JP3931103B2 - Method for preventing surface cracking of hot rolled steel strip - Google Patents

Description

【００１０】
加熱炉より１２５０℃で抽出したスラブを図２に示すように、２基の粗圧延機にて、粗圧延ミル１ではリバース圧延で５パス、粗圧延ミル２では１パス通して圧延を行い、粗圧延ミル１を通過する1パス目の入側と３パス目の入側では５〜７秒、粗圧延ミル２を通過する前では１２秒間、スラブにノズル３より噴射圧力１６０Ｋｇ／ｃｍ²の冷却水を噴射してデスケーリングを行った。図中、４はスラブの圧延ラインを示す。また各パスでのスラブの板厚及び水平圧下量を以下の表２に示す。
尚、前記加熱炉抽出温度は脆化温度域を越える温度であればよいが、必要以上に高温にすると燃焼コストアップとなり、低すぎると粗圧延終了前に脆化温度域にまでスラブ温度が低下することから、スラブの抽出温度は１２５０℃とした。
【００１１】
【表２】

【００１２】
その後、仕上圧延機前のテーブルローラ上で上記ホットバーをジョギングして空冷し、粗圧延ミル２の出側に設置した温度計によりホットバーを測温し、ホットバー温度を１０５０℃未満〜１０００℃まで冷却したのち仕上圧延を行った。尚、ホットバー温度が上記温度より下がり過ぎると仕上圧延終了時にＡｒ₃変態点以上の仕上温度確保が困難となり、コイルでの加工性が得られなくなる。併せて、仕上圧延機への熱間変形抵抗が大きくなり、設備的にも好ましくない。得られたコイルの表面を目視にて点検し、焼き割れ疵による不良率を求めたところ、２．１％であった。図３に本実施例の不良率を示す。
ここで不良率は、不良部分の重量割合を示す。
【００１３】
比較例
実施例と同じアルミキルド鋼のスラブを加熱炉にて加熱し、粗圧延を行った。加熱炉の各加熱帯及び均熱帯でのスラブ温度を表１に示す。
また粗圧延ミル１ではリバース圧延で５パス、粗圧延ミル２では１パス通して圧延を行ったときのスラブの板厚及び水平圧下量を表２に示し、デスケーリングを各パスごとに行った。加熱炉の各加熱帯及び均熱帯での加熱条件、スラブの板厚及び水平圧下量、デスケーリングの回数は従来と同じである。
【００１４】
粗圧延後、従来と同様にして仕上げ圧延を行い、得られたコイルの表面を目視にて点検した。そして焼き割れ疵による不良率を実施例と同じ基準で求めたところ、３７％であった。
図３に示されるように、実施例に示される方法によると、比較例の従来法によるものと比べ、焼き割れ疵による不良率が大幅に減少した。
【００１５】
【発明の効果】
本発明によると、粗圧延の水平圧下量をパスが進むにつれ減少させるようにしたことにより、抽出温度の変動、粗圧延工程での温度変動により粗圧延が脆化温度域で行われるようなことがあっても圧延歪を最小限にすることができる。
【図面の簡単な説明】
【図１】アルミキルド鋼の焼き割れ疵試験の結果を示すグラフ。
【図２】圧延機の概略図。
【図３】焼き割れ疵による不良率のグラフ。
【符号の説明】
１・・粗圧延ミル１
２・・粗圧延ミル２
３・・ノズル
４・・圧延ライン[0001]
[Technical field to which the invention belongs]
In the present invention, by weight%, C: 0.17 to 0.20%, Si ≦ 0.04%, Mn: 0.60 to 0.90%, P ≦ 0.030%, S ≦ 0.025 %, Cu ≦ 0.23%, hot rolling of aluminum killed steel containing 0.020 to 0.040%, avoiding rolling in the embrittlement temperature region and preventing micro cracks on the coil surface (hereinafter “ It relates to a method for preventing “baked cracks”.
[0002]
[Prior art]
When the slab is hot-rolled, the slab is heated from room temperature to a specified temperature of around 1200 ° C. in a heating furnace, and after the extraction in the heating furnace, it is rolled to a predetermined plate thickness in a rough rolling and finish rolling process.
The above-mentioned aluminum killed steel was heated and extracted at around 1160 ° C in a heating furnace, but in the embrittlement temperature range, thermal stress increased due to rapid heating in the heating furnace, and subsequent rolling gave excessive strain to the slab. As a result, generation of cracks on the coil surface after rolling was observed.
[0003]
[Problems to be solved by the invention]
As a result of various investigations on the cause of the occurrence of the above-mentioned burnt cracks, the present inventors have found that the above-described aluminum killed steel has an embrittlement temperature range of 1050 to 1150 ° C.
[0004]
That is, the present inventors rapidly tested a specimen (JIS-Z2201-14 specimen) obtained by sampling from the above-mentioned aluminum killed steel slab for 10 minutes in a heating furnace in an argon atmosphere until reaching 900 to 1200 ° C. After heating, soaking was continued for 15 minutes to maintain the temperature, and then a tensile load was applied until the strain rate reached 10% / sec and the strain amount reached 25%. Then, after slowly cooling in the furnace for 30 minutes, the cracks on the surface of the test piece are visually inspected, and a five-step evaluation is performed with a test temperature of 900. Various changes were made within the range of ˜1200 ° C. As a result, it was found from the average value of the measured values measured n times at each temperature that there is an embrittlement temperature region at 1050 to 1150 ° C. as shown in FIG.
The present invention has been made based on these findings.
[0005]
[Means for solving problems]
That is, the present invention includes C: 0.17 to 0.20%, Si ≦ 0.04%, Mn: 0.60 to 0.90%, P ≦ 0.030%, S ≦ 0.025%, 0 < Cu ≦ 0.23%, Al: 0.020 to 0.040% , the remainder is extracted from the furnace with an aluminum killed steel slab composed of Fe and unavoidable impurities at a temperature other than the embrittlement temperature range of 1050 to 1150 ° C. At the same time, rough rolling and finish rolling are performed, and the rough rolling is preferably performed by reducing the horizontal reduction amount as the pass proceeds so as to prevent thermal cracks and cracking due to rolling distortion. In the invention, the extraction temperature from the heating furnace is set to a temperature exceeding the embrittlement temperature range, and the rough rolling is terminated before the temperature is lowered to the embrittlement temperature, and the finish rolling is less than the embrittlement temperature. And at a temperature above the Ar ₃ transformation point. The Extraction temperature from the heating furnace, due to variations in temperature drop in the rough rolling process, when rough rolling does not end before it falls to the embrittlement temperature range, that is, rough rolling is performed in the embrittlement temperature range However, according to the present invention, the rolling distortion can be minimized.
[0006]
In order to finish the rough rolling of the slab before the slab temperature falls to the above-mentioned embrittlement temperature range, for example, the rolling speed is increased, the use path of water descaling is decreased, for example, the water descaling is changed from 6 passes to 3 passes. In order to perform finish rolling at a temperature less than the above embrittlement temperature, the slab after rough rolling (hereinafter referred to as a hot bar) is, for example, jogged on a roller table up to the finishing mill. It is better to cool by air cooling.
[0008]
【Example】
C: 0.17 to 0.20%, Si ≦ 0.04%, Mn: 0.60 to 0.90%, P ≦ 0.030%, S ≦ 0.025%, Cu ≦ 0.23%, A slab of aluminum killed steel containing Al: 0.020 to 0.040% was heated through a heating furnace composed of first to third heating zones and a soaking zone. Table 1 shows the slab temperature in each heating zone and in the soaking zone. Conventionally, the rate of temperature increase from the charging of the heating furnace to the embrittlement temperature range to the second heating zone was about 300 ° C./hr, but in the practice of the present invention, it is changed to about 250 ° C./hr to generate thermal stress. We tried to reduce it.
[0009]
[Table 1]

[0010]
As shown in FIG. 2, the slab extracted from the heating furnace at 1250 ° C. is rolled by two rough rolling mills, with 5 passes in reverse rolling in the rough rolling mill 1 and 1 pass in the rough rolling mill 2, An injection pressure of 160 kg / cm ² is applied to the slab from the nozzle 3 for 5 to 7 seconds on the first pass and the third pass on the rough rolling mill 1 and for 12 seconds before passing the rough rolling mill ² . The descaling was performed by injecting cooling water. In the figure, 4 indicates a slab rolling line. Table 2 below shows the thickness of the slab and the amount of horizontal reduction in each pass.
The heating furnace extraction temperature may be any temperature that exceeds the embrittlement temperature range, but if it is higher than necessary, the combustion cost will increase, and if it is too low, the slab temperature will fall to the embrittlement temperature range before the end of rough rolling. Therefore, the extraction temperature of the slab was set to 1250 ° C.
[0011]
[Table 2]

[0012]
Thereafter, the hot bar is jogged on the table roller before the finish rolling mill, air-cooled, the hot bar is measured with a thermometer installed on the exit side of the rough rolling mill 2, and the hot bar temperature is less than 1050 ° C. to 1000 ° C. After cooling to ° C., finish rolling was performed. If the hot bar temperature is too lower than the above temperature, it is difficult to secure a finishing temperature above the Ar ₃ transformation point at the end of finishing rolling, and workability in the coil cannot be obtained. In addition, the hot deformation resistance to the finishing mill increases, which is not preferable in terms of equipment. The surface of the obtained coil was visually inspected, and the defect rate due to burnt cracks was found to be 2.1%. FIG. 3 shows the defect rate of this example.
Here, the defective rate indicates the weight ratio of the defective portion.
[0013]
Comparative Example The same aluminum killed steel slab as in the example was heated in a heating furnace and subjected to rough rolling. Table 1 shows the slab temperature in each heating zone and soaking zone of the heating furnace.
In addition, Table 2 shows the slab thickness and horizontal rolling amount when rolling is performed through 5 passes for reverse rolling in the rough rolling mill 1 and 1 pass for the rough rolling mill 2, and descaling was performed for each pass. . The heating conditions in the heating zone and the soaking zone of the furnace, the slab thickness and the horizontal reduction amount, and the number of descaling are the same as before.
[0014]
After rough rolling, finish rolling was performed in the same manner as before, and the surface of the obtained coil was visually inspected. And when the defect rate by the burning crack flaw was calculated | required on the same basis as an Example, it was 37%.
As shown in FIG. 3, according to the method shown in the example, the defect rate due to burning cracks was significantly reduced as compared with the conventional method of the comparative example.
[0015]
【The invention's effect】
According to the present invention, the horizontal rolling reduction amount of the rough rolling is reduced as the pass proceeds, so that the rough rolling is performed in the embrittlement temperature region due to the fluctuation of the extraction temperature and the temperature fluctuation in the rough rolling process. Even if there is, the rolling distortion can be minimized.
[Brief description of the drawings]
FIG. 1 is a graph showing the results of an aluminum killed steel baking cracking test.
FIG. 2 is a schematic view of a rolling mill.
FIG. 3 is a graph showing a defect rate due to burning cracks.
[Explanation of symbols]
1. Rough rolling mill 1
2. Rough rolling mill 2
3 ・ ・ Nozzle 4 ・ ・ Rolling line

Claims (2)

In weight%, C: 0.17-0.20%, Si ≦ 0.04%, Mn: 0.60-0.90%, P ≦ 0.030%, S ≦ 0.025%, 0 < Cu ≦ 0.23%, Al: 0.020 to 0.040% , the remainder is extracted from the furnace with an aluminum killed steel slab composed of Fe and unavoidable impurities at a temperature other than the embrittlement temperature range of 1050 to 1150 ° C. In addition, a method for preventing surface cracking of a hot-rolled steel strip is characterized in that rough rolling and finish rolling are performed, and the horizontal rolling reduces the horizontal reduction amount as the pass proceeds .   The extraction temperature from the heating furnace is set to a temperature exceeding the embrittlement temperature range, the rough rolling is finished before the temperature is lowered to the embrittlement temperature, and the finish rolling is performed below the embrittlement temperature. The method for preventing surface cracking of a hot-rolled steel strip according to claim 1.

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