JP5870874B2 - Method for producing alloyed hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more - Google Patents
Method for producing alloyed hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more Download PDFInfo
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- 238000004519 manufacturing process Methods 0.000 title claims description 21
- 229910001335 Galvanized steel Inorganic materials 0.000 title claims description 10
- 239000008397 galvanized steel Substances 0.000 title claims description 10
- 229910000831 Steel Inorganic materials 0.000 claims description 67
- 239000010959 steel Substances 0.000 claims description 67
- 238000005275 alloying Methods 0.000 claims description 50
- 229910001566 austenite Inorganic materials 0.000 claims description 30
- 238000000137 annealing Methods 0.000 claims description 24
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 10
- 238000001816 cooling Methods 0.000 claims description 10
- 239000012535 impurity Substances 0.000 claims description 3
- 229910052742 iron Inorganic materials 0.000 claims description 3
- 238000010438 heat treatment Methods 0.000 description 16
- 229910000734 martensite Inorganic materials 0.000 description 7
- 238000000034 method Methods 0.000 description 7
- 238000007747 plating Methods 0.000 description 7
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- 238000007254 oxidation reaction Methods 0.000 description 2
- 241000219307 Atriplex rosea Species 0.000 description 1
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
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- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 1
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Description
本発明は、引張強度が９８０ＭＰａ以上である高強度合金化溶融亜鉛めっき鋼板の製造方法に関するものである。 The present invention relates to a method for producing a high-strength galvannealed steel sheet having a tensile strength of 980 MPa or more.
近年、地球環境保全の見地から、自動車の燃費向上が重要な課題となっている。これに伴い、車体材料の高強度化により薄肉化を図り、車体そのものを軽量化しようとする動きが活発となってきている。軽量化と共に車体防錆性の要求も高いために、高強度鋼板に溶融亜鉛めっきを施した高強度溶融亜鉛めっき鋼板のニーズが高まっている。 In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of global environmental conservation. Along with this, there is an active movement to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself. The demand for high-strength hot-dip galvanized steel sheets in which high-strength steel sheets have been hot-dip galvanized has been increasing because the demand for car body rust prevention is high as well as weight reduction.
鋼板の強度を高めるために、Ｓｉ、Ｍｎ、Ｃを高濃度に含有させることが行われるが、周知のようにＳｉ、Ｍｎ、Ｃを高濃度に含有させると溶融亜鉛めっき鋼板の製造時に種々の問題を生じる。例えば、Ｓｉ酸化物やＭｎ酸化物の表面濃化によりめっき性が低下するという問題がある。この問題に対しては、特許文献１に示されるように、焼鈍加熱時に前段で酸化を行い、後段で還元を行って表面にＦｅ酸化物の生成を促進させる方法や、特許文献２に示されるように、焼鈍時の露点を−４０℃以下にしてＳｉ、Ｍｎの酸化を抑制する方法が知られている。 In order to increase the strength of the steel sheet, Si, Mn, and C are contained at a high concentration. As is well known, when Si, Mn, and C are contained at a high concentration, various kinds of galvanized steel sheets are produced. Cause problems. For example, there is a problem that the plating property is lowered due to the surface concentration of Si oxide or Mn oxide. To solve this problem, as shown in Patent Document 1, a method of performing oxidation at the front stage during annealing and performing reduction at the rear stage to promote the formation of Fe oxide on the surface, or Patent Document 2 shows. Thus, a method is known in which the dew point during annealing is set to −40 ° C. or lower to suppress the oxidation of Si and Mn.
本発明者らは、Ｓｉ、Ｍｎ、Ｃを高濃度に含有した高強度合金化溶融亜鉛めっき鋼板の製造に関して、上述したようなめっき性の低下という問題以外に、新たな課題に直面した。それは合金化時に必要とする合金化度（めっき皮膜のＦｅ含有率）に達しないという問題である。通常、合金化溶融亜鉛めっき鋼板を製造する際の亜鉛めっきの合金化はＩＨ加熱により行う。その場合、合金化度の制御はライン速度とＩＨ出力により行うが、ＩＨ出力を最大にしても目標とする合金化度に達しない場合が生じた。また、目標とする合金化度に達する場合であっても、Ｓｉ、Ｍｎ、Ｃを高濃度に添加しない場合に比べて顕著に高いＩＨ出力にしなければならない場合や、ライン速度を顕著に低下させなければならない場合が発生し、生産コストの顕著な増大を招くという問題が生じた。 The present inventors faced a new problem in addition to the above-described problem of a decrease in plating properties regarding the production of a high-strength galvannealed steel sheet containing Si, Mn, and C at high concentrations. That is the problem that the degree of alloying (Fe content of the plating film) required at the time of alloying is not reached. Usually, alloying of galvanizing at the time of manufacturing an alloyed hot-dip galvanized steel sheet is performed by IH heating. In this case, the degree of alloying is controlled by the line speed and the IH output, but the target degree of alloying may not be reached even if the IH output is maximized. Even when the target degree of alloying is reached, when the IH output must be significantly higher than when Si, Mn, and C are not added at a high concentration, the line speed is significantly reduced. In some cases, the production cost has to be increased, resulting in a significant increase in production cost.
したがって本発明の目的は、以上のような課題を解決し、通常の合金化加熱条件やライン速度の下で、所定の合金化度を有する高強度合金化溶融亜鉛めっき鋼板を安定して製造することができる高強度合金化溶融亜鉛めっき鋼板の製造方法を提供することにある。 Therefore, the object of the present invention is to solve the above problems and stably produce a high-strength galvannealed steel sheet having a predetermined degree of alloying under normal alloying heating conditions and line speed. An object of the present invention is to provide a method for producing a high-strength galvannealed steel sheet that can be used.
上記課題を解決するための本発明の要旨は以下のとおりである。
［1］Ｃ：０．０５〜０．２５mass％、Ｓｉ：１．０〜３．０mass％、Ｍｎ：１．５〜３．５mass％、Ｐ：０．１mass％以下、Ｓ：０．０１mass％以下、Ａｌ：０．１mass％以下を含有し、残部が鉄及び不可避的不純物からなり、且つ下記（b）式で表される［Ｃ当量］が１．８以上である鋼板を焼鈍した後、溶融亜鉛めっきし、引き続き合金化処理を行う合金化溶融亜鉛めっき鋼板の製造方法であって、
下記（a）式で表される合金化処理時の鋼板のオーステナイト分率γ（vol％）を３０〜７０vol％とすることを特徴とする、引張強度が９８０ＭＰａ以上である合金化溶融亜鉛めっき鋼板の製造方法。
γ＝［−0.15Ｙ＋0.2（Ｔ−800）＋225［Ｃ当量］−374］/1.93 …（a）
但し Ｔ：鋼板の焼鈍温度（℃）
Ｙ：焼鈍後の鋼板の冷却速度（℃／sec）
［Ｃ当量］＝3.73×〔Ｃ％〕＋0.05×〔Ｓｉ％〕＋0.65×〔Ｍｎ％〕 …（b）
但し 〔Ｃ％〕: 鋼板のＣ含有量（mass％）
〔Ｓｉ％〕：鋼板のＳｉ含有量（mass％）
〔Ｍｎ％〕：鋼板のＭｎ含有量（mass％）
［2］上記［1］の製造方法において、鋼板が、さらに、Ｔｉ：０．０１〜０．１mass％を含有する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。
［3］上記［1］の製造方法において、鋼板が、さらに、Ｂ：０．０００３〜０．００５０mass％を含有し、（a）式で表される合金化処理時の鋼板のオーステナイト分率γ（vol％）を３０〜５０vol％とする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。
The gist of the present invention for solving the above problems is as follows.
[1] C: 0.05 to 0.25 mass%, Si: 1.0 to 3.0 mass%, Mn: 1.5 to 3.5 mass%, P: 0.1 mass% or less, S: 0.01 mass% Hereinafter, after annealing a steel sheet containing Al: 0.1 mass% or less, the balance being iron and inevitable impurities, and [C equivalent] represented by the following formula (b) being 1.8 or more, A method for producing an galvannealed steel sheet, which is galvanized and subsequently alloyed,
An alloyed hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more, wherein the austenite fraction γ (vol%) of the steel sheet during the alloying treatment represented by the following formula (a) is 30 to 70 vol% Manufacturing method.
γ = [− 0.15Y + 0.2 (T−800) +225 [C equivalent] −374] /1.93 (a)
T: annealing temperature of steel sheet (° C)
Y: Cooling rate of steel plate after annealing (° C / sec)
[C equivalent] = 3.73 × [C%] + 0.05 × [Si%] + 0.65 × [Mn%] (b)
However, [C%]: C content of steel sheet (mass%)
[Si%]: Si content of steel sheet (mass%)
[Mn%]: Mn content of steel sheet (mass%)
[2] The method for producing an alloyed hot-dip galvanized steel sheet, wherein the steel sheet further contains Ti: 0.01 to 0.1 mass % in the production method of [1].
[3] In the manufacturing method of [1], the steel sheet further contains B: 0.0003 to 0.0050 mass%, and the austenite fraction γ of the steel sheet during the alloying treatment represented by the formula (a) (Vol%) is 30-50 vol%, The manufacturing method of the galvannealed steel plate characterized by the above-mentioned.
本発明によれば、通常の合金化加熱条件やライン速度の下で、所定の合金化度を有する高強度合金化溶融亜鉛めっき鋼板を安定して製造することができる。 According to the present invention, a high-strength galvannealed steel sheet having a predetermined degree of alloying can be stably produced under normal alloying heating conditions and line speed.
本発明者らは、上述した課題、すなわち、合金化時に必要とする合金化度に達しないという問題の発生原因について検討を行い、その結果、以下のような事実を知見した。すなわち、高強度化のために鋼板中に高濃度に含有させたＭｎとＣが、焼鈍炉内での加熱・均熱過程でオーステナイト変態を起こし、その結果、溶融亜鉛めっき後の合金化加熱する際に未変態オーステナイトが残留した状態にある。そして、このオーステナイトは非磁性であるため、ＩＨ加熱での加熱発熱が少なくなり、目標とする合金化度に達しないことが判った。未変態オーステナイト量（体積分率）は、Ｍｎ、Ｃ量が多くなるほど増加する傾向が認められた。 The present inventors have studied the cause of the problem described above, that is, the problem of not reaching the degree of alloying required at the time of alloying, and as a result, have found the following facts. That is, Mn and C contained in a high concentration in the steel sheet for high strength cause austenite transformation in the heating and soaking process in the annealing furnace, and as a result, alloying heating is performed after hot dip galvanization. In this case, untransformed austenite remains. And since this austenite is nonmagnetic, it turned out that the heat_generation | fever heat_generation | fever by IH heating decreases and it does not reach the target degree of alloying. The untransformed austenite amount (volume fraction) tended to increase as the amount of Mn and C increased.
図１に、引張強度９８０ＭＰａと１１８０ＭＰａの合金化溶融亜鉛めっき鋼板の製造において、合金化処理のＩＨ加熱効率（＝［ＩＨ出力実績〔電流値〕／合金化に必要なＩＨ出力計算値〔電流値〕］×100）と、合金化処理する際の鋼板のオーステナイト分率γとの関係を示す。ここで、合金化溶融亜鉛めっき鋼板のマルテンサイト分率と合金化処理時のオーステナイト分率とは相関すると考えられる。すなわち、本発明が対象とする鋼板は高Ｍｎ成分であるため、合金化処理によるオーステナイトの分解（炭化物の生成）はきわめて生じにくい。そのため、合金化時のオーステナイト分率と最終組織のマルテンサイト分率はほぼ等しくなると考えられる。
ＩＨ加熱効率は高いことが望ましいが、５０％以上であれば実製造では許容範囲内である。ＩＨ加熱効率が５０％未満では合金化不良となりやすい。
FIG. 1 shows the IH heating efficiency of alloying treatment (= [IH output performance [current value] / calculated value of IH output required for alloying [current value]. ]] × 100) and the austenite fraction γ of the steel sheet during alloying. Here, it is considered that the martensite fraction of the galvannealed steel sheet correlates with the austenite fraction during the alloying treatment. That is, since the steel sheet which is the object of the present invention has a high Mn component, the decomposition of austenite (generation of carbides) due to alloying treatment is extremely difficult to occur. Therefore, it is considered that the austenite fraction at the time of alloying and the martensite fraction in the final structure are almost equal.
Although it is desirable that the IH heating efficiency be high, if it is 50% or more, it is within an allowable range in actual production. If the IH heating efficiency is less than 50%, alloying failure tends to occur.
ここで、合金化処理する際のオーステナイト分率γについて検討した結果、オーステナイト分率γは鋼板のＣ当量が高いほど、また焼鈍温度Ｔ（℃）が高いほど高くなり、焼鈍後の鋼板の冷却速度Ｙが速いほど低くなることが判った。これらの相関関係から、合金化処理時のオーステナイト分率γ（vol％）を求める下記（a）式を導出した。この（a）式の導出に当たっては、オーステナイト分率γとＣ当量、焼鈍温度Ｔ、冷却速度Ｙの関係（方向性）から関係式γ＝［aＹ＋b（Ｔ−800）＋c［Ｃ当量］−d］/eを想定し、実験結果からa，b，c，d，eを求めた。また、(Ｔ−800)と設定したのは、８００℃以上ではオーステナイトが生成しやすいためである。
γ＝［−0.15Ｙ＋0.2（Ｔ−800）＋225［Ｃ当量］−374］/1.93 …（a）
但し Ｔ：鋼板の焼鈍温度（℃）
Ｙ：焼鈍後の鋼板の冷却速度（℃／sec）
Here, as a result of examining the austenite fraction γ during the alloying treatment, the austenite fraction γ increases as the C equivalent of the steel sheet increases and the annealing temperature T (° C.) increases, and the steel sheet after annealing is cooled. It was found that the higher the speed Y, the lower. From these correlations, the following equation (a) for obtaining the austenite fraction γ (vol%) during the alloying treatment was derived. In deriving this equation (a), the relationship γ = [aY + b (T−800) + c [C equivalent] −d from the relationship (directionality) of the austenite fraction γ and C equivalent, annealing temperature T, and cooling rate Y. ], And a, b, c, d, and e were obtained from the experimental results. Moreover, (T-800) is set because austenite is easily generated at 800 ° C. or higher.
γ = [− 0.15Y + 0.2 (T−800) +225 [C equivalent] −374] /1.93 (a)
T: annealing temperature of steel sheet (° C)
Y: Cooling rate of steel plate after annealing (° C / sec)
なお、Ｃ当量は下記（b）式で表される。このＣ当量はマルテンサイトの生成指標となるものであり、本発明では、焼鈍中でのオーステナイト分率の確保の観点と実際の実験データとの整合性を考慮し、この当量式を用いる。
［Ｃ当量］＝3.73×〔Ｃ％〕＋0.05×〔Ｓｉ％〕＋0.65×〔Ｍｎ％〕 …（b）
但し 〔Ｃ％〕: 鋼板のＣ含有量（mass％）
〔Ｓｉ％〕：鋼板のＳｉ含有量（mass％）
〔Ｍｎ％〕：鋼板のＭｎ含有量（mass％）
In addition, C equivalent is represented by the following (b) formula. This C equivalent is a martensite formation index. In the present invention, this equivalent formula is used in consideration of the consistency between the austenite fraction during annealing and the actual experimental data.
[C equivalent] = 3.73 × [C%] + 0.05 × [Si%] + 0.65 × [Mn%] (b)
However, [C%]: C content of steel sheet (mass%)
[Si%]: Si content of steel sheet (mass%)
[Mn%]: Mn content of steel sheet (mass%)
合金化処理する際のオーステナイト分率γ（未変態オーステナイト分率）は、３０〜７０vol％とすることが必要である。図１に示されるように、合金化処理する際のオーステナイト分率γが７０vol％を超えると、合金化処理時のＩＨ加熱効率を５０％以上に維持できず、合金化度が不十分となりやすい。また、合金化処理する際のオーステナイト分率γを６０vol％以下にすれば、合金化処理時のＩＨ加熱効率を６０％以上に維持することができるので、より望ましい。さらに、合金化処理する際のオーステナイト分率γを５０vol％以下にすれば、合金化処理時のＩＨ加熱効率を７５％以上に維持できるので、特に望ましい。
一方、合金化処理する際のオーステナイト分率γが３０vol％未満では、十分な引張強度を確保できない場合がある。
The austenite fraction γ (untransformed austenite fraction) at the time of alloying treatment needs to be 30 to 70 vol%. As shown in FIG. 1, when the austenite fraction γ during the alloying process exceeds 70 vol%, the IH heating efficiency during the alloying process cannot be maintained at 50% or more, and the degree of alloying tends to be insufficient. . Moreover, if the austenite fraction γ during the alloying treatment is 60 vol% or less, the IH heating efficiency during the alloying treatment can be maintained at 60% or more, which is more desirable. Furthermore, if the austenite fraction γ during the alloying treatment is 50 vol% or less, the IH heating efficiency during the alloying treatment can be maintained at 75% or more, which is particularly desirable.
On the other hand, if the austenite fraction γ during alloying is less than 30 vol%, sufficient tensile strength may not be ensured.
本発明では、鋼板を焼鈍した後、溶融亜鉛めっきし、引き続き合金化処理（ＩＨ加熱による合金化）を行うことで合金化溶融亜鉛めっき鋼板を製造するが、鋼板の成分組成から［Ｃ当量］を事前に求めておき、上記（a）式に基づき、鋼板の焼鈍温度Ｔと焼鈍後の鋼板の冷却速度Ｙを調整することにより、合金化処理する際のオーステナイト分率γを上記の範囲に制御することができ、これにより合金化処理時のＩＨ加熱効率を所望のレベルに維持し、必要な合金化度を確保することができる。一般に、合金化溶融亜鉛めっき鋼板の合金化度は８〜１５％程度の範囲で目標とする値が設定される。 In the present invention, after annealing a steel sheet, hot dip galvanizing and subsequently alloying treatment (alloying by IH heating) is performed to produce an alloyed hot dip galvanized steel sheet. And by adjusting the annealing temperature T of the steel sheet and the cooling rate Y of the steel sheet after annealing based on the above formula (a), the austenite fraction γ during alloying treatment is within the above range. Thus, the IH heating efficiency during the alloying process can be maintained at a desired level, and a necessary degree of alloying can be ensured. Generally, the target value is set in the range of about 8 to 15% for the degree of alloying of the galvannealed steel sheet.
また、一般に、連続溶融亜鉛めっき設備における鋼板の焼鈍温度は７００〜
９００℃程度、焼鈍後の鋼板の冷却速度は３〜３０℃／sec程度であり、したがって、オーステナイト分率γを制御するための焼鈍温度Ｔと冷却速度Ｙの調整は、上記の範囲でなされることが好ましい。一般に、焼鈍後の鋼板の冷却は、ガスジェットによりなされるので、冷却速度はファンの風量により調整することができる。
Moreover, generally the annealing temperature of the steel plate in a continuous hot-dip galvanization equipment is 700-
The cooling rate of the steel sheet after annealing at about 900 ° C. is about 3 to 30 ° C./sec. Therefore, the adjustment of the annealing temperature T and the cooling rate Y for controlling the austenite fraction γ is made within the above range. It is preferable. Generally, cooling of the steel plate after annealing is performed by a gas jet, so that the cooling rate can be adjusted by the air volume of the fan.
次に、鋼板の成分組成について説明する。
・Ｃ：０．０５〜０．２５mass％
Ｃはオーステナイト生成元素であり、鋼の強化に不可欠な元素である。Ｃ量が０．０５mass％未満では、所望の強度確保が難しい。一方、Ｃを０．２５mass％を超えて過剰に添加すると、マルテンサイト相の面積率が大幅に増大し、加工性が低下する。よって、Ｃは０．０５〜０．２５mass％とし、好ましくは０．０５〜０．２０mass％とする。
Next, the component composition of the steel sheet will be described.
-C: 0.05-0.25 mass%
C is an austenite-forming element and is an essential element for strengthening steel. If the amount of C is less than 0.05 mass%, it is difficult to secure desired strength. On the other hand, when C is added excessively exceeding 0.25 mass%, the area ratio of the martensite phase increases significantly, and the workability decreases. Therefore, C is 0.05 to 0.25 mass%, preferably 0.05 to 0.20 mass%.
・Ｓｉ：１．０〜３．０mass％
Ｓｉはフェライト生成元素であり、また、固溶強化に有効な元素でもある。そして、強度と延性のバランスの改善及びフェライト地の強度確保のためには１．０mass％以上の添加が必要である。しかし、Ｓｉの過剰な添加は、赤スケール等の発生による表面性状の劣化や、めっき付着・密着性の劣化を引き起こす。よって、Ｓｉは１．０〜３．０mass％とし、好ましくは１．２〜２．０mass％とする。
・ Si: 1.0-3.0mass%
Si is a ferrite forming element and is also an element effective for solid solution strengthening. And in order to improve the balance between strength and ductility and secure the strength of the ferrite ground, it is necessary to add 1.0 mass% or more. However, excessive addition of Si causes deterioration of surface properties due to generation of red scale and the like, and deterioration of adhesion and adhesion of plating. Therefore, Si is set to 1.0 to 3.0 mass%, preferably 1.2 to 2.0 mass%.
・Ｍｎ：１．５〜３．５mass％
Ｍｎは、鋼の強化に有効な元素である。また、オーステナイトを安定化させる元素であり、第二相の分率調整に必要な元素である。このため、Ｍｎは１．５mass％以上の添加が必要である。一方、Ｍｎを３．５mass％を超えて過剰に添加すると、第二相中のマルテンサイト面積率が増加し、良好な加工性の確保が困難となる。また、近年Ｍｎの合金コストが高騰しているため、コストアップの要因にも繋がる。よって、Ｍｎは１．５〜３．５mass％とし、好ましくは２．２〜３．０mass％とする。
・Ｐ：０．１mass％以下
Ｐは、鋼の強化に有効な元素であるが、０．１mass％を超えて過剰に添加すると、粒界偏析により脆化を引き起こし、耐衝撃性を劣化させる。また、０．１mass％を超えると合金化速度を大幅に遅延させる。よって、Ｐは０．１mass％以下とする。
・ Mn: 1.5-3.5mass%
Mn is an element effective for strengthening steel. In addition, it is an element that stabilizes austenite, and is an element necessary for adjusting the fraction of the second phase. For this reason, Mn needs addition of 1.5 mass% or more. On the other hand, when Mn is added excessively exceeding 3.5 mass%, the martensite area ratio in the second phase increases, and it becomes difficult to ensure good workability. Moreover, since the alloy cost of Mn has soared in recent years, it also leads to a cost increase factor. Therefore, Mn is 1.5 to 3.5 mass%, preferably 2.2 to 3.0 mass%.
-P: 0.1 mass% or less P is an element effective for strengthening steel, but if added in excess of 0.1 mass%, it causes embrittlement due to segregation at the grain boundaries and deteriorates impact resistance. Moreover, if it exceeds 0.1 mass%, the alloying speed is significantly delayed. Therefore, P is set to 0.1 mass% or less.
・Ｓ：０．０１mass％以下
Ｓは、ＭｎＳなどの介在物となって、耐衝撃性の劣化や溶接部のメタルフローに沿った割れの原因となるので極力低い方がよいが、製造コストの面からＳは０．０１mass％以下とする。
・Ａｌ：０．１mass％以下
Ａｌは、フェライト生成元素であり、製造時におけるフェライト生成量をコントロールするのに有効な元素である。しかし、Ａｌの過剰な添加は製鋼時におけるスラブ品質を劣化させる。よって、Ａｌは０．１mass％以下とする。
・ S: 0.01 mass% or less S is an inclusion such as MnS, and it is better to be as low as possible because it causes degradation of impact resistance and cracks along the metal flow of the weld. From the surface, S is set to 0.01 mass% or less.
-Al: 0.1 mass% or less Al is a ferrite forming element, and is an effective element for controlling the amount of ferrite generated during production. However, excessive addition of Al deteriorates the slab quality during steelmaking. Therefore, Al is 0.1 mass% or less.
残部はＦｅ及び不可避的不純物である。但し、上記成分元素に加えて、以下の元素のうちの１種以上を必要に応じて添加することができる。
・Ｔｉ：０．０１〜０．１mass％
Ｔｉは鋼の析出強化に有効で、その効果は０．０１mass％以上の添加量で得られる。しかし、添加量が０．１mass％を超えると加工性及び形状凍結性が低下する。また、コストアップの要因にもなる。したがって、Ｔｉを添加する場合には、その添加量を０．０１〜０．１mass％とすることが好ましい。
The balance is Fe and inevitable impurities. However, in addition to the above component elements, one or more of the following elements can be added as necessary.
・ Ti: 0.01-0.1mass%
Ti is effective for precipitation strengthening of steel, and the effect is obtained with an addition amount of 0.01 mass% or more. However, when the addition amount exceeds 0.1 mass%, the workability and the shape freezing property decrease. In addition, the cost increases. Therefore, when adding Ti, it is preferable that the addition amount shall be 0.01-0.1 mass%.
・Ｂ：０．０００３〜０．００５０mass％
Ｂはオーステナイト粒界からのフェライトの生成・成長を抑制する作用を有するので、必要に応じて添加することができる。その効果は、０．０００３mass％以上の添加量で得られる。しかし、添加量が０．００５０mass％を超えると加工性が低下する。また、コストアップの要因にもなる。したがって、Ｂを添加する場合には、その添加量を０．０００３〜０．００５０mass％とすることが好ましい。
・ B: 0.0003 to 0.0050 mass%
B has the effect of suppressing the formation and growth of ferrite from the austenite grain boundaries, and can be added as necessary. The effect is acquired with the addition amount of 0.0003 mass% or more. However, if the addition amount exceeds 0.0050 mass%, the workability decreases. In addition, the cost increases. Therefore, when adding B, it is preferable to make the addition amount into 0.0003-0.0050 mass%.
また、鋼板のＣ当量は１．８以上とする。Ｃ当量は上記（b）式で表される。鋼板のＣ当量が１．８未満では、引張強度９８０ＭＰａ以上の高強度を確保することが難しい。
本発明で製造される合金化溶融亜鉛めっき鋼板の引張強度は９８０ＭＰａ以上である。
The C equivalent of the steel sheet is 1.8 or more. C equivalent is represented by the above formula (b). If the C equivalent of the steel sheet is less than 1.8, it is difficult to ensure a high strength with a tensile strength of 980 MPa or more.
The tensile strength of the galvannealed steel sheet produced by the present invention is 980 MPa or more.
連続溶融亜鉛めっき設備において、表１に示す成分組成を有し、板厚が１．０〜１．４ｍｍ、引張強度が１３００ＭＰａの鋼板に対して加熱、焼鈍、冷却、溶融亜鉛めっき、合金化処理を順次施し、合金化溶融亜鉛めっき鋼板を製造した。溶融亜鉛めっきは、めっき浴温：４８０℃、めっき浴中Ａｌ濃度：０．１２mass％の条件で行った。その後、ＩＨ加熱により合金化処理を行った。 In a continuous hot dip galvanizing facility, a steel plate having the composition shown in Table 1, a plate thickness of 1.0 to 1.4 mm, and a tensile strength of 1300 MPa is heated, annealed, cooled, hot dip galvanized, and alloyed. Were sequentially applied to produce a galvannealed steel sheet. Hot dip galvanization was performed under the conditions of a plating bath temperature of 480 ° C. and an Al concentration in the plating bath of 0.12 mass%. Thereafter, an alloying treatment was performed by IH heating.
鋼板の成分組成からＣ当量を事前に求め、上記（a）式に従って、鋼板の焼鈍温度Ｔと焼鈍後の鋼板の冷却速度Ｙを調整することで、合金化処理時のオーステナイト分率γを制御した。得られた合金化溶融亜鉛めっき鋼板のめっき皮膜を酸で剥離し、剥離液中の鉄と亜鉛を定量して皮膜中のＦｅ質量％を求め、合金化度とした。また、得られた合金化溶融亜鉛めっき鋼板のマルテンサイト分率は、鋼板の圧延方向に平行な板厚断面（Ｌ断面）を研磨後、３％ナイタールで腐食し、板厚１／４位置（鋼板表面から深さ方向で板厚の１／４に相当する位置）についてＳＥＭ（走査型電子顕微鏡）を用いて２０００倍の倍率で１０視野観察し、得られた組織画像をMedia Cybernetics社の「Image-Pro」を用いて解析して、マルテンサイトの分率を１０視野分算出し、得られた各値を平均することにより求めた。それらの結果を、製造条件などとともに表２に示す。 The C equivalent is obtained in advance from the component composition of the steel sheet, and the austenite fraction γ during the alloying process is controlled by adjusting the annealing temperature T of the steel sheet and the cooling rate Y of the steel sheet after annealing according to the above formula (a). did. The plating film of the obtained galvannealed steel sheet was peeled with an acid, and the amount of Fe in the film was determined by quantifying iron and zinc in the stripping solution, and the degree of alloying was obtained. In addition, the martensite fraction of the obtained galvannealed steel sheet was obtained by corroding a plate thickness section (L section) parallel to the rolling direction of the steel sheet, corroding with 3% nital, and a thickness of 1/4 position ( 10 positions at a magnification of 2000 times were observed using a scanning electron microscope (SEM) at a position corresponding to ¼ of the plate thickness in the depth direction from the surface of the steel plate, and the resulting tissue image was “Media Cybernetics” Analysis was performed using “Image-Pro”, the martensite fraction was calculated for 10 visual fields, and the obtained values were averaged. The results are shown in Table 2 together with the production conditions.
Claims (3)
下記（a）式で表される合金化処理時の鋼板のオーステナイト分率γ（vol％）を３０〜７０vol％とすることを特徴とする、引張強度が９８０ＭＰａ以上である合金化溶融亜鉛めっき鋼板の製造方法。
γ＝［−0.15Ｙ＋0.2（Ｔ−800）＋225［Ｃ当量］−374］/1.93 …（a）
但し Ｔ：鋼板の焼鈍温度（℃）
Ｙ：焼鈍後の鋼板の冷却速度（℃／sec）
［Ｃ当量］＝3.73×〔Ｃ％〕＋0.05×〔Ｓｉ％〕＋0.65×〔Ｍｎ％〕 …（b）
但し 〔Ｃ％〕: 鋼板のＣ含有量（mass％）
〔Ｓｉ％〕：鋼板のＳｉ含有量（mass％）
〔Ｍｎ％〕：鋼板のＭｎ含有量（mass％） C: 0.05-0.25 mass%, Si: 1.0-3.0 mass%, Mn: 1.5-3.5 mass%, P: 0.1 mass% or less, S: 0.01 mass% or less, Al : After hot-dip galvanization after annealing a steel sheet containing 0.1 mass% or less, the balance being iron and inevitable impurities, and [C equivalent] represented by the following formula (b) being 1.8 or more And a method for producing an alloyed hot-dip galvanized steel sheet that is subsequently subjected to alloying treatment,
An alloyed hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more, wherein the austenite fraction γ (vol%) of the steel sheet during the alloying treatment represented by the following formula (a) is 30 to 70 vol% Manufacturing method.
γ = [− 0.15Y + 0.2 (T−800) +225 [C equivalent] −374] /1.93 (a)
T: annealing temperature of steel sheet (° C)
Y: Cooling rate of steel plate after annealing (° C / sec)
[C equivalent] = 3.73 × [C%] + 0.05 × [Si%] + 0.65 × [Mn%] (b)
However, [C%]: C content of steel sheet (mass%)
[Si%]: Si content of steel sheet (mass%)
[Mn%]: Mn content of steel sheet (mass%)
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