JP3661691B2

JP3661691B2 - Continuous casting method of high carbon special steel

Info

Publication number: JP3661691B2
Application number: JP2003152313A
Authority: JP
Inventors: 滋小倉; 朋典神山
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2003-05-29
Filing date: 2003-05-29
Publication date: 2005-06-15
Anticipated expiration: 2016-11-11
Also published as: JP2003305539A

Description

【０００１】
【発明の属する技術分野】
本発明は、高炭素特殊鋼の連続鋳造方法に関するものである。
【０００２】
【従来の技術】
従来、省エネルギーや工程省略等の観点から、連続鋳造鋳片を常温まで低下させずに温片あるいは熱片のまま熱延（熱間圧延）工程に送り、加熱時間を短縮しあるいは加熱を省略して熱延するいわゆる直送圧延プロセスが検討され、これまでに特許文献１、特許文献２公報などに開示されるような種々の方法が提案されて、現在では、鋳片表面に割れの発生しにくいアルミキルド鋼等はこの直送圧延プロセスで製造されるようになっている。
【０００３】
しかし、Ｃ濃度0.10重量％程度以上でかつCr、Ni、Mo等を含む鋼（この鋼を以下「高炭素特殊鋼」と呼ぶ）の連続鋳造鋳片は、表面割れが特に発生しやすいため、熱延工程に送る前に表面を検査し、欠陥部が検出されればその部位を研削して表面欠陥を除去することが一般的に行われ、かかる検査・除去工程は作業の性格上、常温で実施せざるを得ず、必然的に鋳片は常温まで冷却されなければならず、直送圧延プロセスで製造することが困難であった。
【０００４】
高炭素特殊鋼の鋳造時の表面欠陥の発生を抑制する方法として、一つには鋳片に生じる歪みを極小化するために垂直型連続鋳造機にて鋳造する方法が挙げられる。しかし、この方法は、鋳片の内部品質からは有利だが、生産性の観点からは不利である。そこで、現在は曲げ部、もしくは曲げ部と矯正部を有する連続鋳造機（垂直曲げ型連続鋳造機という）も使用されている。
【０００５】
垂直曲げ型連続鋳造機を使用する場合には、例えば特許文献３、特許文献４に提案されるように、二次冷却帯の水量分布（二次冷却パターン）や具体的なスプレー方法を規定することで鋳造時の鋳片表面割れ防止が図られている。これらの対策は既に常法となっており、▲１▼曲げ部・矯正部を通過する鋳片が脆化温度域に入らないよう二次冷却水量パターンを制御して弱冷却する、▲２▼特に冷えやすく熱応力起因の引張歪みが生じやすい鋳片コーナ部への注水を制限して局部的に表面温度を上昇させる、の二つに要約される。なお、▲１▼は「二次冷却の弱冷化」、▲２▼は「二次冷却の幅切り」と称される。
【０００６】
しかし、これら二つの対策を推し進めても、垂直曲げ型連続鋳造機にて鋳造される高炭素特殊鋼では鋳造時の割れ発生を十分には抑えきれず、相変わらず相当程度の鋳片表面手入れが必要であり、直送圧延に供することができるまでに至っていない。
【０００７】
【特許文献１】
特公昭56−21330 号公報
【特許文献２】
特公昭56−24018 号公報
【特許文献３】
特公平２−18936 号公報
【特許文献４】
特開平１−95801 号公報
【０００８】
【発明が解決しようとする課題】
そこで本発明は、高炭素特殊鋼鋳造時の鋳片表面の割れ発生を従来以上に抑制できる連続鋳造方法を提供し、もって、高炭素特殊鋼を直送圧延プロセスに供用できるようにすることを目的とする。
【０００９】
【課題を解決するための手段】
本発明は、垂直曲げ型連続鋳造機を用いてＣ濃度0.12重量％以上でかつCr, Ni, Moの一種または二種以上を含有する高炭素特殊鋼を鋳造するにあたり、二次冷却水の温度を38℃以上に保つことを特徴とする高炭素特殊鋼の連続鋳造方法である。
【００１０】
そして、本発明に係る高炭素特殊鋼の鋳片は無研削、もしくは無検査かつ無研削にて熱延に供されるものであることが最適である。
【００１１】
【発明の実施の形態】
本発明において「高炭素特殊鋼」とは、Ｃ濃度0.10重量％程度以上でかつCr、Ni、Mo等を含む鋼を意味する。
本発明者らは、垂直曲げ型連続鋳造機を用いて高炭素特殊鋼を鋳造する際の鋳片表面欠陥の発生を種々の角度から検討した結果、前記した従来の二つの対策において唯一の制御要因であった二次冷却水の水量以外に、二次冷却水の温度（以下単に「水温」ともいう）も鋳片の割れ発生を左右する大きな要因であることがわかった。
【００１２】
例えば図１は、高炭素特殊鋼の一種であるＪＩＳ−ＳＫ２相当の1.2 重量％Ｃ鋼を垂直曲げ型連続鋳造機により鋳造した時の水温と鋳片表面の割れ発生率との関係を示すグラフである。この鋳造において、前記従来の割れ防止対策（二次冷却の弱冷化および幅切り）は当然ながら実施されている。なお、割れ発生率の定義は、割れ発生スラブ枚数／鋳造スラブ枚数である。
【００１３】
同図に示すように、水温38℃未満では発生率の高い割れが、38℃以上で極端にその発生率を下げ、40℃以上ではほぼ皆無となる。
水温が冬期の気温低下に伴って低下し、特に冬期の定期整備工事などによる長時間の鋳造停止中には外気温度と同等にまで低下することは以前からわかっていたのであるが、この水温が、高炭素特殊鋼の鋳造時の割れ発生にこれほどまでに大きく影響するというのは従来になかった知見である。
【００１４】
この新知見に基づき完成された本発明によれば、垂直曲げ型連続鋳造機を用いて高炭素特殊鋼を鋳造する際に、従来の割れ防止対策（前記▲１▼、▲２▼）に重ねて二次冷却水の温度を38℃以上に規制することにより、鋳造時の鋳片表面の割れ発生を従来に比べて格段に低減させることができ、高炭素特殊鋼の表面検査・欠陥除去工程が省略でき、高炭素特殊鋼を直送圧延プロセスに供することが可能となる。
【００１５】
また、図２は水温28℃でのCr、Ni、Moの少なくともいずれかを含む高炭素特殊鋼のＣ濃度と割れ発生率の関係を示すグラフである。これらの鋼種はいずれも水温上昇（38℃以上）により割れ発生率が10％程度以下に抑えられた。従って同図縦軸は水温上昇措置による割れ発生率の低下代を示すものでもある。
図示のように、水温上昇に伴う割れ発生率の低下代はＣ濃度が高いところで大きいがＣ濃度の低下につれて小さくなり、Ｃ濃度が0.12重量％未満で割れ発生率の低下代が小さくなることから、本発明方法は、Ｃ濃度0.12重量％以上の鋼種に対して実施するのが好ましい。
【００１６】
なお、水温を38℃以上に上昇させる手段は特に限定されるものではなく、例えば、タンク内に貯蔵されたあるいは供給本管内を流れる二次冷却水を、電気ヒータで加熱する、あるいは高温水蒸気等の高温物と熱交換させて加熱するなど様々な手段が採用できる。
また、本発明により鋳造された高炭素特殊鋼鋳片を無手入れで熱延工程に直送し、加熱炉に熱片装入して抽出後熱延して得た熱延板は、従来の高炭素特殊鋼熱延板に比較して欠陥が少ない傾向が認められた。
【００１７】
この理由は詳らかでないが、従来は鋳片が鋳造されてから加熱炉に装入されるまでの間に常温まで冷却されるので、この間に各鋼種ごとに特有の脆化温度域に到達したとき、ハンドリングその他に起因して、常温で発見できずしかも下流での欠陥発生の原因となるような表面直下割れが発生するのに対し、本発明により鋳造された鋳片は、脆化温度域まで冷却されずに加熱炉に熱片装入され次いで熱延されるので、かような割れが発生しにくくなることによるのであろう。
【００１８】
【実施例】
垂直部2.5mを有する垂直曲げ型連続鋳造機を用いて、Ｃ濃度0.10〜1.30重量％の高炭素特殊鋼を、比水量0.6 〜1.6l/kg-steel の範囲で、常法である曲げ部・矯正部での鋳片表面温度管理（成分系によって異なる脆化温度域の回避）ならびに鋳片コーナより50〜200mm の範囲に対する幅切りを採用した二次冷却を行いつつ、鋳込み速度0.6 〜2.0m/minで鋳造し、厚み200mm および260mm 、幅650 〜1650mmの鋳片を製造するにあたり、タンク内の二次冷却水に水蒸気を吹き込んで水温を40℃±１℃に保ち、実施例とした。また、水温調節を行わず水温が30〜35℃の範囲であった以外は実施例と同じ鋳造条件を従来例とした。
【００１９】
なお、本実施例の高炭素特殊鋼は前記Ｃ以外に、その鋼種に要求される材料特性に応じてCr、Ni、Moを含有する他、Si, Mn, Al, Ｐ，Ｓ，Ｎ，Ｏを通常のＪＩＳ規格範囲内に成分調整されて含むものである。
実施例と従来例の鋳片表面の割れ発生率を表１に示す。同表から明らかなように、実施例は従来例に比べ格段に割れ発生率が低下すること、さらに、割れ発生率の低減効果は、高炭素特殊鋼のうちでも、Cr、Ni、Moを含むＣ濃度0.12重量％以上の鋼で特に顕著であることが確認された。
【００２０】
【表１】

【００２１】
【発明の効果】
本発明によれば、従来は鋳片の表面割れ発生を抑制できず全て常温まで冷却して検査・欠陥除去を行った後に熱延工程の加熱炉に装入せざるを得なかった高炭素特殊鋼の、連続鋳造時の表面割れ発生が大幅に低減し、高炭素特殊鋼を直送圧延プロセスに供しうるから、工程・納期が大幅に短縮するとともにエネルギーが一段と節減できるという優れた効果を奏する。
【図面の簡単な説明】
【図１】 1.2 重量％Ｃ鋼を垂直曲げ型連続鋳造機により鋳造した時の水温と割れ発生率との関係を示すグラフである。
【図２】水温28℃での高炭素特殊鋼のＣ濃度と割れ発生率の関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a continuous casting method for high-carbon special steel.
[0002]
[Prior art]
Conventionally, from the viewpoint of energy saving and process omission, continuous cast slabs are sent to the hot rolling (hot rolling) process without reducing the temperature to room temperature, shortening the heating time or omitting heating. So-called direct-rolling processes for hot rolling have been studied, and various methods as disclosed in Patent Document 1, Patent Document 2 and the like have been proposed so far, and cracks are hardly generated on the slab surface at present. Aluminum killed steel and the like are manufactured by this direct rolling process.
[0003]
However, continuous casting slabs of steel with a C concentration of about 0.10% by weight or more and containing Cr, Ni, Mo, etc. (hereinafter referred to as “high carbon special steel”) are particularly susceptible to surface cracks. The surface is inspected before being sent to the hot rolling process, and if a defect is detected, the surface is generally ground to remove the surface defect. This inspection / removal process is performed at room temperature due to the nature of the work. Inevitably, the slab must be cooled to room temperature, and it was difficult to manufacture by a direct rolling process.
[0004]
One method for suppressing the occurrence of surface defects during the casting of high-carbon special steel is a method of casting with a vertical continuous casting machine in order to minimize distortion generated in the slab. However, this method is advantageous in terms of the internal quality of the slab, but is disadvantageous in terms of productivity. Therefore, a continuous casting machine having a bending portion or a bending portion and a straightening portion (referred to as a vertical bending type continuous casting machine) is currently used.
[0005]
When using a vertical bending type continuous casting machine, for example, as proposed in Patent Document 3 and Patent Document 4, the water amount distribution (secondary cooling pattern) in the secondary cooling zone and a specific spraying method are specified. Thus, the slab surface cracking during casting is prevented. These countermeasures have already become ordinary methods. (1) Weak cooling is performed by controlling the secondary cooling water amount pattern so that the slab passing through the bending part and the straightening part does not enter the embrittlement temperature range. (2) In particular, the surface temperature is locally increased by limiting the water injection to the slab corner portion where it is easy to cool and tensile strain due to thermal stress is likely to occur. Note that (1) is called “weakening of secondary cooling”, and (2) is called “secondary cooling”.
[0006]
However, even if these two countermeasures are promoted, high-carbon special steel cast by a vertical bending type continuous casting machine cannot sufficiently suppress cracking during casting, and still requires considerable slab surface maintenance. It has not yet reached the point where it can be subjected to direct rolling.
[0007]
[Patent Document 1]
Japanese Patent Publication No. 56-21330 [Patent Document 2]
Japanese Patent Publication No.56-24018 [Patent Document 3]
Japanese Patent Publication No.2-18936 [Patent Document 4]
JP-A-1-95801
[Problems to be solved by the invention]
Accordingly, the present invention provides a continuous casting method capable of suppressing the occurrence of cracks on the surface of a slab during casting of high carbon special steel more than before, and has the object of enabling use of high carbon special steel in a direct feed rolling process. And
[0009]
[Means for Solving the Problems]
When casting a high carbon special steel having a C concentration of 0.12% by weight or more and containing one or more of Cr, Ni, and Mo using a vertical bending type continuous casting machine, the temperature of the secondary cooling water Is a continuous casting method of high-carbon special steel characterized by maintaining the temperature at 38 ° C. or higher.
[0010]
The slab of high carbon special steel according to the present invention is optimally subjected to hot rolling without grinding, or without inspection and without grinding.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, “high carbon special steel” means a steel having a C concentration of about 0.10% by weight or more and containing Cr, Ni, Mo and the like.
As a result of examining the occurrence of slab surface defects from various angles when casting high carbon special steel using a vertical bending type continuous casting machine, the present inventors have found that the only control in the above two conventional countermeasures. In addition to the amount of secondary cooling water that was a factor, the temperature of the secondary cooling water (hereinafter also simply referred to as “water temperature”) was found to be a major factor affecting the occurrence of cracks in the slab.
[0012]
For example, Fig. 1 is a graph showing the relationship between the water temperature and the crack occurrence rate on the slab surface when a 1.2 wt% C steel equivalent to JIS-SK2 which is a kind of high carbon special steel is cast by a vertical bending type continuous casting machine. It is. In this casting, the conventional crack prevention measures (weakening of secondary cooling and width cutting) are naturally implemented. The definition of the crack occurrence rate is the number of cracked slabs / the number of cast slabs.
[0013]
As shown in the figure, high-cracking cracks occur at a water temperature of less than 38 ° C, and the cracking rate is drastically reduced at 38 ° C or higher, and almost zero at 40 ° C or higher.
It has been known for a long time that the water temperature decreases as the temperature drops in the winter, especially when the casting is stopped for a long time due to regular maintenance work in the winter. It is an unprecedented finding that it has such a great influence on the occurrence of cracks during casting of high carbon special steel.
[0014]
According to the present invention completed based on this new knowledge, when casting a high carbon special steel using a vertical bending type continuous casting machine, it is layered on the conventional crack prevention measures (1) and (2) above. By controlling the temperature of the secondary cooling water to 38 ° C or higher, cracks on the slab surface during casting can be significantly reduced compared to conventional methods, and surface inspection and defect removal processes for high-carbon special steel Can be omitted, and the high-carbon special steel can be subjected to the direct feed rolling process.
[0015]
FIG. 2 is a graph showing the relationship between the C concentration and crack generation rate of high-carbon special steel containing at least one of Cr, Ni, and Mo at a water temperature of 28 ° C. In all of these steel types, the crack generation rate was suppressed to about 10% or less due to an increase in water temperature (above 38 ° C). Therefore, the vertical axis in the figure also shows the margin of decrease in the crack generation rate due to the measures for increasing the water temperature.
As shown in the figure, the margin of decrease in the crack generation rate with increasing water temperature is large at higher C concentrations, but decreases as the C concentration decreases, and when the C concentration is less than 0.12% by weight, the crack generation rate decreases. The method of the present invention is preferably performed on a steel type having a C concentration of 0.12% by weight or more.
[0016]
The means for raising the water temperature to 38 ° C. or higher is not particularly limited. For example, the secondary cooling water stored in the tank or flowing in the supply main pipe is heated with an electric heater, or high-temperature steam is used. Various means such as heating by exchanging heat with high temperature materials can be adopted.
In addition, the hot-rolled sheet obtained by directly feeding the high-carbon special steel cast slab cast according to the present invention directly to the hot-rolling process, charging it into a heating furnace and hot-rolling it after extraction is a conventional high-rolling sheet. There was a tendency for fewer defects compared to hot rolled sheet of carbon special steel.
[0017]
The reason for this is not clear, but conventionally it is cooled to room temperature from when the slab is cast until it is charged into the heating furnace, so during this time when a specific embrittlement temperature range is reached for each steel type. Due to handling, etc., cracks directly below the surface that cannot be found at room temperature and cause downstream defects occur, whereas the slab cast according to the present invention has a brittle temperature range. This is probably because such a crack is less likely to occur because it is not cooled and is charged into a heating furnace and then hot-rolled.
[0018]
【Example】
Using a vertical bending type continuous casting machine with a vertical section of 2.5m, a high-carbon special steel with a C concentration of 0.10 to 1.30% by weight, with a specific water volume in the range of 0.6 to 1.6l / kg-steel, is a conventional bending section. -Casting speed of 0.6 to 2.0 while controlling the slab surface temperature in the straightening part (avoiding embrittlement temperature range that varies depending on the component system) and secondary cooling adopting a width cut from 50 to 200mm from the slab corner. Casting at m / min to produce slabs with thicknesses of 200mm and 260mm and widths of 650 to 1650mm, steam was blown into the secondary cooling water in the tank, and the water temperature was kept at 40 ° C ± 1 ° C. . Further, the same casting conditions as in the examples were used as conventional examples except that the water temperature was not adjusted and the water temperature was in the range of 30 to 35 ° C.
[0019]
In addition to the C, the high carbon special steel of this example contains Cr, Ni, and Mo according to the material properties required for the steel type, Si, Mn, Al, P, S, N, and O. The component is adjusted within the normal JIS standard range.
Table 1 shows the incidence of cracks on the slab surface of Examples and Conventional Examples. As is clear from the table, the crack generation rate in the example is significantly lower than that in the conventional example, and the effect of reducing the crack generation rate includes Cr, Ni, and Mo among high carbon special steels. It was confirmed that this was particularly noticeable in steel having a C concentration of 0.12% by weight or more.
[0020]
[Table 1]

[0021]
【The invention's effect】
According to the present invention, a high carbon special material that has conventionally been unable to suppress the occurrence of surface cracks in a slab and had to be charged to a heating furnace in a hot rolling process after all were cooled to room temperature and subjected to inspection and defect removal. The occurrence of surface cracks during continuous casting of steel is greatly reduced, and high-carbon special steel can be used for the direct feed rolling process, so that the process and delivery time are greatly shortened and energy can be further reduced.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between water temperature and crack generation rate when 1.2 wt% C steel is cast by a vertical bending type continuous casting machine.
FIG. 2 is a graph showing the relationship between C concentration and crack generation rate of high carbon special steel at a water temperature of 28 ° C.

Claims

When casting a high carbon special steel with a C concentration of 0.12% by weight or more and containing one or more of Cr, Ni, and Mo using a vertical bending type continuous casting machine, the temperature of the secondary cooling water is 38 ° C or more. A continuous casting method for high-carbon special steel, characterized by

The continuous casting method according to claim 1, wherein the cast slab of high carbon special steel is subjected to hot rolling without grinding or without inspection and grinding.