EP3216889B1 - Tôle d'acier à haute teneur en carbone laminée à froid et son procédé de fabrication - Google Patents

Tôle d'acier à haute teneur en carbone laminée à froid et son procédé de fabrication Download PDF

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EP3216889B1
EP3216889B1 EP16836913.0A EP16836913A EP3216889B1 EP 3216889 B1 EP3216889 B1 EP 3216889B1 EP 16836913 A EP16836913 A EP 16836913A EP 3216889 B1 EP3216889 B1 EP 3216889B1
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Prior art keywords
mass
steel sheet
rolled steel
cold
carbide
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German (de)
English (en)
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EP3216889A4 (fr
EP3216889A1 (fr
Inventor
Eiji Tsuchiya
Yuta Matsumura
Yoshihiro Hosoya
Yuka Miyamoto
Takashi Kobayashi
Yasunobu Nagataki
Kazuhiro Seto
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JFE Steel Corp
TOKUSHU KINZOKU EXCEL CO Ltd
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JFE Steel Corp
TOKUSHU KINZOKU EXCEL CO Ltd
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    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C21D1/26—Methods of annealing
    • C21D1/32—Soft annealing, e.g. spheroidising
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties 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
    • 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
    • C21D8/0268—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment between cold rolling steps
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/02—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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high carbon cold-rolled steel sheet manufactured by quenching-tempering treatment for a material for various machine parts.
  • the present invention relates to a high carbon cold-rolled steel sheet having a thickness of less than 1.0 mm, which has both of a sufficient hardness (600 to 750 HV) and an excellent impact property (toughness) after quenched by a short time solution treatment and then subjected to a low temperature tempering treatment, capable of being suitably applied to a knitting needle or the like which further has strict requirements in durability, wear resistance, etc.
  • the short time solution treatment refers to a treatment in a temperature range of from 760 to 820°C for a time of 3-15 minutes
  • the low temperature tempering treatment refers to a treatment in a temperature range of from 200 to 350°C.
  • carbon steels for machine structural use SxxC
  • carbon tool steels SK
  • JIS carbon tool steels
  • the quenching-tempering treatment is conducted after formed into a shape of component through punching or various plastic deformation.
  • a predetermined hardness and toughness impact property
  • a knitting needle for knitting a knit fabric knits a knit fabric with hauling a thread, in a repeated reciprocating motion in high speed.
  • a knitting needle requires a sufficient strength and a wear resistance in a butt portion of a main body of the needle which contacts with a rotary driving part, and requires an excellent impact property of the end portion, in addition to a sufficient wear resistance, in a hook portion which rubs against the thread.
  • a high carbon cold-rolled steel sheet to be used as a material for knitting needles is used in knitting needles for flat knitting machines when a thickness thereof is 1.0 mm or more, and for knitting needles for circular knitting machines or warp knitting machines when a thickness thereof is less than 1.0 mm.
  • a knitting needle for a circular knitting machine or a warp knitting machine a material having a thickness of 0.4 to 0.7 mm is often used, since such knitting needle knits a thread of small diameter at high speed.
  • a material for knitting needles is required to have, in addition to an excellent cold workability (hereinbelow also referred to as secondary workability), a sufficient hardness and a sufficient toughness in the needle end portion after formed into a shape of needle (the secondary working) and quenched and tempered.
  • secondary workability an excellent cold workability
  • Carbon steels for machine structural use (SxxC) or carbon tool steels (SK), so-called high-carbon steels, prescribed in JIS have minutely categorized usages according to amount of C.
  • the C content is less than 0.8 mass%, namely, of steels having a hypo-eutectoid composition, fraction of ferrite phase is high, and therefore, cold workability is excellent, while it is difficult to obtain a sufficient quenched hardness. Therefore, a steel having a hypo-eutectoid composition is not suitable for a use in knitting needles or the like that requires a wear resistance in a hook portion or a durability in a main body of the needle.
  • a class of 0.8 mass% or more namely, of high carbon steels having a C content of more than 1.1 mass% among steels having a hyper-eutectoid composition
  • hardenability is excellent, while cold workability is extremely inferior due to carbides (cementite) contained in large amount. Therefore, the class is not suitable for a use in knitting needles or the like, where a precise and minute process, such as a grooving process is conducted.
  • Use of the high carbon steels having C content of more than 1.1 mass% is limited to components having a simple shape and requiring a high hardness, such as cutlery or cold forming dies.
  • a material for manufacturing a knitting needle needs to have a hardness characteristic and an impact property (toughness) after a quenching-tempering treatment, which is required during a time of an actual use as a needle, in addition to have a sufficient workability (the secondary workability) during a material processing in a manufacturing process pf a needle.
  • a material is subjected to a quenching- tempering treatment, in order to secure a predetermined hardness characteristic.
  • a quenching- tempering treatment generally, a low temperature tempering treatment in a temperature range of from 200 to 350°C is employed.
  • addition content of Mn or Cr which is effective for hardenability is increased, or another third element is added in a large amount, giving weight on hardness characteristic, tempering of a martensite phase is not sufficiently done, causing an insufficient enhancement of impact property (toughness), or scattered toughness values, in some cases.
  • Patent Literatures 1 and 2 discloses technologies of refining a microstructure by adding a carbo-nitride forming element such as Ti, Nb, V, and using a fine carbo-nitride of those elements.
  • carbo-nitride forming element such as Ti, Nb, V
  • these elements have generally been added as measures for enhancing toughness of a steel of a hypo-eutectoid composition containing 0 . 8 mass% or less of carbon.
  • Patent Literature 1 targets a hypoeutectoid steel containing C: 0.5 to 0.7 mass%, adds a carbo-nitride forming element such as V, Ti, Nb thereto, in order to refining prior austenite grains, to thereby enhance toughness (impact property).
  • Patent Literature 2 targets steels having a wide range of carbon content, from a hypoeutectoid steel to a hypereutectoid steel containing C: 0.60 to 1.30 mass%; adds one or two or more kinds of Ni: 1.8 mass% or less, Cr: 2.0 mass% or less, V: 0.5 mass% or less, Mo: 0.5 mass% or less, Nb: 0.3 mass% or less, Ti: 0.3 mass% or less, B: 0.01mass% or less, and Ca: 0.01 mass% or less thereto, as needed; and controls volume fraction (Vf) of undissolved carbides to be in a range where (15.3 ⁇ C mass% - Vf) makes more than 8.5 and less than 10.0, to thereby enhance impact property.
  • Vf volume fraction
  • Patent Literature 1 is limited to hypoeutectoid steels, and is a technique of adding a carbo-nitride forming element such as V, Ti, Nb, etc. , expecting fine carbo-nitride thereof to refine prior austenite grains.
  • the technique described in Patent Literature 1 also is a technique in which formability of ferrite matrix is improved, since carbon level is that of a hypo-eutectoid composition. Therefore, it is difficult to apply this technique to machine parts which require a high hardness, such as knitting needles.
  • Patent Literature 2 Mo, V, Ti, Nb, B, or the like are added to a hypoeutectoid steel having a carbon content in a range of from 0.67 to 0.81 mass%.
  • the addition of Mo, V, Ti, Nb, B, or the like clearly is an addition with the intention of improving a characteristic of a hypoeutectoid steel.
  • Patent Literature 2 does not include any disclosure regarding action of each of the third elements in steels having a carbon content exceeding 0.81 mass%, and optimization thereof.
  • Patent Literature 2 only defines an upper limit value of addition amount of the third elements where an impact value is not influenced adversely by the third elements, and does not define lower limit value thereof . From these facts, it can be said that Patent Literature 2 does not include disclosure of a technique of adding a third element in an intended range, with positively expecting an effect of the added element to enhance impact property.
  • Patent Literature 1 and Patent Literature 2 do not include disclosure of a technique which advantageously improves a desired impact property and a predetermined hardness by a quenching after a short solution treatment soaking time such as 3 to 15 minutes, and a low temperature tempering of 200 to350°C; and do not include disclosure of a technique which evaluated an impact property of a steel plate having a thickness of less than 1.0 mm.
  • the purpose of the present invention is to provide a high carbon cold-rolled steel sheet (hereinbelow also simply referred to as "cold-rolled steel sheet”), having a thickness of less than 1.0 mm, capable of exhibitng a mechanical characteristic with an impact value of 5 J/cm 2 or more, and a hardness in a range of from 600 to 750 HV, after subjected to a short time solution treatment and subsequent quenching and low temperature tempering treatment.
  • a high carbon cold-rolled steel sheet hereinbelow also simply referred to as "cold-rolled steel sheet”
  • the present inventors have extensively researched a proper addition ranges of chemical components of a high carbon cold-rolled steel sheet, and particle diameter or presence form of a carbide in a steel, in order to solve the problem described above.
  • the present invention limits a carbon content to C: 0.85 mass% or more and 1.10 mass% or less which is preferred to a knitting needle, from viewpoints of workability, hardenability, and, hardness and toughness after a low temperature tempering, etc. ; and a core of the present technique is an obtained knowledge that, in order for the objective characteristic to be exhibited, it is effective to add Nb as the third element in a predetermined range within the range of carbon content, and to control an average particle diameter and a degree of spheroidizing of a carbide.
  • the present inventors have developed a new test method (new impact test method) for toughness evaluation, targeting steel sheets having a thickness of less than 1.0 mm which has conventionally been difficult to be evaluated for toughness .
  • the new test method (new impact test method) is shown in Fig. 1 and Fig. 2 .
  • the present inventors have earnestly researched to solve the above problem, and have found that it is possible to obtain a high carbon cold-rolled steel sheet having both of an excellent hardenability and an excellent toughness, by essentially adding 0.005 to 0.020 mass% Nb to a high-carbon steel containing fundamental components prescribed to be in a range of C: 0.85 to 1.10 mass%, Mn: 0.50 to 1.0 mass%, Si: 0.10 to 0.35 mass%, P: 0.030 mass% or less, S: 0.030 mass% or less, and Cr: 0.35 to 0.45 mass%; and controlling a spheroidizing and an average particle diameter of a carbide to be in predetermined ranges; and have also found that it is possible to shorten a time for quenching treatment, or to lower a tempering temperature . Furthermore, it has become possible to prescribe proper chemical components, a spheroidization rate and an average particle diameter of a carbide, by employing the test method for properly evaluating an impact property of a thin plate.
  • a cold-rolled steel sheet (less than 1 mm thickness) was produced in such a manner that a hot-rolled steel plate (4 mm thickness) having a composition containing 1.01% of C-0.26% of Si-0.73% of Mn-0.42% of Cr-0.02% of Mo, by mass%, which is further added with Nb, with changing the amount thereof to 0%, 0.010%, 0.020%, 0.055%, and containing Fe and inevitable impurities as the remainder is repeatedly subjected to a cold rolling (rolling reduction rate: 25 to 65%, last: 3 to 50%), a softening annealing, and a spheroidizing annealing (640 to 700°C) each 5 times.
  • a cold rolling rolling reduction rate: 25 to 65%, last: 3 to 50%
  • a softening annealing a softening annealing
  • a spheroidizing annealing (640 to 700°C) each 5 times.
  • the obtained cold-rolled steel sheet was subjected to a solution treatment in which soaking time was changed within a range of from 0 to 16 minutes, at two levels of heating temperature of 780°C and 800°C, and after that, the sheet was oil-quenched, and then measured for Vickers hardness (HV).
  • the obtained results are shown in Fig. 3 (heating temperature: 800°C) and Fig. 4 (heating temperature: 780°C), in terms of a relationship between a soaking time (minute) of the solution treatment and a quenched hardness (HV).
  • a cold-rolled steel sheet having Nb content of 0.010 mass% can secure a quenched hardness exceeding 700 HV, with the shortest soaking time.
  • Nb content increases exceeding 0.010 mass%, the increase of hardness in a short soaking time slows down.
  • a soaking time in which a quenched hardness reached 700 HV, when the heating temperature of the solution treatment was 780°C was obtained from the results shown in Fig. 4 , and is shown in Fig. 5 in terms of a relationship with Nb content.
  • Fig. 5 shows that when Nb content is 0.020 mass% or more, the soaking time of the solution treatment in which a quenched hardness reaches 700 HV is substantially constant.
  • Nb content is in a range of from 0.005 to 0.015 mass%, a soaking time of the solution treatment for securing a desired quenched hardness (700 HV) becomes the shortest, and at the same time, it is possible to secure a stable hardenability. It is further possible, with the Nb content in this range, to shorten a soaking time of a solution treatment.
  • Nb content to be in the range of from 0.005 to 0.015 mass% is effective as a measure capable of preventing an uneven expansion by quenching and a warp by quenching which have been problems in needle processing makers.
  • the cold-rolled steel sheets having various Nb content were subjected to a solution treatment with a heating temperature: 800°C and soaking time: 10 minutes, and were oil-quenched, and further subjected to a tempering treatment.
  • tempering temperatures were varied temperatures of 150°C, 200°C, 250°C, 300°C and 350°C, and holding time was set to one hour.
  • impact property was checked. Incidentally, the impact property was conducted by using the new test method as shown in Fig. 1 and Fig. 2 . The obtained results are shown in Fig. 6 .
  • the impact value was the highest in the case where Nb content was 0.010 mass%, when the tempering temperature was 200°C or more.
  • Tempering temperature at which an impact value: 5 J/cm 2 can be obtained was obtained from Fig. 6 , and shown in Fig. 7 in terms of a relationship with Nb content.
  • Fig. 7 shows that the tempering temperature at which the impact value: 5 J/cm 2 can be obtained is the lowest in a case of a steel plate having an Nb content: 0.010 mass%.
  • the tempering temperature at which the impact value : 5 J/cm 2 can be obtained comes to the high temperature side.
  • the tempering temperature becomes high temperature hardness lowers and durability as a needle lowers. It was also found that when Nb content was less than 0.005 mass%, it was necessary to make a tempering temperature to be a high temperature, in order to secure a desired impact value.
  • Fig. 5 and Fig. 7 show that a lower limit of the Nb content is 0.005 mass% and a higher limit of the Nb content is 0.020 mass%, in order to obtain both of a high hardness and an excellent impact property after a tempering.
  • the upper limit of Nb content be set to 0.015 mass %, in order to make the soaking time of the solution treatment to be a short time.
  • the high carbon cold-rolled steel sheet of the present invention is a thin high carbon cold-rolled steel sheet having a thickness of less than 1.0 mm, particularly thickness of from 0.4 to 0. 7 mm, and is a steel sheet in which an average particle diameter of a carbide is controlled to be a size of from 0.2 to 0.7 ⁇ m, and at the same time, a spheroidization rate is controlled to be 90% or more.
  • this steel sheet By subjecting this steel sheet to a heat treatments of quenching and tempering, it is possible to obtain a good impact property (impact value: 5 J/cm 2 or more) and good hardness characteristic (600 to 750 HV) by a heat treatments of quenching and low temperature tempering, even by a solution treatment having a short soaking time such as 3 to 15 minutes.
  • the high carbon cold-rolled steel sheet of the present invention exhibits a definite advantage to conventional high carbon cold-rolled steel sheets, in the point of a balance of hardness and impact property (toughness), under a condition that the steel sheet is subjected to a short time solution treatment, and subsequent quenching so as to be converted into a martensite phase containing an inevitable retained ⁇ phase, and then subjected to a so-called low temperature tempering of from 200 to 350°C.
  • the high carbon cold-rolled steel sheet according to the present invention it is possible to obtain machine-parts made by high carbon steel which are excellent in toughness after a quenching-tempering, with securing the excellent hardenability.
  • the cold-rolled steel sheet disclosed in the present invention is preferred in a use which requires not only a balance of hardness and toughness, but also a wear resistance or a fatigue resistance, such as a use in knitting needles which requires an excellent durability under a severe use condition.
  • the steel sheet according to the present invention is obtained as a high carbon cold-rolled steel sheet having a thickness of less than 1.0 mm, in such a manner that a hot-rolled steel plate is subjected to a softening annealing as needed, and repeatedly subjected to a cold rolling and a spheroidizing annealing alternately.
  • this high carbon cold-rolled steel sheet is subjected to a predetermined secondary working and solution treatment, and then subjected to a quenching and a tempering treatment, so as to be used in a member (machine part), such as a knitting needle.
  • the C is an essential element to obtain a sufficient hardness after a heat treatment of the high carbon cold-rolled steel sheet.
  • the lower limit value thereof was determined so as to secure a hardness of 600 to 750 HV in a precision part such as a knitting needle, and the upper limit value was determined so as to be capable of controlling amount of carbides to be in a level where various and variety of cold works were not inhibited.
  • the lower limit value was specified to be 0.85 mass%, in order to stably secure a hardness of 600 HV in a short time quenching-tempering treatment.
  • the upper limit value was specified to be 1.10 mass% as an upper limit for being capable of resisting to wide variety of plastic deformation, such as punching property, swaging property, bending property, cutting property, or the like.
  • a cold workability is improved when a spheroidizing treatment of carbides is conducted by repeating a cold rolling and a spheroidizing annealing.
  • C exceeds 1.10 mass%, problems in the manufacturing process become revealed, such that a rolling load becomes high in a hot rolling process or a cold rolling process, or frequency of occurrence of crack in a coil end portion becomes significantly high. Therefore, C was defined to be in a range of from 0.85 to 1.10 mass% . Preferably, the range is from 0.95 to 1.05 mass% .
  • Mn is an element which is effective in deoxidization of a steel, and at the same time, an element capable of stably obtaining a predetermined hardness by enhancing hardenability of a steel.
  • the effect of the present invention becomes notable by 0.50 mass% or more of Mn.
  • the lower limit value was specified to be 0.50 mass%.
  • MnS precipitates in a large amount and becomes coarse during the hot rolling, causing a frequent occurrence of a crack during parts processing. Therefore, the upper limit value was specified to be 1.0 mass%.
  • Mn was defined to be in a range of from 0.50 to 1.0 mass%. Preferably, the range is from 0.50 to 0.80 mass%.
  • Si is a deoxidizing element for steels, and therefore is an element effective in smelting a clean steel. Si also is an element providing a temper- softening resistance of a martensite.
  • the lower limit value was specified to be 0.10 mass%.
  • the upper limit value thereof was specified to be 0.35 mass%, since when added in a large amount, tempering of a martensite becomes insufficient in a low temperature tempering, causing a deteriorated impact property. Therefore, Si was defined to be in a range of from 0.10 to 0.35 mass%.
  • P and S inevitably present in a steel as impurity elements, both adversely affecting an impact property (toughness), and therefore, preferably are lowered as much as possible.
  • Containing of P up to 0.030 mass% or S up to 0.030 mass% makes no problem in an actual use. From such fact, content of P was specified to be 0.030 mass% or less, and content of S was specified to be 0.030 mass% or less.
  • content of P is set to 0.020 mass% or less, and the content of S is set to 0.010 mass% or less.
  • Cr is an element which enhances a hardenability of a steel.
  • Cr solid-solutes into a carbide (cementite) to cause a delay in redissolution of a carbide in the heating step, and thus, Cr inhibits a hardenability on the contrary, when added in a large amount.
  • the upper limit value of Cr was specified to be 0.45 mass%.
  • a lower limit value of Cr was specified to be 0.35 mass%, considering a balance of hardness and impact property after a quenching-tempering. In view of the above, Cr was defined to be in a range of from 0.35 to 0.45 mass%.
  • Nb has conventionally been known as an element that enlarges an unrecrystallization temperature region of a steel during a hot-rolling, and at the same time, known as an element that precipitates as NbC and contribute to a refinement of austenite grains. Therefore, Nb is sometimes also added to a high carbon steel, expecting the refining effect to a microstructure after a cold rolling process.
  • Nb is added in an amount of 0.005 to 0.020 mass%, for the main purpose of recovering toughness by a low temperature tempering after a quenching. When Nb is added in an extremely small amount, Nb is in a dilute solid solution state, without forming enough NbC to contribute to the refinement of a microstructure.
  • the upper limit of the Nb addition amount was specified to be 0.020 mass%.
  • the upper limit is 0.015 mass% or less.
  • the lower limit of the Nb addition amount was specified to be 0.005 mass%. In view of the above, Nb was specified to be in a range of from 0.005 to 0.020 mass%.
  • Mo and V can inevitably be contained in an amount of Mo: less than 0.001 mass%, and V: less than 0.001 mass%.
  • Mo and V may be added in an amount more than the level contained inevitably, in order to enhance the hardenability or the impact property after a tempering.
  • Mo or V is added more than a predetermined amount, the effect of addition of Nb is lost. Therefore, it is preferred that when Mo or V is added, content thereof be controlled to be in the following range, so that the effect of addition of Nb is exerted at maximum.
  • Mo is an element which is effective in enhancing a hardenability.
  • an addition amount is excessive, an impact property can be deteriorated in some cases of low temperature tempering of 200 to 350°C. Therefore, when Mo is added, the amount thereof was specified to be 0.001 mass% or more which is larger than the level contained inevitably, and less than 0.05 mass% which is a range in which an impact property is not inhibited.
  • addition of Mo is 0.01 to 0.03 mass%.
  • V 0.001 mass % or more and less than 0.05 mass%
  • V is an element which is effective in enhancing an impact property by refining steel microstructure, but is an element which can sometimes deteriorates a hardenability. Therefore, when V is added, the amount thereof was specified to be 0.001 mass% or more which is larger than the level contained inevitably, and less than 0.05 mass% which is a range in which a hardenability is not inhibited. Preferably, addition of V is 0.01 to 0.03 mass%. The remainder other than the components described above contains Fe and inevitable impurities.
  • the average particle diameter (d av ) ( ⁇ m) of the equation (1) is an average value of diameters of each circle, when a circle having an area equivalent to that of each carbide observed on a cross section of the steel sheet is supposed (circle equivalent diameter) .
  • An average particle diameter (d av ) in this range causes an excellent impact property and an effect of easily achieving a desired quenched hardness even by a short time solution treatment. From experience, an average particle diameter (d av ) less than 0.2 ⁇ m causes an increased load at the time of a secondary working which is a process of making a shape of needle. An average particle diameter (d av ) exceeding 0.7 ⁇ m is not preferred, because it makes it difficult to achieve a desired enhancement of hardenability by a short time solution treatment.
  • the present invention further defined a spheroidization rate which is a spheroidization rate of carbides, by N TC and N SC of the equation (2).
  • N TC refers to a total number of carbides per an observed area of 100 ⁇ m 2 .
  • N SC refers to a number of carbides which can be deemed as spheroidized in the same observation view and which satisfying the condition of d L /d S : 1.4 or less.
  • d L represented a major axis
  • d S represented a minor axis.
  • a carbide cannot be said to be formed into a perfect spherical shape, but observed as an oval shape in many cases, depending on a face observed. Therefore, a spheroidization degree was defined by a ratio of a major axis and a minor axis (d L /d S ). According such circumstances, the present invention deemed a carbide which satisfied the condition of d L /d S : 1.4 or less as spheroidized, and defined N SC as a number thereof.
  • the spheroidization rate (N SC /N TC ) ⁇ 100 was specified to be 90% or more, because it has been found, from experience, that the spheroidization rate in this range enhances a secondary workability of a steel plate.
  • the measurement of the average particle diameter and the spheroidization rate of a carbide as described above was conducted by observing secondary electron microscope images at a magnification of 2,000 times by using a scanning electron microscope.
  • test pieces in a shape of plate were cut out from a sample before subjected to the heat treatment, in the direction vertical to the rolled direction.
  • a treatment such as a resin embedding was conducted, and the carbides were measured for the circle equivalent diameter, the d L /d S ratio, and the N TC and N SC within the observed area of 100 ⁇ m 2 around the center portion of the thickness, and an average value of five views was calculated.
  • an image analysis software "winroof" (trade name) which is commercially available was used.
  • a hot-rolled steel plate used in the present invention may be those obtainable in an ordinary manufacturing condition.
  • a hot-rolled steel plate to be used in the present invention may be manufactured by heating steel pieces (slabs) having the chemical composition described above to 1,050 to 1, 250°C, hot-rolling the heated slabs at a finishing temperature of 800 to 950°C, and coiling the resultant at a coiling temperature of from 600 to 750°C.
  • a thickness of the hot-rolled steel plate may appropriately be set, so as to obtain a preferred cold-rolling reduction rate, on the basis of a thickness of a desired cold-rolled steel sheet.
  • a high carbon cold-rolled steel sheet having a thickness of less than 1.0 mm is manufactured by repeating a cold rolling (25 to 65%) and a spheroidizing annealing (640 to 720°C) two or more times. It is preferred that the cold rolling (25 to 65%) and the spheroidizing annealing (640 to 720°C) be each conducted 2 to 5 times.
  • the cold rolling (25 to 65%) and the spheroidizing annealing (640 to 720°C) are repeated for two or more times.
  • the reason is to control an average particle diameter (d av ) and a spheroidization rate (N SC /N TC ) ⁇ 100 of a carbide to satisfy the above described equation (1) and equation (2), respectively, as set forth below.
  • Particularly preferred is 2 to 5 times of cold rolling and 2 to 5 times of spheroidizing annealing.
  • a rolling reduction rate of cold rolling is in a range of from 25 to 65%.
  • the last cold rolling treatment is at a rolling reduction rate of 3 to 50%.
  • the temperature for a spheroidizing annealing is in a range of from 640 to 720°C.
  • a holding time for the spheroidizing annealing may appropriately be selected within a range of from 9 to 30 hours, at a temperature in this range.
  • the same temperature range is also preferred for a softening annealing for the purpose of softening a hot-rolled steel plate before the cold rolling.
  • the above is a method of manufacturing the high carbon cold-rolled steel sheet according to the present invention.
  • the following heat treatment be conducted after the steel sheet is processed into a predetermined shape.
  • the heating temperature is set to 760 to 820°C
  • the soaking time is set to a short time such as 3 to 15 minutes.
  • An oil is preferably used in the quenching (rapid cooling).
  • the tempering temperature it is preferred that the tempering temperature be set to 200 to 350°C. More preferably, the tempering temperature is 250 to 300°C. In this manner, it is possible to manufacture various machine parts having a hardness of 600 to 750 HV.
  • a soaking time of the solution treatment When a soaking time of the solution treatment is longer than 15 minutes, carbides dissolve in excessively, and austenite grains become coarse, resulting in a coarse martensite phase after the quenching, which deteriorates an impact property. Therefore, it is preferred that an upper limit of the soaking time of solution treatment be 15 minutes. On the other hand, when the soaking time is shorter than 3 minutes, carbides dissolve in insufficiently, which makes a quenching hard. Therefore, it is preferred that a lower limit of the soaking time of solution treatment be 3 minutes. A range of from 5 to 10 minutes is more preferred.
  • a tempering temperature When a tempering temperature is less than 200°C, recovery of toughness of a martensite phase is insufficient. On the other hand, when the tempering temperature exceeds 350°C, although the impact value is recovered, a hardness lowers 600 HV, causing a problem in durability and wear resistance. Thus, it is preferred that a proper range of the tempering temperature be set to 200 to 350°C. More preferred is from 250 to 300°C. A holding time for a tempering may be selected from a range of from 30 minutes to 3 hours.
  • these cold-rolled steel sheets were subjected to a solution treatment (soaked in a furnace of 800°C for 10 minutes) under the conditions as shown in Table 2, and subsequently, oil-quenched, and tempered (tempering temperature: 250°C).
  • Predetermined test pieces were sampled from the steel sheets after the tempering treatment, which were then subjected to an impact test and a hardness measurement test.
  • the hardness test is conducted under a condition of 5 kg load (testing force: 49.0N) as measured by a Vickers hardness testing machine, in conformity with the regulation of JIS Z 2244.
  • the impact property was evaluated by Charpy impact test.
  • a U-notch test piece having a notch width of 0.2 mm (notch depth: 2.5 mm; notch radius: 0.1 mm) was used.
  • Fig. 1 shows the state of a testing machine on which the test piece is set, and
  • Fig. 2 shows a shape of the test piece.
  • Such test piece and test method were employed for the following reason.
  • This testing machine was used, with the distance between the supporting beds adjusted from 60 mm to 40 mm.
  • the distance between the supporting beds was adjusted from 60 mm to 40 mm in the present testing machine, in order to obtain a condition close to that of the JIS Standard (JIS Z 2242) which is a Charpy impact test method for metal materials.
  • test piece Used as the test piece was a test piece on which a U notch is formed by an electric discharge machining, so as to obtain a notch depth of 2.5 mm, a notch radius of 0.1 mm (a notch width of 0.2 mm), as shown in Fig. 2 .
  • the notch radius was made small in order to minimize the deflection of the plate during the Charpy impact test, by increasing a stress concentration factor, so that a stable impact value was obtained. It has been confirmed that, by employing this test method and this shape of test piece, it is possible to obtain an impact property in a condition close to an actual use environment.
  • the case having an amount of C out of the lower limit value (steel type No. 1) exhibited an impact value and a quenched-tempered hardness out of the target values.
  • the case having an amount of C out of the upper limit value (steel type No. 6) exhibited a quenched-tempered hardness exceeding the target value 600 to 750 HV, and an impact value lowering the target value 5 J/cm 2 .
  • both of the case having an amount of C of 0.85 mass% (steel type No. 2, Comparative Example) and the case having an amount of C of 1.10 mass% (steel type No.
  • the steel sheets having chemical components corresponding to those of the example of the invention (steel type Nos. 15, 16, 17, 19, and21) all exhibited a quenched-tempered hardness fulfilling the target value 600 to 750 HV, and an excellent impact property.
  • the case without added with Nb (steel type No. 11), the case without added with Nb and with a V addition amount of 0.05 mass% (steel type No. 12), the case without added with Nb and with an Mo addition amount exceeding 0.05 mass% (steel type No. 13), the case with a combined addition of Nb + Mo, the Nb addition amount being less than 0.005 mass% (steel type No.
  • Cold-rolled steel sheets having thicknesses as shown in Table 5 were obtained by using a hot-rolled steel plate having the chemical components of the steel type No. 3 (Table 3), under the manufacturing conditions of cold rolling and spheroidizing treatment described in Table 1 altered.
  • Table 5 shows spheroidization rates and average particle diameters of carbides of the obtained cold-rolled steel sheets.
  • the obtained cold-rolled steel sheets were further subjected to an oil-quenching and a low temperature tempering, under the condition shown in Table 2, in the same manner as in Example 1. Hardnesses of cross section and impact values of the obtained cold-rolled steel sheets after subjected to the solution treatment and the subsequent quenching-tempering were measured in the same manner as in Example 1, and shown in Table 5.
  • Cold-rolled steel sheets having thicknesses as shown in Table 6 were obtained by using a hot-rolled steel plate having the chemical components of the steel type No. 16 (Table 4), and the manufacturing conditions described in Table 1 altered.
  • the spheroidization rates, average particle diameters of carbides of the obtained cold-rolled steel sheets are shown in Table 6.
  • the obtained cold-rolled steel sheets were further subjected to an oil-quenching and a low temperature tempering, under the condition shown in Table 2, in the same manner as in Example 1. Hardnesses of cross section and impact values of the obtained cold-rolled steel sheets after subjected to the solution treatment and the subsequent quenching- tempering were measured in the same manner as in Example 1, and shown in Table 6.
  • a steel sheet having chemical components in the range of the present invention has an enhanced hardenability by an addition of Nb, and an improved impact property after a heat treatment, and therefore, suitable to be used as a hypereutectoid steel in a machine parts which are used in a severe environment.
  • a hypereutectoid steel containing C of 0.85 to 1.10 mass% is suitable for a use where a balance of hardness and toughness is required under a severe use environment, such as a use in knitting needles.

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Claims (3)

  1. Tôle d'acier riche en carbone laminée à froid, dans laquelle
    une composition chimique de la tôle d'acier comprend C : 0,85 à 1,10 % en masse, Mn : 0,50 à 1,0% en masse, Si: 0,10 à 0,35 % en masse, P: 0,030 % en masse ou moins, S : 0,030 % en masse ou moins, Cr: 0,35 à 0,45 % en masse, Nb : 0,005 à 0,020 % en masse, et facultativement une ou deux sortes choisies parmi Mo et V : chacun à 0,001 % en masse ou plus et moins de 0,05 % en masse, avec Fe et une impureté inévitable en tant que reste,
    avec un diamètre moyen de particule (dav) et un taux de sphéroïdisation (NSC/NTC) × 100% d'un carbure dispersé dans la tôle d'acier satisfaisant respectivement à une équation (1) et une équation (2) ci-après, et une épaisseur de la tôle d'acier étant inférieure à 1,0 mm ; 0,2 ≤ d av ≤ 0,7 μm
    Figure imgb0011
     N SC / N TC × 100 ≥ 90 %
    Figure imgb0012
     où le diamètre moyen de particule (dav) de l'équation (1) est une valeur moyenne des diamètres de chaque cercle, lorsqu'un cercle ayant une aire équivalente à celle de chaque carbure observé sur une coupe transversale de la tôle d'acier est supposé (diamètre équivalent d'un cercle), et
    où NTC et NSC dans l'équation (2) représentent respectivement NTC: nombre total de carbure par aire observée de 100 µm2, et NSC: nombre de carbure qui satisfait à une condition selon laquelle dL/dS est 1,4 ou moins, dL étant un axe majeur et dS étant un axe mineur d'un carbure.
  2. Procédé de fabrication d'une tôle d'acier riche en carbone laminée à froid :
    (a) en fournissant une plaque d'acier laminée à chaud comprenant la composition chimique telle que décrite dans la revendication 1 ;
    (b) facultativement, en soumettant la plaque d'acier laminée à chaud de l'étape (a) à un traitement de recuit ramollissant ;
    (c) en soumettant de façon répétée la plaque d'acier laminée à chaud de l'étape (a) ou de l'étape (b) à un traitement de laminage à froid à un taux de réduction par laminage de 25 à 65 % et à un traitement de recuit sphéroïdisant à une température de 640 à 720 °C de façon à fournir une tôle d'acier laminée à froid, dans laquelle le nombre de soumissions de façon répétée de la plaque d'acier laminée à chaud au traitement de laminage à froid et au traitement de recuit sphéroïdisant est de deux ou plus; et
    (d) en soumettant la tôle d'acier laminée à froid de l'étape (c) à un dernier traitement de laminage à froid à un taux de réduction par laminage de 3 à 50 %,
    dans lequel un diamètre moyen de particule (dav) et un taux de sphéroïdisation (NSC/NTC) d'un carbure dispersé dans la tôle d'acier riche en carbone laminée à froid satisfont respectivement à l'équation (1) et à l'équation (2) ci-après ; et une épaisseur de la tôle d'acier riche en carbone laminée à froid est inférieure à 1,0 mm ; 0,2 ≤ d av ≤ 0,7 μm
    Figure imgb0013
     N SC / N TC × 100 ≥ 90 %
    Figure imgb0014
     où le diamètre moyen de grain (dav) de l'équation (1) est une valeur moyenne des diamètres de chaque cercle, lorsqu'un cercle ayant une aire équivalente à celle de chaque carbure observé sur une coupe transversale de la tôle d'acier est supposé (diamètre équivalent d'un cercle), et
    où NTC et NSC dans l'équation (2) représentent respectivement NTC : nombre total de carbure par aire observée de 100 µm2, et NSC : nombre de carbure qui satisfait à une condition selon laquelle dL/dS est 1,4 ou moins, dL représentant un axe majeur et dS représentant un axe mineur du carbure.
  3. Procédé de fabrication d'une tôle d'acier riche en carbone laminée à froid selon la revendication 2, dans lequel le nombre de soumissions de façon répétée de la plaque d'acier laminée à chaud au traitement de laminage à froid et au traitement de recuit sphéroïdisant est de deux à cinq.
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