US20180327880A1 - Method of producing steel material, apparatus that cools steel material, and steel material - Google Patents

Method of producing steel material, apparatus that cools steel material, and steel material Download PDF

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Publication number
US20180327880A1
US20180327880A1 US15/573,885 US201615573885A US2018327880A1 US 20180327880 A1 US20180327880 A1 US 20180327880A1 US 201615573885 A US201615573885 A US 201615573885A US 2018327880 A1 US2018327880 A1 US 2018327880A1
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Prior art keywords
cooling
steel material
rail
longitudinal direction
conveyance
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US15/573,885
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English (en)
Inventor
Kenji Okushiro
Hiroyuki Fukuda
Hideo Kijima
Yoshikazu Yoshida
Hiroshi Ishikawa
Sadanori Nakano
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: NAKANO, SADANORI, ISHIKAWA, HIROSHI, YOSHIDA, YOSHIKAZU, KIJIMA, HIDEO, OKUSHIRO, Kenji, FUKUDA, HIROYUKI
Publication of US20180327880A1 publication Critical patent/US20180327880A1/en
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    • 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
    • C21D9/04—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rails
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/08—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling structural sections, i.e. work of special cross-section, e.g. angle steel
    • B21B1/085—Rail sections
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
    • B21B45/02—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for lubricating, cooling, or cleaning
    • B21B45/0203—Cooling
    • 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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
    • B21B45/02—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for lubricating, cooling, or cleaning
    • B21B45/0203—Cooling
    • B21B45/0209—Cooling devices, e.g. using gaseous coolants
    • B21B2045/0212—Cooling devices, e.g. using gaseous coolants using gaseous coolants
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
    • B21B45/02—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for lubricating, cooling, or cleaning
    • B21B45/0203—Cooling
    • B21B45/0209—Cooling devices, e.g. using gaseous coolants
    • B21B45/0215—Cooling devices, e.g. using gaseous coolants using liquid coolants, e.g. for sections, for tubes
    • B21B2045/0221—Cooling devices, e.g. using gaseous coolants using liquid coolants, e.g. for sections, for tubes for structural sections, e.g. H-beams
    • 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
    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • This disclosure relates to a method of producing a steel material, an apparatus that cools steel material, and a steel material.
  • a rail for railways One of the longest steel materials is a rail for railways.
  • a rail where a rail head section has a pearlite structure high in hardness is, for example, produced as follows.
  • bloom cast by continuous casting is reheated to 1100° C. or more and, thereafter, hot rolled by rough rolling and finish rolling to have a predetermined rail shape.
  • the rolling method in each rolling step is performed by a combination of caliber rolling and universal rolling, or by only caliber rolling, and such rough rolling is performed for a plurality of passes and such finish rolling is performed for a plurality of passes or a single pass.
  • the rail here usually has a length of about 50 m to 200 m by hot rolling.
  • an unsteady section at an end of the rail hot rolled is hot sawn (hot sawing step).
  • a heat treatment apparatus is here limited with respect to the length, further sawing is performed so that a predetermined length (for example, 25 m) is achieved.
  • a coolant air, water, mist, or the like
  • a cooling apparatus thereby performing forced cooling (heat treatment step).
  • the rail is restricted by a restraint apparatus such as a clamp, and the coolant is sprayed to a head section, a foot section, and also, if necessary, a web.
  • the cooling apparatus usually performs cooling until the temperature of the head section of the rail reaches 650° C. or less. After such forced cooling is completed, the rail is released from the restraint apparatus, and further conveyed to a cooling bed and cooled to 100° C. or less.
  • the rail for railways is, for example, a rail for use in a severe environment where heavy goods such as coal and iron ore are transported from mines having natural resources such as coal
  • a rail is demanded to have high wear resistance and high toughness and, therefore, the heat treatment step is required.
  • the heat treatment is performed, thereby enabling the rail to be high in hardness and decreasing the amount of wear in use and, therefore, the effects of increasing the rail replacement period and decreasing the life-time cost are achieved.
  • the variation in hardness is large in the longitudinal direction of the rail, however, is not preferable because the amount of wear is larger at a low-hardness section than a high-hardness section, thereby not only increasing the vibration in train running, but also decreasing the replacement period.
  • a heat treatment method that allows the rail to be small in the variation in hardness and high in hardness.
  • JP H03-166318 A discloses a method of suppressing a cooling rate to 7° C./sec or less, as a method of decreasing the variation in hardness of a rail.
  • J P 2003-193126 A discloses a method of oscillating an H-shaped steel in an amount obtained by an Equation with the pitch between nozzles being adopted as a parameter in accelerated cooling of the H-shaped steel, as a method of uniformly cooling a steel material.
  • J P 2006-55864 A discloses a method of oscillating a steel material at a distance 5 times to 10 times the distance in the longitudinal direction of the material of a guide roller, as a method of uniformly cooling a steel material.
  • JP '318 can decrease the influence of the variation in temperature at the start of a heat treatment in the longitudinal direction of a steel material on the variation in hardness.
  • the variation in cooling rate is caused in the longitudinal direction of a steel material, uniform hardness is not achieved. Therefore, it is difficult to produce a steel material uniform in material properties in the longitudinal direction.
  • JP '126 and JP '864 can alleviate the reduction in cooling rate due to a weak cooling section generated in cooling equipment, it is difficult to provide a uniform cooling rate when the variation in cooling rate is caused between cooling headers in the longitudinal direction of a steel material. Therefore, it is difficult to produce a steel material uniform in material properties such as hardness in the longitudinal direction.
  • a method of producing a steel material wherein, when a cooling apparatus having a plurality of cooling sections disposed side by side in the longitudinal direction of a steel material cools the steel material hot worked or cooled/reheated, the steel material is conveyed at a conveyance distance L o (m) satisfying Equation (1), in one direction along with the longitudinal direction of the steel material, in the cooling apparatus:
  • L o conveyance distance (m) of steel material.
  • m natural number
  • L h length (m) of cooling sections in longitudinal direction of steel material.
  • An apparatus that cools steel material hot worked or cooled/reheated including: a plurality of cooling sections disposed side by side in the longitudinal direction of the steel material; and a conveyance section that conveys the steel material at a conveyance distance L o (m) satisfying Equation (1), in one direction along with the longitudinal direction of the steel material in the cooling apparatus, during cooling of the steel material in the cooling sections.
  • FIG. 1 is a schematic view illustrating a cooling apparatus according to one example.
  • FIG. 2 is a cross-section view illustrating each section of a rail.
  • FIG. 3 is a plan view illustrating peripheral equipment of the cooling apparatus.
  • FIG. 4A and FIG. 4B are schematic views illustrating a conveyance operation of the cooling apparatus.
  • FIG. 5 is a plan view illustrating peripheral equipment of a cooling apparatus in Examples.
  • FIG. 6 is a schematic view illustrating a conveyance state on a discharge table in Examples.
  • a rail 1 is produced as a steel material.
  • the cooling apparatus 2 is used in a heat treatment step performed after a hot rolling step or a hot sawing step described below, and forcedly cools a rail 1 having a high temperature.
  • the rail 1 when viewed cross-sectionally perpendicular to the longitudinal direction, includes a head section 11 and a foot section 13 extending in the width direction and opposite to each other in the vertical direction, and a web section 12 connecting the head section 11 disposed above and the foot section 13 disposed below and extending in the vertical direction, as illustrated in FIG. 2 .
  • the cooling apparatus 2 includes head section-cooling headers 21 a to 21 c , a foot section-cooling header 22 , a pair of clamps 23 a and 23 b , a thermometer 24 in the apparatus, and a conveyance section 25 .
  • the head section-cooling headers 21 a to 21 c , and the foot section-cooling header 22 serve as cooling sections to cool the rail 1 , and a plurality of the respective headers are provided continuously side by side in the y-axis direction serving as the longitudinal direction of the rail 1 .
  • the head section-cooling headers 21 a to 21 c , and the foot section-cooling header 22 are also collectively called cooling headers.
  • the head section-cooling headers 21 a to 21 c have coolant-spraying outlets arranged at pitches of several mm to 100 mm, and the coolant-spraying outlets of each of the head section-cooling headers 21 a to 21 c are provided oppositely on each of the head top surface (end surface in the z-axis positive direction) and the head side surfaces (both end surfaces in the x-axis positive direction) of the head section 11 .
  • the head section-cooling headers 21 a to 21 c each spray a coolant supplied from a supply section not illustrated, to the head top surface and the head side surface of the head section 11 , thereby subjecting the head section 11 to forced cooling.
  • the coolant to be used is air, spray water, mist or the like.
  • Respective pressure measurement apparatuses 211 a to 211 c are also provided on coolant supply pathways of the head section-cooling headers 21 a to 21 c , and the coolant spray pressure is monitored.
  • the foot section-cooling header 22 has coolant-spraying outlets arranged at pitches of several mm to 100 mm, and the coolant-spraying outlets are provided opposite to the lower surface (end surface in the z-axis negative direction) of the foot section 13 .
  • the foot section-cooling header 22 sprays a coolant supplied from a supply section not illustrated, to the lower surface of the foot section 13 , thereby subjecting the foot section 13 to forced cooling, as in the head section-cooling headers 21 a to 21 c .
  • the coolant to be used is air, spray water, mist or the like, as in the head section-cooling headers 21 a to 21 c .
  • a pressure apparatus 221 is also provided on a coolant supply pathway of the foot section-cooling header 22 , and the coolant spray pressure is monitored.
  • an increase in the number of the cooling headers provided in the z-axis direction for a decrease in the length of the cooling headers is not preferable because there are required many feed ports of the coolant as well as many measurement devices and control devices of the amount of coolant spray (for example, a pressure gauge, a flow meter, and a flow regulator) mounted to the cooling headers and a pipe arrangement.
  • the length of the cooling headers in the z-axis direction is needed to be a proper length, and is preferably 0.5 m or more and 4 m or less.
  • the head section-cooling headers 21 a to 21 c and the foot section-cooling header 22 provided side by side in the y-axis direction are preferably provided as close as possible so that any cooling irregularity is not caused.
  • the pair of clamps 23 a and 23 b is an instrument that sandwiches each of both ends of the foot section 13 in the x-axis direction to thereby support and restrain the rail 1 .
  • the pair of clamps 23 a and 23 b is plurally provided over the entire length in the longitudinal direction of the rail 1 and are several meters apart.
  • the thermometer 24 in the apparatus is a non-contact thermometer such as a radiation thermometer, and measures the surface temperature of at least one point on the head top surface of the head section 11 .
  • the conveyance section 25 is a conveyance mechanism connected to the pair of clamps 23 a and 23 b , and is an apparatus that conveys the pair of clamps 23 a and 23 b in the y-axis direction, thereby conveying the rail 1 in the cooling apparatus 2 .
  • the detail of a conveyance operation of the conveyance section 25 is described below.
  • the amount of the coolant sprayed from each of the head section-cooling headers 21 a to 21 c and the foot section-cooling header 22 is adjusted by a control section not illustrated.
  • the control section here acquires the temperature measurement result of the thermometer 24 in the apparatus, and the amount sprayed is adjusted, as needed, based on the temperature measurement result acquired.
  • a carrying-in table 3 and a discharge table 4 are provided on the periphery of the cooling apparatus 2 .
  • the carrying-in table 3 is a table that conveys the rail 1 from a preceding step such as the hot rolling step to the cooling apparatus 2 .
  • the discharge table 4 is a table that conveys the rail 1 heat-treated in the cooling apparatus 2 , to a next step such as a cooling bed or an examination instrument.
  • An exit side thermometer 5 is a non-contact thermometer that measures the surface temperature of the head section 11 of the rail 1 , as in the thermometer 24 in the apparatus, and that measures the temperature of the rail 1 discharged from the cooling apparatus 2 after the heat treatment.
  • a perlite-based rail 1 is produced as a steel material.
  • the rail 1 that can be used is, for example, steel including the following chemical component composition.
  • Equation by “%” with respect to each chemical component means “% by mass,” unless especially noted.
  • C carbon
  • the C content is thus preferably 0.60% or more, more preferably 0.70% or more.
  • an increase in the amount of the cementite can be achieved to result in increases in hardness and strength, but deterioration in ductility is conversely caused.
  • an increase in the C content expands the temperature range of the y+0 region, and promotes softening of a welded heat affected zone.
  • the C content is preferably 1.05% or less, more preferably 0.97% or less.
  • Si 0.1% or more and 1.5% or less
  • Si silicon
  • the Si content is preferably 0.1% or more, more preferably 0.2% or more.
  • the Si content is preferably 1.5% or less, more preferably 1.3% or less.
  • Mn 0.01% or more and 1.5% or less
  • Mn manganese
  • Mn has the effects of decreasing the temperature of perlite transformation and finning the perlite lamellar spacing and, therefore, is an element effective to maintain high hardness inside the rail 1 . If the Mn content is less than 0.01%, however, the effects are less exerted. Therefore, the Mn content is preferably 0.01% or more, more preferably 0.3% or more. If the Mn content is more than 1.5%, the equilibrium transformation temperature (TE) of perlite is lowered, and martensitic transformation easily occurs in the structure. Therefore, the Mn content is preferably 1.5% or less, more preferably 1.3% or less. P: 0.035% or less
  • the P content is preferably made lower. Specifically, the P content is preferably 0.035% or less, more preferably 0.025% or less. If special refining or the like is here performed to decrease the P content as much as possible, cost rise is caused in smelting. Therefore, the P content is preferably 0.001% or more.
  • the S content is preferably made lower. Specifically, the S content is preferably 0.030% or less, more preferably 0.015% or less. If the S content is here decreased as much as possible, cost rise in smelting is remarkably caused due to increases in smelting treatment time and the amount of a solvent. Therefore, the S content is preferably 0.0005% or more.
  • Cr chromium
  • TE equilibrium transformation temperature
  • Cr when used in combination with Sb, is also effective in inhibiting a decarburization layer from being generated. Therefore, the Cr content is preferably 0.1% or more, more preferably 0.2% or more. If the Cr content is more than 2.0%, not only the possibility of the occurrence of welding defects is increased, but also hardenability is increased, and generation of martensite is promoted. Therefore, the Cr content is preferably 2.0% or less, more preferably 1.5% or less.
  • the total content of Si and Cr is desirably 2.0% or less. The reason is because, if the total content of Si and Cr is more than 2.0%, an excessive increase in scale adhesiveness can inhibit scale peeling and promote decarburization.
  • Sb antimony
  • Sb has a remarkable effect of preventing decarburization during heating of a rail steel material in a heating furnace.
  • Sb is added together with Cr, to thereby have the effect of reducing generation of a decarburization layer, when the Sb content is 0.005% or more. Therefore, the Sb content is preferably 0.005% or more, more preferably 0.01% or more. If the Sb content is more than 0.5%, the effect is saturated. Therefore, the Sb content is preferably 0.5% or less, more preferably 0.3% or less.
  • the steel for use as the rail 1 may further contain, in addition to the chemical composition, one or more elements of Cu: 0.01% or more and 1.0% or less, Ni: 0.01% or more and 0.5% or less, Mo: 0.01% or more and 0.5% or less, V: 0.001% or more and 0.15% or less, and Nb: 0.001% or more and 0.030% or less.
  • Cu copper is an element that can provide much higher hardness by solid solution strengthening. Cu also has the effect of suppressing decarburization. To expect such an effect, the Cu content is preferably 0.01% or more, more preferably 0.05% or more. If the Cu content is more than 1.0%, surface cracking due to embrittlement in continuous casting and/or rolling easily occurs. Therefore, the Cu content is preferably 1.0% or less, more preferably 0.6% or less.
  • Ni 0.01% or more and 0.5% or less
  • Ni nickel is an element effective to enhance toughness and ductility. Moreover, Ni is an element also effective to suppress Cu cracking by addition as a composite with Cu. Therefore, when Cu is added, Ni is desirably added, and the Ni content is more preferably 0.05% or more. If the Ni content is less than 0.01%, however, such effects are not exerted. Therefore, the Ni content is preferably 0.01% or more. If the Ni content is more than 0.5%, hardenability is increased, and generation of martensite is promoted. Therefore, the Ni content is preferably 0.5% or less, more preferably 0.3% or less.
  • Mo mobdenum
  • Mo is an element effective for an increase in strength, but such an effect is less exerted if the content is less than 0.01%. Therefore, the Mo content is preferably 0.01% or more, more preferably 0.05% or more. If the Mo content is more than 0.5%, an increase in hardenability causes martensite to be generated, resulting in extreme deterioration in toughness and ductility. Therefore, the Mo content is preferably 0.5% or less, more preferably 0.3% or less.
  • Nb 0.001% or more and 0.030% or less
  • Nb niobium
  • the Nb content is preferably 0.001% or more, more preferably 0.003% or more. If the Nb content is more than 0.030%, Nb carbonitride is crystalized in the course of solidification in casting of a rail steel material such as bloom, resulting in deterioration in cleanliness. Therefore, the Nb content is preferably 0.030% or less, more preferably 0.025% or less.
  • the balance other than the above components is configured from Fe (iron) and inevitable impurities.
  • Fe iron
  • the Al content is desirably 0.001% or less and the Ti content is desirably 0.001% or less.
  • the bloom heated is rolled in each of a break-down roller, a rough roller and a finish roller for one or more passes, and finally rolled to the rail 1 having a shape illustrated in FIG. 2 (hot rolling step).
  • the length in the longitudinal direction of the rail 1 rolled is here about 50 m to 200 m, and is, if necessary, hot sawn to have a length of, for example, 25 m (hot sawing step).
  • a shorter length in the longitudinal direction of the rail 1 here causes the subsequent heat treatment step to be involuntarily affected by the coolant sprayed onto the end surface in the longitudinal direction during cooling.
  • the length in the longitudinal direction of the rail 1 for use in the heat treatment step is three times or more the height from the head top surface of the head section 11 of the rail 1 to the lower surface of the foot section 13 thereof.
  • the upper limit of the length in the longitudinal direction of the rail 1 for use in the heat treatment step is defined as the length of rolling (the maximum rolling length in the hot rolling step).
  • the hot rolled or hot sawn rail 1 is conveyed to the cooling apparatus 2 by the carrying-in table 3, and cooled in the cooling apparatus 2 (heat treatment step).
  • the temperature of the rail 1 here conveyed to the cooling apparatus 2 is desirably in the austenite temperature region.
  • a rail for use in a mine or a curved section is needed to have high hardness and, therefore, rapid acceleration is needed in the cooling apparatus 2 after rolling.
  • Such acceleration fines the perlite lamellar spacing, thereby providing a high-hardness structure, and an increase in the degree of supercooling in transformation, namely, an increase in the cooling rate in transformation can provide such a high-hardness structure. If the structure of the rail 1 , however, is transformed before cooling in the cooling apparatus 2 , such transformation progresses at an extremely low cooling rate in spontaneous cooling and, therefore, cannot provide a high-hardness structure.
  • the rail 1 when the temperature of the rail 1 is equal to or lower than the lowest temperature in the austenite temperature region at the start of cooling in the cooling apparatus 2 , the rail 1 is preferably reheated to any temperature in the austenite temperature region and thereafter subjected to the heat treatment step.
  • the cooling headers may be close to the rail 1 to achieve a high cooling rate in cooling of the rail 1 having a high temperature.
  • the cooling headers are heated by radiation from the rail 1 and/or heat conduction of air, and therefore thermally deformed. Only surfaces of the cooling headers, the surfaces being closer to the steel material, are heated and thermally expended and, therefore, the cooling headers are usually warped so that end portions thereof are away from the rail 1 .
  • the conveyance section 25 conveys the clamps 23 a and 23 b together with the rail 1 restrained, with oscillation at a predetermined amplitude, in cooling.
  • Such oscillation here means an operation that conveys the rail 1 alternately in the y-axis positive direction and in the y-axis negative direction by a predetermined conveyance distance L o .
  • the conveyance distance L o serving as the amplitude of oscillation corresponds to the distance (m) satisfying Equation (1).
  • Equation (1) m represents a natural number
  • L h represents the length (m) of the cooling headers, being the length of the cooling sections in the longitudinal direction of the rail 1 (y-axis direction), respectively:
  • the conveyance operation of the rail 1 by the conveyance section 25 is described with reference to FIG. 4 .
  • the conveyance distance L o in the heat treatment step is a length twice the length L h of the cooling headers (head section-cooling header 21 a and foot section-cooling header 22 ) serving as the cooling sections.
  • the conveyance section 25 then conveys the rail 1 in the state illustrated in FIG. 4A at the conveyance distance L o in the y-axis negative direction.
  • the rail 1 is in the state illustrated in FIG. 4B from the state illustrated in FIG. 4A .
  • the conveyance section 25 conveys the rail 1 in the state illustrated in FIG. 4B at the conveyance distance L o in the y-axis positive direction.
  • the rail 1 is again in the state illustrated in FIG. 4A from the state illustrated in FIG. 4B .
  • Such operations are repeated to perform the conveyance operation.
  • V L h /( T ⁇ n ) (2).
  • a final structure made of 100% of perlite, or a final structure having 5% or less of pro-eutectoid ferrite and pro-eutectoid cementite and the balance being perlite or a final structure where perlite and bainite are mixed is obtained.
  • the bainite phase and the cementite phase are impaired in toughness, therefore, a structure made of 100% of the perlite phase is preferable to not generate any failures caused by deterioration in toughness such as sharing, and a final structure is determined depending on the intended use.
  • a high-hardness structure is obtained by allowing transformation to occur in the heat treatment and, therefore, the heat treatment completion temperature is needed to be achieved after completion of transformation. While the depth necessary for such a high-hardness structure, however, varies depending on the intended use of the rail 1 and the heat treatment completion temperature cannot be thus clearly limited, cooling is needed to be performed at least until the temperature of the surface of the head section 11 reaches 650° C. or less.
  • the rail 1 is conveyed to the cooling bed by the discharge table 4, and cooled thereon to a temperature ranging from room temperature to 100° C. Thereafter, the rail 1 is straightened by roller straightening to decrease warpage. The rail 1 then undergoes an examination and thereafter is shipped. Since a section non-straightened is generated at an end in the longitudinal direction of the rail 1 in straightening by roller straightening, cold sawing may also be performed after straightening by roller straightening, without sawing to the length of a final product in hot sawing.
  • the end in the longitudinal direction of the rail 1 , in cold sawing, here corresponds to each of both ends in the rolling length and, therefore, any section not-straightened is decreased and warpage is decreased.
  • a rail 1 uniform in material properties in the longitudinal direction can be produced through the above steps.
  • the rail 1 is used as the steel material in the example, but the disclosure is not limited to such an example.
  • the steel material to be produced may be any other steel material product such as a thick plate or a shaped steel.
  • the chemical component composition of the steel material product, the configuration of the cooling apparatus 2 and the like are not limited to the examples. Even when the steel material to be produced is the rail 1 , any steel having a different chemical component composition from that in the example may be used.
  • the minimum length in the longitudinal direction of the steel material product is three times or more the thickness of the thickest portion of a steel material such as a shaped steel, or three times or more the thickness of a plate material representative of a thick plate, and the maximum length thereof is the rolling length.
  • the conveyance distance L o satisfies Equation (1) in the example, the conveyance distance L o is preferably a value closer to the integral multiple of the length L of the cooling sections, and preferably satisfies Equation (3):
  • the conveyance section 25 conveys the rail 1 with the rail 1 being oscillated in the heat treatment step in the example, this disclosure is not limited to such an example.
  • the conveyance section 25 may be configured to convey the rail 1 at the conveyance distance L o in only any one direction of the y-axis positive direction and the y-axis negative direction with the rail 1 being not oscillated.
  • the conveyance operation of the rail 1 in the cooling apparatus 2 by the conveyance section 25 in the heat treatment step is continuously performed during cooling of the rail 1 in the example
  • this disclosure is not limited to such an example.
  • the conveyance operation of the rail 1 in the example may be performed for a time more than half of the cooling time T, after cooling of the rail 1 .
  • the conveyance operation is here performed at the conveyance distance L o satisfying Equation (1), for a predetermined time (time more than half of the cooling time T) from the start of cooling of the rail 1 . Thereafter, the conveyance operation is preferably continuously performed for the remaining time of the cooling time T, but the conveyance distance L o does not necessarily satisfy Equation (1).
  • the time for which uniform cooling can be made can be at least half of the heat treatment time, thereby decreasing the variation in cooling rate.
  • the conveyance velocity V does not necessarily satisfy Equation (2) and, therefore, application to a cooling apparatus 2 that cannot be changed in the conveyance velocity V can also be made.
  • the cooling headers of the cooling apparatus 2 are needed therefor to be cooled by being closer to the steel material.
  • the cooling headers are here heated by radiation or the like from the steel material, and the cooling headers are deformed to be warped in the longitudinal direction. If cooling is performed in such a state, the difference in distance from the steel material is caused in the longitudinal direction of the cooling headers, and thus the variation in the cooling rate (in a strong cooling section and a weak cooling section) is caused in each cooling header unit, resulting in the occurrence of the variation in hardness of the steel material.
  • the rail 1 may be usually cooled with being oscillated at a lower amplitude than that in the example, in the longitudinal direction.
  • the cooling rate is here higher at a position immediately below each coolant-spraying outlet and lower at a position away from the position immediately below each coolant-spraying outlet and, therefore, the rail can be at least conveyed at a distance (several mm to 100 mm) between coolant-spraying outlets, thereby uniformly passing through the position immediately below each coolant-spraying outlet, higher in the cooling rate, and the position away therefrom, lower in the cooling rate.
  • Such conventional oscillation conveyance operation
  • Such a configuration can allow a long total conveyance distance to be achieved even when the length of the cooling apparatus does not have sufficient margin relative to the length in the longitudinal direction of the steel material.
  • the Steel Material is a Rail Material.
  • Such a configuration can allow a rail material less in the variation in material properties in the longitudinal direction to be obtained as a rail material being a long steel material.
  • the rail material is a high-hardness rail 1
  • the variation in cooling in the heat treatment step can be suppressed within 20° C. or less, and as a result, the variation in hardness can be suppressed within an HV of 13 or less at a depth position of 1 mm from the surface and within an HV of 10 or less at a depth position of 5 mm therefrom.
  • An apparatus 2 that cools steel material is a cooling apparatus 2 that cools steel material hot worked or cooled/reheated, including a plurality of cooling sections (head section-cooling headers 21 a to 21 c , and a foot section-cooling header 22 ) disposed side by side in the longitudinal direction of the steel material, and a conveyance section 25 that conveys the steel material at the conveyance distance L o (m) satisfying Equation (1), in the longitudinal direction of the steel material in the cooling apparatus 2 , during cooling of the steel material in the cooling sections.
  • a cooling apparatus 2 that cools steel material hot worked or cooled/reheated, including a plurality of cooling sections (head section-cooling headers 21 a to 21 c , and a foot section-cooling header 22 ) disposed side by side in the longitudinal direction of the steel material, and a conveyance section 25 that conveys the steel material at the conveyance distance L o (m) satisfying Equation (1), in the longitudinal direction of the steel material in the cooling apparatus 2 , during cooling
  • Such a configuration can allow the same effect as in configuration (1) above to be obtained.
  • a steel material according to one example is a steel material produced by hot working or cooling/reheating and thereafter cooling in a cooling apparatus 2 having a plurality of cooling sections (head section-cooling headers 21 a to 21 c , and a foot section-cooling header 22 ) disposed side by side in the longitudinal direction, wherein, during cooling in the cooling apparatus 2 , the steel material is produced with being conveyed at the conveyance distance L o (m) satisfying Equation (1), in one direction along with the longitudinal direction of the steel material in the cooling apparatus 2 .
  • Such a configuration can allow the steel material to be uniformly cooled in the longitudinal direction, thereby providing a steel material uniform in material properties in the longitudinal direction.
  • Example 1 is described. First, before Example 1, a rail 1 being a steel material was produced in a different conveyance distance L o condition from the example, as Conventional Examples, and the material properties thereof were evaluated.
  • a bloom of a chemical component composition in Condition A represented in Table 1 was cast by using a continuous casting method.
  • the balance of the chemical component composition of the bloom was here substantially Fe, specifically Fe and inevitable impurities.
  • the bloom cast was reheated to 1100° C. or more in a heating furnace, thereafter taken out from the heating furnace, and hot rolled through a break-down roller, a rough roller and a finish roller so that the cross-sectional shape was the final shape (rail shape illustrated in FIG. 2 ).
  • the rail 1 was rolled at an inverted position where a head section 11 and a foot section 13 were in contact with a conveyance stage.
  • the spray pressure of the coolant was set at 1.3 kPa to 130 kPa so that the cooling rate at 670° C. to 770° C. at a depth position of 5 mm from the surface layer was 3° C./sec to 7° C./sec, and cooling was performed until the surface temperature of the head section 11 reached 530° C. or less, while temperature measurement was performed by a thermometer 24 in the apparatus.
  • the rail 1 was taken out from the cooling apparatus 2 onto a discharge table 4, and the surface temperature of the head section 11 of the rail 1 after cooling was measured by use of an exit side thermometer 5 provided on the discharge table 4 as illustrated in FIGS. 5 and 6 .
  • the exit side thermometer 5 was here used to measure the temperature at a plurality of positions over the entire length in the longitudinal direction of the rail 1 , and the variation in temperature after cooling was calculated from the maximum value and the minimum value of the measurement results.
  • the rail 1 was conveyed to a cooling bed and cooled in the cooling bed until the temperature reached room temperature to 100° C. and, thereafter, straightening was performed by a roller straightening machine to produce a rail 1 being a final product. Thereafter, the rail 1 produced was cold sawn to thereby take a sample, and the hardness of the sample taken was measured.
  • the sample was here taken at a pitch of 1 m relative to the total length of the rail 1 , and the Vickers hardness test was performed as hardness measurement at depth positions of 1 mm and 5 mm from the surface at the center in the width direction of the head section 11 of the rail 1 .
  • Example 1 first, a bloom of each of chemical component compositions with respect to A to C represented in Table 1 was cast by using a continuous casting method.
  • the balance of the chemical component composition of the bloom was substantially Fe, and specifically Fe and inevitable impurities.
  • the bloom cast was reheated to 1100° C. or more in a heating furnace, and thereafter taken out from the heating furnace and hot rolled through a break-down roller, a rough roller and a finish roller so that the cross-sectional shape was the final shape, in the same manner as in the Conventional Examples.
  • the rail 1 was rolled at an inverted position where the head section 11 and the foot section 13 were in contact with a conveyance stage.
  • the rail 1 hot rolled was conveyed to the cooling apparatus 2 , and the rail 1 was cooled in the same manner as in the example (heat treatment step). Since the rail 1 was here rolled at the inverted position as a rolling position, the rail 1 , when carried in the cooling apparatus 2 , was inverted, and allowed to be at an erect position illustrated in FIG. 2 , where the foot section 13 was located below in the vertical direction and the head section 11 was located above in the vertical direction, and the rail 1 was restrained by clamps 23 a and 23 b . Cooling was then performed by spraying of a coolant from each cooling header. During such cooling, the coolant was any of air, mist or spray water, and the distance between the cooling headers and the rail was 20 mm.
  • the spray pressure of the coolant was 5 kPa to 50 kPa, and when the coolant was mist or spray water, 15% of a spray outlet was changed to a mist nozzle or a spray nozzle, and the coolant was sprayed through such a nozzle at a spray pressure of 500 kPa or 300 kPa.
  • the coolant was mist or spray water, air was sprayed through 85% of the remaining outlet, and the pressure of air was 30 kPa. Cooling was performed with the spray pressure of the coolant being changed depending on the condition in the heat treatment step. Furthermore, cooling was performed in the heat treatment step until the surface temperature of the head section 11 reached 530° C. or less, while temperature measurement was performed by the thermometer 24 in the apparatus, in the same manner as in the Conventional Examples.
  • cooling was performed in the heat treatment step in conditions of the length L h of the cooling headers, where the conveyance distance L o and the total conveyance distance (m) serving as the total distance of conveyance in cooling were changed within the scope of the example.
  • the rail 1 was taken out from the cooling apparatus 2 onto the discharge table 4, and the surface temperature of the head section 11 of the rail 1 after cooling was measured by use of the exit side thermometer 5 provided on the discharge table 4, as illustrated in FIG. 5 and FIG. 6 .
  • the exit side thermometer 5 was here used to measure the temperature at a plurality of positions over the entire length in the longitudinal direction of the rail 1 , and the variation in temperature after cooling was calculated from the maximum value and the minimum value of the measurement results.
  • conveyance distance L o was set to 4 m in the condition of Comparative Example 1-3, conveyance was made by only up to 3.0 m during cooling of the rail 1 , and while the conveyance distance L o was set to 2 m in the condition of Comparative Example 1-4, conveyance was made by only up to 1.0 m during cooling of the rail 1 .
  • the variation in temperature in the entire length was within 20° C. in the conditions of Examples 1-1 to 1-17, and the variation in temperature in the entire length was smaller and was within 5° C. in the condition where the oscillation distance L o was n times the cooling header length L h .
  • the variation in temperature was within 20° C. or more in the condition where the oscillation distance L o indicated in Comparative Examples 1-1 to 1-4 was shorter than the cooling header length L h or in the condition where the total conveyance distance in the heat treatment was less than the cooling header length L h .
  • Example 1-1 A 0.5 0.5 4.0 Air 30
  • Example 1-2 A 1 1 4.0 Air 30
  • Example 1-3 A 2 2 4.0 Air 30
  • Example 1-4 A 4 4 4.0 Air 30
  • Example 1-5 A 2 4 4.0 Air 30
  • Example 1-6 A 2 8 8.0 Air 30
  • Example 1-7 A 2 2 2.0 Air 30
  • Example 1-8 A 2 2 5.0 Air 30
  • Example 1-9 A 2 2 2.5 Air 30
  • Example 1-10 B 1 1 3.0 Air 30
  • Example 1-12 A 2 2 6.0 Air 5
  • Example 1-13 A 2 2 8.0 Air 50
  • Example 1-14 A 2 2 10.0 Air 10 ⁇ 30
  • Example 1-15 A 2 12.0 Air 30 ⁇ 10 ⁇ 30
  • Example 1-16 A 4 4 8.0 Mist 500
  • Example 1-17 A 4 4 8.0 Spray water 300 Comparative Example 1-1 A 4 1 8.0 Air 30 Comparative Example 1-2 A 2 1 8.0 Air 30 Comparative Example 1-3 A 4 4 3.0
  • the average hardness was as low as an HV of 398 at a depth position of 1 mm and as low as an HV of 379 at a depth position of 5 mm, while the variation in hardness could be reduced, in the condition where the cooling header length L h was 4 m, as compared to the condition where the cooling header length L h was shorter.
  • Example 1-10 and 1-11 where the component was changed, in Examples 1-12 and 1-13 where the spray pressure was changed, and in Examples 1-14 and 1-15 where the spray pressure was changed halfway that the variations in temperature and hardness were reduced as in Examples 1-1 to 1-9.
  • the average cooling rate in cooling was 4° C./sec in Example 1-12 where the spray pressure was the lowest, and the average cooling rate in cooling was 8.5° C./sec in Example 1-13 where the spray pressure was the highest. Therefore, we confirmed that, when the coolant is air, the desired effects can be exerted at least from 4° C./sec to 8.5° C./sec.
  • Example 2 a bloom of a different chemical component composition from that in Example 1 was used to produce a rail 1 in the same manner as in Example 1 in the condition where the conveyance distance L o in the example was adopted, and material properties of the rail 1 were evaluated.
  • a bloom of each chemical component composition of Conditions D to F represented in Table 4 was cast by using a continuous casting method.
  • the balance of the chemical component composition of the bloom was here substantially Fe, specifically Fe and inevitable impurities.
  • the bloom cast was reheated to 1100° C. or more in a heating furnace, thereafter hot rolled, and subsequently cooled (heat treatment step) in the same manner as in Example 1 described above. Measurement of the surface temperature of the rail 1 and cooling in the cooling bed after completion of the heat treatment, and furthermore straightening with a roller straightening machine, sampling and hardness measurement were also in the same conditions as in Example 1. The same manner was also conducted in Comparative Example 2 where the condition of the conveyance distance L o was different from that of the example, for comparison with Example 2, and material properties of a rail 1 produced were evaluated.
  • Example 2 The cooling conditions and the evaluation results of material properties in Example 2 and Comparative Example 2 are represented in Table 5.
  • the conveyance distance L o was n times the cooling header length L h in the conditions of Examples 2-1 to 2-4, and therefore the variation in temperature in the entire length was within 5° C. and was smaller.
  • the variation in hardness was an HV of 2 or less at a depth position of 1 mm from the surface and an HV of 2 at a depth position of 5 mm therefrom in the conditions of Examples 2-1 to 2-4.

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  • Organic Chemistry (AREA)
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  • Crystallography & Structural Chemistry (AREA)
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  • Heat Treatment Of Steel (AREA)
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US11111555B2 (en) 2017-03-21 2021-09-07 Jfe Steel Corporation Method for producing rail
CN113977211A (zh) * 2021-10-28 2022-01-28 攀钢集团攀枝花钢铁研究院有限公司 一种中等强度钢轨及其生产方法

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JPH08260058A (ja) * 1995-03-27 1996-10-08 Daido Steel Co Ltd 鋼材の冷却方法
JP3945545B2 (ja) * 1996-02-27 2007-07-18 Ｊｆｅスチール株式会社 レールの熱処理方法
JP4126908B2 (ja) * 2001-12-28 2008-07-30 Ｊｆｅスチール株式会社 鋼材の冷却方法
WO2013118236A1 (ja) * 2012-02-06 2013-08-15 Ｊｆｅスチール株式会社 軌条熱処理装置および軌条熱処理方法

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US11111555B2 (en) 2017-03-21 2021-09-07 Jfe Steel Corporation Method for producing rail
CN113977211A (zh) * 2021-10-28 2022-01-28 攀钢集团攀枝花钢铁研究院有限公司 一种中等强度钢轨及其生产方法

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