JP2020131203A - CONTINUOUS CASTING METHOD FOR Ni-CONTAINING LOW ALLOY STEEL - Google Patents
CONTINUOUS CASTING METHOD FOR Ni-CONTAINING LOW ALLOY STEEL Download PDFInfo
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本願はNi含有低合金鋼の連続鋳造方法等を開示する。 The present application discloses a method for continuously casting Ni-containing low alloy steel.
近年、機械構造用合金鋼などの製品において、Nb、V、Niなどの合金元素を含有させる、あるいはC、Nを富化するなどの手段によって機械的性質の改善を図ろうとする試みが広く行われている。これらを含有する合金鋼を湾曲型または垂直曲げ型の連続鋳造機を用いて鋳造する場合、いわゆる横割れや横ひび割れと呼ばれる割れが鋳片の表面に発生しやすい。この割れは、連続鋳造機の矯正点において鋳片の曲げが矯正される際に、鋳片の表面に作用する応力が鋼に固有の限界応力を超えるために発生する。特に、上記の合金鋼においては、鋳型から引き抜かれた後の二次冷却過程において、オーステナイト粒界にAlNやNbCなどの窒化物や炭化物が析出しやすい。これらの析出物が析出したオーステナイト粒界は、鋳片に応力が作用した場合に割れの起点となりやすい。 In recent years, in products such as alloy steels for machine structural use, attempts have been widely made to improve mechanical properties by means such as containing alloying elements such as Nb, V, and Ni, or enriching C and N. It has been. When alloy steel containing these is cast using a curved or vertical bending type continuous casting machine, so-called lateral cracks and lateral cracks are likely to occur on the surface of the slab. This crack occurs because the stress acting on the surface of the slab exceeds the limit stress inherent in steel when the bending of the slab is straightened at the straightening point of the continuous casting machine. In particular, in the above alloy steel, nitrides and carbides such as AlN and NbC are likely to precipitate at the austenite grain boundaries in the secondary cooling process after being drawn from the mold. The austenite grain boundaries where these precipitates are deposited tend to be the starting points of cracks when stress is applied to the slabs.
また、矯正の際に鋳片の表面割れが起こる原因の1つに、鋳造ままのγ粒が製品段階と比較して極めて大きいため、応力が脆弱部に集中しやすいということが挙げられる。そこで、鋳型の直下から一定温度以下まで鋳片表面を強く冷却し、続いて一定温度以上に復熱させ、表層のオーステナイト粒を微細化してから矯正する技術が提案されている。 In addition, one of the causes of surface cracking of the slab during straightening is that the as-cast γ grains are extremely large compared to the product stage, so that stress tends to concentrate on the fragile portion. Therefore, a technique has been proposed in which the surface of the slab is strongly cooled from directly below the mold to a certain temperature or less, and then reheated to a certain temperature or more to refine the austenite particles on the surface layer and then correct the austenite particles.
たとえば、特許文献1には、鋳片の表面をその温度がAr3以上の温度域からAr1以下の温度域になるまで300℃/s以上の冷却速度で冷却し、その後、再び鋳片の表面温度をAr3以上の温度域まで復熱させることを特徴とする連続鋳造鋳片の表面割れ防止方法が提案されている、 For example, in Patent Document 1, the surface of a slab is cooled at a cooling rate of 300 ° C./s or more from a temperature range of Ar3 or higher to a temperature range of Ar1 or lower, and then the surface temperature of the slab is again cooled. A method for preventing surface cracking of continuously cast slabs has been proposed, which comprises reheating the material to a temperature range of Ar3 or higher.
特許文献2には、連続鋳造の鋳型直下から矯正点の手前の冷却過程において、鋼の連続冷却変態線図でのベイナイト、フェライトあるいはパーライト変態開始温度を下回る温度まで鋳片の表層部を冷却し、次いでAc3以上の温度まで3℃/s以上50℃/s以下の昇温速度にて復熱させる、もしくはAr3−100℃を下回る温度まで鋳片の表層部を冷却し、次いでAc3以上の温度まで1.4℃/s以下の昇温速度にて復熱させることを特徴とする鋼の連続鋳造方法が提案されている。 In Patent Document 2, the surface layer of the slab is cooled to a temperature lower than the bainite, ferrite or pearlite transformation start temperature in the continuous cooling transformation diagram of steel in the cooling process from directly below the mold for continuous casting to just before the straightening point. Then, the surface layer of the slab is cooled to a temperature of 3 ° C./s or higher and 50 ° C./s or lower at a heating rate of 3 ° C./s or higher, or to a temperature lower than Ar3-100 ° C., and then a temperature of Ac3 or higher. A method for continuous casting of steel has been proposed, which comprises reheating at a heating rate of 1.4 ° C./s or less.
特許文献3には、連続鋳造の鋳型直下において鋼の連続冷却変態線図におけるフェライト−パーライト変態終了温度未満かつベイナイト変態開始温度を超える温度域までの鋳片の表層部を冷却し、その後、前記連続冷却変態線図におけるフェライト−パーライト変態のノーズを通る一定速度の冷却曲線と交差するまで、フェライト−パーライト変態終了温度未満かつベイナイト変態開始温度を超える温度域に保持することを特徴とする連続鋳造方法が提案されている。 In Patent Document 3, the surface layer portion of the slab is cooled to a temperature range below the ferrite-pearlite transformation end temperature and above the bainite transformation start temperature in the continuous cooling transformation diagram of steel immediately under the mold for continuous casting, and then the above-mentioned Continuous casting characterized by holding in a temperature range below the ferrite-pearlite transformation end temperature and above the bainite transformation start temperature until crossing a constant rate cooling curve through the nose of the ferrite-pearlite transformation in the continuous cooling transformation diagram. A method has been proposed.
特許文献1〜3に開示された技術はいずれも、鋼の相変態を利用して結晶粒を微細化し、矯正点における延性を改善させることを目的とした技術であるが、鋼の組成によっては2次冷却帯に相当する冷却速度においてフェライト−パーライト変態せずAr3を定義できない、すなわち上記技術では十分な表面割れの抑制指針が得られない鋼種も実際に生産されている。そのような鋼種として、例えば、Ni含有低合金鋼が挙げられる。Ni含有低合金鋼の連続鋳造において鋳片の矯正を行う際に発生する鋳片表面の横割れやひび割れを安定して抑制することが可能な新たな技術が必要である。 All of the techniques disclosed in Patent Documents 1 to 3 are techniques aimed at improving the ductility at the straightening point by refining the crystal grains by utilizing the phase transformation of the steel, but depending on the composition of the steel. Steel grades in which Ar3 cannot be defined without ferrite-pearlite transformation at a cooling rate corresponding to the secondary cooling zone, that is, a sufficient guideline for suppressing surface cracks cannot be obtained by the above technique, are actually produced. Examples of such steel types include Ni-containing low alloy steels. In the continuous casting of Ni-containing low alloy steel, a new technique capable of stably suppressing lateral cracks and cracks on the surface of the slab generated when the slab is straightened is required.
本願は上記課題を解決するための手段の一つとして、質量%で、C:0.1〜0.4%、Si:0.01〜0.5%、Mn:0.3〜1.4%、Cr:0.8〜2.0%、Mo:0.6%以下、Ni:0.5〜2.0%の組成を有する鋼の鋳片を、矯正点を有する連続鋳造機を用いて連続的に鋳造する方法であって、鋳型の直下から前記矯正点に至る前において、前記鋳片の表面温度が350〜475℃の間にある時間をTL(s)、600〜675℃にある時間をTH(s)として、下記式(a)で定められるTA(s)が60以上となるように前記鋳片を冷却し、次いで前記矯正点に至る前までに、前記鋳片の表面温度をAc3以上の温度域まで復熱させることを特徴とする、Ni含有低合金鋼の連続鋳造方法を開示する。 In this application, as one of the means for solving the above-mentioned problems, in mass%, C: 0.1-0.4%, Si: 0.01-0.5%, Mn: 0.3-1.4. %, Cr: 0.8 to 2.0%, Mo: 0.6% or less, Ni: 0.5 to 2.0% of steel slabs, using a continuous casting machine with straightening points. a continuous casting methods Te, before reaching right under the mold to the correct point, a time in which the surface temperature of the slab is between 350~475 ℃ T L (s), 600~675 ℃ as T H (s) of time in, the cast strip is cooled as T a defined by the following formula (a) (s) is more than 60, then up before reaching the correcting point, the cast A method for continuously casting a Ni-containing low alloy steel, which comprises reheating the surface temperature of a piece to a temperature range of Ac3 or higher, is disclosed.
TA=TL+TH×0.04×[Ni]−2.32 ・・・(a)
(式(a)において[Ni]は鋼におけるNiの濃度(質量%)である。)
T A = T L + T H × 0.04 × [Ni] -2.32 ··· (a)
(In the formula (a), [Ni] is the concentration (mass%) of Ni in the steel.)
本開示のNi含有低合金鋼の連続鋳造方法において、前記鋳片は、質量%で、Al:0.1%以下、Ti:0.1%以下、V:0.4%以下、Ca:0.01%以下、Mg:0.01%以下、REM:0.01%以下、Nb:0.05%以下、B:0.004%以下、N:0.025%以下の組成を有していてもよい。 In the continuous casting method of Ni-containing low alloy steel of the present disclosure, the slabs are, in mass%, Al: 0.1% or less, Ti: 0.1% or less, V: 0.4% or less, Ca: 0. It has a composition of 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, Nb: 0.05% or less, B: 0.004% or less, N: 0.025% or less. You may.
本開示の方法によれば、連続鋳造時に鋳片の矯正を行う際に発生する、鋳片の表面の横割れや横ひび割れを安定して抑制することができる。そのため、本開示の方法で製造した鋳片を熱間圧延することにより、表面割れ等の発生が抑制された鋼板や鋼片を得ることができる。 According to the method of the present disclosure, lateral cracks and lateral cracks on the surface of the slab, which occur when the slab is straightened during continuous casting, can be stably suppressed. Therefore, by hot rolling the slabs produced by the method of the present disclosure, it is possible to obtain steel plates and steel slabs in which the occurrence of surface cracks and the like is suppressed.
図1を参照しつつ本開示のNi含有低合金鋼の連続鋳造方法について説明する。図1においては分かり易さのため冷却スプレーノズル等を省略して示している。冷却スプレーノズルは、例えば、鋳型10の直下から矯正点20に至る前までの間のサポートロール間に備えられ、鋳片1の両面側から冷却水を噴射し得る。図1においては垂直曲げ型の連続鋳造機100を例示したが、本開示の連続鋳造方法は矯正点を有するいずれの連続鋳造機を用いた場合にも適用可能である。例えば、湾曲型の連続鋳造機を用いてもよい。尚、「矯正点」とは、鋳片1の鋳造方向を湾曲から水平方向に矯正するために歪を加える点をいう。なお、矯正は複数個所で行ってもよい。鋳型10、矯正点20等を備える連続鋳造機100の構成そのものについては従来公知の構成と同様とすればよいことから、ここでは詳細な説明を省略する。 The continuous casting method of the Ni-containing low alloy steel of the present disclosure will be described with reference to FIG. In FIG. 1, the cooling spray nozzle and the like are omitted for the sake of clarity. The cooling spray nozzle is provided, for example, between the support rolls from directly below the mold 10 to before reaching the straightening point 20, and can inject cooling water from both sides of the slab 1. Although the vertical bending type continuous casting machine 100 is illustrated in FIG. 1, the continuous casting method of the present disclosure can be applied to any continuous casting machine having a correction point. For example, a curved continuous casting machine may be used. The "correction point" means a point where strain is applied to correct the casting direction of the slab 1 from the curve to the horizontal direction. The correction may be performed at a plurality of places. Since the configuration itself of the continuous casting machine 100 including the mold 10, the straightening point 20, and the like may be the same as the conventionally known configuration, detailed description thereof will be omitted here.
図1に示すように、本開示のNi含有低合金鋼の連続鋳造方法は、質量%で、C:0.1〜0.4%、Si:0.01〜0.5%、Mn:0.3〜1.4%、Cr:0.8〜2.0%、Mo:0.6%以下、Ni:0.5〜2.0%の組成を有する鋼の鋳片1を、矯正点20を有する連続鋳造機100を用いて連続的に鋳造する方法であって、鋳型10の直下から矯正点20に至る前において、鋳片1の表面温度が350〜475℃の間にある時間をTL(s)、600〜675℃にある時間をTH(s)として、下記式(a)で定められるTA(s)が60以上となるように鋳片1を冷却し、次いで矯正点20に至る前までに、鋳片1の表面温度をAc3以上の温度域まで復熱させることを特徴とする。 As shown in FIG. 1, the continuous casting method of the Ni-containing low alloy steel of the present disclosure is C: 0.1 to 0.4%, Si: 0.01 to 0.5%, Mn: 0 in mass%. .3 to 1.4%, Cr: 0.8 to 2.0%, Mo: 0.6% or less, Ni: 0.5 to 2.0% Steel slab 1 is straightened. A method of continuously casting using a continuous casting machine 100 having a 20. A time during which the surface temperature of the slab 1 is between 350 and 475 ° C. before reaching the straightening point 20 from directly below the mold 10. T L (s), as T H (s) of time in the 600-675 ° C., cooling the slab 1 as following equation T a defined by (a) (s) is 60 or more, then corrective The surface temperature of the slab 1 is reheated to a temperature range of Ac3 or higher before reaching the point 20.
TA=TL+TH×0.04×[Ni]−2.32 ・・・(a)
(式(a)において[Ni]は鋼におけるNiの濃度(質量%)である。)
T A = T L + T H × 0.04 × [Ni] -2.32 ··· (a)
(In the formula (a), [Ni] is the concentration (mass%) of Ni in the steel.)
1.鋼種
本開示の連続鋳造方法において、鋳造対象となる鋼にはFe以外にC、Si、Mn、Cr及びNiが必須で含まれる。また、任意成分として、例えば、Mo、Al、Ti、V、Ca、Mg、REM、Nb、B及びNから選ばれる少なくとも1つが含まれていてもよい。また、不可避不純物として、例えば、PやSが含まれていてもよい。
1. 1. Steel type In the continuous casting method of the present disclosure, C, Si, Mn, Cr and Ni are indispensably contained in the steel to be cast in addition to Fe. Further, as an optional component, for example, at least one selected from Mo, Al, Ti, V, Ca, Mg, REM, Nb, B and N may be contained. Further, as unavoidable impurities, for example, P and S may be contained.
1.1 C
Cは鋼の静的強度だけでなく、疲労強度、靭性、延性に影響する最も基本的な元素である。Cが0.1質量%未満では静的強度および疲労強度が不十分である。よって下限を0.1質量%以上とする。また、0.4質量%を超えると靭性が劣化する。よって上限を0.4質量%以下とする。
1.1 C
C is the most basic element that affects not only the static strength of steel but also fatigue strength, toughness and ductility. If C is less than 0.1% by mass, the static strength and fatigue strength are insufficient. Therefore, the lower limit is set to 0.1% by mass or more. Further, if it exceeds 0.4% by mass, the toughness deteriorates. Therefore, the upper limit is set to 0.4% by mass or less.
1.2 Si
SiはCに次いで固溶強化能が大きい重要な元素である。Siが0.01質量%未満では十分な強度を得ることができない。よって下限を0.01質量%以上とする。また、0.5質量%を超えると靭性や加工性を著しく劣化させる。よって上限を0.5質量%以下とする。
1.2 Si
Si is an important element having the second largest solid solution strengthening ability after C. If Si is less than 0.01% by mass, sufficient strength cannot be obtained. Therefore, the lower limit is set to 0.01% by mass or more. Further, if it exceeds 0.5% by mass, the toughness and workability are significantly deteriorated. Therefore, the upper limit is set to 0.5% by mass or less.
1.3 Mn
Mnは焼入れ性を向上させ、冷却速度が不十分な場合でも部品の内部まで硬度を確保するのに重要な元素である。Mnが0.3質量%未満では必要な強度が確保できない。よって下限を0.3質量%以上とする。また、1.4質量%を超えると靭性および加工性が劣化する。よって上限を1.4質量%以下とする。
1.3 Mn
Mn is an important element for improving hardenability and ensuring hardness even inside parts even when the cooling rate is insufficient. If Mn is less than 0.3% by mass, the required strength cannot be secured. Therefore, the lower limit is set to 0.3% by mass or more. If it exceeds 1.4% by mass, the toughness and workability deteriorate. Therefore, the upper limit is set to 1.4% by mass or less.
1.4 Cr
CrはMnと同様、鋼の焼入れ性を向上する有用な元素であり、低合金鋼を構成する重要な元素の1つである。0.8質量%未満ではこの効果が十分得られない。よって下限を0.8質量%以上とする。また、2.0質量%を超えると効果がほぼ飽和し、コストの増大を招く。よって上限を2.0質量%以下とする。
1.4 Cr
Like Mn, Cr is a useful element for improving the hardenability of steel and is one of the important elements constituting low alloy steel. If it is less than 0.8% by mass, this effect cannot be sufficiently obtained. Therefore, the lower limit is set to 0.8% by mass or more. Further, if it exceeds 2.0% by mass, the effect is almost saturated, which leads to an increase in cost. Therefore, the upper limit is set to 2.0% by mass or less.
1.5 Mo
Moはその炭窒化物を微細に析出させることにより、焼戻し時に鋼を硬化させる、いわゆる2次硬化を起こす元素であり、疲労強度の改善にも有効である。また、焼入れ性向上効果も大きい。しかし1.5質量%を超えると焼入れ熱処理時に未溶解の炭化物が残存しやすくなり、靭性を劣化させる虞がある。靭性の劣化を十分に抑制するためには、上限を0.6質量%以下とすることが好ましい。下限は特に限定されないが、0質量%以上、好ましくは0.004質量%以上とする。
1.5 Mo
Mo is an element that causes so-called secondary hardening, which hardens steel during tempering by finely precipitating the carbonitride, and is also effective in improving fatigue strength. In addition, the effect of improving hardenability is also great. However, if it exceeds 1.5% by mass, undissolved carbides tend to remain during quenching heat treatment, which may deteriorate toughness. In order to sufficiently suppress the deterioration of toughness, the upper limit is preferably 0.6% by mass or less. The lower limit is not particularly limited, but is 0% by mass or more, preferably 0.004% by mass or more.
1.6 Ni
Niは強度及び靭性の確保に有効であり、焼入れ性の向上効果も大きい。Niの量を0.5質量%以上とすることで、この効果が一層顕著となる。一方、2.0質量%を超えると効果が飽和して、コストの増大を招く。よって上限を2.0質量%以下とする。
1.6 Ni
Ni is effective in ensuring strength and toughness, and has a great effect of improving hardenability. By setting the amount of Ni to 0.5% by mass or more, this effect becomes more remarkable. On the other hand, if it exceeds 2.0% by mass, the effect is saturated and the cost increases. Therefore, the upper limit is set to 2.0% by mass or less.
1.7 Al
Alは脱酸目的で最も広く用いられる元素であり、またAlNを生成して結晶粒の粗大化を抑制する効果がある。しかし、0.1質量%を超えると、Al2O3の凝集合に伴い鋳造中にノズル詰まりが発生したり、鋼中に残存するAl2O3が性能を劣化させたりするなどの不具合が生じる虞がある。よって上限を0.1質量%以下とすることが好ましい。下限は特に限定されないが、0質量%以上、好ましくは0.016質量%以上とする。
1.7 Al
Al is the most widely used element for deoxidizing purposes, and has the effect of producing AlN to suppress coarsening of crystal grains. However, when it exceeds 0.1 wt%, nozzle clogging during casting due to agglomeration if may occur in Al 2 O 3, a defect of the Al 2 O 3 or the like remaining in the steel or degrade the performance It may occur. Therefore, the upper limit is preferably 0.1% by mass or less. The lower limit is not particularly limited, but is 0% by mass or more, preferably 0.016% by mass or more.
1.8 Ti
TiはAlと同様に窒化物を生成し得る元素であり、熱的安定性に優れ、より高温まで結晶粒粗大化抑制効果を持続させる。ただし、0.1質量%を超えるとTiNが粗大に成長しやすくなり、疲労強度を低下させる虞がある。よって上限を0.1質量%以下とする。下限は特に限定されないが、0質量%以上、好ましくは0.002質量%以上とする。
1.8 Ti
Like Al, Ti is an element capable of forming a nitride, has excellent thermal stability, and maintains the effect of suppressing grain grain coarsening up to a higher temperature. However, if it exceeds 0.1% by mass, TiN tends to grow coarsely, which may reduce the fatigue strength. Therefore, the upper limit is set to 0.1% by mass or less. The lower limit is not particularly limited, but is 0% by mass or more, preferably 0.002% by mass or more.
1.9 V
VはTi及びAlと同様に窒化物を生成し得る元素であり、強度改善のために用いられる。しかし、0.4質量%を超えるとVNが粗大に成長しやすくなり、疲労強度を低下させる虞がある。よって上限を0.4質量%以下とすることが好ましい。下限は特に限定されず、0質量%であってもよいが、0.002質量%以上とした場合に強度改善の効果が得られ易い。
1.9 V
V is an element capable of forming a nitride like Ti and Al, and is used for improving the strength. However, if it exceeds 0.4% by mass, the VN tends to grow coarsely, which may reduce the fatigue strength. Therefore, the upper limit is preferably 0.4% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.002% by mass or more, the effect of improving the strength can be easily obtained.
1.10 Ca
CaはAl2O3を改質し、酸化物系介在物の粗大化を抑制する効果がある。しかし、0.01質量%を超えるとCaO−Al2O3を主成分とする却って粗大な酸化物系介在物を形成し、疲労破壊の基点となる虞がある。よって上限を0.01質量%以下とすることが好ましい。下限は特に限定されず、0質量%であってもよいが、0.0002質量%以上とした場合に酸化物粗大化を抑制する効果が得られ易い。
1.10 Ca
Ca has the effect of modifying Al 2 O 3 and suppressing the coarsening of oxide-based inclusions. However, if it exceeds 0.01% by mass, it may form coarse oxide-based inclusions containing CaO-Al 2 O 3 as a main component and serve as a base point for fatigue failure. Therefore, the upper limit is preferably 0.01% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.0002% by mass or more, the effect of suppressing oxide coarsening can be easily obtained.
1.11 Mg
MgはCa同様、Al2O3を改質し、酸化物系介在物の粗大化を抑制する効果がある。また、硫化物系介在物にも作用し、アスペクト比を低下させる効果がある。しかし、0.01質量%を超えるとMgOを主成分とする粗大なクラスター状酸化物系介在物を形成し、疲労破壊の基点となる虞がある。よって上限を0.01質量%以下とすることが好ましい。下限は特に限定されず、0質量%であってもよいが、0.0002質量%以上とした場合に酸化物系介在物の粗大化を抑制する効果等が得られ易い。
1.11 Mg
Like Ca, Mg has the effect of modifying Al 2 O 3 and suppressing the coarsening of oxide-based inclusions. It also acts on sulfide-based inclusions and has the effect of lowering the aspect ratio. However, if it exceeds 0.01% by mass, coarse cluster-like oxide-based inclusions containing MgO as a main component may be formed, which may serve as a base point for fatigue fracture. Therefore, the upper limit is preferably 0.01% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.0002% by mass or more, the effect of suppressing the coarsening of oxide-based inclusions can be easily obtained.
1.12 REM
REMもまたAl2O3を改質し、酸化物系介在物の粗大化を抑制する効果がある。しかし、0.01質量%を超えると鋼の清浄性を低下させ、母材の靭性を劣化させる虞がある。よって上限を0.01質量%以下とすることが好ましい。下限は特に限定されず、0質量%であってもよいが、0.0002質量%以上とした場合に酸化物系介在物の粗大化を抑制する効果等が得られ易い。なお、ここでREMとはLaやCe等の希土類元素を表すが、そのうちの任意の1種類、あるいは2種類以上のREMを用いることができる。
1.12 REM
REM also has the effect of modifying Al 2 O 3 and suppressing the coarsening of oxide-based inclusions. However, if it exceeds 0.01% by mass, the cleanliness of the steel may be lowered and the toughness of the base metal may be deteriorated. Therefore, the upper limit is preferably 0.01% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.0002% by mass or more, the effect of suppressing the coarsening of oxide-based inclusions can be easily obtained. Here, the REM represents a rare earth element such as La or Ce, and any one type or two or more types of REM can be used.
1.13 Nb
Nbは強度および靭性の改善に効果がある。しかし、0.05質量%を超えると効果が飽和する。Nbの含有量をあまりに多くし過ぎると、鋳造時に割れが発生する虞もある。よって上限を0.05質量%以下とすることが好ましい。下限は特に限定されず、0質量%であってもよいが、0.001質量%以上とした場合に強度改善効果や靭性改善効果が得られ易い。
1.13 Nb
Nb is effective in improving strength and toughness. However, if it exceeds 0.05% by mass, the effect is saturated. If the Nb content is too high, cracks may occur during casting. Therefore, the upper limit is preferably 0.05% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.001% by mass or more, the strength improving effect and the toughness improving effect can be easily obtained.
1.14 B
Bは少量で大きな焼入れ性向上効果がある。しかし、0.004質量%を超えると効果が飽和する。Bの含有量をあまりに多くし過ぎると、鋳造時に割れが発生する虞もある。よって上限を0.004質量%以下とすることが好ましい。下限は特に限定されず、0質量%であってもよいが、0.0002質量%以上とした場合に焼入れ性向上効果が得られ易い。
1.14 B
B has a large hardenability improving effect with a small amount. However, if it exceeds 0.004% by mass, the effect is saturated. If the B content is too high, cracks may occur during casting. Therefore, the upper limit is preferably 0.004% by mass or less. The lower limit is not particularly limited and may be 0% by mass, but when it is 0.0002% by mass or more, the effect of improving hardenability can be easily obtained.
1.15 N
NはTiN、AlN等の窒化物を生成し、結晶粒粗大化抑制効果を発現させる。しかし、0.025質量%を超えると窒化物の粗大化を招き、疲労強度を低下させる虞がある。また、熱間延性を低下させ、鋳造時あるいは圧延時に表面疵の要因となる虞がある。よって上限を0.025質量%以下とすることが好ましい。鋼材清浄性の観点から、0.02質量%以下とするとより好ましい。下限は特に限定されないが、0質量%以上、好ましくは0.0036質量%以上とする。
1.15 N
N produces nitrides such as TiN and AlN, and exerts an effect of suppressing grain grain coarsening. However, if it exceeds 0.025% by mass, the nitride may become coarse and the fatigue strength may be lowered. In addition, the hot ductility is lowered, which may cause surface defects during casting or rolling. Therefore, the upper limit is preferably 0.025% by mass or less. From the viewpoint of steel cleanliness, 0.02% by mass or less is more preferable. The lower limit is not particularly limited, but is 0% by mass or more, preferably 0.0036% by mass or more.
2.鋳片1の2次冷却
図1に示すように、本開示の連続鋳造方法においては、上記組成を有する鋼の鋳片1を鋳型10から連続的に引き抜き、鋳型10の直下から矯正点20に至るまでに、鋳片1の表面に冷却水を噴射する等して鋳片1の2次冷却を行う。ここで、本開示の連続鋳造方法においては、鋳型10の直下から矯正点20に至る前において、鋳片1の表面温度が350〜475℃の間にある時間をTL(s)、600〜675℃にある時間をTH(s)として、上記式(a)で定められるTA(s)が60以上となるように鋳片1を冷却することが重要である。
2. 2. Secondary cooling of the slab 1 As shown in FIG. 1, in the continuous casting method of the present disclosure, the steel slab 1 having the above composition is continuously drawn from the mold 10 and reaches the straightening point 20 from directly below the mold 10. By then, the surface of the slab 1 is secondarily cooled by injecting cooling water or the like. Here, in the continuous casting method of the present disclosure, the time during which the surface temperature of the slab 1 is between 350 and 475 ° C. is set to TL (s), 600 to before reaching the straightening point 20 from directly below the mold 10. the time in the 675 ° C. as T H (s), it is important to cool the slab 1 so that the formula T a defined by (a) (s) is 60 or more.
鋼の連続鋳造においては、粗大な鋳造組織のオーステナイト(もしくは旧オーステナイト)粒界に沿って鋳片の表面割れが発生し易い。これは、温度降下に伴って介在物あるいは軟質なフェライトがオーステナイト(もしくは旧オーステナイト)粒界上に優先的に生成することに起因する。これを防止するために、鋳型直下の2次冷却帯において鋳片の冷却を行うことで、鋳片表層の組織をフェライト−パーライトあるいはベイナイトに変態させ、その後にAc3以上まで復熱させて逆変態させることにより、鋳片表層のオーステナイト組織を微細化させる技術が開発されてきた。 In continuous steel casting, surface cracks in the slab are likely to occur along the austenite (or former austenite) grain boundaries of the coarse cast structure. This is due to the preferential formation of inclusions or soft ferrite on the austenite (or former austenite) grain boundaries with temperature drop. In order to prevent this, the slab is cooled in the secondary cooling zone directly under the mold to transform the structure of the slab surface layer into ferrite-pearlite or bainite, and then reheat to Ac3 or higher to reverse transformation. A technique for refining the austenite structure on the surface layer of the slab has been developed.
例えば前述した特許文献1にあるとおり、鋳片表層組織をフェライト−パーライトあるいはベイナイトに変態させる(以後必要に応じてオーステナイトの分解と称する)ことを目的として、2次冷却帯における冷却目標温度としてAr1、あるいは特定の温度が提案されてきた。しかし、上記技術は主に炭素鋼を企図して提案されたものである。Cr等を所定量含有する低合金鋼は炭素鋼と比較してフェライト−パーライト変態が遅れる傾向があるため、上記技術では鋳片割れを十分に抑制できないという課題が存在する。 For example, as described in Patent Document 1 described above, Ar1 is set as the cooling target temperature in the secondary cooling zone for the purpose of transforming the surface layer structure of the slab into ferrite-pearlite or bainite (hereinafter referred to as decomposition of austenite as necessary). , Or a specific temperature has been proposed. However, the above technique was proposed mainly for carbon steel. Since low alloy steel containing a predetermined amount of Cr or the like tends to delay the ferrite-pearlite transformation as compared with carbon steel, there is a problem that slab cracking cannot be sufficiently suppressed by the above technique.
当該課題についてTTT線図を用いて詳しく説明する。図2に炭素鋼のTTT線図の例を、図3に低炭合金鋼(JIS:SCM420)のTTT線図の例を示す。炭素鋼のTTT線図は700℃以下の温度域でのフェライト−パーライト変態が短時間で進行するのに対し、低炭合金鋼のTTT線図は温度により変態の進行が大きく変化する。低炭合金鋼においては、含有するCrやMoの影響によりフェライト−パーライト変態が起こりづらいことが知られており、特に500〜600℃においてはベイナイト変態も遅滞するためこのような形状となる。すなわち、例えば炭素鋼においてオーステナイトの分解が迅速に進む温度域であっても鋼種によっては分解が十分に進まないことが一般的に起こり得る。よって、鋼の連続鋳造時に、鋳型直下の2次冷却帯において鋳片を急冷、復熱することにより鋳片表層のオーステナイト組織を微細化して矯正点での割れを回避するためには、鋳造する鋼種の金属学的性質に応じた冷却方法を採用することが必要不可欠である。 The subject will be described in detail using a TTT diagram. FIG. 2 shows an example of a TTT diagram of carbon steel, and FIG. 3 shows an example of a TTT diagram of low carbon alloy steel (JIS: SCM420). In the TTT diagram of carbon steel, the ferrite-pearlite transformation proceeds in a short time in a temperature range of 700 ° C. or lower, whereas in the TTT diagram of low carbon alloy steel, the transformation progress changes greatly depending on the temperature. It is known that ferrite-pearlite transformation is unlikely to occur in low-carbon alloy steel due to the influence of Cr and Mo contained in it, and especially at 500 to 600 ° C., bainite transformation is also delayed, resulting in such a shape. That is, for example, in carbon steel, even in a temperature range in which austenite decomposition proceeds rapidly, it is generally possible that decomposition does not proceed sufficiently depending on the steel type. Therefore, during continuous casting of steel, the austenite structure on the surface layer of the slab is refined by quenching and reheating the slab in the secondary cooling zone directly under the mold, and casting is performed in order to avoid cracking at the straightening point. It is indispensable to adopt a cooling method according to the metallurgical properties of the steel type.
一方、TTT線図は様々なデータブックが知られているが、そのほとんどは図2および図3に示すように、オーステナイト化温度1000℃以下によるものである。鋳片冷却中の変態挙動はオーステナイト粒径に大きく依存するため、これらのデータブックをそのまま利用することはできない。本発明者らは上記の組成を有する低合金鋼の変態挙動を踏まえて鋭意研究を進めた結果、2次冷却帯において上記式(a)で定められるTA(s)を60以上とすることで、上記の組成を有するNi含有低合金鋼の鋳片表層組織を効果的に変態させることができることを知見した。 On the other hand, various data books are known for the TTT diagram, but most of them are based on the austenitization temperature of 1000 ° C. or lower as shown in FIGS. 2 and 3. Since the transformation behavior during slab cooling largely depends on the austenite particle size, these data books cannot be used as they are. The present inventors have a result of our intensive studies in light of the transformation behavior of low alloy steel having the above composition has the above formula in the secondary cooling zone the T defined by (a) A (s) 60 or more It was found that the surface structure of slabs of Ni-containing low alloy steel having the above composition can be effectively transformed.
尚、本開示の連続鋳造方法においては、TLの温度範囲下限を350℃としているが、鋼種によってはこの温度がマルテンサイト変態開始温度を下回り、組織の一部あるいは全部がマルテンサイトになることも考えられる。しかしながら、その場合でも逆変態後には表層組織の微細化が可能であり、所望の効果を得ることが可能である。 In the continuous casting method of the present disclosure, the lower limit of the temperature range of TL is set to 350 ° C., but depending on the steel type, this temperature may be lower than the martensitic transformation start temperature, and part or all of the structure may become martensite. Is also possible. However, even in that case, the surface layer structure can be miniaturized after the reverse transformation, and a desired effect can be obtained.
実機の連続鋳造機内においては、鋳片1が鋳型10の直下から矯正点20に至る前までの間において、鋳片1の表面の温度が350〜475℃、600〜675℃の領域を複数回通過することもあり得る。この場合は、TL、THはそれぞれの温度領域を通過した時間の和で表される。例えば図4に示すような熱履歴において、TL、THは以下の式(b)、(c)で求めることができる。 In the continuous casting machine of the actual machine, the surface temperature of the slab 1 is 350 to 475 ° C. and 600 to 675 ° C. multiple times before the slab 1 reaches the straightening point 20 from directly under the mold 10. It is possible to pass. In this case, T L and T H are represented by the sum of the times passed through the respective temperature regions. For example, in the thermal history as shown in FIG. 4, TL and TH can be obtained by the following equations (b) and (c).
TL=t4−t3 ・・・(b)
TH=(t2−t1)+(t6−t5) ・・・(c)
T L = t 4- t 3 ... (b)
TH = (t 2- t 1 ) + (t 6 −t 5 ) ・ ・ ・ (c)
2次冷却帯において鋳片1を冷却する方法としては、上述した冷却スプレーノズルを用いて冷却水を噴射する方法のほか、気流を用いる方法、特別な冷却設備を備えず放冷する方法等いずれも有効である。さらに、これらを組み合わせて冷却する方法でも構わない。鋳片1の冷却速度は特に限定されるものではなく、いずれの冷却速度であっても所望の効果が発揮される。 As a method of cooling the slab 1 in the secondary cooling zone, in addition to the method of injecting cooling water using the cooling spray nozzle described above, a method of using an air flow, a method of allowing the slab to cool without any special cooling equipment, etc. Is also valid. Further, a method of cooling by combining these may be used. The cooling rate of the slab 1 is not particularly limited, and a desired effect can be exhibited at any cooling rate.
3.鋳片1の復熱
本開示の連続鋳造方法において、2次冷却帯でオーステナイトを分解した後は、矯正点20に至る迄に鋳片1の表面温度をAc3以上の温度に復熱させる。この復熱は、鋳片1の表層組織を微細なオーステナイト組織にする、いわゆる逆変態組織を得るために必須である。復熱温度がAc3に満たない場合、逆変態が起こらない場所が残存する。このような組織は矯正歪に対して割れを呈しやすい鋳造まま組織の影響を有するため、Ac3以上にまで復熱させ、オーステナイト単相組織とすることが割れ発生抑制に有効である。尚、矯正点20に至る迄に鋳片1の表面温度を一旦Ac3以上にまで復熱していれば、その後は鋳片1の表面の熱間延性が高く保たれるため、矯正点20において温度が低下しても表面割れは問題とはならない。
3. 3. Reheating of slab 1 In the continuous casting method of the present disclosure, after decomposing austenite in the secondary cooling zone, the surface temperature of slab 1 is reheated to a temperature of Ac3 or higher by the time the correction point 20 is reached. This reheating is indispensable for obtaining a so-called reverse transformation structure in which the surface layer structure of the slab 1 is made into a fine austenite structure. When the reheat temperature is less than Ac3, a place where reverse transformation does not occur remains. Since such a structure has the influence of the structure as it is cast, which is prone to cracks due to the correction strain, it is effective to reheat the structure to Ac3 or higher to form an austenite single-phase structure in order to suppress the occurrence of cracks. If the surface temperature of the slab 1 is once reheated to Ac3 or higher by the time the straightening point 20 is reached, the hot ductility of the surface of the slab 1 is maintained high thereafter, so that the temperature at the straightening point 20 is maintained. Surface cracking does not matter even if the temperature decreases.
Ac3まで復熱させることにより、鋳片1の表層組織は改質され、適正な2次冷却と組み合わせて表面割れの少ない鋳片を得ることができる。鋳片1内の表面温度や組織のバラつきを一層抑える観点からは、復熱後の最高温度をAc3+30℃以上とすることが好ましい。また、復熱温度が高すぎるとオーステナイト結晶粒が再び粗大化する虞があることから、1200℃以下とすることが好ましい。 By reheating to Ac3, the surface structure of the slab 1 is modified, and a slab with less surface cracking can be obtained in combination with appropriate secondary cooling. From the viewpoint of further suppressing the variation in the surface temperature and structure in the slab 1, it is preferable that the maximum temperature after reheating is Ac3 + 30 ° C. or higher. Further, if the reheating temperature is too high, the austenite crystal grains may be coarsened again, so the temperature is preferably 1200 ° C. or lower.
尚、Ac3は、変態点記録測定装置(フォーマスター装置)等を用いて測定することができる。或いは、先行文献(邦武立郎: 熱処理, 43, p. 100(2003))で提案されている以下の式(d)を用いてAc3を特定することもできる。 Ac3 can be measured using a metamorphosis point recording measuring device (four master device) or the like. Alternatively, Ac3 can be specified using the following formula (d) proposed in the prior art (Tatsuro Kunitake: Heat Treatment, 43, p. 100 (2003)).
Ac3=(32[Si]+17[Mo])-(231[C]+20[Mn]+40[Cu]+18[Ni]+15[Cr])+912 ・・・(d)
(式(d)中の[Si]、[Mo]、[C]、[Mn]、[Cu]、[Ni]、[Cr]は、それぞれの成分の濃度(質量%)を表す。)
Ac3 = (32 [Si] +17 [Mo])-(231 [C] +20 [Mn] +40 [Cu] +18 [Ni] +15 [Cr]) +912 ・ ・ ・ (d)
([Si], [Mo], [C], [Mn], [Cu], [Ni], and [Cr] in the formula (d) represent the concentration (mass%) of each component.)
鋳片1表面の復熱は、鋳片1の内部から伝わる熱量が鋳片1の表面から放出される熱量を上回ることによっておこる現象である。鋳片1の表面の復熱は、2次冷却帯の冷却を緩和させることで比較的簡単に行うことができる。或いは、鋳造ラインの周囲に熱源や高周波誘導加熱設備を配し、表面を加熱してもよい。鋳片1の復熱速度(昇温速度)は特に限定されるものではなく、いずれの復熱速度であっても所望の効果が発揮される。 The reheat of the surface of the slab 1 is a phenomenon that occurs when the amount of heat transferred from the inside of the slab 1 exceeds the amount of heat released from the surface of the slab 1. The reheating of the surface of the slab 1 can be relatively easily performed by relaxing the cooling of the secondary cooling zone. Alternatively, a heat source or high-frequency induction heating equipment may be arranged around the casting line to heat the surface. The reheating rate (heating rate) of the slab 1 is not particularly limited, and a desired effect can be exhibited at any reheating rate.
以下に示す実施例は、本開示の鋼の連続鋳造方法の一例を示したものである。本開示の鋼の連続鋳造方法は以下に示す例に限定されるものではない。 The examples shown below show an example of the steel continuous casting method of the present disclosure. The continuous steel casting method of the present disclosure is not limited to the examples shown below.
1.モデル実験1
2次冷却および復熱による鋳片表層組織微細化効果を十分得るための条件を解明するために、変態点記録測定装置(フォーマスター装置)を用いたモデル実験を実施した。
1. 1. Model experiment 1
In order to elucidate the conditions for obtaining a sufficient effect of slab surface layer structure miniaturization by secondary cooling and reheating, a model experiment using a transformation point recording and measuring device (Formaster device) was carried out.
下記表1に示す鋼組成を有するサンプルを1400℃まで加熱し、平均粒径1.5mm以上のオーステナイト組織とした後、ヘリウムガス気流中で350〜750℃の種々の温度まで急冷した。急冷したサンプルを30〜20000秒間等温保持した後、910℃まで20℃/sで再加熱し、組織を全量オーステナイトとしてから0.2℃/sで室温まで冷却した。得られたサンプルの断面はナイタール液で腐食、SEMで観察した。 A sample having the steel composition shown in Table 1 below was heated to 1400 ° C. to form an austenite structure having an average particle size of 1.5 mm or more, and then rapidly cooled to various temperatures of 350 to 750 ° C. in a helium gas stream. The rapidly cooled sample was held isothermal for 30 to 20,000 seconds and then reheated to 910 ° C. at 20 ° C./s to make the whole tissue austenite and then cooled to room temperature at 0.2 ° C./s. The cross section of the obtained sample was corroded with a nital solution and observed by SEM.
観察例を図5に示す。急冷および温度保持によってオーステナイトが分解していたと推定されるサンプルでは図5左の写真Aのように、粗大な旧オーステナイト粒界は見られず、フェライトが多数分散する形態となった(以下、この組織を「組織A」と称する)。一方、オーステナイトが分解していないと推定されるサンプルでは図5右の写真Bのように、分散したフェライトのみならず旧オーステナイト粒界上にフェライトはほとんど見られなかった(以下、この組織を「組織B」と称する)。 An example of observation is shown in FIG. In the sample in which austenite was presumed to have been decomposed by quenching and temperature maintenance, as shown in Photo A on the left of FIG. 5, coarse austenite grain boundaries were not observed, and a large number of ferrites were dispersed (hereinafter, this form). The organization is referred to as "organization A"). On the other hand, in the sample in which austenite is presumed not decomposed, as shown in Photo B on the right of FIG. 5, not only dispersed ferrite but also ferrite was hardly observed on the old austenite grain boundaries (hereinafter, this structure is referred to as "this structure" Tissue B ").
鋼種N05、N10、N20のそれぞれについて、変態点記録測定装置(フォーマスター装置)で付した熱処理パターンと、得られた組織との関係を図6、7、8に示す。350〜475℃においては、サンプル内全面に組織Aを得るための保持時間が短く、いずれも60秒以下であった。一方、500〜700℃においては組織Aが得られる保持時間が鋼種により大きく異なった。この温度域においても組織Aは得られるものの、350〜475℃の5〜125倍の保持時間が必要であった。 For each of the steel types N05, N10, and N20, the relationship between the heat treatment pattern attached by the transformation point recording measuring device (four master device) and the obtained structure is shown in FIGS. 6, 7, and 8. At 350 to 475 ° C., the holding time for obtaining the tissue A on the entire surface of the sample was short, and all were 60 seconds or less. On the other hand, at 500 to 700 ° C., the holding time for obtaining the structure A greatly differed depending on the steel type. Although the tissue A was obtained in this temperature range, a retention time of 5 to 125 times that of 350 to 475 ° C. was required.
2.モデル実験2
上記のモデル実験から、粗大なオーステナイトの分解が比較的迅速に進む温度領域は、350〜475℃および600〜675℃の2領域であると考えられ、それらの温度域ではそれぞれベイナイト(マルテンサイトを含む可能性あり)、フェライトおよびパーライトが生成していると推定される。本発明者らは上記実験に引き続き、実際の連続鋳造機にて鋳片表層がこれら2つの温度域を両方跨ぐ際の変態挙動を把握すべく、以下の実験を実施した。
2. 2. Model experiment 2
From the above model experiment, it is considered that there are two temperature regions in which the decomposition of coarse austenite proceeds relatively rapidly, 350 to 475 ° C and 600 to 675 ° C, and bainite (martensite) is considered to be in those temperature regions, respectively. May be included), presumed to be ferrite and pearlite. Following the above experiment, the present inventors carried out the following experiment in order to understand the transformation behavior when the slab surface layer straddles both of these two temperature ranges in an actual continuous casting machine.
上記モデル実験と同様のサンプルを1400℃まで加熱し、平均粒径1.5mm以上のオーステナイト組織とした後、ヘリウムガス気流中で425℃まで急冷し、15秒又は45秒等温保持した。引き続き625℃まで20℃/sで加熱し、40〜5400秒等温保持した。その後、さらに910℃まで10℃/sで再加熱し、組織を全量オーステナイトとしてから0.2℃/sで室温まで冷却した。得られたサンプルの断面はナイタール液で腐食、SEMで観察した。 A sample similar to the above model experiment was heated to 1400 ° C. to form an austenite structure having an average particle size of 1.5 mm or more, then rapidly cooled to 425 ° C. in a helium gas stream, and maintained at an isothermal temperature for 15 seconds or 45 seconds. It was subsequently heated to 625 ° C. at 20 ° C./s and kept isothermal for 40-5400 seconds. Then, the tissue was further reheated to 910 ° C. at 10 ° C./s to make the whole tissue austenite, and then cooled to room temperature at 0.2 ° C./s. The cross section of the obtained sample was corroded with a nital solution and observed by SEM.
熱処理パターンと得られた組織の関係を下記表2に示す。下記表2において「×」は全面が組織Bであったことを意味し、「△」は組織Aと組織Bとが混在していたことを意味し、「○」は全面が組織Aであったことを意味する。 The relationship between the heat treatment pattern and the obtained structure is shown in Table 2 below. In Table 2 below, "x" means that the entire surface was organization B, "△" means that organization A and organization B were mixed, and "○" means that the entire surface was organization A. It means that.
表2に示す結果から明らかなように、425℃で45秒保持したサンプルは続く625℃での保持が鋼種N05で80秒以上、鋼種N20で1800秒以上で全面組織Aを呈したのに対し、425℃で15秒保持したサンプルは続く625℃での保持が鋼種N05で240秒、鋼種N20では5400秒を要した。この時、粗大なオーステナイトは425℃保持中にベイナイトに、625℃保持中にはこの時点で未変態のオーステナイトがフェライト−パーライトにそれぞれ変態していると推定される。すなわち、ベイナイト変態量とフェライトーパーライト変態量の和が変態前のオーステナイト量と釣り合った時点でオーステナイトの分解が完了し、続く910℃までの復熱を介して徐冷中に組織Aを呈すると考えられる。 As is clear from the results shown in Table 2, the sample held at 425 ° C. for 45 seconds exhibited the entire structure A in the steel type N05 for 80 seconds or more and in the steel type N20 for 1800 seconds or more. The sample held at 425 ° C. for 15 seconds was subsequently held at 625 ° C. for 240 seconds for steel type N05 and 5400 seconds for steel type N20. At this time, it is presumed that the coarse austenite is transformed into bainite while being held at 425 ° C., and the untransformed austenite is transformed into ferrite-pearlite while being held at 625 ° C. That is, it is considered that the decomposition of austenite is completed when the sum of the bainite transformation amount and the ferrite-pearlite transformation amount is balanced with the austenite amount before transformation, and the tissue A is exhibited during slow cooling through the subsequent reheating to 910 ° C. ..
以上の結果より、全面組織Aを呈するための条件として、以下の式(a)で示されるTAが60秒以上であることが見いだされた。
TA=TL+TH×0.04×[Ni]−2.32 ・・・(a)
(式(a)において、TL:試料が350℃以上475℃以下にある時間、TH:試料が600℃以上675℃以下にある時間であり、[Ni]は鋼におけるNiの濃度(質量%)である。)
These results, as a condition for exhibiting entire tissue A, T A of formula (a) below has been found to be at least 60 seconds.
T A = T L + T H × 0.04 × [Ni] -2.32 ··· (a)
(In the formula (a), T L: time the sample is in the 475 ° C. or less 350 ° C. or higher, T H: sample is a time in the 600 ° C. or higher 675 ° C. or less, [Ni] is the concentration of Ni in the steel (mass %).)
上記関係式は他のNi含有低合金鋼を用いて実施した実験においても成立することが確認されている。すなわち、質量%で、C:0.1〜0.4%、Si:0.01〜0.5%、Mn:0.3〜1.4%、Cr:0.8〜2.0%、Mo:0.6%以下、Ni:0.5〜2.0%の組成を有する鋼を鋳造対象とした場合、上記式(a)を満たすように鋳片の2次冷却を行うことで、鋳片表層組織の微細化が可能であり、矯正点における鋳片表面割れを抑制することができる。 It has been confirmed that the above relational expression also holds in experiments carried out using other Ni-containing low alloy steels. That is, in terms of mass%, C: 0.1 to 0.4%, Si: 0.01 to 0.5%, Mn: 0.3 to 1.4%, Cr: 0.8 to 2.0%, When a steel having a composition of Mo: 0.6% or less and Ni: 0.5 to 2.0% is targeted for casting, the slab is secondarily cooled so as to satisfy the above formula (a). The surface layer structure of the slab can be miniaturized, and cracks on the surface of the slab at the straightening point can be suppressed.
尚、上記式(a)で示されるとおり、Ni含有量が高い鋼においては、600〜675℃の温度域のみでオーステナイトの分解を行おうとすると非常に長い時間を要する。すなわち、このような鋼種を製造する際は生産性の観点から350〜475℃の温度域を重視することが望ましいといえる。 As shown in the above formula (a), in a steel having a high Ni content, it takes a very long time to decompose austenite only in the temperature range of 600 to 675 ° C. That is, when producing such a steel grade, it is desirable to emphasize the temperature range of 350 to 475 ° C. from the viewpoint of productivity.
3.実機試験
転炉−LFプロセスにて下記表3に示す組成の溶鋼を溶製し、曲率半径12.0mの湾曲型連鋳機において、220mm×256mmのサイズの鋳片を鋳造した。鋳造速度は1.0〜1.6m/minである。鋳型から引き抜いた鋳片は鋳型直下に設置したゾーン長さ1mのスプレー急冷装置にて急冷した。ゾーン通過後は通常の2次冷却スプレーの水量を調整し、復熱を制御した。鋳片はガス切断機にて5.0±0.2mの長さに切断後、表面の観察に供した。
3. 3. Actual machine test A molten steel having the composition shown in Table 3 below was melted by a converter-LF process, and a slab having a size of 220 mm × 256 mm was cast in a curved continuous casting machine having a radius of curvature of 12.0 m. The casting speed is 1.0 to 1.6 m / min. The slabs drawn from the mold were rapidly cooled by a spray quenching device having a zone length of 1 m installed directly under the mold. After passing through the zone, the amount of water in the normal secondary cooling spray was adjusted to control the reheat. The slab was cut to a length of 5.0 ± 0.2 m with a gas cutting machine and then subjected to surface observation.
なお、鋳片表面の温度は伝熱凝固計算により算出した鋳片L面中心の温度である。伝熱凝固計算により算出した温度は、連続鋳造機内に設置した鋳片表面温度系のデータと比較により十分高い精度であることを検証した。また、復熱後最高到達温度の目標となるAc3の値は上記式(d)で特定した。 The temperature of the slab surface is the temperature at the center of the slab L surface calculated by heat transfer solidification calculation. It was verified that the temperature calculated by the heat transfer solidification calculation was sufficiently high accuracy by comparing with the data of the slab surface temperature system installed in the continuous casting machine. Further, the value of Ac3, which is the target of the maximum temperature reached after reheating, was specified by the above formula (d).
得られた鋼片の表面割れを目視観察した。冷却ゾーンにおいて鋳片表層温度が350〜475℃、475〜600℃、600〜675℃の間にあった時間、および復熱が始まってから矯正点に至るまでの最高温度と併せて調査結果を下記表4に示す。下記表4において、表面割れの評価は、割れがないものを「○」、割れの深さがいずれも0.5mm未満かつ割れの数が鋳片1本当たり10箇所以下であったものを「△」、いずれにも該当しないものを「×」として表した。 The surface cracks of the obtained steel pieces were visually observed. The table below shows the survey results together with the time when the slab surface temperature was between 350 to 475 ° C, 475 to 600 ° C, and 600 to 675 ° C in the cooling zone, and the maximum temperature from the start of reheating to the correction point. Shown in 4. In Table 4 below, the evaluation of surface cracks was "○" for those without cracks, and "○" for those with a crack depth of less than 0.5 mm and a number of cracks of 10 or less per slab. Those that do not correspond to "Δ" are indicated as "x".
実施例1〜9については、いずれも割れ発生のない良好な表面品位の鋳片が得られた。一方、比較例1〜9のように式(a)で表されるTAを60秒未満とした場合や、比較例10〜13のように復熱が始まってから矯正点に至るまでの最高温度がAc3未満であった場合、いずれも鋳片表面に割れを呈した。 In Examples 1 to 9, slabs having good surface quality without cracking were obtained. On the other hand, the highest and when the T A represented by the formula (a) as in Comparative Examples 1 to 9 and less than 60 seconds, since the start of recuperation as in Comparative Example 10 to 13 up to the correct point When the temperature was lower than Ac3, cracks were exhibited on the surface of the slab.
1 鋳片
10 鋳型
20 矯正点
100 連続鋳造機
1 slab 10 mold 20 straightening point 100 continuous casting machine
Claims (2)
鋳型の直下から前記矯正点に至る前において、前記鋳片の表面温度が350〜475℃の間にある時間をTL(s)、600〜675℃にある時間をTH(s)として、下記式(a)で定められるTA(s)が60以上となるように前記鋳片を冷却し、
次いで前記矯正点に至る前までに、前記鋳片の表面温度をAc3以上の温度域まで復熱させることを特徴とする、
Ni含有低合金鋼の連続鋳造方法。
TA=TL+TH×0.04×[Ni]−2.32 ・・・(a)
(式(a)において[Ni]は鋼におけるNiの濃度(質量%)である。) By mass%, C: 0.1 to 0.4%, Si: 0.01 to 0.5%, Mn: 0.3 to 1.4%, Cr: 0.8 to 2.0%, Mo: A method of continuously casting steel slabs having a composition of 0.6% or less and Ni: 0.5 to 2.0% using a continuous casting machine having a straightening point.
Before reaching right under the mold to the correct point, a time in which the surface temperature of the slab is between 350~475 ℃ T L (s), as T H (s) of time in the 600-675 ° C., the cast strip is cooled as defined by the following formula (a) T a (s) is 60 or more,
Next, the surface temperature of the slab is reheated to a temperature range of Ac3 or higher before reaching the correction point.
Continuous casting method for Ni-containing low alloy steel.
T A = T L + T H × 0.04 × [Ni] -2.32 ··· (a)
(In the formula (a), [Ni] is the concentration (mass%) of Ni in the steel.)
請求項1に記載のNi含有低合金鋼の連続鋳造方法。 The slabs are mass%, Al: 0.1% or less, Ti: 0.1% or less, V: 0.4% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM. : 0.01% or less, Nb: 0.05% or less, B: 0.004% or less, N: 0.025% or less.
The method for continuously casting a Ni-containing low alloy steel according to claim 1.
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