Classifications

• • 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
• • 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
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • 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
  • C21D8/0221—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
  • C21D8/0236—Cold 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
  • C21D8/0263—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
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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

Definitions

• the present disclosure relates to a high-strength steel plate and a method for manufacturing the same, and more specifically, to a high-strength steel plate having excellent tensile strength and yield strength, including 90 area% or greater of martensite, by rapidly cooling a hot-rolled steel plate immediately after the hot rolling without adding a large amount of carbon or manganese to an alloy composition, and a method for manufacturing the same.
• Steel plates are widely used in all industrial fields, and are mainly used as materials for automobile parts and industrial parts that are subject to repeated stress or deformation.
• high-carbon steel plates are widely used for various parts because of their strength.
• automobile parts that use the high-carbon steel plates include clutch parts, seat belt spring parts, or the like.
• industrial parts that use the high-carbon steel plates include industrial spring parts, tool parts, or the like.
• heat treatment is generally performed after hot rolling or cold rolling to secure desired materiality.
• the present disclosure is to provide a high-strength steel plate having excellent tensile strength and yield strength.
• the present disclosure is to provide a high-strength steel plate that may be applied to automobile parts, industrial parts, and the like.
• the present disclosure is to provide a method for manufacturing a high-strength steel plate having excellent eco-friendly characteristics by reducing a manufacturing cost and carbon emissions.
• a steel plate according to the present disclosure includes, in weight%, C: 0.1 1 to 0.30%, Mn: 0.10 to 3.00%, Si: greater than 0% and equal to or smaller than 0.50%, Al: greater than 0% and equal to or smaller than 0.10%, P: greater than 0% and equal to or smaller than 0.05%, S: greater than 0% and equal to or smaller than 0.03%, N: greater than 0% and equal to or smaller than 0.03%, remainder Fe and other inevitable impurities, and has a martensite microstructure of 90 to 100 area% with respect to a total 100 area%.
• the steel plate may further include B: 0.0003 to 0.005wt%.
• an area percentage of the martensite elongated along the rolling direction in the martensite may be equal to or greater than 60%. Further, an area percentage of packets having a long axis to a short axis ratio equal to or greater than 2:1 in the martensite may be equal to or greater than 50%.
• the steel plate may include 0 to 10 area% of microstructures of one or more of ferrite and retained austenite with respect to the total 100 area%. Further, the steel plate may include 0 to 5 area% of microstructures of one or more of pearlite and bainite with respect to the total 100 area%.
• a tensile strength may be equal to or greater than 1800 MPa and a yield strength may be equal to or greater than 1700 MPa.
• a tensile strength may be equal to or greater than 1910 MPa and a yield strength may be equal to or greater than 1880 MPa.
• a yield strength (MPa) and alloying elements (%) of the steel plate may satisfy following Relational Expression 1.
• Relational Expression 1 [yield strength/ ⁇ C+(Mn ⁇ 0.05) ⁇ ] ⁇ 6,000
• a method for manufacturing a steel plate according to the present disclosure includes (a) heating a slab including, in wt%, C: 0.11 to 0.30%, Mn: 0.10 to 3.00%, Si: greater than 0% and equal to or smaller than 0.50%, Al: greater than 0% and equal to or smaller than 0.10%, P: greater than 0% and equal to or smaller than 0.05%, S: greater than 0% and equal to or smaller than 0.03%, N: greater than 0% and equal to or smaller than 0.03%, remainder Fe and other unavoidable impurities, (b) hot-rolling the heated slab, (c) rapidly cooling the hot-rolled slab at a cooling rate of 50 to 1000°C/sec within 5 seconds after the hot rolling and then coiling the cooled slab, and (d) cold-rolling the slab after the coiling.
• the heating may be performed at 1050 to 1350°C.
• the hot rolling may be performed at 800 to 950°C.
• each of the rapid cooling and the coiling may be performed at 100 to 350°C.
• the cold rolling may be performed at a reduction percentage of 50 to 80%.
• the high-strength steel plate according to the present disclosure has the excellent tensile strength and yield strength, including 90 area% or greater of the martensite microstructure, by cooling the hot-rolled steel plate immediately after the hot rolling under the rapid cooling conditions without adding the large amount of carbon or manganese to the alloy composition.
• martensite formed by the rapid cooling has the form that is elongated by the cold rolling, so that the strength of the steel plate is significantly improved.
• the manufactured steel plate may exhibit the high strength, so that it may be applied to the automotive parts such as the clutch parts and the industrial spring parts, and the industrial parts such as the spring parts and the tool parts.
• first component when a first component is described to be disposed "on (or under)" a second component, the first component may not only be disposed in contact with a top surface (or a bottom surface) of the second component, but also be disposed on the second component with a third component interposed therebetween.
• the present inventor studied the steel plate for application to high-carbon parts such as an industrial spring.
• the present inventor lowered a carbon content to 0.30% or lower and a manganese content to 3.0% or lower, and controlled process conditions.
• the high-temperature heat treatment was omitted, and rapidly cooling was performed at a cooling rate of 50 to 1000°C/sec within 5 seconds immediately after the hot rolling to secure martensite of a hard structure by 90 area% or more, so that high tensile strength and yield strength were able to be exhibited simultaneously.
• the present inventor was able to manufacture the high-strength steel plate that may effectively improve a strength of automobile parts and industrial parts by omitting the high-temperature heat treatment and controlling the cooling conditions of the hot-rolled steel plate without adding the large amount of carbon or manganese to the alloy composition.
• the high-strength steel plate manufactured according to the present disclosure may exhibit materiality effects of tensile strength equal to or greater than 1800MPa and yield strength equal to or greater than 1700MPa, as well as environmental friendliness resulted from the reduced carbon content.
• the steel plate according to the present disclosure may contain, in wt%, C: 0.11 to 0.30%, Mn: 0.10 to 3.00%, Si: greater than 0% and equal to or smaller than 0.50%, Al: greater than 0% and equal to or smaller than 0.10%, P: greater than 0% and equal to or smaller than 0.05%, S: greater than 0% and equal to or smaller than 0.03%, N: greater than 0% and equal to or smaller than 0.03%, remainder Fe and other inevitable impurities, and may further contain B: 0.0003 to 0.005wt%.
• the steel plate includes 90 to 100 area% of martensite microstructure with respect to a total 100 area%.
• the % indicating a content of each element is based on wt%.
• Carbon (C) is an element that effectively contributes to improving the strength of steel, so that, in the present disclosure, a certain level or more of carbon (C) may be included to secure the strength of the steel plate.
• the carbon (C) content When the carbon (C) content is not at the certain level, a large number of low-temperature structures such as pearlite and bainite are formed during the cooling after the hot rolling, so that a microstructure that the present disclosure aims for may not be secured.
• the carbon (C) content may be equal to or greater than 0.110%. Preferably, it may be equal to or greater than 0.150%, and more preferably, may be equal to or greater than 0.200% or greater than 0.200%.
• the carbon (C) content may be equal to or smaller than 0.300%, and preferably, may be equal to or smaller than 0.295%.
• the steel plate may have the tensile strength (TS) equal to or greater than 1910 MPa and the yield strength (YS) equal to or greater than 1880 MPa.
• Manganese (Mn) is an element that effectively contributes to improving the strength and hardenability of the steel.
• manganese (Mn) forms MnS by combining with sulfur (S), which is inevitably introduced during the steel manufacturing process, and is therefore an element that effectively prevents cracking caused by sulfur (S).
• the manganese (Mn) content may be equal to or greater than 0.10%.
• a preferable manganese (Mn) content may be equal to or greater than 0.300%, and a more preferable manganese (Mn) content may be equal to or greater than 0.500%.
• the manganese (Mn) content may be equal to or smaller than 3.00%. Preferably, it may be equal to or smaller than 2.90%, and more preferably, may be equal to or smaller than 2.80%.
• Silicone (Si) is an element with a strong affinity for oxygen, so that when added in a large amount, it may cause a deterioration of a surface quality caused by surface scale, and it may also be not preferable in terms of the weldability. Therefore, regarding an upper limit, the silicon (Si) content may be equal to or smaller than 0.50%, and preferably, may be equal to or smaller than 0.45%.
• silicon (Si) not only acts as a deoxidizer but also is an element that contributes to improving the strength of the steel
• addition of silicon (Si) is not completely excluded and thus 0% is able to be excluded from a lower limit of the content thereof.
• a lower limit of a preferable silicon (Si) content may be equal to or greater than 0.03%.
• Aluminum (Al) is an element that combines with oxygen in the steel and acts as a deoxidizer. In the present disclosure, for such an effect, aluminum (Al) may be added, and 0% may be excluded from a lower limit of the content thereof. A lower limit of a preferable aluminum (Al) content may be equal to or greater than 0.01%.
• the aluminum (Al) content may be equal to or smaller than 0.10%, and preferably, may be equal to or smaller than 0.08%.
• Phosphorus (P) is a major element that segregates at a grain boundary and causes a decrease in toughness of the steel, so that it is desirable to control the phosphorus (P) content as low as possible. Therefore, it is theoretically advantageous to set the phosphorus (P) content to 0%.
• phosphorus (P) is an impurity that is inevitably introduced during the steelmaking process, and controlling the phosphorus (P) content to 0% is able to cause an excessive process load.
• the phosphorus (P) content may be greater than 0% and equal to or smaller than 0.05%, and preferably, may be greater than 0% and equal to or smaller than 0.03%.
• S Sulfur
• Sulfur (S) is an element that forms MnS, increases an amount of precipitates, and embrittles the steel, so that it is desirable to control the sulfur (S) content as low as possible. Therefore, it is theoretically advantageous to limit the sulfur (S) content to 0%.
• sulfur (S) is also an impurity that is inevitably introduced during the steelmaking process, and controlling the content thereof to 0% is able to cause the excessive process load.
• the sulfur (S) content may be greater than 0% and equal to or smaller than 0.03%, and preferably may be greater than 0% and equal to or smaller than 0.01%.
• nitrogen (N) is an element that causes cracks in a slab by forming nitrides during continuous casting, it is desirable to control the content thereof as low as possible. Therefore, it is theoretically advantageous to limit the nitrogen (N) content to 0%.
• nitrogen (N) is also an impurity that is inevitably introduced during the steelmaking process, and controlling the content thereof to 0% is able to cause the excessive process load.
• the nitrogen (N) content may be greater than 0% and equal to or smaller than 0.03%, and preferably, may be greater than 0% and equal to or smaller than 0.01%.
• Boron (B) is an element that effectively contributes to improving the hardenability of the steel. Even with a small amount of addition, boron (B) is an element that may effectively suppress transformation into the low-temperature structure such as ferrite and pearlite during the cooling after the hot rolling.
• the boron (B) content may be equal to or greater than 0.0003%, and preferably, may be equal to or greater than 0.001%.
• boron (B) when boron (B) is added excessively, boron (B) may react with iron (Fe) and cause grain boundary embrittlement.
• the boron (B) content may be equal to or smaller than 0.005%, and preferably, may be equal to or smaller than 0.0045%.
• the steel plate of the present disclosure may contain remainder Fe and other inevitable impurities in addition to the aforementioned components.
• unintended impurities may inevitably be mixed from a raw material or a surrounding environment during the normal manufacturing process, the unintended impurities are not able to be completely excluded. Because such impurities are known to anyone with ordinary knowledge in the present technical field, not all of them are specifically mentioned in the present document.
• additional addition of effective components other than the aforementioned components is not completely excluded.
• the steel plate of the present disclosure includes martensite as a matrix structure.
• a fraction of martensite may be 90 to 100 area% relative to the total 100 area% of the steel plate, and preferably 95 to 100 area%. Because the steel plate of the present disclosure includes martensite, which is a hard structure, as the matrix structure, high strength and yield ratio may be secured at the same time.
• Martensite included in the steel plate of the present disclosure is formed by the rapid cooling after the hot rolling and is elongated by the subsequent cold rolling, so that an area percentage of martensite elongated along a rolling direction in entire martensite included in the steel plate may be 60 to 100%, and preferably may be 80 to 95%.
• martensite elongated along the rolling direction means that a long axis direction of a martensite packet is aligned within 45° from the rolling direction, and preferably means that the long axis direction of the packet is aligned parallel to the rolling direction.
• a packet refers to a crystal unit in which blocks, which are a collection of laths, are arranged in parallel with each other. Further, the packet is a group of laths whose densest crystal planes are parallel to each other, and one packet refers to a plurality of blocks divided.
• the long axis refers to an axis along which the martensite packet is extended in the rolling direction
• a short axis refers to an axis aligned in a direction perpendicular to the rolling direction.
• the long and short axes are not distinguished from each other as shown in FIG. 2 in the martensite packet that is initially formed before the rolling, but when the packet is rolled, the packet is extended in the rolling direction, forming the long axis in the rolling direction and the short axis perpendicular to the rolling direction.
• the long and short axes may be measured by distinguishing the packet on a scanning electron microscope (SEM), and then setting the longest axis of the packet as the long axis and setting the longest axis among axes perpendicular to the long axis as the short axis.
• SEM scanning electron microscope
• an area ratio of packets with a long axis:short axis length ratio of 5:1 to 20:1 may be 56 to 91%.
• the steel plate of the present disclosure not only includes martensite, which is the hard structure, as the matrix structure, but also controls martensite included in the steel plate to have the form that is elongated by the cold rolling, so that the strength of the steel plate and a part manufactured using the same may be more effectively improved.
• the steel plate of the present disclosure does not completely exclude inclusion of structures other than martensite. However, because ferrite, pearlite, bainite, retained austenite, and the like are not desirable for securing the strength and the durability, it is necessary to control fractions thereof within a certain range.
• the steel plate may include 0 to 10 area% of one or more microstructures of ferrite and retained austenite, with respect to the total 100 area%, and preferably 1 to 5 area%.
• the steel plate may include 0 to 5 area% of one or more microstructures of pearlite and bainite, with respect to the total 100 area%, and preferably 1 to 2 area%.
• the steel plate may include a case in which a total fraction of ferrite, retained austenite, pearlite, and bainite is 0 area%.
• the steel plate of the present disclosure may further include cementite, precipitates, and the like as a residual structure in addition to the aforementioned microstructure, but the present disclosure may not be limited thereto.
• the steel plate of the present disclosure satisfies the aforementioned alloy composition and satisfies 90 area% or greater of the martensite microstructure, so that the tensile strength (TS) may be 1800 to 2500 MPa and the yield strength (YS) may be 1700 to 2500 MPa.
• TS tensile strength
• YS yield strength
• a preferable tensile strength (TS) may be 1830 to 2200 MPa and a yield strength (YS) may be 1810 to 2100 MPa.
• the tensile strength (TS) may be equal to or greater than 1910 MPa and the yield strength (YS) may be equal to or greater than 1880 MPa.
• the yield strength (MPa) and alloying elements (%) of the steel plate satisfy following Relational Expression 1, so that the present disclosure may secure very high strength without adding a large amount of alloying elements of carbon or manganese.
• 10,100 ⁇ [yield strength/ ⁇ C+(Mn ⁇ 0.05) ⁇ ] ⁇ 6,000 may be satisfied, and more preferably, 7,800 ⁇ [yield strength/ ⁇ C+(Mn ⁇ 0.05) ⁇ ] ⁇ 6,000 may be satisfied.
• FIG. 1 is a flowchart showing a method for manufacturing a high-strength steel plate according to the present disclosure.
• the method for manufacturing the steel plate according to the present disclosure may further include heating a slab including, in wt%, C: 0.11 to 0.30%, Mn: 0.10 to 3.00%, Si: greater than 0% and equal to or smaller than 0.50%, Al: greater than 0% and equal to or smaller than 0.10%, P: greater than 0% and equal to or smaller than 0.05%, S: greater than 0% and equal to or smaller than 0.03%, N: greater than 0% and equal to or smaller than 0.03%, remainder Fe and other unavoidable impurities, hot-rolling the heated slab, rapidly cooling the hot-rolled slab at a cooling rate of 50 to 1000°C/sec within 5 seconds after the hot rolling and then coiling the cooled slab, and cold-rolling the slab after the coiling.
• the steel plate may further include B: 0.0003 to 0.005wt%.
• a slab steel composition of the present disclosure is the same as the steel composition of the steel plate described above, so that it will be omitted.
• Slab manufacturing conditions may not be particularly limited, and slab manufacturing conditions used for manufacturing general steel plates may be applied.
• the prepared slab is heated at 1050 to 1350°C.
• the slab may be heated in a temperature range equal to or higher than 1100°C for sufficient homogenization treatment.
• an upper limit of the slab heating temperature may be equal to or lower than 1350°C.
• the heated slab may be hot rolled under normal hot rolling conditions, but a finishing rolling temperature may be 800 to 950°C, preferably 850 to 930°C, and more preferably 856 to 930°C, for controlling a rolling load and reducing the surface scale.
• the cooling under rapid cooling conditions may be performed on the hot-rolled steel plate immediately after the hot rolling.
• the present disclosure aims to strictly control the microstructure of the steel plate, it is preferable that the cooling starts within 5 seconds immediately after end of the hot rolling. This is because, when a time period from the end of the hot rolling to the start of the cooling exceeds 5 seconds, unintended ferrite, pearlite, and bainite may be formed by air cooling in atmosphere. A preferable time period from the end of the hot rolling to the start of the cooling may be greater than 0 seconds and equal to or smaller than 3 seconds.
• the cooling rate of the hot-rolled steel plate may be adjusted.
• the hot-rolled steel plate immediately after the hot rolling may be cooled to a cooling end temperature equal to or lower than 350°C at a cooling rate of 50 to 1,000°C/s, preferably may be cooled to 100 to 300°C at a cooling rate of 100 to 300°C/s, and more preferably may be cooled to 150 to 295°C at a cooling rate of 100 to 300°C/s.
• a lower limit of the cooling end temperature may be equal to or higher than 100°C, and preferably may be equal to or higher than 150°C.
• the transformation into ferrite, pearlite, and bainite occurs during the cooling, so that it may be difficult to secure the microstructure that the present disclosure aims for.
• the present disclosure performs the cooling under the rapid cooling conditions on the hot-rolled steel plate immediately after the hot rolling, so that 90 area% or greater of martensite may be secured in the hot-rolled steel plate state before the application of the cold rolling.
• a typical method for manufacturing the steel plate includes performing the heat treatment immediately after the hot rolling, cold-rolling the heat-treated hot-rolled steel plate, and then performing the quenching heat treatment to form the martensite structure.
• the present disclosure may exhibit the high-strength steel plate, including 90 area% or greater of martensite, and particularly, 60 area% or greater of elongated martensite, by controlling the alloy composition ratio and the conditions of rapid cooling and cold rolling despite omitting the heat treatment immediately after the hot rolling and the quenching after the cold rolling.
• the hot-rolled steel plate after the end of the cooling may be coiled into a hot-rolled coil.
• a coiling temperature may be equal to or lower than 350°C similar to the cooling conditions, preferably, may be 100 to 300°C, and more preferably, may be 110 to 250 °C.
• cold rolling may be performed at a reduction percentage of 50 to 80%.
• the cold rolling reduction percentage When the cold rolling reduction percentage is smaller than 50%, sufficient elongation of martensite is not achieved, and thus desired high strength characteristics is not able to be secured. Therefore, regarding a lower limit, the cold rolling reduction percentage may be equal to or greater than 50%, and preferably equal to or greater than 60%.
• the cold rolling reduction percentage may be equal to or smaller than 80%, and preferably, may be equal to or smaller than 75%.
• the high-strength steel plate manufactured through the above-described manufacturing method may include 90 volume% or greater of martensite as the microstructure, and may have the tensile strength equal to or greater than 1800 MPa and the yield strength equal to or greater than 1700 MPa.
• steel plate specimens were manufactured by applying process conditions in Table 2 below. Each slab was heated in a temperature range of 1050 to 1350°C and homogenized.
• Alloy compositions in Table 1 are identified via a dry analysis method, a wet analysis method, and the like in a cold-rolled steel plate state.
• Steel type Alloy composition (wt%) C Mn Si Al P S N B A 0.223 0.979 0.08 0.03 0.005 0.002 0.004 0.0013 B 0.126 1.09 0.05 0.04 0.009 0.003 0.003 0.0021 C 0.171 1.21 0.06 0.02 0.012 0.005 0.005 0.0018 D 0.292 0.88 0.08 0.03 0.014 0.004 0.004 0.0016 E 0.219 2.8 0.07 0.02 0.011 0.003 0.005 0.0015 F 0.227 1.06 0.41 0.03 0.009 0.004 0.003 0.0041 G 0.226 1.13 0.03 0.07 0.019 0.006 0.005 0.0022 H 0.102 0.99 0.07 0.02 0.012 0.006 0.004 0.0018 I 0.218 1.03 0.06 0.03 0.015 0.004 0.007
• each specimen was cut in a direction parallel to the rolling direction, and then a specimen for observing the microstructure was collected from a cut surface at a 1/4 point of a plate thickness.
• the specimens collected as such were polished and etched with a nital etchant, and the microstructure of each specimen was observed using an optical microscope and the scanning electron microscope (SEM).
• a microstructure fraction was measured as an area percentage of each phase in a 200x optical microscope image and 500x and 5000x SEM images.
• each martensite packet having a long axis direction parallel to the rolling direction within 45° in the scanning electron microscope image was measured.
• an area of each martensite packet having a length ratio of the long axis to the short axis within a range of 2:1 to 20:1 in the scanning electron microscope image was measured.
• M represents martensite
• F represents ferrite
• R- ⁇ represents retained austenite
• P represents pearlite
• B represents bainite.
• F R- ⁇ P B 1 98 0 1 0 1 81 91 2 100 0 0 0 0 80 87 3 100 0 0 0 0 0 86 77 4 98 0 1 0 1 94 75 5 97 0 1 1 1 1 84 67 6 99 0 1 0 0 91 77 7 95 0 0 2 3 83 75 8 95 1 0 2 2 88 81 9 100 0 0 0 0 95 70 10 98 0 2 0 0 81 86 11 100 0 0 0 0 80 75 12 100 0 0 0 0 85
• FIG. 2 is a photograph showing a 100 area% martensite microstructure before cold rolling according to the present disclosure. Referring to FIG. 2 , it may be identified that the steel plate includes martensite, the hard structure, as the matrix structure.
• FIG. 3 is a photograph showing a martensite microstructure after cold rolling with a 70% reduction percentage according to the present disclosure. Referring to FIG. 3 , it may be identified that martensite included in the steel plate has the form that is elongated by the cold rolling.
• FIG. 3 is a high-magnification measurement of the martensite packet, it is difficult to measure the long and short axes of the packet from FIG. 3 .
• Specimens 1 to 12 which satisfy both the alloy composition and the manufacturing conditions of the present disclosure, show a martensite fraction of 95 to 100 area% and a ratio of martensite elongated in the rolling direction of 72 to 95%.
• a ratio of martensite packets with a long-short axis ratio of 2:1 to 20:1 is 56 to 91%.
• Specimens 1 to 12 show a tensile strength of 1,839 to 2,133 MPa and a yield strength of 1,811 to 2,088 MPa, and the yield strength and the ratio of alloying elements both satisfy Relational Expression 1.
• Specimens 1 to 6 and Specimens 9 to 12 satisfy the alloy composition of the steel plate of C: 0.18 to 0.30%, and Specimens 7 and 8 satisfy C: smaller than 0.18%. Specimens 1 to 6 and Specimens 9 to 12 show further improved tensile strength and yield strength compared to Specimens 7 and 8.
• Specimens 13 to 20 which do not satisfy at least one of the alloy composition and the manufacturing conditions of the present disclosure, do not satisfy at least one of the fraction of martensite, the ratio of martensite elongated in the rolling direction, and the ratio of martensite packets having the long-short axis ratio equal to or greater than 2:1, which are proposed by the present disclosure.
• Specimen 13 is a specimen in which the cooling was started 5 seconds after the end of the rolling. It may be identified that desired strength and durability are not secured in Specimen 13 because of a high fraction of ferrite.
• Specimen 14 is a case in which the rolling end temperature is low
• Specimen 16 is a case in which the cooling rate is low. It may be identified that the martensite fraction that the present disclosure aims for is not secured and the aimed strength is not secured in these Specimens because of high fractions of pearlite and bainite.
• Specimen 15 is a case in which the cooling end temperature and the coiling temperature are high. It may be identified that the aimed strength is not secured in Specimen 15 because of the high fraction of bainite.
• Specimen 17 is a case in which the cold rolling reduction rate is low. It may be identified that the aimed strength is not secured in Specimen 17 because of low fraction of elongated martensite and ratio of martensite packets with the long-short axis ratio equal to or greater than 2:1.
• Specimen 18 is a case in which the carbon (C) content is low
• Specimen 19 is a case in which the boron (B) content is low. It may be identified that aimed levels of strength and durability are not secured in these Specimens because of significantly low fractions of martensite.
• Specimen 20 is a case in which the manganese (Mn) content is high. Because the transformation into martensite did not occur sufficiently, a large amount of residual austenite was formed. It may be identified that Specimen 20 has excellent tensile strength and yield strength because of the high manganese content, but has low efficiency of the alloying elements because the yield strength and the ratio of the alloying elements do not satisfy Relational Expression 1.
• the high-strength steel plate of the present disclosure has the excellent tensile strength and yield strength, including 90 area% or greater of hard martensite, by lowering the carbon and/or manganese content and controlling the alloy composition ratio and the conditions of the rapid cooling and the cold rolling, despite the omission of the heat treatment immediately after the hot rolling and immediately after the cold rolling.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=91589441

Family Applications (1)

Country Status (5)

Family Cites Families (5)

• 2022
  • 2022-12-21 KR KR1020220180619A patent/KR20240098659A/ko active Pending
• 2023
  • 2023-12-13 CN CN202380087427.7A patent/CN120500552A/zh active Pending
  • 2023-12-13 EP EP23907567.4A patent/EP4640880A1/de active Pending
  • 2023-12-13 JP JP2025536074A patent/JP2025541887A/ja active Pending
  • 2023-12-13 WO PCT/KR2023/020553 patent/WO2024136281A1/ko not_active Ceased

Also Published As

Similar Documents

Legal Events