JP2004169130A - Method for manufacturing steel slab without any surface defect caused by edging - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel slab manufacturing method without crack defects by edging when a carbon steel cast slab is edged. <P>SOLUTION: In the method for manufacturing a steel slab by edging the steel cast slab obtained from molten steel by the continuous casting, the cast slab obtained by continuously casting the molten steel containing, by mass, 0.001-0.5% C, 0.1-1.9% Mn, 0.005-2.0% Si, 0.001-0.1% P, 0.001-0.1% S, 0.0005-0.005% O, and 0.002-0.005% N, further containing 0.005-0.05% Ti, 0.005-0.03% Al, and 0.0005-0.01% Ca, with the ratio Mn/S below 40, and the balance iron with inevitable impurities is rolled down in the width direction. <P>COPYRIGHT: (C)2004,JPO

Description

【００１６】
【表２】

【００１７】
その結果を図１に示すが、Ｓ脆化の指標として、１０５０℃における引張り試験材の破断面の絞り値とした。図より、Ｔｉ、Ａｌ、Ｃａを適量添加した場合（Ｌ４、Ｌ５、Ｌ６）に、１０５０℃の絞り値が著しく改善しているのが判る。これらの材料を組織解析してみると、ベース材や他の水準の材料と比較して、γ結晶粒が非常に細かくなっていることが判った。これにより、γ粒界にＳの液膜が生じても絞り値が低下しなかったものと考えられる。γ粒界を詳細に観察すると、Ｔｉ、Ａｌ、Ｃａを含んだ微細な酸化物が観察された。これが、結晶粒の成長を抑制したものと考えられる。
【００１８】
次に、発明の条件を規定した理由とこの発明の具体的な適用法について説明する。対象となる鋼種は、Ｍｎ／Ｓが４０未満の炭素鋼である。このＭｎ／ＳのものはＳ脆化により表面割れを生じやすい。以下、構成成分について、具体的に記す。
【００１９】
Ｃは、鋼の強度を持たす為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。
【００２０】
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％とした。一般に上限は特殊用途で使用される場合の最大値３質量％であるが、Ｍｎがあまり高い場合は、Ｍｎ／Ｓが高くなってＳ脆化を生じなくなる。ここではＭｎ濃度の最大を、Ｓが高くてもＭｎ／Ｓが４０未満となる、１．９質量％とした。
【００２１】
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２．０質量％とした。
【００２２】
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。
【００２３】
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、粗大なＭｎＳを生成させないために０．１質量％とした。粗大なＭｎＳは製品での割れを引き起こす。
【００２４】
Ｔｉ、Ａｌ、Ｃａは本発明に関係する元素である。まず、Ｔｉは、一般に材料の強度や靭性を上げるために用いられているが、本発明では、微細な酸化物を生成させるために用いる。その効果を得るための最低量として、下限を０．００５質量％、上限は材質に悪影響を与える０．０５質量％とした。Ａｌは、脱酸元素として一般的に使用されているが、本発明では、Ｔｉ、Ｃａと共同して微細な酸化物を生成させるために用いられる。この観点から下限は、効果を発揮する最低限の０．００５質量％とした。また、効果を得るためには、ＴｉやＣａと共存することが重要であり、この観点から実験結果も考慮して上限を０．０３質量％とした。
【００２５】
Ｃａも一般に脱酸元素や硫化物の無害化のために用いられているが、本発明では、微細な酸化物を生成させるために、効果を発揮する最低限の０．０００５質量％とした。また、効果を得るためには、ＴｉやＡｌと共存することが重要であり、この観点から実験結果も考慮して上限を０．０１質量％とした。
【００２６】
なお、Ｔｉ、Ａｌ、Ｃａは脱酸材も兼ねており、また、生成する酸化物を微細にするには、Ｔｉ、Ａｌ、Ｃａの順で添加することが望ましい。
【００２７】
酸素は非金属介在物生成の原因となるため、極力低い方が望ましいが、本発明では酸化物を利用するので、下限は、効果を得るために０．０００５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、０．００５質量％とした。
【００２８】
更に、Ｎは、他の合金元素と結合して、材料の靱性や強度を上げるために用いられるが、下限は、不可避的に混入する０．００２質量％とし、上限は、窒化物の析出で表面欠陥が発生しないよう、０．００５質量％とした。
【００２９】
その他、鋼の用途に応じてＣｒ，Ｍｏ，Ｎｉ，Ｃｕ，Ｖ，Ｂ，Ｚｒの一種または二種以上をそれぞれ０．５質量％以下含んでも構わない。
【００３０】
【実施例】
表３および表４に示す成分の炭素鋼を表５に示す製造条件で連続鋳造および幅圧下をし、得られた鋼片で割れを調査した。
【００３１】
【表３】

【００３２】
【表４】

【００３３】
【表５】

【００３４】
割れの調査方法としては、表６に示すように、鋼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。更に鋼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。
【００３５】
【表６】

【００３６】
結果を表４に示す。表より、本発明例であるＡ１、Ｂ１、Ｃ１、Ｄ１、Ｅ１、Ｆ１、Ｇ１、Ｈ１の場合の条件を満たす場合には、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。
【００３７】
一方、Ａ２、Ｂ２、Ｃ２、Ｄ２、Ｅ２、Ｆ２、Ｇ２、Ｈ２のいずれの比較例においても、鋳片の目視やカラーチェック検査で割れが観察された。すなわち、Ａ２ではＴｉが下限値を下回っており、Ｂ２ではＡｌが下限値を下回っており、Ｃ２では、Ｃａが下限値を下回ったために、本発明を満たさなかった。また、Ｄ２では、ＴｉとＡｌが上限値を超え、Ｅ２では、ＡｌとＣａが上限値を超えたために、本発明を満たさなかった。更に、Ｆ２では、Ｔｉが下限値を下回っており、かつＡｌとＣａが上限値を超えている。
【００３８】
Ｇ２では、Ｔｉが上限値を超えており、かつＡｌが下限値を下回っている。Ｈ２では、ＡｌとＣａが下限値を切ったために、本発明を満たさず、割れが生じた。
【００３９】
【発明の効果】
以上のように本発明により、Ｍｎ／Ｓ比が低い鋼においても幅圧下時の割れが発生しなくなり、表面疵のない良好な鋼片が得られることが可能となる。
【図面の簡単な説明】
【図１】元素添加と絞り値の関係を表した図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a steel slab by reducing the width of a slab produced by a continuous casting method, and more particularly to a method for producing a steel slab having no surface defects.
[0002]
In order to achieve mass production of steel, a process combining a continuous casting machine and a width reduction mill is known. In this process, in continuous casting, the width of the slab is changed by casting with a constant slab width and then reducing the width in a subsequent rolling mill, and the time loss associated with changing the mold width in the continuous casting machine is reduced. It can be expected to improve productivity and increase the yield of the width changing part due to the absence of the above. However, in the case where S is contained in the slab, there is a problem that cracks are generated at the time of width reduction depending on conditions, which leads to surface defects.
[0003]
It has been found that this embrittlement is a liquid film embrittlement due to the concentration of S in the γ crystal grain boundary occurring at a temperature near 1000 to 1100 ° C. The present invention belongs to the technical field of preventing liquid film embrittlement due to S.
[0004]
[Prior art]
As a conventional technique, there is an invention in which the Mn / S ratio in steel is specified to be 50 or more in order to suppress the occurrence of surface flaws in a high-strength steel sheet (for example, see Patent Document 1).
[0005]
Also, there is an invention in which the Mn / S ratio in steel is specified to be 50 or more in order to prevent surface defects in the direct rolling process (for example, see Patent Document 2).
[0006]
However, none of the techniques can prevent surface defects when the Mn / S ratio is low. To increase the Mn / S ratio, it is necessary to increase the amount of Mn or decrease the amount of S. Increasing the amount of Mn leads to an increase in manufacturing cost, and depending on the type of steel, the strength is too high, so that the amount of Mn is suppressed to a low level. In addition, since S is originally contained in the hot metal, a desulfurization treatment is required to reduce the amount of S, and the production cost increases.
[0007]
On the other hand, for a steel having a Mn / S ratio of less than 20, after heating the billet to 1200 to 1300 ° C, the surface is rapidly cooled to 900 to 950 ° C, then reheated to 1000 ° C or more, and held for 1 minute or more. There is a technique of preventing surface cracks by rolling after the rolling (for example, see Patent Document 3).
[0008]
In this case, a large-scale quenching facility is required, or a large energy loss due to the rapid cooling occurs. In the case of rolling at around 1000 ° C., the required finishing temperature and winding temperature may not be able to be secured due to the temperature drop in the latter half.
[0009]
[Patent Document 1]
JP 2000-319747 A [Patent Document 2]
JP 2001-152255 A [Patent Document 3]
JP 05-237508 A
[Problems to be solved by the invention]
The present invention provides a condition for preventing the occurrence of surface flaws due to S embrittlement without defining temperature history conditions such as quenching and recuperation, thereby reducing the width of steel having a low Mn / S ratio. Another object of the present invention is to provide a manufacturing method capable of preventing surface cracks.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the present invention has the following features.
(1) In a method of producing a steel slab by reducing the width from a slab obtained by continuous casting from molten steel, C: 0.001 to 0.5% by mass as a component,
Mn: 0.1 to 1.9%,
Si: 0.005 to 2.0%,
P: 0.001-0.1%,
S: 0.001-0.1%,
Oxygen: 0.0005 to 0.005%,
N: 0.002-0.005%
further,
Ti: 0.005 to 0.05%
Al: 0.005 to 0.03%
Ca: 0.0005 to 0.01%, and the Mn / S ratio is less than 40;
A method for producing a steel slab free of surface defects due to width reduction, characterized in that a slab obtained by continuously casting molten steel comprising the balance of iron and unavoidable impurities is reduced in the width direction.
(2) The surface defect caused by width reduction according to (1), further including one or more of Cr, Mo, Ni, Cu, V, B, and Zr in an amount of 0.5% or less by mass%. Method for producing billets that does not cause cracks.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors first studied the conditions for preventing S embrittlement by adding an element, and as a result, found that when appropriate amounts of the three elements Ti, Al, and Ca were added, embrittlement, that is, generation of cracks was prevented. I found what I could do.
[0013]
The details of the present invention are described below.
[0014]
The present inventors first focused on the fact that S embrittlement is embrittlement of austenite (γ) crystal grain boundaries, and predicted that making the γ crystal grain size smaller would cause less embrittlement. It was investigated. Some candidates are listed as elements that may reduce the γ crystal grain size. Ingots were produced by lab experiments, test pieces were cut out of the materials, and tensile tests were performed. The components are shown in Table 1. The Mn / S ratio was set to 10 as a low value at which S embrittlement easily occurs. Table 2 shows the experimental conditions.
[0015]
[Table 1]

[0016]
[Table 2]

[0017]
The results are shown in FIG. 1, and as an index of S embrittlement, the aperture value of the fracture surface of the tensile test material at 1050 ° C. was used. From the figure, it can be seen that when appropriate amounts of Ti, Al, and Ca are added (L4, L5, L6), the aperture value at 1050 ° C. is significantly improved. Microstructural analysis of these materials showed that the γ crystal grains were very fine compared to the base material and other levels of material. It is considered that the aperture value did not decrease even if a liquid film of S was formed at the γ grain boundary. When the γ grain boundaries were observed in detail, fine oxides containing Ti, Al, and Ca were observed. This is considered to suppress the growth of crystal grains.
[0018]
Next, the reason for defining the conditions of the invention and the specific application method of the invention will be described. The target steel type is a carbon steel having Mn / S of less than 40. In the case of Mn / S, surface cracking is liable to occur due to S embrittlement. Hereinafter, the constituent components will be specifically described.
[0019]
Since C is an element indispensable for imparting the strength of steel, the lower limit was set to 0.001% by mass, and the upper limit was set to 0.5% by mass as the maximum amount of carbon used in the sheet material.
[0020]
Also, Mn is necessary for obtaining strength, and the lower limit is set to 0.1% by mass in order to obtain the effect. In general, the upper limit is the maximum value of 3% by mass when used for special applications. However, when Mn is too high, Mn / S increases and S embrittlement does not occur. Here, the maximum of the Mn concentration was set to 1.9% by mass at which Mn / S was less than 40 even when S was high.
[0021]
Si is unnecessary in some applications, but is inevitably mixed, so the lower limit is 0.005% by mass, and the upper limit is 2.0% by mass used for special applications.
[0022]
Since P is an element harmful to steel, its upper limit is set to 0.1% by mass, and it is desirable that P be as small as possible. However, since it is inevitably mixed, the lower limit of 0.001% by mass is practical.
[0023]
Similarly, S also often impairs the product characteristics, and is desirably as low as possible. However, the lower limit is 0.001% by mass because it is inevitably mixed. The upper limit is set to 0.1% by mass so as not to generate coarse MnS. Coarse MnS causes cracking in the product.
[0024]
Ti, Al, and Ca are elements related to the present invention. First, Ti is generally used to increase the strength and toughness of a material. In the present invention, Ti is used to generate a fine oxide. As a minimum amount for obtaining the effect, the lower limit is set to 0.005% by mass, and the upper limit is set to 0.05% by mass which adversely affects the material. Al is generally used as a deoxidizing element. In the present invention, Al is used in combination with Ti and Ca to generate a fine oxide. From this viewpoint, the lower limit is set to the minimum value of 0.005 mass% at which the effect is exhibited. Further, in order to obtain the effect, it is important to coexist with Ti and Ca. From this viewpoint, the upper limit is set to 0.03% by mass in consideration of the experimental results.
[0025]
Ca is also generally used for detoxification of deoxidizing elements and sulfides. However, in the present invention, in order to generate fine oxides, the minimum content of 0.0005% by mass is effective. Further, in order to obtain the effect, it is important to coexist with Ti and Al. From this viewpoint, the upper limit is set to 0.01% by mass in consideration of the experimental results.
[0026]
Note that Ti, Al, and Ca also serve as a deoxidizer, and it is desirable to add Ti, Al, and Ca in this order in order to make the generated oxide finer.
[0027]
Oxygen causes the formation of nonmetallic inclusions, so it is desirable that oxygen be as low as possible. However, in the present invention, since oxides are used, the lower limit is 0.0005% by mass to obtain the effect, and the upper limit is inclusions. Is too large, it causes a product defect. Therefore, the content was set to 0.005% by mass.
[0028]
Further, N is used to increase the toughness and strength of the material by combining with other alloying elements. The lower limit is 0.002% by mass which is inevitably mixed, and the upper limit is the precipitation of nitride. The content was set to 0.005% by mass so that surface defects did not occur.
[0029]
In addition, one or more of Cr, Mo, Ni, Cu, V, B, and Zr may be contained in an amount of 0.5% by mass or less, respectively, depending on the use of the steel.
[0030]
【Example】
The carbon steels having the components shown in Tables 3 and 4 were subjected to continuous casting and width reduction under the production conditions shown in Table 5, and the obtained steel pieces were examined for cracks.
[0031]
[Table 3]

[0032]
[Table 4]

[0033]
[Table 5]

[0034]
As shown in Table 6, as shown in Table 6, the upper and lower surfaces of the steel slab were subjected to scarf cutting by 2 mm to 10 mm, and the surface was visually observed. Furthermore, a sample was cut out from the steel slab, and the state of cracks in the cross section was examined by color check.
[0035]
[Table 6]

[0036]
Table 4 shows the results. From the table, when the conditions of A1, B1, C1, D1, E1, F1, G1, and H1 of the present invention were satisfied, no cracks were detected by visual observation and color check.
[0037]
On the other hand, in any of the comparative examples A2, B2, C2, D2, E2, F2, G2, and H2, cracks were observed by visual inspection and color check inspection of the slab. That is, Ti was lower than the lower limit in A2, Al was lower than the lower limit in B2, and Ca was lower than the lower limit in C2, so that the present invention was not satisfied. Further, in D2, Ti and Al exceeded the upper limit, and in E2, Al and Ca exceeded the upper limit, so that the present invention was not satisfied. Further, in F2, Ti is below the lower limit, and Al and Ca are above the upper limit.
[0038]
In G2, Ti is above the upper limit and Al is below the lower limit. In H2, the present invention was not satisfied and cracks occurred because Al and Ca fell below the lower limit values.
[0039]
【The invention's effect】
As described above, according to the present invention, even in a steel having a low Mn / S ratio, cracking during width reduction does not occur, and a good steel slab without surface flaws can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between element addition and aperture value.

Claims (2)

In a method for producing a steel slab by reducing the width from a slab obtained by continuous casting from molten steel, C: 0.001 to 0.5% by mass as a component,
Mn: 0.1 to 1.9%,
Si: 0.005 to 2.0%,
P: 0.001-0.1%,
S: 0.001-0.1%,
Oxygen: 0.0005 to 0.005%,
N: 0.002-0.005%
further,
Ti: 0.005 to 0.05%
Al: 0.005 to 0.03%
Ca: 0.0005 to 0.01%, and the Mn / S ratio is less than 40;
A method for producing a steel slab free of surface defects due to width reduction, characterized in that a slab obtained by continuously casting molten steel comprising the balance of iron and unavoidable impurities is reduced in the width direction. 2. The method according to claim 1, further comprising 0.5% or less of one or more of Cr, Mo, Ni, Cu, V, B, and Zr in mass%. A method for producing billets that does not occur.

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