JP5483916B2 - High-strength galvannealed steel sheet with excellent bendability - Google Patents
High-strength galvannealed steel sheet with excellent bendability Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 42
- 239000010959 steel Substances 0.000 title claims description 42
- 229910000734 martensite Inorganic materials 0.000 claims description 40
- 229910000859 α-Fe Inorganic materials 0.000 claims description 20
- 239000002245 particle Substances 0.000 claims description 15
- 229910052758 niobium Inorganic materials 0.000 claims description 14
- 239000000203 mixture Substances 0.000 claims description 12
- 229910001563 bainite Inorganic materials 0.000 claims description 11
- 229910052719 titanium Inorganic materials 0.000 claims description 11
- 238000005096 rolling process Methods 0.000 claims description 10
- 229910052799 carbon Inorganic materials 0.000 claims description 8
- 239000002184 metal Substances 0.000 claims description 6
- 229910052751 metal Inorganic materials 0.000 claims description 6
- 239000012535 impurity Substances 0.000 claims description 5
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 229910052750 molybdenum Inorganic materials 0.000 claims description 3
- 229910052698 phosphorus Inorganic materials 0.000 claims description 3
- 229910052720 vanadium Inorganic materials 0.000 claims description 3
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 claims 1
- 229910052725 zinc Inorganic materials 0.000 claims 1
- 239000011701 zinc Substances 0.000 claims 1
- 238000005452 bending Methods 0.000 description 16
- 238000010438 heat treatment Methods 0.000 description 12
- 238000001816 cooling Methods 0.000 description 11
- 238000005246 galvanizing Methods 0.000 description 7
- 238000005275 alloying Methods 0.000 description 6
- 238000012360 testing method Methods 0.000 description 6
- 229910001566 austenite Inorganic materials 0.000 description 5
- 238000000034 method Methods 0.000 description 5
- 238000007747 plating Methods 0.000 description 5
- 229910001335 Galvanized steel Inorganic materials 0.000 description 4
- 230000000052 comparative effect Effects 0.000 description 4
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- 230000006872 improvement Effects 0.000 description 4
- 238000000137 annealing Methods 0.000 description 3
- 239000002131 composite material Substances 0.000 description 3
- 238000007598 dipping method Methods 0.000 description 3
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- 150000001247 metal acetylides Chemical class 0.000 description 3
- 229910001562 pearlite Inorganic materials 0.000 description 3
- 238000005554 pickling Methods 0.000 description 3
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- 238000007670 refining Methods 0.000 description 3
- 230000000717 retained effect Effects 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
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- 229910045601 alloy Inorganic materials 0.000 description 2
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- 230000015572 biosynthetic process Effects 0.000 description 2
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- 238000005098 hot rolling Methods 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 238000001953 recrystallisation Methods 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 229920006395 saturated elastomer Polymers 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- -1 MnS Chemical compound 0.000 description 1
- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 239000004566 building material Substances 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 239000003153 chemical reaction reagent Substances 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 238000005097 cold rolling Methods 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000000593 degrading effect Effects 0.000 description 1
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- 238000010586 diagram Methods 0.000 description 1
- 230000009977 dual effect Effects 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
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- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 238000010791 quenching Methods 0.000 description 1
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- 238000003303 reheating Methods 0.000 description 1
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
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- Coating With Molten Metal (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Description
本発明は、自動車，建築，電気機器等の部材として有用な高強度鋼板、特に曲げ加工性に優れた高強度の合金化溶融亜鉛めっき鋼板に関する。 The present invention relates to a high-strength steel sheet useful as a member for automobiles, buildings, electrical equipment, and the like, and particularly to a high-strength galvannealed steel sheet having excellent bending workability.
合金化溶融亜鉛めっき鋼板は、耐食性，塗装性，塗装後密着性，溶接性に優れていることから、自動車用車体，家電製品を始めとする種々の分野で防錆鋼板として汎用されている。このような用途では、必要形状に成形加工して使用されることから，耐食性に加えて加工性に優れていることも重要である。
加えて、特に、自動車用の分野では、衝突安全性の向上や軽量化による燃費向上の観点から高強度化が求められており、７８０ＭＰａ以上の高強度鋼板も使用され始めている。また、建築分野にあっても、コスト削減の観点から、薄肉高強度化が求められている。
Alloyed hot-dip galvanized steel sheets are widely used as rust-proof steel sheets in various fields including automobile bodies and home appliances because they are excellent in corrosion resistance, paintability, adhesion after coating, and weldability. In such applications, it is important to have excellent workability in addition to corrosion resistance because it is molded into the required shape.
In addition, particularly in the field of automobiles, high strength is demanded from the viewpoint of improving collision safety and improving fuel efficiency by reducing weight, and high strength steel plates of 780 MPa or more are also being used. Even in the construction field, from the viewpoint of cost reduction, there is a demand for thin and high strength.
鋼板においては、一般的に、高強度化するにつれて加工性が低下する傾向にある。このため、高強度鋼板の適用範囲を拡大するにあたっては、加工性が良好な高強度鋼板を製造することが必要である。
例えば、質量％で、Ｃ：０.０５〜０.２０％、Ｓｉ：０.３〜１.８％、Ｍｎ：１.０〜３.０％を含み、残部Ｆｅおよび不可避的不純物からなる組成を有する鋼板に、（Ａｃ3変態点−８０℃）〜（Ａｃ3変態点＋１００℃）の温度域で５ｓｅｃ以上保持する一次加熱処理後、Ｍｓ点以下の温度まで５℃／ｓ以上１０℃／ｓ未満で冷却する一次工程と、２相域での二次加熱処理後、５００℃以下の温度まで急冷する二次工程と、さらに溶融亜鉛めっき処理を施し、３００℃まで急冷する三次工程とを順次施し、６０％以上のフェライトと、５％以上のマルテンサイトと、２％以上の残留オーステナイトを含み、フェライトの平均結晶粒径が５μｍ以上の複合組織とすることが提案されている（特許文献１）。
In steel plates, generally, workability tends to decrease as the strength increases. For this reason, when expanding the application range of a high strength steel plate, it is necessary to manufacture a high strength steel plate with good workability.
For example, a composition comprising, in mass%, C: 0.05-0.20%, Si: 0.3-1.8%, Mn: 1.0-3.0%, the balance being Fe and inevitable impurities After the primary heat treatment in which the steel sheet is held at a temperature range of (Ac 3 transformation point−80 ° C.) to (Ac 3 transformation point + 100 ° C.) for 5 seconds or more, the temperature is 5 ° C./s or more to 10 ° C./s to the temperature below Ms point. a primary step of cooling at less than s, a secondary step of quenching to a temperature of 500 ° C. or lower after the secondary heat treatment in the two-phase region, and a tertiary step of further cooling to 300 ° C. by performing hot dip galvanizing treatment It has been proposed to sequentially apply a composite structure containing 60% or more of ferrite, 5% or more of martensite, and 2% or more of retained austenite and having an average crystal grain size of ferrite of 5 μm or more (Patent Document). 1).
また、Ｃ：０.０４〜０.２５質量％，Ｓｉ：０.２〜２.０質量％，Ｍｎ：０.５〜３.０質量％を含む鋼板に、付着量３〜１５ｇ／ｍ２のＦｅ系めっき層を形成した後、ガス還元焼鈍し、４２０℃以上４９０℃未満の溶融亜鉛めっき浴に浸漬して溶融亜鉛めっきを施し、再加熱なしに、又は溶融めっき後に５３０℃未満の合金化熱処理を施して鋼板表面に合金化溶融亜鉛めっき層を形成することも提案されている（特許文献２）。 Moreover, the adhesion amount is 3 to 15 g / m 2 on a steel plate containing C: 0.04 to 0.25 mass%, Si: 0.2 to 2.0 mass%, and Mn: 0.5 to 3.0 mass%. After forming the Fe-based plating layer, gas reduction annealing, dipping in a hot dip galvanizing bath at 420 ° C. or higher and lower than 490 ° C. to perform hot dip galvanizing, and an alloy having less than 530 ° C. without reheating or after hot dip plating It has also been proposed to form an alloyed hot-dip galvanized layer on the steel sheet surface by performing a heat treatment (Patent Document 2).
前記特許文献１で提案された鋼板は、残留オーステナイトを２％以上残させることによって延性を向上させている。また、特許文献２の方法で製造される鋼板も、残留オーステナイト量が３％以上となって優れた延性を呈している。
ところで、７８０ＭＰａを超えるような高強度鋼板は、曲げ加工に供される場合が多く、加工性の中でも曲げ性に優れることが要求される場合が多い。しかし、前記特許文献１，２で提供される鋼板は延性に優れているため、絞り加工等のプレス成形により成形される部材には好適に用いられるが、曲げ加工性に優れているとは言えないので、単に曲げ加工して用いられる部材としては好適ではない。
The steel sheet proposed in Patent Document 1 has improved ductility by leaving 2% or more of retained austenite. Moreover, the steel plate manufactured by the method of Patent Document 2 also exhibits excellent ductility with a retained austenite amount of 3% or more.
By the way, a high-strength steel plate exceeding 780 MPa is often subjected to bending work, and it is often required to have excellent bendability among workability. However, since the steel sheets provided in Patent Documents 1 and 2 are excellent in ductility, they are preferably used for members formed by press forming such as drawing, but they are excellent in bending workability. Therefore, it is not suitable as a member that is simply bent and used.
本発明は、このような問題を解消すべく案出されたものであり、７８０ＭＰａ以上の引張強さを呈する合金化溶融亜鉛めっき鋼板であっても、組織を細かく調整することにより曲げ性に優れた高強度合金化溶融亜鉛めっき鋼板を提供することを目的とする。 The present invention has been devised to solve such a problem, and even in an alloyed hot-dip galvanized steel sheet exhibiting a tensile strength of 780 MPa or more, it is excellent in bendability by finely adjusting the structure. Another object of the present invention is to provide a high-strength galvannealed steel sheet.
本発明の曲げ性に優れた高強度合金化溶融亜鉛めっき鋼板は、その目的を達成するため、Ｃ：０.０５〜０.１３質量％、Ｓｉ：０.３〜０.８質量％、Ｍｎ：１.５〜２.３質量％、Ｐ：０.０３質量％以下、Ｓ：０.０１質量％以下、Ｂ：０.０００５〜０.００５質量％、Ｔｉ：０.０５〜０.２０質量％、Ｎｂ：０.０１〜０.１０質量％を含み、かつＴｉとＮｂ、Ｃ量が下記（１）式を満足し、残部がＦｅおよび不可避的不純物からなる成分組成と、主相としてのフェライトと副相としてマルテンサイトまたはマルテンサイトとベイナイトからなり、しかも、マルテンサイトまたはマルテンサイトとベイナイトの面積率が１５％以上４５％未満、前記フェライトが５.０μｍ以下の平均粒径を、前記マルテンサイトが３.０μｍ以下の平均粒径と０.７以上の平均アスペクト比を有する金属組織を備え、圧延方向板厚断面の、板厚１／４線上の硬さの最大値と最小値の差を硬さ変動としたとき、４０Ｈｖ以下のビッカース硬さの変動、および７８０ＭＰａ以上の引張強さを呈することを特徴とする。
｛Ｔｉ%＋（Ｎｂ%／２）｝／Ｃ% ＞ １．２ ・・・（１）
なお（１）式で、Ｔｉ％、Ｎｂ％、Ｃ％は、それぞれの成分の質量％を示す。
The high-strength alloyed hot-dip galvanized steel sheet with excellent bendability according to the present invention has the following objectives: C: 0.05 to 0.13 mass%, Si: 0.3 to 0.8 mass%, Mn : 1.5-2.3 mass%, P: 0.03 mass% or less, S: 0.01 mass% or less, B: 0.0005-0.005 mass%, Ti: 0.05-0.20 Ingredient composition comprising N%: Nb: 0.01-0.10 mass%, Ti, Nb, and C content satisfying the following formula (1), the balance being Fe and inevitable impurities, and the main phase The martensite or martensite and bainite as a secondary phase, and the area ratio of martensite or martensite and bainite is 15% or more and less than 45%, and the ferrite has an average particle size of 5.0 μm or less. Martensite has an average particle size of 3.0 μm or less and 0.7 or more Comprising a metal structure having an average aspect ratio, the rolling direction thickness section, when the hardness change the difference in hardness maximum and minimum values of a thickness of 1/4 line, variation of the following Vickers hardness 40Hv And a tensile strength of 780 MPa or more.
{Ti% + (Nb% / 2)} / C%> 1.2 (1)
In the formula (1), Ti%, Nb%, and C% indicate mass% of each component.
成分組成としては、さらにＣｒ：０．０１〜１.０質量％、Ｍｏ：０.０１〜１.０質量％、Ｖ：０.０１〜０.５質量％の１種または２種以上を含むものであってもよい。
前記主相としてのフェライトが５５％以上８５％未満、第２相としてのとしてマルテンサイトまたはマルテンサイトとベイナイトが１５％以上４５％未満の割合で占めているものが好ましい。
The component composition further includes one or more of Cr: 0.01 to 1.0% by mass, Mo: 0.01 to 1.0% by mass, and V: 0.01 to 0.5% by mass. It may be a thing.
It is preferable that the ferrite as the main phase is 55% or more and less than 85%, and the martensite or martensite and bainite as the second phase accounts for 15% or more and less than 45%.
本発明によれば、成分組成および金属組織を細かく設定したことにより、７８０ＭＰａ以上の引張強さを呈する高強度合金化溶融亜鉛めっき鋼板であっても、ビッカース硬さの変動を小さくすることができ、その結果、曲げ性に優れた鋼板となっている。
したがって、本発明により、ピラー、ロッカー、メンバー等、特性の優れた部品が、簡便な曲げ加工を施すことにより低コストで提供できることになる。
According to the present invention, since the component composition and the metal structure are set finely, even a high-strength galvannealed steel sheet exhibiting a tensile strength of 780 MPa or more can reduce fluctuations in Vickers hardness. As a result, the steel sheet has excellent bendability.
Therefore, according to the present invention, parts having excellent characteristics such as pillars, lockers, members, and the like can be provided at low cost by performing simple bending.
７８０ＭＰａを超えるような高強度を得るには、主相フェライトに副相マルテンサイトを配したDual Phase組織を有する、いわゆるＤＰ鋼板とすることが有効である。しかしながら、ＤＰ鋼板は一般的には曲げ性が悪いために、曲げ加工が施されて用いられる部材への適用が進んでいない。
そこで、本発明者らは、ＤＰ鋼板の自動車分野や建材分野への利用拡大のために、曲げ加工性向上策について鋭意検討を重ね、本発明に到達した。
以下に、その詳細を説明する。
In order to obtain a high strength exceeding 780 MPa, it is effective to use a so-called DP steel sheet having a Dual Phase structure in which subphase martensite is arranged in the main phase ferrite. However, since a DP steel sheet generally has poor bendability, application to a member used after being subjected to bending has not progressed.
Accordingly, the present inventors have made extensive studies on measures for improving the bending workability in order to expand the use of DP steel sheets in the automobile field and building materials field, and have reached the present invention.
The details will be described below.
ＤＰ鋼板は、主相フェライトに副相としてマルテンサイトが分散した複合組織を有する鋼板であり、軟質なフェライトを有するが故に延性に優れている。そして、マルテンサイトが材料の強度を高めている。しかしながら、フェライトとマルテンサイトの硬度差が大きいために変形能に差異があり、一方向への変形を行おうとすると２相の境界部に亀裂が発生するため、その結果として曲げ加工性が劣ることになる。
そこで、本発明では、曲げ加工性向上策として、組織の均一・微細化により２相の境界部での亀裂発生を抑制するために、フェライトおよびマルテンサイト粒径を細かく、かつマルテンサイトの形状を等軸状にすることにした。
The DP steel sheet is a steel sheet having a composite structure in which martensite is dispersed as a subphase in the main phase ferrite, and is excellent in ductility because it has soft ferrite. And martensite increases the strength of the material. However, there is a difference in deformability due to the large hardness difference between ferrite and martensite, and cracking occurs at the boundary between the two phases when deforming in one direction, resulting in poor bending workability. become.
Therefore, in the present invention, as a measure for improving the bending workability, the ferrite and martensite grain sizes are made fine and the shape of the martensite is reduced in order to suppress the occurrence of cracks at the boundary between the two phases by making the structure uniform and fine. I decided to make it equiaxed.
具体的には、ＤＰ鋼板を構成する副相としてマルテンサイトを、３.０μｍ以下の平均粒径と０.７以上、好ましくは０.８〜１.０の平均アスペクト比を有するものとした。
鋼板全体としては、Ｃ：０.０５〜０.１３質量％、Ｓｉ：０.３〜０.８質量％、Ｍｎ：１.５〜２.３質量％、Ｐ：０.０３質量％以下、Ｓ：０.０１質量％以下、Ｂ：０.０００５〜０.００５質量％、Ｔｉ：０.０５〜０.２０質量％、Ｎｂ：０.０１〜０.１０質量％、さらに必要に応じてＣｒ：０.０１〜１.０質量％、Ｍｏ：０.０１〜１.０質量％、Ｖ：０.０１〜０.５質量％の１種または２種以上を含み、かつＴｉとＮｂ、Ｃ量が下記（１）式を満足し、残部がＦｅおよび不可避的不純物からなる成分組成を有するＤＰ鋼板であって、平均粒径が５μｍ以下である主相フェライト中に、副相として分散しているマルテンサイトを３.０μｍ以下の平均粒径と０.７以上、好ましくは０.８〜１.０の平均アスペクト比を有するものとした。
｛Ｔｉ%＋（Ｎｂ%／２）｝／Ｃ% ＞ １．２ ・・・（１）
なお（１）式で、Ｔｉ％、Ｎｂ％、Ｃ％は、それぞれの成分の質量％を示す。
このような、成分組成、金属組織とすることにより、ビッカースの硬さ変動が４０Ｈｖ以下の均一な組織とすることができ、曲げ性に優れた高強度合金化溶融亜鉛めっき鋼板が得られる。
Specifically, martensite as an auxiliary phase constituting the DP steel sheet has an average particle size of 3.0 μm or less and an average aspect ratio of 0.7 or more, preferably 0.8 to 1.0.
As a whole steel plate, C: 0.05-0.13 mass%, Si: 0.3-0.8 mass%, Mn: 1.5-2.3 mass%, P: 0.03 mass% or less, S: 0.01 mass% or less, B: 0.0005-0.005 mass%, Ti: 0.05-0.20 mass%, Nb: 0.01-0.10 mass%, and further as needed Cr: 0.01-1.0% by mass, Mo: 0.01-1.0% by mass, V: 0.01-0.5% by mass, including one or more, and Ti and Nb, A DP steel sheet having a composition in which the amount of C satisfies the following formula (1) and the balance is composed of Fe and inevitable impurities, and is dispersed as a secondary phase in the main phase ferrite having an average particle size of 5 μm or less. The martensite having an average particle size of 3.0 μm or less and an average aspect ratio of 0.7 or more, preferably 0.8 to 1.0.
{Ti% + (Nb% / 2)} / C%> 1.2 (1)
In the formula (1), Ti%, Nb%, and C% indicate mass% of each component.
By setting it as such a component composition and a metal structure, it can be set as the uniform structure | tissue whose Vickers hardness fluctuation | variation is 40 Hv or less, and the high-strength galvannealed steel plate excellent in bendability is obtained.
本発明鋼板を構成する各要件の限定理由について説明する。まず、成分組成から説明する。なお、以下、組成における質量％は単に％と記す。
Ｃ：０.０５〜０.１３％
Ｃは鋼板の高強度化に必要不可欠な元素である。含有量が０.０５％未満では、７８０ＭＰａ以上の引張強度を得るのが困難である。ただし、０.１３％を超える添加は、組織の不均一性が顕著となり、曲げ性が劣化する。そのため、Ｃは０.０５〜０.１３％の範囲とする。
The reason for limitation of each requirement which comprises this invention steel plate is demonstrated. First, the component composition will be described. Hereinafter, mass% in the composition is simply referred to as%.
C: 0.05-0.13%
C is an essential element for increasing the strength of steel sheets. If the content is less than 0.05%, it is difficult to obtain a tensile strength of 780 MPa or more. However, when the content exceeds 0.13%, the structure non-uniformity becomes remarkable and the bendability deteriorates. Therefore, C is set in the range of 0.05 to 0.13%.
Ｓｉ：０.３〜０.８％
Ｓｉは曲げ性をあまり劣化させることなく、強度向上に寄与する元素であり、本発明では０.３％以上のSi添加が必要となる。しかし、過剰に添加すると、めっきラインでの加熱時に酸化物を形成し、めっき性を劣化させるため、Ｓｉ量は０.３〜０.８％とする。
Si: 0.3 to 0.8%
Si is an element that contributes to strength improvement without significantly degrading bendability. In the present invention, Si addition of 0.3% or more is required. However, if added excessively, an oxide is formed during heating in the plating line, and the plating property is deteriorated, so the Si amount is set to 0.3 to 0.8%.
Ｍｎ：１.５〜２.３％
Ｍｎはオーステナイトを安定化させるとともに、加熱後の冷却時にパーライトが生成するのを抑制することで、マルテンサイトの生成に寄与する。含有量が１.５％未満では、７８０ＭＰａ以上の高強度を得るために必要なマルテンサイト量が確保できない。ただし、２.３％を超えるとバンド組織が顕著となり、不均一な組織となるため、曲げ性が劣化する。そのため、Ｍｎは１.５〜２.３％の範囲とする。
Mn: 1.5 to 2.3%
Mn stabilizes austenite and contributes to the formation of martensite by suppressing the formation of pearlite during cooling after heating. If the content is less than 1.5%, the amount of martensite necessary to obtain a high strength of 780 MPa or more cannot be ensured. However, if it exceeds 2.3%, the band structure becomes prominent and the structure becomes non-uniform, so that the bendability deteriorates. Therefore, Mn is set to a range of 1.5 to 2.3%.
Ｐ：０.０３％以下
Ｐは不可避的不純物元素であるが、過剰にＰが含まれると溶接性等が劣化するため、０.０３％以下とする。
P: 0.03% or less P is an inevitable impurity element, but if P is excessively contained, weldability and the like deteriorate, so the content is made 0.03% or less.
Ｓ：０.０１％以下
ＳはＭｎＳ等の硫化物として存在し、多量に存在すると曲げ性が劣化する。そのため、Ｓ量は出来るだけ低い方が望ましいが、０.０１％以下であれば、曲げ性に及ぼす影響は小さい。
S: 0.01% or less S exists as a sulfide such as MnS, and if it is present in a large amount, the bendability deteriorates. For this reason, the S content is preferably as low as possible, but if it is 0.01% or less, the effect on bendability is small.
Ｂ：０.０００５〜０.００５％
Ｂは微量添加で強度向上に寄与し、かつ曲げ性の劣化も小さい元素である。本発明では、少なくとも０.０００５％以上のＢ添加が必要である。ただし、０.００５％を超える添加は、ホウ化物が生成し、曲げ性が劣化するため、０.０００５〜０.００５％とする。
B: 0.0005-0.005%
B is an element that contributes to strength improvement by addition of a small amount and has little deterioration in bending property. In the present invention, at least 0.0005% or more of B must be added. However, addition exceeding 0.005% generates boride and deteriorates bendability, so 0.0005 to 0.005%.
Ｔｉ：０.０５〜０.２０％
Ｔｉは組織の微細化によって組織の均一性を向上させるとともに、炭化物の析出強化により、曲げ性を劣化させることなく、強度向上に寄与する元素である。組織微細化により、曲げ性を向上させるには、０.０５％以上の添加が必要である。ただし、０.２０％を超えて添加しても強度上昇の効果が飽和するだけでなく、コスト上昇にもつながるため、０.０８〜０.２０％とする。
Ti: 0.05-0.20%
Ti is an element that improves the uniformity of the structure by refining the structure and contributes to the improvement of strength without deteriorating the bendability by precipitation strengthening of carbides. In order to improve bendability by refining the structure, addition of 0.05% or more is necessary. However, even if added over 0.20%, not only the effect of increasing the strength is saturated, but also the cost is increased, so 0.08 to 0.20%.
Ｎｂ：０.０１〜０.１０％
ＮｂもＴｉと同様に、組織の微細化によって組織の均一性を向上させるとともに、炭化物の析出強化により、曲げ性を劣化させることなく、強度向上に寄与する元素である。その機能を発現させるには、少なくとも０.０１％の含有を必要とする。ただし、０.１０％を超えて添加しても強度上昇の効果が飽和するだけでなく、コスト上昇にもつながるため、０.０１〜０.１０％とする。
Nb: 0.01 to 0.10%
Similarly to Ti, Nb is an element that improves the uniformity of the structure by refining the structure and contributes to the improvement of strength without deteriorating the bendability by precipitation strengthening of carbides. In order to express the function, the content of at least 0.01% is required. However, even if added over 0.10%, not only the effect of increasing the strength is saturated but also the cost is increased, so 0.01 to 0.10%.
｛Ｔｉ%＋（Ｎｂ%／２）｝／Ｃ% ＞ １．２
曲げ性を向上させるには、組織の微細化・均一化が重要である。組織の均一化にはＴｉ、Ｎｂの添加が有効であるが、組織の均一性はＣ量にも影響を受け、Ｃ量の増加にともない組織の不均一性は増加する。そのため、これらのことを考慮し、Ｔｉ、Ｎｂを添加する必要がある。
Ｔｉ、Ｎｂ、Ｃ量と曲げ性の関係を種々検討した結果、｛Ｔｉ%＋（Ｎｂ%／２）｝／Ｃ% ＞ １．２ を満足すれば、良好な曲げ性が得られることが明らかとなった。
なお、上の式で、Ｔｉ％、Ｎｂ％、Ｃ％は、それぞれの成分の質量％を示す。
{Ti% + (Nb% / 2)} / C%> 1.2
In order to improve bendability, it is important to make the structure finer and uniform. Addition of Ti and Nb is effective for homogenizing the tissue. However, the uniformity of the tissue is affected by the amount of C, and the non-uniformity of the tissue increases as the amount of C increases. Therefore, it is necessary to add Ti and Nb in consideration of these matters.
As a result of various investigations on the relationship between Ti, Nb, C content and bendability, it is clear that satisfactory bendability can be obtained if {Ti% + (Nb% / 2)} / C%> 1.2. It became.
In the above formula, Ti%, Nb%, and C% indicate mass% of each component.
Ｃｒ：０.０１〜１.０％、Ｍｏ:０.０１〜１.０％、Ｖ：０.０１〜０.５％
これらは、高強度化に有効な元素である。必要に応じて添加される。７８０ＭＰａ以上の強度を得るために、Ｃｒ，Ｍｏ，Ｖの１種または２種以上の添加が有効である。
Cr: 0.01-1.0%, Mo: 0.01-1.0%, V: 0.01-0.5%
These are elements effective for increasing the strength. It is added as necessary. In order to obtain a strength of 780 MPa or more, it is effective to add one or more of Cr, Mo, and V.
続いて金属組織について説明する。
本発明の合金化溶融めっき鋼板は、主相フェライトに副相としてマルテンサイトまたはマルテンサイトとベイナイトが分散した複合組織を持つＤＰ鋼板を基材とするものである。
基材鋼板の主相フェライトに分散する副相としてのマルテンサイトまたはマルテンサイトとベイナイトは、合計で１５％以上４５％未満とする。１５％に満たないと７８０ＭＰａなる引張強さは得られない。逆に４５％以上になると硬くなりすぎて加工し難くなる。好ましくは２５〜３５％の範囲である。
なお、副相としてはマルテンサイトのみが好ましいが、部分的にベイナイトが分散していても良い。
Next, the metal structure will be described.
The alloyed hot-dip steel sheet of the present invention is based on a DP steel sheet having a composite structure in which martensite or martensite and bainite are dispersed as a secondary phase in the main phase ferrite.
Martensite or martensite and bainite as subphases dispersed in the main phase ferrite of the base steel sheet are made 15% or more and less than 45% in total. If it is less than 15%, a tensile strength of 780 MPa cannot be obtained. On the other hand, if it is 45% or more, it becomes too hard and difficult to process. Preferably it is 25 to 35% of range.
In addition, although only a martensite is preferable as a subphase, the bainite may disperse | distribute partially.
フェライトの平均粒径：５μm以下、マルテンサイトの平均粒径：３μm以下
本発明では、組織を微細にすることにより曲げ性を向上させている。フェライトおよびマルテンサイトの平均粒径が、それぞれ５μｍおよび３μｍを超える程に大きくなると、不均一組織となりやすく、曲げ性が劣化する。
Average particle diameter of ferrite: 5 μm or less, average particle diameter of martensite: 3 μm or less In the present invention, the bendability is improved by making the structure fine. If the average particle sizes of ferrite and martensite become larger than 5 μm and 3 μm, respectively, a non-uniform structure tends to be formed and the bendability deteriorates.
マルテンサイトの平均アスペクト比：０.７以上
本発明では、組織を微細にすることの他に、マルテンサイトを丸くしている。マルテンサイトのアスペクト比（短軸／長軸）は曲げ性と関係があり、アスペクト比が小さいマルテンサイトが増えると、組織の不均一性が増し、曲げ性は劣化する。そのため、０.７以上とする。好ましくは０.８〜１.０である。
Average aspect ratio of martensite: 0.7 or more In the present invention, in addition to making the structure fine, the martensite is rounded. The aspect ratio (short axis / major axis) of martensite is related to bendability. When martensite with a small aspect ratio increases, the non-uniformity of the structure increases and the bendability deteriorates. Therefore, it is set to 0.7 or more. Preferably it is 0.8-1.0.
以上のような成分組成を具備し、またその金属組織を、残留オーステナイトのない主相としてのフェライトと副相としてのマルテンサイト、またはマルテンサイトとベイナイトからなるものとすることにより、７８０ＭＰａ以上の引張強さと４０Ｈｖ以下のビッカース硬さの変動特性を発揮することができる。
なお、ビッカース硬さの変動を４０Ｈｖ以下に抑えることができた結果、軟質部への変形の集中が抑制されて曲げ性が向上する。
By having the component composition as described above and having the metal structure composed of ferrite as a main phase without residual austenite and martensite as a subphase, or martensite and bainite, The fluctuation characteristics of strength and Vickers hardness of 40 Hv or less can be exhibited.
In addition, as a result of suppressing the fluctuation | variation of Vickers hardness to 40 Hv or less, the concentration of a deformation | transformation to a soft part is suppressed and bendability improves.
次に、本発明に係る高強度合金化溶融亜鉛めっき鋼板の製造方法について簡単に説明する。
上記の本発明成分からなる鋼を、転炉、電気炉等の通常の方法によって溶製し、成分調整を行った後、通常の鋳造工程、必要に応じて分塊圧延工程を経た後、熱延と酸洗、その後の冷延と焼鈍を行い、その後に、溶融亜鉛めっき処理とその後の合金化処理を行う。
冷延板焼鈍は溶融めっきラインで行い、所定の温度に所定時間加熱して所定の冷却速度で冷却した後、連続して溶融亜鉛めっき浴に浸漬し、さらに所定の温度に加熱して合金化処理することが好ましい。
熱延およびその後の酸洗工程までには格段の注意点はない。従前どおり、熱延前の加熱は、炭化物がマトリックス中に十分に固溶されるような温度とするべきである。
Next, a method for producing a high-strength galvannealed steel sheet according to the present invention will be briefly described.
After the steel comprising the above-described components of the present invention is melted by a usual method such as a converter or an electric furnace and the components are adjusted, a normal casting process is performed, and a bulk rolling process is performed as necessary. Rolling and pickling, followed by cold rolling and annealing, followed by hot dip galvanizing and subsequent alloying.
Cold-rolled sheet annealing is performed in a hot dipping line, heated to a predetermined temperature for a predetermined time, cooled at a predetermined cooling rate, continuously immersed in a hot dip galvanizing bath, and further heated to a predetermined temperature to form an alloy. It is preferable to process.
There are no special precautions until hot rolling and the subsequent pickling process. As before, the heating prior to hot rolling should be at a temperature such that the carbides are sufficiently dissolved in the matrix.
めっきラインでの加熱温度：８００〜９００℃
本発明ではＴｉ、Ｎｂを含有しているため、再結晶温度が高い。加熱温度が８００℃に満たないと、再結晶が完了せず未再結晶組織が残存するため、良好な曲げ性が得られない。また、９００℃を超えると組織が粗大化し、曲げ性が劣化するので、８００〜９００℃が好ましい。
Heating temperature in the plating line: 800-900 ° C
Since Ti and Nb are contained in the present invention, the recrystallization temperature is high. If the heating temperature is less than 800 ° C., recrystallization is not completed and an unrecrystallized structure remains, so that good bendability cannot be obtained. Moreover, since a structure | tissue will coarsen and bendability will deteriorate when it exceeds 900 degreeC, 800-900 degreeC is preferable.
加熱後の冷却速度：５℃/s以上
加熱後の冷却速度が５℃/s未満では、一部パーライトが生成し、７８０ＭＰａ以上の高強度を得ることが困難となる。また、フェライト粒径の微細化の点からも、冷却速度は５℃/s以上が好ましい。本発明では、所定のＴｉとＮｂを含有していることにより、加熱後の冷却速度をこのように選定することでフェライトの平均粒径が５μｍ以下となる。
その後、従前どおり、溶融亜鉛めっき浴に浸漬して溶融亜鉛めっきを施す。そして合金化処理をする。
Cooling rate after heating: 5 ° C./s or more If the cooling rate after heating is less than 5 ° C./s, some pearlite is generated, and it becomes difficult to obtain high strength of 780 MPa or more. In addition, the cooling rate is preferably 5 ° C./s or more from the viewpoint of reducing the ferrite grain size. In the present invention, by containing predetermined Ti and Nb, the average particle diameter of ferrite becomes 5 μm or less by selecting the cooling rate after heating in this way.
Then, as before, it is immersed in a hot dip galvanizing bath and hot dip galvanized. Then, alloying treatment is performed.
合金化温度：５３０℃以下
合金化温度が５３０℃を超えると、一部パーライトが生成し、７８０ＭＰａ以上の高強度を得ることが困難となるため、合金化温度は５３０℃以下とする。
また、この合金化処理の後の冷却時にマルテンサイト相が生成し、その平均粒径やアスペクト比は所定のものとなる。
Alloying temperature: 530 ° C. or less When the alloying temperature exceeds 530 ° C., a part of pearlite is generated and it is difficult to obtain a high strength of 780 MPa or more, so the alloying temperature is 530 ° C. or less.
Further, a martensite phase is generated during cooling after the alloying treatment, and the average particle size and aspect ratio thereof are predetermined.
表１に示す化学組成を有するスラブを加熱温度：１２５０℃、仕上げ圧延温度：８８０℃にて熱間圧延を行った後、５５０℃にて巻取り、板厚２.４ｍｍの熱延鋼板を得た。
熱延鋼板を酸洗後、板厚１.４ｍｍ（圧下率：４３％）まで冷間圧延し、冷延鋼板を得た後、連続溶融めっきラインにて加熱温度：８５０℃、冷却速度：１０℃／ｓで冷却した後、溶融亜鉛めっき浴に浸漬し、さらに５１０℃で合金化処理を行い、合金化溶融亜鉛めっき鋼板を得た。
A slab having the chemical composition shown in Table 1 was hot-rolled at a heating temperature of 1250 ° C. and a finish rolling temperature of 880 ° C., and then wound at 550 ° C. to obtain a hot-rolled steel plate having a thickness of 2.4 mm. It was.
After pickling the hot-rolled steel sheet, it is cold-rolled to a thickness of 1.4 mm (rolling rate: 43%) to obtain a cold-rolled steel sheet, and then heated at a continuous hot dipping line: 850 ° C., cooling rate: 10 After cooling at ° C./s, it was immersed in a hot dip galvanizing bath and further alloyed at 510 ° C. to obtain an alloyed hot dip galvanized steel sheet.
得られた合金化溶融亜鉛めっき鋼板について、組織観察、硬さ測定、引張試験、曲げ試験を実施した。
組織のフェライト、ベイナイト、マルテンサイトは、圧延方向の板厚断面についてレペラ試薬による着色エッチングにして識別し、画像解析によって各相の面積率、平均粒径、アスペクト比を求めた。
それらの測定結果を表２に示す。なお、表２には示していないが、ベイナイトが観察されたのはNo.２の試料のみであり、その面積率は３％であった。
The obtained galvannealed steel sheet was subjected to structure observation, hardness measurement, tensile test, and bending test.
Ferrite, bainite, and martensite in the structure were identified by color etching with a repeller reagent on the sheet thickness section in the rolling direction, and the area ratio, average particle diameter, and aspect ratio of each phase were determined by image analysis.
The measurement results are shown in Table 2. Although not shown in Table 2, bainite was observed only in the No. 2 sample, and the area ratio was 3%.
硬さ試験は、荷重を１００ｇとしたマイクロビッカース硬さ試験機で、圧延方向の板厚断面において、板厚１／４線上を１５０μｍの間隔で３０点測定を行い、得られた硬さの最大値と最小値の差を硬さ変動とした。
引張試験は、合金化溶融亜鉛めっき鋼板から圧延方向と直角な方向を長手方向とするＪＩＳ５号引張試験片を採取して行い、降伏強さ（ＹＳ）、引張強さ（ＴＳ）、全伸び（Ｔ.Ｅｌ）を算出した。
曲げ試験は、圧延方向と直角な方向を長手方向とする曲げ試験片を採取し、９０°のＶブロック曲げ試験を実施した。試験後に目視にて割れが認められない最小の先端Ｒを限界曲げＲとして求めた。
それらの評価結果を合わせて表３に示す。また、本発明例のNo.１と比較例のNo.１０の３０点の測定位置でのビッカースの硬さ変動状況を図１に示す。
The hardness test is a micro Vickers hardness tester with a load of 100 g. In the thickness cross section in the rolling direction, 30 points are measured at intervals of 150 μm on the thickness 1/4 line, and the maximum hardness obtained. The difference between the value and the minimum value was taken as the hardness variation.
The tensile test is performed by taking a JIS No. 5 tensile test piece having a longitudinal direction perpendicular to the rolling direction from the galvannealed steel sheet, yield strength (YS), tensile strength (TS), total elongation ( T.El) was calculated.
In the bending test, a bending test piece having a longitudinal direction perpendicular to the rolling direction was taken, and a 90 ° V-block bending test was performed. The minimum tip R where no crack was visually observed after the test was determined as the limit bending R.
The evaluation results are shown in Table 3. FIG. 1 shows the Vickers hardness fluctuations at 30 measurement positions, No. 1 of the present invention and No. 10 of the comparative example.
本発明例のNo.１と比較例のNo.１０のビッカースの硬さ変動を示した図１からも分かるように、発明例のNo.１は硬さ変動が２０Ｈｖ程度と小さいのに対し、比較例のNo.１０は硬さ変動が５０Ｈｖ以上と大きく、不均一な組織となっている。
また、表２に示すように、本発明鋼板は引張強度が７８０ＭＰａ以上で、フェライトの平均粒径が５μｍ以下、マルテンサイトの平均粒径が３μｍ以下でかつマルテンサイトの平均アスペクト比が０．７以上、ビッカースの硬さ変動が４０Ｈｖ以下と均一微細な組織となっている。そのため、９０°Ｖブロック曲げにおける限界曲げＲも０.５ｍｍ以下と優れた曲げ性を有している。
As can be seen from FIG. 1 showing the fluctuation in hardness of the Vickers No. 1 of the invention example and No. 10 of the comparative example, the hardness fluctuation of No. 1 of the invention example is as small as about 20 Hv, No. 10 of the comparative example has a large hardness variation of 50 Hv or more, and has a non-uniform structure.
Further, as shown in Table 2, the steel sheet of the present invention has a tensile strength of 780 MPa or more, an average ferrite particle size of 5 μm or less, an average martensite particle size of 3 μm or less, and an average aspect ratio of martensite of 0.7. As described above, the hardness change of Vickers is a uniform fine structure of 40 Hv or less. Therefore, the limit bending R in the 90 ° V block bending is 0.5 mm or less and has excellent bendability.
これに対して、比較例であるNo.１０〜１５は鋼成分が本発明範囲から外れているため、No.１０、No.１２、No.１３、No.１５は曲げ性が悪く、No.１１、No.１４は７８０ＭＰａ以上の強度を得ることができない。 On the other hand, since No.10-15 which is a comparative example has a steel component outside the scope of the present invention, No.10, No.12, No.13 and No.15 have poor bendability and No.10. 11, No. 14 cannot obtain a strength of 780 MPa or more.
Claims (2)
｛Ｔｉ%＋（Ｎｂ%／２）｝／Ｃ% ＞ １．２ ・・・（１）
上の式で、Ｔｉ％、Ｎｂ％、Ｃ％は、それぞれの成分の質量％を示す。 C: 0.05-0.13 mass%, Si: 0.3-0.8 mass%, Mn: 1.5-2.3 mass%, P: 0.03 mass% or less, S: 0.01 % By mass or less, B: 0.0005 to 0.005% by mass, Ti: 0.05 to 0.20% by mass, Nb: 0.01 to 0.10% by mass, and Ti, Nb, and C content. The following composition (1) is satisfied, the balance is composed of Fe and unavoidable impurities, ferrite as the main phase and martensite or martensite and bainite as subphases, and martensite or martensite and bainite. A metal structure having an area ratio of 15% to less than 45%, the ferrite having an average particle diameter of 5.0 μm or less, the martensite having an average particle diameter of 3.0 μm or less and an average aspect ratio of 0.7 or more. comprising, in the rolling direction thickness section, the sheet thickness 1/4 line hard When the difference between the hardness variation of the maximum value and the minimum value, a high strength alloyed molten zinc excellent in bendability, characterized in that exhibits variations in the following Vickers hardness 40Hv, and 780MPa or more tensile strength Plated steel sheet.
{Ti% + (Nb% / 2)} / C%> 1.2 (1)
In the above formula, Ti%, Nb%, and C% indicate mass% of each component.
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| JP4962594B2 (en) * | 2010-04-22 | 2012-06-27 | Ｊｆｅスチール株式会社 | High-strength hot-dip galvanized steel sheet excellent in workability and manufacturing method thereof |
| JP5807624B2 (en) * | 2012-07-30 | 2015-11-10 | 新日鐵住金株式会社 | Cold rolled steel sheet and method for producing the same |
| JP5910396B2 (en) * | 2012-07-30 | 2016-04-27 | 新日鐵住金株式会社 | Hot-dip galvanized steel sheet and manufacturing method thereof |
| KR101978072B1 (en) * | 2017-06-27 | 2019-05-13 | 현대제철 주식회사 | Steel for taylor welded blank and method of manufacturing hot stampig component using the same |
| CN113122769B (en) * | 2019-12-31 | 2022-06-28 | 宝山钢铁股份有限公司 | Low-silicon and low-carbon equivalent gigapascal multiphase steel sheet/strip and its manufacturing method |
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