JP2010209416A - Method for manufacturing martensitic high chromium steel cooling slab, and the cooling slab - Google Patents
Method for manufacturing martensitic high chromium steel cooling slab, and the cooling slab Download PDFInfo
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本発明は、マルテンサイト系高Ｃｒ鋼について、鋳造後の冷却パターンに特徴を有する冷却スラブの製造方法、およびその冷却スラブに関する。 The present invention relates to a method for manufacturing a cooling slab having a feature in a cooling pattern after casting, and a cooling slab for the martensitic high Cr steel.
一般に、鋼の連続鋳造後のスラブは加熱炉に装入され、高温に加熱保持された後、熱間圧延に供される。多くの場合、加熱に要するエネルギーを極力低減することを目的として、温度ができるだけ低下しないうちにスラブは加熱炉に装入される。しかし、熱間圧延工場にて操業トラブル等が発生し、長時間にわたり生産が停止した場合には、鋳造後すぐにスラブを加熱炉に装入できなくなり、スラブが常温にまで冷却されてしまうことがある。ＳＵＳ３０４、ＳＵＳ４３０などの一般的な高Ｃｒ鋼においては、スラブが常温にまで冷却されても特に問題は生じない。 In general, a slab after continuous casting of steel is charged into a heating furnace, heated and held at a high temperature, and then subjected to hot rolling. In many cases, in order to reduce the energy required for heating as much as possible, the slab is charged into the heating furnace before the temperature is lowered as much as possible. However, if operation troubles occur in a hot rolling mill and production stops for a long time, the slab cannot be charged into the heating furnace immediately after casting, and the slab is cooled to room temperature. There is. In general high Cr steels such as SUS304 and SUS430, there is no particular problem even if the slab is cooled to room temperature.
ところが、ＳＵＳ４２０Ｊ２などに代表されるマルテンサイト系高Ｃｒ鋼では、スラブの温度が常温まで低下する場合には冷却の過程で表層部から内部に向けて順次マルテンサイト変態が起こり、それに起因してスラブに割れが生じやすい。この割れはいわゆる「焼割れ」と呼ばれる。割れたスラブを熱延すると熱延鋼帯が破断する恐れがある。熱延工程で鋼が破断すると、ただちに操業が停止され、工場の生産性が著しく低下する。そのため、常温付近にまで冷却されて割れが発生したマルテンサイト系高Ｃｒ鋼のスラブは、そのままでは熱延に供することができず、スクラップ処理されることが多い。 However, in the martensitic high Cr steel represented by SUS420J2, etc., when the temperature of the slab decreases to room temperature, martensitic transformation occurs sequentially from the surface layer to the inside during the cooling process, resulting in the slab Are prone to cracking. This crack is called a so-called “burn crack”. When a cracked slab is hot rolled, the hot rolled steel strip may break. If the steel breaks during the hot rolling process, the operation is immediately stopped and the productivity of the factory is significantly reduced. Therefore, a slab of martensitic high Cr steel that has been cooled to near room temperature and cracked cannot be used for hot rolling as it is, and is often scrapped.
また近年、連鋳スラブを他のメーカーあるいは他の事業所に供給し、熱延を鋳造とは別の工場で実施するケースが増加している。その場合は船舶や車両での運搬となるが、マルテンサイト系高Ｃｒ鋼では焼割れの問題があるため、鋳造後のスラブを常温にまで冷却せずに、焼割れが生じない温度域に保温した状態で運搬することが行われており、コスト増大や工程の煩雑化を招いている。 In recent years, the number of cases in which continuous cast slabs are supplied to other manufacturers or other offices and hot rolling is performed in a factory other than casting is increasing. In that case, it will be transported by ship or vehicle, but martensitic high Cr steel has a problem of burning cracks, so the slab after casting is not cooled to room temperature and kept at a temperature range where burning cracks do not occur. It has been carried out in a state of being carried out, resulting in increased costs and complicated processes.
ステンレス鋼のスラブ割れ防止に関しては、例えば特許文献１に、フェライト単相系ステンレス鋼を対象として、スラブ冷却過程での冷却速度を遅くすべく、対象とするスラブの上下に温片スラブを配置して重ねる手法が開示されている。しかし、この方法をマルテンサイト系鋼種に適用しても、冷却過程でのマルテンサイト変態を回避することはできず、スラブの焼割れは防止できない。 Regarding prevention of slab cracking of stainless steel, for example, in Patent Document 1, hot slabs are arranged above and below the target slab in order to slow down the cooling rate in the slab cooling process for ferritic single-phase stainless steel. A method of overlapping is disclosed. However, even if this method is applied to a martensitic steel grade, martensitic transformation during the cooling process cannot be avoided, and slab cracking cannot be prevented.
本発明は、マルテンサイト系高Ｃｒ鋼において、鋳造後に常温付近までスラブを冷却した場合でも、割れの生じない健全なスラブが得られる新たな手法によって、熱間加工性の良好なスラブを提供することを目的とする。 The present invention provides a slab with good hot workability by using a new technique for obtaining a healthy slab without cracking even when the slab is cooled to near room temperature after casting in martensitic high Cr steel. For the purpose.
上記目的は、質量％で、Ｃ：０.０５〜０.５％、Ｓｉ：１％以下、Ｍｎ：１％以下、Ｐ：０.０４％以下、Ｓ：０.０４％以下、Ｃｒ：８〜１８％、Ｎ：０.１％以下であり、必要に応じてさらにＴｉ、Ｎｂの１種以上を合計０.０５％以下の範囲で含有し、残部がＦｅおよび不可避的不純物であるマルテンサイト系高Ｃｒ鋼の鋳造スラブが、鋳造後の冷却過程にある段階において、スラブ表面温度がＭｓ点まで降下しないうちにスラブを水槽に浸漬し、スラブ表面温度をＭｓ点以下にまで降下させてスラブ表層部をマルテンサイト組織としたのち、スラブを水槽から取り出し、スラブの中心部と表層部の温度差を利用してスラブ表面温度を２５０℃以上に復熱させることによりスラブ表層部のマルテンサイト組織を軟質化し、その後、常温空気中または保温カバー内にスラブを保持するマルテンサイト系高Ｃｒ鋼冷却スラブの製造方法によって達成される。 The purpose is mass%, C: 0.05-0.5%, Si: 1% or less, Mn: 1% or less, P: 0.04% or less, S: 0.04% or less, Cr: 8 Martensite containing -18%, N: 0.1% or less, and further containing one or more of Ti and Nb in a total range of 0.05% or less as necessary, with the balance being Fe and inevitable impurities In the stage where the cast slab of the high Cr steel is in the cooling process after casting, the slab is immersed in a water tank before the slab surface temperature falls to the Ms point, and the slab surface temperature is lowered to the Ms point or less. After the surface layer part has a martensite structure, the slab is taken out of the water tank, and the slab surface temperature is reheated to 250 ° C. or more by utilizing the temperature difference between the center part and the surface layer part of the slab, thereby martensite structure of the slab surface part. Soften, then at room temperature in air Others are achieved by a method for manufacturing a martensitic high Cr steel cooling slab to hold the slab in the heat insulation cover.
ここで、「鋳造スラブ」とは鋳造によって得られたスラブを意味する。「冷却スラブ」とは、鋳造後に表面温度が２５０℃以下にまで冷却された状態の鋳造スラブを意味する。「スラブ表層部をマルテンサイト組織とする」とはスラブの表面から少なくとも１０ｍｍ深さまでの領域をマルテンサイト組織とすることをいう。 Here, “cast slab” means a slab obtained by casting. The “cooled slab” means a cast slab in a state where the surface temperature is cooled to 250 ° C. or less after casting. “The slab surface layer portion has a martensite structure” means that a region from the slab surface to a depth of at least 10 mm is a martensite structure.
上記において、厚さ１５０ｍｍ以上のスラブにおいて、スラブを水槽から取り出すタイミングをスラブ表面温度が１００℃を下回らない時期とすることができる。 In the above, in the slab having a thickness of 150 mm or more, the timing at which the slab is taken out from the water tank can be set as the time when the slab surface temperature does not fall below 100 ° C.
また本発明では、上記の化学組成を有するマルテンサイト系高Ｃｒ鋼の鋳造スラブであって、スラブ広面の表面から少なくとも深さ１０ｍｍ以内の表層部が厚さ中心部より軟化しており、スラブ厚さ方向中心部の断面硬さＨ0（ＨＶ）とスラブ広面の表面からの深さが１０ｍｍ位置の断面硬さＨ1（ＨＶ）との差Ｈ0−Ｈ1が１００ＨＶ以上である熱間加工性の良好なマルテンサイト系高Ｃｒ鋼冷却スラブが提供される。ここで、「スラブ広面」はスラブの厚さ方向両端部の表面である。 Further, in the present invention, a martensitic high Cr steel cast slab having the above chemical composition, the surface layer portion at least within a depth of 10 mm from the surface of the wide surface of the slab is softened from the thickness center portion, and the slab thickness hot working is a difference H 0 -H 1 is 100HV or more and a cross-sectional hardness H 0 of the direction center portion is (HV) and cross-sectional hardness of depth 10mm position from the surface of the slab broad surface H 1 (HV) A martensitic high Cr steel cooling slab with good properties is provided. Here, the “slab wide surface” is the surface of both ends in the thickness direction of the slab.
従来、マルテンサイト系高Ｃｒ鋼の鋳造スラブは常温付近にまで冷却すると、その冷却過程で割れを生じるという問題があったが、本発明によればこの問題が解消され、常温まで冷却した場合でも割れのない健全なスラブが得られるようになった。しかも、従来一般的な連続鋳造工程のライン構成をそのまま利用することが可能であり、新たな強制加熱も必要ない。したがった本発明は、マルテンサイト系高Ｃｒ鋼の製造性向上および製造コスト低減に寄与するものである。 Conventionally, when a cast slab of martensitic high Cr steel is cooled to near room temperature, there is a problem that cracking occurs in the cooling process, but according to the present invention, this problem is solved, even when cooled to room temperature. A healthy slab without cracks can be obtained. In addition, it is possible to use the line configuration of a conventional continuous casting process as it is, and no new forced heating is required. Therefore, the present invention contributes to improving the manufacturability and reducing the manufacturing cost of martensitic high Cr steel.
一般に、マルテンサイト系高Ｃｒ鋼の焼割れ発生機構は以下のように考えられている。
図１に、鋳造後のスラブを空冷した場合の冷却曲線と、スラブの鋳造方向に垂直な断面（以下単に「スラブ断面」ということがある）における応力状態を模式的に示す。図では簡単のために、スラブ断面を「表面側」と「厚み中心側」の２つの領域に分けて示している（後述図２においても同様）。本発明で対象とするマルテンサイト系高Ｃｒ鋼は、高温ではスラブ表面側と厚み中心側いずれもオーステナイト（γ）組織を呈する。高温から冷却される過程では、スラブの表面側の方が厚み中心側よりも冷却速度が速く、温度が低くなる（ａ）。スラブ表面側がＭｓ点（マルテンサイト変態開始温度）に達すると、スラブ表面側ではマルテンサイト（α’）変態が起こる。マルテンサイト変態が起こると鋼は膨張するので、厚み中心側は、表面側の膨張によって引張応力を受ける（ｂ）。ただし、厚み中心側は周囲を拘束されているので、この段階で厚み中心側に割れが生じることはない。その後さらに冷却が進行すると、スラブの厚み中心側がＭｓ点に達してその部分でのマルテンサイト変態が起こり、厚み中心側が膨張するので、今度は表面側の方が厚み中心側の膨張によって引張応力を受ける（ｃ）。この場合、表面側はすでに高硬度で靭性に乏しいマルテンサイト組織となっているので、引張応力に起因して割れを生じてしまう。
In general, the mechanism for the occurrence of cracking in martensitic high Cr steel is considered as follows.
FIG. 1 schematically shows a cooling curve when a slab after casting is air-cooled, and a stress state in a cross section perpendicular to the casting direction of the slab (hereinafter sometimes simply referred to as “slab cross section”). In the drawing, for the sake of simplicity, the slab cross section is divided into two regions of “surface side” and “thickness center side” (the same applies to FIG. 2 described later). The martensitic high Cr steel targeted in the present invention exhibits an austenite (γ) structure on both the slab surface side and the thickness center side at high temperatures. In the process of cooling from a high temperature, the surface side of the slab has a faster cooling rate and a lower temperature than the thickness center side (a). When the slab surface side reaches the Ms point (martensitic transformation start temperature), martensitic (α ′) transformation occurs on the slab surface side. Since the steel expands when martensitic transformation occurs, the thickness center side receives tensile stress due to the expansion on the surface side (b). However, since the periphery of the thickness center side is constrained, cracks do not occur on the thickness center side at this stage. When cooling further proceeds, the thickness center side of the slab reaches the Ms point, the martensitic transformation occurs at that portion, and the thickness center side expands, so this time the surface side is subjected to tensile stress due to expansion on the thickness center side. Receive (c). In this case, since the surface side has already a martensitic structure with high hardness and poor toughness, cracking occurs due to tensile stress.
そこで本発明では、表面側に先に生じたマルテンサイト組織を、復熱によって軟質化し、いわゆる焼戻しマルテンサイト組織と同様な比較的靭性のある組織状態とすることによって、上記のような表面側の割れを防止する。そのためには表面側のみを中心側より大きい冷却速度で冷却し、中心側と表面側の温度差を大きくすることが必要である。本発明ではその手段として、スラブを水槽に浸漬した後、中心側と表面側の温度差が十分に大きくなった状態で水槽から取り出す手順をとる。 Therefore, in the present invention, the martensite structure generated on the surface side first is softened by recuperation, and is made into a relatively tough structure state similar to a so-called tempered martensite structure. Prevent cracking. For this purpose, it is necessary to cool only the surface side at a higher cooling rate than the center side, and to increase the temperature difference between the center side and the surface side. In the present invention, as a means for this, after the slab is immersed in the water tank, a procedure for taking it out from the water tank in a state where the temperature difference between the center side and the surface side is sufficiently large is taken.
図２に、鋳造後のスラブを水槽に浸漬した後、取り出した場合の冷却曲線と、スラブ断面における応力状態を模式的に示す。高温から冷却される過程ではスラブ表面側と厚み中心側いずれもオーステナイト（γ）組織を呈し、表面側の方が厚み中心側よりも温度が低い状態で冷却されることは図１の場合と同じである（ａ）。本発明では表面温度がＭｓ点に達する前にスラブを水槽に浸漬する。このとき表面側のみが急激に冷却され、厚み中心側との間に大きな温度差が生じる（ｂ）。この段階で表面側はマルテンサイト組織、厚み中心側はオーステナイト組織である。厚み中心側には引張応力が付与されるが、周囲を拘束されているので割れは生じない。水槽に浸漬した後、表面側と厚み中心側との温度差が十分に大きいうちにスラブを水槽から取り出す。そうすると両者の温度差によって、表面側の部位は表面へ逃げる熱流量より厚み中心側から受ける熱流量の方が大きくなり、温度が上昇する（ｃ）。本明細書では、この現象を「復熱」と呼んでいる。復熱によって表面側のマルテンサイト組織が２５０℃以上に昇温されると焼戻し現象が起こり、表面側のマルテンサイト組織は軟質化して靭性が付与された状態となる。その後、空冷あるいは保温カバー内での徐冷を行うと、厚み中心部がＭｓ点以下の温度となってマルテンサイトに変態した際に、表面側は引張応力を受ける（ｄ）。しかし、表面側のマルテンサイト組織は既に軟質化され靭性が付与されているので、割れの発生が回避される。 FIG. 2 schematically shows a cooling curve when the slab after casting is immersed in a water tank and then taken out, and a stress state in the cross section of the slab. In the process of cooling from a high temperature, both the slab surface side and the thickness center side exhibit an austenite (γ) structure, and the surface side is cooled at a lower temperature than the thickness center side as in the case of FIG. (A). In the present invention, the slab is immersed in the water tank before the surface temperature reaches the Ms point. At this time, only the surface side is rapidly cooled, and a large temperature difference occurs between the thickness center side (b). At this stage, the surface side has a martensite structure and the thickness center side has an austenite structure. Although tensile stress is applied to the thickness center side, cracks do not occur because the periphery is constrained. After being immersed in the water tank, the slab is taken out from the water tank while the temperature difference between the surface side and the thickness center side is sufficiently large. Then, due to the temperature difference between the two, the heat flow received from the thickness center side becomes larger than the heat flow that escapes to the surface on the surface side portion, and the temperature rises (c). In the present specification, this phenomenon is called “recuperation”. When the surface-side martensite structure is heated to 250 ° C. or higher by recuperation, a tempering phenomenon occurs, and the surface-side martensite structure becomes soft and is given toughness. Thereafter, when air cooling or gradual cooling in the heat insulating cover is performed, the surface side is subjected to tensile stress when transformed into martensite at the thickness center portion at a temperature lower than the Ms point (d). However, since the martensitic structure on the surface side is already softened and toughened, cracking is avoided.
図３に、本発明の適用対象であるマルテンサイト系ステンレス鋼ＳＵＳ４２０Ｊ２の連続鋳造スラブについて、常温までの冷却過程を空冷のみとした場合、および本発明に従って水槽に浸漬したのち取り出す工程を挿入した場合の、得られたスラブの表面外観写真を示す。写真の上下方向がスラブの長手方向（鋳造方向）に対応する。空冷のみを行ったスラブ（図３（ａ））には表面に大きな割れが認められたのに対し、本発明に従って得られたスラブ（図３（ｂ））には割れは観測されなかった。なお、図３（ｂ）の例は後述表２に示すＡ−２である。 FIG. 3 shows a case where a continuous casting slab of martensitic stainless steel SUS420J2 to which the present invention is applied is only cooled by air cooling to the room temperature and a step of taking out after being immersed in a water bath according to the present invention is inserted. The surface appearance photograph of the obtained slab is shown. The vertical direction of the photograph corresponds to the longitudinal direction (casting direction) of the slab. Large cracks were observed on the surface of the slab subjected to only air cooling (FIG. 3A), whereas no cracks were observed in the slab obtained according to the present invention (FIG. 3B). In addition, the example of FIG.3 (b) is A-2 shown in Table 2 mentioned later.
図４は、図３に示した各スラブについて断面硬さを測定し、表面（広面）からの距離で整理したものである。空冷したスラブ（●プロット）は表面近傍から中心部まで硬さは６４０ＨＶ程度でほぼ一定である。一方、本発明にしたがって表面側を復熱により再昇温させたスラブ（○プロット）では、表面から深さ１５ｍｍまでの表層部では硬さが５１０ＨＶ程度に軟質化しており、深さ５０ｍｍ程度で空冷のみのスラブと同等の硬さとなる。発明者らの詳細な調査によれば、スラブ厚さ方向中心部の断面硬さをＨ0（ＨＶ）、スラブ広面の表面からの深さが１０ｍｍ位置の断面硬さをＨ1（ＨＶ）とするとき、両者の差Ｈ0−Ｈ1が１００ＨＶ以上となるように軟質化された表層部を持つマルテンサイト系高Ｃｒ鋼の鋳造スラブは、常温まで冷却された段階で熱間圧延の障害となるような表面の割れがなく、また熱間圧延の加熱に供するまでの間の取扱いにおいてスラブに衝撃が加えられた場合でも一般的なオーステナイト系やフェライト系の鋼種と同様に良好な表面性状が維持されるので、良好な熱間加工性が実現できる。 FIG. 4 shows the cross-sectional hardness measured for each slab shown in FIG. 3 and organized by the distance from the surface (wide surface). The air-cooled slab (● plot) has a substantially constant hardness of about 640 HV from the vicinity of the surface to the center. On the other hand, in the slab (○ plot) in which the surface side is reheated by reheating according to the present invention, the hardness is softened to about 510 HV in the surface layer portion from the surface to a depth of 15 mm, and the depth is about 50 mm. Hardness equivalent to air-cooled slabs. According to the inventors' detailed investigation, the cross-sectional hardness at the center of the slab thickness direction is H 0 (HV), and the cross-sectional hardness at a depth of 10 mm from the surface of the slab wide surface is H 1 (HV). When the slab of martensitic high Cr steel having a surface layer softened so that the difference H 0 -H 1 between the two becomes 100 HV or more is an obstacle to hot rolling when it is cooled to room temperature. Even if the slab is subjected to an impact during handling until it is subjected to heating during hot rolling, it has good surface properties like general austenitic and ferritic steel types. Since it is maintained, good hot workability can be realized.
図５に、スラブ表面温度について、水冷引上げ直前温度Ｔ1（℃）と復熱最高温度Ｔ2（℃）の関係を調査した結果を例示する。常温まで冷却後にスラブ表面に割れが生じていたものを×印、割れが生じていなかったものを○印でプロットしてある。図５のデータは後述表２のＡ鋼の例である。水冷引上げ直前温度Ｔ1は、スラブを水槽から取り出す直前のスラブ表面温度である。水冷引上げ直前温度Ｔ1が高いほど（すなわち水槽中の浸漬時間が短いほど）、復熱最高温度Ｔ2が高くなる。しかし、スラブ表面の復熱最高温度Ｔ2が２５０℃に達しなかった場合には焼割れを安定して回避することができない。表面温度２５０℃未満では表層部のマルテンサイト組織が十分に軟質化されず、靭性が改善されないからである。 FIG. 5 illustrates the results of investigating the relationship between the temperature T 1 (° C.) immediately before the water cooling pull-up and the recuperated maximum temperature T 2 (° C.) for the slab surface temperature. Those with cracks on the surface of the slab after cooling to room temperature are plotted with crosses, and those with no cracks are plotted with circles. The data in FIG. 5 is an example of steel A in Table 2 described later. The temperature T 1 immediately before pulling up the water cooling is the slab surface temperature just before the slab is taken out from the water tank. The higher the temperature T 1 immediately before the water cooling pull-up (that is, the shorter the immersion time in the water tank), the higher the recuperated maximum temperature T 2 . However, when the maximum reheat temperature T 2 on the slab surface does not reach 250 ° C., it is impossible to stably avoid the burning crack. This is because if the surface temperature is less than 250 ° C., the martensitic structure of the surface layer portion is not sufficiently softened and the toughness is not improved.
ただし、マルテンサイト組織を単に２５０℃以上の温度に保持すれば靭性が改善されるわけではない。例えばＳＵＳ４２０Ｊ２の場合Ｍｓ点は３００℃付近にあるので、鋳造後の冷却を空冷のみで行った場合（図１のような冷却曲線の場合）でも、Ｍｓ点で生じたマルテンサイトはその後しばらくの間２５０℃以上の温度域に保持されることになる。しかし、そのような熱履歴ではマルテンサイト組織を軟質化することはできない。そのことは、図４の●プロットからわかるとおりである。したがって、軟質化されたマルテンサイト組織を得るためには、マルテンサイト組織を再度「昇温」させる熱履歴が必要となるのである。厚さ１５０ｍｍ以上のスラブであれば、水槽に浸漬して、水冷引上げ直前温度Ｔ1を１００℃以上とすることによって復熱最高温度Ｔ2は２５０℃以上とすることが可能となる。Ｔ2とＴ1の温度差Ｔ2−Ｔ1は１００℃以上とすることが好ましい。スラブ厚さが１５０ｍｍ以上の場合、Ｔ2が２５０℃以上となるような温度パターンにおいては、通常、自然にＴ2−Ｔ1は１００℃以上となる。 However, if the martensite structure is simply maintained at a temperature of 250 ° C. or higher, the toughness is not improved. For example, in the case of SUS420J2, the Ms point is in the vicinity of 300 ° C. Therefore, even when cooling after casting is performed only by air cooling (in the case of the cooling curve as shown in FIG. 1), martensite generated at the Ms point remains for a while after that. It will be kept in a temperature range of 250 ° C. or higher. However, such a thermal history cannot soften the martensite structure. This can be seen from the ● plot in FIG. Therefore, in order to obtain a softened martensite structure, a heat history for “heating” the martensite structure again is required. If the slab has a thickness of 150 mm or more, the recuperated maximum temperature T 2 can be set to 250 ° C. or higher by immersing it in a water tank and setting the temperature T 1 immediately before water cooling to 100 ° C. or higher. The temperature difference T 2 -T 1 of T 2 and T 1 is preferably set to 100 ° C. or higher. When the slab thickness is 150 mm or more, in a temperature pattern in which T 2 is 250 ° C. or more, T 2 −T 1 is usually 100 ° C. or more naturally.
一方、スラブ表面の引上げ直前温度Ｔ1がＭｓ点（図５の例では約３００℃）より高い場合は、復熱時に表層部がオーステナイト組織のままであり、その後の冷却過程で図１と同様のメカニズムによって割れが生じる。復熱最高温度Ｔ2の上限は特に定める必要はないが、例えば５００℃以下の範囲に管理しても構わない。 On the other hand, when the temperature T 1 immediately before the slab surface is raised is higher than the Ms point (about 300 ° C. in the example of FIG. 5), the surface layer portion remains in the austenite structure at the time of recuperation, and the subsequent cooling process is the same as FIG. Cracks occur due to the mechanism. The upper limit of the maximum recuperation temperature T 2 need not be determined, but may be managed within a range of 500 ° C. or less, for example.
スラブを水槽から取り出すタイミング（すなわち浸漬時間）は、予めその製造ラインにおいて予備実験により冷却曲線のデータを求めておき、それに基づいて設定することができる。 The timing at which the slab is taken out from the water tank (that is, the immersion time) can be set based on the data of the cooling curve obtained in advance through preliminary experiments in the production line.
本発明で対象とする鋼種は、上記の化学組成を有するマルテンサイト系高Ｃｒ鋼である。この組成域には公知の種々のマルテンサイト系ステンレス鋼などが含まれる。 The steel type targeted in the present invention is martensitic high Cr steel having the above chemical composition. This composition range includes various known martensitic stainless steels.
表１に示すマルテンサイト系高Ｃｒ鋼を溶製し、連続鋳造設備を用いて厚さ２００ｍｍ、幅１０４０ｍｍの断面寸法で鋳造し、連続鋳造設備の後段で長さ７ｍ間隔で溶断して、鋳造スラブを得た。スラブの冷却過程において種々の条件で水槽に浸漬したのち取り出す操作を行い、その後、空気中で常温まで放冷して、冷却スラブを得た。Ｍｓ点は各鋼種とも３００℃前後である。水槽に浸漬を開始したときの表面温度は４００〜７００℃の範囲であり、各例ともＭｓ点より高い温度である。表面温度の測定は放射温度計を用いて行った。別途熱電対をスラブ表面に取り付けて測定した測温データを基に放射率等の測定条件を較正してある。各冷却スラブについて表面を観察して割れの有無を調べ、割れが認められなかったものを○、割れが認められたものを×と評価し、○評価を合格とした。表２にスラブの冷却条件および結果を示す。なお、断面硬さを調べた結果、本発明例のものはいずれも前述のＨ0−Ｈ1の値が１００ＨＶ以上であることを確認している。 The martensitic high Cr steel shown in Table 1 is melted, cast using a continuous casting facility, with a cross-sectional dimension of 200 mm in thickness and 1040 mm in width, and melted at intervals of 7 m at the subsequent stage of the continuous casting facility. I got a slab. In the cooling process of the slab, an operation of taking out after being immersed in a water bath under various conditions was performed, and then it was allowed to cool to room temperature in the air to obtain a cooled slab. The Ms point is around 300 ° C. for each steel type. The surface temperature at the start of immersion in the water bath is in the range of 400 to 700 ° C., and in each example, the temperature is higher than the Ms point. The surface temperature was measured using a radiation thermometer. Measurement conditions such as emissivity are calibrated based on temperature measurement data measured separately by attaching a thermocouple to the slab surface. The surface of each cooling slab was observed for the presence or absence of cracks. The case where no cracks were observed was evaluated as “○”, the case where cracks were observed was evaluated as “×”, and the evaluation was “good”. Table 2 shows slab cooling conditions and results. As a result of examining the cross-sectional hardness, it was confirmed that the values of H 0 -H 1 described above were 100 HV or more in all of the examples of the present invention.
表２から判るように、本発明例では、表層部をマルテンサイト組織とした後の復熱によりＴ2を２５０℃以上にしたことにより、割れのない健全な冷却スラブが得られた。表面温度の上昇量Ｔ2−Ｔ1は１００℃以上であった。 As can be seen from Table 2, in the present invention example, a healthy cooling slab without cracks was obtained by setting T 2 to 250 ° C. or higher by reheating after the surface layer portion had a martensite structure. The rise amount T 2 -T 1 of the surface temperature was 100 ° C. or higher.
これに対し、比較例であるＡ−４、Ｂ−４、Ｃ−４、Ｄ−４は空冷のみで常温まで冷却したことによりスラブに割れが生じた。Ａ−５、Ｂ−５、Ｃ−５、Ｄ−５は水槽への浸漬時間が短すぎたのでＴ1がＭｓ点より高くなり、水槽中でマルテンサイト変態が起こらなかったことによりその後の冷却過程でスラブに割れが生じた。Ａ−６、Ｂ−６、Ｃ−６、Ｄ−６は水槽への浸漬時間が長すぎたのでＴ2が２５０℃を下回り、表層部のマルテンサイト組織が十分に軟質化しなかったことによりその後の冷却過程でスラブに割れが生じた。 In contrast, A-4, B-4, C-4, and D-4, which are comparative examples, were cracked in the slab because they were cooled to room temperature only by air cooling. Since A-5, B-5, C-5, and D-5 were immersed in the water tank too short, T 1 became higher than the Ms point, and the martensitic transformation did not occur in the water tank. The slab cracked during the process. Since A-6, B-6, C-6, and D-6 were immersed in the water bath for too long, T 2 was below 250 ° C., and the martensite structure of the surface layer portion was not sufficiently softened. Cracks occurred in the slab during the cooling process.
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| JPS53149119A (en) * | 1977-06-01 | 1978-12-26 | Kawasaki Steel Co | Method of cooling martensite stainless steel strip high temerature state |
| JPH08188827A (en) * | 1995-01-09 | 1996-07-23 | Sumitomo Metal Ind Ltd | Production of martensitic stainless steel tube |
| JPH0999352A (en) * | 1995-10-03 | 1997-04-15 | Aichi Steel Works Ltd | Method for cooling continuously cast slab |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JPS53149119A (en) * | 1977-06-01 | 1978-12-26 | Kawasaki Steel Co | Method of cooling martensite stainless steel strip high temerature state |
| JPH08188827A (en) * | 1995-01-09 | 1996-07-23 | Sumitomo Metal Ind Ltd | Production of martensitic stainless steel tube |
| JPH0999352A (en) * | 1995-10-03 | 1997-04-15 | Aichi Steel Works Ltd | Method for cooling continuously cast slab |
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| US10118218B2 (en) * | 2015-01-15 | 2018-11-06 | Nippon Steel & Sumitomo Metal Corporation | Method for continuously casting slab |
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