JP2010215961A - Steel sheet of boron steel superior in hardenability, and manufacturing method therefor - Google Patents
Steel sheet of boron steel superior in hardenability, and manufacturing method therefor Download PDFInfo
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本発明は、良好な加工性と焼入性を兼備し、自動車部品をはじめとする各種機械部品に適したボロン鋼の鋼板およびその製造方法に関する。 The present invention relates to a boron steel plate suitable for various machine parts such as automobile parts, and a method for producing the same, having both good workability and hardenability.
少量のＢを添加した炭素鋼（ボロン鋼）は焼入性に優れることから、板厚中心部まで十分に焼きが入る（マルテンサイト組織となる）ことが要求される種々の部品用途において、低コスト材として使用されている。ボロン鋼の焼鈍鋼板は、打抜き、曲げなどのプレス成形や、鍛造、切削などの工程により所定の部品形状に加工されたのち、焼入れ・焼戻し、浸炭焼入れ等の熱処理を施して使用される。したがって、素材であるボロン鋼鋼板には加工性、焼入性、および用途に応じた熱処理後の特性が要求される。 Since carbon steel (boron steel) to which a small amount of B is added has excellent hardenability, it is low in various parts applications that require sufficient quenching to the center of the plate thickness (becomes a martensite structure). Used as a cost material. An annealed steel sheet of boron steel is used after being processed into a predetermined part shape by press forming such as punching and bending, forging and cutting, and then subjected to heat treatment such as quenching / tempering and carburizing and quenching. Therefore, the boron steel sheet as a material is required to have workability, hardenability, and characteristics after heat treatment according to the application.
しかしながら、ボロン鋼鋼板には以下のような問題が生じている。
（i）鋼板の製造段階、あるいは部品成形後の焼入れ時に施される高温加熱によって、鋼材表層部の固溶Ｂが減少する（脱Ｂ）。
（ii）脱Ｂによって表層部の焼入性が低下し、焼入れの際に異常層を生じて表面硬さが低下する。
（iii）特に板厚が大きい場合、焼入れ時の冷却速度が低下するため、表面硬さの低下が顕著になる。場合によっては板厚中心部付近の断面硬さも低下して素材そのものの強度が不足することもある。
However, the following problems occur in the boron steel sheet.
(I) The solid solution B of the steel surface layer portion is reduced by high-temperature heating performed at the time of manufacturing the steel sheet or at the time of quenching after forming the part (de-B).
(Ii) The hardenability of the surface layer portion is reduced by removing B, an abnormal layer is formed during quenching, and the surface hardness is reduced.
(Iii) Especially when the plate thickness is large, the cooling rate at the time of quenching is lowered, so that the surface hardness is significantly lowered. In some cases, the cross-sectional hardness in the vicinity of the central portion of the plate thickness is also reduced, and the strength of the material itself may be insufficient.
表面硬さの低下について、これまでにも対応策が提案されている。例えば特許文献１には、Ｃ含有量が０.１５〜０.３５重量％のボロン鋼を使用し、保護雰囲気中で雰囲気ガスのカーボンポテンシャルを素材炭素量より０.０４〜０.２５％高く設定し、部品表面を微浸炭させた後、焼入れすることにより、脱Ｂによる表面部の焼入性低下を補う手法が開示されている。しかし、この方法では浸炭処理のための設備が必要となる。また、高周波焼入れ、電子ビーム焼入れなど、短時間加熱の局部焼入れにはこの手法は適用できない。 Countermeasures have been proposed for the reduction in surface hardness. For example, in Patent Document 1, boron steel having a C content of 0.15 to 0.35 wt% is used, and the carbon potential of the atmospheric gas in the protective atmosphere is 0.04 to 0.25% higher than the material carbon content. A method is disclosed that compensates for the hardenability deterioration of the surface portion due to de-B by setting and finely carburizing the surface of the component and then quenching. However, this method requires equipment for carburizing treatment. Further, this method cannot be applied to local quenching such as induction quenching and electron beam quenching for a short time.
また、特許文献２には、Ｃ含有量が０.１〜０.３質量％のボロン鋼を使用し、Ｔｉ、Ｎｂの炭窒化物を微細析出させることによって浸炭時における粗大粒の発生を防止し、さらにＣｒ、Ｍｏを添加して脱Ｂによる焼入性低下を補い、表面から深さ０.２〜０.７ｍｍに生成する不完全焼入れ組織の生成を防止している。しかし、この方法ではＴｉ、Ｎｂの炭窒化物を微細析出させるために８００〜５００℃の温度範囲を１℃／ｓｅｃ以下の冷却速度で徐冷する必要があり、製造性に劣る。また、浸炭を施さない焼入れ時に問題となる異常層については考慮されていない。 Patent Document 2 uses boron steel having a C content of 0.1 to 0.3% by mass, and prevents the formation of coarse grains during carburization by fine precipitation of carbonitrides of Ti and Nb. In addition, Cr and Mo are further added to compensate for the hardenability deterioration due to de-B, thereby preventing the formation of an incompletely hardened structure formed to a depth of 0.2 to 0.7 mm from the surface. However, in this method, in order to finely precipitate carbonitrides of Ti and Nb, it is necessary to gradually cool the temperature range of 800 to 500 ° C. at a cooling rate of 1 ° C./sec or less, resulting in poor productivity. Moreover, the abnormal layer which becomes a problem at the time of hardening which does not carburize is not considered.
上記のように、ボロン鋼の焼入れ時における表面硬さ低下に対しては、浸炭処理を施して対応することが多い。しかし、浸炭処理は設備投資、処理時間、処理コストにおいて問題があり、安価な部品が要求される用途では必ずしも容易に採用できるものではない。
本発明はこのような現状に鑑み、浸炭に頼ることなく、通常の焼入れ処理によって部品表層部の高い焼入れ硬さが得られ、かつ加工性の良いボロン鋼鋼板を提供しようというものである。
As described above, the reduction in surface hardness during quenching of boron steel is often dealt with by carburizing treatment. However, the carburizing process has problems in capital investment, processing time, and processing cost, and is not always easily adopted in applications that require inexpensive parts.
In view of such a current situation, the present invention is intended to provide a boron steel sheet that can obtain a high quenching hardness of a component surface layer portion by a normal quenching process and has good workability without depending on carburization.
上記目的は、質量％で、Ｃ：０.１０〜０.４０％、Ｓｉ：０.５０％以下、Ｍｎ：０.５０〜１.６０％、Ｃｒ：０.０５〜１.５０％、Ｔｉ：０.０１〜０.３０％、Ｂ：０.０００５〜０.００５０％、Ｐ：０.０３％以下、Ｓ：０.０１％以下であり、必要に応じてＭｏ：０.３％以下、Ｎｉ：２.０％以下の１種以上を含有し、残部がＦｅおよび不可避的不純物、かつ下記（１）式で定義されるＸ値が２４以上である組成を有する焼鈍鋼板であって、鋼板の断面組織において、表面からの深さが２５〜７５μｍの表層部領域における炭化物の面積率Ａｓと、板厚中心位置を含む板厚方向長さ５０μｍの中心部領域における炭化物の面積率Ａｃの比Ａｓ／Ａｃが０.９０以上である鋼板表面の焼入性に優れたボロン鋼鋼板によって達成される。
Ｘ＝５.５Ｃ1/2（１＋０.６Ｓｉ）（１＋４.１Ｍｎ）（１＋０.５Ｎｉ）（１＋２.３Ｃｒ）（１＋３.１Ｍｏ） …（１）
ここで、（１）式の元素記号の箇所には質量％で表された当該元素の含有量値が代入され、無添加元素の元素記号の箇所には０（ゼロ）が代入される。
本発明の対象となる鋼板の板厚は、例えば１〜１２ｍｍである。
The purpose is mass%, C: 0.10 to 0.40%, Si: 0.50% or less, Mn: 0.50 to 1.60%, Cr: 0.05 to 1.50%, Ti : 0.01 to 0.30%, B: 0.0005 to 0.0050%, P: 0.03% or less, S: 0.01% or less, Mo: 0.3% or less as required Ni: An annealed steel sheet containing at least 2.0% or less, the balance being Fe and inevitable impurities, and a composition having an X value defined by the following formula (1) of 24 or more, In the cross-sectional structure of the steel sheet, the carbide area ratio As in the surface layer region having a depth from the surface of 25 to 75 μm and the carbide area ratio Ac in the central region having a plate thickness direction length of 50 μm including the plate thickness center position. This is achieved by a boron steel sheet having a ratio As / Ac of 0.90 or more and excellent in hardenability of the steel sheet surface.
X = 5.5C 1/2 (1 + 0.6Si) (1 + 4.1Mn) (1 + 0.5Ni) (1 + 2.3Cr) (1 + 3.1Mo) (1)
Here, the content value of the element expressed in mass% is assigned to the location of the element symbol in the formula (1), and 0 (zero) is assigned to the location of the element symbol of the additive-free element.
The plate | board thickness of the steel plate used as the object of this invention is 1-12 mm, for example.
また本発明では、上記のボロン鋼鋼板の製造方法として、
１１５０〜１３２０℃に加熱されたスラブを仕上温度８００〜９００℃で熱間圧延したのち、仕上温度から７００℃までの温度域の滞在時間を３０ｓｅｃ以下として、巻取温度５００〜６５０℃で巻取り、その後、酸洗を施して熱延酸洗鋼板とする工程（熱延酸洗工程）、
前工程で得られた鋼板を６００℃以上Ａｃ1未満の温度範囲で０.５ｈ以上均熱保持して焼鈍する工程（焼鈍工程）、
を有する手法が提供される。
Moreover, in the present invention, as a manufacturing method of the above boron steel sheet,
A slab heated to 1150 to 1320 ° C. is hot-rolled at a finishing temperature of 800 to 900 ° C., and then the residence time in the temperature range from the finishing temperature to 700 ° C. is set to 30 sec or less, and the slab is wound at a winding temperature of 500 to 650 ° C. Then, the process (hot-roll pickling process) which gives pickling and makes it a hot-rolled pickled steel sheet,
A step (annealing step) of annealing the steel plate obtained in the previous step by holding the steel plate for 0.5 h or more in a temperature range of 600 ° C. or higher and less than Ac 1 ;
An approach is provided.
前記焼鈍工程に代えて、
前工程で得られた鋼板を（Ａｃ1−３０℃）以上Ａｃ1未満の温度範囲で０.５ｈ以上均熱保持したのち、Ａｃ1以上（Ａｃ1＋５０℃）の温度範囲で０.５〜２０ｈ均熱保持し、その保持温度から少なくとも（Ａｒ1−１０℃）までを冷却速度５〜３０℃／ｈで徐冷するヒートパターンで焼鈍する工程（焼鈍工程）、
を採用することがより効果的である。
また、前記熱延酸洗工程と焼鈍工程の間に、
前工程で得られた鋼板を冷間圧延する工程（冷延工程）、
を入れることができる。
Instead of the annealing step,
The steel plate obtained in the previous step is soaked for 0.5 h or more in a temperature range of (Ac 1 -30 ° C.) or more and less than Ac 1 and then 0.5 to 0.5 in a temperature range of Ac 1 or more (Ac 1 + 50 ° C.). A step of annealing for 20 h soaking and annealing at a cooling rate of 5 to 30 ° C./h from the holding temperature to at least (Ar 1 -10 ° C.) (annealing step),
It is more effective to adopt
Also, between the hot-roll pickling process and the annealing process,
Cold rolling the steel plate obtained in the previous process (cold rolling process),
Can be entered.
本発明のボロン鋼鋼板は、従来のボロン鋼鋼板に比べ表層部の焼入性に優れ、また加工性（延性）も良好である。鋼板製造段階および部品製造段階において新たな製造工程を伴うこともないので、製造コストの上昇も抑えられる。したがって本発明は、自動車部品をはじめとする各種機械部品の用途において、ボロン鋼の普及に寄与しうる。 The boron steel sheet of the present invention is superior in hardenability of the surface layer portion and has good workability (ductility) as compared with the conventional boron steel sheet. Since a new manufacturing process is not involved in the steel plate manufacturing stage and the parts manufacturing stage, an increase in manufacturing cost can be suppressed. Therefore, the present invention can contribute to the spread of boron steel in applications of various machine parts including automobile parts.
発明者らの検討によれば、ボロン鋼に特有の脱Ｂ現象そのものを防止することは現状の製造プロセスにおいて極めて困難であり、脱Ｂが生じること自体はある程度許容することが得策である。一方、炭素鋼材の製造プロセスでは、通常、鋼材表層部の炭素も減少する（脱Ｃ）。そこで発明者らは詳細な検討の結果、ボロン鋼の表層部焼入性に関し、以下の知見を得た。 According to the study by the inventors, it is extremely difficult to prevent the de-B phenomenon that is unique to boron steel in the current manufacturing process, and it is advisable to allow the occurrence of de-B to some extent. On the other hand, in the carbon steel material manufacturing process, carbon in the steel material surface layer portion is also usually reduced (de-C). As a result of detailed studies, the inventors have obtained the following knowledge regarding the hardenability of the surface layer of boron steel.
［１］脱Ｂによって生じる焼入性の低下は、化学組成を厳密にコントロールすることにより補完できる。
［２］通常の鋼板製造工程において、脱Ｃは熱延工程の仕上圧延終了後、巻取までの間に最も生じやすい。上記［１］の組成面での対策を講じた上で、熱延工程で不可避的に生じる脱Ｃをできるだけ軽減することにより、浸炭等のＣ濃化手段に頼ることなく、表層部焼入性を十分に確保することができる。
本発明はこのような知見に基づいて完成したものである。以下、本発明を特定するための事項について説明する。
[1] The decrease in hardenability caused by de-B can be complemented by strictly controlling the chemical composition.
[2] In a normal steel sheet manufacturing process, de-C is most likely to occur after finishing rolling in the hot rolling process and before winding. After taking measures on the composition aspect of [1] above, by reducing as much as possible de-C that is unavoidable in the hot rolling process, the surface layer hardenability without relying on C concentration means such as carburizing. Can be secured sufficiently.
The present invention has been completed based on such findings. Hereinafter, matters for specifying the present invention will be described.
〔化学組成〕
本明細書において鋼組成における「％」は特に断らない限り「質量％」を意味する。
Ｃ：０.１０〜０.４０％
Ｃは機械構造用部品としての芯部強度を確保するために必要な元素である。十分な強度を確保するためには０.１０％以上のＣ含有量が必要である。ただし、Ｃ含有量が多くなると焼鈍後の加工性が低下する。本発明では、自動車部品をはじめとする各種機械部品に幅広く適用できる加工性を持たせることを考慮して、Ｃ含有量は０.４０％以下の範囲とする。
[Chemical composition]
In this specification, “%” in the steel composition means “% by mass” unless otherwise specified.
C: 0.10 to 0.40%
C is an element necessary for securing the core strength as a machine structural component. In order to ensure sufficient strength, a C content of 0.10% or more is necessary. However, if the C content increases, the workability after annealing decreases. In the present invention, the C content is set to a range of 0.40% or less in consideration of providing workability that can be widely applied to various machine parts including automobile parts.
Ｓｉ：０.５０％以下
Ｓｉは脱酸効果があるが、過剰に添加すると加工性を低下させる。また焼鈍時に粒界酸化層の形成を助長し、調質後の部材の疲労特性を低下させる。そのためＳｉ含有量は０.５％以下とする。
Si: 0.50% or less Si has a deoxidizing effect, but if added excessively, workability is lowered. Further, it promotes the formation of a grain boundary oxide layer during annealing, and reduces the fatigue characteristics of the tempered member. Therefore, the Si content is set to 0.5% or less.
Ｍｎ：０.５０〜１.６０％
Ｍｎは脱酸・脱硫、および焼入性の向上に有効であり、これらの作用を十分に発揮させるために０.５０％以上のＭｎ含有量を確保する。ただし、過剰のＭｎ含有は焼鈍材を硬質化させ、加工性の低下を招く要因となるので、Ｍｎ含有量は１.６０％以下の範囲に制限される。
Mn: 0.50 to 1.60%
Mn is effective for deoxidation / desulfurization and improvement of hardenability, and a Mn content of 0.50% or more is ensured in order to fully exhibit these functions. However, excessive Mn content hardens the annealed material and causes a decrease in workability, so the Mn content is limited to a range of 1.60% or less.
Ｃｒ：０.０５〜１.５０％
Ｃｒは焼入性を向上させ、強度や耐摩耗性を向上させる作用を有する。ただし０.０５％未満ではその作用は十分に発揮されない。一方、過剰のＣｒ添加は焼鈍材を硬質化させ、加工性の低下を招く要因となる。またＣｒは炭化物を安定化させる作用を有するので、Ｃｒ含有量が多いとＡｃ1点以上への加熱を利用した焼鈍を施しても炭化物を固溶させるのに長時間を要する。種々検討の結果、Ｃｒ含有量は１.５０％以下の範囲に制限される。
Cr: 0.05 to 1.50%
Cr has an effect of improving hardenability and improving strength and wear resistance. However, if it is less than 0.05%, the effect is not sufficiently exhibited. On the other hand, excessive addition of Cr hardens the annealed material and causes a decrease in workability. Further, since Cr has an action of stabilizing carbides, if the Cr content is large, it takes a long time to dissolve the carbides even if annealing is performed using heating to the Ac 1 point or higher. As a result of various studies, the Cr content is limited to a range of 1.50% or less.
Ｔｉ：０.０１〜０.３０％
Ｔｉは鋼中のＮと結合してＴｉＮとして析出する。このためＴｉは、ＢＮの析出を防止して焼入性の向上に有効な固溶Ｂを確保する上で有効である。またＴｉは焼入れ時のオーステナイト結晶粒径を微細化させる作用を有する。これらの作用を十分に発揮させるためには０.０１％以上のＴｉ含有が必要である。ただし、Ｔｉ含有量が多くなるとＴｉＣが過剰に析出し、加工性を低下させる。検討の結果、Ｔｉ含有量は０.３０％以下の範囲とする。
Ti: 0.01 to 0.30%
Ti combines with N in the steel and precipitates as TiN. For this reason, Ti is effective in preventing precipitation of BN and ensuring solid solution B effective in improving hardenability. Ti also has the effect of refining the austenite grain size during quenching. In order to fully exhibit these actions, it is necessary to contain 0.01% or more of Ti. However, when the Ti content is increased, TiC is excessively precipitated and the workability is lowered. As a result of the study, the Ti content is set to a range of 0.30% or less.
Ｂ：０.０００５〜０.００５０％
Ｂは微量の添加によって焼入性を著しく向上させる元素である。鋼中に存在するＢのうち、焼入性の向上に有効な固溶Ｂ量を十分に確保するためには、０.０００５％以上のＢを含有させる必要がある。ただし、過剰のＢ含有は鋼の靭性を阻害する要因となるので、Ｂ含有量は０.００５０％以下の範囲とする。
B: 0.0005 to 0.0050%
B is an element that remarkably improves hardenability by adding a small amount. In order to sufficiently secure a solid solution B amount effective for improving hardenability among B present in steel, it is necessary to contain 0.0005% or more of B. However, since excessive B content becomes a factor which inhibits the toughness of steel, B content shall be 0.0050% or less of range.
Ｐ：０.０３％以下
Ｐは鋼の靭性に悪影響を与える元素であり、含有量は少ないほうが望ましいが、本発明の成分系では０.０３％まで許容される。
P: 0.03% or less P is an element that adversely affects the toughness of steel, and it is desirable that the content be small. However, in the component system of the present invention, up to 0.03% is allowed.
Ｓ：０.０１％以下
Ｓも鋼の靭性に悪影響を与える元素であり、含有量は少ないほうが望ましいが、本発明の成分系では０.０１％まで許容される。
S: 0.01% or less S is also an element that adversely affects the toughness of steel, and it is desirable that the content be small, but in the component system of the present invention, up to 0.01% is allowed.
Ｍｏ：０.３％以下
Ｍｏは焼入性を向上させる元素である。またＮｉとの複合添加で鋼の強度・靭性を高める作用を有する他、特殊炭化物を形成することによって耐摩耗性を向上させる作用もある。このため、本発明では必要に応じてＭｏを含有させることができる。これらの作用を十分に得るためには０.０５％以上のＭｏ含有量を確保することがより効果的である。ただしＭｏは高価な元素であり、Ｍｏを添加する場合は０.３％以下の範囲で行う。
Mo: 0.3% or less Mo is an element that improves hardenability. In addition to the effect of increasing the strength and toughness of steel by the combined addition with Ni, it also has the effect of improving wear resistance by forming special carbides. For this reason, in this invention, Mo can be contained as needed. In order to sufficiently obtain these functions, it is more effective to secure a Mo content of 0.05% or more. However, Mo is an expensive element, and when Mo is added, it is performed within a range of 0.3% or less.
Ｎｉ：２.０％以下
Ｎｉは焼入性・靭性を向上させる元素であり、本発明では必要に応じて添加することができる。その作用を十分に得るためには０.５％以上のＮｉ含有量を確保することがより効果的である。ただし、あまり多量に添加してもコストに見合った靭性改善効果は期待できない。Ｎｉを添加する場合は２.０％以下の範囲で行う。
Ni: 2.0% or less Ni is an element that improves hardenability and toughness, and can be added as necessary in the present invention. In order to obtain the effect sufficiently, it is more effective to secure a Ni content of 0.5% or more. However, even if added too much, the effect of improving toughness commensurate with the cost cannot be expected. When adding Ni, it is performed within a range of 2.0% or less.
本発明では各合金成分の含有量が上記の範囲において、さらに下記（１）式で定義されるＸ値が２４以上となるように成分調整されていることが重要である。
Ｘ＝５.５Ｃ1/2（１＋０.６Ｓｉ）（１＋４.１Ｍｎ）（１＋０.５Ｎｉ）（１＋２.３Ｃｒ）（１＋３.１Ｍｏ） …（１）
このＸ値は、本発明の成分系において、脱Ｂが生じることを前提とした場合の、表層部の焼入性を評価する指標である。発明者らは詳細な検討の結果、このＸ値が２４以上となる成分組成に調整したとき、通常の焼入れ処理後の断面硬さにおいて、表層部の硬さ低下が顕著に抑制できることを見出した。
In the present invention, it is important that the content of each alloy component is adjusted so that the X value defined by the following formula (1) is 24 or more in the above range.
X = 5.5C 1/2 (1 + 0.6Si) (1 + 4.1Mn) (1 + 0.5Ni) (1 + 2.3Cr) (1 + 3.1Mo) (1)
This X value is an index for evaluating the hardenability of the surface layer portion on the assumption that de-B occurs in the component system of the present invention. As a result of detailed studies, the inventors have found that when the X value is adjusted to a component composition of 24 or more, a decrease in the hardness of the surface layer can be significantly suppressed in the cross-sectional hardness after normal quenching treatment. .
〔金属組織〕
本発明のボロン鋼鋼板は、フェライトマトリクス中に炭化物が分散した焼鈍組織を有しているおり、かつ鋼板断面の表層部において脱Ｃが抑制されている。脱Ｃの程度は、表層部と板厚中心部の炭化物量を比較することによって知ることができる。発明者らの検討によれば、前述の成分組成を有する鋼において、表面からの深さが５０μｍ程度の位置における炭化物量が、板厚中心部に対して９０％以上に維持されていれば、焼入れ後に問題となる表層部の硬さ低下は顕著に改善されることがわかった。
[Metal structure]
The boron steel sheet of the present invention has an annealed structure in which carbides are dispersed in a ferrite matrix, and de-C is suppressed at the surface layer portion of the steel sheet cross section. The degree of de-C can be known by comparing the amount of carbide in the surface layer portion and the center portion of the plate thickness. According to the study by the inventors, in the steel having the above-described composition, if the carbide amount at a position where the depth from the surface is about 50 μm is maintained at 90% or more with respect to the center portion of the plate thickness, It turned out that the hardness reduction of the surface layer part which becomes a problem after quenching is remarkably improved.
本発明では、鋼板断面における表層部の炭化物量の指標として、表面からの深さが２５〜７５μｍの領域における炭化物の面積率Ａｓ（％）を採用する。また、板厚中心部の炭化物量の指標として、板厚中心位置を含む板厚方向長さ５０μｍの領域における炭化物の面積率Ａｃ（％）を採用する。そして、表層部と中心部の炭化物面積率の比Ａｓ／Ａｃによって脱Ｃの程度を評価する。このＡｓ／Ａｃ値が０.９０以上であるとき、前述の限定された化学組成との相乗効果により、焼入れ後の表層部の硬化不足（中央部に対する硬さ低下）が顕著に抑制される。その際、良好な加工性も維持される。 In the present invention, the carbide area ratio As (%) in a region having a depth of 25 to 75 μm from the surface is employed as an index of the carbide amount of the surface layer portion in the cross section of the steel sheet. Further, the carbide area ratio Ac (%) in the region of the plate thickness direction length 50 μm including the plate thickness center position is adopted as an index of the carbide amount in the plate thickness center portion. Then, the degree of de-C is evaluated by the ratio As / Ac of the carbide area ratio between the surface layer portion and the central portion. When this As / Ac value is 0.90 or more, due to a synergistic effect with the above-described limited chemical composition, insufficient hardening of the surface layer portion after quenching (decrease in hardness with respect to the central portion) is remarkably suppressed. At that time, good workability is also maintained.
以下、本発明のボロン鋼鋼板を得るための製造方法を例示する。
〔熱延酸洗工程〕
熱延前のスラブ加熱温度は一般的な炭素鋼と同様に１１５０〜１３２０℃とすればよい。熱延仕上温度は８００〜９００℃とする。８００℃を下回ると変形抵抗が大きくなり通板性が低下し、また巻取温度５００℃以上を確保することが難しくなる。９００℃を超えるとオーステナイト粒径が粗大化して熱延材の靱性が低下する。巻取温度は５００〜６５０℃とする。５００℃を下回ると熱延材が硬化して製造性が低下する。６５０℃を上回ると初析フェライトの量が増加し、セメンタイトの分布が不均一になることに加え、パーライトのラメラー間隔が大きくなるため焼鈍によるセメンタイトの球状化が困難になり、焼鈍後の加工性が低下する。
Hereafter, the manufacturing method for obtaining the boron steel plate of this invention is illustrated.
[Hot-roll pickling process]
The slab heating temperature before hot rolling may be 1150 to 1320 ° C. as in the case of general carbon steel. The hot rolling finishing temperature is 800 to 900 ° C. If the temperature is lower than 800 ° C., the deformation resistance is increased, the sheet passing property is lowered, and it is difficult to ensure a winding temperature of 500 ° C. or higher. When it exceeds 900 degreeC, an austenite particle size will coarsen and the toughness of a hot rolled material will fall. The winding temperature is 500 to 650 ° C. When the temperature is below 500 ° C., the hot-rolled material is cured and the productivity is lowered. Above 650 ° C, the amount of pro-eutectoid ferrite increases, the distribution of cementite becomes non-uniform, and the lamellar spacing of pearlite increases, making it difficult to spheroidize cementite, and workability after annealing. Decreases.
ただし、仕上圧延後、巻取までの間において、仕上温度から７００℃までの温度域の滞在時間を３０ｓｅｃ以下とすることが重要である。熱間圧延設備では通常、仕上圧延後、巻取までの間はランアウトテーブル上にて鋼板の表面が直接大気に暴露されており、７００℃以上での暴露時間が３０ｓｅｃを超えると、表層の脱Ｃが進行し、合金元素添加により焼入性を改善しても十分な表面硬さが得られない。すなわち、この過程での冷却が、前述のＡｓ／Ａｃ値を０.９０以上とするために極めて重要である。また、高温での暴露時間が長くなると鋼板表面のスケールが過剰に厚くなることがあり、脱スケールに要するコストの増加につながる。冷却手段としては水冷が一般的である。 However, it is important that the residence time in the temperature range from the finishing temperature to 700 ° C. is 30 sec or less between the finishing rolling and the winding. In a hot rolling facility, the surface of the steel sheet is usually directly exposed to the air on the run-out table after finish rolling until winding, and if the exposure time at 700 ° C. or higher exceeds 30 seconds, the surface layer is removed. Even if C progresses and hardenability is improved by addition of alloy elements, sufficient surface hardness cannot be obtained. That is, cooling in this process is extremely important in order to make the aforementioned As / Ac value 0.90 or more. Moreover, when the exposure time at high temperature becomes long, the scale on the surface of the steel sheet may become excessively thick, leading to an increase in cost required for descaling. Water cooling is generally used as the cooling means.
巻取後は、通常の手法にて酸洗に供される。 After winding, it is subjected to pickling by a normal method.
〔冷延工程〕
板厚調整のために必要に応じて冷間圧延を行うことができる。
[Cold rolling process]
Cold rolling can be performed as necessary to adjust the plate thickness.
〔焼鈍工程〕
前工程において酸洗を終え、必要に応じて冷間圧延が施された鋼板は、焼鈍に供される。
シンプルな焼鈍方法としては、鋼板を６００℃以上Ａｃ1未満の温度範囲で０.５ｈ以上均熱保持する方法が採用できる。これにより、パーライト中の層状炭化物を分断し、球状化して加工性を付与する。保持時間は概ね４８ｈ以下とすればよい。均熱保持とは、板厚中心部まで所定の温度範囲に保持されることをいう。
[Annealing process]
The steel plate that has been pickled in the previous step and cold-rolled as necessary is subjected to annealing.
As a simple annealing method, a method in which the steel sheet is kept soaked for 0.5 h or more in a temperature range of 600 ° C. or higher and less than Ac 1 can be adopted. Thereby, the layered carbide in the pearlite is divided and spheroidized to give processability. The holding time may be approximately 48 hours or less. The soaking is to be held in a predetermined temperature range up to the center of the plate thickness.
さらに加工性の改善に有効な焼鈍として、（ａ）前工程で得られた鋼板を（Ａｃ1−３０℃）以上Ａｃ1未満の温度範囲で０.５ｈ以上均熱保持したのち、（ｂ）Ａｃ1以上（Ａｃ1＋５０℃）の温度範囲で０.５〜２０ｈ均熱保持し、（ｃ）その保持温度から少なくとも（Ａｒ1−１０℃）までを冷却速度５〜３０℃／ｈで徐冷するヒートパターンを採用することができる。（ａ）の加熱保持は概ね２４ｈ以下とすればよい。（ａ）のあとに（ｂ）の加熱を行うことによって、微細な炭化物を溶解させると共に、炭化物の一部を残存させる。その後（ｃ）の冷却過程で上記の残存した炭化物を核として炭化物を成長させ、炭化物を球状かつ粗大にする。この徐冷は変態が完了するまで実施することが望ましく、徐冷の終了温度は、工業的には（Ａｒ1−１０℃）から（Ａｒ1−８０℃）までの間で設定すればよい。その後の冷却速度は任意に設定して構わない。
ここで、Ａｃ1は昇温過程におけるＡ1点（オーステナイト変態開始点）、Ａｒ1は降温過程におけるＡ1点（フェライト＋セメンタイト変態完了点）である。
Further, as an effective annealing for improving the workability, (a) the steel plate obtained in the previous step is maintained at a constant temperature for 0.5 h or more in a temperature range of (Ac 1 -30 ° C.) or more and less than Ac 1 , (b) Hold soaking for 0.5 to 20 h in a temperature range of Ac 1 or higher (Ac 1 + 50 ° C.), and (c) gradually increase from the holding temperature to at least (Ar 1 −10 ° C.) at a cooling rate of 5 to 30 ° C./h. A cooling heat pattern can be employed. The heating and holding in (a) may be approximately 24 h or less. By heating (b) after (a), fine carbides are dissolved and part of the carbides remain. Thereafter, in the cooling process of (c), the carbide is grown using the remaining carbide as a nucleus to make the carbide spherical and coarse. This slow cooling is desirably carried out until the transformation is completed, and the end temperature of slow cooling may be industrially set between (Ar 1 -10 ° C) and (Ar 1 -80 ° C). The subsequent cooling rate may be set arbitrarily.
Here, Ac 1 is an A 1 point (austenite transformation start point) in the temperature raising process, and Ar 1 is an A 1 point (ferrite + cementite transformation completion point) in the temperature lowering process.
以上のようにして本発明のボロン鋼鋼板が得られる。 The boron steel plate of the present invention is obtained as described above.
〔加工、焼入れ焼戻し〕
上記のようにして得られた本発明のボロン鋼鋼板は、一般的な手法により、所定の機械部品に加工され、その後、焼入れ焼戻し処理に供される。焼入れに際しては、特に浸炭等の特殊な雰囲気加熱を行わなくてよい。
[Processing, quenching and tempering]
The boron steel sheet of the present invention obtained as described above is processed into a predetermined mechanical part by a general method, and then subjected to quenching and tempering treatment. When quenching, special atmosphere heating such as carburizing is not necessary.
表１に示す組成の鋼を溶製し、各鋼とも、加熱温度１１５０〜１３２０℃、仕上温度８１０〜８７０℃、巻取温度５５０〜６１０℃、仕上温度から７００℃までの滞在時間１５〜３０ｓｅｃの条件で熱間圧延を施し、酸洗し、その後、７１０℃、均熱保持４０ｈのシンプルなヒートパターンでの焼鈍を施し、板厚５.０ｍｍのボロン鋼鋼板（供試材）を得た。なお、製造条件はいずれも本発明で規定する適正条件である。 Steels having the compositions shown in Table 1 were melted, and each steel had a heating temperature of 1150 to 1320 ° C, a finishing temperature of 810 to 870 ° C, a winding temperature of 550 to 610 ° C, and a residence time from the finishing temperature to 700 ° C of 15 to 30 seconds. The steel sheet was subjected to hot rolling under the conditions of, pickled, and then annealed in a simple heat pattern of 710 ° C. and soaking for 40 hours to obtain a boron steel sheet (test material) having a thickness of 5.0 mm. . The manufacturing conditions are all appropriate conditions defined in the present invention.
各供試材について、圧延方向および板厚方向に平行な断面（Ｌ断面）を光学顕微鏡で観察し、前述の表層部の炭化物面積率Ａｓ（％）、中心部の炭化物面積率Ａｃ（％）を求めた。その際、各供試材のＡｓおよびＡｃの測定において、それぞれ５０μｍ×５０μｍの観察視野を無作為に５視野選択して各視野について炭化物面積率を測定し、その平均値をその供試材のＡｓおよびＡｃとし、Ａｓ／Ａｃ値を求めた。 About each test material, the cross section (L cross section) parallel to the rolling direction and the plate thickness direction is observed with an optical microscope, and the carbide area ratio As (%) of the surface layer part and the carbide area ratio Ac (%) of the center part are described above. Asked. At that time, in the measurement of As and Ac of each test material, 5 observation fields of 50 μm × 50 μm were selected at random, and the carbide area ratio was measured for each field of view, and the average value was calculated for each of the test materials. As / Ac was determined as As and Ac.
加工性を評価するために、供試材からＪＩＳ１３Ｂ号引張試験片を作製し、引張試験を行って全伸びＴ.ＥＬ（％）を求めた。また、ＪＩＳ１３Ｂ号引張試験片の平行部中央の両エッジ部分に２ｍｍＶノッチを形成した切り欠き引張試験片を作製し、引張試験を行って切り欠き引張伸びＥｌｖ（％）を求めた。これらの引張試験では引張方向が圧延方向に一致している。また標点間距離はＴ.ＥＬを求める試験片で５０ｍｍ、Ｅｌｖを求める切り欠き試験片で１０ｍｍとした。 In order to evaluate the workability, a JIS No. 13B tensile test piece was prepared from the test material, and a tensile test was performed to determine the total elongation T.EL (%). Moreover, the notch tensile test piece which formed the 2 mmV notch in the both edge part of the parallel part center of a JIS13B tensile test piece was produced, and the notch tensile elongation Elv (%) was calculated | required. In these tensile tests, the tensile direction coincides with the rolling direction. The distance between the gauge points was 50 mm for the test piece for obtaining T.EL, and 10 mm for the notch test piece for obtaining Elv.
供試材から切り出した試験片に焼入れ処理を施した。焼入れ条件は、ソルトバス炉中で９００℃で１５ｍｉｎ均熱保持したのち、６０℃の油中に焼入れする方法とした。焼入れ後の試料について、表面硬さＨｓ（ＨＶ）および板厚中心部の断面硬さＨｃ（ＨＶ）を測定した。それぞれ無作為に選択した１０点の測定値を平均することによりその供試材のＨｓおよびＨｃとした。そしてΔＨＶ＝Ｈｃ−Ｈｓを算出した。これらの結果を表２に示す。 A test piece cut out from the test material was subjected to quenching treatment. The quenching conditions were a method of soaking in an oil at 60 ° C. after maintaining soaking at 900 ° C. for 15 minutes in a salt bath furnace. About the sample after hardening, surface hardness Hs (HV) and the cross-sectional hardness Hc (HV) of plate thickness center part were measured. The measured values at 10 points selected at random were averaged to obtain Hs and Hc of the test material. ΔHV = Hc−Hs was calculated. These results are shown in Table 2.
表２からわかるように、本発明例のものはＡｓ／Ａｃ値が０.９０以上を満たし、ΔＨＶは１００以下と、表面硬さの低下が少なく、かつＴ.ＥＬが３３％以上、Ｅｌｖが３８％以上という良好な加工性を兼ね備えていた。これに対し、比較例１と表示した鋼種ＡはＸ値が小さく焼入性が低いために十分な焼入れ硬さが得られていない。比較例２と表示した鋼種Ｂ、Ｆ、ＧはＸ値が小さいために表面硬さの低下が著しく、ΔＨＶが大きくなっている。図１に本発明例と比較例２についてΔＨＶとＸ値の関係を示す。Ｘ≧２４の本発明例はΔＨＶが比較例２に比べて明らかに小さくなっている。比較例３と表示した鋼種Ｄ、Ｉ、Ｍ、Ｒは合金元素量が本発明の規定範囲の上限を超えているものであり、ΔＨＶは小さい反面、加工性（特に切り欠き伸び）が低下している。図２に本発明例と比較例３についてΔＨＶとＴ.ＥＬの関係を示す。本発明例のものは、焼入れ時の表面硬さ低下が抑制され、かつ比較例３に比べて焼鈍材の延性に優れている。 As can be seen from Table 2, the examples of the present invention satisfy the As / Ac value of 0.90 or more, ΔHV is 100 or less, there is little decrease in surface hardness, T.EL is 33% or more, and Elv is It had good workability of 38% or more. On the other hand, since the steel type A indicated as Comparative Example 1 has a small X value and low hardenability, sufficient quenching hardness is not obtained. Steel types B, F and G indicated as Comparative Example 2 have a small X value, so the surface hardness is significantly reduced, and ΔHV is large. FIG. 1 shows the relationship between ΔHV and X value for the inventive example and the comparative example 2. In the present invention example where X ≧ 24, ΔHV is clearly smaller than that in Comparative Example 2. Steel types D, I, M, and R indicated as Comparative Example 3 have alloy element amounts exceeding the upper limit of the specified range of the present invention, and ΔHV is small, but workability (particularly notch elongation) is reduced. ing. FIG. 2 shows the relationship between ΔHV and T.EL for the inventive example and the comparative example 3. In the example of the present invention, a decrease in surface hardness during quenching is suppressed, and the ductility of the annealed material is superior to that of Comparative Example 3.
表１の鋼種Ｊを用いて、熱延条件の影響を調査した。熱延の仕上温度、巻取温度、および仕上温度から７００℃までの温度域の滞在時間を種々変えて熱間圧延を行い、その後、実施例１と同様、７１０℃、均熱保持４０ｈのシンプルなヒートパターンでの焼鈍を施し、板厚５.０ｍｍのボロン鋼鋼板（供試材）を得た。得られた供試材について、実施例１と同様の手法でＡｓ／Ａｃ、Ｔ.ＥＬ、Ｅｌｖを求めた。また、供試材から切り出した試験片を用いて実施例１と同様の手法で焼入れ処理を施し、ΔＨＶを求めた。結果を表３に示す。 The effect of hot rolling conditions was investigated using steel type J in Table 1. Hot rolling is performed by changing the hot rolling finishing temperature, the coiling temperature, and the residence time in the temperature range from the finishing temperature to 700 ° C., and then, as in Example 1, the simple temperature of 710 ° C. and soaking is 40 h. An annealing with a proper heat pattern was performed to obtain a boron steel plate (test material) having a thickness of 5.0 mm. About the obtained test material, As / Ac, T.EL, and Elv were calculated | required by the method similar to Example 1. FIG. Further, a test piece cut out from the test material was used for quenching treatment in the same manner as in Example 1 to obtain ΔHV. The results are shown in Table 3.
表３からわかるように、本発明例のものは、Ａｓ／Ａｃ値が０.９０以上を満たし、ΔＨＶが小さく、かつＴ.ＥＬが３３％以上、Ｅｌｖが３８％以上という良好な加工性を兼ね備えていた。これに対し条件４では仕上温度から７００℃までの温度域の滞在時間が長すぎたため脱Ｃが進行し、焼入れ後の表面硬さの低下ΔＨＶが大きくなった。条件５では巻取温度が本発明の規定範囲より高いために炭化物の分布が不均一となり、加工性が低下した。条件８では仕上温度が高いために仕上温度から７００℃までの温度域の滞在時間が長くなり、脱Ｃが進行してΔＨＶが大きくなった。
As can be seen from Table 3, the examples of the present invention have good workability with an As / Ac value of 0.90 or more, a small ΔHV, T.EL of 33% or more, and Elv of 38% or more. Had both. On the other hand, in condition 4, since the residence time in the temperature range from the finishing temperature to 700 ° C. was too long, the de-C progressed, and the decrease in surface hardness after quenching ΔHV became large. Under
表１の鋼種Ｈ、Ｋ、Ｍを用いて、焼鈍条件の影響を調査した。各鋼種とも熱延加熱温度１２３０〜１３２０℃、仕上温度８５０℃、巻取温度５９０〜６３０℃、仕上温度から７００℃までの温度域の滞在時間２０〜３０ｓｅｃの条件で熱延を行い、酸洗して板厚５.０ｍｍの熱延酸洗鋼板を得た。これらについて、以下の２通りの条件で焼鈍を施しボロン鋼鋼板（供試材）を得た。
〔条件Ａ〕７１０℃で均熱保持４０ｈ
〔条件Ｂ〕（ａ）７００℃で均熱保持２０ｈ→（ｂ）７６０℃で均熱保持１０ｈ→（ｃ）６５０℃まで１０℃／ｈで冷却、その後、炉冷
焼鈍条件Ｂにおいて、（ａ）の７００℃は（Ａｃ1−３０℃）以上Ａｃ1未満の範囲にあり、（ｂ）の７６０℃はＡｃ1以上（Ａｃ1＋５０℃）の温度範囲にあり、（ｃ）の６５０℃は（Ａｒ1−１０℃）以下（Ａｒ1−８０℃）以上の温度範囲にある。
焼鈍後の供試材について断面中心硬さ、Ｔ.ＥＬ、Ｅｌｖを測定した。その結果を表４に示す。
Using the steel types H, K, and M in Table 1, the influence of annealing conditions was investigated. Each steel type is hot-rolled under conditions of hot rolling heating temperature of 1230 to 1320 ° C, finishing temperature of 850 ° C, winding temperature of 590 to 630 ° C, residence time in the temperature range from finishing temperature to 700 ° C for 20 to 30 seconds, and pickling Thus, a hot-rolled pickled steel sheet having a thickness of 5.0 mm was obtained. These were annealed under the following two conditions to obtain a boron steel sheet (test material).
[Condition A] Soaking at 710 ° C. for 40 hours
[Condition B] (a) Soaking at 700 ° C. for 20 h → (b) Soaking at 760 ° C. for 10 h → (c) Cooling to 650 ° C. at 10 ° C./h, then in furnace cooling annealing condition B, (a 700 ° C. is in the range of (Ac 1 −30 ° C.) or more and less than Ac 1 , 760 ° C. in (b) is in the temperature range of Ac 1 or more (Ac 1 + 50 ° C.), and 650 ° C. in (c) is It is in a temperature range of (Ar 1 −10 ° C.) or lower (Ar 1 −80 ° C.) or higher.
The center hardness of the cross section, T.EL, and Elv were measured for the specimens after annealing. The results are shown in Table 4.
表４からわかるように、いずれの鋼種においても条件Ｂで焼鈍を施したものは、条件Ａのものと同等またはそれ以上の加工性を示した。 As can be seen from Table 4, any of the steel types annealed under condition B exhibited a workability equivalent to or better than that of condition A.
表１の鋼種Ｆ（比較例）および鋼種Ｋ、Ｌ（本発明例）を用いて板厚の影響を調査した。各鋼種とも熱延加熱温度１２３０〜１３２０℃、仕上温度８５０℃、巻取温度５９０〜６３０℃、仕上温度から７００℃までの温度域の滞在時間２０〜３０ｓｅｃの条件で熱延を行って板厚を２〜１０ｍｍとし、酸洗して種々の板厚の熱延酸洗鋼板を得た。その後、実施例１と同様の条件で焼鈍、および焼入れを施し、焼入れ後の表面硬さおよび板厚中心部の断面硬さを調べた。図３にその結果を示す。 The influence of sheet thickness was investigated using steel type F (comparative example) and steel types K and L (invention example) in Table 1. Each steel type is subjected to hot rolling under conditions of hot rolling heating temperature of 1230 to 1320 ° C., finishing temperature of 850 ° C., winding temperature of 590 to 630 ° C., and residence time of 20 to 30 sec in the temperature range from finishing temperature to 700 ° C. Was made into 2 to 10 mm, and pickled to obtain hot-rolled pickled steel sheets having various thicknesses. Thereafter, annealing and quenching were performed under the same conditions as in Example 1, and the surface hardness after quenching and the cross-sectional hardness at the center of the plate thickness were examined. The result is shown in FIG.
図３からわかるように、比較例である鋼種Ｆにおいて板厚が５ｍｍ以上になると表面硬さの低下が大きくなり、７ｍｍ以上になると板厚中心部の断面硬さも低下する。これに対し、本発明例の鋼種Ｋ（Ｘ値＝３１）では板厚７ｍｍまで、表面硬さ、板厚中心部硬さともにほとんど低下しない。さらにＸ値が大きい鋼種Ｌ（Ｘ値＝３５）では板厚１０ｍｍまで、表面硬さ、板厚中心部硬さともにほとんど低下しない。このように、本発明の規定範囲内にある鋼板は従来よりも肉厚が大きい部材へ適用しても、焼入れ後の表面および板厚中心部硬さの低下を抑制できる。 As can be seen from FIG. 3, in steel type F as a comparative example, when the plate thickness is 5 mm or more, the decrease in surface hardness increases, and when it is 7 mm or more, the cross-sectional hardness at the center of the plate thickness also decreases. On the other hand, in the steel type K (X value = 31) of the example of the present invention, the surface hardness and the central thickness hardness of the plate are hardly lowered up to a plate thickness of 7 mm. Further, in the steel type L (X value = 35) having a large X value, the surface hardness and the central thickness of the plate thickness hardly decrease up to a plate thickness of 10 mm. As described above, even when the steel sheet within the specified range of the present invention is applied to a member having a larger wall thickness than that of the conventional steel sheet, it is possible to suppress a decrease in hardness of the surface after quenching and the thickness center portion.
Claims (5)
Ｘ＝５.５Ｃ1/2（１＋０.６Ｓｉ）（１＋４.１Ｍｎ）（１＋０.５Ｎｉ）（１＋２.３Ｃｒ）（１＋３.１Ｍｏ） …（１）
ここで、（１）式の元素記号の箇所には質量％で表された当該元素の含有量値が代入され、無添加元素の元素記号の箇所には０（ゼロ）が代入される。 By mass%, C: 0.10 to 0.40%, Si: 0.50% or less, Mn: 0.50 to 1.60%, Cr: 0.05 to 1.50%, Ti: 0.01 ~ 0.30%, B: 0.0005 ~ 0.0050%, P: 0.03% or less, S: 0.01% or less, the balance being Fe and inevitable impurities, and defined by the following formula (1) An annealed steel sheet having a composition having an X value of 24 or more, and in the cross-sectional structure of the steel sheet, includes an area ratio As of carbides in the surface layer region having a depth from the surface of 25 to 75 μm and a center position of the sheet thickness. A boron steel sheet excellent in hardenability of the steel sheet surface, in which the ratio As / Ac of the carbide area ratio Ac in the central region having a length in the thickness direction of 50 μm is 0.90 or more.
X = 5.5C 1/2 (1 + 0.6Si) (1 + 4.1Mn) (1 + 0.5Ni) (1 + 2.3Cr) (1 + 3.1Mo) (1)
Here, the content value of the element expressed in mass% is assigned to the location of the element symbol in the formula (1), and 0 (zero) is assigned to the location of the element symbol of the additive-free element.
前工程で得られた鋼板を６００℃以上Ａｃ1未満の温度範囲で０.５ｈ以上均熱保持して焼鈍する工程（焼鈍工程）、
を有する請求項１または２に記載の鋼板表面の焼入性に優れたボロン鋼鋼板の製造方法。 A slab heated to 1150 to 1320 ° C. is hot-rolled at a finishing temperature of 800 to 900 ° C., and then the residence time in the temperature range from the finishing temperature to 700 ° C. is set to 30 sec or less, and the slab is wound at a winding temperature of 500 to 650 ° C. Then, the process (hot-roll pickling process) which gives pickling and makes it a hot-rolled pickled steel sheet,
A step (annealing step) of annealing the steel plate obtained in the previous step by holding the steel plate for 0.5 h or more in a temperature range of 600 ° C. or higher and less than Ac 1 ;
The manufacturing method of the boron steel plate excellent in the hardenability of the steel plate surface of Claim 1 or 2 which has these.
前工程で得られた鋼板を（Ａｃ1−３０℃）以上Ａｃ1未満の温度範囲で０.５ｈ以上均熱保持したのち、Ａｃ1以上（Ａｃ1＋５０℃）の温度範囲で０.５〜２０ｈ均熱保持し、その保持温度から少なくとも（Ａｒ1−１０℃）までを冷却速度５〜３０℃／ｈで徐冷するヒートパターンで焼鈍する工程（焼鈍工程）、
を採用する請求項３に記載のボロン鋼鋼板の製造方法。 Instead of the annealing step,
The steel plate obtained in the previous step is soaked for 0.5 h or more in a temperature range of (Ac 1 -30 ° C.) or more and less than Ac 1 and then 0.5 to 0.5 in a temperature range of Ac 1 or more (Ac 1 + 50 ° C.). A step of annealing for 20 h soaking and annealing at a cooling rate of 5 to 30 ° C./h from the holding temperature to at least (Ar 1 -10 ° C.) (annealing step),
The manufacturing method of the boron steel plate of Claim 3 which employ | adopts.
前工程で得られた鋼板を冷間圧延する工程（冷延工程）、
を有する請求項３または４に記載のボロン鋼鋼板の製造方法。 Between the hot rolling pickling process and the annealing process,
Cold rolling the steel plate obtained in the previous process (cold rolling process),
The manufacturing method of the boron steel plate of Claim 3 or 4 which has these.
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| JP2021021105A (en) * | 2019-07-25 | 2021-02-18 | Ｊｆｅスチール株式会社 | High carbon hot-rolled steel sheet for vacuum carburization and method for producing the same, and carburized steel component |
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