JP2018145525A - Hot rolled steel sheet and production method thereof, cold rolled steel sheet and production method thereof, production method of cold rolled annealed steel sheet, and production method of hot-dip galvanized steel sheet - Google Patents
Hot rolled steel sheet and production method thereof, cold rolled steel sheet and production method thereof, production method of cold rolled annealed steel sheet, and production method of hot-dip galvanized steel sheet Download PDFInfo
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Abstract
Description
本発明は、自動車の外板パネルや家電製品の筐体など高強度とプレス成型後の優れた表面性状が要求される薄鋼板に好適な、プレス加工後の表面性状に優れた冷延鋼板及びその製造方法に関する。また、本発明は、前記冷延鋼板の素材となる熱延鋼板及びその製造方法に関する。また、本発明は、前記冷延鋼板を素材とした冷延焼鈍鋼板及び溶融亜鉛めっき鋼板の製造方法に関する。 The present invention is suitable for thin steel sheets that require high strength and excellent surface properties after press molding, such as outer panels of automobiles and housings of home appliances, and cold-rolled steel plates excellent in surface properties after press processing and It relates to the manufacturing method. The present invention also relates to a hot-rolled steel sheet as a material for the cold-rolled steel sheet and a method for manufacturing the hot-rolled steel sheet. Moreover, this invention relates to the manufacturing method of the cold-rolled annealing steel plate and hot-dip galvanized steel plate which used the said cold-rolled steel plate as a raw material.
自動車の外板パネルや家電製品の筐体など、優れた意匠性と美麗さの要求される部位に適用される薄鋼板には、高い成形性に加えて、プレス成形後に凹みなどの変形によって容易に意匠が毀損しないよう高強度であることが求められてきた。特に近年、自動車業界においてはCO2排出抑制の観点で車体軽量化が最重要課題となっており、外板パネルにおいても薄肉化を達成するため、高強度かつ表面性状に優れた薄鋼板の提供が求められている。 In addition to high formability, thin steel plates applied to parts that require excellent design and beauty, such as automotive outer panel and home appliance housing, can be easily deformed after press forming. Therefore, it has been required to have high strength so that the design is not damaged. In recent years, especially in the automobile industry, weight reduction of vehicle bodies has become the most important issue from the viewpoint of reducing CO 2 emissions, and in order to achieve thinning of the outer panel, the provision of thin steel sheets with high strength and excellent surface properties is provided. Is required.
このような要請に対して、極低炭素鋼にTiやNbなどのC,N親和性の高い合金元素を微量添加して、C，Nをppmオーダーで除去し優れた成形性を発揮するIF（Interstitial Free）鋼は、優れた成形性や非時効性、表面性状を発揮する鋼板として使用されてきた。しかし、その引張強度は300MPa未満程度と低い。そこで、IF鋼をベースとしてMn，P，Siなどの固溶強化元素を添加したり、少量のフリーCを残すことで、加工後の熱処理に伴って降伏強度が上昇するBH（Bake Hardening）特性を付与した高強度鋼板の開発が進められてきた。 In response to these requirements, IF that exhibits excellent formability by removing trace amounts of C and N on the ppm order by adding trace amounts of alloy elements with high C and N affinity such as Ti and Nb to ultra low carbon steel (Interstitial Free) Steel has been used as a steel sheet that exhibits excellent formability, non-aging properties, and surface properties. However, its tensile strength is as low as less than 300 MPa. Therefore, by adding solid solution strengthening elements such as Mn, P, Si, etc. based on IF steel, or leaving a small amount of free C, BH (Bake Hardening) characteristics that yield strength increases with heat treatment after processing. The development of high-strength steel sheets with the added strength has been promoted.
例えば、特許文献１〜３においてはIF鋼をベースにMn，PあるいはSiを添加することで、深絞り性を損なうことなく300MPa超の高強度鋼板を得られる手法が開示されている。
For example,
Mn，P，Siは高強度化に非常に有効な添加元素である。しかし、MnやPは鋳造時に濃度分布の不均一を生じやすく、これが薄鋼板となった後も残存するため、高濃度部と低濃度部の強度差に起因した局所的な変形の不均一が、プレス成形後に鋼板表面に縞状の凹凸形状が生じる要因となる。このような縞状の模様が塗装後にも顕在化すれば、最終製品の廃棄が必要になって歩留りを低下させる。特に近年、表面品質の要求レベルが厳格化するに伴い、以前は問題とされていなかったような非常に軽度のものでも、不合格と判断される場合が出てきた。このため、従来よりも更なる表面品質の向上が要請されている。 Mn, P, and Si are very effective additive elements for increasing the strength. However, Mn and P are likely to cause non-uniform concentration distribution during casting and remain even after becoming a thin steel plate, so local deformation non-uniformity due to the difference in strength between the high concentration portion and the low concentration portion is not observed. It becomes a factor which a striped uneven | corrugated shape arises on the steel plate surface after press molding. If such a striped pattern becomes apparent even after painting, the final product must be discarded and the yield is lowered. Particularly in recent years, as the required level of surface quality has become stricter, there have been cases where even a very mild one that has not been regarded as a problem in the past is judged to be rejected. For this reason, further improvement in surface quality is demanded compared to the prior art.
これまでにも300MPa超の高強度鋼板の表面品質を改善する先行技術が開示されてきた。例えば特許文献１には、成分組成の調整、及び巻取り温度や焼鈍温度の制御によって、FeNbP系の析出物密度を低減することで表面性状を改善する技術が開示されている。
Previously, prior arts for improving the surface quality of high-strength steel sheets exceeding 300 MPa have been disclosed. For example,
また、特許文献２には、スラブに対して高温でのブレークダウン圧延などの塑性加工を行って、ある部位のＰの偏析度αを「α＝その部位のＰ濃度／鋼中のＰの平均濃度」とするとき、鋼板断面における偏析度の最大値αMAX と最小値αMIN との比αMAX ／αMIN が４以下であるプレス成形後の表面性状に優れた深絞り用高強度冷延鋼板を製造する技術が開示されている。 Further, in Patent Document 2, a slab is subjected to plastic working such as breakdown rolling at a high temperature, and the segregation degree α of P in a certain portion is expressed as “α = P concentration in that portion / average of P in steel”. The ratio of the maximum value α MAX and the minimum value α MIN of the segregation degree in the cross section of the steel sheet α MAX / α MIN is 4 or less. High strength cold rolling for deep drawing with excellent surface properties after press forming A technique for manufacturing a steel sheet is disclosed.
特許文献３には、P量とSi,Mn量の添加比率を適正化し、またTi,Nbの炭窒化物の生成を適切に制御することで、表面性状と材質安定性に優れた鋼板の製造方法が記載されている。 Patent Document 3 describes the production of steel sheets with excellent surface properties and material stability by optimizing the addition ratio of P and Si and Mn, and appropriately controlling the formation of carbonitrides of Ti and Nb. A method is described.
しかし、特許文献１では、元素材である熱延板以前の成分元素の分布不均一を制御していないため、鋳造時の元素分布のバラツキに、熱延コイル中の巻取り温度や焼鈍温度のバラツキも相俟って、凹凸模様を十分に抑制するには至っていない。
However,
特許文献２では、Pの偏析度の最大値αMAX と最小値αMIN との比αMAX ／αMIN を４以下に制御しているが、本発明者らが検討したところ、該規定の範囲内でも表面性状が劣化したり、範囲外でも良好な表面性状を有したりする場合があった。また、特許文献２では、Mnの偏析については考慮されていない。また、本技術は、熱延前にスラブをブレークダウン圧延するなど前処理を加える必要があり、製造コストの増大やCO2排出量の増加を招く。 In Patent Document 2, the ratio α MAX / α MIN between the maximum value α MAX and the minimum value α MIN of the segregation degree of P is controlled to 4 or less. In some cases, the surface properties deteriorated, and even outside the range, good surface properties were obtained. In Patent Document 2, no consideration is given to segregation of Mn. In addition, the present technology requires pretreatment such as breakdown rolling of the slab before hot rolling, leading to an increase in manufacturing cost and an increase in CO 2 emissions.
特許文献３では、P,Mnの濃度分布の制御手法については開示されておらず、凹凸模様を十分に抑制するには至っていない。 Patent Document 3 does not disclose a method for controlling the concentration distribution of P and Mn, and does not sufficiently suppress the uneven pattern.
実際、上記特許文献１〜３において、表面性状を評価する際の加工歪は、特許文献１で5％、特許文献２，３で3％と極端に低く、深絞り用高強度鋼板がプレス成形で付与される加工歪のレベル（最大10％程度）よりも甘い評価となっており、近年のより厳格化された表面品質の要請に対して十分な表面品質を保証する技術とは言いがたい。
In fact, in
そこで本発明は、上記課題に鑑み、高いMn，P成分を含有しながらもプレス加工後の表面性状に優れた冷延鋼板及びその製造方法、冷延焼鈍鋼板の製造方法、並びに溶融亜鉛めっき鋼板の製造方法を提供することを目的とする。 Therefore, in view of the above problems, the present invention provides a cold-rolled steel sheet having a high Mn, P component and excellent surface properties after press working, a method for producing the same, a method for producing a cold-rolled annealed steel sheet, and a hot-dip galvanized steel sheet It aims at providing the manufacturing method of.
また、本発明は、高いMn，P成分を含有しながらもプレス加工後の表面性状に優れた冷延鋼板の素材となる熱延鋼板及びその製造方法を提供することをも目的とする。 Another object of the present invention is to provide a hot-rolled steel sheet that is a material of a cold-rolled steel sheet that has a high Mn, P component and has excellent surface properties after press working, and a method for producing the hot-rolled steel sheet.
連続鋳造機によって製造されたスラブ内部、特に厚み中央付近には、PおよびMnの高濃度領域が幅方向に200μｍ以上から数ｍｍの間隔で存在しており、プレス成形時に鋼板表面に凹凸状の模様が生じる原因と推定されている。これに対して、特許文献２のような偏析度の最大値と最小値の比による規定では、近年の厳格な表面品質への要求に対して十分な解決法とは言い難い。そこで、本発明者らが上述した課題を解決するために鋭意研究を行った結果、以下の知見を得た。本発明で特に重要なことは、P,Mnの隣り合った高濃度領域と低濃度領域の濃度差を多数の部位で測定して、その濃度差と分布形態を制御することにある。隣接した幅方向の濃度差が増大するほど、急峻な強度変化が発生するため縞状の模様を形成しやすいため、個々の濃度差を一定の水準以下に制御する必要がある。 Inside the slab manufactured by a continuous casting machine, especially in the vicinity of the thickness center, high concentration regions of P and Mn exist at intervals of 200 μm or more to several mm in the width direction. It is presumed that the pattern is caused. On the other hand, the regulation based on the ratio between the maximum value and the minimum value of the segregation degree as in Patent Document 2 is not a sufficient solution to the recent demand for strict surface quality. Therefore, as a result of intensive studies conducted by the present inventors to solve the above-described problems, the following knowledge was obtained. In the present invention, it is particularly important to measure the density difference between adjacent high and low density regions of P and Mn at a number of sites and control the density difference and distribution form. As the density difference in the adjacent width direction increases, a steep intensity change occurs and a striped pattern is easily formed. Therefore, it is necessary to control each density difference to a certain level or less.
本発明は、上記の知見に基づき完成されたものであり、その要旨構成は、以下のとおりである。
（１）質量％で、C：0.04％以下、Si：1.5％以下、Mn：0.2〜2.0％、P：0.005〜0.060％、S：0.004〜0.020％、Sol.Al：0.003〜1.0％、N：0.0050％以下を含有し、残部がFe及び不可避的不純物からなる成分組成を有し、
板厚中央部におけるMn偏析度Smの幅方向プロファイルにおいて、各極大値についてΔSm及びΔWmを求めたとき、ΔWmが200μｍ以上となるΔSmの平均値が0.10以下かつ標準偏差2σmが0.10以下であり、
板厚中央部におけるP偏析度Spの幅方向プロファイルにおいて、各極大値についてΔSp及びΔWpを求めたとき、ΔWpが200μｍ以上となるΔSpの平均値が0.20以下かつ標準偏差2σpが0.15以下であることを特徴とする熱延鋼板。
ここで、
Sm＝任意の点におけるMn濃度（％）／鋼板の平均Mn濃度（％）
ΔSm：Smの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWm：各極大値に隣接する２つの極小値間の幅方向距離
Sp＝任意の点におけるP濃度（％）／鋼板の平均P濃度（％）
ΔSp：Spの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWp：各極大値に隣接する２つの極小値間の幅方向距離
とする。
The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
(1) By mass%, C: 0.04% or less, Si: 1.5% or less, Mn: 0.2-2.0%, P: 0.005-0.060%, S: 0.004-0.020%, Sol.Al: 0.003-1.0%, N : Containing 0.0050% or less, the balance having a composition composed of Fe and inevitable impurities,
In the width direction profile of the Mn segregation degree Sm in the central part of the plate thickness, when ΔSm and ΔWm were determined for each maximum value, the average value of ΔSm at which ΔWm was 200 μm or more was 0.10 or less and the standard deviation 2σm was 0.10 or less,
In the width direction profile of P segregation degree Sp at the center of the plate thickness, when ΔSp and ΔWp are calculated for each local maximum value, the average value of ΔSp at which ΔWp is 200 μm or more is 0.20 or less and the standard deviation 2σp is 0.15 or less. Hot-rolled steel sheet characterized by
here,
Sm = Mn concentration (%) at an arbitrary point / average Mn concentration (%) of steel sheet
ΔSm: Difference between the maximum value of Sm and the average value of two local minimum values adjacent to the local maximum value ΔWm: Distance in the width direction between two local minimum values adjacent to each local maximum value
Sp = P concentration (%) at any point / Average P concentration (%) of steel sheet
ΔSp: difference between the maximum value of Sp and the average value of two local minimum values adjacent to the local maximum value ΔWp: the distance in the width direction between two local minimum values adjacent to each local maximum value.
（２）前記成分組成が、質量％で、Ti：0.10％以下、Nb：0.10％以下、V：0.05％以下、W：0.1％以下、Ni：1％以下、Cr：1％以下、Cu：1％以下のうち１種又は２種以上をさらに含有する、上記（１）に記載の熱延鋼板。 (2) The above component composition is in mass%, Ti: 0.10% or less, Nb: 0.10% or less, V: 0.05% or less, W: 0.1% or less, Ni: 1% or less, Cr: 1% or less, Cu: The hot-rolled steel sheet according to (1), further containing one or more of 1% or less.
（３）前記成分組成が、質量％で、B：0.0050％以下、Sb：0.03％以下、Sn：0.03％以下のうち１種又は２種以上をさらに含有する、上記（１）又は（２）に記載の熱延鋼板。 (3) The above component composition (1) or (2), wherein the component composition further contains one or more of B: 0.0050% or less, Sb: 0.03% or less, Sn: 0.03% or less. The hot-rolled steel sheet according to 1.
（４）質量％で、C：0.04％以下、Si：1.5％以下、Mn：0.2〜2.0％、P：0.005〜0.060％、S：0.004〜0.020％、Sol.Al：0.003〜1.0％、N：0.0050％以下を含有し、残部がFe及び不可避的不純物からなる成分組成を有し、
板厚中央部におけるMn偏析度Smの幅方向プロファイルにおいて、各極大値についてΔSm及びΔWmを求めたとき、ΔWmが200μｍ以上となるΔSmの平均値が0.10以下かつ標準偏差2σmが0.05以下であり、
板厚中央部におけるP偏析度Spの幅方向プロファイルにおいて、各極大値についてΔSp及びΔWpを求めたとき、ΔWpが200μｍ以上となるΔSpの平均値が0.20以下かつ標準偏差2σpが0.10以下であることを特徴とする冷延鋼板。
ここで、
Sm＝任意の点におけるMn濃度（％）／鋼板の平均Mn濃度（％）
ΔSm：Smの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWm：各極大値に隣接する２つの極小値間の幅方向距離
Sp＝任意の点におけるP濃度（％）／鋼板の平均P濃度（％）
ΔSp：Spの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWp：各極大値に隣接する２つの極小値間の幅方向距離
とする。
(4) By mass%, C: 0.04% or less, Si: 1.5% or less, Mn: 0.2-2.0%, P: 0.005-0.060%, S: 0.004-0.020%, Sol.Al: 0.003-1.0%, N : Containing 0.0050% or less, the balance having a composition composed of Fe and inevitable impurities,
In the width direction profile of Mn segregation degree Sm in the central part of the plate thickness, when ΔSm and ΔWm were determined for each maximum value, the average value of ΔSm at which ΔWm was 200 μm or more was 0.10 or less and the standard deviation 2σm was 0.05 or less,
In the width direction profile of P segregation degree Sp at the center of the plate thickness, when ΔSp and ΔWp are determined for each local maximum value, the average value of ΔSp at which ΔWp is 200 μm or more is 0.20 or less and the standard deviation 2σp is 0.10 or less. Cold-rolled steel sheet characterized by
here,
Sm = Mn concentration (%) at an arbitrary point / average Mn concentration (%) of steel sheet
ΔSm: Difference between the maximum value of Sm and the average value of two local minimum values adjacent to the local maximum value ΔWm: Distance in the width direction between two local minimum values adjacent to each local maximum value
Sp = P concentration (%) at any point / Average P concentration (%) of steel sheet
ΔSp: difference between the maximum value of Sp and the average value of two local minimum values adjacent to the local maximum value ΔWp: the distance in the width direction between two local minimum values adjacent to each local maximum value.
（５）前記成分組成が、質量％で、Ti：0.10％以下、Nb：0.10％以下、V：0.05％以下、W：0.1％以下、Ni：1％以下、Cr：1％以下、Cu：1％以下のうち１種又は２種以上をさらに含有する、上記（４）に記載の冷延鋼板。 (5) The component composition is mass%, Ti: 0.10% or less, Nb: 0.10% or less, V: 0.05% or less, W: 0.1% or less, Ni: 1% or less, Cr: 1% or less, Cu: The cold-rolled steel sheet according to (4), further containing one or more of 1% or less.
（６）前記成分組成が、質量％で、B：0.0050％以下、Sb：0.03％以下、Sn：0.03％以下のうち１種又は２種以上をさらに含有する、上記（４）又は（５）に記載の冷延鋼板。 (6) The above component composition (4) or (5), wherein the component composition further contains one or more of B: 0.0050% or less, Sb: 0.03% or less, and Sn: 0.03% or less. Cold-rolled steel sheet as described in 1.
（７）上記（１）〜（３）のいずれか一項に記載の成分組成を有する溶鋼を連続鋳造してスラブを得る工程と、
前記スラブを、鋳型直下から（6min×Vc）[m]まではDe/Dc：1.1以上1.5以下の条件下で、（6min×Vc）[m]から凝固完了まではDe/Dc：0.7以上1.5以下の条件下で、かつ、二次冷却全体の平均の比水量Pは0.5以上2.5以下とする条件下で、二次冷却する工程と、
前記スラブを熱間圧延して熱延鋼板を得る工程と、
を有することを特徴とする熱延鋼板の製造方法。
ここで、
Dc：スラブの幅方向中央から幅方向1/2位置までの領域のスプレー水の水量密度
De：スラブの幅方向1/2位置から幅方向端部までの領域のスプレー水の水量密度
比水量P＝L/（W×T×Vc×ρ）
L：スプレー水流量(L/min)
W：スラブ幅(m)
T：スラブ厚み(m)
Vc：鋳造速度(m/min)
ρ：溶鋼密度(kg-鋼/ｍ３)
とする。
(7) A step of continuously casting a molten steel having the component composition according to any one of (1) to (3) to obtain a slab;
The slab is subjected to a condition of De / Dc: 1.1 or more and 1.5 or less from directly under the mold to (6 min × Vc) [m], and De / Dc: 0.7 to 1.5 from (6 min × Vc) [m] to the completion of solidification. Under the following conditions, and under the condition that the average specific water amount P of the entire secondary cooling is 0.5 or more and 2.5 or less, the secondary cooling step,
Hot rolling the slab to obtain a hot-rolled steel sheet;
A method for producing a hot-rolled steel sheet, comprising:
here,
Dc: Water density of spray water in the area from the center of the slab in the width direction to 1/2 position in the width direction
De: The amount of spray water in the region from the half-width position of the slab to the edge in the width direction Density specific water amount P = L / (W × T × Vc × ρ)
L: Spray water flow rate (L / min)
W: Slab width (m)
T: Thickness of slab (m)
Vc: Casting speed (m / min)
ρ: Molten steel density (kg-steel / m 3 )
And
（８）上記（７）に記載の熱延鋼板の製造方法における工程に加えて、
前記熱延鋼板を冷間圧延して冷延鋼板を得る工程をさらに有することを特徴とする冷延鋼板の製造方法。
(8) In addition to the steps in the method for producing a hot-rolled steel sheet according to (7) above,
A method for producing a cold-rolled steel sheet, further comprising a step of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet.
（９）上記（８）に記載の冷延鋼板の製造方法における工程に加えて、
前記冷延鋼板を焼鈍して冷延焼鈍鋼板を得る工程をさらに有することを特徴とする冷延焼鈍鋼板の製造方法。
(9) In addition to the steps in the method for producing a cold-rolled steel sheet according to (8) above,
A method for producing a cold-rolled annealed steel sheet, further comprising the step of annealing the cold-rolled steel sheet to obtain a cold-rolled annealed steel sheet.
（１０）上記（８）に記載の冷延鋼板の製造方法における工程に加えて、
前記冷延鋼板を溶融亜鉛めっきして溶融亜鉛めっき鋼板を得る工程とさらに有することを特徴とする溶融亜鉛めっき鋼板の製造方法。
(10) In addition to the steps in the method for producing a cold-rolled steel sheet according to (8) above,
A method for producing a hot-dip galvanized steel sheet, further comprising a step of hot-dip galvanizing the cold-rolled steel sheet to obtain a hot-dip galvanized steel sheet.
本発明の冷延鋼板は、高いMn，P成分を含有しながらもプレス加工後の表面性状に優れる。本発明の冷延鋼板の製造方法、冷延焼鈍鋼板の製造方法、及び溶融亜鉛めっき鋼板の製造方法によれば、高いMn，P成分を含有しながらもプレス加工後の表面性状に優れた冷延鋼板、冷延焼鈍鋼板、及び溶融亜鉛めっき鋼板を製造することができる。 The cold-rolled steel sheet of the present invention is excellent in surface properties after press working while containing high Mn and P components. According to the method for producing a cold-rolled steel sheet, the method for producing a cold-rolled annealed steel sheet, and the method for producing a hot-dip galvanized steel sheet according to the present invention, the cold-rolled steel sheet having high Mn and P components and excellent surface properties after press working. A rolled steel sheet, a cold-rolled annealed steel sheet, and a hot-dip galvanized steel sheet can be manufactured.
本発明の熱延鋼板を素材とすれば、高いMn，P成分を含有しながらもプレス加工後の表面性状に優れた冷延鋼板を得ることができる。本発明の熱延鋼板の製造方法は、高いMn，P成分を含有しながらもプレス加工後の表面性状に優れた冷延鋼板の素材となる熱延鋼板を製造することができる。 If the hot-rolled steel sheet of the present invention is used as a raw material, it is possible to obtain a cold-rolled steel sheet having a high Mn, P component and excellent surface properties after press working. The method for producing a hot-rolled steel sheet according to the present invention can produce a hot-rolled steel sheet that is a material for a cold-rolled steel sheet that has a high Mn, P component and has excellent surface properties after press working.
（熱延鋼板及び冷延鋼板）
＜成分組成＞
以下、本発明の熱延鋼板及び冷延鋼板の成分組成の限定理由を説明する。特に断りのない限り、「％」は鋼中における対象の添加元素の濃度を示す「質量％」を意味する。
(Hot rolled steel sheet and cold rolled steel sheet)
<Ingredient composition>
Hereinafter, the reason for limitation of the component composition of the hot rolled steel sheet and the cold rolled steel sheet of the present invention will be described. Unless otherwise specified, “%” means “mass%” indicating the concentration of the target additive element in the steel.
C：0.04％以下
C含有量が0.04％を超えると、鋼板の延性、深絞り性、伸びフランジ性が著しく低下する。さらに、Cの増大は凝固時の固液共存温度範囲を拡大し、スラブ中の液相の凝固を低温域まで遅延させる。このため、液相に分配されるP,Mn量が増加する結果、スラブのP,Mn濃度の不均一を助長し、表面品質の劣化を招く。このため、C含有量は0.04％以下とする。より好ましくは0.03％以下、さらに好ましくは0.01％以下である。一方、C含有量の下限は定めないが、常法に従った精錬による極低炭素化では0.0005％以上が目安となり、それ未満とするには精錬コストや歩留りが極端に悪化する。よって、C含有量は0.0005％以上とすることが好ましい。
C: 0.04% or less
If the C content exceeds 0.04%, the ductility, deep drawability and stretch flangeability of the steel sheet will be significantly reduced. Furthermore, the increase in C expands the solid-liquid coexistence temperature range during solidification and delays solidification of the liquid phase in the slab to a low temperature range. For this reason, the amount of P and Mn distributed to the liquid phase increases, and as a result, nonuniform P and Mn concentrations in the slab are promoted, resulting in deterioration of the surface quality. Therefore, the C content is set to 0.04% or less. More preferably, it is 0.03% or less, and further preferably 0.01% or less. On the other hand, although the lower limit of the C content is not set, 0.0005% or more is a guideline for ultra-low carbonization by refining according to a conventional method, and refining costs and yields are extremely deteriorated to make it lower. Therefore, the C content is preferably 0.0005% or more.
Si：1.5％以下
Si含有量が1.5％を超えると、熱延時に強固かつ厚みの不均一なスケールが形成して、酸洗後も鋼板表面にスケール残りや凹みが生じ、最終製品の表面品質が著しく劣化する。このため、Si含有量は1.5%以下とする。より好ましくは0.8％以下、さらに好ましくは0.5％以下とする。特に優れた表面品質を得る観点では0.2％以下とする。一方で、Siは固溶強化能の高い安価元素であり、鋼の高強度化に寄与するため少量加えてもよい。この観点から、Si含有量は0.10％以上とすることが好ましい。
Si: 1.5% or less
If the Si content exceeds 1.5%, a strong and uneven thickness scale is formed during hot rolling, and even after pickling, scale residue and dents are formed on the surface of the steel sheet, and the surface quality of the final product is significantly deteriorated. Therefore, the Si content is 1.5% or less. More preferably, it is 0.8% or less, More preferably, it is 0.5% or less. In particular, from the viewpoint of obtaining excellent surface quality, it is 0.2% or less. On the other hand, Si is an inexpensive element having a high solid solution strengthening ability and may be added in a small amount because it contributes to increasing the strength of steel. From this viewpoint, the Si content is preferably set to 0.10% or more.
Mn：0.2〜2.0％
Mnは鋼板を強化する元素であるだけでなく、鋼の高温脆化を招く不純物元素であるSをMnSとして固定する作用もあるため、添加する必要がある。この効果を得るには、Mn含有量は0.2％以上とする必要がある。ただし、2.0％を超えて過剰に添加すると、成形性やめっき性が損なわれる。さらに、Mnの濃度不均一が助長されプレス後の表面品質も顕著に劣化する。このため、Mn含有量は2.0％以下とする。より好ましくは1.8％以下、さらに好ましくは1.5％以下、特に優れた表面品質を得る観点では、1.3％以下とすることが好ましい。
Mn: 0.2-2.0%
Mn is not only an element that strengthens the steel sheet, but also has an effect of fixing S, which is an impurity element that causes high-temperature embrittlement of steel, as MnS, and therefore needs to be added. In order to obtain this effect, the Mn content needs to be 0.2% or more. However, if it exceeds 2.0% and is added excessively, formability and plating properties are impaired. Furthermore, the non-uniform concentration of Mn is promoted, and the surface quality after pressing is significantly deteriorated. For this reason, Mn content shall be 2.0% or less. More preferably, it is 1.8% or less, more preferably 1.5% or less, and from the viewpoint of obtaining particularly excellent surface quality, it is preferably 1.3% or less.
P：0.005〜0.060％
Pは上述したように、Mnと同様に凝固時の濃度不均一を招き、プレス後の表面品質を顕著に毀損する。また、多量のP添加は耐二次加工脆性を低減したり、鋼板のめっき性を劣化したりするなどデメリットが著しい。これらを回避するため、P含有量は0.060％以下とする。より好ましくは0.050％以下、さらに好ましくは0.045％以下である。一方、0.005％未満にPを低減しようとすると、精錬負荷とコストが顕著に増大するため、P含有量の下限を0.005％とする。ただし、Pは安価で固溶強化能の優れた添加元素であることから、300MPa以上の強度を安定確保する観点では0.020％以上添加することができる。
P: 0.005-0.060%
As described above, P causes non-uniform concentration during solidification, as with Mn, and significantly deteriorates the surface quality after pressing. In addition, the addition of a large amount of P has significant demerits such as reducing secondary work brittleness resistance and degrading the plateability of the steel sheet. In order to avoid these, the P content is set to 0.060% or less. More preferably, it is 0.050% or less, More preferably, it is 0.045% or less. On the other hand, if it is attempted to reduce P to less than 0.005%, the refining load and cost increase significantly, so the lower limit of the P content is set to 0.005%. However, since P is an additive element that is inexpensive and has excellent solid solution strengthening ability, it can be added in an amount of 0.020% or more from the viewpoint of stably securing a strength of 300 MPa or more.
S：0.004〜0.020％
Sは不可避的に鋼中に含有され、粒界に偏析することで赤熱脆化によるスラブ・熱延板の割れや、耐二次加工脆性の劣化を招く。このため、S含有量は0.020％以下とする。より好ましくは0.010％以下である。一方、Sは熱延スケールの剥離性を向上し、薄鋼板の表面品質を改善する効果も認められることから少量ならば含有させてもよい。この効果を得るには0.004％以上の添加が好ましい。
S: 0.004 to 0.020%
S is inevitably contained in the steel, and segregates at the grain boundaries, thereby causing cracks in the slab / hot-rolled sheet due to red heat embrittlement and deterioration of secondary work brittleness resistance. For this reason, S content shall be 0.020% or less. More preferably, it is 0.010% or less. On the other hand, S may be contained in a small amount because it improves the peelability of the hot-rolled scale and improves the surface quality of the thin steel sheet. To obtain this effect, 0.004% or more is preferable.
Sol.Al：0.003〜1.0％
Alは溶鋼の脱酸のために積極的に添加される元素であり、この効果を得るためには少なくとも0.003％以上のsol.Al含有量を確保する。より好ましくは0.010％以上である。さらに、Alは凝固時にP,Mnの高濃度領域に濃化せずに、逆にP,Mn低濃度領域に濃化する特性があるため、Alの固溶強化によってミクロ組織間の強度差を軽減し、プレス後の表面品質を改善する効果がある。このため、Pは積極的に含有させてもよい。ただし、1.0％を超える過剰なAlは、Siと同様にスケール欠陥増大やめっき性の低下、さらに溶接性の劣化も招く。それらの問題を回避するため、Al含有量は1.0％以下とする。より好ましくは0.8％以下であり、特に好ましくは0.2％以下である。
Sol.Al: 0.003-1.0%
Al is an element that is actively added for deoxidation of molten steel. To obtain this effect, a sol.Al content of at least 0.003% is ensured. More preferably, it is 0.010% or more. Furthermore, Al does not concentrate in the high concentration region of P and Mn during solidification, but conversely, it concentrates in the low concentration region of P and Mn. It has the effect of reducing and improving the surface quality after pressing. For this reason, P may be positively contained. However, excessive Al exceeding 1.0% causes an increase in scale defects, a decrease in plating properties, and a deterioration in weldability, as in Si. In order to avoid these problems, the Al content is 1.0% or less. More preferably, it is 0.8% or less, and particularly preferably 0.2% or less.
N：0.0050％以下
Nは不可避的に鋼中に含有される不純物であり、多量に含有すると成形性の劣化や時効硬化によるストレッチャストレインの発生をまねく。それらの課題を回避するため、N含有量は0.0050％以下とする。
N: 0.0050% or less
N is inevitably an impurity contained in the steel, and if it is contained in a large amount, it causes deterioration of formability and generation of stretcher strain due to age hardening. In order to avoid these problems, the N content is 0.0050% or less.
上記元素以外の残部はFe及び不可避的不純物からなる。ただし、上記元素に加えて、任意で以下に示す合金元素を１種又は２種以上含有させてもよい。 The balance other than the above elements consists of Fe and inevitable impurities. However, in addition to the above elements, one or more of the following alloying elements may be optionally contained.
Ti：0.10％以下、Nb：0.10％以下
TiやNbは鋼中のフリーC量を適切にコントロールするのに有効な元素である。それにより、最終製品の耐時効性を著しく向上することができるだけでなく、熱延板の微細化とフリーCの低減によって、焼鈍時の再結晶集合組織を発達させて顕著な深絞り性を獲得するのに極めて有効である。加えて、微細な炭化物を形成するため、鋼板の強度上昇にも寄与する。これらの効果を得るために、Ti,Nbを各々0.01％以上で添加しても良い。一方、過剰な添加は組織の硬質化や粗大な炭窒化物の形成によって、成形性の低下を招く。また、表面窒化によるプレス加工後の表面凹凸の形成という問題も生じることがある。このため、Ti,Nb含有量は各々0.10％以下に制限する。好ましくは0.05％以下である。
Ti: 0.10% or less, Nb: 0.10% or less
Ti and Nb are effective elements for appropriately controlling the amount of free C in steel. As a result, not only the aging resistance of the final product can be remarkably improved, but also the recrystallized texture at the time of annealing is developed by refining the hot-rolled sheet and reducing free C to obtain a remarkable deep drawability. It is extremely effective to do. In addition, the formation of fine carbides contributes to an increase in strength of the steel sheet. In order to obtain these effects, Ti and Nb may be added at 0.01% or more, respectively. On the other hand, excessive addition leads to a decrease in moldability due to hardening of the structure and formation of coarse carbonitride. There may also be a problem of formation of surface irregularities after press working by surface nitriding. For this reason, Ti and Nb content are limited to 0.10% or less respectively. Preferably it is 0.05% or less.
V：0.05％以下
Vは微細炭化物を形成するため、鋼板の強度向上のために0.01％以上添加しても良い。ただし、0.05％を超えて添加すると、組織の硬質化や粗大な炭窒化物の形成によって成形性の低下を招くため、0.05％以下とする。
V: 0.05% or less
V forms fine carbides and may be added in an amount of 0.01% or more for improving the strength of the steel sheet. However, if added over 0.05%, the formability is reduced due to the hardened structure and the formation of coarse carbonitrides, so 0.05% or less.
W：0.1％以下
Wは微細炭化物を形成するため、鋼板の強度向上のために0.01％以上添加しても良い。ただし、0.1％を超えて添加すると、組織の硬質化や粗大な炭窒化物の形成によって成形性の低下を招くため、0.1％以下とする。
W: 0.1% or less
W forms fine carbides, so 0.01% or more may be added to improve the strength of the steel sheet. However, if added over 0.1%, the formability is reduced due to the hardened structure and the formation of coarse carbonitrides, so the content is made 0.1% or less.
Ni：1％以下
Niは鋼板の耐食性や低温靭性を向上する効果が得られるため、0.01％以上添加しても良いが、1％を超える過剰な添加はコストの増大を招くため好ましくない。
Ni: 1% or less
Ni is effective for improving the corrosion resistance and low temperature toughness of the steel sheet, so it may be added in an amount of 0.01% or more. However, excessive addition of more than 1% is not preferable because it causes an increase in cost.
Cr：1％以下
Crは鋼板の耐食性の向上や、炭化物の形成による強度向上の効果が得られるため、0.01％以上添加しても良いが、1％を超える過剰な添加はコストの増大を招くため好ましくない。
Cr: 1% or less
Cr can be added in an amount of 0.01% or more because of the effect of improving the corrosion resistance of the steel sheet and the strength improvement due to the formation of carbides. However, excessive addition of more than 1% is not preferable because it increases the cost.
Cu：1％以下
Cuは鋼板の耐食性の向上や、Cu粒子の析出による強度向上効果が得られるため、0.01％以上添加しても良いが、1％を超える過剰な添加は熱間延性の低下によるスラブ・熱延板割れを招くため好ましくない。また、添加時には同量程度のNiを同時添加することが好ましい。
Cu: 1% or less
Cu can improve the corrosion resistance of the steel sheet and improve the strength by precipitation of Cu particles, so it may be added in an amount of 0.01% or more. However, excessive addition of more than 1% causes slab / hot rolling due to a decrease in hot ductility. This is not preferable because it causes cracking. In addition, it is preferable to add the same amount of Ni at the same time.
B：0.0050％以下
BはP,Sよりも結晶粒界に優先的に濃化することで、P,Sによる粒界脆化を抑制できるため、0.0003％以上添加しても良い。ただし、0.0050％を超えて過剰に添加しても、上記の効果は飽和し、逆に熱間変形抵抗を著しく増大するため生産性を阻害したり、仕上げ温度の上昇によってスケール性欠陥の増加を招く。このため、B含有量は0.0050％以下とする。より好ましくは0.0030％以下である。
B: 0.0050% or less
B can be added at 0.0003% or more because it can suppress the grain boundary embrittlement due to P and S by preferentially concentrating at the grain boundary over P and S. However, even if it is added excessively over 0.0050%, the above effect is saturated, and conversely, the hot deformation resistance is remarkably increased, so that productivity is hindered and scale defects are increased by increasing the finishing temperature. Invite. Therefore, the B content is 0.0050% or less. More preferably, it is 0.0030% or less.
Sb：0.03％以下、Sn：0.03％以下
SbおよびSnは、鋼板の表面酸化を抑制する効果があり、スケール性欠陥や表面窒化、脱炭を低減して表面品質を維持するのに有効である。この効果を得るには、SbまたはSnを各々0.005％以上含有することが好ましい。しかし、0.03％を超えて含有しても、効果は飽和し、成形性の劣化やコスト上昇などを招くことから、含有量は各々0.03％以下とする。
Sb: 0.03% or less, Sn: 0.03% or less
Sb and Sn have the effect of suppressing the surface oxidation of the steel sheet, and are effective for maintaining the surface quality by reducing scale defects, surface nitriding and decarburization. In order to obtain this effect, it is preferable to contain 0.005% or more of Sb or Sn. However, even if the content exceeds 0.03%, the effect is saturated and the moldability is deteriorated and the cost is increased. Therefore, the content is 0.03% or less.
＜Mn偏析度及びP偏析度の分布＞
鋼板内のP,Mnは均一に分布していることが理想であるが、上述したように、実際にはスラブ凝固時にデンドライト樹間や最終凝固部へ分配されることで、Mn,Pの高濃度域とそれに隣接した低濃度領域が、数10μｍ〜数ｍｍ間隔で形成される。特にスラブ板厚の中央近傍は最終凝固部であるため、幅200μｍ以上の大きな濃度分布を形成しやすい。板厚中央以外でも内部割れによって局所的な濃度偏差を生じることもある。このような局所的なMn,P濃度差のうち、幅200μｍを超えるサイズの高濃度域の周囲には、大きな強度変化が生じて歪が不均一になるため、薄鋼板がプレス成型された際に視認性の高い縞状模様を誘発することが分かった。
<Mn segregation degree and P segregation degree distribution>
Ideally, P and Mn in the steel sheet are uniformly distributed. However, as described above, in fact, when the slab is solidified, it is distributed to the dendrite trees and to the final solidified part. A concentration region and a low concentration region adjacent thereto are formed at intervals of several tens of μm to several mm. In particular, since the vicinity of the center of the slab thickness is the final solidified portion, it is easy to form a large concentration distribution with a width of 200 μm or more. Other than the center of the plate thickness, local concentration deviations may occur due to internal cracks. Among such local Mn and P concentration differences, a large strength change occurs around the high concentration region with a width exceeding 200 μm, resulting in uneven strain. It was found that a highly visible stripe pattern was induced.
そこで本発明者らが縞状模様を顕在化させないMn,P濃度分布について鋭意検討した結果、幅200μｍ以上の個々の偏析の濃度差とバラツキを適切に制御することで、引張ひずみで10％の大きな塑性加工を受けても縞状模様の視認性が極めて低くなることを知見した。 Therefore, as a result of intensive studies on the Mn and P concentration distributions that do not reveal the striped pattern, the present inventors have appropriately controlled the concentration difference and variation of individual segregation with a width of 200 μm or more, thereby achieving a tensile strain of 10%. It has been found that the visibility of striped patterns is extremely low even when subjected to large plastic working.
具体的には、熱延鋼板においては、以下の条件を満たすことが肝要である。
−板厚中央部におけるMn偏析度Smの幅方向プロファイルにおいて、各極大値についてΔSm及びΔWmを求めたとき、ΔWmが200μｍ以上となるΔSmの平均値が0.10以下かつ標準偏差2σmが0.10以下であること。
−板厚中央部におけるP偏析度Spの幅方向プロファイルにおいて、各極大値についてΔSp及びΔWpを求めたとき、ΔWpが200μｍ以上となるΔSpの平均値が0.20以下かつ標準偏差2σpが0.15以下であること。
ここで、
Sm＝任意の点におけるMn濃度（％）／鋼板の平均Mn濃度（％）
ΔSm：Smの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWm：各極大値に隣接する２つの極小値間の幅方向距離
Sp＝任意の点におけるP濃度（％）／鋼板の平均P濃度（％）
ΔSp：Spの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWp：各極大値に隣接する２つの極小値間の幅方向距離
とする。
Specifically, in a hot-rolled steel sheet, it is important to satisfy the following conditions.
-In the width direction profile of the Mn segregation degree Sm at the center of the plate thickness, when ΔSm and ΔWm are obtained for each maximum value, the average value of ΔSm at which ΔWm is 200 μm or more is 0.10 or less and the standard deviation 2σm is 0.10 or less about.
-In the width direction profile of the P segregation degree Sp at the center of the plate thickness, when ΔSp and ΔWp are determined for each maximum value, the average value of ΔSp at which ΔWp is 200 μm or more is 0.20 or less and the standard deviation 2σp is 0.15 or less about.
here,
Sm = Mn concentration (%) at an arbitrary point / average Mn concentration (%) of steel sheet
ΔSm: Difference between the maximum value of Sm and the average value of two local minimum values adjacent to the local maximum value ΔWm: Distance in the width direction between two local minimum values adjacent to each local maximum value
Sp = P concentration (%) at any point / Average P concentration (%) of steel sheet
ΔSp: difference between the maximum value of Sp and the average value of two local minimum values adjacent to the local maximum value ΔWp: the distance in the width direction between two local minimum values adjacent to each local maximum value.
ΔWmが200μｍ以上となるΔSmの平均値が0.10超え又は標準偏差2σmが0.10超えの場合、熱延鋼板を素材として製造した冷延鋼板をプレス加工した後の表面性状が劣る。また、ΔWpが200μｍ以上となるΔSpの平均値が0.20超え又は標準偏差2σpが0.15超えの場合も、プレス加工後の表面性状が劣る。 When the average value of ΔSm at which ΔWm is 200 μm or more exceeds 0.10 or the standard deviation 2σm exceeds 0.10, the surface properties after the cold-rolled steel plate manufactured using a hot-rolled steel plate as a raw material is inferior. Further, when the average value of ΔSp at which ΔWp is 200 μm or more exceeds 0.20 or the standard deviation 2σp exceeds 0.15, the surface properties after press working are inferior.
また、冷延鋼板においては、以下の条件を満たすことが肝要である。
−板厚中央部におけるMn偏析度Smの幅方向プロファイルにおいて、各極大値についてΔSm及びΔWmを求めたとき、ΔWmが200μｍ以上となるΔSmの平均値が0.10以下かつ標準偏差2σmが0.05以下であること。
−板厚中央部におけるP偏析度Spの幅方向プロファイルにおいて、各極大値についてΔSp及びΔWpを求めたとき、ΔWpが200μｍ以上となるΔSpの平均値が0.20以下かつ標準偏差2σpが0.10以下であること。
Moreover, in the cold rolled steel sheet, it is important to satisfy the following conditions.
-In the width direction profile of the Mn segregation degree Sm at the center of the plate thickness, when ΔSm and ΔWm are obtained for each maximum value, the average value of ΔSm at which ΔWm is 200 μm or more is 0.10 or less and the standard deviation 2σm is 0.05 or less about.
-In the width direction profile of the P segregation degree Sp at the center of the plate thickness, when ΔSp and ΔWp are determined for each maximum value, the average value of ΔSp at which ΔWp is 200 μm or more is 0.20 or less and the standard deviation 2σp is 0.10 or less about.
ΔWmが200μｍ以上となるΔSmの平均値が0.10超え又は標準偏差2σmが0.05超えの場合、冷延鋼板をプレス加工した後の表面性状が劣る。また、ΔWpが200μｍ以上となるΔSpの平均値が0.20超え又は標準偏差2σpが0.10超えの場合も、プレス加工後の表面性状が劣る。 When the average value of ΔSm at which ΔWm is 200 μm or more exceeds 0.10 or the standard deviation 2σm exceeds 0.05, the surface properties after the cold-rolled steel sheet is pressed are inferior. Also, when the average value of ΔSp at which ΔWp is 200 μm or more exceeds 0.20 or the standard deviation 2σp exceeds 0.10, the surface properties after press working are inferior.
（熱延鋼板、冷延鋼板、冷延焼鈍鋼板及び溶融亜鉛めっき鋼板の製造方法）
以下、上記本開示の熱延鋼板及び冷延鋼板を製造するための製造方法について説明する。まず、上記成分組成を有する溶鋼を連続鋳造してスラブを得る。溶鋼を鋳造するに際しては、湾曲型、垂直型または垂直曲げ型の連続鋳造機を使用することが好ましい。これは、幅方向の濃度不均一の制御と生産性を両立するのに好適であるためである。連続鋳造機における銅鋳型での一次冷却条件は、常法に従って鋳片を均一凝固させるよう適切に行う。
(Method for producing hot-rolled steel sheet, cold-rolled steel sheet, cold-rolled annealed steel sheet and hot-dip galvanized steel sheet)
Hereinafter, the manufacturing method for manufacturing the hot-rolled steel sheet and the cold-rolled steel sheet of the present disclosure will be described. First, molten steel having the above composition is continuously cast to obtain a slab. When casting molten steel, it is preferable to use a curved, vertical or vertical bending type continuous casting machine. This is because it is suitable for achieving both control of non-uniform density in the width direction and productivity. The primary cooling condition in the copper mold in the continuous casting machine is appropriately performed so as to uniformly solidify the slab according to a conventional method.
本発明では、熱延鋼板、あるいはこれを素材とする冷延鋼板において上記のMn偏析度及びP偏析度の分布を得るために、二次冷却における鋳型直下から凝固完了までの領域におけるスプレー冷却を制御することが肝要である。 In the present invention, in order to obtain the distribution of the Mn segregation degree and the P segregation degree in the hot-rolled steel sheet or the cold-rolled steel sheet made of the same, spray cooling is performed in a region from immediately below the mold to completion of solidification in the secondary cooling. It is important to control.
具体的には、鋳型直下から（6min×Vc）[m]まではDe/Dc：1.1以上1.5以下の条件下で、（6min×Vc）[m]から凝固完了まではDe/Dc：0.7以上1.5以下の条件下で、かつ、二次冷却全体の平均の比水量Pは0.5以上2.5以下とする条件下で、スラブの二次冷却を行う。上記二次冷却の条件のいずれかを満たさない場合、Mn及び／又はPの偏析を十分に抑制することができず、本発明のMn偏析度及びP偏析度の分布を得ることができない。この理由は明確ではないが、鋳造初期の幅方向の水量密度を敢えてわずかに不均一にすることは、結果的に、スラブの冷却と凝固の進行に伴って凝固終端（いわゆるクレータエンド）形状が不均一になる影響を相殺し均一化することで、幅方向のMnやPといった合金元素のマクロな分布を均一化する効果が得られると考えられる。一方、比水量が乏しい場合は二次冷却のスプレー水の散布領域が不均一になること、過剰な場合にはスプレー水や乗り水が鋳片表面の所々で遷移沸騰現象に伴う過冷却を引き起こすこと、などにより均一な凝固終端形状を得られなくなると考えられる。
ここで、
Dc：スラブの幅方向中央から幅方向1/2位置までの領域のスプレー水の水量密度
De：スラブの幅方向1/2位置から幅方向端部までの領域のスプレー水の水量密度
比水量P＝L/（W×T×Vc×ρ）
L：スプレー水流量(L/min)
W：スラブ幅(m)
T：スラブ厚み(m)
Vc：鋳造速度(m/min)
ρ：溶鋼密度(kg-鋼/ｍ３)
とする。なお、上記スラブの「幅方向1/2位置」とは、幅方向中央と幅方向端部の中間位置である。
Specifically, De / Dc: 1.1 or more and 1.5 or less from directly under the mold to (6 min x Vc) [m], De / Dc: 0.7 or more from (6 min x Vc) [m] to solidification completion Secondary cooling of the slab is performed under conditions of 1.5 or less and an average specific water amount P of the entire secondary cooling of 0.5 to 2.5. If any of the secondary cooling conditions is not satisfied, the segregation of Mn and / or P cannot be sufficiently suppressed, and the distribution of the Mn segregation degree and the P segregation degree of the present invention cannot be obtained. The reason for this is not clear, but the intention is to make the water density in the width direction at the initial stage of casting slightly uneven. As a result, as the slab cools and solidifies, the shape of the solidification end (so-called crater end) is formed. It is considered that the effect of uniforming the macro distribution of alloy elements such as Mn and P in the width direction can be obtained by offsetting and uniforming the effect of non-uniformity. On the other hand, when the specific water amount is low, the spray area of the secondary cooling spray water becomes non-uniform, and when it is excessive, the spray water and the running water cause supercooling due to the transition boiling phenomenon on the slab surface. It is considered that a uniform solidification end shape cannot be obtained due to the above.
here,
Dc: Water density of spray water in the area from the center of the slab in the width direction to 1/2 position in the width direction
De: The amount of spray water in the region from the half-width position of the slab to the edge in the width direction Density specific water amount P = L / (W × T × Vc × ρ)
L: Spray water flow rate (L / min)
W: Slab width (m)
T: Thickness of slab (m)
Vc: Casting speed (m / min)
ρ: Molten steel density (kg-steel / m 3 )
And The “width direction half position” of the slab is an intermediate position between the center in the width direction and the end in the width direction.
また、スラブコーナーの過冷却を回避するために鋳片長辺面の両端部だけスプレー水の噴霧を行わない、いわゆる幅切りを実施する場合、鋳型直下から（6min×Vc）[m]までの幅切り量の平均がスラブコーナーから0.4T(m)以下で、（6min×Vc）[m]から凝固完了までは幅切り量の平均が0.8T(m)以下の領域に実施して良い。それよりも鋳片の幅中央寄りにまで幅切りを実施するとクレーターエンドのコーナー寄りの形状が伸長し、不均一な元素分布を助長するため筋模様を助長するため好ましくない。なお、二次冷却スプレーの幅切りを実施する場合は、水量密度Deを計算する対象となる鋳片表面の面積から幅切り適用領域の面積を除外すればよい。 In addition, in order to avoid overcooling of the slab corner, spraying water is not sprayed on both ends of the long side of the slab, so when performing so-called width cutting, the width from directly under the mold to (6 min x Vc) [m] The average cutting amount may be 0.4 T (m) or less from the slab corner, and the average width cutting amount may be 0.8 T (m) or less from (6 min × Vc) [m] to the completion of solidification. If the slab is further cut toward the center of the width of the slab, the shape near the corner of the crater end is elongated, which promotes a non-uniform element distribution and promotes a streak pattern. In addition, what is necessary is just to exclude the area of a width cutting application area | region from the area of the slab surface used as the object which calculates water quantity density De, when implementing width cutting of a secondary cooling spray.
二次冷却後のスラブを熱間圧延して熱延鋼板を得る。熱間圧延の条件は特に限定されず、常法とすることができる。ただし、熱間圧延での仕上げ圧延出側温度が900℃を超える場合、最終製品におけるスケール性の表面欠陥が増加し表面品質を損なう可能性があることから、仕上げ圧延出側温度は900℃以下が好ましい。一方、仕上げ圧延温度がAr3温度未満となると、オーステナイトの再結晶が不十分なままフェライト生成して圧延加工組織が残存する。これにより延性の低下や圧延皺が押し込まれた熱延性の線状欠陥を誘発するため、仕上げ圧延出側温度はAr3温度以上とすることが好ましい。なお、Ar3温度は以下の式で成分より求める。
Ar3温度=837-475[%C]+56[%Si]-20[%Mn]-16[%Cu]-27[%Ni]-5[%Cr]+38[%Mo]
+125[%V]-136[%Ti]-20[%Nb]+198[%Al]+3315[%B]
ここで、［%M］は元素Ｍの含有量を意味し、添加しない元素の場合にはゼロとする。
The slab after secondary cooling is hot-rolled to obtain a hot-rolled steel sheet. The conditions for hot rolling are not particularly limited, and can be a conventional method. However, if the finish rolling exit temperature in hot rolling exceeds 900 ° C, the surface defect of the scale property in the final product may increase and the surface quality may be impaired. Is preferred. On the other hand, when the finish rolling temperature is lower than the Ar3 temperature, ferrite is formed with insufficient austenite recrystallization, and the rolled structure remains. This induces a drop in ductility and a hot-ductile linear defect in which the rolling bar is pushed in. Therefore, the finish rolling exit temperature is preferably set to the Ar3 temperature or higher. The Ar3 temperature is obtained from the component by the following formula.
Ar3 temperature = 837-475 [% C] +56 [% Si] -20 [% Mn] -16 [% Cu] -27 [% Ni] -5 [% Cr] +38 [% Mo]
+125 [% V] -136 [% Ti] -20 [% Nb] +198 [% Al] +3315 [% B]
Here, [% M] means the content of the element M, and is zero in the case of an element not added.
さらに、熱延鋼板を酸洗、冷間圧延して冷延鋼板を得る。冷間圧延の条件は特に限定されず、常法とすることができる。ただし、冷間圧延率が20％未満では、引き続いての焼鈍時にフェライト母相の再結晶が進まず延性が低下するため、冷間圧延率は20％以上とすることが好ましい。 Further, the hot-rolled steel sheet is pickled and cold-rolled to obtain a cold-rolled steel sheet. The conditions for cold rolling are not particularly limited, and can be a conventional method. However, if the cold rolling rate is less than 20%, the recrystallization of the ferrite matrix does not proceed during subsequent annealing, and the ductility is lowered. Therefore, the cold rolling rate is preferably 20% or more.
そして、冷延鋼板を常法にて連続焼鈍し、さらに調質圧延を施して、薄鋼板とすることができる。なお、連続焼鈍に際しては、連続焼鈍ライン（CAL）又は連続溶融めっきライン（CGL）、あるいはバッチ焼鈍設備(BAF)、またはそれら複数ラインの組み合わせにて焼鈍処理を実施することが好ましい。なお、上記の一般的な焼鈍設備における加熱温度での工業的な処理時間の範疇では、鋳造時に決定したP,Mn元素の濃度分布を低減・消失することは実質的に不可能であり、焼鈍条件が本発明で規定するP,Mnの分布形態と筋状模様に及ぼす影響は少ない。そのため、焼鈍温度および焼鈍過程におけるヒートサイクルは本発明では特に規定せず、所望のミクロ組織と特性を得るための適切な焼鈍条件をそれぞれ採用することできる。ただし、焼鈍温度が700℃未満ではフェライト母相の再結晶と粒成長が不十分で冷圧による圧延加工組織が残存し延性が劣化するため、焼鈍温度は700℃以上が好ましい。CALまたはBAFにおいては冷延焼鈍鋼板、CGLにおいては溶融亜鉛めっき鋼板あるいは合金化溶融亜鉛めっき鋼板を得られる。それらには、鋼帯の形状矯正や表面の色調の変更、あるいは降伏点伸びを調整する観点で、伸長率2.0％以下の調質圧延を施すことができる。伸長率2.0％を超える調質圧延は、延性の低下を招くため好ましくない。 And a cold-rolled steel plate can be made into a thin steel plate by subjecting the cold-rolled steel plate to continuous annealing and further subjecting it to temper rolling. In the case of continuous annealing, it is preferable to perform the annealing process using a continuous annealing line (CAL) or continuous hot dipping plating line (CGL), a batch annealing facility (BAF), or a combination of these multiple lines. In addition, in the category of the industrial treatment time at the heating temperature in the above general annealing equipment, it is practically impossible to reduce or eliminate the concentration distribution of P and Mn elements determined at the time of casting. The conditions have little effect on the distribution pattern and streak pattern of P and Mn defined in the present invention. Therefore, the annealing temperature and the heat cycle in the annealing process are not particularly defined in the present invention, and appropriate annealing conditions for obtaining a desired microstructure and characteristics can be respectively employed. However, if the annealing temperature is less than 700 ° C., the recrystallization and grain growth of the ferrite matrix phase is insufficient, and a rolled work structure remains due to cold pressure and the ductility deteriorates. Therefore, the annealing temperature is preferably 700 ° C. or more. In CAL or BAF, cold-rolled annealed steel sheets can be obtained, and in CGL, hot-dip galvanized steel sheets or galvannealed steel sheets can be obtained. They can be subjected to temper rolling with an elongation of 2.0% or less from the viewpoint of correcting the shape of the steel strip, changing the color of the surface, or adjusting the elongation at the yield point. Temper rolling exceeding an elongation rate of 2.0% is not preferable because it causes a decrease in ductility.
表１に示す成分組成（残部はFe及び不可避的不純物）を有する鋼を転炉で溶製後に、溶鋼を垂直曲げ型の連続鋳造機によって連続鋳造してスラブとした。鋳造速度及び二次冷却条件を表２に示すものとして、表２に示すスラブ幅で、厚みが250ｍｍのスラブを製造した。表２のNo.7の比較鋼においては、熱延前にスラブを一旦1200℃に10時間加熱し、圧下率20％の事前圧延（ブレークダウン圧延）を実施した。 After melting steel having the composition shown in Table 1 (the balance being Fe and inevitable impurities) in a converter, the molten steel was continuously cast by a vertical bending type continuous casting machine to obtain a slab. As the casting speed and secondary cooling conditions are shown in Table 2, a slab having a slab width shown in Table 2 and a thickness of 250 mm was produced. In the comparative steel of No. 7 in Table 2, the slab was once heated to 1200 ° C. for 10 hours before hot rolling, and pre-rolling (breakdown rolling) with a reduction rate of 20% was performed.
次いで、得られたスラブを表２に示す均熱温度×１時間のスラブ再加熱後に、表２に示す仕上げ熱延温度と、表２に示す巻取温度で熱間圧延を施して、3ｍｍ厚の熱延鋼板を得た。 Next, the obtained slab was subjected to hot rolling at the soaking temperature shown in Table 2 and the coiling temperature shown in Table 2 after reheating the slab as shown in Table 2 for 1 hour. A hot rolled steel sheet was obtained.
次いで、得られた熱延鋼板を常法に従い酸洗および冷間圧延（圧下率78％）を施した後に、表３に示す条件で連続焼鈍ライン（CAL）又は連続溶融めっきライン（CGL）にて焼鈍処理を実施し、伸長率1.0％の調質圧延を施して、最終的に0.6ｍｍ厚の薄鋼板を得た。溶融亜鉛めっきを施した鋼板については、片面45±3g/m2のめっきを両面に作製し、550℃で合金化処理を施して被覆中のFe濃度が10±1mass％の合金化溶融亜鉛めっきとした。 Next, the obtained hot-rolled steel sheet was subjected to pickling and cold rolling (rolling rate 78%) according to a conventional method, and then subjected to a continuous annealing line (CAL) or a continuous hot dipping plating line (CGL) under the conditions shown in Table 3. Then, annealing treatment was performed and temper rolling with an elongation rate of 1.0% was performed to finally obtain a thin steel plate having a thickness of 0.6 mm. For hot dip galvanized steel sheets, one side 45 ± 3g / m 2 plating is prepared on both sides, alloyed at 550 ° C, and alloyed hot dip galvanizing with Fe concentration in the coating 10 ± 1 mass% It was.
＜熱延鋼板におけるMn偏析度及びP偏析度の分布の評価＞
各水準において、板幅方向に平行な板厚断面を有する鋼片を、鋼帯中央とエッジから300ｍｍの位置を中心に各100ｍｍ幅以上サンプル採取した。各サンプルの断面を研磨により平滑に仕上げた後、電子線マイクロプローブアナライザー（EPMA）装置により、加速電圧25ｋＶ、電流2.5μＡ、ビーム径5μｍの条件で、板厚中央から厚みの±10％の領域をマッピングして、Mnの定量濃度分布を得た。そして、Mnの定量濃度分布の各値を平均Mn濃度で割ることによって、偏析度Smの分布に変換した。
<Evaluation of distribution of Mn segregation and P segregation in hot-rolled steel sheets>
At each level, a steel slab having a plate thickness cross section parallel to the plate width direction was sampled with a width of 100 mm or more around the center of the steel strip and the position 300 mm from the edge. After the cross section of each sample is smoothed by polishing, an area of ± 10% of the thickness from the center of the plate thickness at the acceleration voltage of 25 kV, current of 2.5 μA, and beam diameter of 5 μm using an electron beam microprobe analyzer (EPMA) device Was mapped to obtain a quantitative concentration distribution of Mn. Then, each value of the quantitative concentration distribution of Mn was divided by the average Mn concentration to convert to a distribution of segregation degree Sm.
このうち、板厚中央部で最もSmの変化が大きい厚み領域50μｍ分のデータを厚み方向に平均化し、更に幅方向にも30μｍ分の移動平均による平滑化を実施して、Smの幅方向プロファイルを得た。これらの幅方向プロファイルにおいて、全ピークについて、極大値とそれに隣接する２つの極小値の値を求めて、各ピークについてΔSm及びΔWmを求めた。そのうち、ΔWmが200μｍ以上となるΔSmについて、平均値と標準偏差を算出した。 Of these, the data for 50 μm thick region with the largest Sm change at the center of the plate thickness is averaged in the thickness direction, and further smoothed by moving average of 30 μm in the width direction to obtain the Sm width profile. Got. In these width direction profiles, the maximum value and the two minimum values adjacent to it were determined for all peaks, and ΔSm and ΔWm were determined for each peak. Among them, an average value and a standard deviation were calculated for ΔSm having ΔWm of 200 μm or more.
PについてもMnと同様にして、板厚中央部におけるP偏析度Spの幅方向プロファイルを得た。そして、各ピークについてΔSp及びΔWpを求めた。そのうち、ΔWpが200μｍ以上となるΔSpについて、平均値と標準偏差を算出した。 For P, the width direction profile of the P segregation degree Sp at the center of the plate thickness was obtained in the same manner as Mn. Then, ΔSp and ΔWp were determined for each peak. Among them, an average value and a standard deviation were calculated for ΔSp at which ΔWp was 200 μm or more.
得られた結果を表２に示す。 The obtained results are shown in Table 2.
＜冷延鋼板におけるMn偏析度及びP偏析度の分布の評価＞
各水準において、板幅方向に平行な板厚断面を有する鋼片を、鋼帯中央とエッジから300ｍｍの位置を中心に各100ｍｍ幅以上サンプル採取した。各サンプルの断面を研磨により平滑に仕上げた後、EPMA装置により、加速電圧25ｋＶ、電流2.5μＡ、ビーム径3μｍの条件で、板厚中央から厚みの±25％の領域をマッピングして、Mnの定量濃度分布を得た。そして、Mnの定量濃度分布の各値を平均Mn濃度で割ることによって、偏析度Smの分布に変換した。
<Evaluation of distribution of Mn segregation and P segregation in cold-rolled steel sheets>
At each level, a steel slab having a plate thickness cross section parallel to the plate width direction was sampled with a width of 100 mm or more around the center of the steel strip and the position 300 mm from the edge. After smoothing the cross section of each sample by polishing, an area of ± 25% of the thickness from the center of the plate thickness is mapped with an EPMA device under the conditions of an acceleration voltage of 25 kV, a current of 2.5 μA, and a beam diameter of 3 μm. A quantitative concentration distribution was obtained. Then, each value of the quantitative concentration distribution of Mn was divided by the average Mn concentration to convert to a distribution of segregation degree Sm.
このうち、板厚中央部で最もSmの変化の大きい厚み領域30μｍ分のデータを厚み方向に平均化し、更に幅方向にも30μｍ分の移動平均による平滑化を実施して、Smの幅方向プロファイルを得た。これらの幅方向プロファイルにおいて、全ピークについて、極大値とそれに隣接する２つの極小値の値を求めて、各ピークについてΔSm及びΔWmを求めた。そのうち、ΔWmが200μｍ以上となるΔSmについて、平均値と標準偏差を算出した。 Of these, the data for 30 μm thick region with the largest Sm change at the center of the plate thickness is averaged in the thickness direction, and further smoothed by moving average of 30 μm in the width direction. Got. In these width direction profiles, the maximum value and the two minimum values adjacent to it were determined for all peaks, and ΔSm and ΔWm were determined for each peak. Among them, an average value and a standard deviation were calculated for ΔSm having ΔWm of 200 μm or more.
PについてもMnと同様にして、板厚中央部におけるP偏析度Spの幅方向プロファイルを得た。そして、各ピークについてΔSp及びΔWpを求めた。そのうち、ΔWpが200μｍ以上となるΔSpについて、平均値と標準偏差を算出した。 For P, the width direction profile of the P segregation degree Sp at the center of the plate thickness was obtained in the same manner as Mn. Then, ΔSp and ΔWp were determined for each peak. Among them, an average value and a standard deviation were calculated for ΔSp at which ΔWp was 200 μm or more.
得られた結果を表３に示す。 The obtained results are shown in Table 3.
＜機械的特性の評価＞
鋼帯の幅中央から圧延方向に対して直角方向にJIS５号引張試験片（JIS Z 2201）を採取し、歪速度が10-3/sとするJIS Z 2241の規定に準拠した引張試験を行い、引張強度（TS）を求めた。全伸び（El）の測定は、まず引張前の試験片に標点間距離L＝50ｍｍの標点をマーキングし、引張試験で破断した試験片の破断面同士を突き合わせて標点間距離の増加量ΔL(ｍｍ)を測定して、全伸びEl(%)=ΔL/L×100として求めた。さらに、別途試験片に2%の予歪を引張変形で加えて170℃×20分の時効処理を施した後に引張試験を実施して時効後の降伏応力を測定し、2%予歪時の応力からの降伏強度の増加分をBHとした。結果を表３に示す。
<Evaluation of mechanical properties>
JIS No. 5 tensile test piece (JIS Z 2201) is taken from the center of the width of the steel strip in a direction perpendicular to the rolling direction, and a tensile test is performed in accordance with the provisions of JIS Z 2241 with a strain rate of 10 -3 / s. The tensile strength (TS) was determined. The total elongation (El) is measured by first marking a test point with a distance L between the gauge points of L = 50 mm on the specimen before tension, and then increasing the distance between the gauge points by matching the fracture surfaces of the specimens that were fractured in the tensile test. The amount ΔL (mm) was measured and determined as total elongation El (%) = ΔL / L × 100. In addition, 2% pre-strain was added to the test piece separately by tensile deformation, and after aging treatment at 170 ° C for 20 minutes, the tensile test was performed to measure the yield stress after aging, and 2% pre-strain The increase in yield strength from stress was defined as BH. The results are shown in Table 3.
＜縞状模様の評価＞
各水準において、鋼帯を長手方向に100ｍｍ分裁断した板を、幅方向に公称歪10％の引張加工を施し、表面に砥石掛けして目視による縞状模様の程度を５段階で評価した。評価基準は以下のとおりとして、N＝10で評点を平均した。結果を表３に示す。
１＝模様は皆無
２＝模様はほとんど識別不可
３＝全幅に薄い模様はあるが、明瞭な模様は認められない
４＝明瞭な模様が３ヶ所以上発生
５＝明瞭な模様がサンプル全幅に発生
<Evaluation of striped pattern>
At each level, a plate obtained by cutting the steel strip by 100 mm in the longitudinal direction was subjected to a tensile process with a nominal strain of 10% in the width direction, and the surface was subjected to a grinding stone, and the degree of the striped pattern was visually evaluated in five stages. The evaluation criteria were as follows, and the scores were averaged at N = 10. The results are shown in Table 3.
1 = No pattern at all 2 = Pattern is almost indistinguishable 3 = There are thin patterns in the entire width, but no clear patterns are recognized 4 = Three or more clear patterns are generated 5 = Clear patterns are generated in the entire sample width
＜めっき不良有無の評価＞
めっきの外観評価は、溶融亜鉛めっきされた鋼帯の外観を長手方向に少なくとも100m以上検査したうえで、不めっき、合金化むらによる外観不良が含まれる確率が長手100mにつき 1ヵ所/未満であり自動車外板として適切な表面品質が確保されている場合を「なし」、それ以上の多数の上記欠陥が認められる場合を「有り」とした。結果を表３に示す。
<Evaluation of plating defects>
In the appearance evaluation of plating, the appearance of hot-dip galvanized steel strip is inspected at least 100 m in the longitudinal direction, and the probability of appearance defects due to non-plating and uneven alloying is less than 1/100 m in length. The case where an appropriate surface quality was secured as an automobile outer plate was judged as “none”, and the case where a large number of the above-mentioned defects were recognized as “present”. The results are shown in Table 3.
（評価結果の説明）
図１に、実施例No.1〜8において、冷延鋼板におけるMn偏析度差ΔSmの平均値及びP偏析度差ΔSpの平均値と、プレス加工後の縞状模様評価の平均値との関係を示す。また、図２に、実施例No.1〜8において、鋳型直下から6Vc[m]までのエッジ／中央の水量密度比De/Dcと、冷延鋼板におけるMn偏析度差ΔSmの平均値及びP偏析度差ΔSpの平均値との関係を示す。図１，２から明らかなように、De/Dc：1.1以上1.5以下の範囲内において、ΔSmの平均値を0.10以下、ΔSpの平均値を0.20以下とすることができ、縞状模様の平均値を2.0未満と、高い表面品質を実現することができた。
(Explanation of evaluation results)
In FIG. 1, in Examples No. 1 to 8, the relationship between the average value of the Mn segregation degree difference ΔSm and the average value of the P segregation degree difference ΔSp in the cold-rolled steel sheet and the average value of the striped pattern evaluation after press working. Indicates. In FIG. 2, in Examples Nos. 1 to 8, the edge / center water density ratio De / Dc from directly under the mold to 6 Vc [m], the average value of the Mn segregation degree difference ΔSm in the cold-rolled steel sheet, and P The relationship with the average value of the segregation degree difference ΔSp is shown. As is apparent from FIGS. 1 and 2, within the range of De / Dc: 1.1 to 1.5, the average value of ΔSm can be 0.10 or less, and the average value of ΔSp can be 0.20 or less, and the average value of the striped pattern A surface quality of less than 2.0 was achieved.
さらに比較として、No.7で、特許文献２に示されるような1200℃×10時間の加熱および圧下率20％のブレークダウン圧延を熱延前に一度実施して、P偏析の軽減を図った結果を示す。しかし、今回の評価方法では良好な表面品質が得られなかった。これは、特許文献２では規定されていないMnが、Pよりも拡散困難で偏析が残存したこと、さらには今回の表面品質の評価における引張ひずみ量が10％と大きいため、特許文献２では顕在化しなかった縞状模様も顕在化したことが理由と考えられる。 Further, as a comparison, in No. 7, the 1200 ° C. × 10 hour heating and the rolling reduction of 20% as shown in Patent Document 2 were performed once before hot rolling to reduce P segregation. Results are shown. However, this evaluation method did not provide good surface quality. This is because Mn, which is not defined in Patent Document 2, is more difficult to diffuse than P and segregation remains, and the amount of tensile strain in this evaluation of surface quality is as large as 10%. This is probably because the striped pattern that did not become apparent was also revealed.
また、表２，３及び図３，４から明らかなように、本発明の成分条件および製造条件を満足した発明鋼では、Mn,Pの偏析度分布が本発明の規定を満足し、かつ300MPa以上のTSと良好なElおよび高いBH特性を示すとともに、縞状模様の評価が平均2.0未満で極めて良好な表面品質を示す。成分が規定範囲内でも鋳造条件が所定の条件から外れた比較鋼では、縞状模様の評価が劣位であった。一方、C、Mn、Pが所定の範囲より過剰な鋼H,I,Kは、製造条件が規定範囲内でも縞状模様が顕在化した。また、Si,Alが上限を超える鋼J、Mではめっき性が劣る。さらに、Pが下限以下の鋼Lは強度が300MPa未満となった。 Further, as is apparent from Tables 2 and 3 and FIGS. 3 and 4, in the invention steel that satisfies the component conditions and production conditions of the present invention, the segregation degree distribution of Mn and P satisfies the provisions of the present invention and is 300 MPa. In addition to the above TS, good El and high BH characteristics, the striped pattern has an average evaluation of less than 2.0 and shows very good surface quality. In the comparative steel in which the casting condition deviated from the predetermined condition even when the component was within the specified range, the evaluation of the striped pattern was inferior. On the other hand, in steels H, I, and K in which C, Mn, and P are in excess of a predetermined range, a striped pattern becomes apparent even when the manufacturing conditions are within the specified range. Further, steels J and M with Si and Al exceeding the upper limit have poor plating properties. Furthermore, the strength of steel L with P below the lower limit was less than 300 MPa.
本発明によれば、プレス加工などの薄板加工に適用できる美麗な表面性状を有する高強度薄鋼板を製造可能である。本発明は、高意匠性で美麗差の要求される自動車ボディや家電製品の筐体に適用でき、製品の付加価値と耐久性を高めると共に、自動車の車体軽量化を通して地球環境の負荷低減にも寄与するなど産業上の利用価値が高い。 According to the present invention, it is possible to manufacture a high-strength thin steel plate having a beautiful surface property that can be applied to thin plate processing such as press processing. The present invention can be applied to automobile bodies and home appliance housings that require a high degree of design and beautiful differences, increase the added value and durability of the products, and reduce the burden on the global environment by reducing the weight of automobile bodies. The industrial utility value is high.
Claims (10)
板厚中央部におけるMn偏析度Smの幅方向プロファイルにおいて、各極大値についてΔSm及びΔWmを求めたとき、ΔWmが200μｍ以上となるΔSmの平均値が0.10以下かつ標準偏差2σmが0.10以下であり、
板厚中央部におけるP偏析度Spの幅方向プロファイルにおいて、各極大値についてΔSp及びΔWpを求めたとき、ΔWpが200μｍ以上となるΔSpの平均値が0.20以下かつ標準偏差2σpが0.15以下であることを特徴とする熱延鋼板。
ここで、
Sm＝任意の点におけるMn濃度（％）／鋼板の平均Mn濃度（％）
ΔSm：Smの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWm：各極大値に隣接する２つの極小値間の幅方向距離
Sp＝任意の点におけるP濃度（％）／鋼板の平均P濃度（％）
ΔSp：Spの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWp：各極大値に隣接する２つの極小値間の幅方向距離
とする。 In mass%, C: 0.04% or less, Si: 1.5% or less, Mn: 0.2-2.0%, P: 0.005-0.060%, S: 0.004-0.020%, Sol.Al: 0.003-1.0%, N: 0.0050% Containing the following, the balance has a component composition consisting of Fe and inevitable impurities,
In the width direction profile of the Mn segregation degree Sm in the central part of the plate thickness, when ΔSm and ΔWm were determined for each maximum value, the average value of ΔSm at which ΔWm was 200 μm or more was 0.10 or less and the standard deviation 2σm was 0.10 or less,
In the width direction profile of P segregation degree Sp at the center of the plate thickness, when ΔSp and ΔWp are calculated for each local maximum value, the average value of ΔSp at which ΔWp is 200 μm or more is 0.20 or less and the standard deviation 2σp is 0.15 or less. Hot-rolled steel sheet characterized by
here,
Sm = Mn concentration (%) at an arbitrary point / average Mn concentration (%) of steel sheet
ΔSm: Difference between the maximum value of Sm and the average value of two local minimum values adjacent to the local maximum value ΔWm: Distance in the width direction between two local minimum values adjacent to each local maximum value
Sp = P concentration (%) at any point / Average P concentration (%) of steel sheet
ΔSp: difference between the maximum value of Sp and the average value of two local minimum values adjacent to the local maximum value ΔWp: the distance in the width direction between two local minimum values adjacent to each local maximum value.
板厚中央部におけるMn偏析度Smの幅方向プロファイルにおいて、各極大値についてΔSm及びΔWmを求めたとき、ΔWmが200μｍ以上となるΔSmの平均値が0.10以下かつ標準偏差2σmが0.05以下であり、
板厚中央部におけるP偏析度Spの幅方向プロファイルにおいて、各極大値についてΔSp及びΔWpを求めたとき、ΔWpが200μｍ以上となるΔSpの平均値が0.20以下かつ標準偏差2σpが0.10以下であることを特徴とする冷延鋼板。
ここで、
Sm＝任意の点におけるMn濃度（％）／鋼板の平均Mn濃度（％）
ΔSm：Smの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWm：各極大値に隣接する２つの極小値間の幅方向距離
Sp＝任意の点におけるP濃度（％）／鋼板の平均P濃度（％）
ΔSp：Spの極大値と、当該極大値に隣接する２つの極小値の平均値との差
ΔWp：各極大値に隣接する２つの極小値間の幅方向距離
とする。 In mass%, C: 0.04% or less, Si: 1.5% or less, Mn: 0.2-2.0%, P: 0.005-0.060%, S: 0.004-0.020%, Sol.Al: 0.003-1.0%, N: 0.0050% Containing the following, the balance has a component composition consisting of Fe and inevitable impurities,
In the width direction profile of Mn segregation degree Sm in the central part of the plate thickness, when ΔSm and ΔWm were determined for each maximum value, the average value of ΔSm at which ΔWm was 200 μm or more was 0.10 or less and the standard deviation 2σm was 0.05 or less,
In the width direction profile of P segregation degree Sp at the center of the plate thickness, when ΔSp and ΔWp are determined for each local maximum value, the average value of ΔSp at which ΔWp is 200 μm or more is 0.20 or less and the standard deviation 2σp is 0.10 or less. Cold-rolled steel sheet characterized by
here,
Sm = Mn concentration (%) at an arbitrary point / average Mn concentration (%) of steel sheet
ΔSm: Difference between the maximum value of Sm and the average value of two local minimum values adjacent to the local maximum value ΔWm: Distance in the width direction between two local minimum values adjacent to each local maximum value
Sp = P concentration (%) at any point / Average P concentration (%) of steel sheet
ΔSp: difference between the maximum value of Sp and the average value of two local minimum values adjacent to the local maximum value ΔWp: the distance in the width direction between two local minimum values adjacent to each local maximum value.
前記スラブを、鋳型直下から（6min×Vc）[m]まではDe/Dc：1.1以上1.5以下の条件下で、（6min×Vc）[m]から凝固完了まではDe/Dc：0.7以上1.5以下の条件下で、かつ、二次冷却全体の平均の比水量Pは0.5以上2.5以下とする条件下で、二次冷却する工程と、
前記スラブを熱間圧延して熱延鋼板を得る工程と、
を有することを特徴とする熱延鋼板の製造方法。
ここで、
Dc：スラブの幅方向中央から幅方向1/2位置までの領域のスプレー水の水量密度
De：スラブの幅方向1/2位置から幅方向端部までの領域のスプレー水の水量密度
比水量P＝L/（W×T×Vc×ρ）
L：スプレー水流量(L/min)
W：スラブ幅(m)
T：スラブ厚み(m)
Vc：鋳造速度(m/min)
ρ：溶鋼密度(kg-鋼/ｍ３)
とする。 A step of continuously casting a molten steel having the component composition according to any one of claims 1 to 3 to obtain a slab;
The slab is subjected to a condition of De / Dc: 1.1 or more and 1.5 or less from directly under the mold to (6 min × Vc) [m], and De / Dc: 0.7 to 1.5 from (6 min × Vc) [m] to the completion of solidification. Under the following conditions, and under the condition that the average specific water amount P of the entire secondary cooling is 0.5 or more and 2.5 or less, the secondary cooling step,
Hot rolling the slab to obtain a hot-rolled steel sheet;
A method for producing a hot-rolled steel sheet, comprising:
here,
Dc: Water density of spray water in the area from the center of the slab in the width direction to 1/2 position in the width direction
De: The amount of spray water in the region from the half-width position of the slab to the edge in the width direction Density specific water amount P = L / (W × T × Vc × ρ)
L: Spray water flow rate (L / min)
W: Slab width (m)
T: Thickness of slab (m)
Vc: Casting speed (m / min)
ρ: Molten steel density (kg-steel / m 3 )
And
前記熱延鋼板を冷間圧延して冷延鋼板を得る工程をさらに有することを特徴とする冷延鋼板の製造方法。 In addition to the steps in the method for producing a hot-rolled steel sheet according to claim 7,
A method for producing a cold-rolled steel sheet, further comprising a step of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet.
前記冷延鋼板を焼鈍して冷延焼鈍鋼板を得る工程をさらに有することを特徴とする冷延焼鈍鋼板の製造方法。 In addition to the steps in the method for producing a cold-rolled steel sheet according to claim 8,
A method for producing a cold-rolled annealed steel sheet, further comprising the step of annealing the cold-rolled steel sheet to obtain a cold-rolled annealed steel sheet.
前記冷延鋼板を溶融亜鉛めっきして溶融亜鉛めっき鋼板を得る工程とさらに有することを特徴とする溶融亜鉛めっき鋼板の製造方法。 In addition to the steps in the method for producing a cold-rolled steel sheet according to claim 8,
A method for producing a hot-dip galvanized steel sheet, further comprising a step of hot-dip galvanizing the cold-rolled steel sheet to obtain a hot-dip galvanized steel sheet.
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