JP5655363B2 - Alloyed hot-dip galvanized steel sheet and method for producing the same - Google Patents
Alloyed hot-dip galvanized steel sheet and method for producing the same Download PDFInfo
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- 238000004519 manufacturing process Methods 0.000 title claims description 15
- 229910001335 Galvanized steel Inorganic materials 0.000 title claims description 13
- 239000008397 galvanized steel Substances 0.000 title claims description 13
- 229910000831 Steel Inorganic materials 0.000 claims description 79
- 239000010959 steel Substances 0.000 claims description 79
- 238000005096 rolling process Methods 0.000 claims description 36
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- 229910052757 nitrogen Inorganic materials 0.000 claims description 8
- 229910052719 titanium Inorganic materials 0.000 claims description 7
- 238000009749 continuous casting Methods 0.000 claims description 6
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- 229910052698 phosphorus Inorganic materials 0.000 claims description 2
- 239000000126 substance Substances 0.000 claims 1
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- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 2
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 2
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- 150000004763 sulfides Chemical class 0.000 description 2
- 239000011593 sulfur Substances 0.000 description 2
- 150000003464 sulfur compounds Chemical class 0.000 description 2
- 229910052725 zinc Inorganic materials 0.000 description 2
- 239000011701 zinc Substances 0.000 description 2
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
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Description
本発明は、自動車の外装板等に使用される、外観ムラが発生しない表面外観に優れた合金化溶融亜鉛めっき鋼板およびその製造方法に関するものである。 The present invention relates to an alloyed hot-dip galvanized steel sheet having excellent surface appearance that does not cause uneven appearance and is used for automobile exterior boards and the like, and a method for producing the same.
従来から、自動車の外装板としては、引張り強さが350MPa未満と軟質で、加工性に優れた冷延鋼板や合金化溶融亜鉛めっき鋼板が多く使用されている。例えば、軟質で加工性に優れた冷延鋼板としては、炭窒化物形成元素を含有する極低炭素鋼を熱間圧延し、熱延鋼板の段階で炭窒化物を生成させ、鋼中の固溶Cおよび固溶Nを低減させた後、冷間圧延および再結晶焼鈍を経て製造される冷延鋼板、いわゆるIF(Interstitial Free)鋼板が知られている。 Conventionally, cold rolled steel sheets and alloyed hot dip galvanized steel sheets, which are soft with a tensile strength of less than 350 MPa and excellent in workability, are widely used as exterior panels for automobiles. For example, as a cold-rolled steel sheet that is soft and excellent in workability, an ultra-low carbon steel containing a carbonitride-forming element is hot-rolled to produce carbonitride at the stage of the hot-rolled steel sheet, thereby A cold-rolled steel sheet, so-called IF (Interstitial Free) steel sheet, which is manufactured through cold rolling and recrystallization annealing after reducing dissolved C and solute N is known.
このようなIF鋼板のうち、炭窒化物形成元素としてTiを添加したIF鋼板は、特に深絞り性などの加工性に優れるという特徴がある。しかしながら、Tiは炭窒化物のみならず、硫化物や炭硫化物を形成し、これらのうち、微細な析出物が再結晶および再結晶後の粒成長を阻害することがあるため、部分的に未再結晶粒が残存するという問題があった。合金化溶融亜鉛めっきを施す際に、鋼板表層部に未再結晶粒の残存部が存在すると、合金化速度にムラが生じ、外観ムラの原因ともなる。
これらの問題を解決する手法として、例えば、特許文献１には、溶融亜鉛めっき処理を行うに際し、鋼板表面に、炭素化合物、窒素化合物およびホウ素化合物の中から選択される１種または２種以上をC、N、B量として0.1〜1000mg/m2付着させ、かつ硫黄または硫黄化合物をS量として0.1〜1000mg/m2付着させた後、水素を含む非酸化性雰囲気で680℃以上の温度で焼鈍する方法が開示されている。
また、特許文献２には、スジムラと呼ばれる表面外観不均一を解決するために、連続鋳造直後のスラブをその表面温度が1000℃以上になるように保持して仕上圧延工程に導き、Ar3点以上の温度で仕上げる方法が開示されている。
Among such IF steel sheets, IF steel sheets to which Ti is added as a carbonitride-forming element are particularly characterized by excellent workability such as deep drawability. However, Ti forms not only carbonitrides but also sulfides and carbonitrides. Among these, fine precipitates may hinder recrystallization and grain growth after recrystallization. There was a problem that unrecrystallized grains remained. When the alloyed hot dip galvanizing is performed, if the remaining portion of the non-recrystallized grains is present in the surface layer portion of the steel sheet, unevenness occurs in the alloying speed, which causes uneven appearance.
As a technique for solving these problems, for example, in Patent Document 1, when performing hot dip galvanizing treatment, one or more selected from a carbon compound, a nitrogen compound, and a boron compound are added to the steel sheet surface. C, N, B amount 0.1 to 1000 mg / m 2 adhered to as, and sulfur or after the sulfur compound to 0.1 to 1000 mg / m 2 is deposited as a S content, at a temperature above 680 ° C. in a non-oxidizing atmosphere containing hydrogen A method of annealing is disclosed.
Further, in Patent Document 2, in order to solve the uneven surface appearance called “straight unevenness”, the slab immediately after continuous casting is held at a surface temperature of 1000 ° C. or higher and led to the finishing rolling process, and Ar3 point or higher A method of finishing at the following temperature is disclosed.
しかしながら、特許文献１に記載の方法では、硫黄または硫黄化合物をS量として0.1〜1000mg/m2付着させる工程が必要となり、生産性の低下やコストの増大を招くという問題がある。
特許文献２に記載の方法では、スラブの表面を溶削するなどして表面欠陥を防止する、いわゆるスラブ手入れを行うことができず、特に美麗な表面外観を要求される自動車外装板用途に用いるには不適当である。
However, the method described in Patent Document 1 requires a step of depositing 0.1 to 1000 mg / m 2 of sulfur or a sulfur compound as the amount of S, and there is a problem that the productivity is lowered and the cost is increased.
In the method described in Patent Document 2, so-called slab care that prevents surface defects by cutting the surface of the slab or the like cannot be performed, and it is used for an automobile exterior plate that requires a particularly beautiful surface appearance. Inappropriate for
本発明は、かかる事情に鑑み、深絞り性に優れたTi添加IF鋼板において、特殊な処理を施さずに、表面外観に優れた合金化溶融亜鉛めっき鋼板およびその製造方法を提供することを目的とする。 In view of such circumstances, the present invention aims to provide an alloyed hot-dip galvanized steel sheet having an excellent surface appearance and a method for producing the same in a Ti-added IF steel sheet excellent in deep drawability without performing a special treatment. And
発明者らは、上記問題点を解決するため、表面欠陥として現出する欠陥の発生メカニズムと抑制対策について、鋭意研究調査を重ねた。
その結果、上記問題を生じる鋼板には、極表層に未再結晶粒が残存すること、そしてこれらの未再結晶粒を調査した結果、鋼板表面から10μmの領域における析出物の析出状態に特徴があることを見出した。
本発明は、以上の知見に基づいてなされたものであり、その要旨は以下のとおりである。
［１］mass％で、C：0.0005〜0.0100%、Si：0.10%以下、Mn：0.05〜0.50%、P：0.030%以下、S：0.008〜0.030%、Ti：0.020〜0.050%、Al：0.010〜0.080%、N： 0.0050%以下、Cu：0．03%以下であり、かつ、Ti*=（Ti%）−3.4×（N%）−1.5×（S%）−4×（C%）で示されるTi*を、0＜Ti*＜0.02を満たす範囲で含有し、残部はFeおよび不可避的不純物から成る鋼組成を有し、以下の式（1）を満足する鋼板の表面に、合金化溶融亜鉛めっき層を具えることを特徴とする加工後の表面外観に優れた合金化溶融亜鉛めっき鋼板。
（S%）≧0.008＋（0.8×（Cu%）−0.01） 式（1）
ただし、（Ti%）、（N%）、（S%）、（C%）、（Cu%）は、それぞれTi、N、S、C、Cuの含有量（mass％）を示す。
［２］前記［１］において、さらに、mass％で、Nb : 0.001〜0.010%、B : 0.0002〜0.0015%のうち、いずれか一種または二種を含有することを特徴とする冷延鋼板。
［3］前記［１］または［２］に記載の成分組成を有する鋼を連続鋳造によりスラブとし、該スラブに対して、加熱温度が1000℃以上1200℃未満で、かつ、1000℃以上の温度域での加熱時間が4.0時間以下の条件で加熱し、スケール除去および粗圧延を施し、次いで、鋼板表面温度が（Ar3変態点−300℃）以上Ar3変態点以下の範囲となるよう冷却した後、仕上げ圧延終了時の鋼板表面温度がAr3変態点以上の温度となるように仕上げ圧延し、650℃以上の温度で巻取り、次いで、酸洗、冷間圧延後、焼鈍し、溶融亜鉛めっきおよび合金化処理を行うことを特徴とする合金化溶融亜鉛めっき鋼板の製造方法。
なお、本明細書において、鋼の成分を示す％は、すべてmass％である。
In order to solve the above-mentioned problems, the inventors have conducted intensive research and investigations on the generation mechanism of the defects that appear as surface defects and the countermeasures to suppress them.
As a result, unrecrystallized grains remain in the extreme surface layer of the steel plate causing the above problems, and as a result of investigating these unrecrystallized grains, the precipitation state of the precipitate in the region of 10 μm from the steel sheet surface is characterized. I found out.
This invention is made | formed based on the above knowledge, The summary is as follows.
[1] In mass%, C: 0.0005 to 0.0100%, Si: 0.10% or less, Mn: 0.05 to 0.50%, P: 0.030% or less, S: 0.008 to 0.030%, Ti: 0.020 to 0.050%, Al: 0.010 ~ 0.080%, N: 0.0050% or less, Cu: 0.03% or less, and Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%) Is contained in a range satisfying 0 <Ti * <0.02, with the balance being a steel composition composed of Fe and inevitable impurities, and on the surface of the steel sheet satisfying the following formula (1), an alloy An alloyed hot-dip galvanized steel sheet excellent in surface appearance after processing, characterized by comprising a hot-dip galvanized layer.
(S%) ≧ 0.008 + (0.8 × (Cu%) − 0.01) Formula (1)
However, (Ti%), (N%), (S%), (C%), and (Cu%) indicate the contents (mass%) of Ti, N, S, C, and Cu, respectively.
[2] A cold-rolled steel sheet according to [1], further comprising at least one of mass percentages of Nb: 0.001 to 0.010% and B: 0.0002 to 0.0015%.
[3] The steel having the component composition described in [1] or [2] is made into a slab by continuous casting, and the heating temperature is 1000 ° C. or more and less than 1200 ° C. and the temperature is 1000 ° C. or more. After heating under the condition where the heating time in the zone is 4.0 hours or less, applying descaling and rough rolling, and then cooling the steel sheet surface temperature so that it is in the range of (Ar3 transformation point-300 ° C) to Ar3 transformation point , Finish rolling so that the steel sheet surface temperature at the end of finish rolling is equal to or higher than the Ar3 transformation point, winding at a temperature of 650 ° C or higher, then pickling, cold rolling, annealing, hot dip galvanizing and A method for producing an alloyed hot-dip galvanized steel sheet, characterized by performing an alloying treatment.
In addition, in this specification, all% which shows the component of steel is mass%.
本発明によれば、表面外観に優れた合金化溶融亜鉛めっき鋼板が得られる。 According to the present invention, an alloyed hot-dip galvanized steel sheet having an excellent surface appearance can be obtained.
以下に、本発明の詳細を説明する。
従来の自動車の外装板用のＴｉ添加ＩＦ鋼板（合金化溶融亜鉛めっき鋼板）では外観ムラが生じる場合があった。
そこで、このような外観ムラが生じる鋼板について詳細に調査した。その結果、上記問題が生じる鋼板には、板厚表層部に部分的に未再結晶粒が残存することを知見した。また、未再結晶粒が表層付近に残存した場合には、合金化処理時に合金化速度が局部的に異なるため外観ムラが生じることもわかった。
上記知見を受けて、本発明者らは、次に、表層付近に未再結晶粒が残存する原因を詳細に検討した。その結果、未再結晶粒が残存する部分には大きさが20nm未満のごく微細な析出物が多く存在することがわかった。このような微細な析出物は、自動車外装板用鋼板に施される一般的な焼鈍条件では固溶せずに残存し、残存することでいわゆるピン止め効果によって表層付近では再結晶が容易に進まず、未再結晶粒が残存するものと考えられる。
そこで、このような問題を解決するために、様々な製造条件での実験を繰返し実施して種々の鋼板を得、得られた鋼板について表層付近の状態を調査した。そうしたところ、特定の組成の鋼では、表層付近に未再結晶粒が多く残存せず、合金化溶融亜鉛めっき鋼板における外観ムラが生じないことを見出した。そして、この鋼板の板厚最表層付近、具体的には鋼板両面の表面から10μmまでの領域では、20nm未満の析出物量が低減され、未再結晶粒が多く残存しない鋼板を得られることを知見した。
Details of the present invention will be described below.
In the conventional Ti-added IF steel sheet (alloyed hot-dip galvanized steel sheet) for an automobile exterior plate, appearance unevenness may occur.
Therefore, the steel sheets in which such appearance irregularities were generated were investigated in detail. As a result, it was found that unrecrystallized grains partially remain in the plate thickness surface layer portion of the steel plate in which the above problem occurs. It was also found that when non-recrystallized grains remain in the vicinity of the surface layer, unevenness of appearance occurs because the alloying speed differs locally during the alloying treatment.
In view of the above findings, the present inventors then examined in detail the cause of the remaining non-recrystallized grains in the vicinity of the surface layer. As a result, it was found that there were many very fine precipitates having a size of less than 20 nm in the portion where the non-recrystallized grains remained. Such fine precipitates remain undissolved under the general annealing conditions applied to the steel sheet for automobile exterior plates, and the recrystallization easily proceeds near the surface due to the so-called pinning effect. First, it is considered that unrecrystallized grains remain.
Therefore, in order to solve such a problem, experiments under various production conditions were repeatedly performed to obtain various steel plates, and the state of the surface layer of the obtained steel plates was investigated. As a result, it has been found that in a steel having a specific composition, many unrecrystallized grains do not remain in the vicinity of the surface layer, and the appearance unevenness in the galvannealed steel sheet does not occur. In addition, in the vicinity of the outermost layer thickness of this steel sheet, specifically in the region from the surface of both surfaces of the steel sheet to 10 μm, the amount of precipitates of less than 20 nm is reduced, and it is found that a steel sheet with a large amount of unrecrystallized grains remaining can be obtained. did.
特に、鋼中（Ｓ）含有量（mass%）が鋼中（Cu）含有量に対して下記の式（1）を満足する事によって外観ムラ発生を大きく低減できる事がわかった。なお、式（１）は、発明者らが、後述するTiやCuの析出物と外観ムラとの検討結果に基づき、鋼中のCu含有量、S含有量と外観ムラとの関係について検討した結果、得たものである。
（S%）≧0.008＋（0.8×（Cu%）−0.01） 式（1）
ただし、（S%）、（Cu%）は、それぞれS、Cuの含有量（mass％）を示す。
熱間圧延冷却工程では、Ti(Mn)S、CuS、Ti4C2S2、TiCの順で析出する。CuSが析出した後に、Sの量が充分であればTi4C2S2が次に析出される。しかし、Sの量が不足した場合、析出されるTi4C2S2は減少してしまい、固溶Tiが余ることになる。そして、この余剰Tiにより、次いで析出するTiCの量が増大する。このTiCは微細析出物であり、表層付近に存在する。そのため、表層付近はこのTiCのピン止め効果によって再結晶が容易に進まず、未再結晶粒が残存することになり、結果として、合金化溶融亜鉛めっき鋼板における外観ムラが生じることになる。
そこで、本発明では、上記検討結果を鑑み、（Cu）含有量に対して、上記式（１）を満たす範囲に（Ｓ）含有量を調整することによって、Cuの量に対して当量以上で、かつ、Ti4C2S2が充分に析出されるSの量を含有することとする。その結果、TiCの生成増大を抑止し、合金化溶融亜鉛めっき鋼板における外観ムラが防止できる。
In particular, it was found that the occurrence of unevenness in appearance can be greatly reduced when the (S) content (mass%) in the steel satisfies the following formula (1) with respect to the (Cu) content in the steel. In addition, Formula (1) examined the relationship between Cu content in steel, S content, and external appearance nonuniformity based on the examination result of the precipitate of Ti and Cu which are mentioned later, and external appearance nonuniformity. As a result, it is obtained.
(S%) ≧ 0.008 + (0.8 × (Cu%) − 0.01) Formula (1)
However, (S%) and (Cu%) indicate S and Cu contents (mass%), respectively.
In the hot rolling cooling step, Ti (Mn) S, CuS, Ti 4 C 2 S 2 and TiC are deposited in this order. After CuS is precipitated, Ti 4 C 2 S 2 is then precipitated if the amount of S is sufficient. However, when the amount of S is insufficient, the deposited Ti 4 C 2 S 2 decreases, and solid solution Ti remains. Then, the amount of TiC that subsequently precipitates increases due to this surplus Ti. This TiC is a fine precipitate and exists in the vicinity of the surface layer. Therefore, near the surface layer, recrystallization does not easily proceed due to the pinning effect of TiC, and unrecrystallized grains remain, and as a result, appearance irregularities in the galvannealed steel sheet occur.
Therefore, in the present invention, in view of the above examination results, by adjusting the (S) content in a range satisfying the above formula (1) with respect to the (Cu) content, the Cu content is equal to or more than the equivalent amount. And Ti 4 C 2 S 2 contains an amount of S sufficiently precipitated. As a result, it is possible to suppress an increase in the production of TiC and prevent appearance irregularities in the galvannealed steel sheet.
次に、本発明の鋼板の成分組成（鋼組成）の限定理由について説明する。
C：0.0005〜0.0100%
Cは、固溶強化元素であり、降伏強度の上昇に寄与し、面内剛性の向上には有利であるが、優れた深絞り性を得るためには、極力低減することが好ましい。しかし、0.0005%未満では、結晶粒径が著しく粗大化して降伏強度が大きく低下するため、面内剛性が低下して腰折れなどの欠陥が発生しやすくなる。また、脱炭コストの増大を招く。よって、0.0005%を下限とする。一方、Cを多量に含有すると鋼中でのTiC量が増加し、表層部での析出物量が増加して、未再結晶粒の残存量が増大するため、0.0100%を上限とする。
Next, the reason for limiting the component composition (steel composition) of the steel sheet of the present invention will be described.
C: 0.0005 to 0.0100%
C is a solid solution strengthening element and contributes to an increase in yield strength and is advantageous for improving the in-plane rigidity. However, in order to obtain excellent deep drawability, C is preferably reduced as much as possible. However, if it is less than 0.0005%, the crystal grain size becomes extremely coarse and the yield strength is greatly reduced, so that the in-plane rigidity is lowered and defects such as hip breakage tend to occur. Moreover, the decarburization cost increases. Therefore, 0.0005% is set as the lower limit. On the other hand, when a large amount of C is contained, the amount of TiC in the steel increases, the amount of precipitates in the surface layer portion increases, and the residual amount of unrecrystallized grains increases, so 0.0100% is made the upper limit.
Si:0.10%以下
Siは、加工性を劣化することなく固溶強化により鋼を強化するのに有用な元素であるが、本発明のような溶融亜鉛めっき鋼板では焼鈍時に表面に濃化して溶融亜鉛めっき性を著しく阻害するため、0.10%以下とする。
Si: 0.10% or less
Si is a useful element for strengthening steel by solid solution strengthening without degrading workability. However, in hot dip galvanized steel sheets such as the present invention, it is concentrated on the surface during annealing and the hot dip galvanizing property is remarkably increased. In order to inhibit, it is 0.10% or less.
Mn: 0.05〜0.50%
Mnは、固溶強化元素として鋼強度を増大させる。鋼板剛性確保のため、0.05%以上の添加が必要である。所望の強度を得るために適宜添加することができるが、過剰な添加は加工性を阻害するため、0.50%以下とする。
Mn: 0.05 ~ 0.50%
Mn increases the steel strength as a solid solution strengthening element. Addition of 0.05% or more is necessary to secure the steel plate rigidity. Although it can be added appropriately in order to obtain a desired strength, excessive addition inhibits workability, so it is 0.50% or less.
P：0.030%以下
Pは固溶強化元素であり、鋼の強化と降伏強度向上には有効である。しかし、過度に添加すると、熱間、冷間割れの原因となるばかりでなく、溶融亜鉛めっきの合金化反応を阻害するため、0.030%以下とする。
P: 0.030% or less
P is a solid solution strengthening element and is effective in strengthening steel and improving yield strength. However, if added excessively, it not only causes hot and cold cracking, but also inhibits the alloying reaction of hot dip galvanizing, so it is made 0.030% or less.
S：0.008〜0.030%
Sは本発明において重要な元素である。Sは通常、不可避的不純物として鋼中に存在し、極力低減すべきものとされるが、本発明では敢えてその存在量を0.008%以上確保する。すなわち、0.008%未満では、上述したように、鋼中Cuと容易に結びついてCuSが生成した後に、S量不足によりピン止め効果となりやすい微細なTiCが比較的多量に析出し、外観ムラの原因となる。このような微細析出物の影響を低減するため、0.008%以上とする。好ましくは0.010%以上である。一方0.030%超えでは、鋼板製造時の熱間割れが生じ易くなり、生産性が阻害されるとともに表面性状を劣化させる。よって、0.030%以下とする。
S: 0.008 to 0.030%
S is an important element in the present invention. S is usually present in steel as an unavoidable impurity and should be reduced as much as possible. In the present invention, S is intentionally secured in an amount of 0.008% or more. That is, if it is less than 0.008%, as described above, after CuS is easily combined with Cu in the steel, a relatively large amount of fine TiC that tends to become a pinning effect due to insufficient amount of S precipitates, causing uneven appearance. It becomes. In order to reduce the influence of such fine precipitates, the content is made 0.008% or more. Preferably it is 0.010% or more. On the other hand, if it exceeds 0.030%, hot cracking is likely to occur during the production of the steel sheet, which hinders productivity and deteriorates the surface properties. Therefore, it is 0.030% or less.
Ti：0.020〜0.050%、Ti*=（Ti%）−3.4×（N%）−1.5×（S%）−4×（C%）で示されるTi*を、0＜Ti*＜0.02
Tiは本発明における最も重要な元素のひとつである。Tiは、鋼中のC、N、Sを析出物として固定することにより、加工性向上効果を有する。0.020%未満では、このような効果を得ることができない。一方、Tiを0.050%を超えて添加してもそれ以上の効果が望めないばかりでなく、板内部に異常組織の形成を招き、加工性を低下させる。
また、前述したように、鋼中のTiは、鋼中のC、N、Sと析出物を形成するため、これらの成分に対して、当量以上添加することにより、加工性を向上させることができる。そのためには、下記（１）式で示されるTi*を0より大きくする必要がある。一方で、固溶Tiを過剰に存在させると、微細なTiCを生成する場合があり、この微細なTiCは表層において未再結晶粒残存を助長するため好ましくない。よって、Ti*を0.02未満とする必要がある。
Ti*=（Ti%）−3.4×（N%）−1.5×（S%）−4×（C%）・・・（１）
ただし、（Ti%）、（N%）、（S%）、（C%）は、それぞれTi、N、S、Cの含有量（mass％）を示す。
Ti: 0.020 to 0.050%, Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%) Ti * is expressed as 0 <Ti * <0.02
Ti is one of the most important elements in the present invention. Ti has an effect of improving workability by fixing C, N, and S in steel as precipitates. If it is less than 0.020%, such an effect cannot be obtained. On the other hand, if Ti is added in excess of 0.050%, not only a further effect cannot be expected, but an abnormal structure is formed inside the plate, and the workability is lowered.
In addition, as described above, Ti in steel forms precipitates with C, N, and S in steel, so adding these elements in an equivalent amount or more can improve workability. it can. For that purpose, it is necessary to make Ti * shown by the following formula (1) larger than 0. On the other hand, if excessive solid solution Ti is present, fine TiC may be generated, and this fine TiC is not preferable because it promotes the remaining of non-recrystallized grains in the surface layer. Therefore, Ti * needs to be less than 0.02.
Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%) (1)
However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively.
Al：0.010〜0.080%
Alは脱酸剤として添加する元素であり、0.010%以上必要であるが、多量に添加してもより一層の脱酸効果は得られずコストアップにつながるため、0.080%以下とする。
Al: 0.010 to 0.080%
Al is an element to be added as a deoxidizer and needs to be 0.010% or more. However, even if it is added in a large amount, a further deoxidation effect cannot be obtained, leading to an increase in cost.
N： 0.0050%以下
Nは少ないほど加工性には有利であるので、少ないほど望ましい。0.0050％を超えて、過剰に添加すると、成形性の著しい低下と固溶Ti量の低下につながるので、上限を0.0050％とする。
N: 0.0050% or less
The smaller N, the better the workability, so the smaller N is desirable. If over 0.0050% is added in excess, it leads to a significant decrease in formability and a decrease in the amount of dissolved Ti, so the upper limit is made 0.0050%.
Cu：0.03%以下
Cuは本発明における最も重要な元素の１つである。Cuは通常は不可避的不純物として鋼中に存在する。しかし、近年CO2ガス排出規制に伴う製鋼工程でのスクラップ配合率の上昇により、Cu含有量は高くなる傾向がある。特にＨ２と呼ばれる品位の低い市中屑の使用比率が高いとその傾向は顕著である。
Cu含有量が高くなると、CuSを多く析出することになり、微細析出物であるTiCが析出しやすい傾向となる。その結果、鋼板表層部に未再結晶粒が残存する原因となる。よって、この影響を低減させるには0.03%以下とするのが好ましい。Cu含有量は低いほど好ましいが、あまりに低くすると、製造コストが非常に大きくなるため、Cu含有量の下限は例えば0.01％程度とすることが好ましい。
Cu: 0.03% or less
Cu is one of the most important elements in the present invention. Cu is usually present in steel as an inevitable impurity. However, the Cu content tends to increase due to an increase in the scrap mixing ratio in the steelmaking process in recent years due to CO 2 gas emission regulations. The tendency is remarkable especially when the usage rate of the low-grade market waste called H2 is high.
When the Cu content is high, a large amount of CuS is precipitated, and TiC, which is a fine precipitate, tends to precipitate. As a result, non-recrystallized grains remain in the steel sheet surface layer. Therefore, 0.03% or less is preferable to reduce this influence. The lower the Cu content, the better. However, if the content is too low, the manufacturing cost becomes very high. Therefore, the lower limit of the Cu content is preferably about 0.01%, for example.
さらに、本発明では、次の添加元素からいずれか一種または二種を添加することが好ましい。
Nb : 0.001〜0.010%
NbはTiと同様に炭窒化物を形成して加工性を向上させるのに有利な元素である。特に、前述した（１）式のTi*が0.005未満の場合には添加することが望ましく、加工性向上効果を得るためには、0.001%以上添加する必要がある。しかし、0.010%を超えて添加すると、結晶粒が微細化され、深絞り性などの加工性を劣化させる場合がある。よって、添加する場合は、0.001％以上0.010%以下とする。
B : 0.0002〜0.0015%
B はIF鋼板の粒界強化に有効な元素であり、耐二次加工脆性が必要とされる場合に0.0002%以上添加すると効果的である。しかし、過剰に添加すると、鋼板製造時の表面性状を劣化させる恐れがあるため、添加する場合は0.0015%以下とする。
Furthermore, in this invention, it is preferable to add any 1 type or 2 types from the following additional elements.
Nb: 0.001 to 0.010%
Nb, like Ti, is an element that is advantageous for forming a carbonitride to improve workability. In particular, it is desirable to add Ti * in the above formula (1) when it is less than 0.005. In order to obtain the workability improvement effect, it is necessary to add 0.001% or more. However, if added over 0.010%, the crystal grains are refined, and workability such as deep drawability may be deteriorated. Therefore, when adding, it shall be 0.001% or more and 0.010% or less.
B: 0.0002-0.0015%
B is an element effective for strengthening the grain boundary of IF steel sheet. If secondary work brittleness resistance is required, it is effective to add 0.0002% or more. However, if added in excess, the surface properties during steel plate production may be deteriorated.
残部はFeおよび不可避的不純物である。
本発明の合金化溶融亜鉛めっき鋼板は、上記した成分組成の鋼板の表面に合金化溶融亜鉛めっき層を具えるものである。
The balance is Fe and inevitable impurities.
The alloyed hot-dip galvanized steel sheet of the present invention comprises an alloyed hot-dip galvanized layer on the surface of the steel sheet having the above-described composition.
次に、本発明の合金化溶融亜鉛めっき鋼板の製造方法について説明する。 Next, the manufacturing method of the galvannealed steel plate of this invention is demonstrated.
本発明の合金化溶融亜鉛めっき鋼板は、上記のような成分組成を有する鋼を連続鋳造によりスラブとし、該スラブに対して、加熱温度が1000℃以上1200℃未満で、かつ1000℃以上の温度域での加熱時間が4.0時間以下の条件で加熱し、スケール除去および粗圧延を施し、次いで、鋼板表面温度が（Ar3変態点−300℃）以上Ar3変態点以下の範囲となるよう冷却した後、仕上げ圧延終了時の鋼板表面温度がAr3変態点以上の温度となるように仕上げ圧延し、冷却し、650℃以上の温度で巻取り、次いで、酸洗、冷間圧延後、焼鈍し、溶融亜鉛めっきおよび合金化処理を行うことにより得られる。 The alloyed hot-dip galvanized steel sheet of the present invention is a slab formed by continuous casting of steel having the above composition, and the heating temperature is 1000 ° C. or more and less than 1200 ° C., and a temperature of 1000 ° C. or more. After heating under the condition where the heating time in the zone is 4.0 hours or less, applying descaling and rough rolling, and then cooling the steel sheet surface temperature so that it is in the range of (Ar3 transformation point-300 ° C) to Ar3 transformation point Finished and rolled so that the steel sheet surface temperature at the end of finish rolling is equal to or higher than the Ar3 transformation point, cooled, wound at a temperature of 650 ° C or higher, then pickled, cold-rolled, annealed, and melted It is obtained by galvanizing and alloying treatment.
加熱温度が1000℃以上1200℃未満で、かつ1000℃以上の温度域での加熱時間が4.0時間以下の条件でスラブ加熱
熱間圧延工程で上記条件でスラブを加熱する。加熱温度が1000℃未満では、圧延温度が低下して仕上げ圧延後の鋼板表面温度をAr3変態点以上とすることが困難である。一方、加熱温度が1200℃を超えると、連続鋳造時に生成したTiMnSなどのTiを含有する硫化物が短時間で多く固溶し、後に続く工程において微細析出物が多く生成するため、好ましくない。
さらに、加熱温度が1200℃未満であっても、長時間保持すると、上記同様に、Tiを含有する硫化物の固溶が進み、後に続く工程において微細析出物が多く生成するため好ましくない。よって、1000℃以上の温度域での加熱時間は4.0時間以下とする。
The slab is heated under the above conditions in the slab heating hot rolling step under the condition that the heating temperature is 1000 ° C. or more and less than 1200 ° C. and the heating time in the temperature range of 1000 ° C. or more is 4.0 hours or less. When the heating temperature is less than 1000 ° C., the rolling temperature is lowered, and it is difficult to set the steel sheet surface temperature after finish rolling to the Ar3 transformation point or higher. On the other hand, if the heating temperature exceeds 1200 ° C., a large amount of sulfides containing Ti such as TiMnS generated during continuous casting is dissolved in a short time, and a lot of fine precipitates are generated in the subsequent process, which is not preferable.
Further, even if the heating temperature is less than 1200 ° C., holding for a long time is not preferable because, as described above, the solid solution of the sulfide containing Ti proceeds, and many fine precipitates are generated in the subsequent steps. Therefore, the heating time in the temperature range of 1000 ° C. or higher is 4.0 hours or less.
鋼板表面温度が（Ar3変態点−300℃）以上Ar3変態点以下の範囲となるよう冷却
加熱されたスラブに対して、スケール除去および粗圧延を施した後、仕上げ圧延を行う前に表面温度が（Ar3変態点−300℃）以上Ar3変態点以下の範囲となるように冷却する。
通常の製造方法では、熱間圧延工程における仕上げ圧延後に冷却することでフェライト変態が始まる。しかし、本発明では、仕上げ圧延前に鋼板表面を冷却して表面温度を一旦Ar3変態点以下とする。このように仕上げ圧延前に所定の温度まで表面を冷却することで、表層部のみはフェライト変態を開始してTi4C2S2析出物が生成し始め、粗大に成長しやすくなる。その結果、微細析出物の析出物量が低減され、未再結晶粒が多く残存することなく、均一な外観を有し、かつ、プレス加工後の形状均一性に優れた冷延鋼板が得られることになる。
なお、仕上げ圧延中に、表層部は、板厚中央からの復熱および加工発熱によって温度上昇する。
仕上げ圧延前の表面温度が低すぎると、仕上げ圧延終了時の表面温度がAr3変態点以下となり表層部に加工フェライト組織が生成し均一性が損なわれるため、仕上げ圧延前の表面温度は（Ar3変態点−300℃）以上とする必要がある。
このように仕上げ圧延前に表面を一旦冷却して表面温度を制御することは、本発明の製造方法において特に重要な要件であり、特徴である。
仕上げ圧延前に表面を冷却する方法としては、例えば、通常スケール除去に用いられる高圧水噴射装置などを用いて、表面が適切な温度域となるよう冷却することができる。
なお、Ar3変態点は以下のようにして求めることができる。各組成の鋼を1000〜1200℃の温度に加熱し、その後冷却しながら温度と体積変化を測定することによりオーステナイト→フェライト変態による体積膨張が生じる温度（Ar3変態点）を得ることができる。
The surface temperature of the slab cooled and heated so that the steel sheet surface temperature is in the range of Ar3 transformation point to Ar3 transformation point is less than the Ar3 transformation point, and after surface removal before rough rolling. Cool so that it is in the range of (Ar3 transformation point-300 ° C) and below Ar3 transformation point.
In a normal manufacturing method, the ferrite transformation starts by cooling after finish rolling in the hot rolling process. However, in the present invention, the surface of the steel sheet is cooled to the Ar3 transformation point or less once before the finish rolling. In this way, by cooling the surface to a predetermined temperature before finish rolling, only the surface layer portion starts ferrite transformation and Ti 4 C 2 S 2 precipitates start to be formed, and it becomes easy to grow coarsely. As a result, the amount of fine precipitates is reduced, a large number of unrecrystallized grains do not remain, and a cold rolled steel sheet having a uniform appearance and excellent shape uniformity after pressing is obtained. become.
During finish rolling, the temperature of the surface layer portion increases due to recuperation from the center of the plate thickness and processing heat generation.
If the surface temperature before finish rolling is too low, the surface temperature at the end of finish rolling will be below the Ar3 transformation point, and the processed ferrite structure will be generated in the surface layer and the uniformity will be impaired. Point -300 ° C) or higher.
Thus, it is a particularly important requirement and a feature in the manufacturing method of the present invention to control the surface temperature by once cooling the surface before finish rolling.
As a method for cooling the surface before finish rolling, for example, a high-pressure water injection device usually used for scale removal can be used to cool the surface so as to be in an appropriate temperature range.
The Ar3 transformation point can be obtained as follows. By heating the steel of each composition to a temperature of 1000 to 1200 ° C. and then measuring the temperature and volume change while cooling, a temperature (Ar3 transformation point) at which volume expansion due to austenite → ferrite transformation occurs can be obtained.
仕上げ圧延終了時の鋼板の表面温度がAr3変態点以上の温度となるように仕上げ圧延
仕上げ圧延終了時の鋼板表面温度がAr3変態点を下回ると、鋼板表層部に加工フェライト組織が生成し、均一性が損なわれる。このため、仕上げ圧延終了時の鋼板表面温度がAr3変態点以上となるように、制御する必要がある。
なお、仕上げ圧延終了後に鋼板表面温度がAr3変態点以上に長時間保持されると、成長した比較的粗大な析出物が再固溶して、微細析出物量が増加しやすくなるため好ましくない。そのため、鋼板表面温度がAr3変態点以上の温度で仕上げ圧延が終了した後、ただちに冷却してフェライト変態を促進することが好ましい。冷却開始までの時間は、好ましくは1秒以内である。
When the surface temperature of the steel sheet at the end of finish rolling finish rolling is lower than the Ar3 transformation point so that the surface temperature of the steel plate at the end of finish rolling is equal to or higher than the Ar3 transformation point, a processed ferrite structure is generated in the surface layer of the steel sheet and uniform. Sexuality is impaired. For this reason, it is necessary to control so that the steel plate surface temperature at the end of finish rolling is equal to or higher than the Ar3 transformation point.
If the steel sheet surface temperature is maintained at a temperature higher than the Ar3 transformation point after finishing rolling for a long time, it is not preferable because the grown relatively coarse precipitates are re-dissolved and the amount of fine precipitates tends to increase. Therefore, it is preferable that the steel sheet surface temperature is equal to or higher than the Ar3 transformation point, and after finishing rolling is finished, the steel sheet is immediately cooled to promote the ferrite transformation. The time to start cooling is preferably within 1 second.
650℃以上で巻取り
冷却後、650℃以上で巻取る。巻取り温度が650℃を下回ると、析出物の成長速度が小さくなり、微細析出物量が増加する。巻取り温度の上限は特に規定するものではないが、高すぎると表層のスケールが成長して表面欠陥の原因となりやすいため、800℃未満とすることが望ましい。
Winding and cooling at 650 ° C or higher, then winding at 650 ° C or higher. When the coiling temperature is lower than 650 ° C., the growth rate of precipitates is reduced and the amount of fine precipitates is increased. The upper limit of the coiling temperature is not particularly specified, but if it is too high, the scale of the surface layer tends to grow and cause surface defects.
巻取り後、酸洗、冷間圧延、洗浄した後、焼鈍、溶融亜鉛めっき、合金化処理を行う。
酸洗、冷間圧延および焼鈍条件は特に限定する必要は無く、常法に従えばよい。
例えば、巻き取り後の鋼板は、表面に生成したスケールを除去するために酸洗し、次いで冷間圧延を行うが、冷間圧延率（冷間圧延圧下率）は自動車用外板を製造する際に通常行われている50％〜90％程度とすればよい。なお、冷間圧延率は加工性（ｒ値）向上の観点からは70％以上とするのが望ましい。次いで、冷間圧延後の鋼板は、圧延油の脱脂や汚れを除くため洗浄した後、再結晶焼鈍される。なお、焼鈍温度は、Ac3変態点を超えると加工性（ｒ値）が低下しやすいため、Ac3変態点以下とすることが好ましい。なお、下限温度は、700℃程度とすることが、再結晶焼鈍を行う上で好ましい。焼鈍後、引き続き溶融亜鉛めっきおよび合金化処理を行う。なお、焼鈍前に軽酸洗を行ってもよい。溶融亜鉛めっき条件、合金化条件は特に限定する必要は無く、常法に従えばよい。
また、合金化処理後、表面粗度の調整などのため調質圧延を行うことが好ましい。この際、調質圧延の圧延率（伸長率）は、0.5％〜1.5％程度とすることが好ましい。
以上により、本発明の合金化溶融亜鉛めっき鋼板が得られる。
After winding, pickling, cold rolling, and washing, annealing, hot dip galvanizing, and alloying treatment are performed.
Pickling, cold rolling, and annealing conditions are not particularly limited, and may be performed in accordance with ordinary methods.
For example, the steel sheet after winding is pickled to remove the scale formed on the surface, and then cold-rolled, but the cold rolling rate (cold rolling reduction rate) produces an automobile outer plate. It may be about 50% to 90%, which is usually performed. The cold rolling rate is preferably 70% or more from the viewpoint of improving workability (r value). Next, the cold-rolled steel sheet is cleaned to remove the degreasing and dirt of the rolling oil, and then recrystallized and annealed. In addition, since the workability (r value) tends to decrease when the annealing temperature exceeds the Ac3 transformation point, it is preferable to set the annealing temperature below the Ac3 transformation point. The lower limit temperature is preferably about 700 ° C. for performing recrystallization annealing. After annealing, hot dip galvanizing and alloying treatment are subsequently performed. In addition, you may perform light pickling before annealing. Hot dip galvanizing conditions and alloying conditions need not be particularly limited, and may be in accordance with conventional methods.
Further, after the alloying treatment, it is preferable to perform temper rolling in order to adjust the surface roughness. At this time, the rolling rate (elongation rate) of temper rolling is preferably about 0.5% to 1.5%.
As described above, the galvannealed steel sheet of the present invention is obtained.
以下に本発明による効果を具体的に示す。
まず、表1に示す成分組成からなる溶鋼を、真空脱ガス処理後、連続鋳造によりスラブとした。次いで上記スラブを加熱し、スケール除去後、板厚40mmまで粗圧延した。次いで、スケール除去装置で鋼板表層を冷却した後、3.5ｍｍ厚まで仕上げ圧延し、巻取り温度700℃でコイルに巻き取った。なお、この時のスラブの加熱条件、仕上げ圧延前の冷却後の鋼板表層温度、仕上げ圧延温度を表2に示す。なお、仕上げ圧延温度（仕上げ圧延時の鋼板表面温度）は全てAr3変態点以上の温度とし、B4、C4以外は、仕上げ圧延前冷却後表面温度は（Ar3変態点−300℃）〜Ar3変態点の温度域内の温度とした。
次いで、巻取り後の鋼板を酸洗後、0.70mmまで冷間圧延（冷間圧延率：80％）して供試材とし、前処理として脱脂、酸洗した後、溶融亜鉛めっきラインで焼鈍、溶融亜鉛めっき処理、合金化処理、伸長率1.0%の調質圧延を行い、合金化溶融亜鉛めっき鋼板を得た。なお、前記焼鈍時の雰囲気は水素を含む非酸化性ガスとし、各供試材の焼鈍温度はAc3変態点以下である830℃とした。溶融亜鉛めっき処理は、Alを0.12%含む460℃亜鉛めっき浴を用いて、侵入板温460℃、浸漬時間3秒にて行った。合金化処理は、めっき後、N2ガスワイパーを用いて亜鉛付着量を片面当たり60g/m2に調整し、510℃で20秒で行った。合金化処理を行った鋼板に伸長率1.0%の調質圧延を施し、加工後形状均一性評価および外観評価を行った。
The effect by this invention is shown concretely below.
First, molten steel having the composition shown in Table 1 was made into a slab by continuous casting after vacuum degassing treatment. Next, the slab was heated, scale-removed, and then roughly rolled to a plate thickness of 40 mm. Next, the steel sheet surface layer was cooled with a scale removing device, and then finish-rolled to a thickness of 3.5 mm and wound on a coil at a winding temperature of 700 ° C. Table 2 shows the heating conditions of the slab at this time, the surface temperature of the steel sheet after cooling before finish rolling, and the finish rolling temperature. Note that the finish rolling temperature (steel surface temperature during finish rolling) is all equal to or higher than the Ar3 transformation point, and the surface temperature after cooling before finish rolling (Ar3 transformation point -300 ° C) to Ar3 transformation point, except for B4 and C4 The temperature was within the temperature range.
Next, the steel sheet after winding is pickled and cold-rolled to 0.70 mm (cold rolling rate: 80%) as a test material, degreased and pickled as pretreatment, and then annealed in a hot-dip galvanizing line Then, hot dip galvanizing treatment, alloying treatment, and temper rolling with an elongation of 1.0% were performed to obtain an alloyed hot dip galvanized steel sheet. The atmosphere during the annealing was a non-oxidizing gas containing hydrogen, and the annealing temperature of each test material was 830 ° C., which is lower than the Ac 3 transformation point. The hot dip galvanizing treatment was performed using a 460 ° C. zinc plating bath containing 0.12% Al at an intrusion plate temperature of 460 ° C. and an immersion time of 3 seconds. The alloying treatment was carried out at 510 ° C. for 20 seconds after the plating by adjusting the zinc adhesion amount to 60 g / m 2 per side using an N 2 gas wiper. The alloyed steel sheet was subjected to temper rolling with an elongation of 1.0%, and the post-processing shape uniformity evaluation and appearance evaluation were performed.
上記の製造方法により得られた合金化溶融亜鉛鋼板に対して、下記の評価を行った。 The following evaluation was performed on the galvannealed steel sheet obtained by the above production method.
機械的特性
成形性は、引張特性とｒ値の機械的特性により評価した。引張特性は、ＪＩＳＺ 2201記載の５号試験片に加工した後、ＪＩＳＺ 2241記載の試験方法に従って行った。また平均ｒ値は、15％の引張予歪を与えた後、3点法にて測定し、鋼板の圧延方向に対して、90°方向、45°方向、０°方向のｒ値の平均=(r(0°)+2×ｒ(45°)+r(90°))/4として求めた。
Mechanical properties Formability was evaluated by tensile properties and r-value mechanical properties. Tensile properties were measured according to the test method described in JISZ 2241 after being processed into a No. 5 test piece described in JISZ 2201. The average r value was measured by a three-point method after giving a tensile pre-strain of 15%, and the average r value in the 90 ° direction, 45 ° direction, and 0 ° direction with respect to the rolling direction of the steel sheet = It was determined as (r (0 °) + 2 × r (45 °) + r (90 °)) / 4.
めっき後外観
合金化溶融亜鉛めっきを施したものについては、外観ムラの有無を目視にて観察し、ムラの生じたものを×、ムラなく均一な外観であったものを○とした。
Appearance of the alloyed hot-dip galvanized after plating was visually observed for the presence or absence of unevenness in appearance.
本発明例は、深絞り性の指標である平均r値が1.5以上であり、かつ、外観ムラがなく均一で自動車外装板用途に適した性能を有していた。
一方、比較例では、外観が劣り、自動車外装板用途に適した性能を満足しなかった。
The examples of the present invention had an average r value that is an index of deep drawability of 1.5 or more, had no appearance unevenness, and had a performance suitable for automobile exterior plate applications.
On the other hand, in a comparative example, the external appearance was inferior and the performance suitable for a car exterior board use was not satisfied.
本発明の鋼板は、自動車の外装板を中心に、優れた成形後表面品質を必要とする各種電気機器、自動車などの部品に対して好適に使用できる。 The steel sheet of the present invention can be suitably used for various electrical equipment and parts such as automobiles that require excellent post-molding surface quality, centering on automobile exterior panels.
Claims (2)
（Ｓ％）≧０．００８＋（０．８×（Ｃｕ％）−０．０１） 式（１）
ただし、（Ｔｉ％）、（Ｎ％）、（Ｓ％）、（Ｃ％）、（Ｃｕ％）は、それぞれＴｉ、Ｎ、Ｓ、Ｃ、Ｃｕの含有量（ｍａｓｓ％）を示す。 In mass%, C: 0.0005 to 0.0100%, Si: 0.10% or less, Mn: 0.05 to 0.50%, P: 0.030% or less, S: 0.008 to 0.00. 030%, Ti: 0.020 to 0.050%, Al: 0.010 to 0.080%, N: 0.0050% or less, Cu: 0.03% or less, and Ti * = (Ti %) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%), Ti * is contained in a range satisfying 0 <Ti * <0.02, and the balance and the slab, with respect to the slab, the heating temperature is below 1000 ° C. or higher 1200 ° C., the continuous casting of steel having the Ri consists of Fe and unavoidable impurities, the chemical composition satisfying formula (1) below, Heating in the temperature range of 1000 ° C or higher is performed under conditions of 4.0 hours or less, removing scale and rough rolling Then, after cooling so that the steel sheet surface temperature is in the range of (Ar3 transformation point−300 ° C.) or more and below the Ar3 transformation point, finishing is performed so that the steel sheet surface temperature at the end of finish rolling becomes the Ar3 transformation point or more. A method for producing an alloyed hot-dip galvanized steel sheet, which is rolled, wound at a temperature of 650 ° C. or higher, then pickled, cold-rolled, annealed, hot-dip galvanized and alloyed .
(S%) ≧ 0.008 + (0.8 × (Cu%) − 0.01) Formula (1)
However, (Ti%), (N%), (S%), (C%), and (Cu%) indicate the contents (mass%) of Ti, N, S, C, and Cu, respectively.
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