WO2020039485A1 - Tôle d'acier et son procédé de production - Google Patents

Tôle d'acier et son procédé de production Download PDF

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Publication number
WO2020039485A1
WO2020039485A1 PCT/JP2018/030676 JP2018030676W WO2020039485A1 WO 2020039485 A1 WO2020039485 A1 WO 2020039485A1 JP 2018030676 W JP2018030676 W JP 2018030676W WO 2020039485 A1 WO2020039485 A1 WO 2020039485A1
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Prior art keywords
steel sheet
temperature
less
center
thickness
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PCT/JP2018/030676
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English (en)
Japanese (ja)
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史寿 高峰
仁志 古谷
紀正 川端
拓海 三宅
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日本製鉄株式会社
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Priority to KR1020197037386A priority Critical patent/KR102115277B1/ko
Priority to AU2018430608A priority patent/AU2018430608B2/en
Priority to PCT/JP2018/030676 priority patent/WO2020039485A1/fr
Priority to JP2018562695A priority patent/JP6493645B1/ja
Priority to CN201880041321.2A priority patent/CN111083928B/zh
Priority to EP18922100.5A priority patent/EP3633060B1/fr
Publication of WO2020039485A1 publication Critical patent/WO2020039485A1/fr

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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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties 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 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
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

  • the present invention relates to a steel sheet and a method for producing the same.
  • Giant gears are used for a rotating mechanism of a large industrial machine represented by a rotary kiln.
  • the steel plate used as the material is required to have hardness and toughness from the viewpoint of the fatigue resistance and durability of the gear.
  • a steel sheet used as a material has HB350 or more in the surface layer and the center of the sheet thickness and vE ⁇ 20 ° C. ⁇ 47 J in the center of the sheet thickness. This is because the characteristics of the central portion of the sheet thickness are emphasized because the gear is manufactured by cutting the steel material to the central portion of the sheet thickness.
  • Ceq is represented, for example, by the following equation (1).
  • the element symbols included in the formula (1) indicate the content (% by mass) of each element in the chemical components of the steel material.
  • Patent Document 1 is a giant gear material used for a rotating mechanism of a large industrial machine, and has an object to provide a thick steel plate having a plate thickness of more than 200 mm and a small difference in hardness between a surface layer and a center and a method of manufacturing the same.
  • the average of the three points of Charpy in the C direction at ⁇ 20 ° C. at the three points is 20 J or more
  • the hardness of the surface layer is 330 or more in HB
  • the hardness of the central part of the sheet thickness is 300 or more in HB
  • a thick steel plate having a hardness difference ⁇ HB of 30 or less is provided.
  • Patent Document 1 does not aim at stably increasing the hardness at the center of the plate thickness to HB350 or more.
  • the Ceq expressed by the following equation is 0.800% or less, and the Ceq is 0.750% for the sake of securing the hardness at the center of the sheet thickness.
  • a steel sheet having a hardness of HB350 or more at the surface layer and the center part of the sheet thickness and an absorption energy at -20 ° C. of 47 J or more at the center part of the sheet thickness and a method of manufacturing the same are provided.
  • the gist of the present invention is as follows.
  • the chemical components are C: 0.16 to 0.20%, Si: 0.50 to 1.00%, and Mn: 0.90 to 100% by mass. 1.50%, P: 0.010% or less, S: 0.0020% or less, Cu: 0 to 0.40%, Ni: 0.20 to 1.00%, Cr: 0.60 to 0.99 %, Mo: 0.60 to 1.00%, V: 0 to 0.050%, Al: 0.050 to 0.085%, N: 0.0020 to 0.0070%, B: 0.0005 to 0.0020%, Nb: 0 to 0.050%, Ti: 0 to 0.020%, Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030%, And the balance: Fe and impurities, wherein the total area ratio of martensite and bainite is 99% or more at the center of the plate thickness, and In the center, the average value
  • a method for producing a steel sheet according to another aspect of the present invention is the method for producing a steel sheet according to the above (I), wherein a step of heating a slab and a step of hot-rolling the slab to reduce a sheet thickness are performed.
  • a step of obtaining a steel sheet of more than 200 mm, a step of cooling the steel sheet, a step of precipitating the steel sheet, a step of quenching the steel sheet, and a step of tempering the steel sheet, and the chemical composition of the slab Is a unit mass%, C: 0.16 to 0.20%, Si: 0.50 to 1.00%, Mn: 0.90 to 1.50%, P: 0.010% or less, S: 0.0020% or less, Cu: 0 to 0.40%, Ni: 0.20 to 1.00%, Cr: 0.60 to 0.99%, Mo: 0.60 to 1.00%, V: 0 to 0.050%, Al: 0.050 to 0.085%, N: 0.0020 to 0.0070%, B: 0.000 To 0.0020%, Nb: 0 to 0.050%, Ti: 0 to 0.020%, Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030% And the remainder: Fe and impurities, Ceq of
  • the step of tempering the steel sheet is performed by heating the steel sheet to a tempering temperature of 500 to 550 ° C and then cooling it to 150 ° C or less.
  • Ceq C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5: Equation (1)
  • f 4 ⁇ C + Si + 2 ⁇ Mn + Ni + 2 ⁇ Cr + 5 ⁇ Mo:
  • Formula (2) g 2 ⁇ Cr + 3 ⁇ Mo + 5 ⁇ V: Equation (3)
  • Ts 7400 / (1.95 ⁇ log 10 (Al ⁇ N)) ⁇ 273:
  • tq 0.033 ⁇ (950-Tq ) 2 + (1.5 ⁇ f) 2/10:
  • Ac1 750-25 ⁇ C + 22 ⁇ Si-40 ⁇ Mn-30 ⁇ Ni + 20 ⁇ Cr + 25 ⁇ Mo: Formula
  • the present invention even in a steel sheet having a thickness of more than 200 mm, it is possible to provide a steel sheet having excellent hardness of the surface layer and the central part of the sheet thickness, excellent impact absorption energy performance of the central part of the sheet thickness, and suppressed Ceq to 0.800% or less. It can be used for the rotating mechanism of large industrial machines such as rotary kilns.
  • FIG. 4 is a diagram illustrating a relationship between a quenching holding temperature Tq and a quenching holding time tq and a center part hardness obtained as a result of an experiment using component A6 of an example.
  • FIG. 4 is a diagram illustrating a relationship between a quenching holding temperature Tq and a quenching holding time tq and a center part hardness, obtained as a result of an experiment using component A2 of an example. It is a flowchart which shows the manufacturing method of the steel plate which concerns on this embodiment.
  • the mechanical properties of both the central part of the thickness of the steel sheet (may be simply referred to as “center part”) and the surface layer of the steel sheet (may be simply referred to as “surface layer”) are controlled.
  • the thickness center portion 11 of the steel sheet 1 is a plane having a depth of 3 of the thickness T of the steel sheet 1 from the rolling surface 13 which is the outermost surface of the steel sheet 1 and a rolling surface 13. From the plane having a depth of 5/8 of the plate thickness T. The center plane of the plate thickness center portion 11 of the steel sheet 1 and the center plane of the steel sheet 1 coincide.
  • the surface layer 12 of the steel plate 1 is a region between the rolled surface 13 of the steel plate 1 and a surface having a depth of 1 mm and a surface having a depth of 5 mm.
  • a region from the outermost surface of the steel sheet 1 to a depth of 1 mm is excluded from the surface layer 12 of the steel sheet 1. This is because this region corresponds to a decarburized layer and a portion removed during processing.
  • a test piece for a mechanical test, a microstructure observation, and the like is to be collected from a portion apart from the end of the steel plate in the length direction and the width direction, in principle, from a portion separated by a thickness or more.
  • the following (1) to (7) have important meanings.
  • the component parameter formula (3) And the precipitation treatment (5) are important.
  • the upper and lower limits of the amount of C for satisfying both the center hardness and the center toughness (under the conditions described below).
  • the center hardness is HB350 or more
  • the surface layer also needs to have HB350 or more. It can be secured.
  • C in order to keep the hardness at the center of HB 350 or more even after tempering at 500 ° C. or more, C needs to be 0.16% or more, as shown in FIG.
  • the Ceq of the steel sheet according to the present embodiment is set to 0.800% or less. Ceq may be 0.790% or less, 0.785% or less, or 0.780% or less.
  • Ceq C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5: Equation (1)
  • the element symbols included in the formula (1) indicate the content (% by mass) of each element in the chemical components of the steel sheet.
  • FIG. 3 plots steel whose hardness at the center of the plate thickness is insufficient even though Ceq is 0.750% or more. The reason why the hardness at the center of the plate thickness was insufficient in this steel is that no precipitation treatment was performed.
  • Equation (3) The element symbols included in the formulas (2) and (3) indicate the content (% by mass) of each element in the chemical composition of the steel sheet.
  • the element related to the parameter f is an element that enhances the hardenability of the steel sheet by forming a solid solution in the matrix during quenching.
  • the element related to the parameter g is an element that reduces the toughness of the steel sheet by forming a precipitate during tempering. That is, these elements improve the hardenability, while lowering the toughness due to the formation of precipitates during tempering.
  • 4 ⁇ f / g is large indicates that the hardenability is increased while reducing the elements precipitated during tempering.
  • the Cr precipitates, Mo precipitates, and V precipitates at the time of tempering in the steel sheet according to the present embodiment are so fine that they cannot be observed without a transmission electron microscope. Therefore, it is industrially impractical to define the distribution of the precipitate itself. From this, it can be understood that the usefulness of controlling the precipitates by the parameter expression 4 ⁇ f / g is understood.
  • 4 ⁇ f / g may be 9.20 or more, 9.50 or more, or 9.80 or more.
  • the upper limit of 4 ⁇ f / g does not need to be particularly defined, but may be, for example, 11.00, 10.70, 10.50, 10.00, or 9.90.
  • the Al content is less than 0.050% or the Al ⁇ N content is less than 2.0 ⁇ 10 ⁇ 4 , the prior austenite grain size becomes coarse and the low-temperature toughness of the central portion of the steel sheet deteriorates. This is considered to be because the total amount of AlN became insufficient.
  • AlN acting as pinning particles in the steel sheet according to the present embodiment is very fine and therefore difficult to observe. Therefore, it is industrially unrealistic to define the distribution state of AlN itself acting as pinning particles. From this, the usefulness of controlling AlN acting as pinning particles by the parameter Al ⁇ N can be understood.
  • Al ⁇ N may be 2.2 ⁇ 10 ⁇ 4 or more, 2.5 ⁇ 10 ⁇ 4 or more, or 3.0 ⁇ 10 ⁇ 4 or more. It is not necessary to particularly define the upper limit of Al ⁇ N, but the upper limit of Al ⁇ N may be 5.95 ⁇ 10 ⁇ 4 , which is the product of the respective upper limits of the Al content and the N content described below. . Al ⁇ N may be 5.7 ⁇ 10 ⁇ 4 or less, 5.5 ⁇ 10 ⁇ 4 or less, 5.2 ⁇ 10 ⁇ 4 or less, or 4.8 ⁇ 10 ⁇ 4 or less.
  • Solution treatment and precipitation treatment before quenching to secure center hardness and toughness
  • Process requirements for obtaining the AlN pinning effect include solution treatment and precipitation treatment.
  • the slab is heated to a temperature equal to or higher than the AlN solid solution temperature Ts calculated by the following equation (4). Hot rolling is performed after the solution treatment.
  • the precipitation treatment in order to finely precipitate Al and N dissolved in the matrix by the above-mentioned solution as AlN, after hot rolling and before quenching heating, to a precipitation treatment temperature Tp of more than 550 ° C. and less than Ac1.
  • the hot-rolled steel sheet is heated and maintained at the precipitation temperature Tp for the precipitation time tp.
  • Ts 7400 / (1.95 ⁇ log 10 (Al ⁇ N)) ⁇ 273: Formula (4) Log 10 (tp) + 0.012 ⁇ Tp ⁇ 8.7: Equation (5)
  • Ts in the formula (4) is the solid solution temperature (° C.) of AlN
  • Al and N are the contents (% by mass) of Al and N, respectively.
  • Tp is a deposition temperature (° C.)
  • tp” is a deposition time (hour). It should be noted that slight temperature fluctuations are allowed during the temperature maintenance of the precipitation treatment.
  • the steel sheet is extracted from the heat treatment furnace after the temperature at the center of the thickness of the steel sheet finally exceeds the “maximum temperature of the center of the steel sheet in the middle of the precipitation processing ⁇ 40 ° C.”. Up to the average temperature of the steel sheet in the center of the thickness.
  • the precipitation temperature Tp is a value calculated by the following equation (8).
  • the precipitation time tp is the temperature of the center of the sheet thickness of the steel sheet. Is defined as the time from the last time exceeding the “maximum temperature at the center of the thickness of the steel sheet during the precipitation process ⁇ 40 ° C.” until the steel sheet is extracted from the heat treatment furnace (ie, “t B ⁇ t A ”). I do.
  • the precipitation processing temperature Tp obtained by applying the time history of the temperature at the time of the precipitation processing at the center of the thickness of the steel sheet to the above equation (8) is a temperature of more than 550 ° C. and less than Ac1, and the precipitation processing temperature Tp and the precipitation temperature If the processing time tp satisfies the expression (5), it is determined that a suitable precipitation processing has been performed.
  • the present inventors measured vE -20 ° C. of steel sheets manufactured by applying various precipitation treatment times tp and precipitation treatment temperatures Tp to steels having the component A4 in Examples described later.
  • FIG. 5 shows the result. It can be seen from FIG. 5 that in order to obtain the pinning action of AlN, it is necessary to carry out the deposition treatment at an appropriate deposition temperature Tp and deposition time tp.
  • FIG. 5 is a plot of each steel sheet, with the horizontal axis representing the precipitation treatment temperature Tp of each steel sheet and the vertical axis representing Log 10 (tp) of each steel sheet.
  • the unit of tp is time (Hr).
  • the steel sheet plotted with a cross indicates that the vE ⁇ 20 ° C. is less than 47 J
  • the steel sheet plotted with a ⁇ indicates that the vE ⁇ 20 ° C. is 47 J or more. It can be seen from FIG. 5 that toughness cannot be ensured under the processing condition of Log 10 (tp) + 0.012 ⁇ T ⁇ 8.7.
  • Quenching conditions for securing center hardness
  • a quenching holding temperature Tq of 900 ° C. or more and 950 ° C. or less
  • the hot-rolled steel sheet is held at this temperature for a quenching holding time tq (min) or more represented by the following equation (6).
  • Tq 0.033 ⁇ (950-Tq ) 2 + (1.5 ⁇ f) 2/10: Formula (6)
  • Tq is a quenching holding temperature (° C.)
  • f is a value obtained by the above equation (2). Note that the quenching holding temperature Tq indicates not the set temperature of the heat treatment furnace but the temperature at the center of the thickness of the steel sheet. It should be noted that slight temperature fluctuations are allowed during the quenching temperature maintenance. Further, temperature fluctuation may occur in actual operation.
  • the quenching holding temperature Tq is determined from the last time the temperature at the center of the thickness of the steel sheet exceeds the “maximum temperature at the center of the thickness of the steel sheet being quenched ⁇ 40 ° C.” until the steel sheet is extracted from the heat treatment furnace. Is defined as the average temperature of the steel sheet at the center of the thickness. Specifically, the quenching holding temperature Tq is a value calculated by the following equation (9).
  • Tq ⁇ [t 1 ⁇ t 2 ] T (t) dt ⁇ / (t 2 ⁇ t 1 ): Equation (9)
  • t 1 the time when the temperature at the center of the thickness of the steel sheet last exceeds “the maximum temperature at the center of the thickness of the steel sheet being hardened ⁇ 40 ° C.”
  • t 2 the time when the steel sheet is extracted from the heat treatment furnace
  • T (t ) Temporal change of temperature at the center of thickness of steel sheet (time history of temperature) ⁇ [t 1 ⁇ t 2 ] T (t) dt: integrated value of the change with time in the center of the thickness of the steel sheet from t 1 to t 2
  • the value calculated by equation (8) may be described as “actual Tq”.
  • the quenching holding time of the steel sheet as an actual value is such that the steel sheet is not subjected to the heat treatment furnace after the temperature at the center of the steel sheet thickness exceeds the “maximum temperature at the center of the steel sheet during quenching ⁇ 40 ° C.” last. (I.e., "t 2 -t 1 ").
  • the quenching holding time of the steel sheet as an actual value defined as “t 2 ⁇ t 1 ” may be described as “actual tq”.
  • the quenching holding time tq calculated from the equation (6) may be described as “necessary tq”. It is necessary that the performance tq is equal to or greater than the required tq as a manufacturing condition of the steel sheet according to the present embodiment.
  • the quenching holding temperature Tq may be controlled based on a value measured by inserting a thermocouple near the center of the sheet thickness of the steel sheet, or by heat conduction calculation based on the furnace temperature and the sheet thickness. This value may be controlled based on the estimated value.
  • An example of the actual quenching method is shown below.
  • a quenching holding temperature (target Tq) and a quench holding time (target tq) as target values that satisfy Expression (6) are determined in advance.
  • the steel sheet is inserted into a heat treatment furnace, and the steel sheet is heated to a temperature range within a target Tq ⁇ 20 ° C. and maintained at that temperature. After maintaining the temperature of the steel sheet within the target Tq ⁇ 20 ° C.
  • the actual result Tq is calculated by applying the time history T (t) of the actual temperature (actually measured value or estimated value) at the center of the thickness of the steel sheet to the above-described equation (8). Further, from the time t 1 the temperature of the center of plate thickness of the steel sheet exceeds the "maximum temperature -40 °C the center of plate thickness of the steel sheet during quenching" Finally, until time t 2 which steel is extracted from the heat treatment furnace Is the actual time tq. Next, the required Tq is calculated by substituting the actual result Tq for Tq in the equation (6).
  • the performance tq is not smaller than the required tq (that is, if the performance tq ⁇ the required tq), it is determined that the appropriate quenching process has been performed. It should be noted that the determination in the same procedure is required in the precipitation treatment.
  • FIG. 6A shows the results of an experiment using steel having the component A6 in the examples described later
  • FIG. 6B shows the results of experiments using the steel having the component A2 in the examples described later.
  • the present inventors apply various temperature holding times (time during which the temperature of the central part of the hot-rolled steel sheet is maintained at the quench holding temperature Tq isothermally) and quench holding temperature Tq to these steels to produce various steel sheets. Then, the hardness at the center was measured.
  • 6A and 6B are plots of each steel sheet, with the horizontal axis representing the quenching holding temperature Tq of each steel sheet and the vertical axis representing the temperature holding time of each steel sheet. 6A and 6B, the steel plates plotted with a cross mark have a center hardness of less than 350 HB, and the steel plates plotted with a ⁇ mark have a center hardness of 350 HB or more. .
  • a steel plate (a steel plate plotted below the curves in FIGS. 6A and 6B) whose temperature holding time is shorter than the quenching holding time tq represented by the above-described formula (6) has a center part hardness lower than HB350. Can be seen from FIGS. 6A and 6B. This is presumably because the alloy for improving the hardenability was not sufficiently dissolved in the matrix, so that the hardenability could not be secured. The reason that the quenching holding time tq becomes a function of f is that the longer the amount of the alloy, the longer it takes to form a solid solution.
  • tempering temperature to ensure hardness and toughness of the center portion
  • the temperature needs to be 500 ° C. or higher.
  • the tempering temperature needs to be 500 ° C. or higher.
  • the steel sheet according to the present embodiment may have a sudden decrease in hardness due to tempering at more than 550 ° C. For this reason, the tempering temperature needs to be 550 ° C. or less. After this tempering, the steel sheet is cooled to 150 ° C. or less.
  • the total area ratio of martensite and bainite is 99% or more.
  • the remainder of the structure is not particularly defined, for example, ferrite, pearlite, retained austenite, and the like can be considered. These other tissues are acceptable if less than 1 area%.
  • the above structure is achieved by quenching under conditions free of ferrite and tempering at a sufficiently high temperature. Specifically, it is achieved by quenching the steel sheet having a component of Ceq ⁇ 0.750% or more under the above conditions after the precipitation treatment under the above conditions, and performing the tempering under the above conditions. .
  • Ferrite is a factor in reducing the hardness of steel materials.
  • ferrite is likely to be formed at the center of the sheet thickness where the quenching and cooling rate is slow. To secure the center hardness, the amount of ferrite must be as low as possible.
  • pearlite Although pearlite is effective in securing hardness, it becomes a brittle fracture origin because of its hardness. Therefore, the amount of pearlite must be as low as possible. Pearlite is generated by enriching C discharged during ferrite precipitation. Therefore, by avoiding ferrite precipitation, pearlite formation is also suppressed at the same time.
  • Retained austenite becomes the starting point of brittle fracture and lowers the toughness of steel. Therefore, the amount of retained austenite must be as low as possible. If tempering is performed at a tempering temperature of 500 ° C. or more, generation of retained austenite is suppressed.
  • the unit “%” of the content of the alloy element means mass%.
  • C 0.16 to 0.20% C is an element effective for increasing the hardness because it increases the hardness of the quenched structure. Based on the experimental results shown in FIG. 2 described above, the lower limit of the C content is 0.16%. On the other hand, an excessive amount of C impairs the toughness of the steel sheet and causes a difference in hardness between the surface layer and the central portion. Therefore, the upper limit of the C content is set to 0.20% based on the experimental results shown in FIG.
  • the C content may be 0.17% or more, 0.18% or more, or 0.19% or more.
  • the C content may be 0.19% or less, 0.18% or less, or 0.17% or less.
  • Si 0.50-1.00% Si has a deoxidizing effect. Further, Si is also an effective element for improving the strength of the steel sheet, and can enhance hardenability without increasing Ceq. Therefore, the content of Si is set to 0.50% or more. However, a large amount of Si promotes temper embrittlement and lowers the toughness of the steel sheet. Therefore, the Si content is preferably reduced, and the upper limit is set to 1.00%.
  • the Si content may be 0.60% or more, 0.65% or more, or 0.70% or more.
  • the Si content may be 0.90% or less, 0.85% or less, or 0.80% or less.
  • Mn 0.90-1.50% Mn has a deoxidizing effect. Further, Mn is an element that improves hardenability and is effective for improving the strength of a steel sheet. Therefore, the Mn content is set to 0.90% or more. On the other hand, excessive Mn promotes tempering embrittlement and lowers the toughness of the steel sheet. Therefore, the upper limit of the Mn content is set to 1.50%.
  • the Mn content may be 1.00% or more, 1.05% or more, or 1.10% or more.
  • the Mn content may be 1.40% or less, 1.35% or less, or 1.30% or less.
  • P 0.010% or less
  • P is an impurity element contained in steel.
  • P is a harmful element that promotes grain boundary embrittlement and lowers the toughness of the steel sheet. Therefore, the P content is preferably as small as possible. Therefore, the P content is reduced to 0.010% or less. Since P is not required by the steel sheet according to the present embodiment, the lower limit of the P content is 0%. However, from the viewpoint of refining cost and productivity, the P content may be specified as 0.001% or more.
  • the P content may be 0.002% or more, 0.003% or more, or 0.005% or more.
  • the P content may be 0.008% or less, 0.007% or less, or 0.006% or less.
  • S is an impurity element contained in steel.
  • S is an element that lowers the toughness of the steel sheet through segregation and sulfide formation. Therefore, it is preferable that the S content is as small as possible. Therefore, the S content is reduced to 0.0020% or less. Since S is not required by the steel sheet according to the present embodiment, the lower limit of the S content is 0%. However, from the viewpoint of refining cost and productivity, the S content may be set to 0.0004% or more.
  • the S content may be 0.0005% or more, 0.0006% or more, or 0.0007% or more.
  • the S content may be 0.0018% or less, 0.0015% or less, or 0.0010% or less.
  • Cu 0 to 0.40%
  • Cu is an element that can increase the strength of steel without impairing low-temperature toughness.
  • a large amount of Cu may cause cracks in the steel sheet during hot working.
  • a large amount of Cu may reduce the toughness of the steel sheet through precipitation of metallic Cu or the like. Therefore, the upper limit of the Cu content is set to 0.40%.
  • Cu contributes to the suppression of ferrite by increasing Ceq, but is not essential for the steel sheet according to the present embodiment because it can be replaced by another alloy element. Therefore, the lower limit of the Cu content is set to 0%.
  • 0.01% or 0.02% may be set as the lower limit of the Cu content.
  • the Cu content may be 0.03% or more, 0.05% or more, or 0.10% or more.
  • the Cu content may be 0.35% or less, 0.30% or less, or 0.20% or less.
  • Ni 0.20 to 1.00%
  • Ni is an element effective for improving the strength and toughness of steel. Therefore, the Ni content is set to 0.20% or more. On the other hand, even if the amount of Ni is excessive, the effect is saturated, and an increase in the amount of the expensive alloy, Ni, leads to a decrease in manufacturing cost. Therefore, the upper limit of the Ni content is set to 1.00%.
  • the Ni content may be 0.25% or more, 0.30% or more, or 0.40% or more.
  • the Ni content may be 0.90% or less, 0.80% or less, or 0.70% or less.
  • Mo: 0.60 to 1.00% Cr and Mo have a function of improving the hardenability and increasing the hardness of the central part.
  • Cr and Mo also have the effect of raising the hardness of the surface layer and the central part by precipitation hardening. Therefore, the content of each of Cr and Mo is set to 0.60% or more.
  • excessive amounts of Cr and Mo may lower the toughness due to the formation of alloy carbides. Therefore, the upper limit of the Cr content is set to 0.99%, and the upper limit of the Mo content is set to 1.00%.
  • the Cr content may be 0.65% or more, 0.70% or more, or 0.75% or more.
  • the Cr content may be 0.95% or less, 0.90% or less, or 0.80% or less.
  • the Mo content may be 0.65% or more, 0.70% or more, or 0.75% or more.
  • the Mo content may be 0.95% or less, 0.90% or less, or 0.80% or less.
  • V 0 to 0.050%
  • V improves the base metal strength through the formation of carbides.
  • a large amount of V causes a decrease in toughness due to alloy carbide formation. Therefore, the upper limit of the V content is set to 0.050%.
  • V contributes to the suppression of ferrite by increasing Ceq
  • V is not essential for the steel sheet according to the present embodiment because V is an expensive alloy element and can be replaced by another alloy. Therefore, the lower limit of the V content is set to 0%.
  • the lower limit of the V content may be 0.003%.
  • the V content may be 0.005% or more, 0.010% or more, or 0.015% or more.
  • the V content may be 0.045% or less, 0.040% or less, or 0.035% or less.
  • Al 0.050-0.085%
  • Al is an element effective as a deoxidizing material. Further, Al combines with N in steel to form AlN, and contributes to grain refinement of the structure. In addition, Al becomes AlN in the precipitation treatment and contributes to the decomposition of BN, thereby stabilizing the hardenability of B. Therefore, the Al content is 0.050% or more. However, excess Al forms coarse AlN, causing a decrease in toughness and cracking of the slab. Therefore, the upper limit of the Al content is set to 0.085%.
  • the Al content may be 0.055% or more, 0.060% or more, or 0.065% or more.
  • the Al content may be 0.080% or less, 0.075% or less, or 0.070% or less.
  • N 0.0020 to 0.0070%
  • N forms nitrides and carbonitrides with alloying elements, and contributes to refinement of the structure of the steel sheet. Therefore, the lower limit of the N content is 0.0020%.
  • the upper limit of the N content is 0.0070%.
  • the N content may be 0.0025% or more, 0.0030% or more, or 0.0035% or more.
  • the N content may be 0.0065% or less, 0.0060% or less, or 0.0050% or less.
  • Al ⁇ N (the product of the Al content and the N content) needs to be 2.0 ⁇ 10 ⁇ 4 or more. This is to utilize the pinning effect of AlN, which contributes to the refinement of the structure of the steel sheet.
  • B 0.0005 to 0.0020%
  • B is an element that improves the hardenability of steel and improves the strength. Therefore, the B content is 0.0005% or more. However, when B becomes excessive, metal boride is formed and hardenability is reduced. Therefore, the upper limit of the B content is set to 0.0020%.
  • the B content may be 0.0007% or more, 0.0008% or more, or 0.0010% or more.
  • the B content may be 0.0018% or less, 0.0016% or less, or 0.0015% or less.
  • the contents of the following elements that affect toughness are specified as selected elements.
  • the lower limit of the content of each selected element is 0%.
  • Nb 0 to 0.050%
  • Nb is an element that contributes to grain refinement of the internal structure of steel by forming carbonitrides and affects toughness. Therefore, 0.001% or more of Nb can be contained. However, coarse carbonitrides generated by a large amount of Nb rather reduce toughness. Therefore, the upper limit of the Nb content is set to 0.050%.
  • the Nb content may be 0.002% or more, 0.005% or more, or 0.008% or more.
  • the Nb content may be 0.045% or less, 0.040% or less, or 0.035% or less.
  • Ti 0 to 0.020% Ti / N ⁇ 3.4 Ti is an element that contributes to grain refinement of the structure by forming a stable nitride and affects toughness. Therefore, 0.001% or more of Ti can be contained. However, excessive Ti causes a decrease in toughness due to coarse nitride. Therefore, the upper limit of the Ti content is 0.020%.
  • the Ti content may be 0.002% or more, 0.005% or more, or 0.008% or more.
  • the Ti content may be 0.018% or less, 0.016% or less, or 0.012% or less.
  • Ti / N may be set to 3.3 or less, 3.2 or less, or 3.0 or less. It is not necessary to define the lower limit of Ti / N, but since the lower limit of the Ti content is 0%, the lower limit of Ti / N may be defined as 0%. Ti / N may be 0.2 or more, 0.5 or more, or 1.0 or more.
  • Ca, Mg, and REM all combine with harmful impurities such as S to form harmless inclusions. Accordingly, Ca, Mg, and REM can all improve mechanical properties such as toughness of steel. Therefore, the contents of Ca, Mg, and REM can be made 0.0001% or more. However, when the contents of Ca, Mg, and REM are excessive, the effect is not only saturated, but also promotes erosion of refractories such as casting nozzles. Therefore, the upper limits of the contents of Ca, Mg, and REM are set to 0.0030%.
  • the content of each of Ca, Mg, and REM may be 0.0002% or more, 0.0005% or more, or 0.0010% or more.
  • the content of each of Ca, Mg, and REM may be 0.0025% or less, 0.0020% or less, or 0.0015% or less.
  • the term "REM" refers to a total of 17 elements consisting of Sc, Y and lanthanoid, and the "content of REM” means the total content of these 17 elements.
  • the balance of the chemical components of the steel sheet according to the present embodiment contains iron and impurities.
  • the impurity is a component mixed in due to various factors in a manufacturing process or a raw material such as ore or scrap when a steel material is industrially manufactured.
  • the average value of the prior austenite grain size at the center of the thickness of the steel sheet according to the present embodiment is less than 80 ⁇ m.
  • the center of the sheet thickness has high toughness.
  • the prior austenite grain size at the center of the sheet thickness may be 76 ⁇ m or less, 73 ⁇ m or less, 70 ⁇ m or less, or 68 ⁇ m or less. Refinement of the prior austenite grain size at the center of the thickness of the steel sheet according to the present embodiment is achieved mainly by the pinning effect of fine AlN as described above.
  • the thickness of the steel sheet according to the present embodiment is more than 200 mm.
  • a steel sheet having a thickness of more than 200 mm can be used as a material for a giant gear used in a rotating mechanism of a large industrial machine represented by a rotary kiln, and thus has high industrial applicability.
  • the steel sheet according to the present embodiment has good hardness and low-temperature toughness even when the sheet thickness is 200 mm or less.
  • the thickness of the steel sheet may be 205 mm or more, 210 mm or more, or 220 mm or more.
  • the upper limit of the thickness of the steel sheet is not particularly limited, but the thickness may be 250 mm or less, 240 mm or less, or 230 mm or less.
  • the Charpy absorbed energy (vE ⁇ 20 ° C. ) of ⁇ 20 ° C. measured in the C direction at the center of the thickness of the steel sheet according to the present embodiment is 47 J or more.
  • the Charpy absorbed energy is a three-point average of values measured in accordance with ASTM (American Society for Testing and Materials) A370-2017a.
  • ASTM American Society for Testing and Materials
  • a steel sheet that satisfies the above-mentioned requirements for the Charpy absorbed energy has high low-temperature toughness even in the central part of the sheet thickness where control of mechanical properties is normally difficult.
  • the Charpy absorbed energy at ⁇ 20 ° C. measured in the C direction at the center of the thickness of the steel sheet according to this embodiment may be 50 J or more, 55 J or more, or 60 J or more.
  • it is not necessary to define the upper limit of the Charpy absorbed energy at ⁇ 20 ° C. measured in the C direction at the center of the thickness of the steel sheet according to the present embodiment for example, it may be defined as 400 J, 380 J, or 350 J. Good.
  • the hardness of the surface layer and the center of the thickness of the steel sheet according to the present embodiment is HB350 or more.
  • the hardness of the steel sheet according to the present embodiment is an average of five points of HBW10 / 3000 (indenter diameter 10 mm, test force 3000 kgf) specified in JIS Z 2243-1: 2018.
  • a steel sheet that satisfies the above requirements for hardness has high hardness even in the central part of the thickness where it is normally difficult to secure high hardness, but the hardness of the surface layer does not become excessive, so it is used as steel for machine structural use. High value.
  • the hardness of the surface layer of the steel sheet according to the present embodiment may be HB360 or more, HB370 or more, or HB380 or more.
  • the hardness at the center of the thickness of the steel sheet according to the present embodiment may be HB360 or more, HB370 or more, or HB380 or more. Although it is not necessary to define the upper limit value of the hardness of the surface layer of the steel sheet according to the present embodiment, it may be defined as HB450, HB420, or HB400, for example. Although it is not necessary to define the upper limit of the hardness at the center of the thickness of the steel sheet according to the present embodiment, for example, it may be defined as HB450, HB420, or HB400.
  • the components of the steel sheet are in a 1 / 4T portion of the steel sheet (from the rolling surface of the steel sheet to a position at a depth of 1/4 of the thickness T of the steel sheet and its vicinity). , Measured by the usual method. Based on the measured values, Ceq, Al ⁇ N, Ti / N, and 4 ⁇ f / g of the steel sheet are calculated. If the molten steel analysis value of the slab used as the material of the steel sheet is known, it may be regarded as the chemical composition of the steel sheet.
  • the -20 ° C Charpy absorbed energy (vE- 20 ° C ) measured in the C direction at the center of the sheet thickness is measured according to ASTM A370-2017a.
  • the test piece is a V-notch test piece. Three test pieces are collected from the center of the thickness of the steel sheet. At the time of collecting the test piece, the longitudinal direction of the test piece and the C direction of the steel sheet (the direction perpendicular to the rolling direction and the thickness direction) match. The average value of vE ⁇ 20 ° C. of these three test pieces is defined as the Charpy absorbed energy at ⁇ 20 ° C. measured in the C direction at the center of the thickness of the steel sheet.
  • HBW10 / 3000 is determined by setting the indenter diameter to 10 mm and the test force to 3000 kgf.
  • the surface hardness is measured by pressing an indenter into a surface formed by removing a region from the rolled surface of the steel sheet to a depth of at least 1 mm.
  • the average value of the surface layer hardness measurement results at five points is defined as the surface layer hardness of the steel sheet.
  • the measurement of the hardness at the center of the thickness of the steel sheet is performed by pressing an indenter into a portion corresponding to the center of the thickness on a surface formed by cutting the steel sheet in parallel with the rolling surface.
  • the average value of the measurement results of the hardness at the center of the thickness at five points is defined as the hardness at the center of the thickness of the steel sheet.
  • the method of measuring the area ratio of martensite and bainite at the center of the plate thickness is as follows.
  • the observation surface is a surface parallel to the rolling direction of the steel sheet, and is polished and nital etched. This observation surface is observed with an optical microscope of 500 times. Based on the optical micrograph, the total area ratio of martensite and bainite can be measured.
  • the total area of the observation visual field is 0.300 mm 2 or more.
  • the method of measuring the average value of the prior austenite grain size at the center of the sheet thickness is as follows.
  • the observation surface is a surface parallel to the rolling direction of the steel sheet, and is polished and picric acid etched.
  • the average section length is measured by the section method, and the average section length is defined as the average old ⁇ particle size.
  • the length of the section at the time of measurement is 1000 ⁇ m (1 mm) or more.
  • the steel sheet according to the present embodiment can be obtained according to the manufacturing conditions described below. However, even a steel sheet obtained under conditions other than the manufacturing conditions described below falls under the steel sheet according to the present embodiment as long as the above-described requirements are satisfied.
  • the method for manufacturing a steel sheet according to the present embodiment includes a step S1 of heating a slab, a step S2 of hot rolling the slab to obtain a steel sheet, a step S3 of cooling the steel sheet, and a step S3 of cooling the steel sheet. , A step S5 of quenching the steel sheet, and a step S6 of tempering the steel sheet.
  • the manufacturing conditions in these steps are as shown in the table below.
  • the slab is heated to a temperature equal to or higher than the AlN solid solution temperature Ts calculated by the above equation (4).
  • the technical significance of the AlN solid solution temperature Ts is as described above.
  • the components of the slab not only satisfy the upper and lower limits of the respective alloying elements, but also have a Ceq of 0.750 to 0.800% and an Al ⁇ N of 2.0 ⁇ 10 ⁇ 4 or more, as in the case of the steel sheet.
  • Ti / N needs to be 3.4 or less and 4 ⁇ f / g needs to be 9.00 or more.
  • the preferred numerical ranges of the content of each alloy element, Ceq, Al ⁇ N, Ti / N, and 4 ⁇ f / g are the same as those of the steel sheet. If the molten steel analysis value of the slab is known, that value may be considered as the chemical composition of the slab.
  • the step S2 of hot rolling the heated slab is not particularly limited.
  • the thickness of a steel sheet (hot-rolled steel sheet) obtained by hot rolling is more than 200 mm.
  • step S3 of cooling the steel sheet it is preferable to complete the transformation of the structure of the steel sheet from austenite to another structure by cooling the steel sheet to 500 ° C or lower, preferably 150 ° C or lower.
  • the steel sheet is heated to the precipitation treatment temperature Tp, and the temperature is maintained at this temperature T.
  • the precipitation treatment temperature Tp is a temperature exceeding 550 ° C. and less than Ac1, and satisfies the above-mentioned expression (5).
  • the deposition time tp also satisfies the expression (5).
  • the technical significance of the precipitation treatment conditions is as described above.
  • the steel sheet may be cooled to 500 ° C. or lower, preferably 150 ° C. or lower (for example, normal temperature), or may be directly heated for subsequent quenching. Good.
  • the steel sheet is heated to a quenching holding temperature Tq (° C.) of 900 ° C. or more and 950 ° C. or less, and the temperature is held for the quenching holding time tq (min) or more represented by the above-described formula (6). And then water-cooled.
  • Tq quenching holding temperature
  • tq quenching holding time
  • the technical significance of the quenching holding temperature Tq and the quenching holding time tq is as described above.
  • the cooling means of the steel sheet after completion of the temperature holding is assumed to be water-cooled or a cooling rate at the same level as this.
  • the quenching end temperature is, for example, 150 ° C. or less.
  • step S6 of tempering the steel sheet it is preferable to temper at a tempering temperature of 500 ° C. or more and 550 ° C. or less, and then cool to 150 ° C. or less.
  • the technical significance of the tempering temperature is as described above.
  • the hardness of the surface layer of the steel sheet and the hardness of the center part of the thickness of the steel sheet were measured based on JIS Z 2243-1: 2018. HBW10 / 3000 was determined by setting the indenter diameter to 10 mm and the test force to 3000 kgf. The surface hardness was measured by pressing an indenter into a surface formed by removing a region from the rolled surface of the steel sheet to a depth of at least 1 mm. The average value of the measurement results of the surface layer hardness at five points was defined as the surface layer hardness of the steel sheet (Table 4 “HB surface layer”).
  • the measurement of the hardness at the center of the thickness of the steel sheet was performed by pressing an indenter into a portion corresponding to the center of the thickness of the surface formed by cutting the steel sheet in parallel with the rolling surface.
  • the average value of the measurement results of the hardness at the center of the thickness at five points was defined as the hardness at the center of the thickness of the steel sheet (Table 4, "HB center”).
  • the test piece of the steel plate was extract
  • the Charpy absorbed energy at -20 ° C (vE- 20 ° C ) measured in the C direction at the center of the sheet thickness was measured according to ASTM A370-2017a.
  • the method of measuring the area ratio of martensite and bainite at the center of the plate thickness was as follows.
  • the observation surface was a surface parallel to the rolling direction of the steel sheet, and polishing and nital etching were performed on the observation surface. This observation surface was observed with a 500-fold optical microscope. Based on the optical micrograph, the total area ratio of martensite and bainite was measured. The total area of the observation visual field was 0.300 mm 2 or more.
  • the method of measuring the average value of the prior austenite grain size at the center of the sheet thickness was as follows.
  • the observation surface was a surface parallel to the rolling direction of the steel sheet, and was polished and picric acid etched.
  • the average section length was measured by the section method (section length: 1000 ⁇ m or more and 2000 ⁇ m or less), and the average section length was defined as the average value of the prior austenite grain size at the center of the sheet thickness (Table 4 “Old ⁇ grain size”). .
  • Precipitation treatment temperature Tp is a value obtained by applying the heat history during the precipitation treatment to the above equation (8).
  • the “precipitation treatment time tp” described in the table indicates that the steel sheet was not subjected to the heat treatment furnace after the temperature at the center of the sheet thickness of the steel sheet last exceeded “the maximum temperature of the sheet thickness central part of the steel sheet being subjected to the precipitation treatment ⁇ 40 ° C.” It is the time until it is extracted from
  • the “quenching holding temperature Tq” described in the table is a value obtained by applying the heat history during the quenching process to the above equation (9).
  • the “actual quenching retention time” in the table is based on the fact that the temperature of the steel sheet at the center of the thickness exceeds the “maximum temperature at the center of the steel sheet during quenching ⁇ 40 ° C.” lastly, Is the time until it is extracted from (i.e., the result tq).
  • the “quenching holding time tq” shown in the table is a value obtained by substituting the quenching holding temperatures Tq and f shown in the table into the above-described equation (6).
  • 2.2E-04 means 2.2 ⁇ 10 ⁇ 4 .
  • Test Nos. 1 to 10 satisfy the chemical component range and suitable production conditions of the present invention. All of these steels have a total area ratio of martensite and bainite of 99% or more, an average value of the prior austenite grain size at the center of 80 ⁇ m or less, surface hardness, center hardness, and center shock absorption. Energy meets the target.
  • Test Nos. 11 and 12 C deviates from the chemical composition range of the present invention.
  • C was insufficient, and the hardness at the time of quenching was insufficient, so that the hardness did not satisfy the target value even after tempering.
  • Test No. 12 is an example in which C is excessive, and the impact absorption energy is low due to the influence of the precipitation of a hard carbide serving as a fracture starting point.
  • Test Nos. 13 and 14 Si deviates from the chemical composition range of the present invention. In Test No. 13, Si was insufficient and hardenability could not be ensured, so that the center hardness could not satisfy the target value. On the other hand, Test No. 14 is an example in which Si is excessive, and although the hardness is sufficient, the impact absorption energy does not satisfy the target due to the promotion of temper embrittlement by Si.
  • Test Nos. 15 and 16 Mn was outside the range of the chemical composition of the present invention. In Test No. 15, Mn was insufficient, and the hardness at the time of quenching was not sufficient. Therefore, even after tempering, the center portion hardness did not satisfy the target value. On the other hand, Test No. 16 is an example in which Mn is excessive, and the impact absorption energy does not satisfy the target value due to the promotion of temper embrittlement.
  • the Ni content was low outside the range of the chemical components of the present invention, and was lower than the amount for improving the toughness. Therefore, in Test No. 20, the impact absorption energy does not satisfy the target.
  • Test Nos. 21 and 22 are examples in which Cr deviated from the chemical composition range of the present invention.
  • Cr was insufficient, and sufficient hardenability and precipitation strengthening action were not obtained.
  • the center hardness did not satisfy the target, and the impact absorption energy did not achieve the target.
  • the amount of Cr was excessive, and the precipitation effect of coarse Cr carbides was remarkable.
  • the impact absorption energy did not satisfy the target.
  • Test Nos. 23 and 24 are examples in which Mo deviated from the chemical composition range of the present invention.
  • Mo was insufficient, and sufficient hardenability and precipitation strengthening action were not obtained.
  • the center hardness did not satisfy the target, and the impact absorption energy did not achieve the target.
  • Mo was excessive and the effect of precipitation of coarse Mo carbide was remarkable.
  • the impact absorption energy did not satisfy the target value.
  • V was high outside the range of the chemical composition of the present invention, and coarse carbides and nitrides of V were the starting points of brittle fracture. Thus, in Test No. 25, the impact absorption energy did not satisfy the target.
  • Test Nos. 26 and 27 are examples in which Al deviated from the chemical composition range of the present invention.
  • Test No. 26 is an example in which Al was insufficient, in which AlN effective for pinning could not be secured, and excess N was linked with B, thereby reducing the effect of improving hardenability. Therefore, in Test No. 26, the structure other than martensite and bainite became excessive, and the grain size of retained austenite was coarsened. As a result, in Test No. 26, the center hardness and the impact absorption energy did not satisfy the targets.
  • Test No. 25 was an example in which Al was excessive, and became a brittle fracture starting point due to excessive coarsening of AlN. Therefore, in Test No. 25, the impact absorption energy did not satisfy the target.
  • Test Nos. 28 and 29 are examples in which N deviated from the chemical composition range of the present invention.
  • Test No. 28 is an example in which N was insufficient and Al ⁇ N was less than a predetermined range. The pinning effect was weak due to insufficient generation of nitrides and carbonitrides, and the crystal grains were coarse. Granulation occurred. Thus, in Test No. 28, the impact absorption energy did not meet the target.
  • Test No. 29 is an example in which N is excessive, and excessive coarsening of nitrides, carbonitrides, and the like occurred. As a result, in Test No. 29, the impact absorption energy did not meet the target.
  • Test Nos. 30 and 31 B deviates from the chemical composition range of the present invention.
  • Test No. 30 is an example in which B was insufficient, and the amount of solid solution B required for hardenability could not be secured.
  • Test No. 30 the structure other than martensite and bainite was excessive, and the center hardness and the impact absorption energy could not satisfy the targets.
  • Test No. 31 is an example in which B was excessively contained, and the impact absorption energy did not satisfy the target due to precipitation of a metal element boride.
  • Test No. 32 indicates that although the component range of each alloy element is within the scope of the present invention, Ceq is low outside the preferred range of the present invention. In Test No. 32, as a result of the formation of ferrite in the structure due to a decrease in hardenability, the center portion hardness and the impact absorption energy could not satisfy the targets.
  • Test Nos. 33 and 34 indicate that although the component ranges and Ceq of the individual alloy elements are within the scope of the present invention, the parameter formula 4 ⁇ f / g is low outside the preferred range of the present invention. In Test Nos. 33 and 34, the hardening action by the precipitated element was larger than the improvement in hardenability. Therefore, in Test Nos. 33 and 34, the impact absorption energy did not satisfy the target.
  • Test No. 35 indicates that the heating temperature before rolling was lower than the solid solution temperature Ts, although the component ranges of the individual alloy elements and various indices derived from the chemical components were within the scope of the present invention.
  • undissolved coarse AlN remained and became a brittle fracture starting point. Therefore, in Test No. 35, the prior austenite particle size was coarsened, and the absorbed energy did not satisfy the target.
  • Test Nos. 36 and 37 show that, although the component ranges of the individual alloy elements and various indices derived from the chemical components are within the scope of the present invention, the precipitation treatment temperature is out of the preferred range of the present invention.
  • Test No. 36 is an example in which the deposition temperature was low, and sufficient AlN was not deposited, and AlN effective for pinning could not be secured. For this reason, in Test No. 36, the prior austenite grain size became coarse, and the absorbed energy did not satisfy the target.
  • Test No. 37 is an example in which the precipitation treatment temperature exceeded Ac1, in which AlN was locally coarsened by holding in the ⁇ - ⁇ two-phase region. Therefore, in Test No. 37, the absorbed energy did not meet the target.
  • Test No. 38 shows that, although the component ranges of the individual alloy elements and various indices derived from the chemical components are within the scope of the present invention, the temperature and time of the precipitation treatment are in the preferred ranges of the present invention. Is not satisfied. In Test No. 38, sufficient AlN was not deposited, and AlN effective for pinning could not be secured. For this reason, in Test No. 38, the prior austenite grain size became coarse, and the absorbed energy did not satisfy the target.
  • Test No. 39 is an example in which the quenching temperature was below the preferred range of the present invention, although the component ranges of the individual alloy elements and various indices derived from the chemical components were within the range of the present invention.
  • hardenability was low due to insufficient solid solution of the alloy element, and ferrite was excessively generated.
  • the center hardness and the absorbed energy could not satisfy the targets.
  • Test No. 40 is an example in which the quenching holding temperature Tq exceeded the preferred range of the present invention, although the component ranges of the individual alloy elements and various indices derived from the chemical components were within the range of the present invention.
  • the crystal grains were excessively coarsened. For this reason, in Test No. 40, the prior austenite grain size became coarse, and the absorbed energy could not satisfy the target.
  • Test No. 41 shows that although the range of components of each alloy element and various indices derived from chemical components are within the scope of the present invention, the actual quenching holding time tq, which is the preferred range of the present invention, is defined as This is an example in which the solid solution of the alloy element was not sufficiently obtained. From this, in Test No. 41, hardenability was low and ferrite was excessively generated. As a result, in Test No. 41, the center hardness and the absorbed energy could not satisfy the targets.
  • Test No. 42 is an example in which the tempering temperature was lower than the preferred range, although the component ranges of the individual alloy elements and various indices derived from the chemical components were within the scope of the present invention. In test number 42, tempering embrittlement occurred. For this reason, in Test No. 42, the absorbed energy did not meet the target.
  • Test No. 43 is an example in which the tempering temperature was higher than the preferred range, although the component ranges of the individual alloy elements and various indices derived from the chemical components were within the scope of the present invention. In test number 43, the precipitation hardening effect of the alloy carbide was reduced. Therefore, in Test No. 43, the center hardness could not satisfy the target.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Inorganic Compounds Of Heavy Metals (AREA)
  • Crystals, And After-Treatments Of Crystals (AREA)

Abstract

Un mode de réalisation de la présente invention concerne une tôle d'acier ayant une composition chimique qui est comprise dans une plage prescrite : la partie centrale de la tôle d'acier dans la direction de l'épaisseur contenant de la martensite et de la bainite selon une fraction de surface totale d'au moins 99 % ; dans la partie centrale dans la direction de l'épaisseur de la feuille, la valeur moyenne des grains d'austénite antérieurs étant inférieure à 80 µm, Ceq étant égale à 0,750 % à 0,800 %, Al × N étant d'au moins 2,0 × 10-4, Ti/N étant d'au plus 3,4, et 4 × f ≥ 9,00 étant satisfaite ; dans la partie centrale dans la direction de l'épaisseur de la feuille , la moyenne en trois points d'un essai Charpy dans la direction C n'étant pas inférieure à 47 J à -20 °C ; et une couche superficielle et la partie centrale de direction d'épaisseur de feuille ayant une dureté HB d'au moins 350 et une épaisseur de feuille supérieure à 200 mm.
PCT/JP2018/030676 2018-08-20 2018-08-20 Tôle d'acier et son procédé de production WO2020039485A1 (fr)

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AU2018430608A AU2018430608B2 (en) 2018-08-20 2018-08-20 Steel plate and method of manufacturing the same
PCT/JP2018/030676 WO2020039485A1 (fr) 2018-08-20 2018-08-20 Tôle d'acier et son procédé de production
JP2018562695A JP6493645B1 (ja) 2018-08-20 2018-08-20 鋼板およびその製造方法
CN201880041321.2A CN111083928B (zh) 2018-08-20 2018-08-20 钢板及其制造方法
EP18922100.5A EP3633060B1 (fr) 2018-08-20 2018-08-20 Plaque d'acier et son procédé de fabrication

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WO2016079978A1 (fr) * 2014-11-18 2016-05-26 Ｊｆｅスチール株式会社 Feuille d'acier épaisse, haute dureté, haute ténacité ayant une excellente uniformité de matière et son procédé de fabrication
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CN114645182B (zh) * 2022-03-23 2022-10-14 承德建龙特殊钢有限公司 一种齿轮钢及其制备方法与应用

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JPWO2020039485A1 (ja) 2020-08-27
CN111083928B (zh) 2020-11-20
EP3633060A1 (fr) 2020-04-08
EP3633060A4 (fr) 2020-05-06
EP3633060B1 (fr) 2021-12-29
JP6493645B1 (ja) 2019-04-03
KR102115277B1 (ko) 2020-05-26

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