JP4966485B2 - High tensile hot dip galvanized steel sheet and its manufacturing method - Google Patents
High tensile hot dip galvanized steel sheet and its manufacturing method Download PDFInfo
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- 229910001335 Galvanized steel Inorganic materials 0.000 title claims description 48
- 239000008397 galvanized steel Substances 0.000 title claims description 48
- 238000004519 manufacturing process Methods 0.000 title claims description 22
- 229910000831 Steel Inorganic materials 0.000 claims description 160
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- 238000005275 alloying Methods 0.000 claims description 17
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- 229910001566 austenite Inorganic materials 0.000 claims description 13
- 238000005097 cold rolling Methods 0.000 claims description 10
- 229910052750 molybdenum Inorganic materials 0.000 claims description 10
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- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 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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- Coating With Molten Metal (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Description
本発明は主として自動車の足回り部品やメンバー等の補強部材のように、優れた曲げ加工性、疲労特性および耐食性が要求される用途に好適な高張力溶融亜鉛めっき鋼板およびその製造方法に関する。 The present invention relates to a high-tensile hot-dip galvanized steel sheet suitable for applications that require excellent bending workability, fatigue characteristics, and corrosion resistance, such as reinforcement members for automobile undercarriage parts and members, and a method for producing the same.
近年、地球環境保護のため、自動車の燃費向上が求められており、自動車用鋼板においては、車体の軽量化および安全性確保のため、５９０ＭＰａ以上の引張強度（ＴＳ）を有する高強度鋼板へのニーズが高まっている。 In recent years, improvement in fuel efficiency of automobiles has been demanded in order to protect the global environment, and in steel sheets for automobiles, high-strength steel sheets having a tensile strength (TS) of 590 MPa or more are required to reduce the weight of the vehicle body and ensure safety. Needs are growing.
しかしながら、ただ単に高強度であればよいわけではなく、例えば、成形性の観点からは高延性および良曲げ性が求められており、製品寿命の観点からは優れた疲労特性が求められている。特に疲労特性に関しては、一般に部材はボルト止めや、部品取り付けのために穴加工がされることが多く、その穴部の耐切欠き疲労特性が要求される。また、防錆性の観点からは、溶融亜鉛めっきを施した鋼板が求められている。 However, high strength is not necessarily required. For example, high ductility and good bendability are required from the viewpoint of formability, and excellent fatigue characteristics are required from the viewpoint of product life. In particular, with regard to fatigue characteristics, members are generally often drilled for bolting or mounting of parts, and notch fatigue characteristics of the holes are required. Moreover, the steel plate which hot-dip galvanized was calculated | required from a viewpoint of rust prevention property.
鋼の強化手法としては固溶強化、析出強化および変態強化が知られており、これらを組み合わせることにより、所定の引張強度を達成することができる。これらの組合せにより、同一引張強度であっても延性、曲げ加工性および疲労特性等が異なってくる。したがって、自動車用途に代表される高張力鋼板に要求される性能を高度に発現するためには、強化手法を適正に組み合わせることが重要となる。 Solid solution strengthening, precipitation strengthening and transformation strengthening are known as steel strengthening methods, and by combining these, a predetermined tensile strength can be achieved. By these combinations, ductility, bending workability, fatigue characteristics, and the like differ even with the same tensile strength. Therefore, in order to express the performance required for high-tensile steel plates typified by automobile applications at a high level, it is important to combine strengthening methods appropriately.
一般に、固溶強化技術は、ＳｉやＰなどの添加によるフェライト相の強化を用いる手段であり、３９０ＭＰａ級鋼板や４４０ＭＰａ級鋼板などの一般強化材にも広く使われている。 In general, the solid solution strengthening technique is a means using strengthening of a ferrite phase by adding Si or P, and is widely used for general reinforcing materials such as a 390 MPa class steel plate and a 440 MPa class steel plate.
また、上述の強化手法のうち、変態強化を用いると、比較的容易に高強度化を達成することができる。例えば、特許文献１にはＭｎ、Ｃｒ、Ｍｏを添加し、さらに冷却速度を制御することによって、フェライト・ベイナイト・マルテンサイト混合組織とすることで、ＴＳ≧７８０ＭＰａを達成することが開示されている。また、特許文献２には、焼戻しマルテンサイト組織とすることにより、曲げ加工性と高強度化を達成することが開示されている。 Further, among the above-described strengthening methods, when transformation strengthening is used, it is possible to achieve high strength relatively easily. For example, Patent Document 1 discloses that TS ≧ 780 MPa is achieved by adding Mn, Cr, and Mo and further controlling the cooling rate to obtain a mixed structure of ferrite, bainite, and martensite. . Patent Document 2 discloses that bending workability and high strength are achieved by using a tempered martensite structure.
析出強化にはＴｉやＮｂを主に添加するが、その中で安価かつ添加量に対する強度上昇量が多いＴｉを多く添加することが一般的になっている。さらに、Ｔｉの添加はフェライト粒を微細化する効果があり、Ｔｉ炭窒化物による析出強化に加え、フェライト粒の微細化による高強度化を図れるメリットもある。Ｔｉによる析出強化を用いた高張力溶融亜鉛めっき鋼板は、特許文献３、特許文献４および特許文献５に開示されている。 Ti and Nb are mainly added for precipitation strengthening, but it is common to add a large amount of Ti which is inexpensive and has a large increase in strength with respect to the added amount. Furthermore, the addition of Ti has the effect of refining the ferrite grains, and has the advantage of increasing the strength by refining the ferrite grains in addition to precipitation strengthening by Ti carbonitride. High-tensile hot-dip galvanized steel sheets using precipitation strengthening by Ti are disclosed in Patent Document 3, Patent Document 4, and Patent Document 5.
しかしながら、Ｓｉ、Ｐなどの固溶強化のみで５９０ＭＰａ以上の高強度化を達成するには、多量のＳｉ、Ｐ添加が必要であり、めっきの濡れ性および合金化処理性が阻害される欠点がある。 However, in order to achieve a high strength of 590 MPa or more only by solid solution strengthening of Si, P, etc., a large amount of Si, P is necessary, and there is a drawback that the wettability of plating and the alloying processability are hindered. is there.
また、上記の変態強化技術によると、フェライトと硬質相の混合組織とすることにより、比較的容易に高強度化が図れるが、マルテンサイトを多く含む混合組織を用いると、組織間の硬度差が大きくなり、その組織界面から曲げ加工初期に亀裂が発生するために曲げ性に劣る欠点がある。 In addition, according to the transformation strengthening technique described above, the strength can be increased relatively easily by using a mixed structure of ferrite and a hard phase. However, if a mixed structure containing a large amount of martensite is used, the hardness difference between the structures is increased. There is a disadvantage that the bendability is inferior because the crack is generated at the initial stage of the bending process from the structure interface.
さらに、Ｔｉを添加することによるフェライトの微細化や析出強化の技術を利用すると、フェライトの微細化や強化に寄与する、数十ｎｍの微細な析出物であるＴｉ系炭化析出物、窒化析出物、炭窒化析出物の他に粗大な晶出系ＴｉＮが生成する。特に、鋼塊凝固中に生成する晶出系ＴｉＮは高温で生成するために、その大きさは０．１〜２０μｍと粗大となり、フェライトの微細化や強度上昇には全く寄与しない。 Furthermore, Ti-based carbonized precipitates and nitrided precipitates, which are fine precipitates of several tens of nm, contribute to the refinement and strengthening of ferrite by utilizing the technology of refinement and precipitation strengthening of ferrite by adding Ti. In addition to the carbonitride precipitate, coarse crystallized TiN is formed. In particular, since the crystallization TiN generated during solidification of the steel ingot is generated at a high temperature, the size becomes as coarse as 0.1 to 20 μm, and does not contribute to the refinement of ferrite and the increase in strength at all.
本発明は、上記現状に鑑みてなされたもので、その目的は、自動車や各種の産業機械に用いられる構造部材の素材、特に自動車のメンバーや足廻り部品に代表される構造部材の素材として好適な、良好な曲げ加工性及び高い切欠き疲労強度を有し、耐食性に優れた高張力溶融亜鉛めっき鋼板及びその製造方法を提供することである。 The present invention has been made in view of the above-described situation, and the object thereof is suitable as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and undercarriage parts. Another object of the present invention is to provide a high-tensile hot-dip galvanized steel sheet having excellent bending workability and high notch fatigue strength and excellent corrosion resistance, and a method for producing the same.
特に、粗大な晶出系ＴｉＮの粒子寸法及び量ならびに析出強化に寄与する微細なＴｉ系炭窒化析出物の粒子寸法及び分布を適正化することで、Ｔｉの析出強化及びＴｉによるフェライトの微細化を利用し、高強度で加工性を確保しつつ、曲げ加工性及び耐切欠き疲労特性に優れた引張強度５９０ＭＰａ以上の高張力溶融亜鉛めっき鋼板及びその製造方法を提供することである。 In particular, by optimizing the particle size and amount of coarse crystallized TiN particles and the particle size and distribution of fine Ti carbonitride precipitates that contribute to precipitation strengthening, Ti precipitation strengthening and ferrite refinement by Ti Is used to provide a high-tensile hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more, which is excellent in bending workability and notch fatigue resistance, while ensuring high strength and workability, and a method for producing the same.
本発明は高張力溶融亜鉛めっき鋼板及びその製造方法に係るものであって、次の(1)から(7)までのいずれかに記載の高張力溶融亜鉛めっき鋼板に係るものと、(8)から(12)までのいずれかに記載の高張力溶融亜鉛めっき鋼板の製造方法に係るものである。以下、それぞれ、本発明(1)〜本発明(12)という。なお、高張力溶融亜鉛めっき鋼板に係る本発明(1)〜(7)と、高張力溶融亜鉛めっき鋼板の製造方法に係る本発明(8)〜(12)を総称して、本発明ということがある。 The present invention relates to a high-tensile hot-dip galvanized steel sheet and a method for producing the same, and relates to a high-tensile hot-dip galvanized steel sheet according to any one of (1) to (7) below, and (8) To (12), the method for producing a high-tensile hot-dip galvanized steel sheet. Hereinafter, the present invention (1) to the present invention (12), respectively. Note that the present invention (1) to (7) related to the high-tensile hot-dip galvanized steel sheet and the present invention (8) to (12) related to the method for producing the high-tensile hot-dip galvanized steel sheet are collectively referred to as the present invention. There is.
(1)鋼板の表面に溶融亜鉛めっき層を備える溶融亜鉛めっき鋼板において、前記鋼板の化学組成が、質量％で、Ｃ：０．０２％を超え０．２０％以下、Ｓｉ：０．０１〜２．０％、Ｍｎ：０．１〜３．０％、Ｐ：０．００３〜０．１０％、Ｓ：０．０２０％以下、Ａｌ：０．００１〜１．０％、Ｎ：０．０００４〜０．０１５％、Ｔｉ：０．０３〜０．２％を含有し、残部がＦｅおよび不純物であるとともに、前記鋼板の金属組織がフェライトを面積率で３０〜９５％含有し、残部の第２相がマルテンサイト、ベイナイト、パーライト、セメンタイトおよび残留オーステナイトのうちの１種または２種以上からなり、かつマルテンサイトを含有しないか又は含有する場合にマルテンサイトの面積率は５０％以下であり、そして、前記鋼板が粒径２〜３０ｎｍのＴｉ系炭窒化析出物を平均粒子間距離３０〜３００ｎｍで含有し、かつ粒径３μｍ以上の晶出系ＴｉＮを平均粒子間距離５０〜５００μｍで含有し、前記溶融亜鉛めっき層の付着量が３〜８００ｇ／ｍ２であることを特徴とする高張力溶融亜鉛めっき鋼板。
(1) In a hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the steel sheet, the chemical composition of the steel sheet is, by mass%, C: more than 0.02% and 0.20% or less, Si: 0.01 to 2.0%, Mn: 0.1-3.0%, P: 0.003-0.10%, S: 0.020% or less, Al: 0.001-1.0%, N: 0.00. 0004 to 0.015%, Ti: 0.03 to 0.2%, the balance is Fe and impurities, and the metal structure of the steel sheet contains 30 to 95% ferrite in area ratio, When the second phase is composed of one or more of martensite, bainite, pearlite, cementite, and retained austenite and does not contain martensite or contains it, the martensite area ratio is 50% or less . And the steel plate has a grain size -30 nm Ti-carbonitride precipitates with an average interparticle distance of 30-300 nm and crystallization TiN particles with a particle size of 3 μm or more with an average interparticle distance of 50-500 μm. A high-tensile hot-dip galvanized steel sheet characterized by an amount of 3 to 800 g / m 2 .
(2)鋼板が、Ｆｅの一部に代えて、Ｎｂ：０．１％以下、Ｖ：０．５％以下およびＷ：０．５％以下のうちの１種または２種以上を含有することを特徴とする、上記(1)に記載の高張力溶融亜鉛めっき鋼板。 (2) The steel sheet contains one or more of Nb: 0.1% or less, V: 0.5% or less, and W: 0.5% or less, instead of part of Fe. The high-tensile hot-dip galvanized steel sheet according to (1) above,
(3)鋼板が、Ｆｅの一部に代えて、Ｃｒ：１．０％以下、Ｍｏ：１．０％以下、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下およびＢ：０．０１％以下のうちの１種または２種以上を含有することを特徴とする、上記(1)または(2)に記載の高張力溶融亜鉛めっき鋼板。 (3) The steel plate is replaced with a part of Fe, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B: 0.00. The high-tensile hot-dip galvanized steel sheet according to (1) or (2) above, containing one or more of 01% or less.
(4)鋼板が、Ｆｅの一部に代えて、ＲＥＭ：０．１％以下、Ｍｇ：０．０１％以下およびＣａ：０．０１％以下のうちの１種または２種以上を含有することを特徴とする、上記(1)〜(3)のいずれかに記載の高張力溶融亜鉛めっき鋼板。 (4) The steel sheet contains one or more of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less instead of part of Fe. The high-tensile hot-dip galvanized steel sheet according to any one of (1) to (3) above.
(5)鋼板が、Ｆｅの一部に代えて、Ｚｒ：０．０１％以下を含有することを特徴とする、上記(1)〜(4)のいずれかに記載の高張力溶融亜鉛めっき鋼板。 (5) The high-strength hot-dip galvanized steel sheet according to any one of (1) to (4) above, wherein the steel sheet contains Zr: 0.01% or less instead of part of Fe .
(6)鋼板の表面から深さ５０μｍまでの鋼板表層部におけるフェライトの平均粒径が２〜１０μｍであることを特徴とする、上記(1)〜(5)のいずれかに記載の高張力溶融亜鉛めっき鋼板。 (6) The high-tensile fusion as described in any one of (1) to (5) above, wherein the ferrite has an average particle diameter of 2 to 10 μm from the surface of the steel sheet to a depth of 50 μm Galvanized steel sheet.
(7)引張り強度が５９０ＭＰａ以上、１８０度曲げにおける限界曲げ半径が板厚の２倍以下であり、かつ切欠き曲げ疲労特性の耐久比が０．３８以上であることを特徴とする、上記(1)〜(6)のいずれかに記載の高張力溶融亜鉛めっき鋼板。 (7) The tensile strength is not less than 590 MPa, the limit bending radius in 180-degree bending is not more than twice the plate thickness, and the durability ratio of notched bending fatigue characteristics is not less than 0.38. The high-tensile hot-dip galvanized steel sheet according to any one of 1) to (6).
(8)次の〔Ａ１〕から〔Ａ４〕までの工程を備えることを特徴とする高張力溶融亜鉛めっき鋼板の製造方法。
〔Ａ１〕請求項１から５までのいずれかに記載された化学組成を有する溶鋼を、液相線温度から固相線温度までの温度域内の平均冷却速度０．３〜１．０℃／秒で凝固させて鋼塊とする鋳造工程。
〔Ａ２〕鋳造工程で得られた鋼塊あるいはさらに鋼塊を分塊圧延して得られた鋼片を温度１１００℃以上とした後、Ａｒ３点以上で熱間仕上げ圧延を実施し、７００℃以下の温度で巻き取りを行う熱間圧延工程。
〔Ａ３〕熱間圧延工程を経て得られる熱間圧延鋼板に酸洗を施す酸洗工程。
〔Ａ４〕酸洗工程を経て得られる熱間圧延鋼板にＡｃ３点〜１０００℃の温度範囲で５秒以上保持する均熱処理を施した後、平均冷却速度１〜４０℃／秒で６００℃まで冷却するか、あるいはさらに平均冷却速度７０℃／秒以下で４４０〜６００℃の温度まで冷却し、その後付着量が３〜８００ｇ／ｍ ２ の溶融亜鉛めっきを施す溶融亜鉛めっき処理工程。
(8) A method for producing a high-tensile hot-dip galvanized steel sheet, comprising the following steps [A1] to [A4].
[A1] An average cooling rate in the temperature range from the liquidus temperature to the solidus temperature of the molten steel having the chemical composition according to any one of claims 1 to 5 is 0.3 to 1.0 ° C / second. Casting process to solidify with steel to make a steel ingot.
[A2] The steel ingot obtained in the casting process or the steel slab obtained by further rolling the steel ingot is brought to a temperature of 1100 ° C. or higher, and then hot finish rolling is performed at 3 or more points of Ar, 700 ° C. A hot rolling process in which winding is performed at the following temperature.
[A3] A pickling step of pickling a hot-rolled steel sheet obtained through the hot rolling step.
[A4] A hot-rolled steel sheet obtained through the pickling step is subjected to soaking treatment for 5 seconds or more in a temperature range of Ac 3 to 1000 ° C, and then to 600 ° C at an average cooling rate of 1 to 40 ° C / second. or cooling, or further until the average cooling rate 70 ° C. / sec a temperature of four hundred forty to six hundred ° C. or less cooled, then coating weight of 3~800g / m 2 of a hot-dip galvanizing subjecting galvanizing treatment step.
(9)次の〔Ｂ１〕から〔Ｂ５〕までの工程を備えることを特徴とする高張力溶融亜鉛めっき鋼板の製造方法。
〔Ｂ１〕請求項１から５までのいずれかに記載された化学組成を有する溶鋼を、液相線温度から固相線温度までの温度域内の平均冷却速度０．３〜１．０℃／秒で凝固させて鋼塊とする鋳造工程。
〔Ｂ２〕鋳造工程で得られた鋼塊あるいはさらに鋼塊を分塊圧延して得られた鋼片を温度１１００℃以上とした後、Ａｒ３点以上で熱間仕上げ圧延を実施し、７００℃以下の温度で巻き取りを行う熱間圧延工程。
〔Ｂ３〕熱間圧延工程を経て得られる熱間圧延鋼板に酸洗を施す酸洗工程。
〔Ｂ４〕酸洗工程を経て得られる熱間圧延鋼板に冷間圧延を施す冷間圧延工程。
〔Ｂ５〕冷間圧延工程を経て得られる冷間圧延鋼板にＡｃ３点〜１０００℃の温度範囲で５秒以上保持する均熱処理を施した後、平均冷却速度１〜４０℃／秒で６００℃まで冷却するか、あるいはさらに平均冷却速度７０℃／秒以下で４４０〜６００℃の温度まで冷却し、その後付着量が３〜８００ｇ／ｍ ２ の溶融亜鉛めっきを施す溶融亜鉛めっき処理工程。
(9) A method for producing a high-tensile hot-dip galvanized steel sheet, comprising the following steps [B1] to [B5].
[B1] An average cooling rate in the temperature range from the liquidus temperature to the solidus temperature of the molten steel having the chemical composition according to any one of claims 1 to 5 is 0.3 to 1.0 ° C / second. Casting process to solidify with steel to make a steel ingot.
[B2] The steel ingot obtained in the casting process or the steel slab obtained by split rolling the steel ingot is brought to a temperature of 1100 ° C. or higher, and then hot finish rolling is performed at 3 or more points of Ar, 700 ° C. A hot rolling process in which winding is performed at the following temperature.
[B3] A pickling process in which a hot-rolled steel sheet obtained through the hot rolling process is pickled.
[B4] A cold rolling process in which cold rolling is performed on a hot rolled steel sheet obtained through the pickling process.
[B5] Cold-rolled steel sheet obtained through the cold rolling process is subjected to soaking treatment for 5 seconds or more in the temperature range of Ac 3 to 1000 ° C, and then 600 ° C at an average cooling rate of 1 to 40 ° C / second. until either cooled or further cooled to a temperature of an average cooling rate of 70 ° C. / sec or less 440-600 ° C., then coating weight of 3~800g / m subjecting 2 galvanizing galvanizing treatment step.
(10)熱間圧延工程において、鋼塊または鋼片に粗圧延を施してシートバーとした後に９５０℃以上に加熱することを特徴とする、上記(8)または(9)に記載の高張力溶融亜鉛めっき鋼板の製造方法。 (10) The high tension according to (8) or (9) above, wherein in the hot rolling step, the steel ingot or steel piece is subjected to rough rolling to form a sheet bar and then heated to 950 ° C. or higher. Manufacturing method of hot dip galvanized steel sheet.
(11)溶融亜鉛めっき工程において、溶融亜鉛めっきを施す前に、４４０〜６００℃の温度で５〜１００秒間保持することを特徴とする、上記(8)〜(10)のいずれかに記載の高張力溶融亜鉛めっき鋼板の製造方法。 (11) In the hot dip galvanizing step, before applying the hot dip galvanizing, it is held at a temperature of 440 to 600 ° C. for 5 to 100 seconds, according to any one of the above (8) to (10) Manufacturing method of high-tensile hot-dip galvanized steel sheet.
(12)溶融亜鉛めっき工程において、溶融亜鉛めっきを施した後に合金化処理を施すことを特徴とする、上記(8)〜(11)のいずれかに記載の高張力溶融亜鉛めっき鋼板の製造方法。 (12) In the hot dip galvanizing step, the alloying treatment is performed after the hot dip galvanizing, and the method for producing the high-tensile hot dip galvanized steel sheet according to any one of (8) to (11) above .
なお、本発明において、「Ｔｉ系炭窒化析出物」とは、Ｔｉを含有する炭化析出物、窒化析出物および炭窒化析出物の総称である。また、「溶融亜鉛めっき鋼板」には溶融亜鉛めっきした後に合金化処理を施す「合金化溶融亜鉛めっき鋼板」も含まれる。さらに、「ベイナイト」には、ベイニティックフェライトも含まれる。 In the present invention, the “Ti-based carbonitride precipitate” is a general term for a carbonized precipitate, a nitrided precipitate, and a carbonitride precipitate containing Ti. “Hot galvanized steel sheet” also includes “alloyed hot dip galvanized steel sheet” which is subjected to alloying treatment after being hot dip galvanized. Further, “bainite” includes bainitic ferrite.
本発明によれば、高強度で加工性を確保しつつ、曲げ加工性及び耐切欠き疲労特性、耐食性に優れた、高張力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ができる。 According to the present invention, it is possible to provide a high-tensile hot-dip galvanized steel sheet and a method for producing the same, which are excellent in bending workability, notch fatigue resistance, and corrosion resistance while ensuring high strength and workability.
この高張力溶融亜鉛めっき鋼板は、自動車や各種の産業機械に用いられる構造部材の素材、特に自動車のメンバーや足廻り部品に代表される構造部材の素材として最適である。 This high-tensile hot-dip galvanized steel sheet is optimal as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and undercarriage parts.
まず、本発明の高張力溶融亜鉛めっき鋼板のめっきの基材である鋼板の規定理由について説明する。以下、組成についての％は質量％を表す。 First, the reason for the regulation of the steel sheet which is the base material for plating the high-tensile hot-dip galvanized steel sheet of the present invention will be described. Hereinafter, “%” for the composition represents “% by mass”.
Ａ．本発明に係る鋼板の化学組成について
Ｃ：０．０２％を超え０．２０％以下
Ｃは、Ｔｉ系炭窒化析出物による析出強化、フェライトの微細化、更には、フェライト以外の第２相による強度確保のために必要な元素である。しかし、その含有量が０．０２％以下では所望の５９０ＭＰａ以上の引張強度が確保できない。一方、０．２０％を超えると溶接性が低下する。したがって、Ｃの含有量を０．０２％を超え０．２０％以下とした。なお、７８０ＭＰａ以上の高強度を得るには、Ｃを０．０４％以上含有させることが望ましく、そして、９８０ＭＰａ以上の高強度を得るには、Ｃを０．０６％以上含有させることが望ましい。
A. Regarding the chemical composition of the steel sheet according to the present invention C: more than 0.02% and not more than 0.20% C is precipitation strengthening by Ti carbonitride precipitates, refinement of ferrite, and further by the second phase other than ferrite It is an element necessary for ensuring strength. However, if the content is 0.02% or less, the desired tensile strength of 590 MPa or more cannot be ensured. On the other hand, if it exceeds 0.20%, the weldability decreases. Therefore, the C content is more than 0.02% and 0.20% or less. In order to obtain a high strength of 780 MPa or more, it is desirable to contain 0.04% or more of C, and in order to obtain a high strength of 980 MPa or more, it is desirable to contain 0.06% or more of C.
Ｓｉ：０．０１〜２．０％
Ｓｉは、固溶強化によって鋼板の強度を高める元素である。その効果を得るには、0.01％以上の含有が必要である。一方、Ｓｉ含有量が多くなると、鋼表面に生成する酸化スケールが過多になって製造上の困難を伴い、特に、その含有量が２．０％を超えると鋼表面に生成する酸化スケールが極めて過多になる。したがって、Ｓｉの含有量を０．０１〜２．０％とした。望ましくは、０．０１〜１．０％である。
Si: 0.01 to 2.0%
Si is an element that increases the strength of the steel sheet by solid solution strengthening. In order to obtain the effect, a content of 0.01% or more is necessary. On the other hand, when the Si content increases, the oxide scale generated on the steel surface becomes excessive, resulting in manufacturing difficulties. In particular, when the content exceeds 2.0%, the oxide scale generated on the steel surface is extremely high. Too much. Therefore, the Si content is set to 0.01 to 2.0%. Desirably, it is 0.01 to 1.0%.
Ｍｎ：０．１〜３．０％
Ｍｎは、鋼の焼入性を高め強度を上昇させるのに有効な元素であるが、その含有量が０．１％未満では十分な強度が得られない。一方、３．０％を超えると焼入性が過大となり、マルテンサイトが多く生成する。そのため、曲げ加工性の著しい低下をきたす。したがって、Ｍｎの含有量を０．１〜３．０％とした。望ましい下限は０．５％であり、望ましい上限は２．５％である。
Mn: 0.1 to 3.0%
Mn is an element effective for increasing the hardenability of steel and increasing the strength, but if its content is less than 0.1%, sufficient strength cannot be obtained. On the other hand, if it exceeds 3.0%, the hardenability becomes excessive and a lot of martensite is generated. Therefore, the bending workability is significantly reduced. Therefore, the Mn content is set to 0.1 to 3.0%. A desirable lower limit is 0.5% and a desirable upper limit is 2.5%.
Ｐ：０．００３〜０．１０％
Ｐは固溶強化として働く元素であり、高強度化のために有効である。しかし、その含有量が０．００３％未満では上記の効果が得難い。一方、Ｐは偏析し易い元素であるため多量に添加した場合には、加工性の低下を招き、特に、その含有量が０．１０％を超えると偏析が著しくなって加工性の低下が極めて大きくなる。したがって、Ｐの含有量を０．００３〜０．１０％とした。望ましい下限は０．００５％であり、望ましい上限は０．０５％である。
P: 0.003-0.10%
P is an element that works as a solid solution strengthening, and is effective for increasing the strength. However, if the content is less than 0.003%, it is difficult to obtain the above effect. On the other hand, since P is an element that easily segregates, when added in a large amount, it causes a decrease in workability. In particular, when its content exceeds 0.10%, the segregation becomes remarkable and the workability is extremely reduced. growing. Therefore, the content of P is set to 0.003 to 0.10%. A desirable lower limit is 0.005% and a desirable upper limit is 0.05%.
Ｓ：０．０２０％以下
Ｓは、曲げ加工性を低下させる硫化物を生成するため、可能な限り低減する必要のある不純物である。本発明においては、他の成分元素添加による曲げ加工性の向上度合と製鋼コストを考慮して、その含有量の上限を０．０２０％とした。望ましくは、０．０１０％以下である。
S: 0.020% or less S is an impurity that needs to be reduced as much as possible in order to generate a sulfide that lowers bending workability. In the present invention, the upper limit of the content is set to 0.020% in consideration of the degree of improvement in bending workability by adding other component elements and the steelmaking cost. Desirably, it is 0.010% or less.
Ａｌ：０．００１〜１．０％
Ａｌは、鋼の脱酸に有用な元素である。その効果を得るには、少なくとも０．００1％の含有量が必要である。一方、その含有量が１．０％を超えると、粗大なアルミナ系介在物が増加して、曲げ加工性及び耐疲労特性が著しく低下する。したがって、Ａｌの含有量を０．００１〜１．０％とした。望ましい下限は０．０１％であり、望ましい上限は０．１％である。
Al: 0.001 to 1.0%
Al is an element useful for deoxidation of steel. In order to obtain the effect, a content of at least 0.001% is necessary. On the other hand, when the content exceeds 1.0%, coarse alumina inclusions increase, and the bending workability and the fatigue resistance are remarkably lowered. Therefore, the Al content is set to 0.001 to 1.0%. A desirable lower limit is 0.01% and a desirable upper limit is 0.1%.
Ｔｉ：０．０３〜０．２％
Ｔｉは、本発明において最も重要な元素である。０．０３％未満では、析出強化に効果のある平均粒径2〜30nmのＴｉ系炭窒化析出物の生成量が少なくて、強度上昇の効果がない。また、０．２％を超えて含有させてもこれらの効果は飽和するだけである。したがって、Ｔｉの含有量を０．０３〜０．２％とした。なお、７８０ＭＰａ以上の高強度を得るにはＴｉを０．０５％以上含有させるのが望ましく、そして、９８０ＭＰａ以上の高強度を得るにはＴｉを０．０７％以上含有させることが望ましい。
Ti: 0.03-0.2%
Ti is the most important element in the present invention. If it is less than 0.03%, the amount of Ti carbonitride precipitates having an average particle size of 2 to 30 nm effective for precipitation strengthening is small, and there is no effect of increasing the strength. Moreover, even if it contains more than 0.2%, these effects are only saturated. Therefore, the Ti content is set to 0.03 to 0.2%. In order to obtain a high strength of 780 MPa or more, it is desirable to contain 0.05% or more of Ti, and in order to obtain a high strength of 980 MPa or more, it is desirable to contain 0.07% or more of Ti.
Ｎ：０．０００４〜０．０１５％
Ｎは、Ｔｉを添加した鋼においてＴｉとともに粒径２〜３０ｎｍのＴｉ系炭窒化析出物および粒径３μｍ以上の晶出系ＴｉＮを形成する。しかし、Ｎの含有量が０．０００４％未満の場合、粒径３μｍ以上の晶出系ＴｉＮがほとんど生成しないので、部材の穴加工部における打ち抜き破面の形状が悪化して、耐切欠き疲労特性が低下する。一方、その含有量が０．０１５％を超えると、粗大な粒径３μｍ以上の晶出系ＴｉＮが多く生成して曲げ加工性が低下し、曲げ加工特性ならびに耐切欠き疲労特性が著しく低下する。したがって、Ｎの含有量を０．０００４〜０．０１５％とした。望ましい下限は０．００２％であり、望ましい上限は０．００８％である。
N: 0.0004 to 0.015%
N forms Ti-based carbonitride with a particle size of 2 to 30 nm and crystallized TiN with a particle size of 3 μm or more together with Ti in steel to which Ti is added. However, when the N content is less than 0.0004%, crystallized TiN having a grain size of 3 μm or more is hardly generated, so that the shape of the punched fracture surface in the hole processed portion of the member deteriorates, and notch fatigue resistance. Decreases. On the other hand, if the content exceeds 0.015%, a large amount of crystallized TiN having a coarse particle size of 3 μm or more is generated, bending workability is lowered, and bending workability and notch fatigue resistance are remarkably lowered. Therefore, the content of N is set to 0.0004 to 0.015%. A desirable lower limit is 0.002%, and a desirable upper limit is 0.008%.
また、鋼塊又は鋳片の加熱時から熱間圧延後の巻き取りにかけて、または、溶融亜鉛めっき前の均熱からその後の冷却過程にかけて、粒径２〜３０ｎｍのＴｉ系炭窒化析出物が生成する。Ｎの含有量が０．０００４％未満の場合、Ｎの含有量が少ないため、粒径２〜３０ｎｍのＴｉ系炭窒化析出物がほとんど生成しないので、析出強化の効果が減少してしまう。また、Ｎの含有量が０．０１５％を超えると、Ｔｉ系炭窒化析出物が粗大となるため、析出強化の効果が減少してしまう。Ｔｉの含有量を０．０５％以上かつＮの含有量を０．００２〜０．００８％にするとともにＣの含有量を０．０４％以上にすることが、析出強化に効果的な粒径２〜３０ｎｍのＴｉ系炭窒化析出物を得る上で、特に好ましい。 In addition, Ti-carbon carbonitride precipitates with a particle size of 2 to 30 nm are generated from the heating of the steel ingot or slab to the winding after hot rolling, or from the soaking before hot dip galvanization to the subsequent cooling process. To do. When the content of N is less than 0.0004%, since the content of N is small, Ti-based carbonitride precipitates having a particle size of 2 to 30 nm are hardly generated, so that the effect of precipitation strengthening is reduced. On the other hand, if the N content exceeds 0.015%, the Ti carbonitride precipitates become coarse, so the effect of precipitation strengthening decreases. It is effective for precipitation strengthening that the Ti content is 0.05% or more and the N content is 0.002 to 0.008% and the C content is 0.04% or more. It is particularly preferable for obtaining a 2-30 nm Ti-based carbonitride precipitate.
以上のＣからＮまでの元素は、本発明に係る高張力溶融亜鉛めっき鋼板の基材となる鋼板を特徴付ける化学成分である。 The above elements from C to N are chemical components that characterize the steel sheet that is the base material of the high-tensile hot-dip galvanized steel sheet according to the present invention.
そして、この鋼板の化学組成の残部がＦｅと不純物であるとともに、この鋼板の結晶組織がフェライトを面積率で３０〜９５％含有し、残部の第２相がマルテンサイト、ベイナイト、パーライト、セメンタイトおよび残留オーステナイトのうちの１種または２種以上からなり、かつマルテンサイトを含有しないか又は含有する場合にマルテンサイトの面積率は５０％以下であり、そして、前記鋼板が粒径２〜３０ｎｍのＴｉ系炭窒化析出物を平均粒子間距離３０〜３００ｎｍで含有し、かつ粒径３μｍ以上の晶出系ＴｉＮを平均粒子間距離５０〜５００μｍで含有するものが、本発明(1)に係る鋼板である。
And the balance of the chemical composition of this steel sheet is Fe and impurities, the crystal structure of this steel sheet contains ferrite in an area ratio of 30 to 95%, and the remaining second phase is martensite, bainite, pearlite, cementite and It consists of one or more of the retained austenite and does not contain or contains martensite , the martensite area ratio is 50% or less , and the steel sheet has a grain size of 2 to 30 nm. A steel plate according to the present invention (1) contains a carbonitrided carbonaceous precipitate at an average interparticle distance of 30 to 300 nm and a crystallization TiN having a particle size of 3 μm or more at an average interparticle distance of 50 to 500 μm. is there.
本発明に係る鋼板は、任意添加元素として、Ｎｂ、Ｖ、Ｗ、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉ、Ｂ、ＲＥＭ、Ｍｇ、ＣａおよびＺｒのうちの１種または２種以上を含有させることができる。 The steel plate according to the present invention can contain one or more of Nb, V, W, Cr, Mo, Cu, Ni, B, REM, Mg, Ca, and Zr as optional additional elements. .
まず、析出強化による強度上昇を目的として、本発明(1)に係る高張力溶融亜鉛めっき鋼板の基材である鋼板のＦｅの一部に代えて、Ｎｂ：０．１％以下、Ｖ：０．５％以下およびＷ：０．５％以下のうちの１種または２種以上を含有させることができる。これが本発明(2)に係る鋼板である。 First, for the purpose of increasing the strength by precipitation strengthening, Nb: 0.1% or less, V: 0 instead of a part of Fe of the steel plate which is the base material of the high-tensile hot-dip galvanized steel plate according to the present invention (1) One or more of 5% or less and W: 0.5% or less can be contained. This is the steel sheet according to the present invention (2).
次に、固溶強化による強度上昇を目的として、本発明(1)または(2)に係る高張力溶融亜鉛めっき鋼板の基材である鋼板のＦｅの一部に代えて、Ｃｒ：１．０％以下、Ｍｏ：１．０％以下、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下およびＢ：０．０１％以下のうちの１種または２種以上を含有させることができる。これが本発明(3)に係る鋼板である。 Next, for the purpose of increasing the strength by solid solution strengthening, instead of part of Fe of the steel sheet which is the base material of the high-tensile galvanized steel sheet according to the present invention (1) or (2), Cr: 1.0 % Or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less can be contained. This is the steel sheet according to the present invention (3).
また、酸化物、硫化物などの形態制御による曲げ加工性向上を目的として、本発明(1)〜(3)のいずれかに係る高張力溶融亜鉛めっき鋼板の基材である鋼板のＦｅの一部に代えて、ＲＥＭ：０．０００５〜０．１％、Ｍｇ：０．０００５〜０．０１％およびＣａ：０．０００５〜０．０１％のうちの１種または２種以上を含有させることができる。これが、本発明(4)に係る鋼板である。 In addition, for the purpose of improving the bending workability by controlling the form of oxides, sulfides, etc., one of Fe of the steel sheet which is the base material of the high-tensile hot-dip galvanized steel sheet according to any one of the present inventions (1) to (3). In place of parts, REM: 0.0005 to 0.1%, Mg: 0.0005 to 0.01% and Ca: 0.0005 to 0.01%, or one or more of them should be contained Can do. This is the steel sheet according to the present invention (4).
さらに、ＭｎＳの生成による曲げ加工性の低下を防止することを目的として、本発明(1)〜(4)のいずれかに係る高張力溶融亜鉛めっき鋼板の基材である鋼板のＦｅの一部に代えて、Ｚｒ：０．０００２〜０．０１％を含有させることができる。これが、本発明(5)に係る鋼板である。 Furthermore, for the purpose of preventing a decrease in bending workability due to the generation of MnS, a part of Fe of the steel sheet that is the base material of the high-tensile hot-dip galvanized steel sheet according to any of the present inventions (1) to (4) Instead of Zr, 0.0002 to 0.01% can be contained. This is the steel sheet according to the present invention (5).
以下、これらの任意添加元素について、詳述する。 Hereinafter, these optional additive elements will be described in detail.
Ｎｂ：０．１％以下、Ｖ：０．５％以下、Ｗ：０．５％以下
Ｎｂ、Ｖ及びＷは、Ｔｉと同様に析出強化によって強度を高める元素であり、強度を一層高める作用を有する。Ｎｂ、Ｖ及びＷは、それぞれを単独で含有させてもよいし、２種以上を複合して含有させてもよい。しかし、Ｎｂは０．１％を超えて含有させると、延性の低下をきたし、そして、Ｖ、Ｗは０．５％を超えて含有させると延性の低下をきたす。さらに、原料コストの上昇も著しくなる。したがって、Ｎｂを添加する場合にはその含有量は０．１％以下とするのがよく、また、Ｖ、Ｗを添加する場合には、それぞれ、０．５％以下とするのがよい。
なお、この効果を確実に得るには、Ｎｂ、Ｖ、Ｗはいずれも０．０１％以上の含有量とすることが好ましい。
Nb: 0.1% or less, V: 0.5% or less, W: 0.5% or less Nb, V and W are elements that increase the strength by precipitation strengthening in the same manner as Ti, and have the effect of further increasing the strength. Have. Nb, V and W may be contained alone or in combination of two or more. However, if Nb is contained in an amount exceeding 0.1%, the ductility is lowered, and if V and W are contained in an amount exceeding 0.5%, the ductility is lowered. In addition, the cost of raw materials increases significantly. Therefore, when Nb is added, the content is preferably 0.1% or less, and when V and W are added, each content is preferably 0.5% or less.
In order to reliably obtain this effect, it is preferable that Nb, V, and W have a content of 0.01% or more.
Ｃｒ：１．０％以下、Ｍｏ：１．０％以下、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下、Ｂ：０．０１％以下
Ｃｒ、Ｍｏ、Ｃｕ、ＮｉおよびＢは、いずれも固溶強化によって強度を一層高める作用を有する。これらの元素は、それぞれを単独で含有させてもよいし、２種以上を複合して含有させてもよい。しかし、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉについては、１．０％を超えて含有させると延性の低下をきたし、そして、Ｂについては０．０１％を超えて含有させると延性の低下をきたす。さらに、原料コストの上昇も著しくなる。したがって、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉを添加する場合にはその含有量はいずれも１．０％以下とするのがよく、そして、Ｂを添加する場合にはその含有量を０．０１％以下とするのがよい。
Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, B: 0.01% or less Cr, Mo, Cu, Ni and B are: All have the effect | action which raises an intensity | strength further by solid solution strengthening. These elements may be contained alone or in combination of two or more. However, when Cr, Mo, Cu, and Ni are contained in excess of 1.0%, the ductility is lowered, and when B is contained in excess of 0.01%, the ductility is lowered. In addition, the cost of raw materials increases significantly. Therefore, when Cr, Mo, Cu, Ni is added, the content should be 1.0% or less, and when B is added, the content is 0.01% or less. It is good to do.
なお、固溶強化の効果を確実に得るには、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉはいずれも０．０５％以上の含有量とするのが好ましく、そして、Ｂについては０．０００３％以上の含有量とするのが好ましい。 In order to surely obtain the effect of solid solution strengthening, it is preferable that Cr, Mo, Cu, and Ni each have a content of 0.05% or more, and B contains 0.0003% or more. An amount is preferred.
ＲＥＭ：０．１％以下、Ｍｇ：０．０１％以下、Ｃａ：０．０１％以下
ＲＥＭ（希土類元素）、ＭｇおよびＣａは、いずれも、酸化物や硫化物を微細に球状化し、曲げ加工性を向上させる効果をもつ。ＲＥＭ、ＭｇおよびＣａは、それぞれを単独で含有させてもよいし、２種以上を複合して含有させてもよい。
REM: 0.1% or less, Mg: 0.01% or less, Ca: 0.01% or less REM (rare earth element), Mg, and Ca are all spheroidized oxides and sulfides and bent. Has the effect of improving the performance. REM, Mg, and Ca may be contained alone or in combination of two or more.
しかし、含有量が、ＲＥＭについては０．１％を超えて含有させると、鋼中に酸化物や硫化物が多く存在してしまい、曲げ加工性は劣化する。また、Ｍｇ、Ｃａについては０．０１％を超えて含有させると鋼中に酸化物や硫化物が多く存在してしまい、曲げ加工性は劣化する。 However, if the content exceeds 0.1% for REM, a large amount of oxides and sulfides are present in the steel, and bending workability deteriorates. Further, if Mg and Ca are contained in excess of 0.01%, many oxides and sulfides are present in the steel, and the bending workability is deteriorated.
したがって、ＲＥＭ：０．１％以下、Ｍｇ：０．０１％以下、Ｃａ：０．０１％以下とするのが好ましい。
それぞれの下限は特に限定しないが、酸化物や硫化物の微細球状化の効果を確実に得るには、ＲＥＭ、Ｍｇ、Ｃａはいずれも０．０００５％以上の含有量とすることが好ましい。
Therefore, REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less are preferable.
Each lower limit is not particularly limited. However, in order to surely obtain the effect of spheroidizing oxides or sulfides, it is preferable that REM, Mg, and Ca have a content of 0.0005% or more.
ここで、ＲＥＭとは、Ｓｃ、Ｙ及びランタノイドの合計１７元素を指し、ランタノイドの場合、工業的にはミッシュメタルの形で添加される。なお、本発明では、ＲＥＭの含有量はこれらの元素の合計含有量を指す。 Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.
Ｚｒ：０．０１％以下
Ｚｒを添加すると、ＭｎＳの生成による曲げ加工性の低下を防止することができる。しかし、その効果は０．０１％の含有量で飽和するので、含有量の上限を０．０１％とすることが好ましい。
下限は特に限定しないが、ＭｎＳの生成による曲げ加工性の低下を防止する効果を確実に得るには、Ｚｒは０．０００２％以上の含有量とすることが好ましい。
Zr: 0.01% or less Addition of Zr can prevent a decrease in bending workability due to generation of MnS. However, since the effect is saturated at a content of 0.01%, the upper limit of the content is preferably 0.01%.
The lower limit is not particularly limited, but Zr is preferably made a content of 0.0002% or more in order to surely obtain an effect of preventing a decrease in bending workability due to the generation of MnS.
Ｂ．本発明に係る鋼板中の析出物について
引張強度が５９０ＭＰａ以上の領域で、良好な曲げ加工性と耐切欠き疲労特性を得るためには、粒径２〜３０ｎｍのＴｉ系炭窒化析出物を平均粒子間距離３０〜３００ｎｍで含有し、かつ粒径３μｍ以上の晶出系ＴｉＮを平均粒子間距離５０〜５００μｍで含有させる必要がある。その理由は、次の(i)及び(ii)に示すとおりである。
B. About the precipitate in the steel sheet according to the present invention In order to obtain good bending workability and notch fatigue resistance in the region where the tensile strength is 590 MPa or more, an average particle size of Ti-based carbonitride precipitate having a particle size of 2 to 30 nm is used. It is necessary to contain crystallization TiN having an inter-particle distance of 30 to 300 nm and a particle size of 3 μm or more at an average inter-particle distance of 50 to 500 μm. The reason is as shown in the following (i) and (ii).
(i)Ｔｉ系炭窒化析出物の粒径と平均粒子間距離について
Ｔｉ系炭窒化析出物は、析出強化に寄与する析出物であるが、そのためには、その粒径を２〜３０ｎｍとする必要がある。Ｔｉ系炭窒化析出物は、粒径が２ｎｍを下回っても、そして、３０ｎｍを上回っても、転位を容易に通過させてしまうので、析出強化に寄与しない析出物となるからである。また、Ｔｉ系炭窒化析出物の平均粒子間距離が３００ｎｍを超えると、析出物の間隔が広すぎるため、転位が容易に通過してしまい、析出強化に寄与しない。
そして、Ｔｉ系炭窒化析出物の平均粒子間距離が３０ｎｍ未満であると、フェライト粒内にＴｉ系炭窒化析出物が多くなりすぎるため曲げ加工性に劣る。
(i) Particle size and average inter-particle distance of Ti-based carbonitride precipitates Ti-based carbonitride precipitates are precipitates that contribute to precipitation strengthening. For that purpose, the particle size is set to 2 to 30 nm. There is a need. This is because the Ti-based carbonitride precipitate is a precipitate that does not contribute to precipitation strengthening because the dislocation is easily passed even if the particle size is less than 2 nm and more than 30 nm. In addition, when the average interparticle distance of the Ti-based carbonitride precipitate exceeds 300 nm, the distance between the precipitates is too wide, so that dislocations pass easily and do not contribute to precipitation strengthening.
When the average inter-particle distance of the Ti-based carbonitride precipitates is less than 30 nm, the amount of Ti-based carbonitride precipitates is excessive in the ferrite grains, resulting in poor bending workability.
したがって、粒径２〜３０ｎｍのＴｉ系炭窒化析出物を平均粒子間距離３０〜３００ｎｍで含有させる必要がある。なお、Ｔｉ系炭窒化析出物の好ましい粒径は５〜２５ｎｍであり、Ｔｉ系炭窒化析出物の好ましい平均粒子間距離は５０〜２５０ｎｍである。 Therefore, it is necessary to contain a Ti-based carbonitride precipitate having a particle size of 2 to 30 nm with an average interparticle distance of 30 to 300 nm. In addition, the preferable particle size of the Ti-based carbonitride precipitate is 5 to 25 nm, and the preferable average interparticle distance of the Ti-based carbonitride precipitate is 50 to 250 nm.
さらに、本発明に係る鋼板は、表面に溶融亜鉛めっき層を備えるものであるが、鋼板表面に存在するＴｉ系炭窒化析出物は、溶融亜鉛めっき層を母材の鋼板に密着させるアンカー効果を発揮する。このアンカー効果を十分に発揮するためには、Ｔｉ系炭窒化析出物の粒径は５〜２５ｎｍが好ましく、また、Ｔｉ系炭窒化析出物の平均粒子間距離は５０〜２５０ｎｍが好ましい。 Furthermore, the steel sheet according to the present invention is provided with a hot dip galvanized layer on the surface, but the Ti-based carbonitride precipitates present on the steel sheet surface have an anchor effect that causes the hot dip galvanized layer to adhere to the base steel sheet. Demonstrate. In order to sufficiently exhibit this anchor effect, the particle size of the Ti-based carbonitride precipitate is preferably 5 to 25 nm, and the average interparticle distance of the Ti-based carbonitride precipitate is preferably 50 to 250 nm.
(ii)晶出系ＴｉＮの粒径と平均粒子間距離について
粒径３μｍ以上の晶出系ＴｉＮは、曲げ加工性及び耐切欠き疲労特性に著しく影響するが、平均粒子間距離が５０μｍを下回ると、曲げ加工性が著しく劣る。この理由としては、粒径３μｍ以上の晶出系ＴｉＮは金属組織に比べ、硬度が非常に高いため、曲げ加工した場合、金属組織とその晶出系ＴｉＮの界面から亀裂が発生して進展するが、晶出系ＴｉＮの平均粒子間隔が５０μｍ以上であると、間隔が広いため、曲げ加工した場合、亀裂が発生しにくいし、発生しても進展しにくい。このため、粒径３μｍ以上の晶出系ＴｉＮの平均粒子間隔が５０μｍ以上であると、曲げ加工性に優れることになる。
(ii) About grain size and average interparticle distance of crystallized TiN Crystallized TiN having a grain size of 3 μm or more remarkably affects bending workability and notch fatigue resistance, but if the average interparticle distance is less than 50 μm The bending workability is extremely inferior. The reason for this is that crystallized TiN having a particle size of 3 μm or more has a very high hardness compared to the metal structure, and when bent, cracks are generated from the interface between the metal structure and the crystallized TiN and progress. However, when the average particle interval of the crystallization TiN is 50 μm or more, the interval is wide. Therefore, when bending is performed, cracks hardly occur, and even if they occur, they do not easily progress. For this reason, it is excellent in bending workability that the average particle | grain space | interval of crystallization type TiN with a particle size of 3 micrometers or more is 50 micrometers or more.
しかしながら、粒径３μｍ以上の晶出系ＴｉＮの粒子間距離が５００μｍを超えると、打ち抜き時亀裂が蛇行し、部材穴加工部の破面の形状が悪化することになる結果、応力集中が生じやすくなり、耐切欠き疲労特性が著しく劣化する。これに対して、粒径３μｍ以上の晶出系ＴｉＮが平均粒子間隔が５００μｍ以下であると、部材の穴加工のための打ち抜き時、晶出系ＴｉＮに沿って亀裂が入るので、亀裂が蛇行しにくいためである。そのため、打ち抜いた端面の破面性状が良好となる。 However, if the inter-particle distance of crystallized TiN having a particle size of 3 μm or more exceeds 500 μm, cracks meander when punching, and the shape of the fracture surface of the member hole processed portion will deteriorate, resulting in stress concentration. As a result, the notch fatigue resistance is significantly deteriorated. On the other hand, when the crystallized TiN having a particle size of 3 μm or more has an average particle spacing of 500 μm or less, cracks occur along the crystallized TiN when punching for drilling a member. This is because it is difficult to do. Therefore, the fractured surface properties of the punched end surface are good.
したがって、強度５９０ＭＰａ以上で良好な延性および曲げ加工性を確保しつつ、更に耐切欠き疲労特性を得るには、粒径３μｍ以上の晶出系ＴｉＮを平均粒子間距離５０〜５００μｍで含有させる必要がある。 Therefore, in order to obtain notch fatigue resistance while ensuring good ductility and bending workability at a strength of 590 MPa or more, it is necessary to contain crystallized TiN having a particle size of 3 μm or more with an average interparticle distance of 50 to 500 μm. is there.
なお、対象とする晶出系ＴｉＮの好ましい粒径は３〜７μｍであり、対象とする晶出系ＴｉＮの好ましい平均粒子間距離は８０〜３２０μｍである。 In addition, the preferable particle size of the target crystallization TiN is 3 to 7 μm, and the preferable average interparticle distance of the target crystallization TiN is 80 to 320 μm.
なお、本発明に係る鋼板は、表面に溶融亜鉛めっき層を備えるものであるが、鋼板表面に晶出系ＴｉＮが存在すると、溶融亜鉛めっき層との濡れ性が悪くなる。したがって、本発明に係る鋼板の溶融亜鉛めっき層と母材の鋼板との間の密着力は、共に鋼板表面に存在して、片やアンカー効果を発揮するＴｉ系炭窒化析出物と、逆に濡れ性を悪くする晶出系ＴｉＮのバランスによって左右されることになる。 In addition, although the steel plate which concerns on this invention equips the surface with a hot dip galvanization layer, when crystallization system TiN exists in the steel plate surface, wettability with a hot dip galvanization layer will worsen. Therefore, the adhesion force between the hot dip galvanized layer of the steel sheet according to the present invention and the base steel sheet is both present on the steel sheet surface, and on the contrary, Ti-based carbonitride precipitates exhibiting a piece and anchor effect, It depends on the balance of the crystallization TiN that deteriorates the wettability.
Ｃ．本発明に係る鋼板の金属組織について
強度５９０ＭＰａ以上で、優れた曲げ加工性と耐切欠き疲労特性を得るには、鋼板表面から鋼板中心部までの金属組織においてフェライトの面積率が３０％〜９５％であることが必要である。
C. About the metal structure of the steel sheet according to the present invention To obtain excellent bending workability and notch fatigue resistance at a strength of 590 MPa or more, the ferrite area ratio is 30% to 95% in the metal structure from the steel sheet surface to the steel sheet center. It is necessary to be.
フェライトの面積率が３０％未満であると、フェライト粒が少ないため、曲げ加工した場合に局部的な延性を保つことができないので、曲げ加工性に劣るからである。そして、フェライトの面積率が９５％を超えると、フェライト粒が多いため、疲労亀裂がフェライト粒内を積極的に進展するので、耐切欠き疲労特性が低下するからである。 This is because if the area ratio of ferrite is less than 30%, the ferrite grains are small, so that local ductility cannot be maintained when bending is performed, and therefore bending workability is poor. And when the area ratio of ferrite exceeds 95%, since there are many ferrite grains, fatigue cracks actively propagate in the ferrite grains, so that the notch fatigue resistance characteristics deteriorate.
また、フェライト以外の残部の第２相は、マルテンサイト、ベイナイト、パーライト、セメンタイトおよび残留オーステナイトの１種または２種以上とすることによって、優れた曲げ加工性が得られる。ただし、残部の第２相としてマルテンサイトを含有するときは、優れた曲げ加工性を得るために、マルテンサイトの面積率を５０％以下とする必要がある。マルテンサイトが面積率で５０％を超えると、曲げ加工初期におけるボイド発生を抑制できないために、曲げ加工初期に発生した亀裂が進展し易いので、曲げ加工性が低下するからである。
In addition, excellent bending workability can be obtained by using the remaining second phase other than ferrite as one or more of martensite, bainite, pearlite, cementite, and retained austenite. However, when martensite is contained as the remaining second phase, the martensite area ratio needs to be 50% or less in order to obtain excellent bending workability. This is because if the martensite exceeds 50% in area ratio, void generation at the initial stage of bending cannot be suppressed, and cracks generated at the initial stage of bending tend to progress, so that bending workability decreases.
本発明では、上述したとおり、Ｔｉ系炭窒化析出物を析出させることによって、フェライト粒を強化している。したがって、フェライトと第２相との硬度差が小さいため、曲げ加工特性もさらに向上する。また、フェライトの析出強化によっても、フェライト粒内において亀裂が進展しにくくなるので、この点からも、より優れた耐切欠き疲労特性を得ることができる。 In the present invention, as described above, ferrite grains are strengthened by depositing Ti-based carbonitride precipitates. Therefore, since the hardness difference between the ferrite and the second phase is small, the bending property is further improved. In addition, since the ferrite precipitation strengthening also makes it difficult for cracks to propagate in the ferrite grains, more excellent notch fatigue resistance can also be obtained from this point.
板表面から板中心部までのフェライトの平均粒径は２〜２０μｍが好ましい。平均粒径が２μｍ以上であると降伏比が適正範囲にあるため、変形し易いため、より優れた曲げ加工性を得ることができる。そして、平均粒径が２０μｍ以下であると、より優れた耐切欠き疲労特性を得ることができる。 The average particle size of ferrite from the plate surface to the plate center is preferably 2 to 20 μm. If the average particle size is 2 μm or more, the yield ratio is in an appropriate range, so that it is easily deformed, so that it is possible to obtain better bending workability. And the more excellent notch fatigue-proof characteristic can be acquired as an average particle diameter is 20 micrometers or less.
特に、鋼板表面から深さ５０μｍまでの鋼板表層部におけるフェライト平均粒径を２〜１０μｍとするのが好ましい。鋼板表層部のフェライトの平均粒径を１０μｍ以下に微細分散化することにより、第２相が微細分散化し、曲げ加工時に、表面に亀裂が入りにくくなる。また、フェライト粒が微細になると、結晶粒界が多くなるため、疲労亀裂が進展しにくくなり、耐切欠き疲労特性が向上する。したがって、優れた曲げ加工性を有するとともに、耐切欠き疲労特性が著しく向上したものを得ることができる。耐切欠き疲労特性を向上させたい場合には、鋼板表層部のフェライトの平均結晶粒径を５μｍ以下にすることが、さらに望ましい。 In particular, it is preferable that the ferrite average particle diameter in the steel sheet surface layer portion from the steel sheet surface to a depth of 50 μm is 2 to 10 μm. By finely dispersing the average particle diameter of ferrite in the steel sheet surface layer portion to 10 μm or less, the second phase is finely dispersed, and the surface is difficult to crack during bending. In addition, when the ferrite grains become finer, the crystal grain boundaries increase, so that fatigue cracks hardly progress and the notch fatigue resistance is improved. Accordingly, it is possible to obtain a material having excellent bending workability and significantly improved notch fatigue resistance. When it is desired to improve the notch fatigue resistance, it is more desirable that the average crystal grain size of ferrite in the surface layer portion of the steel sheet is 5 μm or less.
さらに、本発明に係る鋼板は、表面に溶融亜鉛めっき層を備えるものであるが、鋼板表面のフェライトは、その結晶粒が細かい程、溶融亜鉛めっき層との濡れ性がよいので、めっきの密着性を向上させることができる。さらに、めっき層と母材の鋼板との間で合金化処理を施して、めっきの密着性を向上させることができる。鋼板表面のフェライトは、その結晶粒が細かい程、合金化処理によるめっき密着性がさらに向上する。 Furthermore, the steel sheet according to the present invention is provided with a hot dip galvanized layer on the surface, but the ferrite on the steel sheet surface has better wettability with the hot dip galvanized layer as the crystal grains are finer, so that the adhesion of the plating is improved. Can be improved. Furthermore, an alloying treatment can be performed between the plating layer and the base steel sheet to improve the adhesion of the plating. The finer the crystal grains of the ferrite on the steel sheet surface, the better the plating adhesion by alloying treatment.
Ｄ．本発明に係る溶融亜鉛めっき層について
本発明に係る溶融亜鉛めっき鋼板の溶融亜鉛めっき層の好ましい態様は次のとおりである。
D. About the hot dip galvanized layer concerning the present invention The preferred mode of the hot dip galvanized layer of the hot dip galvanized steel sheet concerning the present invention is as follows.
めっき量としては、３〜８００ｇ／ｍ２を鋼板表面に施す。３ｇ／ｍ２以上とすると、十分に優れた防食作用が発揮され、亜鉛めっきの目的を果たすことができる。また、８００ｇ／ｍ２以下にすると、溶接時にブローホールなどの欠陥が発生しにくくなるためである。耐食性の確保や、めっき量のコストの観点からは、１０〜２００ｇ／ｍ２がさらに好ましい。
As a plating amount, 3 to 800 g / m 2 is applied to the steel sheet surface . When it is 3 g / m 2 or more, a sufficiently excellent anticorrosive action is exhibited and the purpose of galvanization can be achieved. Moreover, if it is 800 g / m 2 or less, defects such as blowholes are less likely to occur during welding. From the viewpoint of ensuring corrosion resistance and the cost of the plating amount, 10 to 200 g / m 2 is more preferable.
また、上記めっきを施した後、合金化処理をすると、めっきの密着性を向上させることができるので、好ましい。めっき皮膜の合金化処理をする際には、合金化度はＦｅ含有量を３〜２０％程度とするのが望ましい。３％以上の合金化度により、合金化処理の効果が得られるが、合金化度が２０％を超えるとパウダリングが劣化する場合がある。合金化処理によるめっき密着性の確保とパウダリング性の観点から、さらに好ましくは、合金化度は７〜１５％である。 Further, it is preferable to perform an alloying treatment after the plating, since the adhesion of the plating can be improved. When alloying the plating film, the degree of alloying is preferably about 3 to 20% of Fe content. Although the effect of alloying treatment can be obtained with an alloying degree of 3% or more, powdering may deteriorate if the alloying degree exceeds 20%. More preferably, the degree of alloying is 7 to 15% from the viewpoint of securing plating adhesion by alloying treatment and powdering properties.
さらに、亜鉛めっき層の表面には、有機系又は無機系の皮膜を施すことができる。この場合でも本発明の効果は損なわれない。 Furthermore, an organic or inorganic film can be applied to the surface of the galvanized layer. Even in this case, the effect of the present invention is not impaired.
Ｅ．本発明に係る鋼板の特性について
本発明の鋼板は、上記の成分からなり、かつ上記のＴｉ系炭窒化析出物と晶出系ＴｉＮを含有してなる溶融亜鉛めっき鋼板であるので、引張り強度が５９０ＭＰａ以上、１８０度曲げにおける限界曲げ半径が板厚の２倍以下であり、かつ切欠き曲げ疲労特性の耐久比が０．３８以上であるものが得られる。
E. Regarding the characteristics of the steel sheet according to the present invention The steel sheet of the present invention is a hot-dip galvanized steel sheet comprising the above components and containing the above Ti-based carbonitride precipitates and crystallized TiN, and therefore has a tensile strength. A material having a bending radius of 590 MPa or more and a limit bending radius of 180 ° bending of not more than twice the plate thickness and a durability ratio of notched bending fatigue characteristics of 0.38 or more is obtained.
さらに、金属組織や溶融亜鉛めっき層を上記の好ましい態様にすると、強度７８０ＭＰａ以上の鋼板であっても、１８０度曲げにおける限界曲げ半径が板厚の２倍以下であり、かつ切欠き曲げ疲労特性の耐久比が０．３８以上であるものが得られる。なお、強度７８０ＭＰａ以上の鋼板とすることによって、特に部材の薄肉化に効力が発揮される。 Furthermore, when the metal structure and the hot-dip galvanized layer are in the above-described preferred embodiment, even if the steel sheet has a strength of 780 MPa or more, the limit bending radius in 180-degree bending is not more than twice the plate thickness, and the notched bending fatigue characteristics. In which the durability ratio is 0.38 or more. In addition, especially by making it the steel plate of intensity | strength 780 Mpa or more, an effect is exhibited for thickness reduction of a member.
Ｆ．本発明に係る鋼板の製造方法における各工程について
（１）鋳造工程
本発明(8)から(12)に係る鋼板の製造方法においては、連続鋳造中における溶鋼の液相線温度から固相線温度までの温度域内の平均冷却速度を０．３〜１．０℃／秒として急冷却する必要がある。溶鋼の液相線温度から固相線温度までの温度域内の平均冷却速度が０．３℃／秒未満の場合、冷却速度が遅いため、粗大な晶出系ＴｉＮが多く析出し、粒径３μｍ未満の晶出系ＴｉＮの平均粒子間距離は広がるが、粒径３μｍ以上の晶出系ＴｉＮの平均粒子間距離は５０μｍを下回る。一方、溶鋼の液相線温度から固相線温度までの温度域内の平均冷却速度が１．０℃／秒を超える場合、冷却速度が速すぎるために、粒径３μｍ未満の晶出系ＴｉＮの平均粒子間距離は狭くなり、粒径３μｍ以上の晶出系ＴｉＮがほとんど析出しないので、晶出系ＴｉＮの平均粒子間距離は５００μｍ超となる。
F. About each process in the manufacturing method of the steel plate which concerns on this invention (1) Casting process In the manufacturing method of the steel plate which concerns on this invention (8)-(12), solidus temperature from the liquidus temperature of the molten steel in continuous casting It is necessary to rapidly cool the average cooling rate in the temperature range up to 0.3 to 1.0 ° C./second. When the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature of the molten steel is less than 0.3 ° C./second, the cooling rate is slow, so that a large amount of coarse crystallization TiN precipitates and the particle size is 3 μm. The average interparticle distance of crystallized TiN having a particle size of less than 1 is widened, but the average interparticle distance of crystallized TiN having a particle diameter of 3 μm or more is less than 50 μm. On the other hand, when the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature of the molten steel exceeds 1.0 ° C./second, the cooling rate is too high, so that the crystallization TiN having a particle size of less than 3 μm The average interparticle distance becomes narrow, and crystallized TiN having a particle size of 3 μm or more hardly precipitates, so the average interparticle distance of crystallized TiN exceeds 500 μm.
（２）熱間圧延
熱間圧延前に鋼塊または鋼片を温度１１００℃以上にて加熱する必要がある。加熱温度が１１００℃未満の場合、凝固時に析出したＴｉ系炭窒化析出物が再固溶しないので、Ｔｉによる析出強化に寄与しない。なお、粒径３μｍ以上の晶出系ＴｉＮは、１４００℃を超えると、再固溶する場合があるから、加熱温度の上限は１４００℃とするのが好ましい。また、加熱温度が１３５０℃を超えると、鋼塊または鋼片が自重で変形してしまって、熱間圧延ができなく場合があるから、その上限は１３５０℃とするのがさらに好ましい。
(2) Hot rolling It is necessary to heat a steel ingot or a steel piece at a temperature of 1100 ° C or higher before hot rolling. When the heating temperature is less than 1100 ° C., Ti-based carbonitride precipitates precipitated during solidification do not re-dissolve, and thus do not contribute to precipitation strengthening by Ti. In addition, since the crystallization type TiN having a particle size of 3 μm or more may re-dissolve when it exceeds 1400 ° C., the upper limit of the heating temperature is preferably 1400 ° C. If the heating temperature exceeds 1350 ° C., the steel ingot or steel slab may be deformed by its own weight, and hot rolling may not be possible. Therefore, the upper limit is more preferably 1350 ° C.
熱間圧延工程における仕上げ圧延は、常法に従って行う。このとき、鋼板の特性変動を抑制するために、粗圧延後、仕上圧延前の粗バーに対して、誘導加熱等により全長の温度を９５０℃以上に均一化を図るのが好ましい。特に、鋼板表面の組織を均一化すると、曲げ加工性においてバラツキが少なくなる利点がある。そして、仕上圧延温度はＡｒ３点以上とするのが望ましい。また、巻取り温度については、スケール疵の発生を防止するために、７００℃以下とするのが望ましい。 Finish rolling in the hot rolling process is performed according to a conventional method. At this time, in order to suppress fluctuations in the characteristics of the steel sheet, it is preferable to make the temperature of the full length equal to or higher than 950 ° C. by induction heating or the like for the rough bar after rough rolling and before finish rolling. In particular, when the structure of the steel sheet surface is made uniform, there is an advantage that variations in bending workability are reduced. And it is desirable that the finish rolling temperature be 3 or more points of Ar. The winding temperature is preferably 700 ° C. or lower in order to prevent the occurrence of scale flaws.
（３）酸洗工程
熱延後の、酸洗工程については常法でかまわない。平坦性を確保するために、酸洗の前もしくは後に、０〜５％程度の軽度の圧延を行って、形状を修正するのが好ましい。
(3) Pickling process The pickling process after hot rolling may be performed in a conventional manner. In order to ensure flatness, it is preferable to perform a mild rolling of about 0 to 5% to correct the shape before or after pickling.
（４）冷間圧延工程
酸洗工程の後に、冷間圧延工程を施してもよい。このときの冷間圧延工程は常法でかまわない。冷間圧延を施す場合は、圧下率３０〜８５％とするのが好ましい。圧下率を高めにすると、焼鈍時のオーステナイトへの変態を促進するので、均熱処理の好適範囲を広げる効果がある。
(4) Cold rolling process You may give a cold rolling process after a pickling process. The cold rolling process at this time may be a conventional method. When cold rolling is performed, the rolling reduction is preferably 30 to 85%. When the rolling reduction is increased, the transformation to austenite at the time of annealing is promoted, which has the effect of widening the preferred range of soaking.
（５）溶融めっきの前処理工程
溶融亜鉛めっき工程の前に、まず、Ａｃ３点〜１０００℃に５秒以上保持する均熱処理を施す。
(5) Pretreatment step of hot dipping Before the hot dipping galvanization step, first, a soaking treatment is performed to hold at Ac 3 points to 1000 ° C for 5 seconds or more.
本発明に係る鋼板は、Ｔｉを大量に含有しているため、加工歪が残存して加工フェライトの再結晶は抑制される。このことにより、加熱時においてオーステナイト域まで加工歪が残存し易くなり、均熱時における加工フェライトからオーステナイトへの相変態が著しく促進される。そのため、わずか５秒以上の均熱処理を施すことにより、加工フェライトがオーステナイトへ相変態し、加工歪みが取り除かれる。なお、本発明に係る鋼板はＴｉを含有しているため、均熱時のオーステナイトの粒成長を効果的に抑制することができるので、均熱時間の上限は５００秒とするのが好ましい。ただし、生産性の観点からは、２００秒以内とするのがより好ましい。 Since the steel plate according to the present invention contains a large amount of Ti, processing strain remains and recrystallization of the processed ferrite is suppressed. This makes it easy for machining strain to remain in the austenite region during heating, and the phase transformation from worked ferrite to austenite during soaking is significantly promoted. Therefore, by performing soaking for only 5 seconds or more, the processed ferrite is transformed into austenite, and the processing strain is removed. In addition, since the steel plate which concerns on this invention contains Ti, since the grain growth of the austenite at the time of soaking can be suppressed effectively, it is preferable that the upper limit of soaking time shall be 500 second. However, from the viewpoint of productivity, the time is more preferably within 200 seconds.
均熱処理時の均熱温度は、Ａｃ３点〜１０００℃とする。均熱温度がＡｃ３点未満の場合、２相域での均熱処理となるため、加工フェライトからオーステナイトの相変態が不十分となる。加えて、フェライトの粒成長が著しく、フェライトの面積率が９５％超になり、耐切欠き疲労特性が低下してしまう。一方、均熱温度が１０００℃超の場合、オーステナイトの粒成長が著しく、その後の冷却過程におけるフェライト粒の生成が低下してしまい、フェライトの面積率が、３０％未満となり、曲げ加工性が劣化してしまう。 The soaking temperature during the soaking is from Ac 3 points to 1000 ° C. When the soaking temperature is less than Ac 3 points, soaking is performed in a two-phase region, so that the phase transformation from processed ferrite to austenite becomes insufficient. In addition, the grain growth of ferrite is remarkable, the ferrite area ratio exceeds 95%, and the notch fatigue resistance characteristics are degraded. On the other hand, when the soaking temperature is higher than 1000 ° C., the austenite grain growth is remarkable, the generation of ferrite grains in the subsequent cooling process is reduced, the area ratio of the ferrite is less than 30%, and the bending workability is deteriorated. Resulting in.
均熱後の冷却については、６００℃までは１〜４０℃/秒とする必要がある。冷却速度を１〜４０℃/秒とすると、Ｔｉを含有していることと相まって、フェライト変態が著しく促進されるので、フェライトの面積率を３０〜９５％にすることができる。なお、冷却速度を１℃／秒以上とするのは、操業効率の観点からでもある。さらに、冷却速度を１〜４０℃/秒とすることにより、析出強化に寄与する粒径２〜３０ｎｍのＴｉ系炭窒化析出物を平均粒子間距離３０〜３００ｎｍで含有させることができる。 About cooling after soaking, it is necessary to set it as 1-40 degree-C / sec to 600 degreeC. When the cooling rate is 1 to 40 ° C./second, the ferrite transformation is remarkably promoted in combination with containing Ti, so that the area ratio of ferrite can be made 30 to 95%. The cooling rate of 1 ° C./second or more is also from the viewpoint of operation efficiency. Furthermore, by setting the cooling rate to 1 to 40 ° C./second, it is possible to contain a Ti-based carbonitride precipitate having a particle size of 2 to 30 nm that contributes to precipitation strengthening at an average interparticle distance of 30 to 300 nm.
均熱後に６００℃まで１〜４０℃/秒で冷却したのちに、めっき浴に浸漬して溶融亜鉛めっきを施す。めっき浴に浸漬する前に、さらに平均冷却速度７０℃/秒以下で４４０〜６００℃まで冷却してもよい。６００℃以下へ冷却する場合は、その平均冷却速度が７０℃/秒を超えると、マルテンサイト面積率が５０％を超えてしまい、曲げ加工性が劣化するおそれがある。 After soaking, it is cooled to 600 ° C. at 1 to 40 ° C./second, and then immersed in a plating bath to perform hot dip galvanization. You may cool to 440-600 degreeC further at an average cooling rate of 70 degrees C / sec or less before being immersed in a plating bath. When cooling to 600 ° C. or lower, if the average cooling rate exceeds 70 ° C./second, the martensite area ratio exceeds 50%, and the bending workability may be deteriorated.
また、６００℃以下へ冷却する場合は、その冷却終了温度を４４０℃以上とする必要がある。冷却終了温度が４４０℃未満であると、Ｔｉ系炭窒化析出物の析出が少なくなり、粒径２〜３０ｎｍのＴｉ系炭窒化析出物の平均粒子間距離が３００ｎｍを超えてしまうため、フェライトによる析出強化に寄与せず、曲げ加工性が劣化してしまう。 When cooling to 600 ° C. or lower, the cooling end temperature needs to be 440 ° C. or higher. When the cooling end temperature is less than 440 ° C., the precipitation of Ti-based carbonitride precipitates decreases, and the average interparticle distance of Ti-based carbonitride precipitates having a particle size of 2 to 30 nm exceeds 300 nm. It does not contribute to precipitation strengthening and bending workability deteriorates.
なお、冷却した後、めっき浸漬前４００〜５５０℃の温度域で５〜１００秒程度保持すると、第２相のうちベイナイト比率を上げることができるので、曲げ加工性をさらに向上させることができるので、好ましい。 In addition, after cooling, if it hold | maintains for about 5 to 100 second in the temperature range of 400-550 degreeC before plating immersion, since a bainite ratio can be raised among 2nd phases, since bending workability can be improved further. ,preferable.
本発明に係る鋼の製造方法においては、鋼塊凝固時の鋼塊表面の冷却速度が速いために、粒径３μｍ未満の晶出系ＴｉＮが、鋼塊表面近傍に、そして、圧延後の鋼板表面近傍に多く微細分散している。そのため、上記の溶融めっきの前処理工程を施した場合、フェライトの核生成サイトであるオーステナイト粒界が多く存在しているので、鋼板表層部のフェライト平均粒径を２〜１０μｍとすることが可能となる。
めっき浴浸漬後については、合金化を実施しても良い。合金化温度は、鋼板表面が４５０〜６００℃の温度で行うことが望ましい。本発明に係る鋼板の場合、鋼板表層に存在する粒径３μｍ以上の晶出系ＴｉＮの平均粒子間距離が５０〜５００μｍであるので、晶出系ＴｉＮによってめっき皮膜中へのＦｅの拡散を阻害されることはなく、優れた合金化処理性を有している
In the method for producing steel according to the present invention, since the cooling rate of the steel ingot surface during solidification of the steel ingot is high, the crystallization TiN having a particle size of less than 3 μm is formed in the vicinity of the surface of the steel ingot, and the steel plate after rolling. Many are finely dispersed near the surface. Therefore, when the pretreatment step of the above hot dipping is performed, there are many austenite grain boundaries that are nucleation sites of ferrite, so the ferrite average particle size of the steel sheet surface layer can be made 2 to 10 μm. It becomes.
Alloying may be performed after immersion in the plating bath. The alloying temperature is desirably performed at a temperature of 450 to 600 ° C. on the steel sheet surface. In the case of the steel sheet according to the present invention, the average interparticle distance of the crystallization TiN having a particle size of 3 μm or more present on the surface layer of the steel sheet is 50 to 500 μm, so that the crystallization TiN inhibits the diffusion of Fe into the plating film. And has excellent alloying processability
表１に示す化学成分を有する鋼を転炉で溶製し、試験連続鋳造機にて連続鋳造を実施し、巾１０００ｍｍで厚み５０〜２７０ｍｍのスラブとした。液相線温度から固相線温度までの温度域内の平均冷却速度の変更は、スラブ厚みの変更ならびに２次スプレー帯の水量にて調整した。 Steel having the chemical components shown in Table 1 was melted in a converter and continuously cast by a test continuous casting machine to obtain a slab having a width of 1000 mm and a thickness of 50 to 270 mm. The change in the average cooling rate within the temperature range from the liquidus temperature to the solidus temperature was adjusted by changing the slab thickness and the amount of water in the secondary spray zone.
試験圧延装置を用いて、得られたスラブを表２に示す条件にて、加熱した後、熱間圧延を実施した。その後、酸洗を実施した。一部の鋼板については、４０〜６０％の圧下率で冷間圧延を行った。得られた熱延鋼板ならびに冷延鋼板に対して、表３で示した条件で、実験室にて均熱処理、冷却処理、そして溶融亜鉛めっき処理を施した。めっき付着量は、２０〜１５０ｇ／ｍ２の範囲で実施した。一部の鋼板においては、めっき後、炉温８００〜１３００℃で合金化処理もおこなった。 Using the test rolling apparatus, the obtained slab was heated under the conditions shown in Table 2 and then hot rolled. Thereafter, pickling was performed. Some steel plates were cold-rolled at a rolling reduction of 40 to 60%. The obtained hot-rolled steel sheet and cold-rolled steel sheet were subjected to soaking treatment, cooling treatment, and hot-dip galvanizing treatment in a laboratory under the conditions shown in Table 3. The amount of plating was 20 to 150 g / m 2 . Some steel plates were also alloyed at a furnace temperature of 800 to 1300 ° C. after plating.
１）スラブ平均冷却速度ならびに析出物の評価
得られたスラブの断面をピクリン酸にてエッチングし、５ｍｍピッチでデンドライト２次アーム間隔λ（μｍ）を測定し、次式に基づいて、その値からスラブの液相線温度〜固相線温度内の冷却速度Ａ(℃／秒)を算出した。なお、平均冷却速度はスラブ表面から厚み方向のスラブ中心部までを５ｍｍピッチで測定した冷却速度の算術計算での平均値とした。
λ＝７１０×Ａ-0.39
晶出系ＴｉＮの平均粒径ならびに平均粒子間距離は、得られた鋼板を走査型電子顕微鏡にて、２０００倍の倍率で、２００視野を撮影し、その画像処理にて算出した。析出強化に寄与する粒径２〜３０ｎｍのＴｉ系炭窒化析出物の平均粒子間距離の測定は、透過型電子顕微鏡を用いて、１０万倍の倍率で５０視野を撮影し、それを画像処理して算出した。
1) Evaluation of slab average cooling rate and precipitates A cross section of the obtained slab was etched with picric acid, and the dendrite secondary arm interval λ (μm) was measured at a pitch of 5 mm. The cooling rate A (° C./sec) within the liquidus temperature to the solidus temperature of the slab was calculated. In addition, the average cooling rate was taken as the average value in the arithmetic calculation of the cooling rate measured from the slab surface to the center part of the slab in the thickness direction at a pitch of 5 mm.
λ = 710 × A -0.39
The average particle diameter and the average interparticle distance of the crystallization TiN were calculated by photographing the obtained steel plate with a scanning electron microscope at a magnification of 2000 and viewing 200 fields. Measurement of the average interparticle distance of Ti-based carbonitride precipitates with a particle size of 2 to 30 nm that contribute to precipitation strengthening was performed by photographing 50 fields of view at a magnification of 100,000 times using a transmission electron microscope and processing the images. And calculated.
２）金属組織の評価
鋼板の圧延方向に平行な断面について、光学顕微鏡または電子顕微鏡を用いて、JIS G 0552に準拠してフェライトの平均結晶粒径を測定した。フェライトの面積率は、画像処理にてもとめた。
2) Evaluation of metal structure About the cross section parallel to the rolling direction of a steel plate, the average crystal grain diameter of the ferrite was measured based on JIS G 0552 using an optical microscope or an electron microscope. The area ratio of ferrite was also determined by image processing.
３）特性評価
得られた鋼板に対して、次に示す、引張試験、限界曲げ試験ならびに切欠き疲労試験を実施した。
3) Characteristic evaluation The following tensile test, limit bending test and notch fatigue test were carried out on the obtained steel sheet.
３）−１ 引張試験
各鋼板の圧延直角方向からJIS 5 号引張試験を採取した。試験方法はJIS Z2241に準じた。降伏点ＹＰ、引張強さＴＳ、伸びＥｌを測定した。
3) -1 Tensile test A JIS No. 5 tensile test was taken from the direction perpendicular to the rolling direction of each steel plate. The test method conformed to JIS Z2241. Yield point YP, tensile strength TS, and elongation El were measured.
３）−２ 限界曲げ試験
各鋼板の圧延直角方向から巾４０ｍｍ、長さ２００ｍｍの試験片を採取した。試験形状ならびに試験方法はJIS Z2248に準じた。曲げ半径は、密着から板厚の１倍、２倍、３倍、４倍にて実施し、その割れが発生しない板厚に対する曲げ半径を限界曲げ半径とした。
3) -2 Limit bending test A test piece having a width of 40 mm and a length of 200 mm was taken from the direction perpendicular to the rolling direction of each steel plate. The test shape and test method conformed to JIS Z2248. The bending radius was 1 to 2, 3 times, and 4 times the plate thickness from close contact, and the bending radius with respect to the plate thickness at which no cracking occurred was defined as the critical bending radius.
３）−３ 切欠き平面曲げ疲労試験
各鋼板からJIS Z2275に記載されている形状にて長さ９０ｍｍ、巾４０ｍｍの試験片を採取した。その後、試験片平行部の中央部に直径１０ｍｍの穴をクリアランス１２％で打ち抜き、それを試験片とした。試験方法は、JIS Z2275に準じた。両振り平面曲げ疲労（応力比：−１）にて実施し、１０７乗回の繰り返し数にて破断しない応力振幅値を疲労限界とし、次式により、ＴＳとの算術計算から耐久比をもとめた。
耐久比＝１０７回で破断した応力振幅値/ＴＳ。
3) -3 Notched Plane Bending Fatigue Test A test piece having a length of 90 mm and a width of 40 mm was collected from each steel plate in the shape described in JIS Z2275. Thereafter, a hole with a diameter of 10 mm was punched out at the center of the parallel part of the test piece with a clearance of 12%, and this was used as a test piece. The test method conformed to JIS Z2275. Performed in double-bending plane bending fatigue (stress ratio: -1), the stress amplitude value that does not break at the number of repetitions of 10 7 times as the fatigue limit, and calculated the durability ratio from the arithmetic calculation with TS by the following formula It was.
Endurance ratio = 10 Stress amplitude value / TS broken at 7 times.
３）−４ めっき付着量およびめっき密着性
各鋼板から、５７．２ｍｍ角の試験片を３枚採取し、付着量を測定した。付着量の測定方法は、JIS H 0401に準じた。めっき密着性は、絞り比１．８にて円筒成形をした後、テープによるめっき剥離試験にて簡易的に実施した。試験結果を表４及び表５に示す。
3) -4 Amount of plating adhesion and plating adhesion From each steel plate, three 57.2 mm square test pieces were sampled and the amount of adhesion was measured. The method for measuring the amount of adhesion was in accordance with JIS H0401. The plating adhesion was simply carried out by a plating peeling test using a tape after cylindrical forming at a drawing ratio of 1.8. The test results are shown in Tables 4 and 5.
本発明である供試材No.１〜１８は、限界曲げ半径が「密着〜２．０ｔ」であり、切欠き疲労耐久比が０．３８以上であった。そのため曲げ加工性、耐疲労特性に優れ、加えて、めっき密着性にも優れていた。なお、ここで、ｔは板厚を表わし、２．０ｔとは、板厚の２倍を意味する。 Sample materials Nos. 1 to 18 according to the present invention had a critical bending radius of “adhesion to 2.0 t” and a notch fatigue durability ratio of 0.38 or more. Therefore, it was excellent in bending workability and fatigue resistance, and in addition, was excellent in plating adhesion. Here, t represents a plate thickness, and 2.0 t means twice the plate thickness.
これに対して、供試材No.１９は、スラブの平均冷却速度が０．２℃／秒と本発明で規定する範囲外であるため、粒径３μｍ以上の晶出系ＴｉＮの平均粒子間距離は４５μｍとなった。限界曲げ半径が４．０ｔであり、曲げ加工性に劣っていた。 On the other hand, sample material No. 19 has an average cooling rate of the slab of 0.2 ° C./second, which is outside the range defined by the present invention, and therefore, between the average particles of crystallized TiN having a particle size of 3 μm or more. The distance was 45 μm. The limit bending radius was 4.0 t, and the bending workability was poor.
供試材No.２０は、スラブの平均冷却速度が１．１℃／秒と本発明で規定する範囲外であるため、粒径３μｍ以上の晶出系ＴｉＮの平均粒子間距離は５２０μｍとなった。切欠き疲労耐久比が０．３２であり、耐切欠き疲労特性に劣っていた。 Since the sample material No. 20 has an average cooling rate of slab of 1.1 ° C./second, which is outside the range specified in the present invention, the average interparticle distance of crystallized TiN having a particle size of 3 μm or more is 520 μm. It was. The notch fatigue durability ratio was 0.32, and the notch fatigue resistance was inferior.
供試材No.２１は、加熱温度が１０８０℃と本発明で規定する範囲外であるため、加熱時において、Ｔｉ系炭窒化析出物の再固溶が少なく、Ｔｉ系炭窒化析出物は、微細析出しなかった。そして、Ｔｉ系炭窒化析出物は、溶融めっき前の前処理工程においても微細析出せず、粒径２〜３０ｎｍのＴｉ系炭窒化析出物の平均粒子間距離は３１０μｍとなった。切欠き疲労耐久比が０．３２であり、耐切欠き疲労特性に劣っていた。 Specimen No. 21 has a heating temperature of 1080 ° C., which is outside the range defined in the present invention, and therefore, during heating, the Ti-based carbonitride precipitate is less re-dissolved, and the Ti-based carbonitride precipitate is No fine precipitation occurred. The Ti-based carbonitride precipitate did not precipitate finely even in the pretreatment step before hot dipping, and the average inter-particle distance of the Ti-based carbonitride precipitate having a particle size of 2 to 30 nm was 310 μm. The notch fatigue durability ratio was 0.32, and the notch fatigue resistance was inferior.
供試材No.２２は、めっき前の冷却終了温度が４３０℃と本発明で規定する範囲外であるため、粒径２〜３０ｎｍのＴｉ系炭窒化析出物の平均粒子間距離は３２０ｎｍとなり、フェライトの析出強化に寄与しなかった。限界曲げ半径が４．０ｔとなり、曲げ加工性に劣っていた。 Since the test material No. 22 has a cooling end temperature before plating of 430 ° C., which is outside the range specified in the present invention, the average interparticle distance of the Ti-based carbonitride precipitate having a particle size of 2 to 30 nm is 320 nm, It did not contribute to the precipitation strengthening of ferrite. The limit bending radius was 4.0 t, which was inferior in bending workability.
供試材No.２３は、熱間圧延時の仕上げ温度が７２０℃とＡｒ３点を下回り、本発明で規定する範囲外であるため、圧延時にフェライト生成による体積変動が起こり、正常な圧延ができなかった。そのため、鋼板表面の品質が悪くて、鋼板の評価ができなかった。 Specimen No. 23 has a finishing temperature of 720 ° C. during hot rolling, which is below the Ar 3 point, and is outside the range specified in the present invention. could not. For this reason, the quality of the steel sheet surface was poor and the steel sheet could not be evaluated.
供試材No.２４は、熱間圧延後の巻き取り温度が７５０℃と本発明で規定する範囲外であるため、巻き取り後にもスケール生成が起こり、スケール疵が多発した。そのため、鋼板表面の品質が悪くて、鋼板の評価ができなかった。 Since test material No. 24 had a winding temperature after hot rolling of 750 ° C., which was outside the range specified in the present invention, scale generation occurred even after winding, and scale flaws occurred frequently. For this reason, the quality of the steel sheet surface was poor and the steel sheet could not be evaluated.
供試材No.２５は、めっき処理工程の前の均熱処理温度が７９０℃とＡｃ３点を下回り、２相域での均熱処理となったために、本発明で規定する範囲外である。そのため、フェライトの面積率が９６％、鋼板表層から深さ５０μｍ鋼板表層部におけるフェライト平均粒径は１１．２μｍとなった。このため、切欠き疲労耐久比が０．３０となり、耐切欠き疲労特性に劣る。 Test material No. 25 was outside the range specified in the present invention because the soaking temperature before the plating treatment step was 790 ° C., which was less than the Ac 3 point, and the soaking was performed in a two-phase region. Therefore, the ferrite area ratio was 96%, and the ferrite average particle diameter in the steel plate surface layer portion having a depth of 50 μm from the steel plate surface layer was 11.2 μm. For this reason, the notch fatigue durability ratio is 0.30, which is inferior to the notch fatigue resistance.
供試材No.２６は、めっき処理工程の前の均熱処理温度が１０２０℃と本発明で規定する範囲外であるため、フェライトの生成がほとんどなく、フェライトの面積率は２８％となった。そのため、限界曲げ半径は４．０ｔとなり、曲げ加工性に劣る。 In the test material No. 26, the soaking temperature before the plating process was 1020 ° C., which was outside the range specified in the present invention, so that almost no ferrite was generated, and the area ratio of ferrite was 28%. Therefore, the limit bending radius is 4.0 t, which is inferior in bending workability.
供試材No.２７は、めっき処理工程の前の均熱処理が４秒と本発明で規定する範囲外であるため、均熱時におけるオーステナイトの生成が不十分であった。そのため、フェライトの面積率は９６％、鋼板表層から深さ５０μｍまでの鋼板表層部におけるフェライト平均粒径は１０．５μｍとなった。そのため、切欠き疲労耐久比が０．２６となり、耐切欠き疲労特性に劣る。 In Test Material No. 27, the soaking process before the plating process was 4 seconds, which was outside the range defined by the present invention, and thus austenite was not sufficiently generated during soaking. Therefore, the ferrite area ratio was 96%, and the ferrite average particle diameter in the steel sheet surface layer portion from the steel sheet surface layer to the depth of 50 μm was 10.5 μm. Therefore, the notch fatigue durability ratio is 0.26, which is inferior to notch fatigue resistance.
供試材No.２８は、６００℃までの平均冷却速度が４２℃／秒と本発明で規定する範囲外であるため、フェライトの生成ならびに粒径２〜３０ｎｍのＴｉ系炭窒化析出物の生成が不十分であった。そのため、フェライトの面積率は２７％、鋼板表層から深さ５０μｍまでの鋼板表層部におけるフェライト平均粒径は１．７μｍとなった。加えて、粒径２〜３０ｎｍのＴｉ系炭窒化析出物の平均粒子間距離は３０５ｎｍとなった。そのため、限界曲げ半径は４．０ｔとなり、曲げ加工性に劣る。また、切欠き疲労耐久比が０．２８となり、耐切欠き疲労特性に劣る。 Sample No. 28 has an average cooling rate up to 600 ° C. of 42 ° C./second, which is outside the range specified in the present invention. Therefore, generation of ferrite and formation of Ti carbonitride precipitates having a particle size of 2 to 30 nm Was insufficient. Therefore, the ferrite area ratio was 27%, and the ferrite average particle diameter in the steel sheet surface layer portion from the steel sheet surface layer to the depth of 50 μm was 1.7 μm. In addition, the average interparticle distance of the Ti-based carbonitride precipitate having a particle diameter of 2 to 30 nm was 305 nm. Therefore, the limit bending radius is 4.0 t, which is inferior in bending workability. Further, the notch fatigue durability ratio is 0.28, which is inferior to the notch fatigue resistance.
供試材No.２９は、６００℃までの平均冷却速度が０．８℃／秒と本発明で規定する範囲外であるため、フェライトの生成ならびに粒径２〜３０ｎｍのＴｉ系炭窒化析出物の生成が過多であった。フェライトの面積率は９７％、鋼板表層から深さ５０μｍまでの鋼板表層部におけるフェライト平均粒径は１１．２μｍとなった。加えて、粒径２〜３０ｎｍのＴｉ系炭窒化析出物の平均粒子間距離は２５ｎｍとなった。そのため、限界曲げ半径は４．０ｔとなり、曲げ加工性に劣る。また、切欠き疲労耐久比が０．２５となり、耐切欠き疲労特性に劣る。 Specimen No. 29 has an average cooling rate up to 600 ° C. of 0.8 ° C./second, which is outside the range specified in the present invention. Therefore, the formation of ferrite and Ti-based carbonitride precipitates having a particle size of 2 to 30 nm The production of was excessive. The area ratio of ferrite was 97%, and the ferrite average particle diameter in the steel sheet surface layer portion from the steel sheet surface layer to the depth of 50 μm was 11.2 μm. In addition, the average interparticle distance of the Ti-based carbonitride precipitate having a particle size of 2 to 30 nm was 25 nm. Therefore, the limit bending radius is 4.0 t, which is inferior in bending workability. Further, the notch fatigue durability ratio is 0.25, which is inferior to the notch fatigue resistance.
供試材No.３０は、６００℃以下における冷却停止温度までの平均冷却速度が７５℃／秒と本発明で規定する範囲外であるため、第２相であるマルテンサイトの面積率が５２％となった。そのため、限界曲げ半径は４．０ｔとなり、曲げ加工性に劣る。 Since the sample material No. 30 has an average cooling rate up to the cooling stop temperature at 600 ° C. or lower and 75 ° C./second, which is outside the range defined in the present invention, the area ratio of martensite as the second phase is 52%. It became. Therefore, the limit bending radius is 4.0 t, which is inferior in bending workability.
本発明の鋼板は、高強度で加工性を確保しつつ、曲げ加工性及び耐切欠き疲労特性、耐食性に優れている。そのため、自動車や各種の産業機械に用いられる構造部材の素材、特に自動車のメンバーや足廻り部品に代表される構造部材の素材として最適である。また安価に製造できるので産業上格段の効果を奏する。
The steel sheet of the present invention is excellent in bending workability, notch fatigue characteristics, and corrosion resistance while ensuring high strength and workability. Therefore, it is optimal as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and undercarriage parts. Moreover, since it can be manufactured at a low cost, it has a remarkable industrial effect.
Claims (12)
〔Ａ１〕請求項１から５までのいずれかに記載された化学組成を有する溶鋼を、液相線温度から固相線温度までの温度域内の平均冷却速度０．３〜１．０℃／秒で凝固させて鋼塊とする鋳造工程。
〔Ａ２〕鋳造工程で得られた鋼塊あるいはさらに鋼塊を分塊圧延して得られた鋼片を温度１１００℃以上とした後、Ａｒ３点以上で熱間仕上げ圧延を実施し、７００℃以下の温度で巻き取りを行う熱間圧延工程。
〔Ａ３〕熱間圧延工程を経て得られる熱間圧延鋼板に酸洗を施す酸洗工程。
〔Ａ４〕酸洗工程を経て得られる熱間圧延鋼板にＡｃ３点〜１０００℃の温度範囲で５秒以上保持する均熱処理を施した後、平均冷却速度１〜４０℃／秒で６００℃まで冷却するか、あるいはさらに平均冷却速度７０℃／秒以下で４４０〜６００℃の温度まで冷却し、その後付着量が３〜８００ｇ／ｍ２の溶融亜鉛めっきを施す溶融亜鉛めっき処理工程。 A method for producing a high-tensile hot-dip galvanized steel sheet, comprising the following steps [A1] to [A4].
[A1] An average cooling rate in the temperature range from the liquidus temperature to the solidus temperature of the molten steel having the chemical composition according to any one of claims 1 to 5 is 0.3 to 1.0 ° C / second. Casting process to solidify with steel to make a steel ingot.
[A2] The steel ingot obtained in the casting process or the steel slab obtained by further rolling the steel ingot is brought to a temperature of 1100 ° C. or higher, and then hot finish rolling is performed at 3 or more points of Ar, 700 ° C. A hot rolling process in which winding is performed at the following temperature.
[A3] A pickling step of pickling a hot-rolled steel sheet obtained through the hot rolling step.
[A4] A hot-rolled steel sheet obtained through the pickling step is subjected to soaking treatment for 5 seconds or more in a temperature range of Ac 3 to 1000 ° C, and then to 600 ° C at an average cooling rate of 1 to 40 ° C / second. A hot dip galvanizing treatment step of cooling or further cooling to a temperature of 440 to 600 ° C. at an average cooling rate of 70 ° C./second or less, and thereafter performing hot dip galvanization with an adhesion amount of 3 to 800 g / m 2 .
〔Ｂ１〕請求項１から５までのいずれかに記載された化学組成を有する溶鋼を、液相線温度から固相線温度までの温度域内の平均冷却速度０．３〜１．０℃／秒で凝固させて鋼塊とする鋳造工程。
〔Ｂ２〕鋳造工程で得られた鋼塊あるいはさらに鋼塊を分塊圧延して得られた鋼片を温度１１００℃以上とした後、Ａｒ３点以上で熱間仕上げ圧延を実施し、７００℃以下の温度で巻き取りを行う熱間圧延工程。
〔Ｂ３〕熱間圧延工程を経て得られる熱間圧延鋼板に酸洗を施す酸洗工程。
〔Ｂ４〕酸洗工程を経て得られる熱間圧延鋼板に冷間圧延を施す冷間圧延工程。
〔Ｂ５〕冷間圧延工程を経て得られる冷間圧延鋼板にＡｃ３点〜１０００℃の温度範囲で５秒以上保持する均熱処理を施した後、平均冷却速度１〜４０℃／秒で６００℃まで冷却するか、あるいはさらに平均冷却速度７０℃／秒以下で４４０〜６００℃の温度まで冷却し、その後付着量が３〜８００ｇ／ｍ２の溶融亜鉛めっきを施す溶融亜鉛めっき処理工程。 A method for producing a high-tensile hot-dip galvanized steel sheet, comprising the following steps [B1] to [B5].
[B1] An average cooling rate in the temperature range from the liquidus temperature to the solidus temperature of the molten steel having the chemical composition according to any one of claims 1 to 5 is 0.3 to 1.0 ° C / second. Casting process to solidify with steel to make a steel ingot.
[B2] The steel ingot obtained in the casting process or the steel slab obtained by split rolling the steel ingot is brought to a temperature of 1100 ° C. or higher, and then hot finish rolling is performed at 3 or more points of Ar, 700 ° C. A hot rolling process in which winding is performed at the following temperature.
[B3] A pickling process in which a hot-rolled steel sheet obtained through the hot rolling process is pickled.
[B4] A cold rolling process in which cold rolling is performed on a hot rolled steel sheet obtained through the pickling process.
[B5] Cold-rolled steel sheet obtained through the cold rolling process is subjected to soaking treatment for 5 seconds or more in the temperature range of Ac 3 to 1000 ° C, and then 600 ° C at an average cooling rate of 1 to 40 ° C / second. Or a hot dip galvanizing treatment step of cooling to a temperature of 440 to 600 ° C. at an average cooling rate of 70 ° C./second or less, and then performing hot dip galvanizing with an adhesion amount of 3 to 800 g / m 2 .
The method for producing a high-tensile hot-dip galvanized steel sheet according to any one of claims 8 to 11, wherein in the hot-dip galvanizing step, alloying treatment is performed after hot-dip galvanizing.
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| KR20080110904A (en) * | 2006-05-16 | 2008-12-19 | 제이에프이 스틸 가부시키가이샤 | High-strength hot-rolled steel plate having excellent stretch properties, stretch flanging properties and tension fatigue properties, and method for production thereof |
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