JP3756804B2 - Continuous cast slabs with no intergranular cracking defects - Google Patents

Description

【００１３】
成分範囲を検討する際には、鋼に溶解しているＮ量、ＮｂＮの析出量に着目した。Ｎｂを含む析出物は一般にＮｂＣＮであるが、ここでは、簡易的にＮｂＮで代表させた。成分指標を表す横軸として、Ｗ（Ｎ）−０．２９２×Ｗ（Ｔｉ）−０．１５２×（Ｗ（Ｎｂ）−６０）とし、析出物の量を表す指標として、Ｗ（Ｎｂ）×Ｗ（Ｎ）を用いて層別して、縦軸となる絞り値との関係を表すと、図１のようになった。（ここで、Ｗ（Ｎ）：Ｎ濃度（ｐｐｍ）、Ｗ（Ｔｉ）：Ｔｉ濃度（ｐｐｍ）、Ｗ（Ｎｂ）：Ｎｂ濃度（ｐｐｍ））。
【００１４】
図より、横軸、すなわち、Ｗ（Ｎ）−０．２９２×Ｗ（Ｔｉ）−０．１５２×（Ｗ（Ｎｂ）−６０）の値が５以下で、かつＷ（Ｎｂ）×Ｗ（Ｎ）の値が９０００以下の場合に、絞り値６０％以上を確保できることが判る。絞り値６０％は、連続鋳造機内で粒化割れが発生しない下限の限界値であり、これ以上の値であれば、連続鋳造機内では、粒化割れは発生しにくい。
【００１５】
次に、本発明の条件を規定した理由と具体的な適用法について説明する。
先ず、実際の鋼材に適用される鋼の成分範囲についてであるが、成分組成としては以下のような成分範囲が好ましい。
Ｃは鋼の強度を持たすために不可欠の元素であるため、下限を０．００１質量％とし、上限は板材で用いられる最大炭素量として０．５質量％とした。
また、Ｍｎも強度を得るために必要でありその効果を出すために下限を０．１質量％とし、上限は特殊用途で使用される場合の最大値３質量％とした。
Ｓｉは用途によっては不要の場合もあるが、不可避的に混入するためその下限を０．００５質量％とし、上限は特殊用途で用いられる２質量％とした。
【００１６】
Ｐは鋼に有害な元素であるため、その上限を０．１質量％とし極力少ないほうが望ましいが、不可避的に混入するため下限値０．００１質量％が現実的である。
Ｓも同様に製品特性に害をなす場合が多く極力低位とすることが望ましいが、不可避的に混入するため下限値０．００１質量％が現実的である。また上限は連続鋳造時の割れを防ぐために０．０５質量％とした。
Ｖは材料の強度や靱性を上げるために用いられているが、その効果を得るための最低量として下限０．０１質量％、上限は材質に悪影響を与える０．１質量％とした。
【００１７】
Ｔｉ，Ｎｂ，Ｎは本発明に関係する元素である。材料の強度や靱性を上げるために用いられているが、本発明の効果を得るためには上限が制限される。また、下限は脆化の発生しない値で規定した。すなわち下限以下であれば、本発明を用いる必要はない。この観点から、それぞれ、Ｔｉ：０．００４〜０．１質量％、Ｎｂ：０．００６〜０．１質量％、Ｎ：０．００２〜０．０１５質量％となる。
Ａｌは脱酸元素として一般的に使用されているが、Ｎと化合してＡｌＮを生成するため、材料の強度を上げる目的で用いられることもある。この観点から下限は不可避的に混入する０．００１質量％とし、上限は材料の強度を上げすぎること、および介在物量が増えすぎる問題から０．１質量％とした。
また、酸素は非金属介在物生成の原因となるため、極力低いほうが望ましいが、下限は不可避的に混入する０．００１質量％とし、上限は介在物があまり多くなると製品欠陥の原因となるので、０．０５０質量％とした。
【００１８】
その他、鋼の用途に応じてＣｒ，Ｍｏ，Ｃｕ，Ｎｉ，Ｂ，Ｚｒ，Ｍｇ，Ｃａの一種または二種以上を０．１質量％以下含ませることができる。
すなわち、Ｃｒ，Ｍｏは焼入れ性を向上させることにより、母材の強度および靱性を向上させるために有効な元素であるが、鋼材ＨＡＺ部においては過剰な添加は靱性を著しく低下させるため０．１質量％を上限とした。
Ｃｕは鋼材の強度を向上させるために有効であるが、ＨＡＺ靱性の低下やＣｕ脆化の問題があるために、０．５質量％を上限とした。
【００１９】
Ｎｉは鋼材の強度および靱性を向上させるために有効であるが、Ｎｉ量の増加は製造コストを上昇させるので０．５質量％を上限とした。
Ｂは鋼材の強度の向上に有効であるが、過剰な含有は靱性を著しく低下させるので上限を０．１質量％とした。
Ｚｒ，Ｍｇ，Ｃａは強力な脱酸元素として、微細酸化物の個数増大に有効な元素であるが、過剰な添加は併せて介在物の粗大化も促進するため０．１質量％を上限とした。
【００２０】
次に、関係式の規定であるが、上述したように、鋼に溶解しているＮ量とＮｂＮの析出量を表す指標であり、その限界値は、ラボ実験での成分条件と絞り値との関係を解析した結果から得られたものである。鋼に溶解している（固溶）Ｎ量の指標としては、以下の［１］式とした。

ここで、Ｗ（Ｎ）：Ｎ濃度（ｐｐｍ）、Ｗ（Ｔｉ）：Ｔｉ濃度（ｐｐｍ）、Ｗ（Ｎｂ）：Ｎｂ濃度（ｐｐｍ）
ただし、Ｗ（Ｎｂ）−６０＜０の時は、［１］式中で、Ｗ（Ｎｂ）−６０＝０とおく
【００２１】
これは、全Ｎ量からＴｉと化合してＴｉＮとして析出する量を除き、更にＮｂと化合してＮｂＮとして析出する量を除いたものである。Ｎｂの場合には、Ｔｉより低温で析出物が生成するので、６０ｐｐｍ分を引いた場合に絞り値との関係がうまく整理できた。ここで、限界値５は、実験での実績値より求めた。また、係数０．２９２、および０．１５２はそれぞれ、ＴｉＮを形成するＮのＴｉに対する質量比、およびＮｂＮを形成するＮのＮｂ対する質量比である。
【００２２】
また、ＮｂＮ析出量を表す指標であるが、下記に示した［２］式とした。
Ｗ（Ｎｂ）×Ｗ（Ｎ）≦９０００ ・・・・・［２］
ここで、Ｗ（Ｎ）：Ｎ濃度（ｐｐｍ）、Ｗ（Ｔｉ）：Ｔｉ濃度（ｐｐｍ）、Ｗ（Ｎｂ）：Ｎｂ濃度（ｐｐｍ）
これは、Ｎｂ濃度とＮ濃度の積が大きい場合にはＮｂＮが析出しやすいという熱力学的性質を利用したものである。
【００２３】
更に、厚板向けの材料では、材質の観点からは、高靭性を維持するために、以下の条件を加えたほうが良い。
Ｗ（Ｔｉ）／Ｗ（Ｎ）≦３．４２ ・・・・・［３］
ここで、Ｗ（Ｎ）：Ｎ濃度（ｐｐｍ）、Ｗ（Ｔｉ）：Ｔｉ濃度（ｐｐｍ）、Ｗ（Ｎｂ）：Ｎｂ濃度（ｐｐｍ）
これは、Ｔｉが対象となる鋼種は、ＴｉとＮｂとＮが含まれている炭素鋼であれば、どんなものでも構わない。
【００２４】
なお、結晶粒界脆化に起因する鋳片の割れは、鋼の組織がγからαに変態する温度（成分によって変化するが、一般には７５０〜８００℃）でも発生する場合がある。この温度領域での割れを防止するには、本発明に加えて連鋳機内での最低温度を８００℃以上に保つことが望ましい。
【００２５】
【実施例】
表２に示す成分の炭素鋼を表３に示す製造条件で連続鋳造し、得られた鋳片の割れを調査した。割れの調査方法としては、表４に示すように、鋳片上面と下面にスカーフ溶削を２ｍｍ〜１０ｍｍ行い、表面を目視観察した。更に鋳片からサンプルを切り出し、断面の割れの状態をカラーチェックで調査した。結果を表５に示す。
【００２６】
【表２】

【００２７】
【表３】

【００２８】
【表４】

【００２９】
【表５】

【００３０】
表５より、本発明の条件を満たす場合（Ａ〜Ｈ）には、目視観察およびカラーチェックとも割れは検出されなかった。更に、Ｗ（Ｔｉ）／Ｗ（Ｎ）≦３．４２（条件式（３））を満足したＣ，Ｄ，Ｅ，Ｆでは、非常に良好な靭性も得られた。
【００３１】
一方、Ｉ〜Ｐのいずれの比較例においては、成分が本発明条件を満たさないために、割れが発生した。すなわち、Ｉ，Ｊ，Ｌ，Ｎでは、条件式［１］を満足しないために、また、Ｋ，Ｍ，Ｏでは、条件式［２］を満足しないために、更に、Ｐでは両方の条件式を満足しないために、鋳片の目視やカラーチェック検査で割れが観察された。
【００３２】
【発明の効果】
以上のように本発明により、ＮｂやＴｉ、Ｎを含む鋼においても連続鋳造時の粒化割れが発生しなくなり、表面疵のない良好な鋳片が得られることが可能となる。
【図面の簡単な説明】
【図１】絞り値と固溶Ｎ指標とＮｂＮ析出指標の関係を表した図[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a slab cast by a continuous casting method, and particularly relates to a continuous cast slab having no surface defects due to grain cracking.
[0002]
[Prior art]
Mass production of steel is mainly carried out in a process including a continuous casting machine. However, when Ti, Nb, V, N, etc. are contained as components, the slab is bent or straightened in the continuous casting machine. When deforming, there is a problem that cracks occur along the γ grain boundaries in the slab.
Countermeasures for this problem have been studied in the past, and the main technologies are listed in Japanese Patent Publication No. 56-46530, Japanese Patent Publication No. 59-28424, Japanese Patent Publication No. 61-11704, Japanese Patent Publication No. 60-56457. There is a gazette.
[0003]
In Japanese Examined Patent Publication No. 56-46530, a technique for preventing cracking of the slab surface during continuous casting is presented using an expression that defines the Nb + V amount and an expression that defines the relationship between the N amount and the Ti amount. Since the relationship between Nb and N is not defined, there is a risk of cracking if both Nb and N are large.
In Japanese Patent Publication No. 59-28424, a technique for preventing cracking is proposed by adding a plastic strain as a characteristic condition in the cooling process following solidification in continuous casting to refine the austenite crystal grains. In the vertical bending type continuous casting machine, since there is a bending point immediately after the mold, it is difficult to give a predetermined strain therebetween. Japanese Patent Application Laid-Open No. 60-56457 also discloses a technique for preventing cracking by imparting processing distortion, but there is a problem for the same reason.
[0004]
Furthermore, Japanese Patent Publication No. 61-11704 discloses a technique for preventing cracking by regulating the amount of water injected into the side surface of a slab in a vertical bending type continuous casting machine. If it increases, the effect is lost, and if the amount of water is reduced too much, the risk of breakout increases. Similarly, a technique for maintaining a high slab temperature by limiting the amount of water poured onto the surface of the slab in the continuous casting machine is also known. Similarly, depending on the risk of breakout and the C amount, There is a problem that the risk of occurrence of internal cracks also increases.
Therefore, the above-described methods have limitations and problems in any case, and it is difficult to completely prevent the grain boundary cracking of the continuous cast slab.
[0005]
[Problems to be solved by the invention]
The present invention presents a component range for preventing cracking regardless of the cooling conditions and strain imparting conditions of continuous casting, and thus does not specifically define the manufacturing conditions in continuous casting, and is continuous due to intergranular cracking. The present invention provides a slab capable of preventing surface cracking of the cast slab.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is characterized by the following configurations.
(1) C: 0.001 to 0.5 mass%, Mn: 0.1 to 3.0 mass%, Si: 0.005 to 2.0 mass%, P: 0.001 to 0.1 mass% , S: 0.001 to 0.05 mass%, oxygen is 0.0005 to 0.0050 mass%, N is 0.002 or more and 0.015 mass% or less, and Al is 0.001 to 0.1 mass%. wherein wt%, and comprises a Nb from 0.006 to .1% by weight and comprises a Ti .004-.1 wt%, Ri Do the balance iron and inevitable impurities, the following ingredients [1 ], [2], [3] Continuous cast slabs that do not cause grain boundary cracking defects adjusted to satisfy the equations .
W (N) -0.292 × W (Ti) -0.152
× (W (Nb) −60) ≦ 5 [1]
W (Nb) × W (N) ≦ 9000 (2)
W (Ti) / W (N) ≦ 3.42 [3]
Here, W (N): N concentration (ppm), W (Ti): Ti concentration (ppm), W (Nb): Nb concentration (ppm)
(2) C: 0.001 to 0.5 mass%, Mn: 0.1 to 3.0 mass%, Si: 0.005 to 2.0 mass%, P: 0.001 to 0.1 mass% , S: 0.001 to 0.05 mass%, oxygen is 0.0005 to 0.0050 mass%, N is 0.002 or more and 0.015 mass% or less, and Al is 0.001 to 0.1 mass%. Inclusive, Nb in an amount of 0.006 to 0.1% by mass, Ti in an amount of 0.004 to 0.1% by mass, V in an amount of 0.01 to 0.1% by mass, and the balance Ri do iron and unavoidable impurities, the following ingredients [1], [2], [3] the continuous casting does not cause intergranular cracking defects, characterized in that adjusted so as to satisfy the equation slab.
W (N) -0.292 × W (Ti) -0.152
× (W (Nb) −60) ≦ 5 [1]
W (Nb) × W (N) ≦ 9000 (2)
W (Ti) / W (N) ≦ 3.42 [3]
Here, W (N): N concentration (ppm), W (Ti): Ti concentration (ppm), W (Nb): Nb concentration (ppm)
[0007]
(3) In addition to the composition of the continuous cast slab, further comprising 0.1% by mass or less of one or more of Cr and Mo, the balance consisting of iron and inevitable impurities (1) or (2) Continuous cast slab without the described grain boundary cracking defect.
(4) In addition to the composition of the continuous cast slab, the grain boundary cracking defect according to any one of (1) to (3), further including 0.5% by mass or less of Cu, and remaining iron and inevitable impurities Continuous cast slab that does not cause
(5) In addition to the composition of the continuous cast slab, the grain boundary cracking defect according to any one of (1) to (4), further including Ni in an amount of 0.5% by mass or less, and remaining iron and inevitable impurities Continuous cast slab that does not cause
[0008]
(6) In addition to the composition of the continuous cast slab, the grain boundary cracking defect according to any one of (1) to (5), further including B in an amount of 0.1% by mass or less and comprising the remaining iron and inevitable impurities Continuous cast slab that does not cause
[0009]
(7) In addition to the composition of the continuous cast slab, it further contains one or more of Zr, Mg, and Ca in an amount of 0.1% by mass or less, and consists of the balance iron and inevitable impurities (1) to (6 The continuous cast slab in which the grain boundary cracking defect according to any one of the above is not generated.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention have focused on the fact that the embrittlement of steel containing Ti, Nb, N, etc. is the embrittlement of austenite (γ) grain boundaries, and studied the conditions under which these embrittlement occurs. As a result, the inventors have come up with the relational expression between the occurrence of embrittlement, that is, cracking, the steel component composition, and the steel component composition.
[0011]
Details of the present invention will be described below.
The present inventors first performed a tensile test using steel with different contents of Ti, Nb, and N.
Using steels having the components shown in Table 1, the γ crystal grains were enlarged by heating to 1400 ° C., then cooled, held at a predetermined temperature, pulled, and the aperture value of the sample was measured. In particular, focusing on the drawing value at 900 ° C., which is the upper limit, in the temperature range in which intergranular cracking occurs in the continuous casting machine, the relationship with the components was examined.
[0012]
[Table 1]

[0013]
When examining the component ranges, attention was paid to the amount of N dissolved in the steel and the amount of NbN precipitated. The precipitate containing Nb is generally NbCN, but here it is simply represented by NbN. The horizontal axis representing the component index is W (N) −0.292 × W (Ti) −0.152 × (W (Nb) −60), and the index representing the amount of precipitate is W (Nb) × FIG. 1 shows the relationship with the aperture value on the vertical axis, stratified using W (N). (W (N): N concentration (ppm), W (Ti): Ti concentration (ppm), W (Nb): Nb concentration (ppm)).
[0014]
From the figure, the horizontal axis, that is, the value of W (N) −0.292 × W (Ti) −0.152 × (W (Nb) −60) is 5 or less and W (Nb) × W (N When the value of) is 9000 or less, it can be seen that an aperture value of 60% or more can be secured. The drawing value of 60% is a lower limit value at which no grain cracking occurs in the continuous casting machine. If the value is larger than this, the grain cracking is less likely to occur in the continuous casting machine.
[0015]
Next, the reason for defining the conditions of the present invention and the specific application method will be described.
First, regarding the component range of steel applied to actual steel materials, the following component ranges are preferable as the component composition.
Since C is an indispensable element for imparting the strength of steel, the lower limit is set to 0.001% by mass, and the upper limit is set to 0.5% by mass as the maximum amount of carbon used in the plate material.
Further, Mn is also necessary for obtaining strength, and in order to exert its effect, the lower limit is set to 0.1% by mass, and the upper limit is set to 3% by mass when used for special purposes.
Although Si may be unnecessary depending on the application, since it is inevitably mixed, the lower limit is set to 0.005 mass%, and the upper limit is set to 2 mass% used for special applications.
[0016]
Since P is an element harmful to steel, the upper limit is preferably 0.1% by mass and is preferably as small as possible. However, since it is inevitably mixed, the lower limit is 0.001% by mass.
S is also harmful to the product characteristics in the same way, and it is desirable to make it as low as possible. However, since it is inevitably mixed, the lower limit of 0.001% by mass is practical. The upper limit was set to 0.05% by mass to prevent cracking during continuous casting.
V is used to increase the strength and toughness of the material. The minimum amount for obtaining the effect is 0.01% by mass, and the upper limit is 0.1% by mass that adversely affects the material.
[0017]
Ti, Nb, and N are elements related to the present invention. Although used to increase the strength and toughness of the material, the upper limit is limited to obtain the effects of the present invention. Further, the lower limit was defined as a value at which embrittlement does not occur. That is, if it is below a lower limit, it is not necessary to use this invention. From this viewpoint, Ti: 0.004 to 0.1 mass%, Nb: 0.006 to 0.1 mass%, and N: 0.002 to 0.015 mass%, respectively.
Al is generally used as a deoxidizing element. However, since it combines with N to produce AlN, it may be used for the purpose of increasing the strength of the material. From this viewpoint, the lower limit is set to 0.001% by mass, and the upper limit is set to 0.1% by mass because the strength of the material is excessively increased and the amount of inclusions is excessively increased.
Also, since oxygen causes non-metallic inclusions, it is desirable that it be as low as possible. However, the lower limit is unavoidably mixed to 0.001% by mass, and the upper limit causes product defects if too much inclusions are present. 0.050% by mass.
[0018]
In addition, Cr, Mo, Cu, Ni, B, Zr, Mg, or Ca may be contained in an amount of 0.1% by mass or less in accordance with the use of steel.
That is, Cr and Mo are effective elements for improving the strength and toughness of the base material by improving the hardenability. However, excessive addition in the steel HAZ part significantly reduces the toughness. The upper limit was mass%.
Cu is effective for improving the strength of the steel material, but due to the problem of reduced HAZ toughness and Cu embrittlement, the upper limit was made 0.5 mass%.
[0019]
Ni is effective for improving the strength and toughness of the steel, but an increase in the amount of Ni increases the manufacturing cost, so the upper limit was made 0.5 mass%.
B is effective in improving the strength of the steel material, but excessive content significantly reduces toughness, so the upper limit was made 0.1% by mass.
Zr, Mg, and Ca are effective deoxidizing elements and are effective elements for increasing the number of fine oxides. However, excessive addition also promotes the coarsening of inclusions, so the upper limit is 0.1% by mass. did.
[0020]
Next, as defined in the relational expression, as described above, it is an index representing the amount of N dissolved in steel and the amount of NbN precipitated, and the limit values are the component conditions and the aperture value in the laboratory experiment. It was obtained from the result of analyzing the relationship. The following formula [1] was used as an index of the amount of N dissolved (solid solution) in steel.

Here, W (N): N concentration (ppm), W (Ti): Ti concentration (ppm), W (Nb): Nb concentration (ppm)
However, when W (Nb) −60 <0, W (Nb) −60 = 0 is set in the formula [1].
This is the total N content except for the amount that combines with Ti and precipitates as TiN, and further excludes the amount that combines with Nb and precipitates as NbN. In the case of Nb, precipitates are formed at a temperature lower than that of Ti, so that the relationship with the aperture value can be well organized when subtracting 60 ppm. Here, the limit value 5 was calculated | required from the actual value in experiment. Coefficients 0.292 and 0.152 are the mass ratio of N forming TiN to Ti and the mass ratio of N forming NbN to Nb, respectively.
[0022]
Moreover, although it is a parameter | index showing NbN precipitation amount, it was set as the following [2] type | formula.
W (Nb) × W (N) ≦ 9000 (2)
Here, W (N): N concentration (ppm), W (Ti): Ti concentration (ppm), W (Nb): Nb concentration (ppm)
This utilizes the thermodynamic property that NbN is likely to precipitate when the product of Nb concentration and N concentration is large.
[0023]
Furthermore, in the material for thick plates, it is better to add the following conditions from the viewpoint of the material in order to maintain high toughness.
W (Ti) / W (N) ≦ 3.42 [3]
Here, W (N): N concentration (ppm), W (Ti): Ti concentration (ppm), W (Nb): Nb concentration (ppm)
Any steel may be used as long as it is a carbon steel containing Ti, Nb and N.
[0024]
Note that cracking of the slab caused by grain boundary embrittlement may occur even at a temperature at which the steel structure transforms from γ to α (although it varies depending on the component, it is generally 750 to 800 ° C.). In order to prevent cracking in this temperature range, it is desirable to keep the minimum temperature in the continuous casting machine at 800 ° C. or higher in addition to the present invention.
[0025]
【Example】
Carbon steels having the components shown in Table 2 were continuously cast under the production conditions shown in Table 3, and cracks of the resulting slabs were investigated. As a method for investigating cracks, as shown in Table 4, scarf cutting was performed on the upper and lower surfaces of the slab by 2 mm to 10 mm, and the surface was visually observed. Further, a sample was cut out from the slab, and the state of the cross-sectional crack was examined by color check. The results are shown in Table 5.
[0026]
[Table 2]

[0027]
[Table 3]

[0028]
[Table 4]

[0029]
[Table 5]

[0030]
From Table 5, when the conditions of the present invention were satisfied (A to H), no crack was detected in both visual observation and color check. Furthermore, very good toughness was obtained with C, D, E, and F satisfying W (Ti) / W (N) ≦ 3.42 (conditional expression (3)).
[0031]
On the other hand, in any of Comparative Examples I to P, cracks occurred because the components did not satisfy the conditions of the present invention. That is, since conditional expression [1] is not satisfied for I, J, L, and N, and conditional expression [2] is not satisfied for K, M, and O, both conditional expressions are used for P. Therefore, cracks were observed by visual inspection and color check inspection of the slab.
[0032]
【The invention's effect】
As described above, according to the present invention, even in a steel containing Nb, Ti, and N, granulation cracking does not occur during continuous casting, and a good slab having no surface defects can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between a drawing value, a solute N index, and an NbN precipitation index.

Claims (7)

C: 0.001 to 0.5 mass%, Mn: 0.1 to 3.0 mass%, Si: 0.005 to 2.0 mass%, P: 0.001 to 0.1 mass%, S: 0.001 to 0.05 mass%, oxygen 0.0005 to 0.0050 mass%, N 0.002 to 0.015 mass%, and Al 0.001 to 0.1 mass% and includes Nb from 0.006 to 0.1 wt%, and contains a Ti from 0.004 to 0.1 wt%, Ri Do the balance iron and inevitable impurities, the following ingredients [1], [ A continuous cast slab that is adjusted so as to satisfy the formulas [2] and [3] and is free from grain boundary cracking defects.
W (N) -0.292 × W (Ti) -0.152
× (W (Nb) −60) ≦ 5 [1]
W (Nb) × W (N) ≦ 9000 (2)
W (Ti) / W (N) ≦ 3.42 [3]
Here, W (N): N concentration (ppm), W (Ti): Ti concentration (ppm), W (Nb): Nb concentration (ppm) C: 0.001-0.5 mass%, Mn: 0.1-3.0 mass%, Si: 0.005-2.0 mass%, P: 0.001-0.1 mass%, S: 0.001 to 0.05 mass%, oxygen 0.0005 to 0.0050 mass%, N 0.002 to 0.015 mass%, and Al 0.001 to 0.1 mass% Nb is contained in an amount of 0.006 to 0.1% by mass, Ti is contained in an amount of 0.004 to 0.1% by mass, and V is contained in an amount of 0.01 to 0.1% by mass. Ri do from impurities, the following ingredients [1], [2], [3] the continuous casting does not cause intergranular cracking defects, characterized in that adjusted so as to satisfy the equation slab.
W (N) -0.292 × W (Ti) -0.152
× (W (Nb) −60) ≦ 5 [1]
W (Nb) × W (N) ≦ 9000 (2)
W (Ti) / W (N) ≦ 3.42 [3]
Here, W (N): N concentration (ppm), W (Ti): Ti concentration (ppm), W (Nb): Nb concentration (ppm)  3. In addition to the composition of the continuous cast slab, the alloy further comprises 0.1% by mass or less of one or more of Cr and Mo, and is composed of the balance iron and inevitable impurities. Continuous cast slab without the described grain boundary cracking defect.  The intergranular crack defect according to any one of claims 1 to 3, further comprising 0.5 mass% or less of Cu in addition to the composition of the continuous cast slab, comprising the balance iron and inevitable impurities. Continuous cast slab that does not cause  The intergranular crack defect according to any one of claims 1 to 4, further comprising 0.5% by mass or less of Ni in addition to the composition of the continuous cast slab, comprising the balance iron and inevitable impurities. Continuous cast slab that does not cause  The grain boundary crack defect according to any one of claims 1 to 5, further comprising 0.1 mass% or less of B in addition to the composition of the continuous cast slab, comprising the balance iron and inevitable impurities. Continuous cast slab that does not cause  In addition to the composition of the continuous cast slab, it further comprises 0.1 mass% or less of one or more of Zr, Mg, and Ca, and consists of the balance iron and inevitable impurities. The continuous cast slab in which the grain boundary cracking defect according to any one of 6 is not generated.

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