JP4066116B2 - Low temperature casting method for steel grades requiring low segregation - Google Patents
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Description
【０００１】
【産業上の利用分野】
本発明は、低偏析度が要求されるＳＵＪ２等の鋼種を連続鋳造する方法に関する。
【０００２】
【従来の技術】
ＳＵＪ２等の高炭素低合金鋼は、硬質で耐摩耗性に優れている特性を活用し、軸受材、各種工具等に使用されている。しかし、合金成分が偏析し易い鋼種であることから、鋳片段階でも偏析を可能な限り低く抑えて安定した要求特性を付与することが要求される。
鋳片にみられる偏析は、連鋳鋳型に注入された溶鋼が凝固するまでの間で合金成分が比重差等によって鋳型内を浮上し、或いは凝固点温度の高い合金成分が結晶粒界に液相のままで濃化されること等に原因がある。偏析の防止には、溶鋼の過熱度（溶鋼温度−液相線温度）を可能な限り低く設定し、連鋳鋳型に注入された溶鋼を短時間で凝固させる低温鋳造が有効である。
【０００３】
しかし、低温鋳造では、鋳造中に溶鋼温度が下がり過ぎ、溶鋼からの晶出物がノズル内面に付着堆積して浸漬ノズルを閉塞し易い。浸漬ノズルが閉塞すると、連続鋳造を停止せざるを得ない。閉塞しないまでもノズル内面に付着堆積した晶出物が多くなると、連鋳鋳型への溶鋼の安定供給を阻害する抵抗として働き、鋳造条件を不安定化させる。そのため、低温鋳造時の過熱度には、安定した鋳造条件を維持するために下限値が設定される。
低温鋳造本来の目的からする過熱度の上限と鋳造条件の安定化に必要な下限値の間は、ごく狭い温度範囲である。なかでも、Ｃｒ含有高炭素低合金鋼にあっては、僅か１４５０〜１４６５℃の狭い温度範囲で溶鋼が連鋳鋳型に注入される。このような狭い温度範囲に溶鋼を安定保持するためには、取鍋からタンディッシュに注湯された溶鋼の温度降下を抑制する必要がある。そこで、誘導加熱，プラズマ加熱等の加熱機構を備えたタンディッシュを用いた連続鋳造法（特開平５−２２８５８９号公報）や、比較的高温に調整した溶鋼をタンディッシュに注湯してタンディッシュ内で冷却材を添加して温度調整する方法等が開発されている。
【０００４】
【発明が解決しようとする課題】
しかし、タンディッシュの周辺に連続鋳造機の各種機械器具が設けられており、この狭隘な空間に加熱機構を付設することは位置の取合い上での制約が多い。また、誘導加熱，プラズマ加熱等でタンディッシュを温度補償しようとすると、電力コストが高くなることは勿論、タンディッシュ内にある溶鋼を均一な温度分布に維持することも困難である。
冷却材の添加でタンディッシュ内の溶鋼を所定温度に調整する場合、普通鋼等の鋼材が冷却材として使用される。そのため、冷却材由来の不純物で溶鋼が汚染され易く、微量合金成分を含む溶鋼にあっては成分変動をきたす虞れもある。
本発明は、このような問題を解消すべく案出されたものであり、前回の連続鋳造時に昇温したタンディッシュの保有熱を低温鋳造に利用することにより、余分な加熱機構を必要とせず、精錬・成分調整された溶鋼を汚染することなく安定条件下で低温鋳造することを目的とする。
【０００５】
【課題を解決するための手段】
本発明は、その目的を達成するため、Ｃ：０．９〜１．３重量％，Ｃｒ：１．０〜６．０重量％を含む低合金鋼溶鋼を過熱度１０〜２５℃で連続鋳造する際、該低合金鋼溶鋼より鋳造温度が高い鋼種の連続鋳造時に加熱されたタンディッシュを高温状態のまま使用し、平均流量１．３〜１．５トン／分の低吐出流量で前記低合金鋼溶鋼を連続鋳造することを特徴とする。
低合金鋼溶鋼の連続鋳造は、式（１）で定義されるタンディッシュの空炉限界時間γ（分）に達する前に開始することが好ましい。
γ＝５０．５×β×（Ｔ0−Ｔ1）0.1／Ｔ0 ・・・・（１）
ただし、Ｔ0：低合金鋼溶鋼の連続鋳造に先立つ鋼種の鋳造温度（℃）
Ｔ1：低合金鋼溶鋼の鋳造温度（℃）
β：低合金鋼溶鋼の連続鋳造に先立つ鋼種の鋳造時間（分）であり、２００分を超える鋳造時間ではβ＝２００に固定する。
【０００６】
【実施の形態】
本発明では、Ｃ：０．９〜１．３重量％，Ｃｒ：１．０〜６．０重量％を含む高炭素低合金鋼（以下、単に高炭素低合金鋼という）を対象としている。この鋼種は、硬質で耐摩耗性に優れていることから、主として軸受材，工具類等に使用されている。しかし、比較的多量のＣに加えてＣを固溶し易いＣｒを多量に含んでいるので、著しく偏析し易い鋼種である。
性質が一定した鋼材を得る上では偏析防止が要求され、そのため低温鋳造で鋳片に製造される。偏析防止に与える低温鋳造の効果は、溶鋼の過熱度を２５℃以下に調整したとき顕著になる。しかし、鋳造条件に及ぼす悪影響を回避するためには、過熱度の下限を１０℃以上に設定することが必要である。
【０００７】
過熱度１０〜２５℃の溶鋼をタンディッシュに注湯すると、溶鋼の保有熱がタンディッシュに奪われ、溶鋼が温度降下し易い。溶鋼の温度効果を防止するため、本発明では、高炭素低合金鋼の連続鋳造に先立って実施された鋳造温度が高い鋼種の連続鋳造で昇温したタンディッシュの高温状態を使用している。高温状態のタンディッシュに高炭素低合金鋼の溶鋼が注湯されるため、溶鋼温度とタンディッシュの温度との間の温度差が小さく、タンディッシュに奪われる溶鋼の保有熱が少なくなる。また、前回の連続鋳造時に使用される溶鋼として、高炭素低合金鋼より鋳造温度が高い鋼種を選択するとき、タンディッシュが所定の高温域に保持されるため、タンディッシュに注湯された高炭素低合金鋼溶鋼の温度降下が大幅に少なくなる。
【０００８】
前回の連続鋳造でタンディッシュに付与された熱を利用するとき、別途のタンディッシュ加熱機構や冷却材の添加を必要とすることなく、タンディッシュに注湯された高炭素低合金鋼の温度降下が抑制される。しかも、前回の連続鋳造によってタンディッシュの全体が均一な温度分布になっているので、タンディッシュに注湯された高炭素低合金鋼に対する保温効果が働き、温度範囲１０〜２５℃の狭い過熱度が維持される。
タンディッシュは、連続鋳造後に空炉状態で放置されると時間経過に伴って温度降下する。そこで、空炉状態での放置時間が高炭素低合金鋼鋳片の偏析に及ぼす影響を種々調査検討した結果、後述する実施例でも説明しているように前掲の式（１）で定義される空炉限界時間γに至る前に高炭素低合金鋼の連続鋳造を開始するとき、タンディッシュの温度降下に起因する影響が抑えられ、前回の連続鋳造で与えられたタンディッシュの保有熱が高炭素低合金鋼の偏析防止に効果的に活用されることが判った。
【０００９】
タンディッシュ内で過熱度１０〜２５℃に保持された高炭素低合金鋼の溶鋼は、全ストランドを合計して得られる平均流量１．３〜１．５トン／分の低吐出流量でタンディッシュから連鋳鋳型に注入される。この条件下では、溶鋼の降温速度が０．２℃／分以下になり、連鋳鋳型内での偏析も防止され、得られた鋳片は合金成分が均一に分散した組織をもつ。吐出流量が１．５トン／分を超えると、連鋳鋳型との接触で生成した凝固シェルが十分に成長することなく、連鋳鋳型から引き出されるため、バルジングや鋳片割れ等のトラブルが発生し易くなる。逆に、１．３トン／分に達しない吐出流量では、生産性が劣ることは勿論、凝固シェルから連鋳鋳型への抜熱速度が大きくなり過ぎるため凝固速度が速くなり、ピンホール，スカム疵等の表面欠陥が鋳片に発生し易くなる。
【００１０】
【実施例】
Ｃ：０．９９重量％，Ｓｉ：０．２５重量％，Ｍｎ：０．３４重量％，Ｓ：０．００５重量％，Ｃｒ：１．３７重量％，残部が不純物を除きＦｅの組成をもつ高炭素低合金鋼（液相線温度１４５６℃）を転炉，ＲＨ脱ガス法で溶製した。高炭素低合金鋼の溶鋼を容量３０トンのタンディッシュに注湯し、過熱度２５℃以下で低温鋳造した。
タンディッシュとしては、低温鋳造の必要がない低炭素普通鋼（Ｃ：０．０４重量％，Ｍｎ：０．２０重量％，Ａｌ：０．０３０重量％）を鋳込み温度１５７０℃，鋳込み時間９８分で連続鋳造することによって平均温度１５６４℃に昇温したタンディッシュを前回の連続鋳造終了後から３．６分経過した時点で再使用した。このときの空炉限界時間γは、４．６分であった。比較のため、前回の連続鋳造から十分な時間が経過して常温近傍まで降温したタンディッシュを併せ使用した。
【００１１】
タンディッシュに注湯された高炭素低合金鋼を吐出流量１．５トン／分で連鋳鋳型に注入しながら、５５分にわたって鋳片に連続鋳造した。連続鋳造中にタンディッシュ内の溶鋼温度を連続測定し、過熱度の時間変化を調査した。図１の調査結果にみられるように、前回の連続鋳造で昇温したタンディッシュを用いた本発明例では、連続鋳造の末期においても１０〜２５℃の温度範囲に過熱度が維持されていた。そのため、安定した鋳造条件下で偏析のない鋳片が得られ、偏析度が１．０４と低く、炭化物の異常析出も検出されなかった。酸素含有量もタンディッシュ段階で１０ｐｐｍ，鋳片段階で８ｐｐｍと低く、清浄度が十分に高い鋳片であった。
これに対し、常温近傍まで降温したタンディッシュを用いた比較例では、連鋳鋳型に供給される溶鋼流が連続鋳造の末期で不安定化した。不安定な溶鋼流は、連続鋳造の開始から４５分程度経過した時点で過熱度が１０℃を下回ったため、浸漬ノズル内部に付着する晶出物が流路抵抗として作用したことに由来するものと推察される。
【００１２】
次いで、同じ低炭素普通鋼を鋳造温度１５７０℃で１２０分連続鋳造した後、連続鋳造の終了から高炭素低合金鋼の鋳造開始までの時間（タンディッシュの空炉時間）を種々変更した。この場合、溶鋼温度１４９３℃の高炭素低合金鋼をタンディッシュに注湯し、目標鋳込み温度１４８０℃で６０分間連続鋳造した。連続鋳造中に、タンディッシュに注湯された高炭素低合金鋼の降温速度を調査した。図２の調査結果にみられるように、タンディッシュの空炉時間が長くなるほど、高炭素低合金鋼の降温速度が大きくなった。
また、同じ低炭素普通鋼を鋳造温度１５７０℃で連続鋳造するときの鋳造時間を変更し、続いて連続鋳造する際の高炭素低合金鋼の降温速度に及ぼす影響を調査した。図３の調査結果にみられるように、前回鋳造時間が長くなるほど、タンディッシュの保有熱が増加したため、高炭素低合金鋼の降温速度が小さくなった。しかし、２００分を超える前回鋳造時間では、前回鋳造の長時間化が高炭素低合金鋼の降温抑制に及ぼす影響はほとんどみられなかった。
【００１３】
更に、種々の鋳込み温度をもつ溶鋼を前回鋳造に使用して鋳造時間１２０分で連続鋳造した後、高炭素低合金鋼を目標鋳込み温度１５７０℃で６０分間連続鋳造した。そして、前回鋳込み温度Ｔ0と今回鋳込み温度Ｔ1との温度差ΔＴ（Ｔ0−Ｔ1）が高炭素低合金鋼の降温速度に及ぼす影響を調査したところ、図４に示すように温度差ΔＴが大きなほど高炭素低合金鋼の降温が抑制されていた。
降温速度をα（℃／分），図２〜４の関係における比例定数（実績値）をそれぞれＡ〜Ｃとすると、図２からα＝Ａ×γ，図３からα＝Ｂ×Ｔ0／β，図４からα＝Ｃ／（Ｔ0−Ｔ1）0.1の関係式が得られる。これら関係式をαに対する比例式でまとめ、その比例係数をＤとすると降温速度αは式α＝Ｄ×γ×Ｔ0／β／（Ｔ0−Ｔ1）0.1で表わされる。この式を空炉限界時間γで整理すると、γ＝α／Ｄ×β×（Ｔ0−Ｔ1）0.1／Ｔ0に書き換えられる。低温鋳造の実現には０．２０℃／分以下の降温速度αが必要であるので、α＝０．２０を代入し比例係数Ｄを実績値から定めることにより前掲の式（１）が得られる。
このように、式（１）で定義される空炉限界時間γに至る前のタンディッシュを使用して高炭素低合金鋼を連続鋳造するとき、連鋳期間中に過熱度が１０〜２５℃の温度範囲に維持され、安定した低温鋳造が可能になった。得られた鋳片も、偏析度が低位に抑えられ、清浄度の高いものであった。
【００１４】
【発明の効果】
以上に説明したように、本発明は、偏析し易い合金成分を含む高炭素低合金鋼を低温鋳造することによって偏析を抑制する際、前回鋳造で昇温したタンディッシュの保有熱を高炭素低合金鋼の保温及び降温防止に有効活用している。そのため、別途の加熱機構の付設や冷却材添加による温度調節を必要とすることなく、高炭素低合金鋼の過熱度が低温鋳造に必要な温度範囲に維持され、ノズル閉塞等のトラブルなしに安定した低温鋳造が可能になる。
【図面の簡単な説明】
【図１】 タンディッシュに注湯された高炭素低合金鋼の過熱度の時間変化を示したグラフ
【図２】 タンディッシュの空炉時間が高炭素低合金鋼の降温速度に及ぼす影響を示したグラフ
【図３】 鋳込み温度の高い溶鋼を用いた前回鋳造時間が高炭素低合金鋼の降温速度に及ぼす影響を示したグラフ
【図４】 前回鋳込み温度と今回鋳込み温度との温度差が高炭素低合金鋼の降温速度に及ぼす影響を示したグラフ[0001]
[Industrial application fields]
The present invention relates to a method for continuously casting a steel type such as SUJ2 that requires a low degree of segregation.
[0002]
[Prior art]
High carbon low alloy steels such as SUJ2 are used for bearing materials, various tools, etc., utilizing the characteristics of being hard and excellent in wear resistance. However, since the alloy components are easily segregated, the segregation is required to be kept as low as possible even at the slab stage to provide stable required characteristics.
The segregation observed in the slab is caused by the alloy component floating in the mold due to the difference in specific gravity until the molten steel injected into the continuous casting mold solidifies, or the alloy component having a high freezing point temperature is in the liquid phase at the grain boundary. The cause is that it is concentrated as it is. In order to prevent segregation, low temperature casting is effective in which the superheat degree (molten steel temperature-liquidus temperature) of molten steel is set as low as possible and the molten steel injected into the continuous casting mold is solidified in a short time.
[0003]
However, in low temperature casting, the temperature of the molten steel is too low during casting, and crystallized substances from the molten steel tend to adhere and accumulate on the inner surface of the nozzle, thereby closing the immersion nozzle. If the immersion nozzle is blocked, continuous casting must be stopped. If the amount of crystallized material deposited and deposited on the inner surface of the nozzle increases without clogging, it acts as a resistance that hinders the stable supply of molten steel to the continuous casting mold and destabilizes the casting conditions. Therefore, a lower limit is set for the degree of superheating during low temperature casting in order to maintain stable casting conditions.
There is a very narrow temperature range between the upper limit of the superheat degree intended for low temperature casting and the lower limit necessary for stabilizing the casting conditions. Among them, in the case of Cr-containing high carbon low alloy steel, molten steel is injected into the continuous casting mold in a narrow temperature range of only 1450 to 1465 ° C. In order to stably hold the molten steel in such a narrow temperature range, it is necessary to suppress the temperature drop of the molten steel poured from the ladle into the tundish. Therefore, a continuous casting method using a tundish equipped with a heating mechanism such as induction heating or plasma heating (Japanese Patent Laid-Open No. 5-228589), or a molten steel adjusted to a relatively high temperature is poured into the tundish to tundish. A method of adjusting the temperature by adding a coolant has been developed.
[0004]
[Problems to be solved by the invention]
However, various machines and equipments of a continuous casting machine are provided around the tundish, and attaching a heating mechanism to this narrow space has many restrictions in terms of position. In addition, when trying to compensate the temperature of the tundish by induction heating, plasma heating, etc., the power cost increases, and it is difficult to maintain the molten steel in the tundish in a uniform temperature distribution.
When the molten steel in the tundish is adjusted to a predetermined temperature by adding a coolant, a steel material such as ordinary steel is used as the coolant. Therefore, the molten steel is easily contaminated with impurities derived from the coolant, and there is a risk that the molten steel containing a trace alloy component may cause a component fluctuation.
The present invention has been devised to solve such a problem, and by using the retained heat of the tundish heated during the previous continuous casting for low temperature casting, an extra heating mechanism is not required. It aims at low temperature casting under stable conditions without contaminating the refined and component-adjusted molten steel.
[0005]
[Means for Solving the Problems]
In order to achieve the object, the present invention continuously casts a low alloy steel molten steel containing C: 0.9 to 1.3% by weight and Cr: 1.0 to 6.0% by weight at a superheat of 10 to 25 ° C. The tundish heated during continuous casting of the steel type having a higher casting temperature than the low alloy steel molten steel is used in a high temperature state, and the low discharge flow rate with an average flow rate of 1.3 to 1.5 ton / min is used. It is characterized by continuously casting molten alloy steel.
The continuous casting of the low alloy steel molten steel is preferably started before reaching the tundish air limit time γ (min) defined by the equation (1).
γ = 50.5 × β × (T 0 −T 1 ) 0.1 / T 0 ... (1)
However, T 0 : Casting temperature (° C) of the steel type prior to continuous casting of molten low alloy steel
T 1 : Casting temperature of low alloy steel molten steel (° C)
β: Casting time (min) of the steel type prior to continuous casting of the low alloy steel molten steel, and β = 200 is fixed when the casting time exceeds 200 minutes.
[0006]
Embodiment
In the present invention, a high carbon low alloy steel (hereinafter, simply referred to as a high carbon low alloy steel) containing C: 0.9 to 1.3 wt% and Cr: 1.0 to 6.0 wt% is targeted. Since this steel type is hard and excellent in wear resistance, it is mainly used for bearing materials, tools and the like. However, since it contains a large amount of Cr that can easily dissolve C in addition to a relatively large amount of C, it is a steel type that is remarkably easily segregated.
In order to obtain a steel material having a constant property, prevention of segregation is required, and for this reason, it is manufactured into a slab by low temperature casting. The effect of low temperature casting on segregation prevention becomes significant when the superheat degree of molten steel is adjusted to 25 ° C. or less. However, in order to avoid adverse effects on casting conditions, it is necessary to set the lower limit of the superheat degree to 10 ° C. or higher.
[0007]
When molten steel having a superheat degree of 10 to 25 ° C. is poured into the tundish, the retained heat of the molten steel is taken away by the tundish, and the molten steel easily falls in temperature. In order to prevent the temperature effect of the molten steel, the present invention uses the high temperature state of the tundish heated by continuous casting of a steel type having a high casting temperature performed prior to continuous casting of the high carbon low alloy steel. Since the molten steel of high carbon low alloy steel is poured into the tundish in a high temperature state, the temperature difference between the molten steel temperature and the tundish temperature is small, and the retained heat of the molten steel taken away by the tundish is reduced. In addition, when selecting a steel grade that has a higher casting temperature than high-carbon low-alloy steel as the molten steel used during the previous continuous casting, the tundish is kept in a predetermined high temperature range, so the high temperature poured into the tundish The temperature drop of molten carbon low alloy steel is greatly reduced.
[0008]
When using the heat applied to the tundish in the previous continuous casting, the temperature drop of the high-carbon low-alloy steel poured into the tundish without the need for a separate tundish heating mechanism or coolant Is suppressed. Moreover, since the entire tundish has a uniform temperature distribution by the previous continuous casting, the heat retention effect on the high carbon low alloy steel poured into the tundish works, and a narrow superheat degree of 10 to 25 ° C. Is maintained.
When the tundish is left in an empty furnace state after continuous casting, the temperature drops with time. Therefore, as a result of various investigations and examinations on the influence of the standing time in the empty furnace state on the segregation of the high-carbon low-alloy steel slab, it is defined by the above formula (1) as described in Examples described later. When continuous casting of high-carbon low alloy steel is started before reaching the furnace limit time γ, the influence caused by the temperature drop of the tundish is suppressed, and the retained heat of the tundish given in the previous continuous casting is high. It was found that it can be effectively used to prevent segregation of carbon low alloy steel.
[0009]
The high-carbon low-alloy steel molten steel maintained at a superheat of 10 to 25 ° C. in the tundish is a tundish with an average flow rate of 1.3 to 1.5 tons / min. Is poured into a continuous casting mold. Under this condition, the temperature drop rate of the molten steel is 0.2 ° C./min or less, segregation in the continuous casting mold is prevented, and the obtained slab has a structure in which alloy components are uniformly dispersed. If the discharge flow rate exceeds 1.5 tons / minute, the solidified shell generated by contact with the continuous casting mold will not be sufficiently grown and will be pulled out from the continuous casting mold, causing problems such as bulging and cracking of the cast slab. It becomes easy. On the other hand, if the discharge flow rate does not reach 1.3 tons / min, the productivity will be inferior, and the heat removal rate from the solidified shell to the continuous casting mold will become too high, resulting in a faster solidification rate, pinholes and scum. Surface defects such as wrinkles are likely to occur in the slab.
[0010]
【Example】
C: 0.99% by weight, Si: 0.25% by weight, Mn: 0.34% by weight, S: 0.005% by weight, Cr: 1.37% by weight, the balance is Fe except for impurities High carbon low alloy steel (liquidus temperature 1456 ° C.) was melted by a converter and RH degassing method. Molten steel of high carbon low alloy steel was poured into a tundish with a capacity of 30 tons and cast at a low temperature at a superheat of 25 ° C. or less.
As a tundish, a low carbon plain steel (C: 0.04% by weight, Mn: 0.20% by weight, Al: 0.030% by weight) that does not require low temperature casting is cast at a temperature of 1570 ° C. and a casting time of 98 minutes. The tundish heated to an average temperature of 1564 ° C. by continuous casting was reused when 3.6 minutes had elapsed since the end of the previous continuous casting. The empty furnace limit time γ at this time was 4.6 minutes. For comparison, a tundish that had been cooled to near room temperature after a sufficient time had elapsed since the previous continuous casting was also used.
[0011]
The high carbon low alloy steel poured into the tundish was continuously cast into a slab for 55 minutes while being poured into a continuous casting mold at a discharge flow rate of 1.5 tons / minute. During continuous casting, the temperature of the molten steel in the tundish was continuously measured, and the temporal change in the degree of superheat was investigated. As can be seen from the investigation results of FIG. 1, in the present invention example using the tundish heated at the previous continuous casting, the degree of superheat was maintained in the temperature range of 10 to 25 ° C. even at the end of the continuous casting. . Therefore, a slab without segregation was obtained under stable casting conditions, the segregation degree was as low as 1.04, and abnormal precipitation of carbide was not detected. The oxygen content was as low as 10 ppm in the tundish stage and as low as 8 ppm in the slab stage, and the slab was sufficiently high in cleanliness.
On the other hand, in the comparative example using the tundish that was cooled to near room temperature, the molten steel flow supplied to the continuous casting mold was destabilized at the end of continuous casting. The unstable molten steel flow is derived from the fact that the crystallized material adhering to the inside of the immersion nozzle acted as a channel resistance because the degree of superheat fell below 10 ° C. after about 45 minutes had passed since the start of continuous casting. Inferred.
[0012]
Next, after the same low carbon plain steel was continuously cast at a casting temperature of 1570 ° C. for 120 minutes, the time from the end of continuous casting to the start of casting of the high carbon low alloy steel (tundish air time) was variously changed. In this case, a high carbon low alloy steel having a molten steel temperature of 1493 ° C. was poured into a tundish and continuously cast at a target casting temperature of 1480 ° C. for 60 minutes. The temperature drop rate of high carbon low alloy steel poured into the tundish during continuous casting was investigated. As can be seen from the results of the investigation in FIG. 2, the temperature drop rate of the high-carbon low-alloy steel increased as the tundish furnace time increased.
In addition, the casting time when continuously casting the same low carbon ordinary steel at a casting temperature of 1570 ° C. was changed, and the influence on the temperature drop rate of the high carbon low alloy steel during the continuous casting was investigated. As can be seen from the results of the investigation in FIG. 3, the longer the previous casting time, the more the heat held in the tundish, and the lowering the temperature of the high carbon low alloy steel became smaller. However, in the previous casting time exceeding 200 minutes, there was almost no effect of the prolonged casting time on the temperature reduction of the high carbon low alloy steel.
[0013]
Further, molten steel having various casting temperatures was used for the previous casting and continuously cast in a casting time of 120 minutes, and then a high carbon low alloy steel was continuously cast at a target casting temperature of 1570 ° C. for 60 minutes. Then, when the influence of the temperature difference ΔT (T 0 −T 1 ) between the previous casting temperature T 0 and the current casting temperature T 1 on the temperature drop rate of the high carbon low alloy steel was investigated, the temperature difference as shown in FIG. The temperature drop of the high carbon low alloy steel was suppressed as ΔT was larger.
The cooling rate alpha (° C. / min), when the respective A~C proportionality constant (actual value) in relation to FIG. 2 to 4, = A × Figures 2 alpha gamma, from FIG. 3 α = B × T 0 / From FIG. 4, a relational expression of α = C / (T 0 −T 1 ) 0.1 is obtained. When these relational expressions are summarized as a proportional expression with respect to α and the proportionality coefficient is D, the cooling rate α is expressed by the equation α = D × γ × T 0 / β / (T 0 −T 1 ) 0.1 . If this equation is arranged by the empty furnace limit time γ, it can be rewritten as γ = α / D × β × (T 0 −T 1 ) 0.1 / T 0 . Since a temperature drop rate α of 0.20 ° C./min or less is necessary to realize low temperature casting, the above formula (1) can be obtained by substituting α = 0.20 and determining the proportionality coefficient D from the actual value. .
Thus, when continuously casting a high carbon low alloy steel using the tundish before reaching the furnace limit time γ defined by the formula (1), the superheat is 10 to 25 ° C. during the continuous casting period. Thus, stable low temperature casting became possible. The obtained slab also had a high degree of cleanliness with a low segregation degree.
[0014]
【The invention's effect】
As described above, when the present invention suppresses segregation by low temperature casting of a high carbon low alloy steel containing an alloy component that easily segregates, the retained heat of the tundish that has been heated in the previous casting is reduced. It is effectively used to keep the alloy steel warm and prevent it from falling. As a result, the temperature of the high-carbon low-alloy steel is maintained within the temperature range required for low-temperature casting without the need for additional heating mechanisms or temperature adjustment by adding coolant, and stable without trouble such as nozzle clogging. Low temperature casting becomes possible.
[Brief description of the drawings]
[Fig. 1] Graph showing temporal change in superheat of high carbon low alloy steel poured into tundish [Fig. 2] Shows the effect of tundish emptying time on temperature drop rate of high carbon low alloy steel Graph [Figure 3] Graph showing the effect of the previous casting time using molten steel with a high casting temperature on the cooling rate of high carbon low alloy steel [Figure 4] The temperature difference between the previous casting temperature and the current casting temperature is high A graph showing the effect of carbon low alloy steel on temperature drop rate
Claims (2)
γ＝５０．５×β×（Ｔ0−Ｔ1）0.1／Ｔ0 ・・・・（１）
ただし、鋳造時間βが２００分を超えるとき、
β＝２００として空炉限界時間γを算出する。When the casting temperature of the steel type prior to continuous casting of the molten low alloy steel is T 0 (° C.), the casting time is β (min), and the casting temperature of the molten low alloy steel is T 1 (° C.), the formula (1 The low-temperature casting method of a steel type requiring a low segregation degree according to claim 1, wherein the low-alloy steel molten steel is continuously cast before reaching the tundish air limit time γ (min) defined in (1).
γ = 50.5 × β × (T 0 −T 1 ) 0.1 / T 0 ... (1)
However, when the casting time β exceeds 200 minutes,
Assuming that β = 200, the furnace limit time γ is calculated.
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