JP3820900B2 - Method for producing hot-dip galvanized steel sheet - Google Patents
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Description
【０００１】
【発明の属する技術分野】
この発明は、溶融亜鉛めっき鋼板、特に外観の良好な溶融亜鉛めっき鋼板を製造する方法に関するものである。
【０００２】
【従来の技術】
溶融亜鉛めっき鋼板は、表面が亜鉛めっき被膜により保護されており、しかも鉄よりも電気化学的に卑な亜鉛が鉄に優先して腐食溶解する、いわゆる犠牲防食作用により、優れた耐食性を有する。したがって、溶融亜鉛めっき鋼板は、耐食性が望まれる建築用材料や自動車用材料をはじめとして幅広く使われている。
【０００３】
このような溶融亜鉛めっき鋼板を自動車の外板に用いる場合には、耐食性に優れることばかりでなく、鋼板表面の美麗さが強く求められている。というのは、鋼板表面の美麗さが、自動車の商品価値に直結するからである。更に、近年では地球環境の保全の機運が高まり、自動車用鋼板においては高張力化による軽量化が指向されている。したがって、高張力溶融亜鉛めっき鋼板が、更には自動車外板用としての外観に優れた溶融亜鉛めっき鋼板が望まれているところである。
【０００４】
ところが、従来の溶融亜鉛めっき鋼板では、溶融亜鉛めっきや合金化溶融亜鉛めっきが被成されている表面に、鋼板の圧延方向に延びる筋状の凹凸が発生して鋼板の外観が阻害されている場合が見られた。かかる筋状の表面欠陥に対しては、鋼板の需要家において溶融亜鉛めっき鋼板の表面に被成させる塗料の膜厚を厚くすることで、筋状欠陥を目立たなくすることが行われてきた。しかし、かかる対処法は、筋状欠陥の本質的な解消法ではないし、近年では生産性の向上及びコスト低減を目指して塗膜膜厚を薄くするようになってきたため、溶融亜鉛めっき鋼板の筋状欠陥そのものをなくすことが求められるようになってきた。
【０００５】
溶融亜鉛めっきの表面の筋状模様の発生を防止する方法に関して、特開平８−２０８５２号公報には、鋼表面から50μｍまでの範囲の表面層に等軸結晶粒と延伸結晶粒からなる混合結晶粒を面積率で10％以下又は90％以上にした合金化溶融亜鉛めっき鋼板が提案されている。しかしながら、上記特開平８−２０８５２号公報に開示された合金化溶融亜鉛めっき鋼板の製造法は、素材の成分組成範囲及び製造工程のいずれも、通常の合金化溶融亜鉛めっき鋼板の製造方法と変わるところがなく、そして、かような通常の合金化溶融亜鉛めっき鋼板の製造方法では、依然として鋼板表面に筋状の欠陥が発生する場合があった。
【０００６】
【発明が解決しようとする課題】
そこで、この発明は上記の問題を有利に解決するもので、製造工程に工夫を加えることにより、前述した筋状欠陥の発生を有利に防止することのできる外観の良好な溶融亜鉛めっき鋼板の製造方法について提案することを目的とする。
【０００７】
【課題を解決するための手段】
発明者らは溶融亜鉛めっき鋼板の筋状欠陥を解決すべく鋼板の組織を調査した結果、筋状欠陥の原因は、鋼板表層の一部が冷間圧延後の熱処理を施しても部分的に未再結晶組織のままであるため、めっき処理時の合金化速度が定常部と異なることにあることを知見した。
そこで、かかる鋼板表層部における未再結晶組織の発生を防止すべく鋭意研究を重ねたところ、この発明を完成するに至った。
【０００８】
すなわち、この発明の要旨構成は、次のとおりである。
(1) Ｃ：0.0050mass％以下、Si：0.10mass％以下、Mn：0.30mass％以下、Ti：0.010 〜0.050 mass％、Nb：0.001 〜0.030 mass％、Al：0.010 〜0.070 mass％、Ｎ：0.0050mass％以下、Ｐ：0.030 mass％以下およびＳ：0.010 mass％以下を含み、残部は鉄及び不可避的不純物よりなる鋼スラブに熱間圧延を施したのち、熱延板の表面から20μｍの深さまでの表層域に歪みを導入してから、冷間圧延を施し、その後熱処理、次いで溶融亜鉛めっき処理を施すことを特徴とする溶融亜鉛めっき鋼板の製造方法。
【０００９】
(2) Ｃ：0.0050mass％以下、Si：0.10mass％以下、Mn：0.30mass％以下、Ti：0.010 〜0.050 mass％、Nb：0.001 〜0.030 mass％、Al：0.010 〜0.070 mass％、Ｎ：0.0050mass％以下、Ｐ：0.030 mass％以下およびＳ：0.010 mass％以下を含み、更にＢ：0.0002 〜 0.0010mass％を含有し、残部は鉄及び不可避的不純物よりなる鋼スラブに熱間圧延を施したのち、熱延板の表面から20μｍの深さまでの表層域に歪みを導入してから、冷間圧延を施し、その後熱処理、次いで溶融亜鉛めっき処理を施すことを特徴とする溶融亜鉛めっき鋼板の製造方法。
【００１０】
(3) 上記(1) または(2) において、ショットブラスト処理を施して表層域に歪みを導入することを特徴とする溶融亜鉛めっき鋼板の製造方法。
【００１１】
(4) 上記(1) ないし(3) のいずれかにおいて、めっき処理の後、合金化処理を行うことを特徴とする溶融亜鉛めっき鋼板の製造方法。
【００１２】
【発明の実施の形態】
発明者らは、筋状の凹凸欠陥の原因である鋼板表層の未再結晶組織の発生原因について調査した結果、以下の推論を得た。
従来の溶融亜鉛めっき鋼板の製造過程においては、熱間圧延の仕上圧延終了温度は、Ar3 変態点以上とされていた。これは、フェライト等の変態相の出現を防止するためなどの理由からである。そして、仕上圧延終了時の結晶粒径を小さくして最終的な鋼板に良好な加工性を与える観点から、熱間圧延の仕上圧延終了温度は、Ar3 変態点直上とするのが通常であった。
【００１３】
ところが、発明者らの調査による知見によれば、圧延中の鋼板は、大気放冷、圧延ロールとの接触伝熱、冷却水との接触などが原因で鋼板表層が鋼板の厚み方向中心部よりも優先的に抜熱されることから、従来の溶融亜鉛めっき鋼板の製造方法のように仕上圧延終了温度をAr3 変態点直上にした場合には、この鋼板表層は仕上圧延終了時にAr3 変態点を下回る温度になっている場合があった。つまり、従来の仕上圧延終了温度は、厚み方向における平均温度をいっているため、Ar3 変態点直上の平均温度で仕上圧延を終了しても、実際には、鋼板表層の温度が部分的にAr3 変態点を下回る場合があったのである。このように、仕上圧延終了温度がAr3 変態点を下回った場合には、鋼板表層部分が、その後に行われる熱処理（焼鈍）によっても再結晶し難い結晶方位となるため、連続焼鈍を行っても未再結晶のまま残存してしまう。また、このように仕上圧延温度がAr3 変態点を下回った部分は、他の領域と比較すると、引き続く冷間圧延時に加工硬化し易い領域でもある。このように、仕上圧延終了温度がAr3 変態点を下回った部分が、他の部分とは異なる結晶組織になっていることから、めっき処理や合金化処理における反応速度に相違が生じ、その結果、表面に凹凸が生じて筋状欠陥になっていたものと考えられる。
【００１４】
かように鋼板表層に未再結晶組織が残存するのは、鋼板表層の温度が部分的にＡr3変態点を下回る場合があることに起因するから、この鋼板表層での再結晶温度を下げることができれば、従前の熱処理によっても鋼板表層における再結晶が可能になり、筋状の表面欠陥は解消すると考えられる。そこで、鋼板表層での再結晶温度を下げる方途を鋭意究明したところ、冷間圧延による歪みに加えて、熱間圧延後の鋼板表層に歪みを導入するのが、極めて有効であることを見出し、この発明を完成するに至ったのである。
【００１５】
以下、この発明の構成要件をより具体的に説明する。
まず、素材の成分組成範囲は、以下の理由により限定している。
Ｃ：0.0050mass％以下
Ｃは、加工性を劣化させる成分であり、加工用鋼板として求められる特性の中でも、最も重要な全伸び(El.) 、ランクフォード値（ｒ値）を向上させるためには、Ｃ量は少ないほどよく、0.0050mass％以下とした。
【００１６】
Si：0.10mass％以下
Siが増加すると、めっきの濡れ性が低下し、表面性状が劣化するため、0.10mass％以下とした。
【００１７】
Mn：0.30 mass ％以下
Mnは、多量に含有させると、ｒ値の低下及びめっき濡れ性の悪化を招くことから、上限を0.30 mass ％とする。
【００１８】
Ti：0.010 〜0.050 mass％
Tiは、炭窒化物形成成分であり、鋼中の固溶ＣやＮなどを析出物として固定し、よって時効劣化やストレッチャーストレインを有利に防止する。かかる作用を効果的に発揮させるには、0.010 mass％以上のTiを含有させるが、過剰に添加すると、品質の改善が飽和する一方コストアップになることから、上限を0.050 mass％とする。
【００１９】
Nb：0.001 〜0.030 mass％
NbはTiと同様に炭窒化物形成成分であり、鋼中の固溶ＣやＮを析出物として固定して鋼を清浄化し、鋼板の加工性を向上させるのに有効な成分である。かかる効果を十分に発揮させるためには、0.001 mass％以上のNb量が必要である。しかし、Nbを過剰に含有させると、鋼の再結晶温度を上昇させてしまうので、Nb量の上限を0.030 mass％とした。
【００２０】
Al：0.010 〜0.070 mass％
Alは脱酸剤の役割を果たし、鋼中の酸素を固定するために添加する。もっとも、過剰な添加は効果が飽和するばかりか、コストアップの要因ともなるため、0.010 〜0.070 mass％の範囲とする。
【００２１】
Ｎ：0.0050mass％以下
Ｎは不純物成分であり、また、加工性を劣化させ、時効劣化やストレッチャーストレインを生じさせる成分であるため、できるだけ低減することが望ましい。もっとも、0.0050mass％以下であれば、過度の悪影響は及ぼさないので、Ｎの低減コストも勘案して、0.0050mass％までは許容する。
【００２２】
Ｐ：0.030 mass％以下
Ｐは、合金化を阻害するため、Ｐは0.030 mass％以下の範囲とする。
【００２３】
Ｓ：0.010 mass％以下
Ｓは、鋼の延性を劣化するため、可能な限り抑制することが好ましいが、この発明の効果を損なわないためには、0.010 mass％以下とする必要がある。
【００２４】
さらに、上記の基本成分に加えて、Ｂを以下の範囲で添加することも可能である。
Ｂ：0.0002〜0.0010mass％、
Ｂは、結晶粒界に濃化することにより結晶粒界の強度を高め、脆性を向上させる成分である。特に、この発明の鋼板において極低炭素Ｐ添加鋼の場合では、Ｐが粒界に濃化して粒界強度が下がり、二次加工脆性割れが発生するおそれがある。そこで、かような成分系の場合は、Ｂを添加することにより、Ｂを優先的に粒界に濃化させ、二次加工脆性を効果的に改善することができる。かかる作用効果を発揮させるためには、Ｂは0.0002mass％以上が必要であるが、過剰に添加すると鋼の再結晶温度を上昇させてしまうため、0.0002〜0.0010mass％の範囲とする。
【００２５】
上記の成分組成になる鋼スラブは、まず熱間圧延に供する。この熱間圧延における条件は、特に規制する必要はなく、通常の条件に従えばよい。例えば、加熱炉抽出温度は1000〜1300℃、熱間圧延終了温度は 800〜1000℃および巻取り温度は 500〜800 ℃の条件で行うことができる。
【００２６】
次いで、熱間圧延後に、熱延板の表面から20μｍの深さまでの表層域に歪みを導入することによって、鋼板表層での再結晶温度を下げて、従前の熱処理によっても鋼板表層における再結晶を可能にすることが肝要である。すなわち、筋状欠陥は、表層の結晶が冷間圧延後の焼鈍でも再結晶しないことが問題であり、鋼板内部まで余計に歪みを与える必要はない。熱延板の表面から20μｍの深さまでの表層域に歪みを導入することによって、冷間圧延後の焼鈍で全て再結晶する。
【００２７】
なお、歪みを導入する手法としては、ショットブラスト処理を用いることが好適である。このショットブラスト処理は、径が 100〜800 μｍ、より好ましくは 297〜710 μｍの鋼粒子を投射量600 kg／ｍ2 ・min 以上および投射時間１min以上の条件で行うことが、有利である。
【００２８】
上記の歪み導入後は、冷間圧延を行う。この冷間圧延の圧下率は、深絞り性を確保する観点から、60％以上とすることが望ましい。
【００２９】
冷間圧延の後は、焼鈍を目的とする熱処理を行う。かかる熱処理は、連続焼鈍ラインにて行うのが好ましく、焼鈍温度は、再結晶が十分完了する温度として800 ℃程度を下限とするのが望ましい。一方、焼鈍温度が高過ぎると粒成長が進み過ぎ、必要以上に軟化してしまう場合があるので、900 ℃程度を上限とするのが望ましい。
その後のめっき処理や、必要に応じて行われる合金化処理は、常法に従って行えばよい。
【００３０】
【実施例】
Ｃ：0.0020mass％、Si：0.004 mass％、Mn：0.12mass％、Ｐ：0.012 mass％、Ｓ：0.005 mass％、Al：0.032 mass％、Ti：0.029 mass％、Nb：0.003 mass％及びＮ：0.0020mass％を含有し、残部は鉄及び不可避的不純物よりなる鋼スラブＡ及び、Ｃ：0.0018mass％、Si：0.002 mass％、Mn：0.10mass％、Ｐ：0.010 mass％、Ｓ：0.006mass ％、Al：0.034mass ％、Ti：0.035mass ％、Nb：0.003 mass％、Ｂ：0.0003mass％及びＮ：0.0022mass％を含有し、残部は鉄及び不可避的不純物よりなる鋼スラブＢを、それぞれ加熱炉抽出温度1000〜1300℃、熱間圧延終了温度 800〜1000℃及び巻取り温度 500〜800 ℃の条件で熱間圧延を行った後、ショットブラスト処理を下記の条件で行って、熱延板の表層（表面から20μｍ深さまでの領域）に歪みを導入し、次いで圧下率80％の冷間圧延、引き続き、連続溶融亜鉛めっきラインにて860 ℃で60秒の焼鈍を行ってから、目付量45g/m2の溶融亜鉛めっき処理、引き続き合金化処理を行った。また、比較として、ショントブラスト処理を行わない以外は、同様の製造工程に従って溶融亜鉛めっき鋼板を製造した。
【００３１】
記
ショット：鋼球（粒子径297 〜710mμｍ）
投射量：625 kg／ｍ2 ・min
投射時間：１min
【００３２】
かくして得られた合金化溶融亜鉛めっき鋼板の表面について、筋状欠陥の有無を目視により観察したところ、いずれの鋼種においても筋状欠陥の発生は皆無であるのに対して、歪み導入を行わない鋼板の表面には筋状欠陥が認められた。
【００３３】
【発明の効果】
かくしてこの発明によれば、自動車外板などのように良好な外観を要求される溶融亜鉛めっき鋼板に関して、筋状欠陥のない、優れた外観を有する溶融亜鉛めっき鋼板を安定して製造することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot dip galvanized steel sheet, particularly a hot dip galvanized steel sheet having a good appearance.
[0002]
[Prior art]
The surface of the hot dip galvanized steel sheet is protected by a galvanized film, and has excellent corrosion resistance due to the so-called sacrificial anticorrosive action in which zinc which is electrochemically lower than iron is preferentially corroded and dissolved over iron. Accordingly, hot dip galvanized steel sheets are widely used, including building materials and automotive materials that require corrosion resistance.
[0003]
When such a hot dip galvanized steel sheet is used for an outer panel of an automobile, not only is it excellent in corrosion resistance, but also the beauty of the steel sheet surface is strongly demanded. This is because the beauty of the steel sheet surface is directly linked to the commercial value of the car. Furthermore, in recent years, the momentum for conservation of the global environment has increased, and in steel sheets for automobiles, weight reduction by increasing the tension has been directed. Therefore, a high-tensile hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet having an excellent appearance for automobile outer plates are desired.
[0004]
However, in the conventional hot dip galvanized steel sheet, streaky irregularities extending in the rolling direction of the steel sheet are generated on the surface on which the hot dip galvanizing or alloying hot dip galvanizing is formed, and the appearance of the steel sheet is obstructed. A case was seen. With respect to such streak-like surface defects, it has been made to make the streak-like defects inconspicuous by increasing the film thickness of a coating to be formed on the surface of a hot-dip galvanized steel sheet by a steel sheet customer. However, such a countermeasure is not an essential method for eliminating streak defects, and in recent years, the coating film thickness has been reduced with the aim of improving productivity and reducing costs. It has become necessary to eliminate the defects of the shape itself.
[0005]
Regarding a method for preventing the occurrence of streaky patterns on the surface of hot dip galvanizing, JP-A-8-20852 discloses a mixed crystal comprising equiaxed grains and stretched grains in a surface layer in a range from the steel surface to 50 μm. An alloyed hot-dip galvanized steel sheet having a grain ratio of 10% or less or 90% or more has been proposed. However, the method for producing an alloyed hot dip galvanized steel sheet disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 8-20852 is different from the usual method for producing an alloyed hot dip galvanized steel sheet in both the component composition range and the production process. However, there are cases where streak-like defects still occur on the surface of the steel sheet in the conventional method for producing a galvannealed steel sheet.
[0006]
[Problems to be solved by the invention]
Therefore, the present invention advantageously solves the above problems, and by adding a device to the production process, it is possible to produce a hot-dip galvanized steel sheet having a good appearance that can advantageously prevent the occurrence of the aforementioned streak defects. The purpose is to propose a method.
[0007]
[Means for Solving the Problems]
As a result of investigating the structure of the steel sheet in order to solve the streak defect of the hot-dip galvanized steel sheet, the inventors have found that the cause of the streak defect is partly even if a part of the steel sheet surface layer is subjected to heat treatment after cold rolling. Since the unrecrystallized structure remains, it has been found that the alloying rate during the plating process is different from that in the stationary part.
Then, when the earnest research was repeated in order to prevent generation | occurrence | production of the non-recrystallized structure in this steel plate surface layer part, it came to complete this invention.
[0008]
That is, the gist of the present invention is as follows.
(1) C: 0.0050 mass% or less, Si: 0.10 mass% or less, Mn: 0.30 mass% or less, Ti: 0.010 to 0.050 mass%, Nb: 0.001 to 0.030 mass%, Al: 0.010 to 0.070 mass%, N: 0.0050 mass% or less, P: 0.030 mass% or less and S: 0.010 mass% or less, the remainder being hot-rolled to a steel slab made of iron and unavoidable impurities, then 20 μm deep from the surface of the hot-rolled sheet A method for producing a hot dip galvanized steel sheet, wherein a strain is introduced into the surface layer region and then cold rolling is performed, followed by heat treatment and then hot dip galvanizing treatment.
[0009]
(2) C: 0.0050 mass% or less, Si: 0.10 mass% or less, Mn: 0.30 mass% or less, Ti: 0.010 to 0.050 mass%, Nb: 0.001 to 0.030 mass%, Al: 0.010 to 0.070 mass%, N: Contains 0.0050 mass% or less, P: 0.030 mass% or less, and S: 0.010 mass% or less, and further contains B: 0.0002 to 0.0010 mass%, and the remainder is hot-rolled to a steel slab made of iron and inevitable impurities. Then, after introducing strain into the surface layer region from the surface of the hot-rolled sheet to a depth of 20 μm, cold rolling, followed by heat treatment and then hot-dip galvanizing treatment, Production method.
[0010]
(3) A method for producing a hot dip galvanized steel sheet according to the above (1) or (2), wherein the shot blast treatment is performed to introduce strain into the surface layer region.
[0011]
(4) The method for producing a hot dip galvanized steel sheet according to any one of the above (1) to (3), wherein an alloying treatment is performed after the plating treatment.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
As a result of investigating the cause of the non-recrystallized structure of the steel sheet surface layer, which is the cause of the streak-like unevenness, the inventors have obtained the following inference.
In the manufacturing process of the conventional hot dip galvanized steel sheet, the finish rolling finishing temperature of the hot rolling is set to the Ar 3 transformation point or higher. This is for the purpose of preventing the appearance of a transformation phase such as ferrite. From the viewpoint of reducing the grain size at the end of finish rolling and giving good workability to the final steel sheet, the finish rolling end temperature of hot rolling is usually just above the Ar 3 transformation point. It was.
[0013]
However, according to the findings of the inventors' investigation, the steel sheet surface during rolling is more than the center in the thickness direction of the steel sheet due to air cooling, contact heat transfer with the rolling roll, contact with cooling water, etc. Therefore, when the finish rolling end temperature is set just above the Ar 3 transformation point as in the conventional method of manufacturing a hot dip galvanized steel sheet, the surface layer of this steel sheet will have an Ar 3 transformation point at the end of finish rolling. In some cases, the temperature was lower than. In other words, the conventional finish rolling end temperature is the average temperature in the thickness direction, so even if finish rolling is finished at the average temperature immediately above the Ar 3 transformation point, the temperature of the steel sheet surface layer is actually partially Ar. In some cases, it was below the transformation point. Thus, when the finish rolling finish temperature is lower than the Ar 3 transformation point, the steel sheet surface layer portion has a crystal orientation that is difficult to recrystallize even by a subsequent heat treatment (annealing). Will remain unrecrystallized. In addition, the portion where the finish rolling temperature is lower than the Ar 3 transformation point as described above is also a region that is easily work-hardened during subsequent cold rolling as compared with other regions. In this way, the part where the finish rolling finish temperature is lower than the Ar 3 transformation point has a different crystal structure from the other parts, so there is a difference in the reaction rate in the plating process and alloying process, and as a result It is considered that irregularities were generated on the surface, resulting in streak defects.
[0014]
The reason why the non-recrystallized structure remains in the steel sheet surface layer is that the temperature of the steel sheet surface layer may be partially lower than the Ar 3 transformation point. Therefore, the recrystallization temperature in the steel sheet surface layer should be lowered. If it is possible, recrystallization on the steel sheet surface layer will be possible even by the conventional heat treatment, and it is considered that the streak-like surface defects are eliminated. Therefore, as a result of diligent investigations on how to lower the recrystallization temperature in the steel sheet surface layer, it was found that it is extremely effective to introduce strain into the steel sheet surface layer after hot rolling in addition to the strain due to cold rolling. The present invention has been completed.
[0015]
Hereinafter, the constituent features of the present invention will be described more specifically.
First, the component composition range of the material is limited for the following reasons.
C: 0.0050 mass% or less C is a component that deteriorates workability, and in order to improve the most important total elongation (El.) And Rankford value (r value) among the characteristics required for steel sheets for processing. The smaller the amount of C, the better, and 0.0050 mass% or less.
[0016]
Si: 0.10 mass% or less
When Si increases, the wettability of the plating decreases and the surface properties deteriorate, so the content was made 0.10 mass% or less.
[0017]
Mn: 0.30 mass% or less
When Mn is contained in a large amount, the r value is lowered and the plating wettability is deteriorated, so the upper limit is made 0.30 mass%.
[0018]
Ti: 0.010 to 0.050 mass%
Ti is a carbonitride-forming component, and fixes solute C, N, and the like in steel as precipitates, and thus advantageously prevents aging deterioration and stretcher strain. In order to exert such an action effectively, 0.010 mass% or more of Ti is contained. However, if excessively added, the improvement in quality is saturated while the cost is increased, so the upper limit is made 0.050 mass%.
[0019]
Nb: 0.001 to 0.030 mass%
Nb is a carbonitride-forming component like Ti, and is an effective component for fixing solid solution C or N in steel as a precipitate to clean the steel and improve the workability of the steel plate. In order to sufficiently exhibit such an effect, an Nb amount of 0.001 mass% or more is necessary. However, if Nb is contained excessively, the recrystallization temperature of the steel is increased, so the upper limit of the Nb content is set to 0.030 mass%.
[0020]
Al: 0.010 to 0.070 mass%
Al serves as a deoxidizer and is added to fix oxygen in the steel. However, excessive addition not only saturates the effect but also increases the cost, so the range is 0.010 to 0.070 mass%.
[0021]
N: 0.0050 mass% or less N is an impurity component, and is a component that deteriorates workability and causes aging deterioration and stretcher strain. Therefore, it is desirable to reduce it as much as possible. However, if it is 0.0050 mass% or less, an excessive adverse effect is not exerted, and therefore, up to 0.0050 mass% is allowed in consideration of the reduction cost of N.
[0022]
P: 0.030 mass% or less P is in the range of 0.030 mass% or less because P inhibits alloying.
[0023]
S: 0.010 mass% or less S is preferably suppressed as much as possible because it deteriorates the ductility of the steel. However, in order not to impair the effects of the present invention, it is necessary to make 0.010 mass% or less.
[0024]
Furthermore, in addition to the above basic components, B can be added in the following range.
B: 0.0002 to 0.0010 mass%,
B is a component that improves the brittleness by increasing the strength of the crystal grain boundary by concentrating the crystal grain boundary. In particular, in the case of an ultra-low carbon P-added steel in the steel sheet according to the present invention, P may be concentrated at the grain boundary to lower the grain boundary strength, and secondary work brittle cracking may occur. Therefore, in the case of such a component system, by adding B, it is possible to preferentially concentrate B at the grain boundary and effectively improve secondary work brittleness. In order to exhibit this effect, B needs to be 0.0002 mass% or more. However, if excessively added, the recrystallization temperature of the steel is raised, so the range is 0.0002 to 0.0010 mass%.
[0025]
The steel slab having the above component composition is first subjected to hot rolling. The conditions for this hot rolling need not be particularly restricted, and may be according to ordinary conditions. For example, the heating furnace extraction temperature can be 1000 to 1300 ° C, the hot rolling end temperature can be 800 to 1000 ° C, and the winding temperature can be 500 to 800 ° C.
[0026]
Next, after hot rolling, by introducing strain into the surface layer area from the surface of the hot-rolled sheet to a depth of 20 μm, the recrystallization temperature in the steel sheet surface layer is lowered, and recrystallization in the steel sheet surface layer is also performed by conventional heat treatment. It is important to make it possible. That is, the streak defect is a problem that the surface layer crystal does not recrystallize even after annealing after cold rolling, and it is not necessary to give extra strain to the inside of the steel sheet. All are recrystallized by annealing after cold rolling by introducing strain into the surface layer region from the surface of the hot-rolled sheet to a depth of 20 μm.
[0027]
Note that shot blasting is preferably used as a technique for introducing distortion. In this shot blasting treatment, it is advantageous to perform steel particles having a diameter of 100 to 800 μm, more preferably 297 to 710 μm, under conditions of a projection amount of 600 kg / m 2 · min or more and a projection time of 1 min or more.
[0028]
After the above strain introduction, cold rolling is performed. The rolling reduction of this cold rolling is desirably 60% or more from the viewpoint of ensuring deep drawability.
[0029]
After the cold rolling, a heat treatment for annealing is performed. Such heat treatment is preferably performed in a continuous annealing line, and the annealing temperature is desirably set to a lower limit of about 800 ° C. as a temperature at which recrystallization is sufficiently completed. On the other hand, if the annealing temperature is too high, the grain growth proceeds excessively and may be softened more than necessary, so it is desirable to set the upper limit at about 900 ° C.
Subsequent plating treatment and alloying treatment performed as necessary may be performed according to a conventional method.
[0030]
【Example】
C: 0.0020 mass%, Si: 0.004 mass%, Mn: 0.12 mass%, P: 0.012 mass%, S: 0.005 mass%, Al: 0.032 mass%, Ti: 0.029 mass%, Nb: 0.003 mass% and N: Steel slab A containing 0.0020 mass%, the balance being iron and inevitable impurities, and C: 0.0018 mass%, Si: 0.002 mass%, Mn: 0.10 mass%, P: 0.010 mass%, S: 0.006 mass% , Al: 0.034mass%, Ti: 0.035mass%, Nb: 0.003mass%, B: 0.0003mass% and N: 0.0022mass%, with the balance heating steel slab B made of iron and inevitable impurities After hot rolling under conditions of furnace extraction temperature 1000-1300 ° C, hot rolling end temperature 800-1000 ° C and coiling temperature 500-800 ° C, shot blasting is performed under the following conditions, hot rolled sheet Strain was introduced into the surface layer (region from the surface to a depth of 20 μm), followed by cold rolling with a reduction rate of 80%, followed by 860 in a continuous hot dip galvanizing line. In after performing annealing for 60 seconds, galvanizing treatment basis weight 45 g / m 2, it was subsequently subjected to alloying treatment. For comparison, a hot dip galvanized steel sheet was manufactured according to the same manufacturing process except that the shot blasting was not performed.
[0031]
Shot: Steel ball (particle size 297 ~ 710mμm)
Projection amount: 625 kg / m 2 · min
Projection time: 1 min
[0032]
The surface of the alloyed hot-dip galvanized steel sheet thus obtained was visually observed for the presence of streak defects. None of the steel types produced streak defects, but no strain was introduced. Streaky defects were observed on the surface of the steel plate.
[0033]
【The invention's effect】
Thus, according to the present invention, a hot-dip galvanized steel sheet having excellent appearance without streak defects can be stably produced with respect to a hot-dip galvanized steel sheet that requires a good appearance such as an automobile outer plate. it can.
Claims (4)
Si：0.10mass％以下、
Mn：0.30mass％以下、
Ti：0.010 〜0.050 mass％、
Nb：0.001 〜0.030 mass％、
Al：0.010 〜0.070 mass％、
Ｎ：0.0050mass％以下、
Ｐ：0.030 mass％以下および
Ｓ：0.010 mass％以下
を含み、残部は鉄及び不可避的不純物よりなる鋼スラブに熱間圧延を施したのち、熱延板の表面から20μｍの深さまでの表層域に歪みを導入してから、冷間圧延を施し、その後熱処理、次いで溶融亜鉛めっき処理を施すことを特徴とする溶融亜鉛めっき鋼板の製造方法。C: 0.0050 mass% or less,
Si: 0.10 mass% or less,
Mn: 0.30mass% or less,
Ti: 0.010 to 0.050 mass%,
Nb: 0.001 to 0.030 mass%,
Al: 0.010 to 0.070 mass%,
N: 0.0050 mass% or less,
P: 0.030 mass% or less and S: 0.010 mass% or less, the remainder being hot rolled on a steel slab made of iron and inevitable impurities, and then in the surface layer area from the surface of the hot rolled sheet to a depth of 20 μm A method for producing a hot dip galvanized steel sheet, comprising introducing a strain and then cold rolling, followed by a heat treatment and then a hot dip galvanizing treatment.
Si：0.10mass％以下、
Mn：0.30mass％以下、
Ti：0.010 〜0.050 mass％、
Nb：0.001 〜0.030 mass％、
Al：0.010 〜0.070 mass％、
Ｎ：0.0050mass％以下、
Ｐ：0.030 mass％以下および
Ｓ：0.010 mass％以下
を含み、更に
Ｂ：0.0002〜0.0010 mass ％
を含有し、残部は鉄及び不可避的不純物よりなる鋼スラブに熱間圧延を施したのち、熱延板の表面から20μｍの深さまでの表層域に歪みを導入してから、冷間圧延を施し、その後熱処理、次いで溶融亜鉛めっき処理を施すことを特徴とする溶融亜鉛めっき鋼板の製造方法。C: 0.0050 mass% or less,
Si: 0.10 mass% or less,
Mn: 0.30mass% or less,
Ti: 0.010 to 0.050 mass%,
Nb: 0.001 to 0.030 mass%,
Al: 0.010 to 0.070 mass%,
N: 0.0050 mass% or less,
Including P: 0.030 mass% or less and S: 0.010 mass% or less, and B: 0.0002 to 0.0010 mass%
After the steel slab consisting of iron and inevitable impurities is hot-rolled, strain is introduced into the surface layer area from the surface of the hot-rolled sheet to a depth of 20 μm, and then cold-rolling is performed. Then, a method for producing a hot-dip galvanized steel sheet, which is subjected to heat treatment and then hot-dip galvanizing treatment.
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