JPH0949026A - Production of high strength hot rolled steel plate excellent in balance between strength and elongation and in stretch-flange formability - Google Patents
Production of high strength hot rolled steel plate excellent in balance between strength and elongation and in stretch-flange formabilityInfo
- Publication number
- JPH0949026A JPH0949026A JP7200897A JP20089795A JPH0949026A JP H0949026 A JPH0949026 A JP H0949026A JP 7200897 A JP7200897 A JP 7200897A JP 20089795 A JP20089795 A JP 20089795A JP H0949026 A JPH0949026 A JP H0949026A
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- Japan
- Prior art keywords
- temperature
- martensite
- balance
- area ratio
- hot
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Abstract
Description
【０００１】[0001]
【発明の属する技術分野】本発明は、強度−伸びバラン
ス及び伸びフランジ性にすぐれる低降伏比高強度熱延鋼
板の製造方法に関し、特に、自動車の足回り部材やホイ
ールディスク等の素材として好適に用いることができる
熱延鋼板の製造方法に関する。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-rolled steel sheet having a low yield ratio, which is excellent in strength-elongation balance and stretch-flangeability, and is particularly suitable as a material for automobile underbody members, wheel discs and the like. The present invention relates to a method for producing a hot rolled steel sheet that can be used for.
【０００２】[0002]
【従来の技術】近年、自動車の安全向上及び燃費向上の
観点から、自動車用鋼板の高強度薄肉化が広く推進され
ている。これらのなかでも、ばね下部材となるホイール
や足回り部材の軽量化は、自動車の燃費向上に極めて有
効な手段であるので、その強度クラスも、従来の引張強
さ４９０〜５９０Ｎ／ｍｍ2 級から、最近では６９０〜
７８０Ｎ／ｍｍ2 級主体へと移行しつつある。2. Description of the Related Art In recent years, from the viewpoint of improving safety and fuel efficiency of automobiles, reduction in strength and thickness of automobile steel sheets has been widely promoted. Among these, reducing the weight of wheels and underbody members that are unsprung members is an extremely effective means for improving the fuel efficiency of automobiles. Therefore, the strength class is also the conventional tensile strength of 490 to 590 N / mm 2 class. So, recently 690
It is shifting to 780 N / mm 2 class main body.
【０００３】ホイールや足回り部材に用いられる加工用
の高強度熱延鋼板としては、フェライト・マルテンサイ
ト複合組織鋼板、所謂デュアル・フェイズ鋼板（ＤＰ鋼
板）が既に知られている。この鋼板は、降伏比が低く、
強度−伸びパランス及び疲労特性にすぐれているが、反
面、伸びフランジ性に劣り、また、溶接熱影響部での軟
化が起こりやすいという問題を有している。As a high-strength hot-rolled steel sheet for working used for wheels and underbody members, a ferrite / martensite composite steel sheet, a so-called dual-phase steel sheet (DP steel sheet) is already known. This steel sheet has a low yield ratio,
It has excellent strength-elongation balance and fatigue characteristics, but on the other hand, it has problems of poor stretch flangeability and softening in the weld heat affected zone.
【０００４】更に、最近では、特開平３−１００４９号
公報に記載されているように、強度−伸びバランスにす
ぐれた鋼板として、残留オーステナイトを含む高強度鋼
板が提案されているが、上記ＤＰ鋼板と同様に、伸びフ
ランジ性の面で劣る問題がある。Further, recently, as described in JP-A-3-10049, a high strength steel sheet containing retained austenite has been proposed as a steel sheet excellent in strength-elongation balance. Similarly, there is a problem in that the stretch flangeability is inferior.
【０００５】一方、伸びフランジ性にすぐれる熱延鋼板
としては、例えば、特開昭５７−１７２９２０号公報に
記載されているように、フェライト・ベイナイト複合組
織鋼板が知られている。しかし、この鋼板は、降伏比が
高く、また、強度−伸びバランスも十分とはいえない。On the other hand, as a hot-rolled steel sheet having excellent stretch-flangeability, a ferrite-bainite composite structure steel sheet is known, for example, as described in JP-A-57-172920. However, this steel sheet has a high yield ratio and the strength-elongation balance is not sufficient.
【０００６】このように、同一強度の鋼板で比較すれ
ば、通常、低降伏比で強度−伸びバランスにすぐれる鋼
板は、伸びフランジ性に劣り、伸びフランジ性にすぐれ
る鋼板は、低降伏比特性及び強度−伸びバランスに劣る
傾向にあり、かくして、従来、両方の特性を兼備させる
のは困難であった。As described above, when comparing steel sheets having the same strength, generally, a steel sheet having a low yield ratio and an excellent strength-elongation balance is inferior in stretch flangeability, and a steel sheet having an excellent stretch flangeability has a low yield ratio. The properties and the strength-elongation balance tend to be inferior, and thus it has heretofore been difficult to combine both properties.
【０００７】このような問題を解決し、強度−伸びバラ
ンス、伸びフランジ性の両方にすぐれる低降伏比の鋼板
として、フェライト・ベイナイト・マルテンサイトから
なる複合組織を最適に調整した所謂トライ・フェイズ鋼
板が特開昭５７−７０２５７号公報、特開昭５７−１４
５９６５号公報等に提案されている。しかし、この鋼板
は、引張強度が６９０Ｎ／ｍｍ2 までは、強度−伸びバ
ランス、伸びフランジ性及び低降伏比特性を兼ね備えて
いるが、最近の更なる高強度化の動きのなかで要求され
ている引張強度６９０Ｎ／ｍｍ2 以上の領域では、すべ
ての面ですぐれた特性を有せしめることは困難である。A so-called tri phase in which a composite structure composed of ferrite, bainite, and martensite is optimally adjusted as a steel plate having a low yield ratio which is excellent in both strength-elongation balance and stretch flangeability by solving such problems. Steel plates are disclosed in JP-A-57-70257 and JP-A-57-14.
It is proposed in Japanese Patent No. 5965. However, this steel sheet has strength-elongation balance, stretch-flangeability, and low yield ratio characteristics up to a tensile strength of 690 N / mm 2 , but it is required in the recent trend toward higher strength. In the region where the tensile strength is 690 N / mm 2 or more, it is difficult to provide excellent properties on all surfaces.
【０００８】[0008]
【発明が解決しようとする課題】本発明者らは、フェラ
イト・ベイナイト・マルテンサイト３相複合組織鋼板の
高強度化に伴う強度−伸びバランス及び伸びフランジ性
の劣化の問題を解決するために鋭意研究した結果、上記
３相組織鋼板において、ベイナイト及びマルテンサイト
の面積率をそれぞれ最適に規制することによって、フェ
ライトとマルテンサイトが直接に接するのを防止して、
フェライトとマルテンサイトの間の歪みの差をなくする
と共に、低降伏比化し、且つ、マルテンサイトの周囲の
長さに占めるフェライト−マルテンサイトの境界長さの
割合を規制することによって、局部変形時の亀裂の発生
を制御して、伸びフランジ性を高めることができること
を見出して、本発明に至ったものである。DISCLOSURE OF THE INVENTION The inventors of the present invention have been keen to solve the problems of deterioration of strength-elongation balance and stretch-flangeability associated with higher strength of a ferrite / bainite / martensite triple-phase composite steel sheet. As a result of research, in the above-described three-phase steel sheet, by controlling the area ratio of bainite and martensite to the optimum, it is possible to prevent direct contact between ferrite and martensite,
At the time of local deformation by eliminating the difference in strain between ferrite and martensite, reducing the yield ratio, and controlling the ratio of the boundary length of ferrite-martensite to the peripheral length of martensite. The inventors of the present invention have found that the occurrence of cracks can be controlled to enhance stretch-flange formability, leading to the present invention.
【０００９】即ち、本発明は、フェライト・ベイナイト
・マルテンサイト３相組織熱延鋼板であって、強度−伸
びバランス及び伸びフランジ性にすぐれる低降伏比高強
度熱延鋼板の製造方法を提供することを目的とする。That is, the present invention provides a method for producing a ferrite-bainite-martensite three-phase hot rolled steel sheet having a low yield ratio and a high strength hot rolled steel sheet having excellent strength-elongation balance and stretch flangeability. The purpose is to
【００１０】[0010]
【課題を解決するための手段】本発明による強度−伸び
バランス及び伸びフランジ性にすぐれる高強度熱延鋼板
の製造方法は、重量％にて Ｃ 0.０５〜0.３０％、 Ｓｉ 0.５〜2.５％ Ｍｎ 0.５〜3.０％ Ａｌ 0.０１〜0.１０％ Ｓ 0.０１％以下を含み、 残部鉄及び不可避的不純物よりなる鋼を仕上温度Ａr3点
以上の温度で熱間圧延し、熱間圧延仕上温度からＡr3点
〜Ａr1点の範囲の温度までを平均冷却速度５〜７０℃／
秒にて冷却し、次いで、その温度から２〜２０秒間、放
冷又は徐冷し、その後、平均冷却速度２０〜７０℃／秒
にて５５０〜３００℃の範囲の温度まで冷却して、巻取
り、その組織がベイナイト面積率（ＶｆＢ）１０〜４０
％及びマルテンサイト面積率（ＶｆＭ）３〜１５％、残
部が実質的にポリゴナルフェライトからなり、且つ、マ
ルテンサイト周囲の長さ（ＬM ）とマルテンサイト−フ
ェライト境界の長さ（ＬM-F ) が次式 （ＬM-F ／ＬM ）＋0.０５×ＶｆＭ≦1.１５ を満足することを特徴とする。ここに、本発明におい
て、上記マルテンサイト周囲の長さ（ＬM ）とマルテン
サイト−フェライト境界の長さ（ＬM-F ) は、いずれも
平均値である。The method for producing a high-strength hot-rolled steel sheet excellent in strength-elongation balance and stretch-flangeability according to the present invention is C 0.05-0.30% by weight% and Si 0.30. 5 to 2.5% Mn 0.5 to 3.0% Al 0.01 to 0.10% S 0.01% or less, steel with balance iron and unavoidable impurities with a finishing temperature Ar of 3 points or more Hot rolling at a temperature, and from the hot rolling finish temperature to a temperature in the range of Ar 3 points to Ar 1 point, an average cooling rate of 5 to 70 ° C /
Cooling for 2 to 20 seconds, and then cooled to a temperature in the range of 550 to 300 ° C. at an average cooling rate of 20 to 70 ° C./second and wound. And its structure is bainite area ratio (VfB) 10-40
% And martensite area ratio (VfM) 3 to 15%, the balance substantially consisting of polygonal ferrite, and the length of the martensite periphery (L M ) and the length of the martensite-ferrite boundary (L MF ). Satisfies the following expression (L MF / L M ) + 0.05 × VfM ≦ 1.15. Here, in the present invention, the martensite perimeter (L M) and martensite - the length of the ferrite boundaries (L MF) are all average values.
【００１１】[0011]
【発明の実施の形態】先ず、本発明による方法において
用いる鋼の化学成分について説明する。Ｃは、鋼の強化
に効果を有し、特に、ベイナイト及びマルテンサイト等
の低温変態生成物を形成するために必要な元素であり、
そのためには少なくとも0.０５％を添加する必要があ
る。しかし、過多に添加するときは、延性の劣化が著し
く、溶接性も低下するので、その上限を0.３０％とす
る。BEST MODE FOR CARRYING OUT THE INVENTION First, the chemical composition of steel used in the method according to the present invention will be described. C is an element which has an effect of strengthening steel and is particularly necessary for forming low temperature transformation products such as bainite and martensite,
For that purpose, it is necessary to add at least 0.05%. However, if excessively added, ductility deteriorates remarkably and weldability also deteriorates, so the upper limit is made 0.30%.
【００１２】Ｓｉは、本発明において重要な元素であ
る。即ち、Ｓｉは、固溶強化元素として、引張強さの向
上に非常に有効であるばかりでなく、フェライトの強度
を高めることによって、第２相との強度差を低減し、フ
ェライトに変形が集中することを防ぐので、伸びフラン
ジ性の向上にも寄与する。また、本発明のような複合組
織鋼板においては、熱延後のγ−α（オーステナイト−
フェライト）変態において、αへの変態を促進すると共
に、α中の固溶炭素をγ中へ排出する作用があり、その
結果、α相の清浄性を高め、強度−延性バランスの向上
にも有効である。これらの効果を発揮させるためには、
少なくとも0.５％以上の添加を必要とするが、しかし、
過多に添加するときは、溶接部の脆化を招くだけでな
く、酸化スケールの生成によって、表面性状が悪化する
ので、添加量は2.５％を上限とする。Si is an important element in the present invention. That is, Si is not only a very effective solid solution strengthening element for improving the tensile strength, but also increases the strength of ferrite, thereby reducing the strength difference from the second phase and concentrating deformation on ferrite. It also prevents the occurrence of stretch and contributes to the improvement of stretch flangeability. Further, in a steel sheet having a composite structure as in the present invention, γ-α (austenite-
In the (ferrite) transformation, it has the effect of promoting the transformation to α and discharging the solid solution carbon in α into γ. As a result, it improves the cleanliness of the α phase and is also effective in improving the strength-ductility balance. Is. In order to exert these effects,
It requires at least 0.5% or more, but
If added in excess, not only will the weld become brittle, but the surface quality will deteriorate due to the formation of oxide scale, so the upper limit of addition is 2.5%.
【００１３】Ｍｎは、焼き入れ性を向上させ、ベイナイ
ト及びマルテンサイトを生成させるために必要な元素で
ある。この効果を有効に発揮させるには、少なくとも0.
５％を添加する必要がある。しかし、過多に添加すると
きは、延性を低下させるのみならず、溶接性をも害する
ので、その上限を3.０％とする。Ｓは、伸びフランジ性
の改善のために、0.０１％以下とする。Mn is an element necessary for improving hardenability and forming bainite and martensite. At least 0 for this effect to work effectively.
It is necessary to add 5%. However, if too much is added, not only the ductility is lowered, but also the weldability is impaired, so the upper limit is made 3.0%. S is 0.01% or less in order to improve stretch flangeability.
【００１４】Ａｌは、脱酸のために添加するが、0.１０
％を超えて加えるときは、アルミナ系の介在物が増加
し、加工性を劣化させるので、Ａｌ量は0.１０％以下と
する。本発明の方法においては、上記元素に加えて、 Ｍｏ 0.０５〜1.０％ Ｖ 0.０１〜0.５％ Ｔｉ 0.０１〜0.３％ Ｎｂ 0.０１〜0.３％ よりなる第１の群から選ばれる少なくとも１種の元素を
鋼に添加することができる。Al is added for deoxidation, but 0.10 is added.
If added in excess of%, alumina-based inclusions increase and workability deteriorates, so the Al content is made 0.10% or less. In the method of the present invention, in addition to the above elements, Mo 0.05-1.0% V 0.01-0.5% Ti 0.01-0.3% Nb 0.01-0.3% At least one element selected from the first group can be added to the steel.
【００１５】これらの元素は、析出強化元素であり、得
られる熱延鋼板の強度上昇に必要であるのみならず、Ｍ
ｎと共存して熱延後の変態組織に影響を与え、低温変態
生成物を得やすくする作用がある。また、これらの元素
は、組織を微細化し、伸びフランジ性を向上させると共
に、溶接後の熱影響部の硬度低下を防止し、疲労強度を
改善するのに役立つ。これらの効果を有効に発揮させる
ために、添加量は上記下限量が必要である。しかし、過
多に添加するときは、降伏比の上昇や延性の低下を招く
ので、それぞれ上記を上限量とする。These elements are precipitation strengthening elements and are not only required for increasing the strength of the hot-rolled steel sheet obtained, but also M
Coexisting with n, it has an effect on the transformation structure after hot rolling and makes it easy to obtain a low temperature transformation product. Further, these elements are useful for refining the structure, improving stretch flangeability, preventing a decrease in hardness of the heat-affected zone after welding, and improving fatigue strength. In order to effectively exhibit these effects, the amount added must be the above lower limit. However, if too much is added, the yield ratio will increase and the ductility will decrease, so the above limits are set for the respective amounts.
【００１６】更に、本発明においては、 Ｃｒ 0.０５〜1.０％、 Ｂ 0.０００５〜0.０１％ よりなる第２の群から選ばれる少なくとも１種の元素を
鋼に添加することができる。Further, in the present invention, at least one element selected from the second group consisting of Cr 0.05 to 1.0% and B 0.0005 to 0.01% may be added to the steel. it can.
【００１７】Ｃｒは、焼き入れ性を向上させて、所望の
組織を有利に与える元素である。この効果を有効に得る
ためには、少なくとも0.０５％を添加する必要がある。
しかし、過多に添加しても、上記効果が飽和するのみな
らず、製造費用の上昇を招くので、添加量の上限を1.０
％とする。Ｂは、焼き入れ性を向上させ、所望の組織を
得るうえで有用な元素であり、この効果を有効に得るた
めには、0.０００５％以上を添加することが必要であ
る。しかし、過多に添加しても、その効果が飽和し、ま
た、経済的にも不利であるので、上限を0.０１％とす
る。Cr is an element that improves the hardenability and advantageously gives a desired structure. In order to obtain this effect effectively, it is necessary to add at least 0.05%.
However, even if added excessively, not only the above effects will be saturated, but also the production cost will rise, so the upper limit of the addition amount is 1.0.
%. B is an element useful for improving hardenability and obtaining a desired structure, and it is necessary to add 0.0005% or more in order to effectively obtain this effect. However, even if added in excess, the effect is saturated and it is economically disadvantageous, so the upper limit is made 0.01%.
【００１８】更に、本発明においては、 Ｃａ 0.００５％以下、 希土類元素 0.００５％以下 よりなる第３の群から選ばれる少なくとも１種の元素を
鋼に添加することができる。Further, in the present invention, at least one element selected from the third group consisting of Ca 0.005% or less and rare earth element 0.005% or less can be added to the steel.
【００１９】これらの元素は、硫化物の形態の制御を通
じて、延性、特に、伸びフランジ性を改善する効果を有
する。このような効果を有効に得るためには、それぞれ
上記の下限量の添加を必要とするが、しかし、過多に添
加しても、上記効果が飽和し、また、製造費用を高める
ので、それぞれ上記を上限とする。本発明においては、
上記第１、第２及び第３の群の元素は、それぞれ単独で
添加してもよく、また、複合添加してもよい。These elements have the effect of improving ductility, especially stretch-flangeability, by controlling the sulfide morphology. In order to effectively obtain such effects, it is necessary to add the respective lower limit amounts described above, however, even if added excessively, the above effects saturate and the manufacturing cost is increased. Is the upper limit. In the present invention,
The elements of the first, second and third groups may be added alone or in combination.
【００２０】本発明の方法によれば、上述したような化
学成分を有する鋼を仕上温度Ａr3点以上の温度で熱間圧
延し、熱間圧延仕上温度からＡr3点〜Ａr1点の範囲の温
度までを平均冷却速度５〜７０℃／秒にて冷却（一次冷
却）し、次いで、その温度から、２〜２０秒間、放冷又
は徐冷し、その後、平均冷却速度２０〜７０℃／秒にて
５５０〜３００℃の範囲の温度まで冷却（二次冷却）し
て、巻取る。According to the method of the present invention, steel having the above-described chemical composition is hot-rolled at a finishing temperature of Ar 3 points or higher, and the hot-rolling finishing temperature is in the range of Ar 3 point to Ar 1 point. To an average cooling rate of 5 to 70 ° C./sec (primary cooling), and then from that temperature is allowed to cool or gradually cool for 2 to 20 seconds, and then the average cooling rate of 20 to 70 ° C. / It is cooled to a temperature in the range of 550 to 300 ° C. in a second (secondary cooling) and wound.
【００２１】本発明の方法において、熱間圧延での仕上
温度は、Ａr3点以上の温度であることが必要である。仕
上温度がＡr3点よりも低いときは、加工フェライト組織
や混粒相が生じて、得られる熱延鋼板が延性において劣
化する。In the method of the present invention, the finishing temperature in hot rolling needs to be a temperature of Ar 3 or higher. When the finishing temperature is lower than the Ar 3 point, a work ferrite structure and a mixed grain phase are generated, and the hot rolled steel sheet obtained is deteriorated in ductility.
【００２２】仕上圧延後、熱間圧延仕上温度からＡr3点
〜Ａr1点の範囲の温度までの冷却（一次冷却）に際し
て、その平均冷却速度は、５〜７０℃／秒の範囲であ
る。平均冷却速度が５℃／秒よりも遅いときは、Ａr3点
までにオーステナイトの再結晶−結晶粒成長が進行し、
フェライト変態促進の効果が低下するうえ、操業上、時
間がかかりすぎるので好ましくない。他方、平均冷却速
度が７０℃／秒を越えるときは、鋼板の温度制御が困難
となる。特に、本発明においては、上記一次冷却におけ
る平均冷却速度は、好ましくは、３０〜７０℃／秒の範
囲である。After finishing rolling, upon cooling from the hot rolling finishing temperature to a temperature in the range of Ar 3 point to Ar 1 point (primary cooling), the average cooling rate is in the range of 5 to 70 ° C./sec. When the average cooling rate is slower than 5 ° C./sec, recrystallization of austenite-grain growth proceeds up to the Ar 3 point,
This is not preferable because the effect of accelerating the ferrite transformation is lowered and the operation takes too long. On the other hand, when the average cooling rate exceeds 70 ° C./sec, it becomes difficult to control the temperature of the steel sheet. Particularly, in the present invention, the average cooling rate in the primary cooling is preferably in the range of 30 to 70 ° C / sec.
【００２３】本発明によれば、このように、Ａr3点〜Ａ
r1点の間の温度まで制御冷却した後、鋼板をその温度か
ら放冷又は徐冷する。この放冷又は徐冷は、フェライト
変態ノーズ付近で行なわれるので、多数のフェライトの
核が生成し、短時間にて所定のフェライト量を得ること
ができる。また、この放冷又は徐冷の間に変態したフェ
ライト相中の固溶炭素は、未変態オーステナイト相中に
濃縮されるが、多数の核によって生成したフェライト
は、未変態オーステナイト中に均一に分散しているの
で、オーステナイト中に濃縮された炭素の量は、どの結
晶粒においても、ほぼ同じ量となる。極端に炭素が濃縮
された未変態オーステナイト粒が存在するときは、その
後の冷却によってベイナイトを生成せず、マルテンサイ
ト変態するため、孤立したマルテンサイトとなり、伸び
フランジ性が劣化する。According to the present invention, as described above, Ar 3 points to A
r After controlled cooling to a temperature between 1 point, the steel sheet is allowed to cool or gradually cool from that temperature. Since this cooling or gradual cooling is performed in the vicinity of the ferrite transformation nose, a large number of ferrite nuclei are generated and a predetermined amount of ferrite can be obtained in a short time. Further, the solid solution carbon in the ferrite phase transformed during the standing cooling or slow cooling is concentrated in the untransformed austenite phase, but the ferrite generated by many nuclei is uniformly dispersed in the untransformed austenite. Therefore, the amount of carbon enriched in austenite is almost the same in every crystal grain. When untransformed austenite grains with extremely concentrated carbon are present, bainite is not formed by subsequent cooling and martensite transformation occurs, resulting in isolated martensite and deterioration of stretch flangeability.
【００２４】上記徐冷又は放冷の時間は、余りに短いと
きは、所定のフェライト量を得ることができず、他方、
長すぎるときは、パーライト変態が起こり、更に、ラン
アウトテーブルの長さによっても、自ずと制限されるこ
とから、放冷又は徐冷時間は、２〜２０秒の間が適当で
ある。次いで、上記放冷又は徐冷後の冷却（二次冷却）
は、未変態のオーステナイトから硬質の低温変態生成物
を得るためであって、その平均冷却速度は、２０〜７０
℃／秒の範囲である。この二次冷却に際して、平均冷却
速度が２０℃／秒よりも遅いときは、未変態オーステナ
イトの一部又は全部がすべてパーライト変態し、他方、
７０℃／秒よりも早いときは、強度−延性バランスを悪
くすると共に、降伏比を高めることとなる。If the gradual cooling or cooling time is too short, a predetermined amount of ferrite cannot be obtained, while
If it is too long, pearlite transformation will occur, and the length of the run-out table will naturally limit it. Therefore, the cooling time or gradual cooling time is suitably between 2 and 20 seconds. Then, cooling after the above cooling or slow cooling (secondary cooling)
Is to obtain a hard low-temperature transformation product from untransformed austenite, and the average cooling rate is 20 to 70.
The range is ° C / sec. In this secondary cooling, when the average cooling rate is slower than 20 ° C./sec, part or all of the untransformed austenite undergoes pearlite transformation, while
When it is faster than 70 ° C / sec, the strength-ductility balance is deteriorated and the yield ratio is increased.
【００２５】このように、本発明によれば、冷却速度が
比較的遅くても、硬質相を得ることができるのは、それ
までの過程において、未変態オーステナイトの安定度が
高められているためであり、かくして、熱間圧延仕上温
度からこの二次冷却開始までの過程が重要である。更
に、本発明によれば、硬質相をベイナイト主体にしてい
ることも、比較的ゆっくりとした冷却を可能とする一因
であって、これによって鋼板の強度−延性バランスの改
善に寄与する。As described above, according to the present invention, the hard phase can be obtained even if the cooling rate is relatively slow, because the stability of the untransformed austenite is increased in the process up to that point. Thus, the process from the hot rolling finish temperature to the start of this secondary cooling is important. Further, according to the present invention, the fact that the hard phase is mainly composed of bainite is one of the factors that enables relatively slow cooling, which contributes to the improvement of the strength-ductility balance of the steel sheet.
【００２６】この二次冷却の後、鋼板を巻取るが、その
温度が３００℃よりも低いときは、過剰で孤立したマル
テンサイトの生成によって伸びフランジ性が劣化する。
他方、５５０℃越える場合には、多量の合金元素を添加
しないときは、パーライト変態が生じて、強度が不足す
ることとなるうえに、伸びフランジ性も却って劣化す
る。かくして、巻取温度は５５０℃以下とする。After the secondary cooling, the steel sheet is wound up, but when the temperature is lower than 300 ° C., stretch flangeability is deteriorated due to the formation of excessive and isolated martensite.
On the other hand, when the temperature exceeds 550 ° C., unless a large amount of alloying element is added, pearlite transformation occurs, resulting in insufficient strength and stretch flangeability rather deteriorates. Thus, the winding temperature is set to 550 ° C or lower.
【００２７】次に、本発明による熱延鋼板の金属組織に
ついて説明する。本発明による熱延鋼板は、その組織が
ベイナイト面積率（ＶｆＢ）１０〜４０％及びマルテン
サイト面積率（ＶｆＭ）３〜１５％、残部が実質的にポ
リゴナルフェライトからなると共に、マルテンサイト周
囲の長さ（ＬM ）とマルテンサイト−フェライト境界の
長さ（ＬM-F ) が次式 （ＬM-F ／ＬM ）＋0.０５×ＶｆＭ≦1.１５ （１） を満足する。以下、上記式（１）の左辺の値を単に式値
ということがある。Next, the metallographic structure of the hot rolled steel sheet according to the present invention will be described. The hot-rolled steel sheet according to the present invention has a bainite area ratio (VfB) of 10 to 40%, a martensite area ratio (VfM) of 3 to 15%, and the balance substantially consisting of polygonal ferrite, and the surroundings of martensite. The length (L M ) and the length of the martensite-ferrite boundary (L MF ) satisfy the following equation (L MF / L M ) + 0.05 × VfM ≦ 1.15 (1). Hereinafter, the value on the left side of the above formula (1) may be simply referred to as a formula value.
【００２８】ベイナイトは、得られる熱延鋼板が所要の
強度を有するように、本発明によれば、その量が面積率
にて１０％以上であることが必要であるが、しかし、面
積率にて４０％を超えるときは、却って強度−延性バラ
ンスが低下するので、その量は、面積率で１０％〜４０
％の範囲とする。According to the present invention, the amount of bainite is required to be 10% or more in area ratio so that the obtained hot-rolled steel sheet has required strength. If it exceeds 40%, the strength-ductility balance is rather decreased, so the amount is 10% to 40% in terms of area ratio.
The range is%.
【００２９】マルテンサイトは、その存在によって、低
降伏比化と強度−伸びバランスに対して好ましい効果を
与える。しかし、マルテンサイト量が面積率で３％より
も少ないときは、低降伏比化の効果が殆どなく、他方、
面積率で１５％を超えるときは、ベイナイトと接するこ
となく、孤立して、フェライト中に存在するマルテンサ
イト量が増加して、伸びフランジ性が著しく低下する。
かくして、本発明においては、マルテンサイト量は、面
積率にて１〜１５％の範囲とする。Due to the presence of martensite, it has a favorable effect on lowering the yield ratio and the strength-elongation balance. However, when the martensite content is less than 3% in area ratio, there is almost no effect of lowering the yield ratio, while
When the area ratio exceeds 15%, the amount of martensite existing in ferrite increases without being in contact with bainite, and the stretch-flange formability significantly decreases.
Thus, in the present invention, the amount of martensite is in the range of 1 to 15% in area ratio.
【００３０】本発明に従って、各相の面積率が上記の範
囲内にあるとき、得られる熱延鋼板は、強度−延性バラ
ンス及び低降伏比特性について満足すべき特性を有する
が、更に、本発明によれば、マルテンサイト周囲の長さ
（ＬM ）とフェライト−マルテンサイト境界の長さ（Ｌ
M-F ) とマルテンサイト面積率ＶｆＭが前記式（１）を
満足するときに、同じベイナイト、マルテンサイト面積
率をもつ鋼板であっても、伸びフランジ性に一層すぐれ
ている。図１に前記式（１）の値とＴＳ（引張強さ）×
λとの積との関係を示す。伸びフランジ性λについて
は、後述する。According to the present invention, when the area ratio of each phase is within the above range, the hot-rolled steel sheet obtained has satisfactory properties in terms of strength-ductility balance and low yield ratio characteristics. According to the results, the length around the martensite (L M ) and the length of the ferrite-martensite boundary (L
When MF ) and the martensite area ratio VfM satisfy the above formula (1), even a steel sheet having the same bainite and martensite area ratio is more excellent in stretch flangeability. The value of the above formula (1) and TS (tensile strength) ×
The relationship with the product of λ is shown. The stretch flangeability λ will be described later.
【００３１】前記式（１）における式値によって、伸び
フランジ性が大幅に変化する理由は、３相複合組織鋼板
の局部変形時の亀裂発生場所が多くの場合、フェライト
−マルテンサイト境界であり、フェライト−ベイナイト
境界とベイナイト−マルテンサイト境界においては局部
変形時の亀裂発生が少ないので、鋼板がマルテンサイト
を同じ面積率で有していても、ＬM-F ／ＬM の値が小さ
ければ、マルテンサイトの周囲の組織がベイナイトが主
体となるので、亀裂の発生が起こり難いためであると考
えられられる。即ち、ベイナイト−マルテンサイト境
界、フェライト−ベイナイト境界はいずれも、フェライ
ト−マルテンサイト境界よりも亀裂が発生し難いからで
ある。これに対して、ＬM-F ／ＬM の値が大きいとき
は、マルテンサイトの周囲の組織が殆どフェライトであ
り、フェライト−マルテンサイト境界から亀裂が発生す
る頻度が高く、このこととから伸びフランジ性が悪い。The reason why the stretch flangeability is greatly changed by the value in the above formula (1) is the ferrite-martensite boundary in many cases where cracks are generated during local deformation of the three-phase composite structure steel sheet, ferrite - bainite boundaries and bainite - because small cracks generation during local deformation in martensite boundaries, even if the steel sheet have the same area ratio of martensite, the smaller the value of L MF / L M, martensite It is considered that this is because the structure around Ba is mainly composed of bainite, and thus cracks are less likely to occur. That is, cracks are less likely to occur in the bainite-martensite boundary and the ferrite-bainite boundary than in the ferrite-martensite boundary. On the other hand, when the value of L MF / L M is large, the structure around the martensite is mostly ferrite, and cracks frequently occur from the ferrite-martensite boundary, which indicates that stretch flangeability is high. Is bad.
【００３２】[0032]
【実施例】以下に実施例を挙げて本発明を説明するが、
本発明はこれら実施例により何ら限定されるものではな
い。表１に示す化学成分を有する鋼を真空溶解炉にて溶
製し、粗圧延により３０ｍｍ厚のスラブとした。次に、
このスラブを１２００℃に加熱し、表２及び表３に示す
熱間圧延条件にて3.５ｍｍ厚さまで圧延し、続いて、熱
間圧延仕上温度からＡr3点〜Ａr1点の間の温度まで一次
冷却し、その冷却停止温度（放冷開始温度）から放冷を
開始し、次いで、５５０〜３００℃の間の温度まで二次
冷却し、巻き取って、供試材を得た。表２及び表３に仕
上温度、一次冷却速度、放冷開始温度、放冷時間、二次
冷却速度及び巻取温度を示す。The present invention will be described below with reference to examples.
The present invention is not limited to these examples. Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace and rough-rolled into a slab having a thickness of 30 mm. next,
This slab is heated to 1200 ° C. and rolled under the hot rolling conditions shown in Tables 2 and 3 to a thickness of 3.5 mm, and then the temperature between the hot rolling finish temperature and the Ar 3 point to Ar 1 point. After that, primary cooling was started, cooling was started from the cooling stop temperature (cooling start temperature), then secondary cooling was performed to a temperature between 550 and 300 ° C., and winding was performed to obtain a test material. Tables 2 and 3 show finishing temperature, primary cooling rate, cooling start temperature, cooling time, secondary cooling rate and winding temperature.
【００３３】その後、得られた熱延鋼板の表裏面を研削
して、板厚２ｍｍにした後、ＪＩＳ５号引張試験、穴拡
げ試験及び両振平面曲げ試験に供した。伸びフランジ性
の評価として、穴拡げ率λは１０ｍｍ径の打抜き穴をあ
け、円錐ポンチにてクラックが板厚を貫通するまで押し
拡げたときの穴系ｄb と初期穴径ｄi とから、次式によ
り求めた。Thereafter, the front and back surfaces of the hot rolled steel sheet obtained were ground to a plate thickness of 2 mm, and then subjected to JIS No. 5 tensile test, hole expansion test, and both-sided plane bending test. As the evaluation of the stretch flangeability, the hole expansion ratio λ was determined from a hole system d b and an initial hole diameter d i when a punched hole having a diameter of 10 mm was punched and expanded by a conical punch until cracks penetrate the plate thickness. It was calculated by the following formula.
【００３４】 λ＝（（ｄb −ｄi ）／ｄi ）×１００（％） 得られた熱延鋼板の組織と機械的性質を表４及び表５に
示す。また、本発明鋼と比較鋼とについて、ＴＳ×Ｅｌ
（伸び）とＴＳ×λとの関係を図２に示す。Λ = ((d b −d i ) / d i ) × 100 (%) Tables 4 and 5 show the structures and mechanical properties of the obtained hot-rolled steel sheet. Further, regarding the steel of the present invention and the comparative steel, TS × El
The relationship between (elongation) and TS × λ is shown in FIG.
【００３５】[0035]
【表１】 [Table 1]
【００３６】[0036]
【表２】 [Table 2]
【００３７】[0037]
【表３】 [Table 3]
【００３８】[0038]
【表４】 [Table 4]
【００３９】[0039]
【表５】 [Table 5]
【００４０】これらの結果から明らかなように、本発明
鋼は、伸び、伸びフランジ性、降伏比（ＹＲ）のすべて
にすぐれている。特に、ＴＳ×Ｅｌが２００００Ｎ／ｍ
ｍ2・％以上、好ましくは、２００００Ｎ／ｍｍ2 ・％
以上であり、ＴＳ×λが４２０００Ｎ／ｍｍ2 ・％以
上、好ましくは、４５０００Ｎ／ｍｍ2 ・％以上であ
る。また、本発明による熱延鋼板は、伸びフランジ性に
すぐれるものであるが、疲労特性にも十分にすぐれてい
ることが示されている。As is clear from these results, the steel of the present invention is excellent in elongation, stretch flangeability and yield ratio (YR). Especially, TS × El is 20000 N / m
m 2 ·% or more, preferably 20000 N / mm 2 ·%
Above, TS × λ is 42000 N / mm 2 ·% or more, preferably 45000 N / mm 2 ·% or more. Further, it has been shown that the hot-rolled steel sheet according to the present invention is excellent in stretch flangeability, but is also sufficiently excellent in fatigue properties.
【００４１】一方、比較鋼のうち、比較鋼１９はＣ量が
下限を下回るので、６９０Ｎ／ｍｍ 2 以上の引張強さを
得ることができず、また、マルテンサイトを含まないた
めに、降伏比が高くなっている。また、比較鋼２０は、
反対にＣ量が上限を超えるので、ＴＳ×Ｅｌ（伸び）、
ＴＳ×λ共に著しく劣っている。比較鋼２１は、Ｓｉが
不足しているために、強度不足であるうえに、ＴＳ×Ｅ
ｌ、ＴＳ×λ共に十分ではない。比較鋼２３は、Ｍｎ量
が不足しているところから、マルテンサイトが生成せ
ず、降伏比が高い。On the other hand, of the comparative steels, comparative steel 19 has a C content.
Since it is below the lower limit, 690 N / mm 2Above tensile strength
Could not be obtained and did not contain martensite
Therefore, the yield ratio is high. Further, the comparative steel 20 is
On the contrary, since the amount of C exceeds the upper limit, TS × El (elongation),
Both TS × λ are extremely inferior. Comparative steel 21 has Si
Insufficient strength due to lack and TS × E
Both l and TS × λ are not sufficient. Comparative steel 23 has Mn content
Martensite is generated from the lack of
No, the yield ratio is high.
【００４２】また、比較鋼２、８、１４及び１５は化学
成分、ＶｆＢ、ＶｆＭのすべてが本発明で規定する範囲
内にあるが、前記式（１）における式値が本発明による
条件を満たさないために、ＴＳ×λの値が低い。比較鋼
６は、ベイナイ面積率が上限を超えているので、特に、
ＴＳ×Ｅｌの値が低い。In Comparative Steels 2, 8, 14 and 15, all of the chemical components, VfB and VfM are within the range specified by the present invention, but the formula value in the above formula (1) satisfies the condition according to the present invention. Since it is not present, the value of TS × λ is low. In Comparative Steel 6, since the Baynai area ratio exceeds the upper limit,
The value of TS × El is low.
【００４３】[0043]
【発明の効果】以上のように、本発明の方法によれば、
フェライト・ベイナイト・マルテンサイト三相複合組織
からなり、強度−伸びバランスと加工性、特に、伸びフ
ランジ性にすぐれた熱延鋼板を得ることができ、このよ
うな熱延鋼板は、例えば、自動車の足回り部材やホイー
ルディスク等の素材として好適に用いることができる。As described above, according to the method of the present invention,
A hot-rolled steel sheet having a ferrite-bainite-martensite three-phase composite structure and excellent in strength-elongation balance and workability, particularly stretch-flangeability, can be obtained. It can be suitably used as a material for an underbody member, a wheel disc, and the like.
【図１】は、式（１）の式値とＴＳ×λの関係を示すグ
ラフである。FIG. 1 is a graph showing the relationship between the expression value of expression (1) and TS × λ.
【図２】は、ＴＳ×ＥｌとＴＳ×λとの関係を示すグラ
フである。FIG. 2 is a graph showing the relationship between TS × El and TS × λ.
───────────────────────────────────────────────────── フロントページの続き (51)Int.Cl.6 識別記号 庁内整理番号 ＦＩ 技術表示箇所 Ｃ２２Ｃ 38/14 Ｃ２２Ｃ 38/14 38/32 38/32 38/38 38/38 ─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. 6 Identification code Internal reference number FI Technical indication C22C 38/14 C22C 38/14 38/32 38/32 38/38 38/38
Claims (7)
よりなる鋼を仕上温度Ａr3点以上の温度で熱間圧延し、
熱間圧延仕上温度からＡr3点〜Ａr1点の範囲の温度まで
を平均冷却速度５〜７０℃／秒にて冷却し、次いで、そ
の温度から２〜２０秒間、放冷又は徐冷し、その後、平
均冷却速度２０〜７０℃／秒にて５５０〜３００℃の範
囲の温度まで冷却して、巻取り、その組織がベイナイト
面積率（ＶｆＢ）１０〜４０％及びマルテンサイト面積
率（ＶｆＭ）３〜１５％、残部が実質的にポリゴナルフ
ェライトからなり、且つ、マルテンサイト周囲の長さ
（ＬM ）とマルテンサイト−フェライト境界の長さ（Ｌ
M-F ) が次式 （ＬM-F ／ＬM ）＋0.０５×ＶｆＭ≦1.１５ を満足することを特徴とする強度−伸びバランス及び伸
びフランジ性にすぐれる高強度熱延鋼板の製造方法。1. In weight%, C 0.05 to 0.30%, Si 0.5 to 2.5% Mn 0.5 to 3.0% Al 0.01 to 0.10% S 0.01 % Of steel, and the balance of iron and unavoidable impurities, is hot-rolled at a finishing temperature Ar of 3 points or more,
From the hot rolling finish temperature to a temperature in the range of Ar 3 point to Ar 1 point at an average cooling rate of 5 to 70 ° C./sec, and then, from that temperature, it is allowed to cool or gradually cool for 2 to 20 seconds, Then, it is cooled to a temperature in the range of 550 to 300 ° C. at an average cooling rate of 20 to 70 ° C./sec and wound, and its structure has a bainite area ratio (VfB) of 10 to 40% and a martensite area ratio (VfM). 3% to 15%, the balance being substantially polygonal ferrite and martensite of perimeter (L M) and martensite - the length of the ferrite boundaries (L
MF ) satisfies the following expression (L MF / L M ) + 0.05 × VfM ≦ 1.15, a method for producing a high-strength hot-rolled steel sheet having excellent strength-elongation balance and stretch-flange formability.
残部鉄及び不可避的不純物よりなる鋼を仕上温度Ａr3点
以上の温度で熱間圧延し、熱間圧延仕上温度からＡr3点
〜Ａr1点の範囲の温度までを平均冷却速度５〜７０℃／
秒にて冷却し、次いで、その温度から２〜２０秒間、放
冷又は徐冷し、その後、平均冷却速度２０〜７０℃／秒
にて５５０〜３００℃の範囲の温度まで冷却して、巻取
り、その組織がベイナイト面積率（ＶｆＢ）１０〜４０
％及びマルテンサイト面積率（ＶｆＭ）３〜１５％、残
部が実質的にポリゴナルフェライトからなり、且つ、マ
ルテンサイト周囲の長さ（ＬM ）とマルテンサイト−フ
ェライト境界の長さ（ＬM-F ) が次式 （ＬM-F ／ＬM ）＋0.０５×ＶｆＭ≦1.１５ を満足することを特徴とする強度−伸びバランス及び伸
びフランジ性にすぐれる高強度熱延鋼板の製造方法。2. In weight%, (a) C 0.05 to 0.30%, Si 0.5 to 2.5% Mn 0.5 to 3.0% Al 0.01 to 0.10% S 0.01% or less, and (b) Mo 0.05-1.0% V 0.01-0.5% Ti 0.01-0.3% Nb 0.01-0.3% Containing at least one element selected from the group consisting of
Steel consisting of balance iron and unavoidable impurities is hot-rolled at a finishing temperature of Ar 3 points or more, and the average cooling rate is 5 to 70 ° C. from the hot rolling finishing temperature to a temperature in the range of Ar 3 points to Ar 1 point. /
Cooling for 2 to 20 seconds, and then cooling to a temperature in the range of 550 to 300 ° C. at an average cooling rate of 20 to 70 ° C./second and winding. And its structure is bainite area ratio (VfB) 10-40
% And martensite area ratio (VfM) 3 to 15%, the balance substantially consisting of polygonal ferrite, and the length (L M ) around the martensite and the length of the martensite-ferrite boundary (L MF ). Satisfies the following formula (L MF / L M ) + 0.05 × VfM ≦ 1.15: A method for producing a high-strength hot-rolled steel sheet having excellent strength-elongation balance and stretch-flange formability.
残部鉄及び不可避的不純物よりなる鋼を仕上温度Ａr3点
以上の温度で熱間圧延し、熱間圧延仕上温度からＡr3点
〜Ａr1点の範囲の温度までを平均冷却速度５〜７０℃／
秒にて冷却し、次いで、その温度から２〜２０秒間、放
冷又は徐冷し、その後、平均冷却速度２０〜７０℃／秒
にて５５０〜３００℃の範囲の温度まで冷却して、巻取
り、その組織がベイナイト面積率（ＶｆＢ）１０〜４０
％及びマルテンサイト面積率（ＶｆＭ）３〜１５％、残
部が実質的にポリゴナルフェライトからなり、且つ、マ
ルテンサイト周囲の長さ（ＬM ）とマルテンサイト−フ
ェライト境界の長さ（ＬM-F ) が次式 （ＬM-F ／ＬM ）＋0.０５×ＶｆＭ≦1.１５ を満足することを特徴とする強度−伸びバランス及び伸
びフランジ性にすぐれる高強度熱延鋼板の製造方法。3. In weight%, (a) C 0.05 to 0.30%, Si 0.5 to 2.5% Mn 0.5 to 3.0% Al 0.01 to 0.10% S 0.01% or less, and further contains (b) at least one element selected from the group consisting of Cr 0.05 to 1.0% and B 0.0005 to 0.01%,
Steel consisting of balance iron and unavoidable impurities is hot-rolled at a finishing temperature of Ar 3 points or more, and the average cooling rate is 5 to 70 ° C. from the hot rolling finishing temperature to a temperature in the range of Ar 3 points to Ar 1 point. /
Cooling for 2 to 20 seconds, and then cooling to a temperature in the range of 550 to 300 ° C. at an average cooling rate of 20 to 70 ° C./second and winding. And its structure is bainite area ratio (VfB) 10-40
% And martensite area ratio (VfM) 3 to 15%, the balance substantially consisting of polygonal ferrite, and the length (L M ) around the martensite and the length of the martensite-ferrite boundary (L MF ). Satisfies the following formula (L MF / L M ) + 0.05 × VfM ≦ 1.15: A method for producing a high-strength hot-rolled steel sheet having excellent strength-elongation balance and stretch-flange formability.
残部鉄及び不可避的不純物よりなる鋼を仕上温度Ａr3点
以上の温度で熱間圧延し、熱間圧延仕上温度からＡr3点
〜Ａr1点の範囲の温度までを平均冷却速度５〜７０℃／
秒にて冷却し、次いで、その温度から２〜２０秒間、放
冷又は徐冷し、その後、平均冷却速度２０〜７０℃／秒
にて５５０〜３００℃の範囲の温度まで冷却して、巻取
り、その組織がベイナイト面積率（ＶｆＢ）１０〜４０
％及びマルテンサイト面積率（ＶｆＭ）３〜１５％、残
部が実質的にポリゴナルフェライトからなり、且つ、マ
ルテンサイト周囲の長さ（ＬM ）とマルテンサイト−フ
ェライト境界の長さ（ＬM-F ) が次式 （ＬM-F ／ＬM ）＋0.０５×ＶｆＭ≦1.１５ を満足することを特徴とする強度−伸びバランス及び伸
びフランジ性にすぐれる高強度熱延鋼板の製造方法。4. In weight%, (a) C 0.05 to 0.30%, Si 0.5 to 2.5% Mn 0.5 to 3.0% Al 0.01 to 0.10% S 0.01% or less, and (b) at least one element selected from the group consisting of Ca 0.005% or less and rare earth element 0.005% or less,
Steel consisting of balance iron and unavoidable impurities is hot-rolled at a finishing temperature of Ar 3 points or more, and the average cooling rate is 5 to 70 ° C. from the hot rolling finishing temperature to a temperature in the range of Ar 3 points to Ar 1 point. /
Cooling for 2 to 20 seconds, and then cooling to a temperature in the range of 550 to 300 ° C. at an average cooling rate of 20 to 70 ° C./second and winding. And its structure is bainite area ratio (VfB) 10-40
% And martensite area ratio (VfM) 3 to 15%, the balance substantially consisting of polygonal ferrite, and the length (L M ) around the martensite and the length of the martensite-ferrite boundary (L MF ). Satisfies the following formula (L MF / L M ) + 0.05 × VfM ≦ 1.15: A method for producing a high-strength hot-rolled steel sheet having excellent strength-elongation balance and stretch-flange formability.
み、残部鉄及び不可避的不純物よりなる鋼を仕上温度Ａ
r3点以上の温度で熱間圧延し、熱間圧延仕上温度からＡ
r3点〜Ａr1点の範囲の温度までを平均冷却速度５〜７０
℃／秒にて冷却し、次いで、その温度から２〜２０秒
間、放冷又は徐冷し、その後、平均冷却速度２０〜７０
℃／秒にて５５０〜３００℃の範囲の温度まで冷却し
て、巻取り、その組織がベイナイト面積率（ＶｆＢ）１
０〜４０％及びマルテンサイト面積率（ＶｆＭ）３〜１
５％、残部が実質的にポリゴナルフェライトからなり、
且つ、マルテンサイト周囲の長さ（ＬM ）とマルテンサ
イト−フェライト境界の長さ（ＬM-F ) が次式 （ＬM-F ／ＬM ）＋0.０５×ＶｆＭ≦1.１５ を満足することを特徴とする強度−伸びバランス及び伸
びフランジ性にすぐれる高強度熱延鋼板の製造方法。5. In weight%, (a) C 0.05 to 0.30%, Si 0.5 to 2.5% Mn 0.5 to 3.0% Al 0.01 to 0.10% S 0.01% or less, and (b) Mo 0.05-1.0% V 0.01-0.5% Ti 0.01-0.3% Nb 0.01-0.3% And at least one element selected from the group consisting of (c) Cr 0.05 to 1.0% and B 0.0005 to 0.01%, with the balance being at least one element selected from the group consisting of Finishing temperature A for steel consisting of iron and unavoidable impurities
r Hot rolling at a temperature of 3 points or more, and from the hot rolling finishing temperature to A
Average cooling rate of 5 to 70 for temperatures in the range of r 3 points to Ar 1 point
C./sec., Then allowed to cool or gradually cool from that temperature for 2 to 20 seconds, then average cooling rate 20 to 70.
The composition is cooled to a temperature in the range of 550 to 300 ° C. at a temperature of 550 ° C./sec and wound, and the structure has a bainite area ratio (VfB) of 1
0-40% and martensite area ratio (VfM) 3-1
5%, the balance consisting essentially of polygonal ferrite,
In addition, the martensite surrounding length (L M ) and the martensite-ferrite boundary length (L MF ) satisfy the following equation (L MF / L M ) + 0.05 × VfM ≦ 1.15. And a method for producing a high-strength hot-rolled steel sheet having excellent strength-elongation balance and stretch-flangeability.
み、残部鉄及び不可避的不純物よりなる鋼を仕上温度Ａ
r3点以上の温度で熱間圧延し、熱間圧延仕上温度からＡ
r3点〜Ａr1点の範囲の温度までを平均冷却速度５〜７０
℃／秒にて冷却し、次いで、その温度から２〜２０秒
間、放冷又は徐冷し、その後、平均冷却速度２０〜７０
℃／秒にて５５０〜３００℃の範囲の温度まで冷却し
て、巻取り、その組織がベイナイト面積率（ＶｆＢ）１
０〜４０％及びマルテンサイト面積率（ＶｆＭ）３〜１
５％、残部が実質的にポリゴナルフェライトからなり、
且つ、マルテンサイト周囲の長さ（ＬM ）とマルテンサ
イト−フェライト境界の長さ（ＬM-F ) が次式 （ＬM-F ／ＬM ）＋0.０５×ＶｆＭ≦1.１５ を満足することを特徴とする強度−伸びバランス及び伸
びフランジ性にすぐれる高強度熱延鋼板の製造方法。6. In weight%, (a) C 0.05 to 0.30%, Si 0.5 to 2.5% Mn 0.5 to 3.0% Al 0.01 to 0.10% S 0.01% or less, and (b) Mo 0.05-1.0% V 0.01-0.5% Ti 0.01-0.3% Nb 0.01-0.3% And at least one element selected from the group consisting of (c) Ca 0.005% or less and rare earth element 0.005% or less, with the balance being iron and inevitable impurities. Made of steel with finishing temperature A
r Hot rolling at a temperature of 3 points or more, and from the hot rolling finishing temperature to A
Average cooling rate of 5 to 70 for temperatures in the range of r 3 points to Ar 1 point
C./sec., Then allowed to cool or gradually cool from that temperature for 2 to 20 seconds, then average cooling rate 20 to 70.
The composition is cooled to a temperature in the range of 550 to 300 ° C. at a temperature of 550 ° C./sec and wound, and the structure has a bainite area ratio (VfB) of 1
0-40% and martensite area ratio (VfM) 3-1
5%, the balance consisting essentially of polygonal ferrite,
In addition, the martensite circumference length (L M ) and the martensite-ferrite boundary length (L MF ) satisfy the following equation (L MF / L M ) + 0.05 × VfM ≦ 1.15. And a method for producing a high-strength hot-rolled steel sheet having excellent strength-elongation balance and stretch-flangeability.
み、残部鉄及び不可避的不純物よりなる鋼を仕上温度Ａ
r3点以上の温度で熱間圧延し、熱間圧延仕上温度からＡ
r3点〜Ａr1点の範囲の温度までを平均冷却速度５〜７０
℃／秒にて冷却し、次いで、その温度から２〜２０秒
間、放冷又は徐冷し、その後、平均冷却速度２０〜７０
℃／秒にて５５０〜３００℃の範囲の温度まで冷却し
て、巻取り、その組織がベイナイト面積率（ＶｆＢ）１
０〜４０％及びマルテンサイト面積率（ＶｆＭ）３〜１
５％、残部が実質的にポリゴナルフェライトからなり、
且つ、マルテンサイト周囲の長さ（ＬM ）とマルテンサ
イト−フェライト境界の長さ（ＬM-F ) が次式 （ＬM-F ／ＬM ）＋0.０５×ＶｆＭ≦1.１５ を満足することを特徴とする強度−伸びバランス及び伸
びフランジ性にすぐれる高強度熱延鋼板の製造方法。7. In weight%, (a) C 0.05 to 0.30%, Si 0.5 to 2.5% Mn 0.5 to 3.0% Al 0.01 to 0.10% S 0.01% or less, and (b) Mo 0.05-1.0% V 0.01-0.5% Ti 0.01-0.3% Nb 0.01-0.3% At least one element selected from the group consisting of: (c) at least one element selected from the group consisting of Cr 0.05 to 1.0% and B 0.0005 to 0.01%; and (d) A steel containing Ca 0.005% or less and at least one element selected from the group consisting of rare earth elements 0.005% or less and the balance iron and unavoidable impurities is used at a finishing temperature A.
r Hot rolling at a temperature of 3 points or more, and from the hot rolling finishing temperature to A
Average cooling rate of 5 to 70 for temperatures in the range of r 3 points to Ar 1 point
C./sec., Then allowed to cool or gradually cool from that temperature for 2 to 20 seconds, then average cooling rate 20 to 70.
The composition is cooled to a temperature in the range of 550 to 300 ° C. at a temperature of 550 ° C./sec and wound, and the structure has a bainite area ratio (VfB) of 1
0-40% and martensite area ratio (VfM) 3-1
5%, the balance consisting essentially of polygonal ferrite,
In addition, the martensite circumference length (L M ) and the martensite-ferrite boundary length (L MF ) satisfy the following equation (L MF / L M ) + 0.05 × VfM ≦ 1.15. And a method for producing a high-strength hot-rolled steel sheet having excellent strength-elongation balance and stretch-flangeability.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP7200897A JPH0949026A (en) | 1995-08-07 | 1995-08-07 | Production of high strength hot rolled steel plate excellent in balance between strength and elongation and in stretch-flange formability |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP7200897A JPH0949026A (en) | 1995-08-07 | 1995-08-07 | Production of high strength hot rolled steel plate excellent in balance between strength and elongation and in stretch-flange formability |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| JPH0949026A true JPH0949026A (en) | 1997-02-18 |
Family
ID=16432077
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP7200897A Pending JPH0949026A (en) | 1995-08-07 | 1995-08-07 | Production of high strength hot rolled steel plate excellent in balance between strength and elongation and in stretch-flange formability |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JPH0949026A (en) |
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| US7615126B2 (en) * | 2000-12-07 | 2009-11-10 | Nippon Steel Corporation | High strength hot rolled steel plate excellent in enlargeability and ductility and method for producing thereof |
| JP2009270171A (en) * | 2008-05-09 | 2009-11-19 | Sumitomo Metal Ind Ltd | Hot-rolled steel sheet and method for producing the same |
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1995
- 1995-08-07 JP JP7200897A patent/JPH0949026A/en active Pending
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| EP1394276A1 (en) * | 2001-06-07 | 2004-03-03 | JFE Steel Corporation | High tensile hot−rolled steel sheet excellent in resistance to scuff on mold and in fatigue characteristics |
| EP1394276A4 (en) * | 2001-06-07 | 2006-01-18 | Jfe Steel Corp | High tensile hot-rolled steel sheet excellent in resistance to scuff on mold andin fatigue characteristics |
| US7485194B2 (en) | 2001-06-07 | 2009-02-03 | Jfe Steel Corporation | High tensile hot-rolled steel sheet excellent in resistance to scuff on mold and in fatigue characteristics |
| AU2005200300C1 (en) * | 2004-01-29 | 2008-03-06 | Jfe Steel Corporation | High strength steel sheet and method for manufacturing same |
| EP2243852A4 (en) * | 2008-02-08 | 2017-04-12 | JFE Steel Corporation | High-strength hot-dip zinc coated steel sheet excellent in workability and process for production thereof |
| JP2009270171A (en) * | 2008-05-09 | 2009-11-19 | Sumitomo Metal Ind Ltd | Hot-rolled steel sheet and method for producing the same |
| EP2484792A1 (en) * | 2009-09-30 | 2012-08-08 | JFE Steel Corporation | Steel plate with low yield ratio, high strength, and high toughness and process for producing same |
| EP2484792A4 (en) * | 2009-09-30 | 2013-03-06 | Jfe Steel Corp | Steel plate with low yield ratio, high strength, and high toughness and process for producing same |
| US8778096B2 (en) | 2009-09-30 | 2014-07-15 | Jfe Steel Corporation | Low yield ratio, high strength and high toughness steel plate and method for manufacturing the same |
| US10167539B2 (en) | 2011-05-25 | 2019-01-01 | Nippon Steel & Sumitomo Metal Corporation | Hot-rolled steel sheet and method for producing same |
| CN102363857A (en) * | 2011-11-02 | 2012-02-29 | 武汉钢铁(集团)公司 | Method for producing color-coated plate for structure with yield strength of more than or equal to 550 MPa |
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