JP5201653B2 - High-strength steel sheet excellent in elongation and stretch flangeability and method for producing the same - Google Patents
High-strength steel sheet excellent in elongation and stretch flangeability and method for producing the same Download PDFInfo
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本発明は、自動車用鋼板に代表される高プレス成形性が求められる高強度鋼板、特に延性及び伸びフランジ性を兼ね備えた高強度鋼板とその製造方法に関する。 The present invention relates to a high-strength steel sheet that is required to have high press formability as typified by a steel sheet for automobiles, and more particularly to a high-strength steel sheet having both ductility and stretch flangeability and a method for producing the same.
近年、自動車用鋼板には、高強度のみならず高プレス成形性すなわち優れた伸び及び伸びフランジ性が要求されるようになってきた。これらの各特性を兼ね備えた鋼として、例えば特許文献１に記載されているように、金属組織がフェライト相とマルテンサイト相からなる複合組織鋼（ＤＰ鋼：Dual phase鋼）が知られている。前記ＤＰ鋼は軟質なフェライトにより延性（伸び）を確保すると共に硬質なマルテンサイトにより強度を確保することができるので、強度と伸び（特に、均一伸び）を兼備するものである。しかし、軟質なフェライトと硬質なマルテンサイトが共存するため、変形時には両相の界面にひずみ（応力）が集中して、界面が破壊の起点となり易くなり、伸びフランジ性（局部伸び）が確保し難いという欠点がある。 In recent years, steel sheets for automobiles have been required to have not only high strength but also high press formability, that is, excellent elongation and stretch flangeability. As steel having these properties, for example, as described in Patent Document 1, a composite structure steel (DP steel: Dual phase steel) whose metal structure is composed of a ferrite phase and a martensite phase is known. Since the DP steel can ensure ductility (elongation) with soft ferrite and strength with hard martensite, it has both strength and elongation (particularly uniform elongation). However, since soft ferrite and hard martensite coexist, strain (stress) concentrates at the interface between both phases during deformation, and the interface tends to be the starting point of fracture, ensuring stretch flangeability (local elongation). There is a drawback that it is difficult.
また、ＤＰ鋼よりも更に高い延性（特に、均一伸び）が期待できる鋼板として、例えば、特許文献２に記載されているように、ＴＲＩＰ（Transformation Induced Plasticity：変態誘起塑性）現象を活用したＴＲＩＰ鋼が知られている。このＴＲＩＰ鋼は、変形中に残留オーステナイトをマルテンサイトに変態させること（加工誘起変態）により、均一伸びを高めた鋼板である。しかし、ＴＲＩＰ鋼の残留オーステナイトが加工中に変態したマルテンサイトは極めて硬質であるため、破壊の起点となり易く、やはり伸びフランジ性の低下を余儀なくされる。 Further, as a steel sheet that can be expected to have higher ductility (particularly uniform elongation) than DP steel, for example, as described in Patent Document 2, TRIP steel utilizing the TRIP (Transformation Induced Plasticity) phenomenon It has been known. This TRIP steel is a steel sheet whose uniform elongation is increased by transforming retained austenite to martensite during deformation (work-induced transformation). However, since martensite in which the retained austenite of TRIP steel is transformed during processing is extremely hard, it tends to be a starting point of fracture, and the elongation flangeability is inevitably lowered.
一方、伸びフランジ性を向上させるためには、均一組織が有効であることが知られている。マルテンサイト単相組織鋼板は均一組織であるため、強度と伸びフランジ性を両立させる鋼板として知られている。しかし、マルテンサイト単相組織鋼板は延性に劣り、伸びが不十分であるという問題がある。
上記のとおり、ＤＰ鋼板、ＴＲＩＰ鋼板、およびマルテンサイト単相組織鋼板は、それぞれ一長一短があるため、高強度と共に優れた伸びおよび伸びフランジ性を兼備した鋼板が求められている。本発明はかかる問題に鑑みなされたもので、優れた伸びおよび伸びフランジ性を兼ね備えた高強度鋼板を提供することを目的とする。 As described above, the DP steel plate, the TRIP steel plate, and the martensite single-phase steel plate have advantages and disadvantages, and therefore, a steel plate that has both high strength and excellent elongation and stretch flangeability is required. The present invention has been made in view of such problems, and an object of the present invention is to provide a high-strength steel sheet having both excellent elongation and stretch flangeability.
本発明者らは高強度を確保しつつ、伸び及び特に伸びフランジ性を改善する組織を種々検討した結果、初期組織として微細なラス状組織であるベイナイトをフェライト＋オーステナイトの二相温度域で焼鈍（以下、「二相域焼鈍」という。）することによって、基地中に生成した微細な焼鈍ベイナイトがオーステナイトの成長を抑制するように作用し、その後の焼き入れ、焼戻しによりオーステナイトから微細な焼戻しマルテンサイトが生じ、組織全体がこれらの微細組織によって形成されるため、伸び、伸びフランジ性が改善されることを知見し、これにより本発明を完成するに至った。 As a result of various studies on the structure that improves the elongation and particularly the stretch flangeability while ensuring high strength, the present inventors annealed bainite, which is a fine lath structure, as an initial structure in the two-phase temperature range of ferrite + austenite. (Hereinafter referred to as “two-phase annealing”), the fine annealed bainite generated in the matrix acts to suppress the growth of austenite, and the fine tempered martensite from austenite by subsequent quenching and tempering. Since a site is formed and the entire structure is formed by these fine structures, it has been found that the elongation and stretch flangeability are improved, thereby completing the present invention.
すなわち、本発明の高強度鋼板は、
化学成分が、mass％で、
Ｃ：０．０５〜０．３％、
Ｓｉ：０．０１〜３．０％、
Ｍｎ：０．５〜３．０％、
Ａｌ：０．０１〜０．１％
を含み、残部Ｆｅ及び不可避的不純物からなり、
焼戻しマルテンサイトと、フェライト＋オーステナイトの二相温度域での焼鈍によりオーステナイトに変態せずに基地中に残存して微細分散したベイナイト（「焼鈍ベイナイト」という。）との占積率が９０％以上の組織を有し、前記焼戻しマルテンサイトの占積率が５０〜９５％で、前記焼鈍ベイナイトの占積率が５〜３０％であり、前記焼戻しマルテンサイトの平均粒径が円相当直径で１０μm 以下であり、引張強度が５９０ＭＰａ以上とされたものである。前記円相当直径とは、焼戻しマルテンサイトの粒と面積が等しい円を想定し、その円の直径を意味するもので、組織写真を画像解析することによって求められる。また、占積率とは体積％を意味し、組織観察試験片をナイタール腐食し、光学顕微鏡観察（１０００倍）し、観察された組織写真を画像解析することによって求められる。また、焼鈍ベイナイトは、結晶構造としては体心立方構造として観察される。
That is, the high-strength steel sheet of the present invention is
Chemical composition is mass%,
C: 0.05-0.3%
Si: 0.01-3.0%,
Mn: 0.5 to 3.0%
Al: 0.01 to 0.1%
Comprising the balance Fe and unavoidable impurities,
And the tempered martensite, ferrite + austenite two-phase temperature region annealing with remaining in the base without transformation to austenite by finely dispersed bainite (referred to as "annealing bainite".) And space factor of 90 %, The space factor of the tempered martensite is 50 to 95%, the space factor of the annealed bainite is 5 to 30%, and the average particle size of the tempered martensite is an equivalent circle diameter. The tensile strength is 590 MPa or more. The circle equivalent diameter means a circle having the same area as that of grains of tempered martensite, and means the diameter of the circle, and is obtained by image analysis of a structure photograph. The space factor means volume%, and is obtained by performing a nital corrosion on a structure observation specimen, observing with an optical microscope (1000 times), and analyzing an image of the observed structure. Annealed bainite is observed as a body-centered cubic structure as a crystal structure.
本発明の高強度鋼板は、上記のとおり、その組織に特徴があるが、上記組織、強度を得やすい化学成分（単位はmass％）として、以下の成分が適用される。すなわち、その化学成分は、Ｃ：０．０５〜０．３％、Ｓｉ：０．０１〜３．０％、Ｍｎ：０．５〜３．０％、Ａｌ：０．０１〜０．１％を含み、残部Ｆｅ及び不可避的不純物からなる。上記基本成分に加えて、下記の(1) から(5) に記載した元素群のいずれか、あるいは複数群から選択された１種又は２種以上の元素を、各元素群に規定した範囲内で含めることができる。
(1) Ｚｒを０．０１〜１％
(2) ＮｉおよびＣｕから選択される１種以上の元素を合計量で１％以下
(3) Ｃｒ：２％以下、Ｍｏ：１％以下のうち１種以上の元素
(4) Ｂを０．０００１〜０．００５％
(5) ＣａおよびＲＥＭから選択される１種以上の元素を合計量で０．００３％以下
High-strength steel sheet of the present invention, as described above, but wherein there Ru in the tissue, the tissue, as easily obtained chemical component strength (in mass%), the following components are applied. That is, the chemical components are: C: 0.05-0.3%, Si: 0.01-3.0%, Mn: 0.5-3.0%, Al: 0.01-0.1% It consists of the remainder Fe and unavoidable impurities. In addition to the above basic components, any one of the element groups described in (1) to (5) below, or one or more elements selected from a plurality of groups, within the range specified for each element group Can be included.
(1) 0.01-1% Zr
(2) 1% or less of the total amount of one or more elements selected from Ni and Cu
(3) One or more elements of Cr: 2% or less, Mo: 1% or less
(4) B is 0.0001 to 0.005%
(5) The total amount of one or more elements selected from Ca and REM is 0.003% or less
また、本発明に係る、上記高強度鋼板の製造方法は、上記化学成分を有し、全組織に対するベイナイトの占積率が９０％以上である素材鋼板を準備し、この素材鋼板を（Ａc3点−１００）℃以上、Ａc3以下の温度で０sec 以上（０sec を含む。）、２４００sec 以下の時間を保持した後、１０℃/sec以上の平均冷却速度でマルテンサイト変態開始温度Ｍｓ点以下まで冷却し、引き続いて３００℃以上、５５０℃以下で６０sec 以上、１２００sec 以下の時間を保持し、引張強度が５９０ＭＰａ以上の鋼板を製造するものである。前記素材鋼板は、前記化学成分の鋼片を熱間圧延あるいはさらに冷間圧延を行うことにより製造することができる。 Further, according to the present invention, a method of manufacturing the high strength steel sheet has the chemical composition, the space factor of the bainitic for all organizations to prepare steel sheet is 90% or more, the steel sheet (Ac 3 Point -100) After maintaining a temperature of 0 sec or more (including 0 sec) at a temperature of 3 ° C. or more and Ac 3 or less, and after maintaining a time of 2400 sec or less, at an average cooling rate of 10 ° C./sec or more to the martensite transformation start temperature Ms or less. The steel sheet is cooled and subsequently manufactured at a temperature of 300 ° C. or more and 550 ° C. or less for 60 seconds or more and 1200 seconds or less, and a steel sheet having a tensile strength of 590 MPa or more is produced. The material steel plate can be produced by hot rolling or further cold rolling the steel slab of the chemical component.
本発明によれば、所定の化学成分を有し、特に焼戻しマルテンサイトと微細分散した焼鈍ベイナイトとの占積率が９０％以上の組織とし、それぞれの占積率を所定量に規定すると共に焼戻しマルテンサイトの平均粒径を１０μm 以下に規定したので、５９０ＭＰａ以上の高強度を有しながら、優れた伸びおよび伸びフランジ性を兼備し、引いては優れたプレス成形性を備えた高強度鋼板を提供することができる。本発明にかかる高強度鋼板はこのように優れたプレス成形性を有するので、自動車用鋼板を始め、様々な鋼製品の素材として好適である。 According to the present invention, a structure having a predetermined chemical component, in particular, a space factor of tempered martensite and finely dispersed annealed bainite is 90% or more, and each space factor is prescribed to a predetermined amount and tempered. Since the average particle size of martensite is specified to be 10 μm or less, a high-strength steel sheet having high elongation of 590 MPa or more, having excellent elongation and stretch flangeability, and having excellent press formability. Can be provided. Since the high-strength steel sheet according to the present invention has such excellent press formability, it is suitable as a material for various steel products including steel sheets for automobiles.
本発明の高強度鋼板は、焼戻しマルテンサイト中に焼鈍ベイナイトが微細分散した組織を主体とし、前記焼戻しマルテンサイトの占積率が５０〜９５％、前記焼鈍ベイナイトの占積率が５〜３０％であり、前記焼戻しマルテンサイトの平均粒径が円相当直径で１０μm 以下であり、引張強度が５９０ＭＰａ以上とされたものである。以下、組織の限定理由を説明する。 The high-strength steel sheet of the present invention is mainly composed of a structure in which annealed bainite is finely dispersed in tempered martensite, the space factor of the tempered martensite is 50 to 95%, and the space factor of the annealed bainite is 5 to 30%. The average particle diameter of the tempered martensite is 10 μm or less in terms of the equivalent circle diameter, and the tensile strength is 590 MPa or more. Hereinafter, the reason for limitation of the organization will be described.
前記焼鈍ベイナイトの占積率が５％未満では、オーステナイトの成長を抑制するピンニング効果が弱く、オーステナイト粒が成長して、引いてはマルテンサイトが大粒となって、良好な伸びを確保することが困難になる。一方、３０％を超えると、伸びフランジ性が低下するようになる。このため、焼鈍ベイナイトの下限を５％、好ましくは７％とし、その上限を３０％、好ましくは２５％とする。 When the space factor of the annealed bainite is less than 5%, the pinning effect for suppressing the growth of austenite is weak, austenite grains grow, and in turn, the martensite becomes large grains, and it is possible to ensure good elongation. It becomes difficult. On the other hand, when it exceeds 30%, stretch flangeability will fall. For this reason, the lower limit of annealing bainite is 5%, preferably 7%, and the upper limit is 30%, preferably 25%.
また、焼戻しマルテンサイトの占積率が５０％未満では強度が低下すると共に、伸びフランジ性が低下し、一方９５％を超えると硬くなり過ぎて伸びが低下するようになる。このため、焼戻しマルテンサイト相の下限を５０％、好ましくは７０％とし、その上限を９５％、好ましくは８５％とする。 Further, when the space factor of tempered martensite is less than 50%, the strength is lowered and the stretch flangeability is lowered. On the other hand, when it exceeds 95%, it becomes too hard and the elongation is lowered. For this reason, the lower limit of the tempered martensite phase is 50%, preferably 70%, and the upper limit is 95%, preferably 85%.
また、前記焼戻しマルテンサイトの平均粒径は、微細分散した焼鈍ベイナイトの量によって左右されるが、相当円直径で１０μm を超えると伸び及び伸びフランジ性が低下するようになる。このため、上限を１０μm とする。 The average particle size of the tempered martensite depends on the amount of finely dispersed annealed bainite. However, if the equivalent circular diameter exceeds 10 μm, the elongation and stretch flangeability are lowered. For this reason, the upper limit is set to 10 μm.
前記焼戻しマルテンサイトと焼鈍ベイナイトとの共存組織は、本発明の高強度鋼板の組織主体を構成するが、主体とは９０％以上、好ましくは９５％以上を意味し、他の組織が１０％程度未満含まれても、伸び、特に伸びフランジ性に対する影響が少ないので許容される。他の組織としては、フェライト、パーライト、残留オーステナイトなどがある。勿論、これらの組織は少ない方がよい。 The coexisting structure of the tempered martensite and the annealed bainite constitutes the structure main body of the high-strength steel sheet of the present invention. The main body means 90% or more, preferably 95% or more, and the other structure is about 10%. Even if it is contained less than, it is acceptable because it has little influence on elongation, particularly stretch flangeability. Examples of other structures include ferrite, pearlite, and retained austenite. Of course, fewer of these organizations are better.
次に、本発明にかかる鋼板の組織、強度を得るのに好適な化学成分（単位はmass％）について説明する。このような化学成分として、Ｃ：０．０５〜０．３％、Ｓｉ：０．０１〜３．０％、Ｍｎ：０．５〜３．０％、Ａｌ：０．０１〜０．１％を含み、残部Ｆｅ及び不可避的不純物からなるものを示すことができる。以下、成分限定理由について説明する。 Next, chemical components (unit: mass%) suitable for obtaining the structure and strength of the steel sheet according to the present invention will be described. As such chemical components, C: 0.05 to 0.3%, Si: 0.01 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to 0.1% And the balance consisting of Fe and unavoidable impurities. Hereinafter, the reason for component limitation will be described.
Ｃ：０．０５〜０．３％
Ｃはマルテンサイトを生成させ、鋼板の強度を高める上で重要な元素である。０．０５％未満ではかかる効果が過少となり、一方、高強度化の観点からはＣ量が多いほど好ましいが、０．３％を超えると残留オーステナイトが多量に生成して伸びフランジ性が低下するようになる。また、溶接性も劣化するようになる。このため、Ｃ量の下限を０．０５％、好ましくは０．０７％とし、その上限を０．３％、好ましくは０．２５％とする。
C: 0.05-0.3%
C is an important element for generating martensite and increasing the strength of the steel sheet. If it is less than 0.05%, such an effect is insufficient. On the other hand, a larger amount of C is preferable from the viewpoint of increasing the strength, but if it exceeds 0.3%, a large amount of retained austenite is generated and stretch flangeability is deteriorated. It becomes like this. Also, the weldability is deteriorated. For this reason, the lower limit of the C amount is 0.05%, preferably 0.07%, and the upper limit is 0.3%, preferably 0.25%.
Ｓｉ：０．０１〜３．０％
Ｓｉは鋼を溶製する際に脱酸元素として作用し、また鋼の延性を劣化させることなく強度を高めるのに有効な元素で、さらに伸びフランジ性を劣化させる粗大な炭化物の析出を抑える作用を有している。０．０１％未満ではこれらの作用が過少であり、３．０％程度を超えて添加しても効果が飽和する。このため、Ｓｉ量の下限を０．０１％、好ましくは０．１％とし、その上限を３．０％、好ましくは２．５％とする。
Si: 0.01-3.0%
Si acts as a deoxidizing element when melting steel, and is an effective element for increasing strength without degrading the ductility of steel, and also suppresses the precipitation of coarse carbides that degrade stretch flangeability. have. If it is less than 0.01%, these effects are too small, and even if added over about 3.0%, the effect is saturated. For this reason, the lower limit of the Si amount is 0.01%, preferably 0.1%, and the upper limit is 3.0%, preferably 2.5%.
Ｍｎ：０．５〜３％
Ｍｎは鋼の焼入れ性を高めて高強度を確保する上で有用な元素であり、０．５％未満ではこうした作用が過少となる。一方、３％を超えると延性を低下させて加工性に悪影響を及ぼす。このため、Ｍｎ量の下限を０．５％、好ましくは０．７％とし、その上限を３％、好ましくは２．５％とする。
Mn: 0.5 to 3%
Mn is an element useful for increasing the hardenability of the steel and ensuring high strength, and if it is less than 0.5%, such an action is insufficient. On the other hand, if it exceeds 3%, the ductility is lowered and the workability is adversely affected. For this reason, the lower limit of the amount of Mn is 0.5%, preferably 0.7%, and the upper limit is 3%, preferably 2.5%.
Ａｌ：０．０１〜０．１％
Ａｌは脱酸作用を有する元素であり、そのためには０．０１％以上添加する必要がある。一方、０．１％超添加しても脱酸効果は飽和し、また非金属系介在物源となって物性や表面性状を劣化させる。このため、Ａｌ量の下限を０．０１％、好ましくは０．０３％とし、その上限を０．１％、好ましくは０．０８％とする。
Al: 0.01 to 0.1%
Al is an element having a deoxidizing action, and for that purpose, it is necessary to add 0.01% or more. On the other hand, even if added over 0.1%, the deoxidation effect is saturated and becomes a non-metallic inclusion source, which deteriorates physical properties and surface properties. For this reason, the lower limit of the Al amount is 0.01%, preferably 0.03%, and the upper limit is 0.1%, preferably 0.08%.
本発明鋼板の好適な化学成分は、上記基本成分のほか、残部Ｆｅ及び製造上不可避的に混入する不純物、例えばＰ、Ｓ、Ｎ、Ｏからなる。もっとも、鋼板の機械的特性を向上させるために下記(1) から(5) に記載した補助元素群のいずれか、あるいは複数群から選択された元素の１種又は２種以上を、各群の添加許容範囲内で添加することができる。
(1) Ｚｒを０．０１〜１％
(2) ＮｉおよびＣｕから選択される１種以上の元素を合計量で１％以下
(3) Ｃｒ：２％以下、Ｍｏ：１％以下のうち１種以上の元素
(4) Ｂを０．０００１〜０．００５％
(5) ＣａおよびＲＥＭから選択される１種以上の元素を合計量で０．００３％以下
A suitable chemical component of the steel sheet of the present invention comprises the above basic components, the remaining Fe, and impurities inevitably mixed in production, such as P, S, N, and O. However, in order to improve the mechanical properties of the steel sheet, one or more of the auxiliary element groups described in the following (1) to (5), or an element selected from a plurality of groups are added to each group. It can be added within the allowable range.
(1) 0.01-1% Zr
(2) 1% or less of the total amount of one or more elements selected from Ni and Cu
(3) One or more elements of Cr: 2% or less, Mo: 1% or less
(4) B is 0.0001 to 0.005%
(5) The total amount of one or more elements selected from Ca and REM is 0.003% or less
Ｚｒ：０．０１〜１％
ＺｒはＣやＮと炭化物、窒化物、炭窒化物などの析出物を形成し、強度向上に寄与するほか、熱延時に結晶粒を微細化して伸びおよび伸びフランジ性を高める作用を有する。合計添加量が０．０１％ではかかる作用が過少となる。一方、１％を超えると伸び、伸びフランジ性が却って低下するようになる。このため、含有量の下限を０．０１％、好ましくは０．０３％とし、その上限を１．０％、好ましくは０．７％とする。
Zr: 0.01 to 1%
Zr forms precipitates such as carbides, nitrides, carbonitrides and the like with C and N and contributes to improving the strength, and also has the effect of refining the crystal grains during hot rolling to increase elongation and stretch flangeability. When the total addition amount is 0.01%, such an effect is insufficient. On the other hand, when it exceeds 1%, it will elongate and stretch flangeability will fall on the contrary. For this reason, the lower limit of the content is 0.01%, preferably 0.03%, and the upper limit is 1.0%, preferably 0.7%.
Ｎｉ、Ｃｕの１種以上：合計量で１％以下
これらの元素は、強度−延性バランスを高く維持したまま、高強度化を実現するのに有効な元素である。こうした効果を有効に発揮させるには０．０５％以上添加することが好ましい。一方、これらの元素の含有量が増加するに従って前記効果も増大するが、これらの元素の１種又は２種以上の合計量が１％を超えるとかかる効果が飽和するようになり、また熱延時に割れが生じるおそれが生じる。このため、合計量の上限を１．０％、好ましくは０．７％とする。
One or more of Ni and Cu: 1% or less in total amount These elements are effective elements for achieving high strength while maintaining a high strength-ductility balance. In order to exhibit such an effect effectively, it is preferable to add 0.05% or more. On the other hand, as the content of these elements increases, the effect increases. However, when the total amount of one or more of these elements exceeds 1%, the effect becomes saturated and hot rolling is performed. Occasionally cracking may occur. For this reason, the upper limit of the total amount is 1.0%, preferably 0.7%.
Ｃｒ：２％以下、Ｍｏ：１％以下の１種又は２種
これらの元素は、いずれもオーステナイト相を安定化し、冷却過程でベイナイトの生成を容易にするのに有効な元素である。その効果は、含有量が増加するほど増大するが、過剰に含有されると延性が却って劣化する。このため、Ｃｒは２．０％以下、より好ましくは１．５％以下とし、Ｍｏは１．０％以下、より好ましくは０．７％以下とする。
One or two of Cr: 2% or less and Mo: 1% or less These elements are effective elements for stabilizing the austenite phase and facilitating the formation of bainite during the cooling process. The effect increases as the content increases. However, when the content is excessive, the ductility deteriorates. For this reason, Cr is 2.0% or less, more preferably 1.5% or less, and Mo is 1.0% or less, more preferably 0.7% or less.
Ｂ：０．０００１〜０．００５％
Ｂは焼き入れ性を向上し、微量で鋼板の強度を高めるのに有効な元素である。こうした効果を発揮させるためには０．０００１％以上含有させることが好ましい。しかし、Ｂの含有が過剰となり、０．００５％を超えると、結晶粒界が脆化して圧延時に割れが生じるおそれがある。このため、上限を０．００５％とする。
B: 0.0001 to 0.005%
B is an element effective in improving the hardenability and increasing the strength of the steel sheet in a small amount. In order to exhibit such an effect, it is preferable to contain 0.0001% or more. However, if the B content is excessive and exceeds 0.005%, the crystal grain boundaries become brittle and cracks may occur during rolling. For this reason, the upper limit is made 0.005%.
Ｃａ、ＲＥＭの１種以上：合計量で０．００３％以下
これらの元素は、鋼中の硫化物の形態を制御し、加工性の向上に有効な元素である。こうした効果はその含有量が増加するにつれて増大するが、過剰に含有されると、上記効果が飽和するので、これらの元素の１種又は２種以上の合計量の上限を０．００３％とする。
One or more of Ca and REM: 0.003% or less in total amount These elements are elements effective in controlling the form of sulfide in steel and improving workability. These effects increase as the content increases. However, if the content is excessive, the above effects are saturated, so the upper limit of the total amount of one or more of these elements is 0.003%. .
次に、本発明の実施形態に係る高強度鋼板の製造方法について説明する。まず、上記化学成分を有し、全組織に対するベイナイトの占積率が９０％以上である素材鋼板を準備する。次に、この素材鋼板に（Ａc3点−１００）℃以上、Ａc3以下の温度で０sec 以上、２４００sec 以下の時間を保持した後、１０℃/sec以上の平均冷却速度でマルテンサイト変態開始温度Ｍｓ点以下まで冷却する焼鈍熱処理を施す。引き続いて３００℃以上、５５０℃以下で６０sec 以上、１２００sec 以下の時間を保持する焼戻し熱処理を施すことによって、引張強度が５９０ＭＰａ以上の前記焼戻しマルテンサイトと焼鈍ベイナイトを主体とする微細組織の鋼板が得られる。 Next, the manufacturing method of the high strength steel plate which concerns on embodiment of this invention is demonstrated. First, a raw steel plate having the above chemical components and having a bainite space factor of 90% or more with respect to the entire structure is prepared. Next, after maintaining a time of 0 sec or more and 2400 sec or less at a temperature of (Ac 3 point−100) ° C. or more and Ac 3 or less on this material steel plate, the martensite transformation start temperature at an average cooling rate of 10 ° C./sec or more. An annealing heat treatment is performed to cool to the Ms point or lower. Subsequently, by performing a tempering heat treatment at a temperature of 300 ° C. or more and 550 ° C. or less for 60 seconds or more and 1200 seconds or less, a fine-structure steel sheet mainly composed of the tempered martensite and annealed bainite having a tensile strength of 590 MPa or more is obtained. It is done.
前記素材鋼板は、以下の工程によって製造することができる。まず、上記化学成分の鋼を溶製し、その鋼スラブを用いて、仕上温度がＡr3点以上となるようにして熱間圧延を終了し、その後、１０℃/sec以上の平均冷却速度でベイナイト変態温度（３５０〜４５０℃程度）まで冷却し、同温度にて巻き取る。仕上温度がＡr3点未満あるいは熱間圧延後の冷却速度が１０℃/sec未満では、熱延鋼板にフェライト相が生成しやすくなって、素材鋼板のベイナイトの占積率が９０％を下回るようになる。また、素材鋼板としては、熱間圧延後に酸洗処理、冷間圧延を施して、冷延鋼板としたものでもよい。なお、Ｔｉ、Ｎｂ、Ｖ、Ｚｒを含む鋼種では、熱延前に生成した前記元素を含む析出物を再固溶させるため、熱延の際に鋼片を高めの温度に加熱保持することが好ましい。 The raw steel plate can be manufactured by the following steps. First, the steel having the above chemical components is melted, and the steel slab is used to finish hot rolling so that the finishing temperature becomes Ar 3 points or higher, and then at an average cooling rate of 10 ° C./sec or higher. It cools to a bainite transformation temperature (about 350-450 degreeC), and winds up at the same temperature. If the finishing temperature is less than 3 points Ar or the cooling rate after hot rolling is less than 10 ° C./sec, a ferrite phase is likely to be generated in the hot-rolled steel sheet so that the bainite space factor of the raw steel sheet is less than 90%. become. In addition, the raw steel plate may be a cold-rolled steel plate by performing pickling treatment and cold rolling after hot rolling. In addition, in the steel type containing Ti, Nb, V, and Zr, in order to re-dissolve the precipitate containing the element generated before hot rolling, the steel slab can be heated and held at a higher temperature during hot rolling. preferable.
前記素材鋼板は、上記熱間圧延条件、冷却条件を満足しない熱延鋼板に対して、下記予備焼鈍を施すことによっても、ベイナイトの占積率を９０％以上にすることができる。この予備焼鈍は、熱延鋼板をＡc3点以上の温度域に５秒程度以上保持した後、１０℃/sec以上の平均冷却速度でベイナイト変態温度まで冷却する熱処理ある。保持温度がＡc3点未満では、鋼板にフェライト相が生成しやすくなって、ベイナイトの占積率が低下し、またＡc3点以上の温度に保持する場合でも５秒程度未満ではオーステナイト化が不十分であるため、やはり占積率が９０％を下回るようになる。前記予備焼鈍を施した場合も、その後に冷間圧延を施して冷延鋼板とし、これを素材鋼板として用いてもよい。 The material steel plate can also have a bainite space factor of 90% or more by subjecting the hot-rolled steel plate not satisfying the hot rolling conditions and cooling conditions to the following pre-annealing. This pre-annealing is a heat treatment in which the hot-rolled steel sheet is held at a temperature range of Ac 3 or higher for about 5 seconds or more and then cooled to the bainite transformation temperature at an average cooling rate of 10 ° C./sec or higher. If the holding temperature is less than Ac 3 point, a ferrite phase tends to be formed on the steel sheet, the bainite space factor is reduced, and even if it is kept at a temperature of Ac 3 point or more, austenitization is not possible if it is held for about 5 seconds or less. Since it is sufficient, the space factor will still fall below 90%. Even when the pre-annealing is performed, cold rolling may be performed thereafter to form a cold-rolled steel sheet, which may be used as a raw steel sheet.
前記素材鋼板を準備した後、次に前記素材鋼板に（Ａc3点−１００）℃以上、Ａc3以下の温度で０sec 以上（０sec を含む。）、２４００sec 以下の時間を保持した後、１０℃/sec以上の平均冷却速度でマルテンサイト変態開始温度Ｍｓ点以下まで冷却する二相域焼鈍を施し、さらに焼戻しを行う。かかる熱処理により、本発明にかかる高強度鋼板の組織が得られる。以下、まず二相域焼鈍の条件について説明する。 After preparing the raw steel plate, the raw steel plate is kept at a temperature of (Ac 3 point-100) ° C. or higher and Ac 3 or lower for 0 sec or longer (including 0 sec) and 2400 sec or shorter, and then 10 ° C. Two-phase region annealing is performed to cool to a martensitic transformation start temperature Ms point or less at an average cooling rate of at least / sec, and further tempering is performed. By such heat treatment, the structure of the high-strength steel sheet according to the present invention is obtained. Hereinafter, the conditions for the two-phase region annealing will be described first.
二相域焼鈍の焼鈍温度を（Ａc3点−１００）℃以上、Ａc3以下とする理由は以下のとおりである。焼鈍温度をオーステナイト単相が安定なＡｃ３点よりも高い温度域に設定すると、素材鋼板においてオーステナイトの結晶粒が成長し、相互に合体して粗大化すると共に微細に分散した焼鈍ベイナイトによるオーステナイトの成長抑制効果（ピニング効果）が得られないようになる。このため、微細な複合組織鋼板を得ることができず、高強度鋼板の伸びフランジ性が低下するようになる。一方、（Ａc3点−１００）℃よりも低い温度で焼鈍すると、オーステナイト化が十分に進まず、熱処理後のマルテンサイトの占積率が５０％未満となって、鋼板の伸びフランジ性が低下するようになる。 The reason why the annealing temperature of the two-phase region annealing is set to (Ac 3 point-100) ° C. or higher and Ac 3 or lower is as follows. When the annealing temperature is set to a temperature range higher than the Ac3 point where the austenite single phase is stable, austenite crystal grains grow in the raw steel plate, coalesce with each other and become coarse, and austenite grows by finely dispersed annealing bainite. The suppression effect (pinning effect) cannot be obtained. For this reason, a fine composite structure steel plate cannot be obtained, and the stretch flangeability of a high-strength steel plate is lowered. On the other hand, when annealing is performed at a temperature lower than (Ac 3 point-100) ° C., austenitization does not proceed sufficiently, the martensite space ratio after heat treatment becomes less than 50%, and the stretch flangeability of the steel sheet decreases. To come.
また、焼鈍時間（加熱保持時間）は、焼鈍温度に昇温するだけでも占積率が５０％程度以上のオーステナイト引いてはマルテンサイトが得られるが、好ましくは１sec 以上、より好ましくは５秒以上とするのがよい。一方、必要以上に長時間保持するとオーステナイト粒が粗大化し、微細なマルテンサイトが得られないようになるので、２４００sec 以下、好ましくは１２００sec 以下に止めるのがよい。 Further, the annealing time (heating holding time) can be obtained by simply raising the temperature to the annealing temperature, but martensite can be obtained by pulling austenite with a space factor of about 50% or more, preferably 1 sec or more, more preferably 5 sec or more. It is good to do. On the other hand, if held for an unnecessarily long time, austenite grains become coarse and fine martensite cannot be obtained. Therefore, it is preferable to keep it at 2400 sec or less, preferably 1200 sec or less.
加熱保持後の平均冷却速度が１０℃/sec未満であったり、冷却停止温度がマルテンサイト変態開始温度Ｍｓ点より高いと、残留オーステナイト相、パーライト相、フェライト相が生成し、またセメンタイト相が析出し、オーステナイトからマルテンサイト以外の組織が多く形成されるため、伸び、伸びフランジ性が低下するようになる。 If the average cooling rate after heating is less than 10 ° C / sec or if the cooling stop temperature is higher than the martensite transformation start temperature Ms point, a retained austenite phase, pearlite phase, and ferrite phase are formed, and a cementite phase is precipitated. And since many structures other than martensite are formed from austenite, elongation and stretch flangeability come to fall.
前記二相域焼鈍後、焼戻し（再加熱処理）が行われるが、これは硬質マルテンサイトを軟化させ、また加工誘起変態してマルテンサイトを生成させる残留オーステナイトを分解することにより、伸び、伸びフランジ性を向上させるための処理である。焼戻し条件は、３００℃以上、５５０℃以下の温度で、６０sec 以上、１２００sec 以下の時間を保持する。保持後の冷却速度は特に制限されない。 After the two-phase region annealing, tempering (reheating treatment) is performed. This is because the hard martensite is softened, and the retained austenite that generates martensite by processing-induced transformation is decomposed to expand and stretch the flange. This is a process for improving the performance. The tempering conditions are a temperature of 300 ° C. or more and 550 ° C. or less, and a time of 60 seconds or more and 1200 seconds or less is maintained. The cooling rate after the holding is not particularly limited.
焼戻し温度が３００℃未満では、マルテンサイトの軟質化が十分でなく、鋼板の伸びおよび伸びフランジ性が低下する。一方、５５０℃よりも高くなると、粗大なセメンタイト相が析出して、鋼板の伸びフランジ性が低下する。このため、３００℃以上、５５０℃以下の温度で焼戻しを行う。 When the tempering temperature is less than 300 ° C., the martensite is not sufficiently softened, and the elongation and stretch flangeability of the steel sheet are deteriorated. On the other hand, when it becomes higher than 550 degreeC, a coarse cementite phase will precipitate and the stretch flangeability of a steel plate will fall. For this reason, tempering is performed at a temperature of 300 ° C. or higher and 550 ° C. or lower.
また焼戻しの保持時間が６０sec 未満では、マルテンサイトの軟質化が十分でく、また１２００sec よりも長くなると、マルテンサイトが軟質化し過ぎて強度の確保が困難になり、またセメンタイトの析出により、鋼板の伸びフランジ性が低下するようになる。このため、焼戻しの際の保持時間は、下限を６０sec 、好ましくは９０sec 以上、より好ましくは１２０sec とし、上限を１２００sec 、好ましくは９００sec 、より好ましくは６００sec とする。 If the tempering holding time is less than 60 seconds, the martensite is not sufficiently softened, and if it is longer than 1200 seconds, the martensite becomes too soft and it is difficult to ensure the strength. Stretch flangeability is reduced. For this reason, the lower limit of the holding time during tempering is 60 sec, preferably 90 sec or more, more preferably 120 sec, and the upper limit is 1200 sec, preferably 900 sec, more preferably 600 sec.
以下、実施例を挙げて本発明をより具体的に説明するが、本発明はかかる実施例によって限定的に解釈されるものではない。 EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limitedly interpreted by this Example.
下記表１に示す化学組成を有する鋼スラブを溶製し、各鋼スラブを１０００〜１１００℃程度に加熱し、下記表２の条件で熱間圧延あるいはさらに予備焼鈍を行い、素材鋼板を製作した。熱延後の平均冷却速度は５０℃/secとした。各素材鋼板から組織観察試験片を採取し、顕微鏡により組織構成を観察すると共にナイタール腐食後の顕微鏡組織写真を画像解析することによってベイナイトの占積率を測定した。表１には成分から公知の計算式により算出したＡc3点、Ｍｓ点の値も参考として示した。また、組織観察結果を表２に併せて示した。そして、得られた各素材鋼板について、下記表３に示した条件で最終焼鈍（二相域焼鈍）および焼戻しを行い、試料鋼板を製作した。 Steel slabs having the chemical composition shown in Table 1 below were melted, each steel slab was heated to about 1000 to 1100 ° C., and hot rolling or further pre-annealing was performed under the conditions shown in Table 2 below to produce a raw steel plate. . The average cooling rate after hot rolling was 50 ° C./sec. A microstructure observation specimen was collected from each material steel plate, and the composition of the bainite was measured by observing the structure of the structure with a microscope and image-analyzing the microscopic structure photograph after nital corrosion. Table 1 also shows the values of the A c3 point and Ms point calculated from the components by a known formula. In addition, the results of tissue observation are also shown in Table 2. And about each obtained raw material steel plate, the final annealing (two-phase region annealing) and tempering were performed on the conditions shown in following Table 3, and the sample steel plate was manufactured.
各試料鋼板の組織（焼鈍ベイナイトの占積率、焼戻しマルテンサイト占積率及び平均粒径）、および機械的特性（引張強さＴＳ、伸びＥＬおよび伸びフランジ性）を以下の要領で測定した。
試料鋼板から組織観察試験片を採取し、ナイタール腐食後の顕微鏡組織写真を画像解析することによって焼鈍ベイナイト、焼戻しマルテンサイトの占積率を求めた。また、焼鈍マルテンサイトの平均粒径は、ＦＥ／ＳＥＭ−ＥＢＳＰによる組織解析によって各粒の面積を測定し、それぞれの粒に相当する円の直径を求め、それらの平均を取ることによて求められた。
また、機械的性質のうち、引張強さ及び伸びは、インストロン杜製の万能引張試験機を使用し、ＪＩＳ５号引張試験片を用いて測定した。伸びフランジ性は、東京衡機社製の２０トン穴拡げ試験機を使用し、鉄鋼連盟規格（ＪＦＳＴ１００１−１９９６）に準拠して穴拡げ率（λ）を求め、これにより評価した。これらの測定結果を表４に併せて示す。表４中、「評価」については、引張強さ（ＴＳ）が５９０ＭＰａ以上、伸び（Ｅｌ）が１０％以上、穴拡げ率（λ）が８０％以上をそれぞれ優れた特性と評価し、３特性のいずれも優れるものを○、３特性中２特性が優れるものを△、３特性中１特性のみが優れるものをＸで表示した。
The structure of each sample steel plate (space ratio of annealed bainite, space ratio of tempered martensite and average particle diameter) and mechanical properties (tensile strength TS, elongation EL and stretch flangeability) were measured as follows.
A microstructure observation specimen was collected from the sample steel plate, and the space ratio of annealed bainite and tempered martensite was determined by image analysis of a microscopic microstructure photograph after nital corrosion. The average particle size of the annealed martensite is obtained by measuring the area of each grain by structural analysis by FE / SEM-EBSP, obtaining the diameter of a circle corresponding to each grain, and taking the average of them. It was.
Of the mechanical properties, the tensile strength and elongation were measured using a universal tensile testing machine manufactured by Instron and using a JIS No. 5 tensile test piece. Stretch flangeability was evaluated by using a 20-ton hole expansion tester manufactured by Tokyo Henki Co., Ltd. and determining the hole expansion rate (λ) in accordance with the Steel Federation Standard (JFST1001-1996). These measurement results are also shown in Table 4. In Table 4, for “evaluation”, the tensile strength (TS) is 590 MPa or more, the elongation (El) is 10% or more, and the hole expansion ratio (λ) is 80% or more, which is evaluated as an excellent characteristic. Each of the three characteristics is excellent, ◯, three of the characteristics are excellent, Δ, and only one of the three characteristics is excellent, indicated by X.
表４より、化学成分、素材鋼板組織、最終焼鈍条件および焼戻し条件のいずれも本発明条件を満足する試料No. １，２，４，５，７，８，１１，１２，１５，１７〜２７の試料鋼板（発明例）は、いずれも引張強さが５９０ＭＰａ以上の高強度、１０％以上の伸び、さらに穴拡げ率が８０％以上の伸びフランジ性を有しており、高強度でありながら、伸び、伸びフランジ性に優れ、優れたプレス成形性を備えていることがわかる。
From Table 4, Sample Nos. 1 , 2 , 4 , 5 , 7 , 8 , 11 , 12 , 15 and 17 to which all of the chemical composition, the raw steel sheet structure, the final annealing condition, and the tempering condition satisfy the conditions of the present invention. The 27 sample steel plates (invention examples) all have high strength with a tensile strength of 590 MPa or more, elongation of 10% or more, and stretch flangeability with a hole expansion ratio of 80% or more. However, it is understood that it has excellent stretchability and stretch flangeability, and has excellent press formability.
Claims (8)
Ｃ：０．０５〜０．３％、
Ｓｉ：０．０１〜３．０％、
Ｍｎ：０．５〜３．０％、
Ａｌ：０．０１〜０．１％
を含み、残部Ｆｅ及び不可避的不純物からなり、
焼戻しマルテンサイトと、フェライト＋オーステナイトの二相温度域での焼鈍によりオーステナイトに変態せずに基地中に残存して微細分散したベイナイト（「焼鈍ベイナイト」という。）との占積率が９０％以上の組織を有し、全組織に対する前記焼戻しマルテンサイトの占積率が５０〜９５％で、前記焼鈍ベイナイトの占積率が５〜３０％であり、前記焼戻しマルテンサイトの平均粒径が円相当直径で１０μm 以下であり、引張強度が５９０ＭＰａ以上である、伸び及び伸びフランジ性に優れた高強度鋼板。 Chemical composition is mass%,
C: 0.05-0.3%
Si: 0.01-3.0%,
Mn: 0.5 to 3.0%
Al: 0.01 to 0.1%
Comprising the balance Fe and unavoidable impurities,
And the tempered martensite, ferrite + austenite two-phase temperature region annealing with remaining in the base without transformation to austenite by finely dispersed bainite (referred to as "annealing bainite".) And space factor of 90 %, The space factor of the tempered martensite with respect to the whole structure is 50 to 95%, the space factor of the annealed bainite is 5 to 30%, and the average particle size of the tempered martensite is A high-strength steel sheet having an equivalent circle diameter of 10 μm or less and an excellent tensile strength and stretch flangeability, having a tensile strength of 590 MPa or more.
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| EP07790799.6A EP2053140B1 (en) | 2006-07-14 | 2007-07-13 | High-strength steel sheets and processes for production of the same |
| PCT/JP2007/064019 WO2008007785A1 (en) | 2006-07-14 | 2007-07-13 | High-strength steel sheets and processes for production of the same |
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