JP4696341B2 - Manufacturing method of thin steel sheet with excellent surface properties - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、熱間圧延時の割れによる表面欠陥の少ない薄鋼板の製造方法に関し、特に連続鋳造後、鋳片を冷却し、再加熱後、または直接、熱間圧延を行う直送圧延プロセスにおいても、熱間圧延時に表面における割れの発生が少ない薄鋼板の製造方法に関する。
【０００２】
【従来の技術】
連続鋳造から熱間圧延まで直接行う直送圧延プロセスにより、薄鋼板を製造する場合、熱間圧延での割れに起因した表面欠陥の発生が問題となることが多い。この種の割れはＣを０．２％以下含有する炭素鋼において発生する。
【０００３】
熱間圧延での割れの発生原因は、連続鋳造時のスラブに曲げ変形が加わる際の引張応力により発生したスラブ表面またはスラブ表層下の割れが、粗圧延で伸展したものと考えられている。
【０００４】
特に、直送圧延プロセスにおいては、スラブ手入が不可能であり、スラブに発生した割れはそのまま熱延鋼帯や冷延鋼帯などの製品に残存し、製品歩留まりを著しく低下させていた。
【０００５】
このような問題に対し、特公平７−７６３７５号公報は、凝固後の冷却過程である１２００〜１０５０℃の温度領域においてスラブを特定時間保持した後、Ａｒ3点以上で圧延を開始し、鋼片の表面割れを防止する熱間圧延法を開示している。
【０００６】
【発明が解決しようとする課題】
しかし、実機において、本技術を適用した場合、表面欠陥の発生を十分に抑制することが出来ず、直送圧延プロセスでは、熱間圧延の温度条件によって割れが発生し、鋳造後、スラブを冷却後再加熱する方法では、その再加熱条件によって表面欠陥が発生していた。尚、熱間圧延での割れは一般に赤熱脆性に起因するとされている。
【０００７】
そこで、本発明では、連続鋳造から、熱間圧延まで直接行う直送圧延プロセス及び連続鋳造機、鋳片を冷却し、再加熱後に粗圧延を行うプロセスを対象に、熱間圧延での割れに起因した表面欠陥の発生が極めて少ない表面性状に優れた薄鋼板、特にＴＳ２７０ＭＰａ級以上の薄鋼板の製造方法を提供することを目的とする。
【０００８】
【課題を解決するための手段】
本発明者等は、上記課題を解決すべく、鋼材の成分組成および連続鋳造機による鋳造後、熱間圧延までの諸条件について検討し、特定の成分組成の鋼を鋳造後、一定時間保持し、その後、曲げ変形を表面温度、表面での歪速度を制御しつつ行ない、粗圧延における圧延条件を規定した場合、熱間圧延での割れに起因した表面欠陥の発生が極めて少ない表面性状に優れた薄鋼板の得られることを見出した。熱延後、冷間圧延、表面処理を行っても本発明の効果は損なわれず、薄鋼板には、冷延鋼板、表面処理鋼板も含むものとする。
【０００９】
本発明はこれらの知見を基に更に検討を加えてなされたものであり、すなわち、本発明は、
１． 以下の工程を備えたことを特徴とする表面性状に優れた薄鋼板の製造方法。
【００１０】
（１）質量％で、Ｃ：０．２％以下、Ｓｉ：１％以下、Ｍｎ：０．４〜２％、Ｐ：０．０８％以下、Ｏ：０．００５％以下、Ｓ：０．０３％以下、Ｎ：０．０１％以下、Ｓｎ：０．００４％以下を含有する鋼を連続鋳造機により鋳造する工程。
【００１１】
（２）鋳造後、鋳片表面の温度：９５０〜１１５０℃において５分以上保持する工程。
【００１２】
（３）鋳片の表面温度が、８５０℃未満、７５０℃超え、となる温度域を避けて、表面での歪速度が１×１０−４〜１×１０−１となる曲げ変形を行う工程。
【００１３】
（４）粗圧延において、歪速度１／秒以上で幅方向の圧下後、歪速度１／秒以上で板厚方向の圧下を１回以上行う工程。
【００１４】
２． 連続鋳造機における鋳造速度を１．５ｍ／ｍｉｎ以上とする１記載の表面性状に優れた薄鋼板の製造方法。
【００１５】
【発明の実施の形態】
本発明では、鋼組成、直送圧延プロセスにおける連続鋳造後から、熱間圧延までの諸条件について規定する。
【００１６】
１．鋼組成
Ｃ
Ｃは鋼板の強度を確保するため添加する。０．２０％を超えると加工性が劣化するため、０．２０％以下とする。表面欠陥低減のために好ましいＣ量は０．１０％以下、より好ましくは０．０６％以下とする。
【００１７】
図１に熱間加工性に及ぼすＣ量の影響を示す。Ｃ量の低下に伴い、８００℃近傍における絞り値の劣化度は減少し、熱間加工性が改善されている。
【００１８】
Ｓｉ
Ｓｉは固溶強化元素であり、鋼板の強度を確保するために添加する。１％を超えると表面性状が劣化するため、１％以下とする。
【００１９】
Ｍｎ
Ｍｎは、連続鋳造時のスラブ表面または、スラブ表層下での割れの発生抑制に有効な元素であり添加する。０．４％未満では、その効果が得られず、一方、２％を超えて含有すると加工性が劣化するため、０．４〜２％（０．４％以上、２％以下）とする。
【００２０】
Ｐ
Ｐは、連続鋳造時のスラブ表面または、スラブ表層下に割れを発生し易くする有害な元素である。０．０８％を超えると連続鋳造時のスラブ表面または、スラブ表層下での割れの発生が著しくなり、熱間圧延での割れの発生頻度が高くなるため、０．０８％以下とする。尚、より好ましくは０．０２％以下とする。
【００２１】
Ｏ
Ｏは、連続鋳造時のスラブ表面または、スラブ表層下での割れの発生を抑制するため、０．００５％以下とする。
【００２２】
Ｓ
Ｓは、連続鋳造時のスラブ表面または、スラブ表層下に割れを発生し易くする有害な元素であり、スラブに割れがない場合でも、熱間圧延時に割れを誘発する。
【００２３】
０．０３％を超えると連続鋳造時のスラブ表面または、スラブ表層下での割れの発生が著しくなり、熱間圧延での割れの発生頻度が高くなるため、０．０３％以下とする。尚、より好ましくは０．０１％以下で、更に好ましくは０．００５％以下とする。
【００２４】
Ｎ
Ｎは、熱間圧延時の割れの発生を抑制するため、０．０１％以下とする。
Ｓｎ
Ｓｎは、連続鋳造時のスラブ表面または、スラブ表層下に割れを著しく発生し易くする極めて有害な元素であり、０．００４％を超えると特にその傾向が強くなるため、０．００４％以下とする。
【００２５】
本発明での鋼の基本成分組成は以上であるが、更に強度特性を向上させるため、必要に応じて、Ｔｉ，Ｎｂ、Ｖ，Ｍｏの一種又は二種以上の合計で、０．０１〜０．２％、Ｃｕ：０．０２〜２％、Ｂ：０．０００１〜０．０１％の一種又は二種以上を添加することができる。
【００２６】
２．製造条件
鋳造後、鋳片表面の温度：９５０〜１１５０℃において５分以上保持する工程
本発明では、熱間加工性を向上させるため、連鋳機による鋳造後、鋳片表面温度が９５０〜１１５０℃の範囲内で、５分以上保持する。
【００２７】
鋳片の表面温度が、８５０℃未満、７５０℃超え、となる温度域を避けて、表面での歪速度が１×１０−４〜１×１０−１となる曲げ変形を行う工程
本発明では、曲げ変形による表面割れを抑制するため、熱間脆性域となる、８５０℃未満、７５０℃超えの温度域を避けて、表面での歪速度が１×１０−４〜１×１０−１となるように曲げ変形を行う。
【００２８】
粗圧延において、幅方向の圧下後、歪速度１／秒以上で板厚方向の圧下を1回以上行う工程。
【００２９】
仕上圧延前に、材料表面、特にコーナ部での組織を微細化し、割れの発生を抑制するため、本発明では、粗圧延において、幅殺し圧下後、歪速度１／秒以上で水平圧下を1回以上行う。幅殺し圧下は、好ましくは歪速度１／秒以上とする。
【００３０】
尚、本発明では連続鋳造機の型式は規定せず、湾曲型連鋳機、垂直曲げ型連鋳機等に適用することが可能である。高能率で鋳片の温度低下を抑制し直送圧延を可能とするためには、鋳造速度は、１．５ｍ／ｍｉｎ以上とすることが好ましい。
【００３１】
また、粗圧延は、曲げ変形後、一旦冷却し、再加熱後に行うか、又は冷却することなく直接行うことが可能で特に規定しない。
【００３２】
【実施例】
表１に示す化学成分を有する鋼Ａ，Ｂ，Ｃを転炉で溶製後、鋳造、粗圧延、仕上圧延により熱延コイルとした。その後、表面検査により、表面欠陥の有無を調査した。表２に製造条件、評価結果を示す。
【００３３】
表２において本発明例であるＮｏ．１〜３は、表面欠陥は検出されず、優れた表面性状を有している。一方、比較例であるＮｏ．４〜６は、鋳片の保持時間、曲げ変形時の表面温度および粗圧延条件のいずれかが本発明範囲外であり、表面欠陥の発生が認められた。
【００３４】
【表１】
【表２】
【発明の効果】
本発明は、鋳片の保持時間、曲げ変形時の表面温度、及び歪速度を調整し、更に粗圧延において幅方向の圧下後、板厚方向の圧下をすることにより、熱間圧延での割れに起因した表面欠陥の発生が極めて少ない表面性状に優れた薄鋼板およびその製造方法が得られ、産業上極めて有用である。
【図面の簡単な説明】
【図１】熱間加工性（高温引張試験における絞り値）に及ぼすＣ量の影響を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a thin steel sheet having few surface defects due to cracks during hot rolling, and in particular, in a direct feed rolling process in which, after continuous casting, a slab is cooled and reheated or directly subjected to hot rolling. The present invention relates to a method for producing a thin steel sheet with less occurrence of cracks on the surface during hot rolling.
[0002]
[Prior art]
When producing a thin steel sheet by a direct feed rolling process that is directly performed from continuous casting to hot rolling, the occurrence of surface defects due to cracks in hot rolling is often a problem. This type of cracking occurs in carbon steel containing 0.2% or less of C.
[0003]
The cause of cracks in hot rolling is considered to be that cracks in the slab surface or under the slab surface layer generated by tensile stress when bending deformation is applied to the slab during continuous casting are extended by rough rolling.
[0004]
In particular, in the direct rolling process, slab maintenance is impossible, and cracks generated in the slab remain in products such as hot-rolled steel strips and cold-rolled steel strips, and the product yield is significantly reduced.
[0005]
In response to such a problem, Japanese Patent Publication No. 7-76375 discloses a method in which rolling is started at an Ar3 point or higher after holding a slab for a specific time in a temperature range of 1200 to 1050 ° C., which is a cooling process after solidification. Discloses a hot rolling method for preventing surface cracking of steel.
[0006]
[Problems to be solved by the invention]
However, when this technology is applied to actual machines, the occurrence of surface defects cannot be sufficiently suppressed, and in the direct feed rolling process, cracks occur due to the temperature conditions of hot rolling, and after casting and after cooling the slab In the reheating method, surface defects were generated depending on the reheating conditions. Note that cracks in hot rolling are generally attributed to red heat embrittlement.
[0007]
Therefore, in the present invention, direct feed rolling process and continuous casting machine directly from continuous casting to hot rolling, continuous casting machine, for the process of cooling the slab and performing rough rolling after reheating, due to cracks in hot rolling It is an object of the present invention to provide a method for producing a thin steel plate excellent in surface properties, in particular, a thin steel plate of TS270 MPa class or higher.
[0008]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors examined the composition of the steel material and various conditions from the casting by the continuous casting machine to the hot rolling, and kept the steel of a specific composition for a certain time after casting. After that, bending deformation is performed while controlling the surface temperature and the strain rate on the surface, and when the rolling conditions in rough rolling are specified, the surface properties are excellent with very few surface defects due to cracks in hot rolling. It was found that a thin steel sheet can be obtained. Even if cold rolling and surface treatment are performed after hot rolling, the effect of the present invention is not impaired, and the thin steel plate includes a cold rolled steel plate and a surface-treated steel plate.
[0009]
The present invention has been made by further study based on these findings, that is, the present invention,
1. A method for producing a thin steel sheet having excellent surface properties, comprising the following steps.
[0010]
(1) By mass%, C: 0.2% or less, Si: 1% or less, Mn: 0.4-2%, P: 0.08% or less, O: 0.005% or less, S: 0.00. A step of casting a steel containing 03% or less, N: 0.01% or less, and Sn: 0.004% or less with a continuous casting machine.
[0011]
(2) A step of maintaining the slab surface temperature at 950 to 1150 ° C. for 5 minutes or more after casting.
[0012]
(3) A step of bending deformation in which the surface temperature of the slab is less than 850 ° C. and over 750 ° C., and the strain rate on the surface is 1 × 10 −4 to 1 × 10 −1. .
[0013]
(4) In rough rolling, after the reduction in the width direction at a strain rate of 1 / second or more, the step of performing the reduction in the sheet thickness direction at a strain rate of 1 / second or more once.
[0014]
2. 2. The method for producing a thin steel sheet having excellent surface properties according to 1, wherein a casting speed in a continuous casting machine is 1.5 m / min or more.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the steel composition and various conditions from continuous casting in the direct rolling process to hot rolling are defined.
[0016]
1. Steel composition C
C is added to ensure the strength of the steel sheet. If it exceeds 0.20%, workability deteriorates, so the content is made 0.20% or less. The preferable amount of C for reducing surface defects is 0.10% or less, more preferably 0.06% or less.
[0017]
FIG. 1 shows the effect of the amount of C on hot workability. As the amount of C decreases, the degree of deterioration of the drawing value near 800 ° C. decreases and the hot workability is improved.
[0018]
Si
Si is a solid solution strengthening element and is added to ensure the strength of the steel sheet. If it exceeds 1%, the surface properties deteriorate, so 1% or less.
[0019]
Mn
Mn is an element effective for suppressing the occurrence of cracks on the surface of the slab during continuous casting or under the surface of the slab. If the content is less than 0.4%, the effect cannot be obtained. On the other hand, if the content exceeds 2%, the workability deteriorates, so 0.4 to 2% (0.4% or more and 2% or less).
[0020]
P
P is a harmful element that easily causes cracks on the surface of the slab during continuous casting or on the surface of the slab. If it exceeds 0.08%, the occurrence of cracks on the surface of the slab during continuous casting or under the surface layer of the slab becomes remarkable, and the frequency of occurrence of cracks in hot rolling increases, so the content is made 0.08% or less. More preferably, the content is 0.02% or less.
[0021]
O
O is 0.005% or less in order to suppress the occurrence of cracks on the surface of the slab during continuous casting or on the surface of the slab.
[0022]
S
S is a harmful element that easily generates cracks on the surface of the slab during continuous casting or on the surface of the slab, and induces cracks during hot rolling even when there is no crack in the slab.
[0023]
If it exceeds 0.03%, the occurrence of cracks on the surface of the slab during continuous casting or under the surface layer of the slab becomes remarkable, and the frequency of occurrence of cracks during hot rolling increases, so the content is made 0.03% or less. In addition, More preferably, it is 0.01% or less, More preferably, you may be 0.005% or less.
[0024]
N
N is set to 0.01% or less in order to suppress the occurrence of cracks during hot rolling.
Sn
Sn is an extremely harmful element that remarkably easily causes cracks on the surface of the slab during continuous casting or under the surface of the slab. When the content exceeds 0.004%, the tendency is particularly strong. To do.
[0025]
Although the basic component composition of the steel in the present invention is as described above, in order to further improve the strength characteristics, if necessary, the total of one or more of Ti, Nb, V, and Mo is 0.01 to 0. .2%, Cu: 0.02 to 2%, B: 0.0001 to 0.01%, or two or more of them may be added.
[0026]
2. Manufacturing conditions Temperature of slab surface after casting: Step of holding at 950 to 1150 ° C. for 5 minutes or more In the present invention, the slab surface temperature is 950 to 1150 after casting by a continuous casting machine in order to improve hot workability. Hold for 5 minutes or more within the range of ° C.
[0027]
In the present invention, the surface temperature of the slab is less than 850 ° C. and exceeds 750 ° C., and the bending deformation is performed so that the strain rate on the surface becomes 1 × 10 −4 to 1 × 10 −1. In order to suppress surface cracking due to bending deformation, avoid the temperature range below 850 ° C. and above 750 ° C., which is a hot brittle region, and the strain rate on the surface is 1 × 10 −4 to 1 × 10 −1 . Bending deformation is performed as follows.
[0028]
In rough rolling, after rolling in the width direction, the step of rolling at least once in the sheet thickness direction at a strain rate of 1 / second or more.
[0029]
In order to refine the structure of the material surface, particularly at the corner, and suppress the occurrence of cracks before finish rolling, in the present invention, in rough rolling, after horizontal squeeze reduction, horizontal reduction is performed at a strain rate of 1 / second or more. Do more than once. The width killing pressure is preferably a strain rate of 1 / second or more.
[0030]
In the present invention, the type of the continuous casting machine is not specified, and it can be applied to a curved continuous casting machine, a vertical bending continuous casting machine, and the like. In order to suppress the temperature drop of the slab and enable direct feed rolling with high efficiency, the casting speed is preferably 1.5 m / min or more.
[0031]
Further, the rough rolling can be performed after the bending deformation and once cooled and reheated or directly without cooling, and is not particularly defined.
[0032]
【Example】
Steels A, B, and C having chemical components shown in Table 1 were melted in a converter, and then formed into hot-rolled coils by casting, rough rolling, and finish rolling. Then, the presence or absence of surface defects was investigated by surface inspection. Table 2 shows manufacturing conditions and evaluation results.
[0033]
In Table 2, No. which is an example of the present invention. In Nos. 1 to 3, no surface defects are detected and the surface properties are excellent. On the other hand, No. which is a comparative example. In Nos. 4 to 6, any of the holding time of the slab, the surface temperature during bending deformation, and the rough rolling conditions were outside the scope of the present invention, and the occurrence of surface defects was observed.
[0034]
[Table 1]
[Table 2]
【The invention's effect】
The present invention adjusts the holding time of the slab, the surface temperature at the time of bending deformation, and the strain rate, and further reduces the crack in hot rolling by rolling in the sheet thickness direction after rolling in the width direction in rough rolling. Thus, a thin steel sheet having excellent surface properties with very few surface defects caused by the above and a method for producing the same can be obtained, which is extremely useful industrially.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of C content on hot workability (drawing value in a high-temperature tensile test).
Claims (2)
（１）質量％で、Ｃ：０．２％以下、Ｓｉ：１％以下、Ｍｎ：０．４〜２％、Ｐ：０．０８％以下、Ｏ：０．００５％以下、Ｓ：０．０３％以下、Ｎ：０．０１％以下、Ｓｎ：０．００４％以下を含有する鋼を連続鋳造機により鋳造する工程。
（２）鋳造後、鋳片表面の温度：９５０〜１１５０℃において５分以上保持する工程。
（３）鋳片の表面温度が、８５０℃未満、７５０℃超え、となる温度域を避けて、表面での歪速度が１×１０-4〜１×１０-1となる曲げ変形を行う工程。
（４）粗圧延において、歪速度１／秒以上で幅方向の圧下後、歪速度１／秒以上で板厚方向の圧下を１回以上行う工程。A method for producing a thin steel sheet having excellent surface properties, comprising the following steps.
(1) By mass%, C: 0.2% or less, Si: 1% or less, Mn: 0.4-2%, P: 0.08% or less, O: 0.005% or less, S: 0.00. A step of casting a steel containing 03% or less, N: 0.01% or less, and Sn: 0.004% or less with a continuous casting machine.
(2) A step of maintaining the slab surface temperature at 950 to 1150 ° C. for 5 minutes or more after casting.
(3) A step of bending deformation in which the surface temperature of the slab is less than 850 ° C. and over 750 ° C., and the strain rate on the surface is 1 × 10 −4 to 1 × 10 −1. .
(4) In rough rolling, after the reduction in the width direction at a strain rate of 1 / second or more, the step of performing the reduction in the sheet thickness direction at a strain rate of 1 / second or more once.
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| JP2000224324A JP4696341B2 (en) | 2000-07-25 | 2000-07-25 | Manufacturing method of thin steel sheet with excellent surface properties |
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| JP2000224324A JP4696341B2 (en) | 2000-07-25 | 2000-07-25 | Manufacturing method of thin steel sheet with excellent surface properties |
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| JP4696341B2 true JP4696341B2 (en) | 2011-06-08 |
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| RU2471877C1 (en) * | 2009-04-06 | 2013-01-10 | Ниппон Стил Корпорейшн | Method of processing steel for electric steel sheet with aligned grain structure and method of making electric steel sheet with aligned grain structure |
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| JPS60262915A (en) * | 1984-06-11 | 1985-12-26 | Sumitomo Metal Ind Ltd | Method for preventing surface cracking of continuous cast billet |
| JPS6240960A (en) * | 1985-08-19 | 1987-02-21 | Sumitomo Metal Ind Ltd | Method for preventing crack in hot direct rolling stage of steel ingot |
| JPH0776375B2 (en) * | 1986-03-13 | 1995-08-16 | 住友金属工業株式会社 | Hot rolling method that prevents surface cracking of billets |
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