JP5440203B2 - Manufacturing method of high carbon hot rolled steel sheet - Google Patents
Manufacturing method of high carbon hot rolled steel sheet Download PDFInfo
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- JP5440203B2 JP5440203B2 JP2010011860A JP2010011860A JP5440203B2 JP 5440203 B2 JP5440203 B2 JP 5440203B2 JP 2010011860 A JP2010011860 A JP 2010011860A JP 2010011860 A JP2010011860 A JP 2010011860A JP 5440203 B2 JP5440203 B2 JP 5440203B2
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- 229910000831 Steel Inorganic materials 0.000 title claims description 114
- 239000010959 steel Substances 0.000 title claims description 114
- 229910052799 carbon Inorganic materials 0.000 title claims description 22
- 238000004519 manufacturing process Methods 0.000 title claims description 21
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 title claims description 13
- 238000001816 cooling Methods 0.000 claims description 88
- 230000009466 transformation Effects 0.000 claims description 44
- 238000000137 annealing Methods 0.000 claims description 20
- 238000004804 winding Methods 0.000 claims description 8
- 229910052796 boron Inorganic materials 0.000 claims description 5
- 229910052802 copper Inorganic materials 0.000 claims description 5
- 239000012535 impurity Substances 0.000 claims description 5
- 239000000203 mixture Substances 0.000 claims description 5
- 229910052750 molybdenum Inorganic materials 0.000 claims description 5
- 229910052759 nickel Inorganic materials 0.000 claims description 5
- 229910052758 niobium Inorganic materials 0.000 claims description 5
- 229910052719 titanium Inorganic materials 0.000 claims description 5
- 229910052720 vanadium Inorganic materials 0.000 claims description 5
- 229910052748 manganese Inorganic materials 0.000 claims description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 229910052698 phosphorus Inorganic materials 0.000 claims description 4
- 229910052717 sulfur Inorganic materials 0.000 claims description 4
- 229910052804 chromium Inorganic materials 0.000 claims description 3
- 229910001567 cementite Inorganic materials 0.000 description 19
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 19
- 238000005098 hot rolling Methods 0.000 description 14
- 230000000694 effects Effects 0.000 description 12
- 229910001562 pearlite Inorganic materials 0.000 description 11
- 229910000677 High-carbon steel Inorganic materials 0.000 description 8
- 238000010438 heat treatment Methods 0.000 description 8
- 238000000034 method Methods 0.000 description 6
- 238000010791 quenching Methods 0.000 description 6
- 230000000171 quenching effect Effects 0.000 description 6
- 238000005096 rolling process Methods 0.000 description 6
- 239000002245 particle Substances 0.000 description 5
- 230000001965 increasing effect Effects 0.000 description 4
- 230000008569 process Effects 0.000 description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- 229910000859 α-Fe Inorganic materials 0.000 description 4
- 229910001566 austenite Inorganic materials 0.000 description 3
- 238000009835 boiling Methods 0.000 description 3
- 238000002347 injection Methods 0.000 description 3
- 239000007924 injection Substances 0.000 description 3
- 239000012298 atmosphere Substances 0.000 description 2
- 229910001563 bainite Inorganic materials 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 239000006185 dispersion Substances 0.000 description 2
- 230000020169 heat generation Effects 0.000 description 2
- 229910000734 martensite Inorganic materials 0.000 description 2
- 239000012299 nitrogen atmosphere Substances 0.000 description 2
- 230000001737 promoting effect Effects 0.000 description 2
- 238000005496 tempering Methods 0.000 description 2
- 229910001208 Crucible steel Inorganic materials 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 239000000498 cooling water Substances 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 230000002708 enhancing effect Effects 0.000 description 1
- 239000012530 fluid Substances 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 229910052738 indium Inorganic materials 0.000 description 1
- 230000006698 induction Effects 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 229910052745 lead Inorganic materials 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 238000005554 pickling Methods 0.000 description 1
- OXNIZHLAWKMVMX-UHFFFAOYSA-N picric acid Chemical compound OC1=C([N+]([O-])=O)C=C([N+]([O-])=O)C=C1[N+]([O-])=O OXNIZHLAWKMVMX-UHFFFAOYSA-N 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 238000003303 reheating Methods 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 239000002344 surface layer Substances 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
- 238000011282 treatment Methods 0.000 description 1
Classifications
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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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- 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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0242—Flattening; Dressing; Flexing
-
- 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
- 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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
-
- 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
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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 Sheet Steel (AREA)
Description
本発明は、高炭素熱延鋼板、特に0.5質量%以上のCを含有し、鋼板内の特性のばらつきが小さく、加工性と焼入性に優れた高炭素熱延鋼板の製造方法に関する。 The present invention relates to a high-carbon hot-rolled steel sheet, particularly a method for producing a high-carbon hot-rolled steel sheet that contains 0.5% by mass or more of C, has little variation in properties in the steel sheet, and is excellent in workability and hardenability.
機械構造部品や工具などに使用される高炭素鋼板は、種々の形状に冷間で成形加工された後、硬質化のために焼入焼戻処理を施される場合が多い。そのため、こうした用途の高炭素鋼板には、優れた加工性と焼入性が要求されており、これまでに種々の技術が提案されている。 High carbon steel sheets used for machine structural parts, tools, etc. are often subjected to quenching and tempering treatments for hardening after being cold-formed into various shapes. Therefore, high workability and hardenability are required for such high carbon steel sheets, and various techniques have been proposed so far.
例えば、特許文献1には、質量%で、C:0.30〜1.20%、Si:2.00%以下、Mn:1.00%以下、P:0.030%以下、S:0.030%以下、Al:0.01〜0.08%、N:0.010%以下で、残部が実質的にFeおよび不可避的合金成分からなる高炭素鋼を、仕上温度を(Ac1変態点+30℃)以上として熱間圧延後、10〜100℃/sの冷却速度で20〜500℃の温度まで冷却し、1〜10s保持後、500℃〜(Ac1変態点+30℃)の温度域へ再加熱し、この温度域で巻き取る成形性の良好な高炭素薄鋼板の製造方法が開示されている。 For example, Patent Document 1 includes mass%, C: 0.30 to 1.20%, Si: 2.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.030% or less, Al: 0.01 to 0.08%, N: 0.010% or less, high carbon steel with the balance being substantially Fe and inevitable alloy components, after hot rolling at a finishing temperature of (Ac 1 transformation point + 30 ° C) or higher, 10-100 ° C / s Cooling at a cooling rate of 20 to 500 ° C, holding for 1 to 10 seconds, reheating to a temperature range of 500 ° C to (Ac 1 transformation point + 30 ° C), and good formability to wind up in this temperature range A method for producing a simple high carbon steel sheet is disclosed.
特許文献2には、Cを0.2〜0.7質量%含有する鋼を、仕上温度(Ar3変態点-20℃)以上で熱間圧延後、冷却速度120℃/s超かつ冷却停止温度650℃以下で冷却を行い、次いで巻取温度600℃以下で巻き取り、酸洗後、焼鈍温度640℃以上Ac1変態点以下で焼鈍する伸びフランジ性に優れた高炭素熱延鋼板の製造方法が開示されている。 In Patent Document 2, a steel containing 0.2 to 0.7% by mass of C is hot-rolled at a finishing temperature (Ar 3 transformation point -20 ° C.) or higher, a cooling rate of 120 ° C./s or higher and a cooling stop temperature of 650 ° C. or lower. A method for producing a high carbon hot-rolled steel sheet with excellent stretch flangeability is disclosed in which the steel sheet is cooled at a coiling temperature of 600 ° C. or lower, pickled, and annealed at an annealing temperature of 640 ° C. or higher and an Ac 1 transformation point or lower. ing.
特許文献3には、質量%で、C:0.2〜0.7%、Si:2%以下、Mn:2%以下、P:0.03%以下、S:0.03%以下、sol.Al:0.01%以下、N:0.01%以下を含有する組成の鋼を、(Ar3変態点-20℃)以上の仕上温度で熱間圧延して熱延鋼板とする工程と、前記熱延鋼板を、60℃/s以上120℃/s未満の冷却速度で650℃以下の温度まで冷却する工程と、前記冷却後の熱延鋼板を、600℃以下の巻取温度で巻き取る工程と、前記巻取後の熱延鋼板を、640℃以上Ac1変態点以下の焼鈍温度で焼鈍する工程とを有する高炭素熱延鋼板の製造方法が開示されている。 Patent Document 3 includes mass%, C: 0.2 to 0.7%, Si: 2% or less, Mn: 2% or less, P: 0.03% or less, S: 0.03% or less, sol.Al: 0.01% or less, N : A step of hot rolling a steel having a composition containing 0.01% or less at a finishing temperature of (Ar 3 transformation point -20 ° C) or higher to form a hot rolled steel plate, A step of cooling to a temperature of 650 ° C. or less at a cooling rate of less than 120 ° C./s, a step of winding the hot-rolled steel sheet after cooling at a coiling temperature of 600 ° C. or less, and the hot-rolled steel sheet after winding And a step of annealing at a temperature of 640 ° C. or more and an Ac 1 transformation point or less, a method for producing a high carbon hot rolled steel sheet is disclosed.
しかしながら、特許文献1〜3に記載の製造方法で製造された高炭素熱延鋼板、特に、Cを0.5質量%以上含有する鋼板では、鋼板内の特性に大きなばらつきが生じやすく、必ずしも優れた加工性や焼入性が得られないといった問題がある。 However, high-carbon hot-rolled steel sheets manufactured by the manufacturing methods described in Patent Documents 1 to 3, particularly steel sheets containing 0.5% by mass or more of C tend to have large variations in characteristics in the steel sheets, and are always excellent in processing. There is a problem that the property and hardenability cannot be obtained.
本発明は、0.5質量%以上のCを含有し、鋼板内の特性のばらつきが小さく、加工性と焼入性にも優れた高炭素熱延鋼板の製造方法を提供することを目的とする。 An object of the present invention is to provide a method for producing a high carbon hot rolled steel sheet containing 0.5% by mass or more of C, having small variations in characteristics in the steel sheet, and excellent in workability and hardenability.
本発明者らは、0.5質量%以上のCを含有し、鋼板内の特性のばらつきが小さく、加工性と焼入性にも優れた高炭素熱延鋼板の製造方法について鋭意検討したところ、熱間圧延後に冷却する際に、550〜650℃の温度域で放冷を行う二段階の急速冷却、すなわち急速冷却-放冷-急速冷却のパターンで冷却して巻き取り、その後セメンタイトの球状化のための焼鈍を行うことが効果的であることを見出した。 The present inventors have earnestly studied a method for producing a high-carbon hot-rolled steel sheet containing 0.5% by mass or more of C, having small variations in characteristics in the steel sheet, and excellent in workability and hardenability. When cooling after hot rolling, it is cooled in a two-stage rapid cooling in the temperature range of 550 to 650 ° C, that is, cooled and wound in the pattern of rapid cooling-cooling-rapid cooling, and then cementite is spheroidized. It has been found that it is effective to perform annealing for the purpose.
本発明は、このような知見に基づいてなされたものであり、質量%で、C:0.5〜1.0%、Si:2.0%以下、Mn:2.0%以下、P:0.03%以下、S:0.03%以下、sol.Al:0.08%以下、N:0.01%以下を含有し、残部がFeおよび不可避的不純物からなる組成の鋼片を、Ar3変態点あるいはArcm変態点以上の仕上温度で熱間圧延し、60℃/s以上の平均冷却速度で550〜650℃の冷却停止温度まで一次冷却後、1.0〜10s間放冷し、次いで、120℃/s以上の平均冷却速度で500〜600℃の冷却停止温度まで二次冷却して巻き取り、その後、640℃以上Ac1変態点以下の温度で焼鈍することを特徴とする高炭素熱延鋼板の製造方法を提供する。 The present invention has been made based on such knowledge, in mass%, C: 0.5-1.0%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.03% In the following, a slab containing sol.Al: 0.08% or less, N: 0.01% or less, with the balance consisting of Fe and inevitable impurities, hot at a finishing temperature equal to or higher than the Ar 3 transformation point or Ar cm transformation point. Rolled, after primary cooling to a cooling stop temperature of 550 to 650 ° C. at an average cooling rate of 60 ° C./s or more, then allowed to cool for 1.0 to 10 s, and then 500 to 600 ° C. at an average cooling rate of 120 ° C./s or more A method for producing a high carbon hot-rolled steel sheet is provided, characterized by performing secondary cooling to the cooling stop temperature of the steel sheet and winding up, followed by annealing at a temperature not lower than 640 ° C. and not higher than the Ac 1 transformation point.
本発明の高炭素熱延鋼板の製造方法では、一次冷却時の平均冷却速度が120℃/s以上であることが好ましい。また、鋼片に含有されるsol.Al量が、質量%で、0.01%以下であったり、鋼片に、さらに、質量%で、Cr:0.1〜2.0%、Mo:0.1〜1.0%、Ni:0.1〜2.0%、Cu:0.1〜1.0%、Ti:0.01〜0.10%、Nb:0.01〜0.10%、V:0.01〜0.10%、B:0.0005〜0.0100%の内から選ばれた少なくとも1種の元素が含有されてもよい。 In the method for producing a high carbon hot-rolled steel sheet of the present invention, the average cooling rate during primary cooling is preferably 120 ° C./s or more. Also, the amount of sol.Al contained in the steel slab is 0.01% or less by mass%, or in the steel slab, further by mass%, Cr: 0.1-2.0%, Mo: 0.1-1.0%, Ni : 0.1-2.0%, Cu: 0.1-1.0%, Ti: 0.01-0.10%, Nb: 0.01-0.10%, V: 0.01-0.10%, B: 0.0005-0.0100% An element may be contained.
本発明により、0.5質量%以上のCを含有し、鋼板内の特性のばらつきが小さく、加工性と焼入性にも優れた高炭素熱延鋼板を製造することが可能となり、産業上格段の効果を奏する。 According to the present invention, it is possible to produce a high carbon hot rolled steel sheet containing 0.5% by mass or more of C, having small variations in characteristics in the steel sheet, and excellent in workability and hardenability, which is remarkably industrial. There is an effect.
本発明である高炭素熱延鋼板の製造方法について、以下に詳細に説明する。 The manufacturing method of the high carbon hot-rolled steel sheet which is this invention is demonstrated in detail below.
(1) 鋼片の組成
以下、成分元素の含有量の単位である%は、特に断らない限り質量%を意味するものとする。
(1) Composition of steel slab Hereinafter, “%” as a unit of content of component elements means “% by mass” unless otherwise specified.
C:0.5〜1.0%
Cは、焼入焼戻処理後の鋼板強度を高めるために必須の元素である。C量が0.5%未満では、機械構造部品や工具の素材として必要な強度が得られない。一方、C量が1.0%を超えると、鋼板が脆くなって加工性が低下する上、焼入後にも残留オーステナイトが存在し易くなり、熱処理後の強度も飽和あるいは減少する。したがって、Cの含有量は0.5〜1.0%に限定する。好ましくは0.6〜0.9%である。
C: 0.5-1.0%
C is an essential element for increasing the strength of the steel sheet after quenching and tempering. If the amount of C is less than 0.5%, the strength required for machine structural parts and tool materials cannot be obtained. On the other hand, if the C content exceeds 1.0%, the steel sheet becomes brittle and the workability is lowered, and retained austenite tends to exist even after quenching, and the strength after heat treatment is saturated or reduced. Therefore, the C content is limited to 0.5 to 1.0%. Preferably it is 0.6 to 0.9%.
Si:2.0%以下
Siは、鋼を脱酸する作用や焼入後の焼戻軟化抵抗を高める作用を有するため、必要に応じて含有できる。ただし、Siの含有は、センメンタイトを黒鉛化して鋼の焼入性を低める作用もあるので、Siの含有量は2.0%以下に限定する。好ましくは0.5%以下である。
Si: 2.0% or less
Si has an effect of deoxidizing steel and an effect of increasing the temper softening resistance after quenching, and therefore can be contained as necessary. However, the Si content has the effect of reducing the hardenability of the steel by graphitizing the cementite, so the Si content is limited to 2.0% or less. Preferably it is 0.5% or less.
Mn:2.0%以下
Mnは、鋼の焼入性を高める作用があり、必要に応じて含有できる。ただし、Mnを過剰に含有すると、鋼の靱性や延性の低下を招くので、Mnの含有量は2.0%以下に限定する。好ましくは1.0%以下である。
Mn: 2.0% or less
Mn has the effect of enhancing the hardenability of steel and can be contained as required. However, if Mn is contained excessively, the toughness and ductility of the steel are reduced, so the Mn content is limited to 2.0% or less. Preferably it is 1.0% or less.
P:0.03%以下
Pは、鋼板の加工性や熱処理後の鋼の靱性を低下させる作用があるため、Pの含有量は0.03%以下に限定する。好ましくは0.02%以下である。
P: 0.03% or less
P has the effect of reducing the workability of the steel sheet and the toughness of the steel after heat treatment, so the P content is limited to 0.03% or less. Preferably it is 0.02% or less.
S:0.03%以下
Sは、鋼板の加工性や熱処理後の鋼の靱性を低下させる作用があるため、Sの含有量は0.03%以下に限定する。好ましくは0.01%以下である。
S: 0.03% or less
Since S has the effect of reducing the workability of the steel sheet and the toughness of the steel after the heat treatment, the S content is limited to 0.03% or less. Preferably it is 0.01% or less.
sol.Al:0.08%以下
Alは、鋼の脱酸のために添加される元素であり、必要に応じて含有できる。ただし、Alの含有量として、鋼中のsol.Al量が0.08%を超えるような添加は、鋼中介在物の増加を招き、鋼板の加工性の低下を招く。そのため、Alの含有量は、sol.Al量で0.08%以下に限定する。好ましくは0.04%以下である。また、鋼が高温に保持される場合、鋼中で固溶Alと固溶Nが化合してAlNが形成され、焼入加熱時にオーステナイト結晶粒の成長を抑制し、鋼板の焼入性を低める場合がある。特に、鋼板を窒素雰囲気中で焼鈍する場合には、雰囲気から鋼中に侵入したNによって上記作用が顕著化する。AlNの形成に起因するこのような鋼板の焼入性低下を避けるためには、Alの含有量を、sol.Al量で0.01%以下とするのがより好ましい。
sol.Al: 0.08% or less
Al is an element added for deoxidation of steel and can be contained as necessary. However, as the Al content, the addition such that the amount of sol.Al in the steel exceeds 0.08% leads to an increase in inclusions in the steel and a decrease in the workability of the steel sheet. Therefore, the Al content is limited to 0.08% or less in terms of sol.Al content. Preferably it is 0.04% or less. Also, when the steel is kept at a high temperature, solute Al and solute N combine to form AlN in the steel, suppressing the growth of austenite crystal grains during quenching heating and reducing the hardenability of the steel sheet. There is a case. In particular, when the steel sheet is annealed in a nitrogen atmosphere, the above-described action becomes noticeable due to N entering the steel from the atmosphere. In order to avoid such a decrease in hardenability of the steel sheet due to the formation of AlN, it is more preferable that the Al content is 0.01% or less in terms of the sol.Al content.
N:0.01%以下
Nの多量の含有は、鋼中でのAlN形成を通じて、鋼板の焼入性を低める場合がある。そのため、Nの含有量は0.01%以下に限定する。好ましくは0.005%以下である。
N: 0.01% or less
If N is contained in a large amount, the hardenability of the steel sheet may be lowered through the formation of AlN in the steel. Therefore, the N content is limited to 0.01% or less. Preferably it is 0.005% or less.
本発明に用いる鋼片は、上記した成分組成を有するものとなる。なお、上記の各元素の含有量を通常実施されている程度の精錬範囲を超えて、例えば0.001%未満にまで低減することは、鋼板の製造コストの増加を招くため、特段の理由がない限り必要ない。 The steel slab used in the present invention has the above-described component composition. It should be noted that reducing the content of each element above the refining range that is normally practiced, for example, to less than 0.001%, increases the manufacturing cost of the steel sheet, unless there is a special reason. unnecessary.
本発明に用いる鋼片の残部は、Feおよび不可避的不純物である。ただし、焼入性の向上や焼戻軟化抵抗の向上を目的として、Cr、Mo、Ni、Cu、Ti、Nb、V、Bの内から1種以上の元素をさらに添加しても、本発明の効果が損なわれることはない。 The balance of the steel slab used in the present invention is Fe and inevitable impurities. However, for the purpose of improving hardenability and resistance to temper softening, the present invention can be applied even if one or more elements of Cr, Mo, Ni, Cu, Ti, Nb, V, and B are further added. The effect of is not impaired.
鋼片に、これらの元素を添加する場合には、Cr:0.1〜2.0%、Mo:0.1〜1.0%、Ni:0.1〜2.0%、Cu:0.1〜1.0%、Ti:0.01〜0.10%、Nb:0.01〜0.10%、V:0.01〜0.10%、B:0.0005〜0.0100%の範囲の含有量とすることが好ましい。それぞれの範囲の下限未満の含有量では、添加の効果が十分に得られない。また、それぞれの範囲の上限を超える含有量では、製造コストの増加を招くとともに、鋼板の加工性や靱性を低下させる場合がある。なお、より好ましい含有量の範囲は、それぞれ、Cr:0.1〜1.0%、Mo:0.1〜0.5%、Ni:0.1〜1.0%、Cu:0.1〜0.5%、Ti:0.01〜0.05%、Nb:0.01〜0.05%、V:0.01〜0.05%、B:0.0005〜0.0050%である。 When these elements are added to the billet, Cr: 0.1-2.0%, Mo: 0.1-1.0%, Ni: 0.1-2.0%, Cu: 0.1-1.0%, Ti: 0.01-0.10%, Nb : It is preferable to set it as content in the range of 0.01-0.10%, V: 0.01-0.10%, B: 0.0005-0.0100%. When the content is less than the lower limit of each range, the effect of addition cannot be sufficiently obtained. In addition, when the content exceeds the upper limit of each range, the manufacturing cost is increased and the workability and toughness of the steel sheet may be lowered. More preferable content ranges are Cr: 0.1 to 1.0%, Mo: 0.1 to 0.5%, Ni: 0.1 to 1.0%, Cu: 0.1 to 0.5%, Ti: 0.01 to 0.05%, Nb: 0.01, respectively. -0.05%, V: 0.01-0.05%, B: 0.0005-0.0050%.
不可避的不純物としては、例えば、O、Sn、Pbなどがある。これらの元素は極力含有しないことが望ましいが、0.01%未満の含有量であれば許容できる。 Examples of inevitable impurities include O, Sn, and Pb. Although it is desirable that these elements are not contained as much as possible, a content of less than 0.01% is acceptable.
2) 製造条件
本発明による高炭素熱延鋼板の製造方法では、上記した成分組成を有する鋼片を、Ar3変態点あるいはArcm変態点以上の仕上温度で熱間圧延し、60℃/s以上の平均冷却速度で550〜650℃の冷却停止温度まで一次冷却後、1.0〜10s間放冷し、次いで、120℃/s以上の平均冷却速度で500〜600℃の冷却停止温度まで二次冷却して巻き取り、その後、640℃以上Ac1変態点以下の温度で焼鈍する。なお、焼鈍前に鋼板表層に形成されているスケールを酸洗等により除去することが好ましい。本発明の製造条件における限定理由を以下に説明する。
2) Production conditions In the method for producing a high carbon hot-rolled steel sheet according to the present invention, a steel slab having the above-described composition is hot-rolled at a finishing temperature not lower than the Ar 3 transformation point or Ar cm transformation point, and 60 ° C / s. After the primary cooling to the cooling stop temperature of 550 to 650 ° C at the above average cooling rate, it is allowed to cool for 1.0 to 10 seconds, and then secondary to the cooling stop temperature of 500 to 600 ° C at an average cooling rate of 120 ° C / s or more. Cooling and winding up, and then annealing at a temperature of 640 ° C. or more and Ac 1 transformation point or less. In addition, it is preferable to remove the scale formed in the steel sheet surface layer by annealing or the like before annealing. The reason for limitation in the production conditions of the present invention will be described below.
熱間圧延の仕上温度：Ar3変態点あるいはArcm変態点以上
熱間圧延の仕上温度がAr3変態点あるいはArcm変態点未満では、初析フェライトあるいは初析セメンタイトが一部析出した状態で圧延され、不均一な鋼板組織となって鋼板内の特性の均一性が低下する。そのため、熱間圧延の仕上温度はAr3変態点あるいはArcm変態点以上とする。
Hot rolling finishing temperature: Ar 3 transformation point or Ar cm transformation point or higher If the hot rolling finishing temperature is lower than Ar 3 transformation point or Ar cm transformation point, the proeutectoid ferrite or proeutectoid cementite is partially precipitated It is rolled and becomes a non-uniform steel sheet structure, and the uniformity of the characteristic in a steel plate falls. Therefore, the finishing temperature of hot rolling is set to the Ar 3 transformation point or Ar cm transformation point or higher.
なお、鋼片のAr3変態点あるいはArcm変態点は、例えば、オーステナイト温度域からの冷却過程における熱収縮曲線を測定し、曲線の変化点から求めることができる。 The Ar 3 transformation point or Ar cm transformation point of the steel slab can be obtained, for example, by measuring a heat shrinkage curve in the cooling process from the austenite temperature range and from the curve change point.
一次冷却の平均冷却速度：60℃/s以上
一次冷却の冷却停止温度(放冷温度)：550〜650℃
熱間圧延後の一次冷却は、熱間圧延後、550〜650℃の範囲の冷却停止温度まで、60℃/s以上の平均冷却速度で行う必要がある。本発明では、鋼板内の特性のばらつきを低減しつつ、低温変態相の生成による製造性の低下を回避するため、熱間圧延後の鋼板組織を均一なパーライトを主体とする組織に調製する。そのため、熱間圧延後の冷却過程において、フェライトやセメンタイトの粗大析出を回避することが必須となる。このためには、熱間圧延後の鋼板を60℃/s以上の平均冷却速度で、550〜650℃の冷却停止温度まで速やかに一次冷却する必要がある。
Average cooling rate for primary cooling: 60 ° C / s or more Cooling stop temperature for primary cooling (cooling temperature): 550 to 650 ° C
The primary cooling after the hot rolling needs to be performed at an average cooling rate of 60 ° C./s or higher up to the cooling stop temperature in the range of 550 to 650 ° C. after the hot rolling. In the present invention, the steel sheet structure after hot rolling is prepared into a structure mainly composed of uniform pearlite in order to avoid a decrease in manufacturability due to the generation of a low-temperature transformation phase while reducing variations in characteristics in the steel sheet. Therefore, it is essential to avoid coarse precipitation of ferrite and cementite in the cooling process after hot rolling. For this purpose, it is necessary to quickly firstly cool the steel sheet after hot rolling to a cooling stop temperature of 550 to 650 ° C. at an average cooling rate of 60 ° C./s or more.
一次冷却の平均冷却速度が60℃/s未満の場合には、一次冷却中に初析フェライトや初析セメンタイトが顕著に生成し、均一なミクロ組織が形成されない。また、冷却停止温度が650℃を超えると、60℃/s以上の平均冷却速度で冷却しても、冷却停止後に初析フェライトや初析セメンタイトが多く生成する。一方、冷却停止温度が550℃より低い場合には、ベイナイトやマルテンサイトといった低温変態相が部分的に生成し、鋼板形状が悪化して以降の冷却むら、ひいては鋼板内の特性のばらつきを生ずる可能性がある。なお、初析相の生成を確実に回避し、より均一な鋼板組織を得るためには、一次冷却の平均冷却速度を120℃/s以上とすることが好ましい。 When the average cooling rate of the primary cooling is less than 60 ° C./s, pro-eutectoid ferrite and pro-eutectoid cementite are remarkably generated during the primary cooling, and a uniform microstructure is not formed. In addition, when the cooling stop temperature exceeds 650 ° C., a large amount of pro-eutectoid ferrite and pro-eutectoid cementite is generated after cooling stop even if cooling is performed at an average cooling rate of 60 ° C./s or more. On the other hand, when the cooling stop temperature is lower than 550 ° C, low-temperature transformation phases such as bainite and martensite are partially generated, resulting in uneven cooling after the deterioration of the steel sheet shape, and in turn, variations in characteristics within the steel sheet. There is sex. In order to reliably avoid the generation of the pro-eutectoid phase and obtain a more uniform steel sheet structure, it is preferable to set the average cooling rate of primary cooling to 120 ° C./s or more.
放冷時間：1.0〜10s
一次冷却後、1.0〜10sの間鋼板を放冷する。一次冷却での急冷に続けて放冷することにより、パーライト変態が短時間で進行し、均一なパーライト組織が形成される。本発明の要点は、焼鈍前の高炭素鋼板のミクロ組織を均一なパーライトを主体とする組織に調製することにあり、放冷によるパーライト変態の促進が非常に重要な役割を果たす。
Cooling time: 1.0-10s
After the primary cooling, the steel plate is allowed to cool for 1.0 to 10 seconds. By allowing to cool after the rapid cooling in the primary cooling, the pearlite transformation proceeds in a short time, and a uniform pearlite structure is formed. The main point of the present invention is to prepare the microstructure of the high carbon steel sheet before annealing into a structure mainly composed of uniform pearlite, and promotion of pearlite transformation by cooling is a very important role.
放冷時間が1.0s未満では、上記の変態促進効果が十分に得られない。0.5質量%以上のCを含有する高炭素鋼では、高いC含有量のために焼入性が高く、容易に低温変態相を生成しやすい。そのため、1.0s未満の短時間の放冷では、パーライト変態の促進効果が不十分となり、所望の組織に調製できない。一方、放冷時間が10sを超える場合には、パーライト変態の進行につれて変態発熱による鋼板温度の上昇が生じ、放冷過程の後期に生成するパーライトが粗大化して、鋼板内の特性の不均一化を招く。そのため、放冷時間は1.0〜10sの範囲に限定する。好ましくは3〜8sである。なお、放冷とは、注水等による強制冷却を行わずに鋼板を大気中に暴露することを意味する。ただし、一次冷却工程で注水した冷却水の払拭のため、圧縮空気などの流体を鋼板に向けて短時間噴射することなどは、噴射による冷却効果が十分に小さく、本発明の効果を損なうことがないので許容できる。 When the cooling time is less than 1.0 s, the above-mentioned transformation promoting effect cannot be obtained sufficiently. A high carbon steel containing 0.5 mass% or more of C has high hardenability due to a high C content, and easily forms a low-temperature transformation phase. For this reason, if it is allowed to cool for a short time of less than 1.0 s, the effect of promoting pearlite transformation becomes insufficient, and the desired structure cannot be prepared. On the other hand, when the cooling time exceeds 10 s, the steel plate temperature rises due to transformation heat generation as the pearlite transformation progresses, and the pearlite generated in the latter stage of the cooling process becomes coarse, resulting in non-uniform characteristics in the steel plate. Invite. Therefore, the cooling time is limited to the range of 1.0 to 10 s. Preferably, it is 3 to 8 s. The term “cooling” means that the steel sheet is exposed to the atmosphere without forced cooling by water injection or the like. However, in order to wipe the cooling water injected in the primary cooling process, injecting a fluid such as compressed air toward the steel sheet for a short time may cause a sufficiently small cooling effect by the injection and impair the effect of the present invention. Not acceptable.
二次冷却の平均冷却速度：120℃/s以上
二次冷却の冷却停止温度(巻取温度)：500〜600℃
所定時間放冷した後の鋼板は、120℃/s以上の平均冷却速度で再度冷却し、500〜600℃の冷却停止温度で冷却を停止して巻き取る。放冷後の鋼板は、変態発熱により温度が上昇しているので、鋼板のミクロ組織の粗大化を抑制するため、500〜600℃の温度にまで再度冷却してから巻き取る。冷却停止温度が600℃を超える場合には、粗大なパーライトが生成しやすくなり、鋼板組織の不均一化を完全に回避することができない。一方、冷却停止温度が500℃未満の場合には、ベイナイトやマルテンサイトといった低温変態相が生成し、鋼板が過度に硬化して巻形状が悪化するとともに、加工性の大幅な低下を招く。低温変態相主体の組織には、焼鈍後にセメンタイトが微細に分散しやすいといった利点もあるが、0.5質量%以上のCを含有する高炭素鋼では、高いC含有量のために低温変態相の硬度が高く、鋼板の製造性や加工性の低下が許容できないので、冷却停止温度は500℃以上に限定する。
Secondary cooling average cooling rate: 120 ° C / s or more Secondary cooling cooling stop temperature (winding temperature): 500-600 ° C
The steel sheet after being allowed to cool for a predetermined time is cooled again at an average cooling rate of 120 ° C./s or more, and the cooling is stopped at a cooling stop temperature of 500 to 600 ° C. Since the temperature of the steel sheet after being allowed to cool is increased by transformation heat generation, the steel sheet is cooled again to a temperature of 500 to 600 ° C. and wound up in order to suppress the coarsening of the microstructure of the steel sheet. When the cooling stop temperature exceeds 600 ° C., coarse pearlite is likely to be generated, and the unevenness of the steel sheet structure cannot be completely avoided. On the other hand, when the cooling stop temperature is less than 500 ° C., a low-temperature transformation phase such as bainite or martensite is generated, the steel sheet is excessively hardened and the winding shape is deteriorated, and the workability is greatly reduced. The structure mainly composed of low-temperature transformation phase has the advantage that cementite tends to be finely dispersed after annealing, but high carbon steel containing 0.5 mass% or more of C has a low C-phase hardness due to its high C content. Therefore, the cooling stop temperature is limited to 500 ° C. or higher.
鋼板組織の均一性を高めるためには、放冷後の平均冷却速度を120℃/s以上にする必要がある。一般的な注水による冷却の場合、500〜600℃の温度域は、膜沸騰から核沸騰への遷移が始まる領域となるため、鋼板の冷却むらが発生し易い。このような温度域では、平均冷却速度が120℃/s以上となるように核沸騰主体の条件で水冷すると、鋼板の冷却むらが発生し難くなり、鋼板内の特性のばらつきを小さく抑えることができる。平均冷却速度が240℃/s以上の水冷であればより好ましい。 In order to improve the uniformity of the steel sheet structure, the average cooling rate after cooling must be 120 ° C./s or more. In the case of cooling by general water injection, since the temperature range of 500 to 600 ° C. is a region where the transition from film boiling to nucleate boiling starts, uneven cooling of the steel sheet is likely to occur. In such a temperature range, if water cooling is performed under conditions mainly consisting of nucleate boiling so that the average cooling rate is 120 ° C./s or more, uneven cooling of the steel sheet is less likely to occur, and variation in characteristics within the steel sheet can be suppressed to a small level. it can. Water cooling with an average cooling rate of 240 ° C./s or more is more preferable.
焼鈍温度：640℃以上Ac1変態点以下
巻き取り後の熱延鋼板は、セメンタイトの球状化を図るために焼鈍する。このとき、焼鈍温度が640℃未満では、セメンタイトの球状化が速やかに進行しない。また、焼鈍温度がAc1変態点を超えると、焼鈍中に鋼板組織が一部再オーステナイト化した後で冷却されるため、焼鈍後の鋼板組織中にパーライト、すなわち球状化されていないセメンタイトが混在し、鋼板内の特性の均一性とともに、加工性や焼入性が低下する。よって、焼鈍温度は、640℃以上Ac1変態点以下の範囲に限定する。好ましくは、680℃以上Ac1変態点以下である。
Annealing temperature: 640 ° C. or higher and Ac 1 transformation point or lower The hot-rolled steel sheet after winding is annealed in order to spheroidize cementite. At this time, when the annealing temperature is less than 640 ° C., spheroidization of cementite does not proceed rapidly. Also, if the annealing temperature exceeds the Ac 1 transformation point, the steel sheet structure is cooled after being partially re-austenitic during annealing, so pearlite, that is, non-spheroidized cementite, is mixed in the steel sheet structure after annealing. However, workability and hardenability are reduced along with the uniformity of characteristics in the steel sheet. Therefore, the annealing temperature is limited to a range of 640 ° C. or higher and Ac 1 transformation point or lower. Preferably, it is 680 ° C. or more and Ac 1 transformation point or less.
本発明では、焼鈍前の熱延鋼板の組織を均一なパーライトを主体とする組織に調製しているため、セメンタイトの球状化が効率的に進行するので、熱延鋼板を焼鈍温度に保持する時間については、10時間以上とればよい。望ましくは15〜35時間である。焼鈍後の鋼板には、鋼板の形状矯正あるいは表面性状調整のため、必要に応じて調質圧延を施すことができる。 In the present invention, since the structure of the hot-rolled steel sheet before annealing is prepared as a structure mainly composed of uniform pearlite, the spheroidization of cementite proceeds efficiently, so the time for holding the hot-rolled steel sheet at the annealing temperature About 10 hours or more. Desirably, it is 15 to 35 hours. The annealed steel sheet can be subjected to temper rolling as necessary for correcting the shape of the steel sheet or adjusting the surface properties.
なお、鋼板のAc1変態点は、例えば、常温からの加熱過程における熱膨張曲線を測定し、曲線の変化点から求めることができる。 The Ac 1 transformation point of the steel sheet can be determined from, for example, a curve expansion point by measuring a thermal expansion curve in a heating process from room temperature.
本発明で用いる高炭素鋼の溶製には、転炉または電炉のどちらも使用可能である。溶製された鋼は、連続鋳造あるいは造塊後の分塊圧延により鋼片(スラブ)とされる。熱間圧延前の鋼片は、製造設備の能力に応じて、所定の仕上温度が確保できる温度に加熱すればよい。連続鋳造された鋼片を常温まで冷却することなく直接あるいは短時間の加熱の後に熱間圧延してもよい。また、バーヒーターやエッヂヒーターのような誘導加熱装置により、熱間圧延途中の鋼片を追加加熱することも可能である。 Either a converter or an electric furnace can be used for melting the high carbon steel used in the present invention. The molten steel is made into a steel slab (slab) by continuous casting or ingot rolling after ingot forming. What is necessary is just to heat the steel slab before hot rolling to the temperature which can ensure a predetermined finishing temperature according to the capability of manufacturing equipment. The continuously cast steel slab may be hot-rolled directly or after heating for a short time without cooling to room temperature. It is also possible to additionally heat the steel slab during hot rolling with an induction heating device such as a bar heater or an edge heater.
表1に示す元素を含有し、残部がFeおよび不可避的不純物からなる組成を有し、同じく表1に示すAr3変態点あるいはArcm変態点、およびAc1変態点を有する鋼片A〜Lを、表2に示す熱延条件にて板厚4.0mmの熱延鋼板とした後、酸洗によりスケールを除去し、同じく表2に示す焼鈍条件にて窒素雰囲気中で焼鈍し、次いで伸張率0.5%の調質圧延を施して、鋼板1〜24を得た。なお、表1中の各変態点は、前記した方法により求めた。 Steel slabs A to L containing the elements shown in Table 1, the balance being composed of Fe and inevitable impurities, and also having the Ar 3 transformation point or Ar cm transformation point and Ac 1 transformation point shown in Table 1. After forming a hot-rolled steel sheet having a thickness of 4.0 mm under the hot rolling conditions shown in Table 2, the scale was removed by pickling, and annealing was performed in a nitrogen atmosphere under the annealing conditions shown in Table 2, followed by the elongation rate. Steel sheets 1 to 24 were obtained by temper rolling of 0.5%. Each transformation point in Table 1 was determined by the method described above.
得られた各鋼板から圧延方向に平行な板厚断面調査用のサンプルを採取し、次のようにして、板厚断面におけるビッカース硬さとセメンタイトの平均径を測定して、鋼板内の特性のばらつき、および加工性と焼入性を評価した。結果を表3に示す。 Samples for the thickness cross-section investigation parallel to the rolling direction were collected from each obtained steel plate, and the Vickers hardness and the average diameter of cementite in the plate thickness cross-section were measured as follows, and the dispersion of the characteristics in the steel plate And processability and hardenability were evaluated. The results are shown in Table 3.
板厚断面におけるビッカース硬さ：圧延方向に平行な板厚断面を鏡面研磨し、板厚の1/4深さの位置にて、JIS Z 2244の規定に準拠し、9.8N(1kgf)の試験力で測定した。各々のサンプルあたり5回以上測定し、それらの平均値をそのサンプルの板厚断面におけるビッカース硬さHVとした。このビッカース硬さHVの測定を、鋼板板幅の1/8、1/4、3/8、1/2、5/8、3/4、7/8の計7位置で採取したサンプルで行い、全7位置のHVの値の最大値(HVmax)と最小値(HVmin)の差(ΔHV=HVmax-HVmin)と全7位置のHVの値の平均値(HVave)を求め、ΔHV/HVaveの値を鋼板内の特性の均一性の指標とした。ΔHV/HVaveの値が0.10以下であれば、鋼板内の特性のばらつきが小さいと評価した。 Vickers hardness in the plate thickness section: The plate thickness section parallel to the rolling direction is mirror-polished and tested at 9.8 N (1 kgf) according to the JIS Z 2244 standard at a depth of 1/4 the plate thickness. Measured by force. Each sample was measured 5 times or more, and the average value thereof was defined as the Vickers hardness HV in the plate thickness section of the sample. This Vickers hardness HV is measured on samples taken at a total of 7 positions: 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, 7/8 of the plate width. Then, the difference between the maximum value (HVmax) and the minimum value (HVmin) (ΔHV = HVmax-HVmin) of the HV values at all 7 positions and the average value (HVave) of the HV values at all 7 positions are calculated, and ΔHV / HVave The value was used as an index of the uniformity of characteristics in the steel sheet. If the value of ΔHV / HVave was 0.10 or less, it was evaluated that the variation in characteristics in the steel sheet was small.
板厚断面におけるセメンタイトの平均径：鋼板板幅の1/4位置で採取したサンプルの圧延方向に平行な板厚断面を鏡面研磨し、ピクラールで腐食した後、板厚の1/4深さの位置を走査型電子顕微鏡にて5000倍の倍率で撮影した組織写真を用いて、個々のセメンタイト粒子の長径と短径の相乗平均を個々のセメンタイト粒子の粒径とし、組織写真の視野内にあるセメンタイト粒子の粒径の平均値をもって、その鋼板のセメンタイトの平均径dとした。このセメンタイトの平均径dは、粒子分散による強化量や加工時の応力集中度、焼入加熱時の分解難易度の目安となるため、加工性および焼入性の指標として利用でき、dの値が0.5〜2.0μmの場合に、加工性および焼入性に優れると評価した。 The average diameter of cementite in the plate thickness section: After the sample thickness sample parallel to the rolling direction of the sample taken at 1/4 position of the plate width is mirror-polished and corroded with Picral, Using a structure photograph taken at a magnification of 5000 times with a scanning electron microscope, the geometric mean of the long diameter and short diameter of each cementite particle is the particle diameter of each cementite particle, and is within the field of view of the structure photograph. The average value of the cementite particles was defined as the average diameter d of the cementite of the steel sheet. The average diameter d of cementite can be used as an index of workability and hardenability because it is a measure of the amount of strengthening due to particle dispersion, the degree of stress concentration during processing, and the difficulty of decomposition during quenching heating. Was 0.5 to 2.0 μm, it was evaluated that it was excellent in workability and hardenability.
表3に示す本発明例の鋼板は、鋼板内の特性の均一性の指標としたΔHV/HVaveの値が0.10以下であって鋼板内の特性のばらつきが小さく、鋼板の加工性と焼入性の指標としたセメンタイトの平均径dの値も0.5〜2.0μmとなっており、加工性と焼入性にも優れた高炭素熱延鋼板となっている。 The steel sheet of the present invention example shown in Table 3 has a ΔHV / HVave value of 0.10 or less as an index of the uniformity of the characteristics in the steel sheet, and there is little variation in the characteristics in the steel sheet, and the workability and hardenability of the steel sheet. The value of the average diameter d of cementite used as an index of the steel is 0.5 to 2.0 μm, and it is a high carbon hot rolled steel sheet excellent in workability and hardenability.
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