties may not be guaranteed.
• a steel slab may be heated in the temperature range of 1050-1250° C.
• the heating temperature of the steel slab may have a range of 1050-1250° C. preferably.
• a lower limit of the heating temperature of the steel slab may more preferably be 1060° C., even more preferably 1070° C., and most preferably 1080° C.
• An upper limit of the heating temperature of the steel slab may more preferably be 1230° C., even more preferably 1200° C., and most preferably 1180° C.
• the reheated steel slab may be roughly rolled in a temperature range of 950-1050° C. to obtain a rough-rolled bar.
• a rolling load may increase and the pressure may be relatively weakened, such that deformation may not be sufficiently transmitted to a center of the slab in a thickness direction, and defects such as voids may not be removed.
• the temperature exceeds 1050° C. recrystallization may simultaneously occur while rolling, and grains may grow, such that initial austenite grains may become excessively coarse. Therefore, in the present disclosure, the rough-rolling temperature may preferably be 950-1050° C.
• a lower limit of the rough-rolling temperature may more preferably be 960° C., even more preferably 970° C., and most preferably 980° C.
• An upper limit of the rough-rolling temperature may more preferably be 1040° C., even more preferably 1020° C., and most preferably 1000° C.
• the rough-rolled bar may be finishing hot-rolled in a temperature range of 850-950° C. to obtain a hot-rolled steel sheet.
• the finishing hot-rolling temperature is less than 850° C.
• the rolling may become two-phase rolling, such that ferrite may be formed in the microstructure.
• the temperature exceeds 950° C. a grain size of the final structure may become coarse such that low-temperature toughness may be deteriorated. Therefore, in the present disclosure, the finishing hot-rolling temperature may be 850-950° C. preferably.
• a lower limit of the finishing hot-rolling temperature may more preferably be 860° C., even more preferably 870° C., and most preferably 880° C.
• An upper limit of the finish hot-rolling temperature may more preferably be 940° C., even more preferably 930° C., and most preferably 920° C.
• the hot-rolled steel sheet may be air-cooled to room temperature, and may be reheated in a temperature range of 860-950° C. for a residence time of 1.3t+10 min ⁇ 1.3t+60 min (t: sheet thickness).
• the reheating may be performed for reverse transformation of the hot-rolled steel sheet including ferrite and pearlite into austenite single phase.
• austenitization may not be sufficiently performed and coarse soft ferrites may be mixed, such that hardness of the final product may degrade.
• austenite grains may become coarse, such that hardenability may increase, but low temperature toughness of the steel may be deteriorated.
• the reheating temperature may preferably be 860-950° C.
• a lower limit of the reheating temperature may more preferably be 870° C., even more preferably 880° C., and most preferably 890° C.
• An upper limit of the reheating temperature may more preferably be 940° C., even more preferably 930° C., and most preferably 920° C.
• the residence time during the reheating may preferably be 1.3t+10 min ⁇ 1.3t+60 min (t: sheet thickness).
• a lower limit of the residence time during reheating may more preferably be 1.3t+12 minutes, even more preferably 1.3t+15 minutes, and most preferably 1.3t+20 minutes.
• An upper limit of the residence time during reheating may more preferably be 1.3t+50 min, even more preferably 1.3t+45 min, and most preferably 1.3t+40 min.
• the reheated hot-rolled steel sheet may be water-cooled to 150° C. or less with reference to a surface layer portion (e.g., the area from the surface to 1 ⁇ 8t (t: sheet thickness (mm)) of the sheet.
• the water-cooling stop temperature exceeds 150° C., a ferrite phase may be formed during cooling or a bainite phase may be excessively formed. Therefore, the water-cooling stop temperature may preferably be 150° C. or less.
• the water-cooling stop temperature may more preferably be 100° C. or less, even more preferably 70° C. or less, and most preferably 40° C. or less.
• the water-cooling rate may preferably be 10° C./s or more.
• a ferrite phase may be formed during cooling or a bainite phase may be excessively formed.
• a cooling rate during the water-cooling may more preferably be 15° C./s or more, and even more preferably 20° C./s or more.
• the higher the cooling rate the more advantageous it may be, and thus, an upper limit of the cooling rate may not be particularly limited, and may be determined in consideration of facility limitations by a person skilled in the art.
• the hot-rolled steel sheet in the present disclosure having gone through the above process conditions may be a thick steel sheet having a thickness of 60 mm or less, and may have a thickness of 8-50 mm more preferably, and 12-40 mm even more preferably. In the present disclosure, it may be preferable to not perform a tempering process on the thick steel sheet.
• a steel slab having alloy compositions as in Tables 1 and 2 below was prepared, and the steel slab heating-rough-rolling-hot-rolling-cooling (room temperature)-reheating-water cooling was performed on the steel slab under the conditions as in Table 3 below to manufacture a hot-rolled steel sheet.
• a microstructure and mechanical properties of the hot-rolled steel sheet were measured, and results thereof are listed in Table 4 below.
• the sample was cut out in an arbitrary size to manufacture a mirror surface, the surface was corroded using a nital etching solution, and a 1 ⁇ 2t position, a center of the thickness, was observed using an optical microscope and an electron scanning microscope.
• Hardness and toughness were measured using the Brinell hardness tester (load 3000 kgf, 10 mm tungsten indentation) and the Charpy impact tester, respectively.
• surface hardness an average value of values obtained by milling the sheet surface by 2 mm and measuring surface hardness three times therefrom was used.
• Charpy impact test result an average value of values obtained by taking a sample from a 1 ⁇ 4t position and measuring toughness three times therefrom at ⁇ 40° C. was used.

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