JP4328037B2 - Method for producing high workability hot-rolled steel sheet without folding back and folds - Google Patents
Method for producing high workability hot-rolled steel sheet without folding back and folds Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、Ｂを添加した低炭素鋼による腰折れ並びに畳じわの発生しない加工用軟質熱延鋼板の製造方法に関するものである。
【０００２】
【従来の技術】
低炭素Ａｌキルド鋼を用いた従来の加工用熱延鋼板の製造方法では、巻取り後にスキンパスラインを通板する。薄手材の場合は形状矯正の目的もあるが、板厚によらず、残留した固溶Ｃや固溶Ｎによる降伏現象が元で発生する腰折れとよばれる縞模様や畳じわとよばれるしわ模様を防止するという目的がある。これらは降伏現象によって生じるが、スキンパスラインを通板することにより、鋼板に均一歪みを与えて可動転位を導入し降伏現象を抑えることにより防止することが可能である。
【０００３】
上記の方法では通板工程が増えるため、製造コストが高くなり設備も余分に必要となるという欠点がある。腰折れや畳じわが発生しなければ、形状矯正の必要のない鋼板を熱延したまま使用する場合や、酸洗ラインでレベラーによって形状矯正する場合は、スキンパスラインを通板する必要がなく、製造コストを低く抑えることができる。
【０００４】
腰折れをなくす手段としては、たとえば、特開昭６１−７３８３６号公報のようにＣ％が０．０１０％以下の極低炭素鋼にＴｉ、Ｎｂ及びＢを添加して固溶ＮをＴｉとＢで固定し、固溶ＣをＴｉとＮｂで固定する方法が知られている。しかし、この方法では、Ｃ％が０．０１０％以下の極低炭素鋼の製造及びＴｉ、Ｎｂの添加により製鋼段階での製造コストが高くなるという欠点がある。
【０００５】
製鋼段階でのコストを低く抑えたまま、腰折れの発生をなくす方法としては、特開平４−５６７３２号公報のように、Ｂ添加鋼を用いてＡｒ3変態点以上で圧延し、圧延後の冷却速度を制御した後３００〜５００℃で巻取るという方法が提案されている。この方法は、圧延中にＢＮを析出させて固溶Ｎを固定し、巻取り後にセメンタイトを析出させて固溶Ｃを固定するものである。しかし、ＢＮの析出ノーズは８００℃程度であるため、本成分系では８３０〜８５０℃程度となるＡｒ3変態点以上の圧延でＢＮを十分に析出させることは困難である。逆にＢＮの析出しやすいＡｒ3変態点以下の温度で圧延すれば、表層に粗大粒が発生して延性の劣化を招く。
【０００６】
【発明が解決しようとする課題】
本発明では、低コストのＢ添加低炭素鋼を用い、固溶Ｃと固溶Ｎを十分に低減して腰折れ及び畳じわの発生をなくした上で十分な延性を確保した加工用熱延鋼板の製造方法の提供を課題とする。
【０００７】
【課題を解決するための手段】
本発明者らは、Ｂ添加低炭素鋼を用いて熱間圧延条件を種々変化させた実験を行った結果、Ａｒ3変態点以上で圧延を終了し、その後の冷却速度を制御した上、５００℃超６３０℃以下で巻取ることにより、腰折れ並びに畳じわが発生しなくなることを知見し、本発明を完成した。本発明の要旨は次の通りである。
【０００８】
質量％で、
Ｃ ：０．０１５〜０．０８％、
Ｍｎ：０．０５〜０．５％、
Ｓｉ：０．００３〜０．５％、
Ｐ ：０．０５％以下、
Ｓ ：０．００５〜０．０２％、
Ａｌ：０．００５〜０．３％、
Ｎ ：０．００６％以下、
Ｂ ：０．８≦Ｂ／Ｎ≦２．５を含み、
残部はＦｅ及び不可避的不純物からなる鋼片を再加熱した後、
Ａｒ3変態点以上で圧延を完了し、
Ａｒ3変態点〜（Ａｒ3変態点−１００℃）の温度範囲を７０℃／秒以上１２０℃／秒以下の冷却速度で冷却して５００℃超６３０℃以下で巻取ることを特徴とする腰折れ並びに畳じわの発生しない高加工性熱延鋼板の製造方法。
【０００９】
【発明の実施の形態】
以下に本発明を詳細に説明する。
【００１０】
腰折れや畳じわの原因となる降伏現象は固溶Ｃや固溶Ｎが転位を固着することにより生じる。よって、固溶Ｃ・固溶Ｎの低減と転位密度の増大を図ることにより防止可能で、両者のバランスが重要である。固溶Ｃに関しては、６００℃以下で巻取ることにより、巻取り後にセメンタイト、すなわちＦｅ3Ｃが十分に析出し、固溶Ｃは０となる。固溶Ｎに関しては、５８０℃以上で巻取ることにより、圧延中に析出しきれなかったＢＮが巻取り後に析出して、固溶Ｎは０となる。コイルに巻取った後は非常に遅い速度で温度が低下するため、ＢＮの析出ノーズから離れている温度域ではあるが、ＢＮが十分に析出可能なのである。
【００１１】
転位密度に関しては、オーステナイトからフェライトへの変態時、つまりＡｒ3変態点〜（Ａｒ3変態点−１００℃）の冷却速度を速めることにより針状フェライトの混入した組織となり、フェライト中に変態転位を導入することが可能である。その冷却速度は、図１に示したように７０℃／秒以上１２０℃／秒以下であることが判明している。しかし、巻取り温度が高いと巻取り後に回復し転位密度が減少するため、転位を残すためには、巻取り温度が低い方が好ましい。なお、図１はＡｒ3〜（Ａｒ3−１００℃）の冷却速度と腰折れ、畳じわ発生状況の関係を示す図である。ただし、他の条件は本発明の範囲内である。また、図２は巻取り温度と固溶Ｃ量・固溶Ｎ量の関係を示す図である。併せて、腰折れ・畳じわ防止に必要な固溶Ｃ量＋固溶Ｎ量の上限も示している。ただし、この場合のＡｒ3〜（Ａｒ3−１００℃）の冷却速度は７０℃／秒以上１２０℃／秒以下である。図２に示したように、転位密度に応じて腰折れ及び畳じわを防止するのに要する固溶Ｃ＋固溶Ｎ量の上限が決まっており、その値は巻取り温度が低くなるほど増加する。それと固溶Ｃ量・固溶Ｎ量との関係から腰折れを防止するための巻取り温度範囲は５００℃超６３０℃以下となる。
【００１２】
Ａｒ3変態点以上で圧延するのは、通常のように材質劣化を防止するということもあるが、腰折れや畳じわの防止の観点からも必要である。Ａｒ3変態点以下で圧延すると鋼板の表層に粗大粒が発生するが、この粗大粒は圧延中に生成したフェライトが歪を受けた後に粗大粒成長するものであり、転位密度は非常に低い。よって、表層にこの粗大粒組織があると、固溶Ｃ・Ｎが低くとも降伏現象が発生するため、腰折れや畳じわが発生する。
【００１３】
次に、本発明が対象とする鋼板の成分及び成分範囲を限定した理由を述べる。
【００１４】
Ｃは、硬化元素であり、Ｃ量が少ない程加工性に有利であるが、Ｃ量を低下させる脱炭処理の経済性を考慮してＣ量の下限を０．０１５％とした。また、Ｃ量が多くなると硬質になり加工性が劣化するので、Ｃ量の上限を０．０８％とした。
【００１５】
Ｍｎは、靭性を付与するために必要な元素であると共に、ＢＮの核となるＭｎＳの構成元素であり、０．０５％以上の量が必要である。また、Ｍｎ量が多くなると硬化して伸びを向上させる効果が飽和するので、上限を０．５％とした。
【００１６】
Ｓｉは、鋼の脱酸剤として添加されるが、多くなると硬化して加工性を劣化させるので、その範囲を０．００３〜０．５％とした。
【００１７】
Ｐは、不純物として不可避的に含有され伸びに悪影響を与えるので、上限を０．０５％とした。
【００１８】
ＳはＢＮの核となるＭｎＳを析出させるのに必要な元素であり、０．００５％以上必要である。しかし、多くなると硬化して加工性を劣化させるので、その上限を０．０２％とした。
【００１９】
Ａｌは、鋼の脱酸剤として添加され鋼中に含有されるが、Ａｌは鋼中の固溶ＮをＡｌＮとして析出させるため、Ｂと共に固溶Ｎ低減のためには重要な元素であって、０．００５％以上必要である。一方、Ａｌ量が多くなるに応じて伸びが向上するが、０．３％を超えると硬化して加工性を劣化させるので、Ａｌは０．００５〜０．３％とした。
【００２０】
Ｎは不可避的不純物として含有されるが、固溶Ｎのまま残留すると腰折れの発生原因となる。Ｂを添加することによってＢＮとして析出させることができるが、Ｎ量が多いとＢＮの量も多くせざるを得ず材質の劣化を招く。そのバランスからみて上限を０．００６％とする。
【００２１】
Ｂは、鋼中のＮをＢＮとして析出させ、更にそれを核としてセメンタイトを析出させるので、本発明においては最も重要な元素である。ＢＮは優先的に析出するが、ＡｌＮも一部析出するので、Ｂ／Ｎが０．８以上であれば固溶Ｎを十分に低減することができる。ただし、Ｂ／Ｎが高すぎても固溶Ｂが増えるだけで、経済性や加工性の低下を招くので上限を２．５とした。
【００２２】
本発明は、加熱炉・粗圧延・仕上圧延・冷却帯・巻取り機からなる通常の連続熱間圧延ラインにおいて実施される。巻取り後は、そのまま出荷してもよいし、直接酸洗ラインを通板してもよい。
【００２３】
溶鋼は通常の高炉法で溶製されたものの他、電炉法のようにスクラップを多量に使用したものでもよい。スラブは、通常の連続鋳造プロセスで製造されたものでもよいし、薄スラブ鋳造で製造されたものでもよい。また、粗圧延後に粗バーを巻取って保持するコイルボックスは、ＭｎＳの析出に有効であって、本発明に有利となる。更に巻取った粗バーを巻き戻す際に先行する粗バーと接合して圧延する、いわゆる熱延連続化プロセスは材質ばらつきや生産性の観点から好ましい。
【００２４】
【実施例】
表１に示す成分の鋼片を用いて、表２に示す条件で加工用熱延鋼板を製造した。その結果を表２中に併せて記載している。
【００２５】
【表１】
【表２】
本発明の成分範囲を外れるＥ〜Ｉの鋼種を用いて本発明で規定する製造条件で製造した比較例Ｎｏ．７〜１１、本発明で規定する成分範囲の鋼種を用いて本発明で規定する製造条件を外れて製造した比較例Ｎｏ．１２〜１５では、いずれも腰折れまたは畳じわが発生している。
【００２８】
それに対して、本発明例のＮｏ．２〜６では腰折れ及び畳じわが発生せず、伸びも良好であった。
【００２９】
【発明の効果】
以上のように、本発明の方法に従えば、腰折れ及び畳じわの発生しない高加工性熱延鋼板を低コストで提供することができ、更にスキンパス工程の省工程化を図ることができる。よって、経済的なメリットは非常に大きい。
【図面の簡単な説明】
【図１】Ａｒ3〜（Ａｒ3−１００℃）の冷却速度と腰折れ・畳じわ発生状況の関係を示す図である。
【図２】巻取り温度と固溶Ｃ量・固溶Ｎ量の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a soft hot-rolled steel sheet for processing that does not cause waist folding and folds with low carbon steel added with B.
[0002]
[Prior art]
In the conventional method for manufacturing a hot-rolled steel sheet for processing using low carbon Al killed steel, a skin pass line is passed after winding. In the case of thin materials, there is also the purpose of shape correction. Regardless of the plate thickness, wrinkles called folds or folds called waist folding that occur due to the yielding phenomenon due to the remaining solid solution C or solid solution N. The purpose is to prevent patterns. These are caused by the yield phenomenon, but can be prevented by passing a skin pass line to give a uniform strain to the steel sheet and introducing movable dislocations to suppress the yield phenomenon.
[0003]
In the above method, since the number of sheet passing steps is increased, there is a disadvantage that the manufacturing cost is increased and extra equipment is required. If there is no hip folding or crease, when using steel plates that do not require shape correction while being hot-rolled, or when correcting the shape with a leveler in the pickling line, it is not necessary to pass the skin pass line. Cost can be kept low.
[0004]
As a means for eliminating hip breakage, for example, as disclosed in JP-A-61-73736, Ti, Nb and B are added to an ultra-low carbon steel having a C% of 0.010% or less to convert solid solution N into Ti and B And fixing the solid solution C with Ti and Nb is known. However, this method has a drawback that the production cost at the steel making stage becomes high due to the production of ultra-low carbon steel having C% of 0.010% or less and the addition of Ti and Nb.
[0005]
As a method for eliminating the occurrence of hip breakage while keeping the cost at the steelmaking stage low, as described in JP-A-4-56732, rolling is performed at an Ar 3 transformation point or higher using B-added steel, and cooling after rolling is performed. A method of winding up at 300 to 500 ° C. after controlling the speed has been proposed. In this method, BN is precipitated during rolling to fix solid solution N, and after winding, cementite is precipitated to fix solid solution C. However, since the precipitation nose of BN is about 800 ° C., it is difficult to sufficiently precipitate BN by rolling above the Ar 3 transformation point, which is about 830 to 850 ° C. in this component system. On the other hand, if rolling is performed at a temperature below the Ar 3 transformation point at which BN is likely to precipitate, coarse grains are generated on the surface layer, resulting in deterioration of ductility.
[0006]
[Problems to be solved by the invention]
In the present invention, low-cost B-added low-carbon steel is used, and hot-rolling for machining that sufficiently reduces solid solution C and solid solution N to eliminate the occurrence of waist folding and folds and ensures sufficient ductility. An object is to provide a method for manufacturing a steel sheet.
[0007]
[Means for Solving the Problems]
As a result of conducting experiments in which the hot rolling conditions were variously changed using the B-added low carbon steel, the present inventors finished rolling at an Ar 3 transformation point or higher and controlled the subsequent cooling rate, and then 500 The present invention was completed by finding out that winding at 630 ° C. or higher and lower than 630 ° C. caused no folding of the hips and no folding. The gist of the present invention is as follows.
[0008]
% By mass
C: 0.015-0.08%,
Mn: 0.05 to 0.5%
Si: 0.003 to 0.5%,
P: 0.05% or less,
S: 0.005 to 0.02%,
Al: 0.005 to 0.3%,
N: 0.006% or less,
B: 0.8 ≦ B / N ≦ 2.5 is included,
The rest is reheated steel pieces consisting of Fe and inevitable impurities,
Completed rolling above the Ar 3 transformation point,
A waist fold characterized by being cooled at a cooling rate of 70 ° C./second to 120 ° C./second and wound up at a temperature higher than 500 ° C. and not higher than 630 ° C. from the Ar 3 transformation point to (Ar 3 transformation point−100 ° C.). In addition, a method for producing a highly workable hot-rolled steel sheet that does not generate folds.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
[0010]
The yield phenomenon that causes hip folding and folding is caused by solid solution C or solid solution N fixing dislocations. Therefore, it can be prevented by reducing the solid solution C and solid solution N and increasing the dislocation density, and the balance between them is important. Regarding the solid solution C, by winding at 600 ° C. or less, cementite, that is, Fe 3 C is sufficiently precipitated after winding, and the solid solution C becomes zero. Regarding solute N, by winding at 580 ° C. or higher, BN that could not be precipitated during rolling is precipitated after winding, and solute N becomes zero. Since the temperature drops at a very slow speed after being wound on the coil, BN can be sufficiently precipitated although it is in a temperature range away from the BN precipitation nose.
[0011]
With regard to the dislocation density, at the time of transformation from austenite to ferrite, that is, by increasing the cooling rate from the Ar 3 transformation point to (Ar 3 transformation point −100 ° C.), the structure becomes mixed with acicular ferrite. It is possible to introduce. It has been found that the cooling rate is 70 ° C./second or more and 120 ° C./second or less as shown in FIG. However, if the coiling temperature is high, it recovers after coiling and the dislocation density decreases. Therefore, in order to leave the dislocation, it is preferable that the coiling temperature is low. Incidentally, FIG. 1 is a diagram showing a relationship between cooling rate and buckling, tatami wrinkle occurrence of Ar 3 ~ (Ar 3 -100 ℃ ). However, other conditions are within the scope of the present invention. FIG. 2 is a graph showing the relationship between the coiling temperature and the amount of dissolved C and the amount of dissolved N. In addition, the upper limit of the amount of solid solution C + the amount of solid solution N necessary for preventing hip folding / folding is also shown. However, the cooling rate of Ar 3 to (Ar 3 −100 ° C.) in this case is 70 ° C./second or more and 120 ° C./second or less . As shown in FIG. 2, the upper limit of the amount of solid solution C + solid solution N required to prevent hip folding and folding is determined according to the dislocation density, and the value increases as the coiling temperature decreases. The winding temperature range for preventing hip breakage from the relationship between the solid solution C amount and the solid solution N amount is more than 500 ° C. and 630 ° C. or less.
[0012]
Rolling at an Ar 3 transformation point or higher may prevent material deterioration as usual, but is also necessary from the viewpoint of preventing hip folding and folding. When rolling below the Ar 3 transformation point, coarse grains are generated in the surface layer of the steel sheet. These coarse grains grow coarse after the ferrite formed during the rolling is strained, and the dislocation density is very low. Therefore, when this coarse-grained structure is present on the surface layer, a yield phenomenon occurs even when the solid solution C · N is low, so that waist folding and folds occur.
[0013]
Next, the reason why the components and the component ranges of the steel plate targeted by the present invention are limited will be described.
[0014]
C is a hardening element, and the smaller the amount of C, the better the workability. However, considering the economic efficiency of the decarburization treatment that lowers the amount of C, the lower limit of the amount of C is set to 0.015%. Moreover, since it will become hard and workability will deteriorate when C content increases, the upper limit of C content was made into 0.08%.
[0015]
Mn is an element necessary for imparting toughness and is a constituent element of MnS serving as a nucleus of BN, and an amount of 0.05% or more is necessary. Moreover, since the effect which hardens | cures and improves elongation will be saturated if the amount of Mn increases, the upper limit was made 0.5%.
[0016]
Si is added as a deoxidizer for steel, but if it increases, it hardens and deteriorates workability, so the range was made 0.003 to 0.5%.
[0017]
Since P is inevitably contained as an impurity and adversely affects elongation, the upper limit was made 0.05%.
[0018]
S is an element necessary for precipitating MnS as a core of BN, and is required to be 0.005% or more. However, since it will harden and workability will deteriorate when it increases, the upper limit was made 0.02%.
[0019]
Al is added as a steel deoxidizer and contained in the steel, but Al precipitates solute N in the steel as AlN, so it is an important element for reducing the solute N together with B. 0.005% or more is necessary. On the other hand, elongation increases as the amount of Al increases, but if it exceeds 0.3%, it hardens and deteriorates workability, so Al was made 0.005 to 0.3%.
[0020]
N is contained as an unavoidable impurity, but if it remains in the form of solid solution N, it causes the occurrence of hip breakage. BN can be precipitated by adding B. However, if the amount of N is large, the amount of BN must be increased, leading to deterioration of the material. In view of the balance, the upper limit is made 0.006%.
[0021]
B is the most important element in the present invention because it precipitates N in steel as BN and further precipitates cementite using it as a nucleus. BN precipitates preferentially, but AlN also partially precipitates. Therefore, if B / N is 0.8 or more, solid solution N can be sufficiently reduced. However, even if B / N is too high, only the amount of solid solution B increases, resulting in a decrease in economy and workability. Therefore, the upper limit is set to 2.5.
[0022]
The present invention is carried out in a normal continuous hot rolling line comprising a heating furnace, rough rolling, finish rolling, cooling zone, and winding machine. After winding, the product may be shipped as it is, or the pickling line may be directly passed through.
[0023]
The molten steel may be one produced by a normal blast furnace method or one using a large amount of scrap as in the electric furnace method. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting. In addition, the coil box that winds and holds the coarse bar after rough rolling is effective for precipitation of MnS, which is advantageous for the present invention. Furthermore, a so-called hot rolling continuous process in which the rolled coarse bar is rolled by joining with the preceding coarse bar when unwinding is preferable from the viewpoint of material variation and productivity.
[0024]
【Example】
Using the steel slabs having the components shown in Table 1, hot-rolled steel sheets for processing were produced under the conditions shown in Table 2. The results are also shown in Table 2.
[0025]
[Table 1]
[Table 2]
Comparative Example No. produced under the production conditions defined in the present invention using E to I steel grades outside the component range of the present invention. Comparative Examples No. 7 to 11 manufactured using the steel grades of the component ranges specified in the present invention and out of the manufacturing conditions specified in the present invention. In 12-15, the waist break or the crease occurred.
[0028]
In contrast, No. of the present invention example. In 2-6, waist folding and folds did not occur, and the elongation was good.
[0029]
【The invention's effect】
As described above, according to the method of the present invention, it is possible to provide a high-workability hot-rolled steel sheet that does not cause folding and folding, and to save the skin pass process. Therefore, the economic merit is very large.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the cooling rate of Ar 3 to (Ar 3 -100 ° C.) and the state of occurrence of hip folding / folding.
FIG. 2 is a diagram showing a relationship between a coiling temperature and a solute C amount / a solute N amount.
Claims (1)
Ｃ ：０．０１５〜０．０８％、
Ｍｎ：０．０５〜０．５％、
Ｓｉ：０．００３〜０．５％、
Ｐ ：０．０５％以下、
Ｓ ：０．００５〜０．０２％、
Ａｌ：０．００５〜０．３％、
Ｎ ：０．００６％以下、
Ｂ ：０．８≦Ｂ／Ｎ≦２．５を含み、
残部はＦｅ及び不可避的不純物からなる鋼片を再加熱した後、
Ａｒ３変態点以上で圧延を完了し、
Ａｒ３変態点〜（Ａｒ３変態点−１００℃）の温度範囲を７０℃／秒以上１２０℃／秒以下の冷却速度で冷却して５００℃超６３０℃以下で巻取ることを特徴とする腰折れ並びに畳じわの発生しない高加工性熱延鋼板の製造方法。% By mass
C: 0.015-0.08%,
Mn: 0.05 to 0.5%
Si: 0.003 to 0.5%,
P: 0.05% or less,
S: 0.005 to 0.02%,
Al: 0.005 to 0.3%,
N: 0.006% or less,
B: 0.8 ≦ B / N ≦ 2.5 is included,
The rest is reheated steel pieces consisting of Fe and inevitable impurities,
Rolling is completed at the Ar 3 transformation point or higher,
The temperature range from the Ar 3 transformation point to (Ar 3 transformation point−100 ° C.) is cooled at a cooling rate of 70 ° C./second or more and 120 ° C./second or less and wound at 500 ° C. or more and 630 ° C. or less. In addition, a method for producing a highly workable hot-rolled steel sheet that does not generate folds.
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| JP4328037B2 true JP4328037B2 (en) | 2009-09-09 |
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