JP2010196115A - High-strength cold-rolled steel sheet excellent in workability and impact resistance and method for manufacturing the same - Google Patents
High-strength cold-rolled steel sheet excellent in workability and impact resistance and method for manufacturing the same Download PDFInfo
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- 239000010960 cold rolled steel Substances 0.000 title claims abstract description 33
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 15
- 238000000034 method Methods 0.000 title abstract description 10
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 55
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 51
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- 238000005098 hot rolling Methods 0.000 claims description 12
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical group [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 11
- 238000005097 cold rolling Methods 0.000 claims description 8
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- 229910000831 Steel Inorganic materials 0.000 description 45
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- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
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Abstract
Description
本発明は、主に自動車の産業分野で使用される骨格部材用および足回り部材用の成形性に優れた高強度冷延鋼板およびその製造方法に関するものである。 The present invention relates to a high-strength cold-rolled steel sheet excellent in formability for skeleton members and suspension members mainly used in the industrial field of automobiles and a method for producing the same.
近年、地球環境の保全の見地から、自動車の燃費向上が重要な課題となっている。このため、車体材料の高強度化により薄肉化を図り、車体そのものを軽量化しようとする動きが活発となってきている。しかしながら、鋼板の高強度化は延性の低下、即ち成形加工性の低下を招くことから、高強度と高加工性を併せ持つ材料の開発が望まれている。 In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of conservation of the global environment. For this reason, a movement to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body has become active. However, increasing the strength of a steel sheet causes a decrease in ductility, that is, a decrease in forming processability, and therefore development of a material having both high strength and high processability is desired.
このような要求に対して、これまでにフェライト、マルテンサイト二相鋼（以下ＤＰ鋼と呼ぶ）や残留オーステナイトの変態誘起塑性を利用したＴＲＩＰ鋼など、種々の複合組織型冷延鋼板が開発されてきた。 In response to such demands, various composite-structure cold-rolled steel sheets have been developed, such as ferrite, martensite duplex steel (hereinafter referred to as DP steel), and TRIP steel using transformation-induced plasticity of retained austenite. I came.
例えば、特許文献１では多量のＳｉを添加することにより残留オーステナイトを確保し高延性を達成する加工性に優れた高強度鋼板の製造方法が開示されている。
しかし、これらＤＰ鋼やＴＲＩＰ鋼は伸び特性には優れるものの穴拡げ性が劣るという問題がある。穴拡げ性は加工穴部を拡張してフランジ成形させるときの加工性を示す指標で、伸び特性と共に高強度鋼板に要求される重要な特性である。
For example, Patent Document 1 discloses a method for producing a high-strength steel sheet excellent in workability that secures retained austenite and achieves high ductility by adding a large amount of Si.
However, although these DP steels and TRIP steels are excellent in elongation characteristics, there is a problem that the hole expandability is inferior. Hole expansibility is an index indicating workability when a processed hole is expanded to form a flange, and is an important characteristic required for high-strength steel sheets together with elongation characteristics.
伸びフランジ性に優れる冷延鋼板の製造方法として、特許文献２には焼鈍均熱後、焼入れ-焼戻しを行いフェライトと焼戻しマルテンサイトの複合組織とすることにより穴拡げ性を向上させる技術が開示されている。しかし、このような技術では高い穴拡げ性が得られるものの、伸びが低いことが問題となる。 As a method for producing a cold-rolled steel sheet having excellent stretch flangeability, Patent Document 2 discloses a technique for improving the hole expandability by annealing and soaking, followed by quenching and tempering to obtain a composite structure of ferrite and tempered martensite. ing. However, although such techniques can provide high hole expansibility, low elongation is a problem.
このように、従来の技術では、優れた伸び特性および伸びフランジ性を兼ね備える冷延鋼板は得られていない。 Thus, in the conventional technology, a cold-rolled steel sheet having both excellent elongation characteristics and stretch flangeability has not been obtained.
本発明は上記の様な問題点に着目してなされたものであって、その目的は延性および伸びフランジ性に優れる高強度冷延鋼板ならびにその製造方法を提供することにある。 The present invention has been made paying attention to the above-described problems, and an object thereof is to provide a high-strength cold-rolled steel sheet excellent in ductility and stretch flangeability, and a method for producing the same.
本発明者らは、上記した課題を達成し、延性および伸びフランジ性に優れる高強度冷延鋼板を製造するため、鋼板の組成およびミクロ組織の観点から鋭意研究を重ねた。その結果、合金元素を適正に調整して、焼鈍過程における均熱温度からの冷却時に１５０〜３５０℃の温度域まで強冷却し、その後再加熱することにより、面積率でフェライトを２０％以上、焼き戻しマルテンサイトを１０〜６０％、体積率で残留オーステナイトを３〜１５％含む組織が得られ、高い延性および伸びフランジ性が可能となることが分った。 In order to achieve the above-described problems and to produce a high-strength cold-rolled steel sheet that is excellent in ductility and stretch flangeability, the present inventors have made extensive studies from the viewpoint of the composition and microstructure of the steel sheet. As a result, the alloy elements are appropriately adjusted, and when cooled from the soaking temperature in the annealing process, it is strongly cooled to a temperature range of 150 to 350 ° C., and then reheated, whereby the ferrite is 20% or more in area ratio, It was found that a structure containing 10 to 60% of tempered martensite and 3 to 15% of retained austenite at a volume ratio was obtained, and high ductility and stretch flangeability became possible.
一般的に残留オーステナイトが存在すると残留オーステナイトのＴＲＩＰ効果により延性が向上する。しかし、歪の付加により残留オーステナイトが変態して生成するマルテンサイトは非常に硬質なものとなり、その結果、主相であるフェライトとの硬度差が大きくなり伸びフランジ性が低下することが知られている。 In general, when retained austenite is present, ductility is improved by the TRIP effect of retained austenite. However, it is known that the martensite produced by the transformation of retained austenite due to the addition of strain becomes very hard, and as a result, the hardness difference from the main phase ferrite increases and the stretch flangeability decreases. Yes.
しかし本発明における成分および組織構成においては、高い延性と高い伸びフランジ性が両立可能となる。残留オーステナイトが存在しても高い伸びフランジ性が可能となる理由について詳細は不明であるが、残留オーステナイトが焼戻しマルテンサイトと共存することにより、残留オーステナイトの伸びフランジ性への悪影響が低減されるためと考えられる。 However, in the components and the structure of the present invention, both high ductility and high stretch flangeability can be achieved. The details of why high stretch flangeability is possible even in the presence of retained austenite are unclear, but the coexistence of retained austenite with tempered martensite reduces the adverse effect of retained austenite on stretch flangeability. it is conceivable that.
さらにマルテンサイト、焼戻しマルテンサイト、残留オーステナイトからなる低温変態相の平均結晶粒径が３μｍ以下の鋼板組織とすることで、高い加工性と併せて耐衝撃特性の向上も可能となることがわかった。
本発明は、上記した知見に基づいてなされたもので、その要旨は以下の通りである。
Furthermore, it was found that the impact resistance characteristics can be improved in addition to high workability by making the steel sheet structure having an average crystal grain size of 3 μm or less of the low temperature transformation phase composed of martensite, tempered martensite and retained austenite. .
The present invention has been made based on the above-described findings, and the gist thereof is as follows.
第一の発明は、質量％で、Ｃ：０．０５〜０．３％、Ｓｉ：０．３〜２．５％、Ｍｎ：０．５〜３．５％、Ｐ：０．００３〜０．１００％、Ｓ：０．０２％以下、Ａｌ：０．０１０〜０．５％を含み、残部が鉄および不可避的不純物からなり、かつ、面積率でフェライトを２０％以上、焼戻しマルテンサイトを１０〜６０％、マルテンサイトを０〜１０％、体積率で残留オーステナイトを３〜１５％含む組織を有する加工性および耐衝撃性に優れた高強度冷延鋼板である。 1st invention is the mass%, C: 0.05-0.3%, Si: 0.3-2.5%, Mn: 0.5-3.5%, P: 0.003-0 100%, S: 0.02% or less, Al: 0.010 to 0.5%, the balance is made of iron and inevitable impurities, ferrite is 20% or more in area ratio, tempered martensite It is a high-strength cold-rolled steel sheet excellent in workability and impact resistance having a structure containing 10 to 60%, martensite 0 to 10%, and volume ratio of retained austenite 3 to 15%.
第二の発明は、前記マルテンサイト、焼戻しマルテンサイト、残留オーステナイトからなる低温変態相の平均結晶粒径が３μｍ以下の組織を有する第一の発明に記載の加工性および耐衝撃性に優れた高強度冷延鋼板である。 The second invention is a highly excellent workability and impact resistance according to the first invention having a structure in which the average crystal grain size of the low-temperature transformation phase comprising the martensite, tempered martensite, and retained austenite is 3 μm or less. It is a strength cold-rolled steel sheet.
第三の発明は、更に、質量%で、Ｃｒ：０．００５〜２．００％、Ｍｏ：０．００５〜２．００％、Ｖ：０．００５〜２．００％、Ｎｉ：０．００５〜２．００％、Ｃｕ：０．００５〜２．００％の中から選ばれる1種または２種以上の元素を含有することを特徴とする第一の発明または第二の発明に記載の加工性および耐衝撃性に優れた高強度冷延鋼板である。 The third invention is further mass%, Cr: 0.005 to 2.00%, Mo: 0.005 to 2.00%, V: 0.005 to 2.00%, Ni: 0.005. The processing according to the first invention or the second invention, characterized by containing one or more elements selected from -2.00%, Cu: 0.005-2.00% It is a high-strength cold-rolled steel sheet excellent in heat resistance and impact resistance.
第四の発明は、更に、質量%で、Ｔｉ：０．０１〜０．２０％、Ｎｂ：０．０１〜０．２０％の中から選ばれる1種または２種の元素を含有することを特徴とする第一〜第三の発明のいずれかに記載の加工性および耐衝撃性に優れた高強度冷延鋼板である。 The fourth invention further contains one or two elements selected from Ti: 0.01-0.20% and Nb: 0.01-0.20% by mass%. A high-strength cold-rolled steel sheet excellent in workability and impact resistance according to any one of the first to third inventions.
第五の発明は、更に、質量％で、Ｂ：０．０００２〜０．００５％を含有することを特徴とする第一〜第四の発明のいずれかに記載の加工性および耐衝撃性に優れた高強度冷延鋼板である。 The fifth invention further comprises B: 0.0002 to 0.005% by mass%, and the workability and impact resistance according to any one of the first to fourth inventions are improved. It is an excellent high-strength cold-rolled steel sheet.
第六の発明は、更に、質量%で、Ｃａ：０．００１〜０．００５％、ＲＥＭ：０．００１〜０．００５％の中から選ばれる1種または2種の元素を含有することを特徴とする第一〜第五の発明のいずれかに記載の加工性および耐衝撃性に優れた高強度冷延鋼板である。 The sixth invention further includes one or two elements selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% by mass%. A high-strength cold-rolled steel sheet excellent in workability and impact resistance according to any one of the first to fifth inventions.
第七の発明は、第一〜第六の発明のいずれかに記載の成分を有するスラブに熱間圧延および冷間圧延を施して製造した冷延鋼板に連続焼鈍を施すに際し、７５０℃以上の温度で１０秒以上保持した後、７５０℃から平均１０℃／ｓ以上の冷却速度で１５０〜３５０℃の温度域まで冷却した後、３５０〜６００℃まで加熱し１０〜６００秒保持した後、室温まで冷却することを特徴とする加工性および耐衝撃性に優れた高強度冷延鋼板の製造方法である。 The seventh invention, when subjecting the slab having the components described in any one of the first to sixth inventions to hot rolling and cold rolling to a cold rolled steel sheet produced by continuous annealing, After holding at temperature for 10 seconds or more, after cooling from 750 ° C. to a temperature range of 150 to 350 ° C. at an average cooling rate of 10 ° C./s or more, heating to 350 to 600 ° C. and holding for 10 to 600 seconds, then room temperature It is a method for producing a high-strength cold-rolled steel sheet excellent in workability and impact resistance, characterized by cooling to a low temperature.
第八の発明は、５００℃〜Ａｃ１変態点における平均加熱速度を１０℃／ｓ以上で昇温することを特徴とする第七の発明に記載の加工性および耐衝撃性に優れた高強度冷延鋼板の製造方法である。 The eighth invention is a high strength excellent in workability and impact resistance according to the seventh invention, characterized in that the average heating rate in the 500 ° C. to Ac 1 transformation point is raised at 10 ° C./s or more. It is a manufacturing method of a cold-rolled steel plate.
本発明によれば加工性に優れた高強度冷延鋼板が得られ、自動車の軽量化と衝突安全性向上との両立を可能とし、自動車車体の高性能化に大きく寄与するという優れた効果を奏する。 According to the present invention, a high-strength cold-rolled steel sheet having excellent workability can be obtained, and it is possible to achieve both the weight reduction of the automobile and the improvement of the collision safety, and the excellent effect of greatly contributing to the enhancement of the performance of the automobile body. Play.
以下、本発明を具体的に説明する。 The present invention will be specifically described below.
１．成分組成について
まず、本発明において鋼の成分組成を上記の範囲に限定した理由について説明する。なお、成分に関する％表示は特に断らない限り質量％を意味するものとする。
1. About a component composition First, the reason which limited the component composition of steel in said range in this invention is demonstrated. In addition, unless otherwise indicated, the% display regarding a component shall mean the mass%.
Ｃ：０．０５〜０．３％
Ｃはオーステナイトを安定化させる元素であり、フェライト以外の相を生成しやすくするため、鋼板強度を上昇させるとともに、組織を複合化してＴＳ-ＥＬバランスを向上させるために必要な元素である。Ｃ量が０．０５％未満では製造条件の最適化を図ったとしてもフェライト以外の相の確保が難しく、ＴＳ×ＥＬが低下する。一方、Ｃ量が０．３％を超えると、溶接部および熱影響部の硬化が著しく、溶接部の機械的特性が劣化する。こうした観点からＣ量は、０．０５〜０．３％の範囲とする。好ましくは０．０８〜０．１５％の範囲である。
C: 0.05-0.3%
C is an element that stabilizes austenite, and is an element necessary for increasing the steel sheet strength and improving the TS-EL balance by increasing the steel sheet strength in order to easily generate phases other than ferrite. If the amount of C is less than 0.05%, it is difficult to secure phases other than ferrite even if the production conditions are optimized, and TS × EL decreases. On the other hand, when the amount of C exceeds 0.3%, the welded portion and the heat affected zone are significantly hardened, and the mechanical properties of the welded portion are deteriorated. From this point of view, the C content is in the range of 0.05 to 0.3%. Preferably it is 0.08 to 0.15% of range.
Ｓｉ：０．３〜２．５％
Ｓｉは鋼の強化に有効な元素である。また、フェライト生成元素であり、オーステナイト中へのＣの濃化促進および炭化物の生成を抑制することから、残留オーステナイトの生成を促進する働きを有する。ここに、Ｓｉ量が０．３％に満たないとその添加効果に乏しくなるので、下限を０．３％とした。ただし過剰な添加は、表面性状、溶接性を劣化させるので、Ｓｉは２．５％以下で含有させるものとした。好ましくは０．７〜２．０％の範囲である。
Si: 0.3-2.5%
Si is an element effective for strengthening steel. Further, it is a ferrite-forming element and has the function of promoting the formation of retained austenite because it promotes the concentration of C in austenite and suppresses the formation of carbides. If the amount of Si is less than 0.3%, the effect of addition becomes poor, so the lower limit was made 0.3%. However, excessive addition deteriorates the surface properties and weldability, so Si was contained at 2.5% or less. Preferably it is 0.7 to 2.0% of range.
Ｍｎ：０．５〜３．５％
Ｍｎは鋼の強化に有効な元素であり、焼戻しマルテンサイト等の低温変態相の生成を促進する。このような作用は、Ｍｎ含有量が０．５％以上で認められる。ただし、Ｍｎを3.5％を超えて過剰に添加すると、第二相分率の過剰な増加や固溶強化によるフェライトの延性劣化が著しくなり成形性が低下する。従って、Ｍｎ量を０．５〜３．５％の範囲とする。好ましくは１．５〜３．０％の範囲である。
Mn: 0.5 to 3.5%
Mn is an element effective for strengthening steel and promotes the generation of low-temperature transformation phases such as tempered martensite. Such an effect is recognized when the Mn content is 0.5% or more. However, if Mn is added excessively exceeding 3.5%, the ductile deterioration of ferrite due to excessive increase of the second phase fraction or solid solution strengthening becomes remarkable and the formability is lowered. Therefore, the Mn content is set to a range of 0.5 to 3.5%. Preferably it is 1.5 to 3.0% of range.
Ｐ：０．００３〜０．１００％
Ｐは鋼の強化に有効な元素であり、この効果は０．００３％以上で得られる。しかし、０．１００％を超えて過剰に添加すると粒界偏析により脆化を引き起こし、耐衝撃性を劣化させる。従って、Ｐ量は０．００３％〜０．１００％の範囲とする。
P: 0.003 to 0.100%
P is an element effective for strengthening steel, and this effect is obtained at 0.003% or more. However, excessive addition over 0.100% causes embrittlement due to grain boundary segregation and degrades impact resistance. Therefore, the P content is in the range of 0.003% to 0.100%.
Ｓ：０．０２％以下
ＳはＭｎＳなどの介在物となって、耐衝撃特性の劣化や溶接部のメタルフローに沿った割れの原因になるので極力低い方が良いが、製造コストの面から０．０２％以下とする。
S: 0.02% or less S is an inclusion such as MnS, which causes deterioration of impact resistance and cracks along the metal flow of the weld. 0.02% or less.
Ａｌ：０．０１０〜０．５％
Ａｌは脱酸剤として作用し、鋼の清浄度に有効な元素であり、脱酸工程で添加することが好ましい。ここに、Ａｌ量が０．０１％に満たないとその添加効果に乏しくなるので、下限を０．０１％とした。しかし多量に添加すると連続鋳造時の鋼片割れ発生の危険性が高まり製造性を低下させる。従ってAlの添加量の上限は０．５％とする。
Al: 0.010 to 0.5%
Al acts as a deoxidizer and is an element effective for the cleanliness of steel, and is preferably added in the deoxidation step. If the amount of Al is less than 0.01%, the effect of addition becomes poor, so the lower limit was made 0.01%. However, if added in a large amount, the risk of steel piece cracking during continuous casting increases and the productivity decreases. Therefore, the upper limit of the amount of Al added is 0.5%.
本発明における高強度冷延鋼板は、上記の成分組成を基本成分とし、残部は鉄および不可避的不純物からなるが、所望の特性に応じて、以下に述べる成分を適宜含有させることができる。 The high-strength cold-rolled steel sheet in the present invention has the above-described component composition as a basic component, and the balance is composed of iron and unavoidable impurities. The following components can be appropriately contained according to desired characteristics.
Ｃｒ：０．００５〜２．００％、Ｍｏ：０．００５〜２．００％、Ｖ：０．００５〜２．００％、Ｎｉ：０．００５〜２．００％、Ｃｕ：０．００５〜２．００％の中から選ばれる1種または２種以上
Ｃｒ、Ｍｏ、Ｖ、Ｎｉ、Ｃｕは焼鈍温度からの冷却時にパーライトの生成を抑制し、低温変態相の生成を促進し鋼の強化に有効に働く。この効果は、Ｃｒ、Ｍｏ、Ｖ、Ｎｉ、Ｃｕ の少なくとも1種を０．００５％以上含有させることで得られる。しかし、Ｃｒ、Ｍｏ、Ｖ、Ｎｉ、Ｃｕ のそれぞれの成分が２．００％を超えるとその効果は飽和し、コストアップの要因となる。従ってＣｒ、Ｍｏ、Ｖ、Ｎｉ、Ｃｕの量はそれぞれ、、０．００５〜２．００％の範囲とする。
Cr: 0.005-2.00%, Mo: 0.005-2.00%, V: 0.005-2.00%, Ni: 0.005-2.00%, Cu: 0.005- One or more selected from 2.00% Cr, Mo, V, Ni, Cu suppresses the formation of pearlite during cooling from the annealing temperature, promotes the formation of low-temperature transformation phase, and strengthens the steel Works effectively. This effect is obtained by adding 0.005% or more of at least one of Cr, Mo, V, Ni, and Cu. However, when each component of Cr, Mo, V, Ni, and Cu exceeds 2.00%, the effect is saturated, resulting in an increase in cost. Accordingly, the amounts of Cr, Mo, V, Ni, and Cu are each in the range of 0.005 to 2.00%.
Ｔｉ：０．０１〜０．２０％、Ｎｂ：０．０１〜０．２０％の中から選ばれる1種または２種
Ｔｉ、Ｎｂは炭窒化物を形成し、鋼を析出強化により高強度化する作用を有する。このような効果はそれぞれ０．０１％以上で認められる。一方、Ｔｉ、Ｎｂはそれぞれ０．２０％を超えて含有しても、過度に高強度化し、延性が低下する。このため、Ｔｉ、Ｎｂの量はそれぞれ０．０１〜０．２０％の範囲とする。
One or two types selected from Ti: 0.01-0.20%, Nb: 0.01-0.20% Ti, Nb forms carbonitrides and strengthens steel by precipitation strengthening Have the effect of Such an effect is recognized at 0.01% or more. On the other hand, even if Ti and Nb are contained in amounts exceeding 0.20%, the strength is excessively increased and the ductility is lowered. For this reason, the amounts of Ti and Nb are each in the range of 0.01 to 0.20%.
Ｂ：０．０００２〜０．００５％
Ｂはオーステナイト粒界からのフェライトの生成を抑制し強度を上昇させる作用を有する。その効果は０．０００２％以上で得られる。しかし、Ｂ量が０．００５％を超えるとその効果は飽和し、コストアップの要因となる。従って、Ｂ量は０．０００２〜０．００５％の範囲とする。
B: 0.0002 to 0.005%
B has the effect of suppressing the formation of ferrite from the austenite grain boundaries and increasing the strength. The effect is obtained at 0.0002% or more. However, if the amount of B exceeds 0.005%, the effect is saturated, which causes a cost increase. Therefore, the B amount is in the range of 0.0002 to 0.005%.
Ｃａ：０．００１〜０．００５％、ＲＥＭ：０．００１〜０．００５％の中から選ばれる1種または2種
Ｃａ、ＲＥＭはいずれも硫化物の形態制御により加工性を改善する効果を有しており、必要に応じてＣａ、ＲＥＭの1種または2種を０．００１％以上含有させることができる。しかしながら過剰な添加は清浄度に悪影響を及ぼす恐れがあるため、それぞれ０．００５％以下とする。
One or two types selected from Ca: 0.001 to 0.005%, REM: 0.001 to 0.005% Ca and REM both have the effect of improving workability by controlling the form of sulfide. If necessary, one or two of Ca and REM can be contained in an amount of 0.001% or more. However, excessive addition may adversely affect cleanliness, so each content is made 0.005% or less.
２．組織について
次に鋼の組織について説明する。
2. About structure Next, the structure of steel will be described.
フェライトの面積率：２０％以上
フェライトの面積率が２０％未満だとＴＳ×ＥＬが低下するため２０％以上とする。好ましくは５０％以上である。
Ferrite area ratio: 20% or more If the area ratio of ferrite is less than 20%, TS × EL decreases, so it is 20% or more. Preferably it is 50% or more.
焼戻しマルテンサイトの面積率：１０〜６０％
焼戻しマルテンサイトとはマルテンサイトをＡｃ１変態点以下、好ましくはＡｃ１変態点よりも低い温度に加熱して得られる転位密度の高いフェライトとセメンタイトとの複合組織を示し、鋼の強化に有効に働く。また、マルテンサイトをＡｃ１変態点を超える温度に加熱して得られる組織は、フェライト中にセメンタイトを含まない組織であり、本願発明で意図する焼戻しマルテンサイトとは基本的に異なるものである。
Tempered martensite area ratio: 10-60%
Hereinafter Ac 1 transformation point martensite and tempered martensite, preferably an complex structure of high ferrite and cementite dislocation density obtained by heating to a temperature lower than the Ac 1 transformation point, effectively strengthening the steel work. Further, the structure obtained by heating martensite to a temperature exceeding the Ac 1 transformation point is a structure not containing cementite in the ferrite, and is basically different from the tempered martensite intended in the present invention.
また、焼戻しマルテンサイトはマルテンサイトに比べて穴拡げ性への悪影響が小さく、顕著な穴拡げ性の低下なしに強度を確保するのに有効な相である。焼戻しマルテンサイトの面積率が１０％未満では強度確保が困難となり、６０％を超えるとＴＳ×ＥＬが低下するため、焼戻しマルテンサイトの面積率は１０〜６０％とする。 Further, tempered martensite has a smaller adverse effect on hole expandability than martensite, and is an effective phase for ensuring strength without a significant decrease in hole expandability. If the area ratio of tempered martensite is less than 10%, it is difficult to ensure strength, and if it exceeds 60%, TS × EL decreases, so the area ratio of tempered martensite is 10 to 60%.
マルテンサイトの面積率：０〜１０％
マルテンサイトは鋼の高強度化に有効に働くが、その面積率が１０％を超えると伸びフランジ性が顕著に低下する。従って、マルテンサイトの面積率は０〜１０％とする。
Martensite area ratio: 0-10%
Martensite works effectively to increase the strength of steel, but if the area ratio exceeds 10%, stretch flangeability is significantly reduced. Therefore, the area ratio of martensite is set to 0 to 10%.
残留オーステナイトの体積率：３〜１５％
残留オーステナイトは鋼の強化に寄与するだけでなく、鋼のＴＳ×ＥＬの向上およびに有効に働く。このような効果は体積率が３％以上で得られる。また、残留オーステナイトが１５％を超えると穴拡げ性が低下する。従って、残留オーステナイトの体積率は３〜１５％とする。
Volume ratio of retained austenite: 3 to 15%
Residual austenite not only contributes to the strengthening of the steel, but also works effectively to improve and improve the TS × EL of the steel. Such an effect is obtained when the volume ratio is 3% or more. On the other hand, when the retained austenite exceeds 15%, the hole expansibility decreases. Therefore, the volume ratio of retained austenite is 3 to 15%.
マルテンサイト、焼戻しマルテンサイト、残留オーステナイトからなる低温変態相の平均結晶粒径：３μｍ以下
マルテンサイト、焼戻しマルテンサイト、残留オーステナイトからなる低温変態相は耐衝撃特性の向上に有効に働く。特に低温変態相を微細に分散させることにより耐衝撃特性が向上し、低温変態相の平均結晶粒径が３μｍ以下でその効果が顕著となる。従って低温変態相の平均結晶粒径を３μｍ以下とする。
Average crystal grain size of low-temperature transformation phase composed of martensite, tempered martensite, and retained austenite: 3 μm or less The low-temperature transformation phase composed of martensite, tempered martensite, and retained austenite works effectively to improve impact resistance. In particular, the impact resistance is improved by finely dispersing the low-temperature transformation phase, and the effect becomes remarkable when the average crystal grain size of the low-temperature transformation phase is 3 μm or less. Therefore, the average crystal grain size of the low temperature transformation phase is set to 3 μm or less.
また、フェライト、焼戻しマルテンサイト、マルテンサイト、残留オーステナイト以外の相としてはパーライトおよびベイナイトを含む可能性があるが、上記相構成を満たしていれば問題ない。ただし、延性および穴拡げ性確保の観点からパーライトは３％以下とすることが望ましい。 Further, phases other than ferrite, tempered martensite, martensite and retained austenite may contain pearlite and bainite, but there is no problem as long as the above phase structure is satisfied. However, from the viewpoint of securing ductility and hole expansibility, pearlite is desirably 3% or less.
３．製造条件について
上記の成分組成に調整した鋼を転炉などで溶製し、連続鋳造法等でスラブとする。この鋼素材を熱間圧延および冷間圧延を施した後、連続焼鈍を行う。鋳造、熱延圧延、冷間圧延については特に製造方法を限定するものではないが、以下に好適な製造方法について説明する
鋳造条件
使用する鋼スラブは、成分のマクロ偏析を防止するために連続鋳造法で製造するのが好ましいが、造塊法、薄スラブ鋳造法で製造してもよい。また、鋼スラブを製造したのち、いったん室温まで冷却し、その後再度加熱する従来法に加え、室温まで冷却しないで、温片のままで加熱炉に挿入する、あるいはわずかの保熱をおこなった後に直ちに圧延する直送圧延・直接圧延などの省エネルギープロセスも問題なく適用できる。
3. Manufacturing conditions Steel adjusted to the above component composition is melted in a converter or the like, and is made into a slab by a continuous casting method or the like. This steel material is subjected to continuous annealing after hot rolling and cold rolling. For casting, hot rolling, and cold rolling, the manufacturing method is not particularly limited, but a suitable manufacturing method is described below. Casting conditions The steel slab used is continuously cast to prevent macro segregation of components. However, it may be produced by an ingot-making method or a thin slab casting method. After manufacturing the steel slab, in addition to the conventional method of cooling to room temperature and then heating again, without cooling to room temperature, insert it into a heating furnace as it is, or carry out slight heat retention Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately, can be applied without any problem.
熱間圧延条件
スラブ加熱温度：１１００℃以上
スラブ加熱温度は、低温加熱がエネルギー的には好ましいが、加熱温度が１１００℃未満では、炭化物が十分に固溶できなかったり、圧延荷重の増大による熱間圧延時のトラブル発生の危険が増大するなどの問題が生じる。なお、酸化重量の増加にともなうスケールロスの増大などから、スラブ加熱温度は１３００℃以下とすることが望ましい。
Hot rolling conditions Slab heating temperature: 1100 ° C or higher As for the slab heating temperature, low-temperature heating is preferable in terms of energy, but if the heating temperature is less than 1100 ° C, carbides cannot be sufficiently dissolved or heat due to an increase in rolling load. Problems such as an increased risk of trouble during hot rolling occur. Note that the slab heating temperature is desirably 1300 ° C. or less because of an increase in scale loss accompanying an increase in oxidized weight.
なお、スラブ加熱温度を低くしても熱間圧延時のトラブルを防止するといった観点から、シートバーを加熱する、いわゆるシートバーヒーターを活用してもよい。 From the viewpoint of preventing troubles during hot rolling even if the slab heating temperature is lowered, a so-called sheet bar heater that heats the sheet bar may be used.
仕上圧延終了温度：Ａｒ３変態点以上
仕上げ圧延終了温度がＡｒ３変態点未満では、圧延中にフェライトとオーステナイトが生成して、鋼板にバンド状組織が生成し易くなり、かかるバンド状組織は冷間圧延後や焼鈍後にも残留し、材料特性に異方性を生じさせたり、加工性を低下させる原因となる場合がある。このため、仕上げ圧延温度はＡｒ３変態点以上とすることが望ましい。
Finish rolling end temperature: Ar 3 transformation point or higher If the finish rolling end temperature is less than the Ar 3 transformation point, ferrite and austenite are generated during rolling, and a band-like structure is easily generated in the steel sheet. It may remain even after hot rolling or after annealing, causing anisotropy in material properties or reducing workability. For this reason, it is desirable that the finish rolling temperature is not less than the Ar 3 transformation point.
巻取り温度：４５０〜７００℃
巻取り温度が４５０℃未満だと巻取り温度の制御が難しく温度ムラが生じやすくなり、その結果冷間圧延性が低下するなどの問題が生じることがある。また巻取り温度が７００℃を超えると地鉄表層で脱炭が生じるなどの問題が起こることがある。このため、巻取り温度は４５０〜７００℃の範囲とするのが望ましい。
Winding temperature: 450-700 ° C
When the coiling temperature is less than 450 ° C., it is difficult to control the coiling temperature, and temperature unevenness is likely to occur. As a result, problems such as a decrease in cold rolling property may occur. In addition, when the coiling temperature exceeds 700 ° C., problems such as decarburization may occur in the surface layer of the railway. For this reason, it is desirable that the coiling temperature be in the range of 450 to 700 ° C.
なお、本発明における熱延工程では、熱間圧延時の圧延荷重を低減するために仕上圧延の一部または全部を潤滑圧延としてもよい。潤滑圧延を行うことは、鋼板形状の均一化、材質の均一化の観点からも有効である。なお、潤滑圧延の際の摩擦係数は０．２５〜０．１０の範囲とすることが好ましい。また、相前後するシートバー同士を接合し、連続的に仕上圧延する連続圧延プロセスとすることが好ましい。連続圧延プロセスを適用することは、熱間圧延の操業安定性の観点からも望ましい。 In the hot rolling process of the present invention, part or all of finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
ついで、好ましくは熱延鋼板の表面の酸化スケールを酸洗により除去した後、冷間圧延に供して所定の板厚の冷延鋼板とする。ここに酸洗条件や冷間圧延条件は特に制限されるものではなく、常法に従えば良い。冷間圧延の圧下率は４０％以上とすることが好ましい。 Next, preferably after removing the oxidized scale on the surface of the hot-rolled steel sheet by pickling, it is subjected to cold rolling to obtain a cold-rolled steel sheet having a predetermined thickness. Here, pickling conditions and cold rolling conditions are not particularly limited, and may be in accordance with conventional methods. The rolling reduction of cold rolling is preferably 40% or more.
５００℃〜Ａｃ１変態点における平均加熱速度：１０℃／ｓ以上
本発明の鋼における再結晶温度域である５００℃からＡｃ１変態点における平均加熱速度を１０℃／ｓ以上とすることで、加熱昇温時の再結晶が抑制され、Ａｃ１変態点以上で生成するオーステナイトの微細化、ひいては焼鈍冷却後の組織の微細化に有効に働き、低温変態相の平均粒経を３μｍ以下とすることが可能となる。
Average heating rate at 500 ° C. to Ac 1 transformation point: the average heating rate in the Ac 1 transformation point from 500 ° C. a recrystallization temperature region at 10 ° C. / s or more steel of the present invention by a 10 ° C. / s or higher, Recrystallization at the time of heating and heating is suppressed, and it effectively works for refining austenite generated at the Ac 1 transformation point or higher, and thus for refining the structure after annealing and cooling, and the average grain size of the low temperature transformation phase is 3 μm or less. It becomes possible.
平均加熱速度が１０℃／ｓ未満では、加熱昇温時にαの再結晶が起こり、フェライト中に導入された歪が開放され十分な微細化が達成できなくなる。従って、５００℃〜Ａｃ１変態点における平均加熱速度を１０℃／ｓ以上とした。該平均加熱速度の好ましい範囲は２０℃／ｓ以上である。 If the average heating rate is less than 10 ° C./s, recrystallization of α occurs at the time of heating and heating, the strain introduced into the ferrite is released, and sufficient refinement cannot be achieved. Therefore, the average heating rate from 500 ° C. to Ac 1 transformation point is set to 10 ° C./s or more. A preferable range of the average heating rate is 20 ° C./s or more.
７５０℃以上の温度で１０秒以上保持
加熱温度が７５０℃未満あるいは保持時間が１０秒未満では、焼鈍時のオーステナイトの生成が不十分となり、焼鈍冷却後に十分な量の低温変態相が確保できなくなる。保持温度および保持時間の上限は特に規定しないが、保持温度が９００℃以上および保持時間が６００秒以上では効果が飽和する上、コストアップにつながるので、保持温度は９００℃未満および保持時間は６００秒未満が好ましい。
Holding at a temperature of 750 ° C. or higher for 10 seconds or longer If the heating temperature is lower than 750 ° C. or holding time is shorter than 10 seconds, austenite is not sufficiently generated during annealing, and a sufficient amount of low-temperature transformation phase cannot be secured after annealing cooling. . The upper limit of the holding temperature and holding time is not particularly specified, but if the holding temperature is 900 ° C. or more and the holding time is 600 seconds or more, the effect is saturated and the cost is increased. Therefore, the holding temperature is less than 900 ° C. and the holding time is 600 Less than a second is preferred.
７５０℃から１０℃／ｓ以上の平均冷却速度で１５０〜３５０℃の温度域まで冷却
７５０℃からの冷却速度が１０℃／ｓ未満ではパーライトが生成し、ＴＳ×ＥＬおよび穴拡げ性が低下する。従って、７５０℃からの冷却速度は１０℃／ｓ以上とする。冷却到達温度条件は本技術で最も重要な条件の一つである。冷却停止時にはオーステナイトの一部がマルテンサイトに変態し、残りは未変態のオーステナイトとなる。そこから再加熱し、めっき・合金化処理後、室温まで冷却することで、マルテンサイトは焼戻しマルテンサイトとなり、未変態オーステナイトは残留オーステナイトまたはマルテンサイトとなる。焼鈍からの冷却到達温度が低いほど、冷却中に生成するマルテンサイト量が増加し、未変態オーステナイト量が減少するため、冷却到達温度の制御により、最終的なマルテンサイトおよび残留オーステナイトと焼戻しマルテンサイトの面積率が決定される。
From 750 ° C. to an average cooling rate of 10 ° C./s or more to a temperature range of 150 to 350 ° C. When the cooling rate from 750 ° C. is less than 10 ° C./s, pearlite is generated, and TS × EL and hole expansibility decrease. . Therefore, the cooling rate from 750 ° C. is set to 10 ° C./s or more. Cooling temperature condition is one of the most important conditions in this technology. When the cooling is stopped, a part of austenite is transformed into martensite, and the rest becomes untransformed austenite. By reheating from there and cooling to room temperature after plating and alloying treatment, martensite becomes tempered martensite and untransformed austenite becomes retained austenite or martensite. The lower the temperature reached from the annealing, the lower the amount of martensite generated during cooling and the lower the amount of untransformed austenite. Is determined.
冷却到達温度が３５０℃より高い温度では、冷却停止時のマルテンサイト変態が不十分で未変態オーステナイト量が多くなり、最終的なマルテンサイトまたは残留オーステナイトが過剰に生成し、穴拡げ性を低下させる。また、冷却到達温度が１５０℃より低くなると、冷却中にオーステナイトがほとんどマルテンサイトに変態し未変態オーステナイト量が減少し、３％以上の残留オーステナイトが得られない。従って冷却到達温度は１５０〜３５０℃の範囲とする。冷却の方法については、目標の冷却速度と冷却停止温度が達成できれば、ガスジェット冷却、ミスト冷却、水冷、メタルクエンチ等のいかなる冷却方法を用いても良い。 When the temperature reached by cooling is higher than 350 ° C., the martensite transformation at the time of cooling stop is insufficient and the amount of untransformed austenite increases, resulting in excessive formation of final martensite or residual austenite, which decreases hole expandability. . On the other hand, when the temperature reached by cooling is lower than 150 ° C., most of the austenite is transformed into martensite during cooling and the amount of untransformed austenite is reduced, and 3% or more of retained austenite cannot be obtained. Therefore, the cooling ultimate temperature is in the range of 150 to 350 ° C. As for the cooling method, any cooling method such as gas jet cooling, mist cooling, water cooling, or metal quenching may be used as long as the target cooling rate and cooling stop temperature can be achieved.
３５０〜６００℃まで加熱し１０〜６００秒保持
１５０〜３５０℃の温度範囲までの冷却後、３５０〜６００℃の温度範囲で１０秒以上保持することで、前記冷却時に生成したマルテンサイトが焼戻され焼戻しマルテンサイトとなることで、穴拡げ性が向上し、さらに前記冷却時にマルテンサイトに変態しなかった未変態オーステナイトが安定化され、最終的に３％以上の残留オーステナイトが得られ、延性が向上する。
After heating to 350 to 600 ° C. and holding for 10 to 600 seconds and cooling to a temperature range of 150 to 350 ° C., the martensite generated during the cooling is tempered by holding at 350 to 600 ° C. for 10 seconds or more. By being tempered martensite, the hole expandability is improved, and untransformed austenite that has not been transformed into martensite at the time of cooling is stabilized, and finally 3% or more of retained austenite is obtained, and ductility is improved. improves.
再加熱保持による未変態オーステナイトの安定化のメカニズムについて詳細は不明であるが、過飽和にＣが固溶したマルテンサイトから未変態オーステナイトにＣが拡散して未変態オーステナイトへのＣの濃化が進み、オーステナイトが安定化されるものと考えられる。その際、マルテンサイト中のセメンタイトの析出がＣの拡散よりも早く進行すると未変態オーステナイトへのＣの濃化が不十分となるため、セメンタイト析出を遅延させることが重要であり、そのために０．３％以上のＳｉ添加が必要となる。 The details of the mechanism of stabilization of untransformed austenite by reheating and holding are unknown, but C is diffused from martensite, which is super-saturated with C in solid solution, to untransformed austenite, and the concentration of C to untransformed austenite proceeds. It is considered that austenite is stabilized. At that time, if the precipitation of cementite in martensite proceeds faster than the diffusion of C, the concentration of C in the untransformed austenite becomes insufficient. Therefore, it is important to delay the precipitation of cementite. It is necessary to add 3% or more of Si.
再加熱温度が３５０℃未満ではマルテンサイトの焼戻しおよびオーステナイトの安定化が不十分となり穴拡げ性および延性が低下する。また再加熱温度が６００℃を超えると、冷却停止時の未変態オーステナイトがパーライトに変態し、最終的に３％以上残留オーステナイトが得られなくなる。従って、加熱温度は３５０〜６００℃とする。 When the reheating temperature is less than 350 ° C., the tempering of martensite and the stabilization of austenite become insufficient, and the hole expansibility and ductility are lowered. On the other hand, when the reheating temperature exceeds 600 ° C., untransformed austenite at the time of cooling stop is transformed into pearlite, and finally 3% or more of retained austenite cannot be obtained. Accordingly, the heating temperature is 350 to 600 ° C.
保持時間が１０秒未満ではオーステナイトの安定化が不十分となり、また６００秒を超えると冷却停止時の未変態オーステナイトがベイナイトに変態し、最終的に３％以上の残留オーステナイトが得られなくなる。従って、再加熱温度は３５０〜６００℃の範囲とし、その温度域での保持時間は１０〜６００秒とする。 If the holding time is less than 10 seconds, the stabilization of austenite becomes insufficient, and if it exceeds 600 seconds, untransformed austenite at the time of cooling stop is transformed into bainite, and finally 3% or more of retained austenite cannot be obtained. Therefore, the reheating temperature is in the range of 350 to 600 ° C., and the holding time in that temperature range is 10 to 600 seconds.
なお、焼鈍後の鋼板には、形状矯正、表面粗度等の調整のため調質圧延を加えてもよい。また、樹脂あるいは油脂コーティング、各種塗装等の処理を施しても何ら不都合はない。 In addition, you may add temper rolling to the steel plate after annealing for adjustment of shape correction, surface roughness, etc. In addition, there is no inconvenience even if treatments such as resin or oil coating and various paintings are applied.
表１に示す成分組成を有し、残部がＦｅおよび不可避的不純物よりなる鋼を転炉にて溶製し、連続鋳造法にて鋳片とした。得られた鋳片を板厚３．０ｍｍまで熱間圧延した。熱間圧延の条件は仕上げ温度９００℃、圧延後の冷却速度１０℃／ｓ、巻取り温度６００℃で行った。次いで、熱延鋼板を酸洗した後、板厚１．２ｍｍまで冷間圧延し、冷延鋼板を製造した。 Steel having the composition shown in Table 1 and the balance being Fe and unavoidable impurities was melted in a converter and made into a slab by a continuous casting method. The obtained slab was hot-rolled to a plate thickness of 3.0 mm. The hot rolling conditions were a finishing temperature of 900 ° C., a cooling rate after rolling of 10 ° C./s, and a winding temperature of 600 ° C. Next, the hot-rolled steel sheet was pickled and then cold-rolled to a thickness of 1.2 mm to produce a cold-rolled steel sheet.
次いで、これら冷延鋼板に、連続焼鈍ラインにて、表２に示す条件で焼鈍処理を施した。
得られた鋼板の断面ミクロ組織、引張特性および穴拡げ性について調査を行い、その結果を表３に示した。
Subsequently, these cold-rolled steel sheets were subjected to an annealing treatment in the continuous annealing line under the conditions shown in Table 2.
The obtained steel sheet was examined for cross-sectional microstructure, tensile properties and hole expansibility, and the results are shown in Table 3.
鋼板の断面ミクロ組織は３％ナイタール溶液（３％硝酸+エタノール）で組織を現出し、走査型電子顕微鏡で深さ方向板厚１／４位置を観察して、撮影した組織写真を用いて、画像解析処理を行ない、フェライト相の分率を定量化した（なお、画像解析処理は市販の画像処理ソフトを用いることができる）。マルテンサイト面積率、焼戻しマルテンサイト面積率は、組織の細かさに応じて１０００〜３０００倍の適切な倍率のＳＥＭ写真を撮影し、画像処理ソフトで定量化した。低温変態相の平均粒径は、観察した視野の低温変態相の面積を低温変態相の個数で割り、平均面積を求め、その１／２乗を平均粒径とした。 The cross-sectional microstructure of the steel sheet is revealed with a 3% nital solution (3% nitric acid + ethanol), the thickness direction 1/4 thickness position is observed with a scanning electron microscope, and the photographed structure photograph is used. Image analysis processing was performed to quantify the ferrite phase fraction (in addition, commercially available image processing software can be used for image analysis processing). The martensite area ratio and tempered martensite area ratio were quantified with image processing software by taking SEM photographs at an appropriate magnification of 1000 to 3000 times depending on the fineness of the structure. The average particle size of the low-temperature transformation phase was obtained by dividing the area of the low-temperature transformation phase in the observed visual field by the number of low-temperature transformation phases to obtain an average area, and the 1/2 power was taken as the average particle size.
残留オーステナイトの体積率は、鋼板を板厚方向の１／４面まで研磨し、この板厚１／４面の回折Ｘ線強度により求めた。入射Ｘ線にはＭｏＫα線を使用し、残留オーステナイト相の｛１１１｝、｛２００｝、｛２２０｝、｛３１１｝面とフェライト相の｛１１０｝、｛２００｝、｛２１１｝面のピークの積分強度の全ての組み合わせについて強度比を求め、これらの平均値を残留オーステナイトの体積率とした。 The volume ratio of retained austenite was determined by polishing the steel plate to a ¼ surface in the plate thickness direction and diffracting X-ray intensity of the ¼ surface thickness. MoKα rays are used as incident X-rays, and the peaks of {111}, {200}, {220}, {311} planes of retained austenite and {110}, {200}, {211} planes of ferrite phases are used. Intensity ratios were determined for all combinations of integrated intensities, and the average value of these ratios was taken as the volume fraction of retained austenite.
また、引張特性は、引張方向が鋼板の圧延方向と直角方向となるようサンプル採取したＪＩＳ５号試験片を用いて、ＪＩＳＺ２２４１に準拠した引張試験を行ない、ＴＳ（引張強さ）、ＥＬ（伸び）を測定し、強度と伸びの積（ＴＳ×ＥＬ）で表される強度ー伸びバランスの値を求めた。 In addition, the tensile properties were determined by performing a tensile test in accordance with JISZ2241, using a JIS No. 5 test piece sampled so that the tensile direction was perpendicular to the rolling direction of the steel sheet, and TS (tensile strength) and EL (elongation). The strength-elongation balance value represented by the product of strength and elongation (TS × EL) was determined.
さらに、伸びフランジ性を評価する指標として、穴拡げ率λを測定した。穴拡げ率λは日本鉄鋼連盟規格ＪＦＳＴ１００１に準じた穴拡げ試験を行ない、打ち抜き時の穴の初期径（１０ｍｍφ）と、これを穴拡げ加工を行なった際に穴縁の亀裂が板厚を貫通した時点の穴の径との比から求めた。 Furthermore, the hole expansion rate λ was measured as an index for evaluating stretch flangeability. The hole expansion rate λ is determined by performing a hole expansion test in accordance with the Japan Iron and Steel Federation Standard JFST1001, and the hole diameter at the time of punching and punching through the hole thickness penetrates the plate thickness. It was determined from the ratio with the diameter of the hole at the time of the test.
衝撃吸収特性は、鋼板の圧延方向と直角方向から採取した平行部の幅５ｍｍ、長さ７ｍｍの試験片を用い、歪速度２０００／ｓで引張試験を行って採取した応力-真歪曲線を歪量０〜１０％の範囲で積分することにより、吸収エネルギーを算出して評価した（鉄と鋼、８３（１９９７）ｐ７４８参照）。 The shock absorption characteristic is obtained by distorting a stress-true strain curve obtained by performing a tensile test at a strain rate of 2000 / s using a test piece having a width of 5 mm and a length of 7 mm of a parallel portion taken from a direction perpendicular to the rolling direction of the steel sheet. Absorption energy was calculated and evaluated by integrating in the range of 0 to 10% (see Iron and Steel, 83 (1997) p748).
本発明例の鋼板はＴＳ×ＥＬが２２０００ＭＰａ・％以上、λが７０％以上の優れた強度、延性および伸びフランジ性を示す。 The steel sheet of the present invention exhibits excellent strength, ductility and stretch flangeability with TS × EL of 22000 MPa ·% or more and λ of 70% or more.
これに対し本発明の範囲をはずれる比較例の鋼板はＴＳ×ＥＬが２２０００ＭＰａ・％未満および（または）λが７０％未満となり、本発明例の鋼板のような優れた強度、延性および伸びフランジ性が得られない。さらに、低温変態相の平均粒径を３μｍ以下とすることで吸収エネルギーとＴＳとの比（ＡＥ／ＴＳ）が０．０６３以上の優れた耐衝撃特性が得られる。 On the other hand, the steel plate of the comparative example which deviates from the scope of the present invention has TS × EL of less than 22000 MPa ·% and / or λ of less than 70%, and has excellent strength, ductility and stretch flangeability like the steel plate of the present invention. Cannot be obtained. Furthermore, by setting the average particle size of the low-temperature transformation phase to 3 μm or less, excellent impact resistance characteristics with a ratio of absorbed energy to TS (AE / TS) of 0.063 or more can be obtained.
本発明は、加工性および耐衝撃性に優れた高強度冷延鋼板として自動車の軽量化、低燃費化に貢献することができる。 INDUSTRIAL APPLICABILITY The present invention can contribute to reducing the weight and fuel consumption of automobiles as a high-strength cold-rolled steel sheet having excellent workability and impact resistance.
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