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JP4954909B2 - Low yield ratio type high-strength cold-rolled steel sheet with excellent bake hardening properties and slow aging at room temperature, and its manufacturing method - Google Patents

Low yield ratio type high-strength cold-rolled steel sheet with excellent bake hardening properties and slow aging at room temperature, and its manufacturing method Download PDF

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JP4954909B2
JP4954909B2 JP2008014999A JP2008014999A JP4954909B2 JP 4954909 B2 JP4954909 B2 JP 4954909B2 JP 2008014999 A JP2008014999 A JP 2008014999A JP 2008014999 A JP2008014999 A JP 2008014999A JP 4954909 B2 JP4954909 B2 JP 4954909B2
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JP2009174019A (en
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純治 土師
司 酒井
薫 川崎
輝昭 山田
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Nippon Steel Corp
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Description

本発明は、自動車の外板及び内板に使用可能な、優れた焼付硬化性と常温遅時効性を有する低降伏比型高強度冷延鋼板及びその製造方法に関するものである。   The present invention relates to a low-yield ratio type high-strength cold-rolled steel sheet having excellent bake hardenability and room temperature slow aging that can be used for an outer plate and an inner plate of an automobile, and a method for producing the same.

近年、自動車用鋼板においては、地球環境問題に端を発する燃費向上の有力手段である車体軽量化と衝突時の乗員保護を目的とした衝突安全性の確保の両立を主な背景として、高強度化が進められている。一般に、鋼板の高強度化は成形性の劣化を招くが、この課題を克服するために、プレス成形時には軟質で成形し易く、プレス後の焼き付け塗装工程において降伏強度が上昇する焼き付け硬化(BH)性を有する鋼板が開発されてきた。このBH鋼板は、固溶Cおよび固溶Nを活用した歪時効現象により、鋼板を強化する技術であるが、一方で、この固溶Cおよび固溶Nは常温においても時効が進行するため、長期間保持された場合には降伏点伸びが生じるようになり、プレス成形時にストレッチャーストレインが発生して外観を損なうことがある。よって、BH鋼板においては、常温遅時効性を保ちつつ、BH量を上げることが課題となる。   In recent years, steel sheets for automobiles have high strength, mainly due to the reduction of vehicle body weight, which is an effective means of improving fuel economy, which is caused by global environmental problems, and the securing of collision safety for the purpose of protecting passengers during collisions. Is being promoted. In general, increasing the strength of steel sheets leads to deterioration of formability, but in order to overcome this problem, bake hardening (BH), which is soft and easy to form during press forming, increases the yield strength in the baking coating process after pressing. Steel sheets having properties have been developed. This BH steel sheet is a technique for strengthening a steel sheet by a strain aging phenomenon utilizing solute C and solute N. On the other hand, aging of the solute C and solute N progresses even at room temperature. When it is held for a long period of time, yield point elongation occurs, and stretcher strain may occur during press molding, which may impair the appearance. Therefore, in a BH steel sheet, it becomes a subject to raise BH amount, maintaining normal temperature slow aging.

常温遅時効性を保ちつつ、高いBH量を確保するための有効な手段として、いわゆるDPハイテンのような、フェライトと低温変態生成相の混合組織を活用するという方法がある。低温変態生成相として重要なのはマルテンサイト相であり、変態時に導入される可動転位が、固溶C、Nにより固着することで高いBH性を確保することができる。また、このような組織の鋼板は、常温遅時効性にも優れる上、低降伏比で形状凍結性に優れ、加工性も良いという特徴をもっている。   As an effective means for ensuring a high BH content while maintaining a slow aging property at room temperature, there is a method of utilizing a mixed structure of ferrite and a low-temperature transformation generation phase, such as so-called DP high ten. What is important as the low temperature transformation generation phase is the martensite phase, and the mobile dislocations introduced at the transformation are fixed by the solute C and N, so that high BH property can be secured. In addition, the steel sheet having such a structure is characterized by excellent room temperature slow aging, low yield ratio, excellent shape freezing property, and good workability.

フェライトと低温変態生成相の混合組織を活用したBH鋼板として、特許文献1や特許文献2の方法が提案されている。これらの方法によれば、常温遅時効性と、少なくとも60MPa以上の優れたBH量を両立することが可能である。また、低降伏比のため、形状凍結性も良好である。しかしながら、成分として、近年価格が高騰しているMoやCrを添加することが必須となっており、製造コストが上がることは避けられない。   As a BH steel sheet using a mixed structure of ferrite and a low temperature transformation generation phase, methods of Patent Document 1 and Patent Document 2 have been proposed. According to these methods, it is possible to achieve both room temperature slow aging and an excellent BH amount of at least 60 MPa or more. Moreover, because of the low yield ratio, the shape freezing property is also good. However, it is indispensable to add Mo and Cr, which have been increasing in price in recent years, as components, and it is inevitable that the manufacturing cost will increase.

特開2006−233294号公報JP 2006-233294 A 特開2006−52465号公報JP 2006-52465 A

本発明では、かかる事情を鑑みてなされたものであり、60MPa以上のBH量と常温遅時効性をもち、60%以下の低降伏比である高強度冷延鋼板を低コストで提供することを課題とする。強度としては、ハイテン化の効果が十分に発揮しつつ、ユーザーでの生産コストを抑制することが可能な440MPa以上590MPa未満を目標とする。   The present invention has been made in view of such circumstances, and provides a high-strength cold-rolled steel sheet having a BH amount of 60 MPa or more and room temperature slow aging and a low yield ratio of 60% or less at a low cost. Let it be an issue. The strength is set to 440 MPa or more and less than 590 MPa, which can suppress the production cost by the user while fully exhibiting the effect of high tensile strength.

本発明者らは、上記課題を解決するための手段を鋭意検討した結果、固溶Nは析出物として完全固定した上で、適量の固溶Cを活用し、組織としては、2μm以下の微細マルテンサイトを適量確保することが最も重要であることを見出した。微細マルテンサイトにより、常温遅時効性と低降伏比を確保することができ、また、適量の固溶Cにより優れた焼付け硬化性をも具備することが可能となる。更に、それらを得るための成分、組織、製造条件を詳細に検討した。その結果、特に、CとMnのバランスが重要であり、C+(Mn/20)≧0.12%の場合に、優れた特性を発揮できることが判明した。以上の知見を総合し、本発明は完成した。
本発明の要旨は、次の通りである。
As a result of earnestly examining the means for solving the above problems, the present inventors have fixed solid solution N as a precipitate, and then used an appropriate amount of solid solution C. It was found that securing the proper amount of martensite is the most important. Fine martensite can ensure normal temperature slow aging and low yield ratio, and can also have excellent bake hardenability due to an appropriate amount of solid solution C. Furthermore, components, structures, and production conditions for obtaining them were examined in detail. As a result, it was found that the balance between C and Mn is particularly important, and excellent characteristics can be exhibited when C + (Mn / 20) ≧ 0.12%. The present invention has been completed by integrating the above findings.
The gist of the present invention is as follows.

本発明は、質量%で、C:0.03〜0.09%、Mn:1.0〜2.0%、Si:1.0%以下、P:0.05%以下、S:0.03%以下、sol.Al:0.005〜0.1%、N:0.01%以下、B:0.001〜0.003%、0.005%≧(14/48)Ti+(14/93)Nb−N≧0を満たす分のTiとNbのうち1種以上を含み、かつ、C+(Mn/20)≧0.12%を満たし、固溶C:1〜7ppmであり、残部はFeおよび不可避的不純物から成り、体積率で、80%以上のフェライトと3〜10%のマルテンサイトから成り、マルテンサイトのうち2%以上は2μm以下の微細マルテンサイトであることを特徴とする焼き付け硬化特性と常温遅時効性に優れた低降伏比型高強度冷延鋼板を提供する。   In the present invention, by mass%, C: 0.03 to 0.09%, Mn: 1.0 to 2.0%, Si: 1.0% or less, P: 0.05% or less, S: 0.00. 03% or less, sol. Al: 0.005-0.1%, N: 0.01% or less, B: 0.001-0.003%, 0.005% ≧ (14/48) Ti + (14/93) Nb−N ≧ It contains one or more of Ti and Nb that satisfy 0, and satisfies C + (Mn / 20) ≧ 0.12%, solid solution C: 1 to 7 ppm, and the balance from Fe and inevitable impurities Bake hardening characteristics and room temperature slow aging characterized in that it consists of 80% or more ferrite and 3 to 10% martensite by volume ratio, and 2% or more of martensite is fine martensite of 2 μm or less A low yield ratio type high-strength cold-rolled steel sheet having excellent properties is provided.

また、上記鋼板において、さらに、Ca、REMのうちの1種または2種を、合計で0.01質量%以下含有することを特徴とする焼き付け硬化特性と常温遅時効性に優れた低降伏比型高強度冷延鋼板を提供する。   Further, in the steel sheet, one or two of Ca and REM are further contained in a total of 0.01% by mass or less, and a low yield ratio excellent in bake hardening characteristics and room temperature slow aging A high strength cold-rolled steel sheet is provided.

また、本発明は、質量%で、C:0.03〜0.09%、Mn:1.0〜2.0%、Si:1.0%以下、P:0.05%以下、S:0.03%以下、sol.Al:0.005〜0.1%、N:0.01%以下、B:0.001〜0.003%、0.005%≧(14/48)Ti+(14/93)Nb−N≧0を満たす分のTiとNbのうち1種以上を含み、かつ、C+(Mn/20)≧0.12%を満たし、残部はFeおよび不可避的不純物から成る鋼片を仕上圧延後、450〜600℃にて巻き取り、酸洗、冷間圧延後、連続焼鈍を行うに際し、(Ac1変態温度+20℃)〜(Ac1変態温度+100℃)にて焼鈍後、2〜10℃/秒で550〜700℃まで一次冷却し、50℃/秒以上で350℃以下まで二次冷却し、250〜350℃で100秒以上の過時効処理を行った後、室温まで冷却することを特徴とする焼き付け硬化特性と常温遅時効性に優れた低降伏比型高強度冷延鋼板の製造方法を提供する。   Moreover, this invention is mass%, C: 0.03-0.09%, Mn: 1.0-2.0%, Si: 1.0% or less, P: 0.05% or less, S: 0.03% or less, sol. Al: 0.005-0.1%, N: 0.01% or less, B: 0.001-0.003%, 0.005% ≧ (14/48) Ti + (14/93) Nb−N ≧ One or more of Ti and Nb satisfying 0 and C + (Mn / 20) ≧ 0.12% are satisfied, and the balance is 450 to 450- after finish rolling a steel slab composed of Fe and inevitable impurities. In continuous annealing after winding at 600 ° C., pickling and cold rolling, after annealing at (Ac1 transformation temperature + 20 ° C.) to (Ac1 transformation temperature + 100 ° C.), 550 at 2-10 ° C./second. Baking and curing characterized by primary cooling to 700 ° C., secondary cooling to 350 ° C. or less at 50 ° C./second or more, and after overaging at 250 to 350 ° C. for 100 seconds or more, followed by cooling to room temperature Low yield ratio type high strength cold-rolled steel sheet with excellent properties and room temperature slow aging To provide a method.

また、上記鋼板の製造方法において、さらに、鋼片がCa、REMのうちの1種または2種を、合計で0.01質量%以下含有することを特徴とする焼き付け硬化特性と常温遅時効性に優れた低降伏比型高強度冷延鋼板の製造方法を提供する。   Further, in the method for producing a steel sheet, the bake hardening characteristic and room temperature slow aging property characterized in that the steel piece further contains one or two of Ca and REM in a total amount of 0.01% by mass or less. A method for producing a low-yield ratio type high-strength cold-rolled steel sheet excellent in the above is provided.

本発明によれば、60MPa以上のBH量と常温遅時効性をもち、60%以下の低降伏比である高強度冷延鋼板を低コストで提供することが可能であり、自動車の外板用及び内板用などとしての用途は広く、産業上のメリットは大きい。   According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet having a BH amount of 60 MPa or more and room temperature slow aging and a low yield ratio of 60% or less at a low cost. In addition, it is widely used as an inner plate and has great industrial advantages.

先ず、本発明が対象とする鋼板の成分及び成分範囲を限定した理由を述べる。なお、以下、組成における質量%は単に%と記す。   First, the reason why the components and the component ranges of the steel sheet targeted by the present invention are limited will be described. Hereinafter, mass% in the composition is simply referred to as%.

Cは、硬化元素であり、マルテンサイトの生成に効果がある。0.03%未満では十分な量のマルテンサイトが得られず、0.09%を超えると成形性や溶接性の劣化を招く。したがって、C量を0.03〜0.09%とした。好ましくは、0.05〜0.07%である。   C is a hardening element and is effective in generating martensite. If it is less than 0.03%, a sufficient amount of martensite cannot be obtained, and if it exceeds 0.09%, formability and weldability are deteriorated. Therefore, the C content is set to 0.03 to 0.09%. Preferably, it is 0.05 to 0.07%.

Mnは、固溶強化により鋼を強化すると共に、焼き入れ性を上げてマルテンサイトの生成を促進する。このような作用を発揮するには1.0%以上必要である。また、2.0%を超えると、スラブコストの著しい上昇と共に、外観不良や成形性の劣化を招く。したがって、Mn量を1.0〜2.0%とした。好ましくは、1.2〜1.8%である。   Mn strengthens the steel by solid solution strengthening and enhances hardenability to promote the formation of martensite. In order to exert such an effect, 1.0% or more is necessary. On the other hand, if it exceeds 2.0%, the slab cost is significantly increased and the appearance is poor and the moldability is deteriorated. Therefore, the amount of Mn is set to 1.0 to 2.0%. Preferably, it is 1.2 to 1.8%.

Siはフェライト安定化元素であり、セメンタイトの析出を阻害し、延性を向上させるので、添加することが望ましい。しかし、1.0%を超えると表面性状及び化成処理性が顕著に劣化する。したがって、Si量を1.0%以下とした。好ましくは、0.5%以下である。   Si is a ferrite stabilizing element, which inhibits cementite precipitation and improves ductility, so it is desirable to add Si. However, if it exceeds 1.0%, the surface properties and the chemical conversion treatment properties are significantly deteriorated. Therefore, the Si amount is set to 1.0% or less. Preferably, it is 0.5% or less.

Pは、不純物として不可避的に含有され伸びに悪影響を与えるので、上限を0.05%とした。好ましくは、0.03%以下である。   Since P is inevitably contained as an impurity and adversely affects elongation, the upper limit was made 0.05%. Preferably, it is 0.03% or less.

Sは、多くなると熱間脆性の原因となり、また、加工性を劣化させるので、その上限を0.03%とした。好ましくは、0.01%以下である。   If S increases, it causes hot brittleness and deteriorates workability, so the upper limit was made 0.03%. Preferably, it is 0.01% or less.

Alは、鋼の脱酸剤として添加され鋼中に含有されsol.Alで0.005%以上必要である。しかし、sol.Alで0.1%を超えると鋼板中に介在物が多くなりすぎ延性を劣化させる。したがって、sol.Alで0.005〜0.1%とした。   Al is added as a steel deoxidizer and contained in the steel. 0.005% or more is necessary for Al. However, sol. If it exceeds 0.1% with Al, the inclusions in the steel sheet will increase and ductility will deteriorate. Therefore, sol. It was made into 0.005-0.1% with Al.

Nは不可避的不純物として含有されるが、N量が多いと加工性の劣化を招くので、上限を0.01%とする。好ましくは、0.05%以下である。   N is contained as an inevitable impurity, but if the amount of N is large, workability is deteriorated, so the upper limit is made 0.01%. Preferably, it is 0.05% or less.

Bは、焼入れ性向上元素であり、マルテンサイトの生成を促進する。極少量で効果があり、低コスト化に有効である。その効果を十分に発揮させるには、0.001%以上必要である。しかし、0.003%を超えると効果が飽和する。よって、B量を0.001〜0.003%とした。好ましくは、0.0015〜0.0025%である。   B is a hardenability improving element and promotes the formation of martensite. Effective in a very small amount, effective for cost reduction. In order to fully exhibit the effect, 0.001% or more is necessary. However, if it exceeds 0.003%, the effect is saturated. Therefore, the B content is set to 0.001 to 0.003%. Preferably, it is 0.0015 to 0.0025%.

Ti及びNbは、Nを析出物として固定するために必要である。しかし、過剰に添加すると、再結晶を遅延させることにより加工性の劣化を招く。よって、0.005%≧(14/48)Ti+(14/93)Nb−N≧0を満たす分のTiとNbのうち1種以上を含むこととした。(14/48)Ti+(14/93)Nb−Nという式は、TiNまたはNbNとして析出可能なN量から実際のN量を引いた値を示す。これが0ならばNが全て析出可能であり、この値が正の値であれば、NよりもTiとNbの方が過剰であることを示す。   Ti and Nb are necessary for fixing N as a precipitate. However, if added excessively, workability is deteriorated by delaying recrystallization. Therefore, one or more of Ti and Nb that satisfy 0.005% ≧ (14/48) Ti + (14/93) Nb−N ≧ 0 are included. The formula (14/48) Ti + (14/93) Nb-N indicates a value obtained by subtracting the actual N amount from the N amount that can be precipitated as TiN or NbN. If this is 0, all of N can be precipitated, and if this value is a positive value, it indicates that Ti and Nb are more excessive than N.

更に、本発明においては、CとMnのバランスが重要である。種々の成分での試験を実施した結果、C+(Mn/20)≧0.12%を満たす場合、製造条件を調整することにより、優れた焼付硬化性と常温遅時効性、加工性を具備することが可能となる判明した。マルテンサイトが十分に効果を発揮するためには、上記の成分条件を満たす必要があるものと推定される。よって、C+(Mn/20)≧0.12%を満たすことを本発明の必須条件とした。C+(Mn/20)という式はいわゆるC当量であるが、ここでは、マルテンサイトの生じ易さ及び周囲に生じる転位量の多さを示す指標である。マルテンサイトの周囲の転位が多い方が、常温非時効性はよくなる。   Furthermore, in the present invention, the balance between C and Mn is important. As a result of conducting tests with various components, when satisfying C + (Mn / 20) ≧ 0.12%, by adjusting the production conditions, excellent bake hardenability, room temperature slow aging and workability are provided. It turned out that it would be possible. It is presumed that the above-mentioned component conditions must be satisfied in order for martensite to exhibit a sufficient effect. Therefore, satisfying C + (Mn / 20) ≧ 0.12% is an essential condition of the present invention. The formula C + (Mn / 20) is a so-called C equivalent, but here it is an index indicating the ease with which martensite is generated and the amount of dislocations generated around it. If there are more dislocations around martensite, the non-aging property at room temperature is improved.

さらに、必要に応じ、上記の化学成分に加え、Ca、REMのうちの1種または2種を、合計で0.01%以下含有することができる。Ca、REMは、硫化物系介在物の形態を制御し、鋼板の伸びフランジ性を向上させる効果を有する。このような作用は、合計で0.01%を超えても効果が飽和し、含有量に見合う効果が期待できない。したがって、Ca、REMのうちの1種または2種を合計で、0.01%以下とする。なお、ここでいうREMとは、Sc、Yおよびランタノイドの合計17元素の総称であり、REMの含有量は、上記元素の合計含有量をさす。   Furthermore, if necessary, in addition to the above chemical components, one or two of Ca and REM can be contained in a total of 0.01% or less. Ca and REM have the effect of controlling the form of sulfide inclusions and improving the stretch flangeability of the steel sheet. Even if such action exceeds 0.01% in total, the effect is saturated, and an effect commensurate with the content cannot be expected. Accordingly, one or two of Ca and REM are made 0.01% or less in total. In addition, REM here is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM indicates the total content of the above elements.

次に、本発明が対象とする鋼板の組織と固溶C量について詳細に説明する。   Next, the structure of the steel plate and the amount of solute C targeted by the present invention will be described in detail.

本発明では、良好な成形性を得るため軟質なフェライトを主体とし、従来のDP鋼と比較してマルテンサイト分率を抑えた組織を得ることが重要である。しかも、マルテンサイトは、変態時に導入される可動転位が固溶Cにより固着されることで高いBH性を得ることができるため、一定量以上のマルテンサイトが必要である。また、その可動転位は、マルテンサイト相周囲のフェライトに生成するため、同じ体積率であっても、マルテンサイトがより微細な方が、より多くの可動転位を確保できる。よって、微細なマルテンサイトを適量確保することにより、上記の高いBH性の他、常温遅時効性や低降伏比も確保することができる。以上のような考え方と実際の実験結果を元に、80%以上のフェライトと3〜10%のマルテンサイトから成り、マルテンサイトのうち2%以上は2μm以下の微細マルテンサイトを含むことを本発明が対象とする鋼板の組織の要件とした。フェライトの体積率は、好ましくは、85%以上である。   In the present invention, it is important to obtain a structure mainly composed of soft ferrite and having a reduced martensite fraction as compared with conventional DP steel in order to obtain good formability. In addition, martensite requires a certain amount or more of martensite because the mobile dislocations introduced at the time of transformation are fixed by solute C so that high BH properties can be obtained. Further, since the movable dislocations are generated in the ferrite around the martensite phase, more movable dislocations can be secured when the martensite is finer even if the volume ratio is the same. Therefore, by securing an appropriate amount of fine martensite, room temperature slow aging and low yield ratio can be ensured in addition to the high BH property. Based on the above concept and actual experimental results, the present invention comprises 80% or more of ferrite and 3 to 10% martensite, and 2% or more of martensite contains fine martensite of 2 μm or less. Is the requirement of the structure of the target steel sheet. The volume fraction of ferrite is preferably 85% or more.

なお、その他の組織として、残留オーステナイト、ベイナイト、炭化物が含まれても良い。特に、残留オーステナイトは、成形性向上や降伏比低下に有効である。   In addition, residual austenite, bainite, and carbide may be included as other structures. In particular, retained austenite is effective in improving the formability and lowering the yield ratio.

製造後の鋼板段階での固溶C量については、BH量及び常温遅時効性を左右する重要な要素である。上記の成分及び組織の要件を満たし、60MPa以上のBH量と常温遅時効性を兼ね備えた鋼板の固溶C量を横振動法の内部摩擦測定装置により測定したところ、1〜7ppmであった。よって、本発明が対象とする鋼板の固溶C量を1〜7ppmとした。固溶Cが少なければBH量が確保できず、多すぎると常温遅時効性が損なわれる。ただし、横振動法の場合、200℃程度までの昇温中に固溶Cが可動転位にトラップされることが推定され、昇温前の固溶C量はもっと多いと考えられる。よって、ここでいう固溶C量は、横振動法で測定した値とする。   About the amount of dissolved C in the steel plate stage after manufacture, it is an important factor which influences the amount of BH and normal temperature slow aging. It was 1-7 ppm when the solid solution C amount of the steel plate which satisfy | fills the requirements of said component and structure | tissue, and has BH amount of 60 Mpa or more and normal temperature slow aging was measured with the internal friction measuring device of the transverse vibration method. Therefore, the solid solution C amount of the steel sheet targeted by the present invention is set to 1 to 7 ppm. If the amount of solute C is small, the amount of BH cannot be ensured. However, in the case of the transverse vibration method, it is estimated that the solid solution C is trapped by the movable dislocation during the temperature rise to about 200 ° C., and it is considered that the amount of the solid solution C before the temperature rise is larger. Therefore, the solid solution C amount here is a value measured by the transverse vibration method.

次に、本発明が対象とする鋼板の製造方法について詳細に説明する。   Next, the manufacturing method of the steel plate which this invention makes object is demonstrated in detail.

溶鋼は通常の高炉法で溶製されたものの他、電炉法のようにスクラップを多量に使用したものでもよい。スクラップ使用によるCu、Ni、Sn、Crなどのいわゆるトランプエレメントの混入は、本鋼板の特性上、特に問題はない。スラブは、通常の連続鋳造プロセスで製造されたものでもよいし、薄スラブ鋳造で製造されたものでもよい。スラブは一旦冷却してから、熱延前の加熱炉で加熱しても良いし、冷却途中で高温まま加熱炉に入れる所謂HCRやDRでも良い。   The molten steel may be one produced by a normal blast furnace method or one using a large amount of scrap as in the electric furnace method. Mixing of so-called trump elements such as Cu, Ni, Sn, and Cr due to the use of scraps is not particularly problematic in terms of the properties of the steel sheet. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting. The slab may be cooled once and then heated in a heating furnace before hot rolling, or may be so-called HCR or DR which is put into a heating furnace at a high temperature during cooling.

熱延においては、仕上圧延後に450〜600℃で巻き取ることにより、熱延後の組織をベイニティックフェライトにすることが重要である。ベイニティックフェライトにすることにより、熱延板段階でのCの分布が均一化し、冷延後の焼鈍の際、微細マルテンサイトの生成と固溶Cの確保が容易になるものと考えられる。巻き取り温度が600℃を超えると、フェライトとパーライトの混合組織となり、Cの分布が局所化するため、焼鈍時のオーステナイトの生成も局所化し、微細マルテンサイトが得にくくなる。巻き取り温度が450℃を下回ると組織が硬くなるだけで、冷間圧延時の圧下力が多大に必要となるだけであり、水の膜沸騰と核沸騰の遷移領域に入るため、巻き取り温度の制御も困難となる。よって、巻き取り温度は、450〜600℃とした。また、熱延は粗圧延後に粗バーを巻き取って保持するコイルボックスを使用しても良い。更に巻き取った粗バーを巻き戻す際に先行する粗バーと接合して圧延する、いわゆる熱延連続化プロセスでも良い。   In hot rolling, it is important that the structure after hot rolling is made bainitic ferrite by winding at 450 to 600 ° C. after finish rolling. By using bainitic ferrite, the distribution of C in the hot-rolled sheet stage is made uniform, and it is considered that the formation of fine martensite and the securing of solid solution C are facilitated during annealing after cold rolling. When the coiling temperature exceeds 600 ° C., it becomes a mixed structure of ferrite and pearlite, and the distribution of C is localized. Therefore, the generation of austenite at the time of annealing is also localized, and it becomes difficult to obtain fine martensite. If the coiling temperature is lower than 450 ° C, the structure becomes hard, and only a large rolling force during cold rolling is required, and it enters the transition region between water film boiling and nucleate boiling. It becomes difficult to control. Therefore, the winding temperature was set to 450 to 600 ° C. Moreover, you may use the coil box which winds and hold | maintains a rough bar after rough rolling for hot rolling. Further, a so-called hot rolling continuous process in which the rolled coarse bar is joined and rolled with the preceding coarse bar when unwinding may be used.

酸洗、冷延については、冷延鋼板における通常の製造条件にて実施される。冷延後の連続焼鈍プロセスでは、まず、(Ac1変態温度+20℃)〜(Ac1変態温度+100℃)にて焼鈍し、フェライトを再結晶させると共に、オーステナイトを生成させる。(Ac1変態温度+20℃)未満では、十分にオーステナイトが生じず、所定の特性を得ることができない。また、(Ac1変態温度+100℃)を超えるとオーステナイトが増えすぎてオーステナイト中のC濃度が低下するため、周囲に十分な可動転位をもったマルテンサイトを得ることが困難となり、所定の特性を得ることができなくなる。逆にいえば、焼鈍時のオーステナイト中のC濃度を上げるため、(Ac1変態温度+20℃)以上の範囲で焼鈍温度が低い方が望ましい。また、焼鈍時の保持時間は、30〜200秒程度が望ましい。   About pickling and cold rolling, it is carried out under normal manufacturing conditions for cold rolled steel sheets. In the continuous annealing process after cold rolling, first, annealing is performed at (Ac1 transformation temperature + 20 ° C.) to (Ac1 transformation temperature + 100 ° C.) to recrystallize ferrite and to generate austenite. If it is less than (Ac1 transformation temperature + 20 ° C.), sufficient austenite is not generated, and predetermined characteristics cannot be obtained. Further, if it exceeds (Ac1 transformation temperature + 100 ° C.), austenite increases too much and the C concentration in the austenite decreases, so that it becomes difficult to obtain martensite having sufficient movable dislocations in the surroundings, and a predetermined characteristic is obtained. I can't do that. Conversely, in order to increase the C concentration in the austenite during annealing, it is desirable that the annealing temperature is lower in the range of (Ac1 transformation temperature + 20 ° C.) or higher. The holding time during annealing is desirably about 30 to 200 seconds.

焼鈍後は、2〜10℃/秒で550〜700℃まで一次冷却し、50℃/秒以上で350℃以下まで二次冷却し、250〜350℃で100秒以上の過時効処理を行った後、室温まで冷却することが必要である。一次冷却は、パーライトを回避しつつフェライト分率を増加させてオーステナイト中にCを濃化させる効果がある。2℃/秒未満ではパーライト変態が生じて成形性が劣化し、10℃/秒を超えると、フェライトとオーステナイトの2相分離が進み難くなり、オーステナイト中へのC濃化が十分に進まず、硬質なマルテンサイトが得にくくなる。一次冷却停止温度については、700℃を超えるとフェライトとオーステナイトの2相分離が進みにくくなり、550℃未満ではベイナイト変態が起こりやすく、所望のマルテンサイト体積率を得ることが困難となる。よって、550〜700℃とする。二次冷却にて、マルテンサイトが生成される。350℃超までの冷却や50℃/秒未満の冷却では、十分にマルテンサイトが生成されず、強度低下や加工性劣化を招く。望ましくは、300℃以下まで冷却すべきである。急冷後は、セメンタイト析出により固溶C量を適切に制御しつつ、マルテンサイトの硬度を保つため、250〜350℃で100秒以上の過時効処理を施す必要がある。   After annealing, primary cooling to 550 to 700 ° C. at 2 to 10 ° C./second, secondary cooling to 50 ° C./second or more to 350 ° C. or less, and overaging treatment at 250 to 350 ° C. for 100 seconds or more was performed. Later, it is necessary to cool to room temperature. The primary cooling has an effect of concentrating C in the austenite by increasing the ferrite fraction while avoiding pearlite. If it is less than 2 ° C./second, pearlite transformation occurs and the formability deteriorates. If it exceeds 10 ° C./second, the two-phase separation of ferrite and austenite is difficult to proceed, and C concentration in austenite does not proceed sufficiently. Hard martensite is difficult to obtain. If the primary cooling stop temperature exceeds 700 ° C., the two-phase separation of ferrite and austenite hardly proceeds, and if it is less than 550 ° C., bainite transformation tends to occur and it becomes difficult to obtain a desired martensite volume fraction. Therefore, it is set as 550-700 degreeC. In the secondary cooling, martensite is generated. Cooling to over 350 ° C. or cooling at less than 50 ° C./second does not sufficiently generate martensite, leading to strength reduction and workability deterioration. Desirably, it should be cooled to below 300 ° C. After quenching, it is necessary to perform an overaging treatment at 250 to 350 ° C. for 100 seconds or more in order to maintain the hardness of martensite while appropriately controlling the amount of dissolved C by cementite precipitation.

連続焼鈍の後の調質圧延は、成形性の観点からはかけない方が望ましいが、形状矯正のためにかけても良い。ただし、最小限とすべきであり、伸び率は1%以下とするのが望ましい。   The temper rolling after the continuous annealing is preferably not performed from the viewpoint of formability, but may be performed for shape correction. However, it should be kept to a minimum, and the elongation is preferably 1% or less.

以上のような熱延の後の各工程、酸洗、冷延、連続焼鈍、調質圧延は、各々独立した工程であってもかまわないし、部分的に連続している工程でもかまわない。生産効率から考えれば、全て連続化していることが理想である。   Each process after hot rolling as described above, pickling, cold rolling, continuous annealing and temper rolling may be independent processes or may be partially continuous processes. From the viewpoint of production efficiency, it is ideal that everything is continuous.

以下、本発明の実施例について説明する。
(実施例1)
表1に示した1番〜15番の成分の鋼を溶製後、連続鋳造によりスラブを製造した。これらのうち、1番〜5番は本発明の範囲内であり、6番〜15番は本発明の範囲外のものである。
Examples of the present invention will be described below.
Example 1
Slabs were produced by continuous casting after melting the steels of No. 1 to No. 15 shown in Table 1. Of these, numbers 1 to 5 are within the scope of the present invention, and numbers 6 to 15 are outside the scope of the present invention.

これらのスラブを、実機連続熱延ラインにおいて、1200℃に再加熱後、粗圧延し、850℃で仕上圧延を終了して板厚3.0mmとし、550℃にて巻き取りコイルとした。この熱延コイルを酸洗−冷延−連続焼鈍−調質圧延まで連続した実機ラインで冷延鋼板とした。板厚1.6mmまで冷延し、760℃で50秒焼鈍後、650℃まで5℃/秒にて一次冷却し、300℃まで100℃/秒で冷却し、300℃にて200秒保持した後、室温まで冷却後酸洗し、伸び率0.5%で調質圧延をかけた。その冷延鋼板の性能を評価した。   These slabs were reheated to 1200 ° C. in an actual continuous hot rolling line, roughly rolled, finished at 850 ° C., finished to a plate thickness of 3.0 mm, and wound into a coil at 550 ° C. This hot-rolled coil was made into a cold-rolled steel sheet by an actual machine line that continued from pickling, cold-rolling, continuous annealing, and temper rolling. Cold-rolled to a thickness of 1.6 mm, annealed at 760 ° C. for 50 seconds, first cooled to 650 ° C. at 5 ° C./second, cooled to 300 ° C. at 100 ° C./second, and held at 300 ° C. for 200 seconds. Then, after cooling to room temperature, pickling and temper rolling was performed at an elongation of 0.5%. The performance of the cold rolled steel sheet was evaluated.

機械的特性は、幅方向からJIS5号引張試験片を採取し、引張試験にて評価した。引張試験の応力−歪曲線より、降伏強度(YP)、引張強度(TS)、全伸び(T−EL)、降伏点伸び(時効前YP−EL)を求め、更に、降伏比(YR=YP/TS×100)、加工性の指標であるTS×T−ELをもとめた。時効前YP−ELは0%を合格とし、YRは60%以下を合格とし、TS×T−ELは17000MPa・%以上を合格とした。   The mechanical characteristics were evaluated by taking a JIS No. 5 tensile test piece from the width direction and performing a tensile test. From the stress-strain curve of the tensile test, yield strength (YP), tensile strength (TS), total elongation (T-EL), yield point elongation (YP-EL before aging) are determined, and yield ratio (YR = YP) / TS × 100), TS × T-EL, which is an index of workability, was obtained. YP-EL before aging made 0% pass, YR made 60% or less pass, and TS × T-EL made 17000 MPa ·% or more pass.

焼き付け硬化性は、幅方向から切り出したJIS5号引張試験片に2%の予歪を付加後、170℃で20分間の熱処理を施し、再度引張試験を行った時のYP増加量で評価した。そのYPの測定については、JISの規定に従った。このYP増加量をBH量とし、60MPa以上を合格とした。   The bake hardenability was evaluated by the amount of YP increase when a JIS No. 5 tensile test piece cut out from the width direction was subjected to a heat treatment at 170 ° C. for 20 minutes after applying a pre-strain of 20% and a tensile test was performed again. The YP was measured according to JIS regulations. The amount of increase in YP was defined as the amount of BH, and 60 MPa or more was regarded as acceptable.

常温遅時効性は、幅方向から切り出したJIS5号引張試験片を、100℃で1時間熱処理した後、引張試験に供し、降伏点伸びを測定することにより評価した。その時効後YP−ELが0.2%以下を合格とした。   The room temperature slow aging was evaluated by subjecting a JIS No. 5 tensile specimen cut out from the width direction to heat treatment at 100 ° C. for 1 hour, and then subjecting it to a tensile test and measuring the elongation at yield. The YP-EL after the aging was determined to be 0.2% or less.

成形試験では、ブランク角160mmのサンプルを採取後、800mmR円筒面のポンチでプレス成形を行って評価した。割れ、しわ、ストレッチャーストレインが発生した場合は×、発生しない場合は○とし、○を合格とした。   In the molding test, a sample with a blank angle of 160 mm was collected and then evaluated by press molding with a punch having an 800 mmR cylindrical surface. When cracks, wrinkles and stretcher strains were generated, x was given.

固溶C量を、横振動法の内部摩擦測定装置により測定した。また、試験片の圧延方向断面を研磨した後、レペラー腐食し、全厚の1/4の位置におげる灰色のフェライトと白色のマルテンサイトの組織分率を測定した。マルテンサイトについては、2μm以下の微細マルテンサイトの組織分率も測定した。   The amount of solute C was measured by an internal friction measuring device using a transverse vibration method. Further, after polishing the cross section in the rolling direction of the test piece, the structure fraction of gray ferrite and white martensite that was subjected to repeller corrosion and placed at a position of 1/4 of the total thickness was measured. For martensite, the structural fraction of fine martensite of 2 μm or less was also measured.

表2に性能の評価結果及び組織等の観察結果を示した。評価項目については不合格の場合を網掛けし、組織等観察結果については、請求項1の範囲から外れる場合を網掛けした。
本発明の範囲内の成分である1番〜5番は、いずれの特性も合格となり、目標とする特性の鋼板が得られている。しかし、本発明の範囲から外れた成分である6番〜15番については、いずれかの特性が不合格となっている。
Table 2 shows the evaluation results of performance and the observation results of the structure and the like. The evaluation items are shaded when they are rejected, and the observation results of the structure and the like are shaded when they fall outside the scope of claim 1.
No. 1 to No. 5 which are components within the scope of the present invention pass all the properties, and a steel plate having the target properties is obtained. However, about the 6th-15th which is a component which remove | deviated from the range of this invention, any characteristic is disqualified.

また、1番〜8番につき、式のC+(Mn/20)と時効後YP−ELの関係をグラフにすると、図1のようになる。このように、時効後YP−ELを0.2%以下とし、常温遅時効性を確保するには、C+(Mn/20)≧0.12%を満たす必要がある。   In addition, for Nos. 1 to 8, a graph showing the relationship between C + (Mn / 20) and YP-EL after aging is shown in FIG. Thus, in order to make YP-EL after aging 0.2% or less and to ensure normal temperature slow aging, it is necessary to satisfy C + (Mn / 20) ≧ 0.12%.

Figure 0004954909
Figure 0004954909

Figure 0004954909
Figure 0004954909

(実施例2)
表1の2番の成分のスラブにつき、表3の製造条件に従って冷延鋼板を製造した。なお、過時効処理については、表3に記載の二次冷却停止温度から過時効処理終了温度まで、直線的に温度変化するものとする。表3に記載した以外の条件は、実施例1と同じとした。また、評価についても実施例1と同じとした。
(Example 2)
A cold-rolled steel sheet was manufactured according to the manufacturing conditions in Table 3 for the slab of No. 2 component in Table 1. In addition, about overaging treatment, it shall change linearly from the secondary cooling stop temperature of Table 3 to overaging treatment completion temperature. Conditions other than those described in Table 3 were the same as in Example 1. The evaluation was also the same as in Example 1.

表4に性能の評価結果及び組織等の観察結果を示した。網掛けに関しては、表2と同様である。本発明の範囲内の製造条件であるNo.1〜5は、いずれの特性も合格となり、目標とする特性の鋼板が得られている。各々の組織分率または固溶Cが、全て請求項1の要件を満たしている。しかし、本発明の範囲から外れた製造条件であるNo.6〜17については、いずれかの特性が不合格となっている。また、これらの場合、各々の組織分率または固溶Cのうち、いずれかが請求項1の要件を満たしていない。   Table 4 shows the performance evaluation results and the observation results of the structure and the like. The shading is the same as in Table 2. No. which is a production condition within the scope of the present invention. As for 1-5, all the characteristics passed, and the steel plate of the target characteristic is obtained. Each of the tissue fractions or solute C satisfies the requirements of claim 1. However, the production conditions deviating from the scope of the present invention are No. About 6-17, either characteristic has failed. Moreover, in these cases, either one of the tissue fractions or the solid solution C does not satisfy the requirement of claim 1.

Figure 0004954909
Figure 0004954909

Figure 0004954909
Figure 0004954909

(実施例1)において、各種鋼板のC+(Mn/20)に対して、時効後YP−ELをプロットしたグラフである。In (Example 1), it is the graph which plotted YP-EL after aging with respect to C + (Mn / 20) of various steel plates.

Claims (4)

質量%で、C:0.03〜0.09%、Mn:1.0〜2.0%、Si:1.0%以下、P:0.05%以下、S:0.03%以下、sol.Al:0.005〜0.1%、N:0.01%以下、B:0.001〜0.003%、0.005%≧(14/48)Ti+(14/93)Nb−N≧0を満たす分のTiとNbのうち1種以上を含み、かつ、C+(Mn/20)≧0.12%を満たし、固溶C:1〜7ppmであり、残部はFeおよび不可避的不純物から成り、体積率で、80%以上のフェライトと3〜10%のマルテンサイトから成り、マルテンサイトのうち2%以上は2μm以下の微細マルテンサイトであることを特徴とする焼き付け硬化特性と常温遅時効性に優れた低降伏比型高強度冷延鋼板。   In mass%, C: 0.03-0.09%, Mn: 1.0-2.0%, Si: 1.0% or less, P: 0.05% or less, S: 0.03% or less, sol. Al: 0.005-0.1%, N: 0.01% or less, B: 0.001-0.003%, 0.005% ≧ (14/48) Ti + (14/93) Nb−N ≧ It contains one or more of Ti and Nb that satisfy 0, and satisfies C + (Mn / 20) ≧ 0.12%, solid solution C: 1 to 7 ppm, and the balance from Fe and inevitable impurities Bake hardening characteristics and room temperature slow aging characterized in that it consists of 80% or more ferrite and 3 to 10% martensite by volume ratio, and 2% or more of martensite is fine martensite of 2 μm or less Low yield ratio type high-strength cold-rolled steel sheet with excellent properties. さらに、Ca、REMのうちの1種または2種を、合計で0.01質量%以下含有することを特徴とする請求項1に記載の焼き付け硬化特性と常温遅時効性に優れた低降伏比型高強度冷延鋼板。   Furthermore, the low yield ratio which was excellent in the bake hardening characteristic and normal temperature slow aging property of Claim 1 characterized by including 1 type or 2 types in total in Ca and REM 0.01 mass% or less Type high strength cold-rolled steel sheet. 質量%で、C:0.03〜0.09%、Mn:1.0〜2.0%、Si:1.0%以下、P:0.05%以下、S:0.03%以下、sol.Al:0.005〜0.1%、N:0.01%以下、B:0.001〜0.003%、0.005%≧(14/48)Ti+(14/93)Nb−N≧0を満たす分のTiとNbのうち1種以上を含み、かつ、C+(Mn/20)≧0.12%を満たし、残部はFeおよび不可避的不純物から成る鋼片を仕上圧延後、450〜600℃にて巻き取り、酸洗、冷間圧延後、連続焼鈍を行うに際し、(Ac1変態温度+20℃)〜(Ac1変態温度+100℃)にて焼鈍後、2〜10℃/秒で550〜700℃まで一次冷却し、50℃/秒以上で350℃以下まで二次冷却し、250〜350℃で100秒以上の過時効処理を行った後、室温まで冷却することを特徴とする焼き付け硬化特性と常温遅時効性に優れた低降伏比型高強度冷延鋼板の製造方法。   In mass%, C: 0.03-0.09%, Mn: 1.0-2.0%, Si: 1.0% or less, P: 0.05% or less, S: 0.03% or less, sol. Al: 0.005-0.1%, N: 0.01% or less, B: 0.001-0.003%, 0.005% ≧ (14/48) Ti + (14/93) Nb−N ≧ One or more of Ti and Nb satisfying 0 and C + (Mn / 20) ≧ 0.12% are satisfied, and the balance is 450 to 450- after finish rolling a steel slab composed of Fe and inevitable impurities. In continuous annealing after winding at 600 ° C., pickling and cold rolling, after annealing at (Ac1 transformation temperature + 20 ° C.) to (Ac1 transformation temperature + 100 ° C.), 550 at 2-10 ° C./second. Baking and curing characterized by primary cooling to 700 ° C., secondary cooling to 350 ° C. or less at 50 ° C./second or more, and after overaging at 250 to 350 ° C. for 100 seconds or more, followed by cooling to room temperature Low yield ratio type high strength cold-rolled steel sheet with excellent properties and room temperature slow aging Method. さらに、鋼片がCa、REMのうちの1種または2種を、合計で0.01質量%以下含有することを特徴とする請求項3に記載の焼き付け硬化特性と常温遅時効性に優れた低降伏比型高強度冷延鋼板の製造方法。   Further, the steel slab contains one or two of Ca and REM in a total amount of 0.01% by mass or less, and is excellent in bake hardening characteristics and room temperature slow aging. Low yield ratio type high strength cold rolled steel sheet manufacturing method.
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