JP3825641B2 - Cold-rolled steel sheet with excellent slow aging and bake hardenability - Google Patents
Cold-rolled steel sheet with excellent slow aging and bake hardenability Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、遅時効性と焼付硬化性に優れた冷延鋼板に関するものである。
【０００２】
【従来の技術】
冷延鋼板の焼付硬化性を向上させる方法としては、例えば、特開昭55−１４１５２６号公報、特開昭55−１４１５５５号公報の如くNb添加において、鋼中のＣ、Ｎ、Ａｌ含有量に応じてNbを添加して、at. ％でNb／（固溶Ｃ＋固溶Ｎ）をある範囲内に制限することより、鋼板中の固溶Ｃ、固溶Ｎを調整し、さらに焼鈍後の冷却速度を制御する方法が開示されている。また、特開昭61−４５６８９号公報の如く、TiとNbの複合添加によって焼付硬化性に優れた鋼板とすることが開示されている。しかし、これらはいずれも極低炭素鋼と呼ばれる鋼種であり、固溶Ｃを低く抑えることにより時効硬化性を改善することを特徴としている。また、このような方法で固溶Ｃを制御しただけでは、焼付硬化性はせいぜい３０ＭＰａ程度の上昇にとどまり、これ以上焼付硬化性を向上させるべく固溶Ｃを残存させれば、時効硬化性が悪化し、長時間保存した後にプレス成型を行うとストレッチャーストレインという縞模様が発生してしまう。
【０００３】
これに対し、特開2000−１７３８６号公報に示されるように、TiおよびNbにより制御された固溶Ｃの残存量によりMoを適正量添加することで、焼付硬化性と時効硬化性を両立させる方法が開示されている。しかし、これはあくまでも極低炭素鋼についてのみ成し得る技術である。いわゆるＡｌキルド鋼といわれる低炭素鋼においては、TiやNbによる固溶Ｃの調整を行わないため、焼鈍後の固溶Ｃの残存量の特定が困難であった。やみくもにMoを添加して、Al−キルド・低炭素鋼の焼付硬化性と時効硬化性を両立させることは、コスト上の観点から問題となり、さらに過剰なMo添加は強度の上昇をもたらし、プレス成形時の割れにつながる。このため、これ以外の方法で、Moの添加量の最適化が重要となる。
【０００４】
【発明が解決しようとする課題】
本発明は、焼付硬化性と遅時効性を両立し、かつ安定した焼付効果量を確保し、また従来大きな焼付硬化性を有するAl−キルド・低炭素鋼を提供するものである。
【０００５】
【課題を解決するための手段】
本発明の特徴とするところは、
（１） 重量％にて、
Ｃ：0.01〜0.20 ％
Si：0.005 〜1.5 ％
Mn：0.01〜2.0 ％
Ｐ：0.01〜0.10 ％
Ｓ：≦0.02 ％
Al：≦0.40 ％
Ｎ：0.001 〜0.01 ％
Mo ： 0.005 〜 0.350 ％
残部 Fe 及び不可避的不純物からなり、
ｋ＝Ａ−3600×ｄｃ×ｎ３／２−17000 ×ｄｃ×（２×ｄｆ−ｄｃ ２)×Ａ
なる定数を定めたとき、ｋ≧８であって、かつ、
9.8 ×10−５×ｋ２＋2.47×10−３×ｋ＋0.12≦ Mo ％≦ 1.73 ×10−６×ｋ２＋2.47×10−４×ｋ−0.016 を満足するようにMo を上記範囲内で
添加することを特徴とする遅時効性と焼付硬化性に優れた冷延鋼板。
ここで、Ａは10×√Ｃ−Ｃ／８にて求まる定数、ただし、Ｃは重量濃度で単位は[ppm] 。ｄｃは粒内セメンタイトの平均粒径[mm]、ｎは粒内セメンタイトの平均個数、ｄｆはフェライトの平均粒径[mm]である。
（２）（１）の鋼板であって、その転位密度が、平面視野１μｍ２あたり、５０本以上、３０００本以下である事を特徴とする遅時効性と焼付硬化性に優れた冷延鋼板にある。
【０００６】
本発明の対象とする冷延鋼板は、冷延鋼板、亜鉛等を溶融めっき又は電気めっきしためっき鋼板等で、鋼の製造方法として、転炉、電気炉、平炉等いずれの方法でもよく、鋳型による鋳造後分塊したスラブ、連続鋳造でスラブとしたもの等その製造方法は問わない。本発明者らは、冷延鋼板の遅時効性と焼付硬化性を向上させるために、種々の研究を重ねた結果、本発明に至ったのである。
【０００７】
従来の極低炭素鋼においては、前述したようにTiやNbの添加により固溶Ｃ残存量の制御・計算が可能であったが、Moと固溶Ｃの相互作用による焼付硬化性と遅時効性の両立が可能であることは既にわかっており、いかにAlキルド鋼・低炭素鋼でのMo添加量を特定するか、本発明者らは各種試験と解析を行い、鋼中の組織と析出セメンタイトの単位体積あたりの存在率から、最適Mo添加量を導き出す下記式へと発展させることが可能であることを見出した。
すなわち、Mo濃度が 0.005≦Mo％≦0.350 であって、さらに、
ｋ＝Ａ−3600×ｄc ×ｎ3/2 −17000 ×ｄc ×（２×ｄf −ｄc 2)×Ａなる定数を定めたとき、ｋ≧８であって、かつ、
9.8 ×10-5×ｋ2 ＋2.47×10-3×ｋ＋0.12≦ Mo ％≦ 1.73 ×10-6×ｋ2 ＋2.47×10-4×ｋ−0.016 を満足するときに、遅時効性と焼付硬化性が両立する。ｋの特定方法は以下の手順と考え方によって求められることを見出した。
【０００８】
すなわち、フェライト粒内および粒界に析出したＣを測定、もしくは予測するものである。フェライト粒内の析出Ｃは析出したセメンタイトの平均粒径と個数により求め、粒界の析出Ｃは、粒界に近い固溶Ｃがある時間経過した後に粒界に析出する量を予測する。この経過時間は粒内の析出セメンタイトの平均粒径から逆算することが可能で、フェライトの粒径、粒内セメンタイトの粒径から求めることができる。これにより、定数ｋを求める式の右辺の第二項では、粒内炭化物（セメンタイト）のＣ量が、第三項では粒界炭化物のＣ量が、各々求まる。これと、トータル添加Ｃ量から求まる定数Ａとから上記式を導いた。
【０００９】
また、ｋを８以上としたのは、それ未満では焼付硬化性が少ないためである。
尚、フェライト粒径およびセメンタイト粒径は、いずれもナイタールエッチングを施した試料を、光学顕微鏡にて５００倍に拡大して測定する。
一方、Mo濃度は9.8 ×10-5×ｋ2 ＋2.47×10-3×ｋ＋0.12未満となると、Moの効果が不十分となり、遅時効性が劣化する、すなわち、降伏点伸びが発生し、プレス成形時にストレッチャーストレインが発生する。また、1.73×10-6×ｋ2 ＋2.47×10-4×ｋ−0.016 を超えると、遅時効性と焼付硬化性の両立は確保できるが、その効果は飽和し、コストが高くなる。
【００１０】
遅時効性と焼付硬化性とが、両立する原因は、本発明者らはMoとＣとが結合力的な相互作用を起こすことにより、MoがＣの拡散を阻害し、Ｃが転移へ固着する速度を遅くしていると考えている。つまり40℃程度の室温レベルのＣの拡散速度では、Moによる拡散速度の低下が、時効を大幅に遅らせるが、170 ℃の塗装焼付時の高温時には、Ｃの拡散速度が増加しMoによる拡散阻害が無視できるレベルになるため、Moの有無にかかわらず、同じ焼付時間でも同等の焼付硬化量が得られるものと考えている。
【００１１】
【発明の実施の形態】
本発明の範囲を、図１に示す。図１において、Ａの部分（但し、境界線を含む）が、本発明の範囲であって、焼付硬化性と遅時効性に優れている。Ｂの部分は、Mo添加量が少ないため遅時効性が悪く、プレス成形によりストレッチャーストレインが発生する。Ｃの領域では、Mo添加量が高すぎるため、強度が高くなりプレス成形時に割れが発生しやすいと共に、コスト面でも問題となる。Ｄの領域では焼付硬化性が少ない。
【００１２】
また、転位分布によりその特性が大きく変わることも、多くの電子顕微鏡観察の結果から明らかとなった。本発明者らは、遅時効性の良好なサンプルを電子顕微鏡観察を行った結果、その転位密度が、平面視野１μm2あたり、５０本以上、３０００本以下の場合、さらに、遅時効性と焼付硬化性が改善される事が判明した。転位密度が５０本未満では、本発明の効果がなくなるわけではないが、５０本以上でさらに焼付硬化性が改善されるものである。転位密度が１μm2あたり３０００本を超える場合には、鋼材の伸びが低下し、プレス時に割れが発生することが明らかとなった。この原因は明らかではないが、転位が歪場を形成し、MoやＣの相互作用に対し、別の相互作用を起こすものと考えられる。
【００１３】
本発明の鋼成分を限定した理由は以下のとおりである。
まず、Ｃ： 0.01 ％未満であると、焼付硬化に必要な十分な固溶Ｃが確保できない。また、0.20％を超えると、硬度が高くなりすぎ、プレス加工性が悪化するため、0.01〜0.20％とした。
【００１４】
Ｓｉ： 0.005％未満にしようとすると、製鋼でのコストアップになり、また、高い焼付硬化性を得られない。また、1.5 ％以下としたのは、それを超えると、強度が高くなりすぎ、加工性を損なうためである。さらに望ましくは、亜鉛めっきを行うときには、密着性を確保するために、0.10％以下とするほうがよい。
【００１５】
Ｍｎ： 0.01 ％以上としたのは、それ未満だと高い焼付硬化性を得られないからである。2.0 ％以下としたのは、それを超えるとMnが鋼の強化元素であり、強度が高くなり、加工性を損なうためである。
【００１６】
Ｐ： 0.01 ％未満としようとすると、製鋼でのコストアップになり、また、高い焼付硬化性を得られないからである。また、0.10％以下としたのは、Ｐが少量でも鋼の強化元素であり、強度が高くなり、加工性を損なうためであり、しかも、Ｐは結晶粒界に濃化して、粒界脆化を起こしやすい元素であり、それを超えて添加することは加工性を損なうためである。
【００１７】
Ｓ： 0.02 ％以下としたのは、Ｓは不純物であり、多すぎると熱間圧延時の割れを引き起こすばかりでなく、平均ランクフォード値の劣化を起こすので極力低減させるべきであるが、製鋼コストも考慮すると、0.02％以下が許容範囲となる。
【００１８】
Ａｌ：上限を0.40％としたのも、それを超えて添加するとコスト的に不利になるからである。しかも強度が高くなり、加工性を損なうためである。
【００１９】
Ｎ： 0.001％未満としようとすると、製鋼でのコストアップになる。また、0.01％以下としたのは、それを超えて添加すれば、Ｎによる時効が生じ、遅時効性が劣化する。
【００２０】
Ｍｏ： 0.005％以上としたのは、それ未満では焼付硬化性を高くする効果がないためである。また、上限を0.35％としたのはそれを超えるとＭｏが鋼の強化元素であり、強度が高くなりすぎ、加工性を損なうためであり、焼付硬化性も飽和してしまうために、高価で経済的に成り立たなくなるためである。
また、さらに、この範囲は、Mo濃度をさらに、
9.8 ×10-5×ｋ2 ＋2.47×10-3×ｋ＋0.12≦ Mo ％≦ 1.73 ×10-6×ｋ2 ＋2.47×10-4×ｋ−0.016
ただし、ｋ＝Ａ−3600×ｄc ×ｎ3/2 −17000 ×ｄc ×（２×ｄf −ｄc 2)×Ａかつｋ≧８を満たす時に焼付硬化性と遅時効性が改善される。
これは、先述したように、ＭｏとＣの相互作用が形成される最適範囲と考えられる。Ｃに対して不要にＭｏ濃度を高くしても、効果は飽和し、またコスト高となり、また、鋼材の伸びが低減する場合があるので、0.35％を上限とした。また、再結晶が起こりにくくなり、伸びが低下する可能性もある。ただし、本発明の効果がなくなるものではない。また、0.005 ％以下のＭｏでは、時効硬化性が改善されず、降伏点伸びが発生してしまう。
【００２１】
【実施例】
表１、２に本発明の実施例を比較例と共に示す。表１、２に示す成分の鋼を転炉にて溶解し、次に連続鋳造によりスラブとなした。このスラブを熱間圧延・冷間圧延し、その後焼鈍を行い、冷延鋼板とした。遅時効性の評価指標として、４０℃の雰囲気中に７０日保持し、その後、引張り試験を行い、この時の降伏点伸びを測定し、0.02％以下を遅時効性が良好であるとした。また、焼付硬化性の測定は、冷延鋼板を２％引張り、その後170 ℃×20min.保持したときのＹＰを測定し、先に２％引張り試験を行ったときの強度との差を測定した。本発明では、いずれも、遅時効性は0.01％以下であり、焼付硬化性は50ＭＰａを超えている。比較例では、Moの少ない物は、遅時効性が悪く0.02％を超えており、また、焼付硬化性も低くなる。また、Moの多い物は、遅時効性と焼付硬化性は良いが、プレス時に割れが発生した。
【００２２】
【表１】
また、表２は、転移密度の効果を示したもので、比較例に比べて焼付硬化性において、同レベルのｋ値に対して30ＭＰａ程度の改善が見られる。表２の転移密度は、冷延鋼板から薄膜試験片を採取し、透過電子顕微鏡にて、各３個の薄膜試験片で通常の観察方法で転移を求め、１μｍ2 の本数に換算し、その平均値とした。本発明では、いずれも、常温時効性は0.02％以下で良好であった。また、焼付硬化性についても、いずれも５０ＭＰａ以上を示し、十分な値を示している。
【００２４】
【表２】
【発明の効果】
本発明により、焼付硬化性と遅時効性の優れた鋼板を得ることができる。
【図面の簡単な説明】
【図１】 本発明のMoとｋ値との関係を示す説明図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a cold-rolled steel sheet excellent in delayed aging and bake hardenability.
[0002]
[Prior art]
As a method for improving the bake hardenability of a cold rolled steel sheet, for example, in the addition of Nb as disclosed in JP-A-55-141526 and JP-A-55-141555, the content of C, N, and Al in the steel is increased. Accordingly, Nb is added, and Nb / (Solution C + Solution N) is limited to a certain range by at.%, So that solute C and solute N in the steel sheet are adjusted, and further after annealing. A method for controlling the cooling rate is disclosed. Further, as disclosed in Japanese Patent Application Laid-Open No. 61-45689, it is disclosed that a steel plate having excellent bake hardenability can be obtained by a combined addition of Ti and Nb. However, these are all steel types called ultra-low carbon steels, and are characterized by improving age hardenability by keeping solute C low. Further, by controlling the solid solution C by such a method, the bake hardenability only rises to about 30 MPa, and if the solid solution C is left to further improve the bake hardenability, the age hardenability is improved. When it is pressed and molded after being stored for a long time, a stripe pattern called stretcher strain occurs.
[0003]
On the other hand, as shown in JP-A-2000-17386, by adding an appropriate amount of Mo based on the residual amount of solute C controlled by Ti and Nb, both bake hardenability and age hardenability are achieved. A method is disclosed. However, this is a technique that can be achieved only for ultra-low carbon steel. In a low carbon steel called so-called Al killed steel, since the solute C is not adjusted with Ti or Nb, it is difficult to specify the residual amount of the solute C after annealing. It is a problem from the viewpoint of cost to add Mo to the glaze and make the bake hardenability and age hardenability of Al-killed low-carbon steel compatible, and excessive Mo addition causes an increase in strength. This leads to cracking during molding. For this reason, it is important to optimize the addition amount of Mo by other methods.
[0004]
[Problems to be solved by the invention]
The present invention provides an Al-killed low-carbon steel that has both bake hardenability and delayed aging properties, ensures a stable bake effect, and has a large bake hardenability.
[0005]
[Means for Solving the Problems]
The feature of the present invention is that
(1) By weight%
C: 0.01 to 0.20%
Si: 0.005 to 1.5%
Mn: 0.01-2.0%
P: 0.01-0.10%
S: ≤ 0.02%
Al: ≤0.40%
N: 0.001 to 0.01%
Mo : 0.005 to 0.350 %
It consists of the balance Fe and inevitable impurities,
k = A-3600 × d c × n 3/2 -17000 × d c × (2 × d f -d c 2) × A
When a constant is determined, k ≧ 8, and
9.8 × 10 -5 × k 2 + 2.47 × 10 -3 × k + 0.12 ≦ Mo% ≦ 1.73 × 10 -6 × k 2 + 2.47 × 10 -4 above the Mo to satisfy × k-0.016 A cold-rolled steel sheet excellent in delayed aging and bake hardenability, characterized by being added within the range .
Here, A is a constant determined by 10 × √C−C / 8, where C is the weight concentration and the unit is [ppm]. It is d c average particle diameter of grain cementite [mm], n is the average number, d f of grain cementite having an average particle size of the ferrite [mm].
(2) A cold rolled steel sheet excellent in delayed aging and bake hardenability, characterized in that the dislocation density is 50 or more and 3000 or less per 1 μm 2 of plane view, the steel sheet of (1). It is in.
[0006]
The cold-rolled steel sheet that is the subject of the present invention is a cold-rolled steel sheet, a plated steel sheet that is hot-plated or electroplated with zinc, etc. Any method of manufacturing such as a slab that has been agglomerated after casting or a slab formed by continuous casting may be used. The inventors of the present invention have arrived at the present invention as a result of conducting various studies in order to improve the slow aging property and bake hardenability of the cold-rolled steel sheet.
[0007]
In the conventional ultra-low carbon steel, the amount of residual solute C can be controlled and calculated by adding Ti and Nb as described above, but the bake hardenability and slow aging due to the interaction between Mo and solute C are possible. It is already known that compatibility can be achieved, and the inventors conducted various tests and analyzes on how to specify the amount of Mo addition in Al killed steel and low carbon steel, and the structure and precipitation in the steel. From the abundance rate of cementite per unit volume, it was found that it can be developed into the following formula that derives the optimum Mo addition amount.
That is, the Mo concentration is 0.005 ≦ Mo% ≦ 0.350, and
k = A-3600 × d c × n 3/2 -17000 × d c × (2 × d f -d c 2) when defining a × A becomes constant, a k ≧ 8, and,
9.8 × 10 -5 × k 2 + 2.47 × 10 -3 × k + 0.12 ≤ Mo% ≤ 1.73 × 10 -6 × k 2 + 2.47 × 10 -4 × k-0.016 Compatibility with bake hardenability. It has been found that the identification method of k is obtained by the following procedure and concept.
[0008]
That is, C precipitated in the ferrite grains and in the grain boundaries is measured or predicted. Precipitation C in the ferrite grains is determined from the average particle diameter and number of cementite precipitated, and the precipitation C at the grain boundaries is predicted to be the amount precipitated at the grain boundaries after a certain time of solid solution C near the grain boundaries. This elapsed time can be calculated back from the average particle size of precipitated cementite in the grains, and can be determined from the particle diameter of ferrite and the particle diameter of intra-particle cementite. Thereby, in the second term on the right side of the equation for obtaining the constant k, the C amount of intragranular carbide (cementite) is obtained, and in the third term, the C amount of grain boundary carbide is obtained. The above formula was derived from this and a constant A obtained from the total amount of added C.
[0009]
The reason why k is 8 or more is that if it is less than that, the bake hardenability is small.
The ferrite particle size and the cementite particle size are measured by enlarging a sample subjected to nital etching 500 times with an optical microscope.
On the other hand, when the Mo concentration is less than 9.8 × 10 −5 × k 2 + 2.47 × 10 −3 × k + 0.12, the effect of Mo becomes insufficient and the delayed aging deteriorates, that is, yield point elongation occurs. In addition, stretcher strain occurs during press molding. On the other hand, if it exceeds 1.73 × 10 −6 × k 2 + 2.47 × 10 −4 × k−0.016, both the delayed aging property and the bake hardenability can be ensured, but the effect is saturated and the cost is increased.
[0010]
The reason why both the slow aging property and the bake hardenability are compatible is that the inventors have caused a binding interaction between Mo and C, so that Mo inhibits the diffusion of C, and C adheres to the transition. I think it is slowing down. In other words, at the room temperature level of about 40 ° C, the decrease in the diffusion rate due to Mo significantly delays aging, but at a high temperature during baking at 170 ° C, the diffusion rate of C increases, which inhibits diffusion by Mo. Therefore, it is considered that an equivalent bake hardening amount can be obtained even with the same baking time regardless of the presence or absence of Mo.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The scope of the present invention is shown in FIG. In FIG. 1, the portion A (including the boundary line) is within the scope of the present invention, and is excellent in bake hardenability and delayed aging. The portion B is poor in slow aging due to the small amount of Mo added, and stretcher strain is generated by press molding. In the region C, since the amount of Mo added is too high, the strength is increased, and cracking is likely to occur during press molding, and there is a problem in terms of cost. In the region D, the bake hardenability is small.
[0012]
In addition, it has been clarified from the results of many electron microscope observations that the characteristics vary greatly depending on the dislocation distribution. As a result of observing a sample with good slow aging by an electron microscope, the present inventors have found that when the dislocation density is 50 or more and 3000 or less per 1 μm 2 of the plane field of view, the delayed aging and seizure are further observed. It was found that the curability was improved. If the dislocation density is less than 50, the effect of the present invention is not lost, but if it is 50 or more, the bake hardenability is further improved. It has been clarified that when the dislocation density exceeds 3000 per 1 μm 2 , the elongation of the steel material is lowered and cracking occurs during pressing. The cause of this is not clear, but it is thought that dislocations form a strain field and cause another interaction with the interaction between Mo and C.
[0013]
The reason for limiting the steel components of the present invention is as follows.
First, if C: less than 0.01%, sufficient solid solution C necessary for bake hardening cannot be secured. Further, if it exceeds 0.20%, the hardness becomes too high and the press workability deteriorates, so the content was made 0.01 to 0.20%.
[0014]
Si: If it is attempted to make it less than 0.005%, the cost of steelmaking increases, and high bake hardenability cannot be obtained. Further, the reason why it is set to 1.5% or less is that if it exceeds that, the strength becomes too high and the workability is impaired. More desirably, when galvanizing is performed, the content is preferably set to 0.10% or less in order to ensure adhesion.
[0015]
The reason why Mn is 0.01% or more is that if it is less than that, high bake hardenability cannot be obtained. The reason why it is set to 2.0% or less is that Mn is a strengthening element of the steel beyond that, and the strength becomes high and the workability is impaired.
[0016]
P: If the content is less than 0.01%, the cost increases in steelmaking, and high bake hardenability cannot be obtained. Further, the 0.10% or less is a steel strengthening element even if a small amount of P is to increase the strength and impair the workability. Moreover, P is concentrated at the grain boundary and becomes brittle at the grain boundary. This is because it is an element that tends to cause erosion, and adding more than that element impairs workability.
[0017]
S: 0.02% or less, S is an impurity. If it is too much, not only will it cause cracking during hot rolling, but it will also cause deterioration of the average rankford value. In consideration of this, the allowable range is 0.02% or less.
[0018]
Al: The upper limit was set to 0.40% because adding it beyond that would be costly. Moreover, the strength is increased and workability is impaired.
[0019]
N: If it is less than 0.001%, the cost of steelmaking will increase. Moreover, the reason why the content is 0.01% or less is that if it is added in excess of that, aging due to N occurs, and the delayed aging deteriorates.
[0020]
The reason why Mo is set to 0.005% or more is that if it is less than 0.005%, there is no effect of increasing bake hardenability. Moreover, the upper limit is set to 0.35%, and if it exceeds the upper limit, Mo is a strengthening element of steel, the strength becomes too high, and the workability is impaired, and the bake hardenability is saturated, which is expensive. This is because it will not be economically viable.
Moreover, this range further increases the Mo concentration,
9.8 x 10 -5 x k 2 + 2.47 x 10 -3 x k + 0.12 ≤ Mo% ≤ 1.73 x 10 -6 x k 2 + 2.47 x 10 -4 x k-0.016
However, bake hardenability and the delay aging property are improved when satisfying k = A-3600 × d c × n 3/2 -17000 × d c × (2 × d f -d c 2) × A and k ≧ 8 The
As described above, this is considered to be the optimum range in which the interaction between Mo and C is formed. Even if the Mo concentration is increased unnecessarily with respect to C, the effect is saturated, the cost is increased, and the elongation of the steel material may be reduced, so 0.35% was made the upper limit. In addition, recrystallization hardly occurs and elongation may decrease. However, the effect of the present invention is not lost. On the other hand, when the Mo content is 0.005% or less, the age hardening is not improved and the yield point elongation occurs.
[0021]
【Example】
Tables 1 and 2 show examples of the present invention together with comparative examples. Steels having the components shown in Tables 1 and 2 were melted in a converter and then converted into slabs by continuous casting. This slab was hot-rolled / cold-rolled and then annealed to obtain a cold-rolled steel sheet. As an evaluation index of slow aging, it was kept in an atmosphere at 40 ° C. for 70 days, and thereafter a tensile test was performed. The elongation at yield at this time was measured, and 0.02% or less was regarded as having good slow aging. In addition, the bake hardenability was measured by measuring the YP when the cold-rolled steel sheet was pulled 2% and then held at 170 ° C. × 20 min., And the difference from the strength when the 2% tensile test was performed first was measured. . In the present invention, the delayed aging property is 0.01% or less, and the bake hardenability exceeds 50 MPa. In the comparative example, those with less Mo have a slow aging effect and exceed 0.02%, and the bake hardenability also becomes low. In addition, the material with a lot of Mo had good slow aging and bake hardenability, but cracks occurred during pressing.
[0022]
[Table 1]
Table 2 shows the effect of the transition density. Compared to the comparative example, the bake hardenability is improved by about 30 MPa with respect to the k value at the same level. For the transition density in Table 2, thin film specimens were taken from cold-rolled steel sheets, and with a transmission electron microscope, the transition was obtained with a normal observation method for each of the three thin film specimens, and converted to the number of 1 μm 2. The average value was used. In the present invention, the aging property at normal temperature was good at 0.02% or less. Moreover, also about bake hardenability, all show 50 Mpa or more and have shown sufficient value.
[0024]
[Table 2]
【The invention's effect】
According to the present invention, a steel sheet having excellent bake hardenability and slow aging can be obtained.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing the relationship between Mo and k value of the present invention.
Claims (2)
Ｃ：0.01〜0.20 ％
Si：0.005 〜1.5 ％
Mn：0.01〜2.0 ％
Ｐ：0.01〜0.10 ％
Ｓ：≦0.02 ％
Al：≦0.40 ％
Ｎ：0.001 〜0.01 ％
Mo ： 0.005 〜 0.350 ％
残部 Fe 及び不可避的不純物からなり、
ｋ＝Ａ−3600×ｄｃ×ｎ３／２−17000 ×ｄｃ×（２×ｄｆ−ｄｃ ２)×Ａ
なる定数を定めたとき、ｋ≧８であって、かつ、
9.8 ×10−５×ｋ２＋2.47×10−３×ｋ＋0.12≦ Mo ％≦ 1.73 ×10−６×ｋ２＋2.47×10−４×ｋ−0.016 を満足するようにMo を上記範囲内で
添加することを特徴とする遅時効性と焼付硬化性に優れた冷延鋼板。
ここで、Ａは10×√Ｃ−Ｃ／８にて求まる定数、ただし、Ｃは重量濃度で単位は[ppm] 。ｄｃは粒内セメンタイトの平均粒径[mm]、ｎは粒内セメンタイトの平均個数、ｄｆはフェライトの平均粒径[mm]である。% By weight
C: 0.01 to 0.20%
Si: 0.005 to 1.5%
Mn: 0.01-2.0%
P: 0.01-0.10%
S: ≤ 0.02%
Al: ≤0.40%
N: 0.001 to 0.01%
Mo : 0.005 to 0.350 %
It consists of the balance Fe and inevitable impurities,
k = A-3600 × d c × n 3/2 -17000 × d c × (2 × d f -d c 2) × A
When a constant is determined, k ≧ 8, and
9.8 × 10 -5 × k 2 + 2.47 × 10 -3 × k + 0.12 ≦ Mo% ≦ 1.73 × 10 -6 × k 2 + 2.47 × 10 -4 above the Mo to satisfy × k-0.016 A cold-rolled steel sheet excellent in delayed aging and bake hardenability, characterized by being added within the range .
Here, A is a constant determined by 10 × √C−C / 8, where C is the weight concentration and the unit is [ppm]. It is d c average particle diameter of grain cementite [mm], n is the average number, d f of grain cementite having an average particle size of the ferrite [mm].
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