JP5811020B2 - High-strength steel sheet with high toughness, high workability and formability, and excellent delayed fracture characteristics due to hydrogen embrittlement - Google Patents
High-strength steel sheet with high toughness, high workability and formability, and excellent delayed fracture characteristics due to hydrogen embrittlement Download PDFInfo
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本発明は、強度が必要とされる自動車の構造部材や補強部材に使用されるような高強度鋼板に関し、特にホットスタンプ後の強度と、靱性、遅れ破壊特性に優れたホットスタンプ用鋼板に関するものである。 The present invention relates to a high-strength steel sheet used for structural members and reinforcing members of automobiles that require strength, and particularly to a steel sheet for hot stamping excellent in strength, toughness, and delayed fracture characteristics after hot stamping. It is.
近年、環境保護及び省資源化の観点から自動車車体軽量化は喫緊の課題であり、それに対して高強度鋼板を適用する検討が積極的に行われており、その鋼材強度も益々高まっている。しかし、鋼板強度が高くなるに伴い加工性が劣化するとともに、形状凍結性への配慮が必要となる。また、通常使用されるプレス加工において、その成形荷重は鋼材強度と共に益々高まっており、プレス能力上も実用化に向けて大きな課題である。 In recent years, the weight reduction of automobile bodies is an urgent issue from the viewpoint of environmental protection and resource saving, and studies on applying high-strength steel sheets are being actively carried out, and the strength of steel materials is also increasing. However, as the steel plate strength increases, workability deteriorates and consideration for shape freezing property is required. Moreover, in the press work normally used, the forming load is increasing with the strength of the steel material, and the pressing ability is also a big issue for practical use.
このような課題を解決するために、温間で成形しその際の熱を利用して強度上昇させる技術が特許文献1に開示されている。この技術では鋼中成分を適切に制御し200〜850℃の温度で保持・成形加工し、この温度域での析出強化を利用して強度を上昇させることを狙っており、いわゆる焼き入れにより強度を確保することはしていない。
また、特許文献2ではプレス成形精度を向上させる目的で温間プレス時での降伏強度を低く、常温での降伏強度を高くする高強度鋼板が提案されている。
しかしながら、これらの技術では得られる強度に限度がある。
In order to solve such a problem,
Patent Document 2 proposes a high-strength steel sheet that has low yield strength during warm pressing and high yield strength at room temperature for the purpose of improving press forming accuracy.
However, these techniques have limitations on the strength that can be obtained.
一方、ホットスタンプは、より高強度を得る目的で、鋼板をオーステナイト域の高温まで加熱した後にプレス成形を実施するものである。そのため、室温で実施する通常のプレス加工に比べ、成形荷重が大幅に低減される。また、プレス加工と同時に、金型内において、いわゆる焼入れ処理を実施することになるため、鋼中に含まれるＣ量に応じた強度を得ることができるため、形状凍結性と強度確保を両立する技術として注目されている。 On the other hand, hot stamping is a method in which a steel sheet is heated to a high temperature in the austenite region for press forming in order to obtain higher strength. For this reason, the molding load is greatly reduced as compared with the normal pressing performed at room temperature. In addition, since the so-called quenching process is performed in the mold at the same time as pressing, strength according to the amount of C contained in the steel can be obtained. It is attracting attention as a technology.
こうしたホットスタンプ技術により、９８０ＭＰａ以上の強度を得る方法として、特許文献３に記載されているものがある。すなわち、ホットスタンプ前の組織を規定し且つ、強度を９８０ＭＰａ以上とすることで耐水素脆性に優れたホットスタンプ用鋼板に関わるものである。しかし、ホットスタンプ前の組織を規定せず、化学成分の適正化としてＳｎ、Ｎｉ、Ｃｕ等の添加、Ｃａ、Ｍｇ、Ｙ、Ａｓ、Ｓｂ、ＲＥＭ、等の硫化物生成元素の添加による衝撃特性の向上と、Ｍｎ、Ｓｉ、Ｎｉ、Ｃｕ、Ｍｏ、Ｔｉの添加により、ホットスタンプを行った際の最適焼入れ可能範囲、ホットスタンプ後に形成される組織として９８％以上のマルテンサイトを形成させるとともに、ホットスタンプ後の強度を１４７０ＭＰａ以上とする本発明とは、全くその狙いが異なるものである。
As a method for obtaining a strength of 980 MPa or more by such a hot stamp technique, there is one described in
また、ホットスタンプ時に金型内へスケールが剥離する事が問題となっており、ホットスタンプ用めっき鋼板及びその製造方法に関わる技術として、特許文献4に記載されている。本技術では、化学成分の適正化としてＣｒ／Ｓｉ比を定めることにより、母材とスケールの密着性を向上させた、めっきレスホットスタンプ用鋼板と、スケール形成を防ぐめっき有ホットプレス鋼板のホットプレス中の冷却速度と、冷却後の組織を規定しており、本発明の思想は全く異なる。 In addition, there is a problem that the scale is peeled into the mold during hot stamping, and is described in Patent Document 4 as a technique related to a hot stamped plated steel sheet and a manufacturing method thereof. In this technology, by setting the Cr / Si ratio as an optimization of the chemical composition, the hot plate of the hot stamped steel plate with plating and the steel plate for platingless hot stamping that improves the adhesion between the base material and the scale and the prevention of scale formation. The cooling rate during pressing and the structure after cooling are defined, and the idea of the present invention is completely different.
本発明は、引張強度が１４７０ＭＰａ以上で、且つ高い靱性、高い加工性および成形性を有し、さらに水素脆化起因による遅れ破壊特性に優れた高強度鋼板、及び、その製造方法を提供することを目的とする。 The present invention provides a high-strength steel sheet having a tensile strength of 1470 MPa or more, high toughness, high workability and formability, and excellent delayed fracture characteristics due to hydrogen embrittlement, and a method for producing the same. With the goal.
上述の如き目的を達成するために、本発明者らは、以下に示す高強度鋼板及びその製造方法を発明した。
（１） 質量%で、
Ｃ：０．２０〜０．３５％、
Ｓｉ：０．０５〜１．００％、
Ｍｎ：０．２０〜３．５０％、
Ｐ：０．０１５％以下、
Ｓ：０．０５％以下、
Ａｌ：０．００５〜０．３００％、
Ｔｉ：０．００１〜０．０５０％、
Ｂ：０．０００２〜０．００５０％、
Ｎ：０．００４０％以下、
Ｖ、Ｃｒのいずれか一方または両方を合計で０．００１〜３．０００％、
Ｓｎ、Ｎｉ、Ｃｕの１種または２種以上を合計で０．００５〜２％、
さらにＣａ、Ｍｇ、Ｙ、Ａｓ、Ｓｂ、ＲＥＭの１種または２種以上を合計で０．０００５〜０．０５％を含有し、残部Fe及び不可避的不純物からなり、
前記元素の含有量から計算される下記の式（１）を満たし、
ホットスタンプ後において、旧オーステナイト粒径が７μｍ以上２０μｍ以下で、９７％以上のマルテンサイトを含み、引張強度が１４７０ＭＰａ以上であることを特徴とする、高い靱性と高い加工性および成形性とを有し、水素脆化起因による遅れ破壊特性に優れた高強度鋼板。
{−1.6×[C%]＋2.62}×{{−2.04×([C%]−0.08)4＋4.50×([C%]−0.08)3−3.84×([C%]−0.08)2＋1.87×([C%]−0.08)}＋{−0.045×[Si%]2＋0.277×[Si%]}＋{0.04×[Mn%]5−0.31×[Mn%]4＋0.93×[Mn%]3−1.36×[Mn%]2＋1.34×[Mn%]}＋{−0.012×[Ni,Cu,Sn%]2＋0.147×[Ni,Cu,Sn%]}＋{0.018×[Cr,V%]5−0.142×[Cr,V%]4＋0.431×[Cr,V%]3−0.709×[Cr,V%]2＋0.901×[Cr,V%]}＋{−0.45×[Mo%]4＋1.22×[Mo%]3−1.43×[Mo%]2＋1.26×[Mo%]}＋{−0.94×[Ti%]}} ＞ ３．０ ・・・（１）
ここで、[Ni,Cu,Sn%]はNi,Cu,Snの合計量、[Cr,V%]はCr,Vの合計量をそれぞれ示す。
（２） さらに鋼中に、質量％で、Ｎｂ：０．００５〜０．５００％、Ｍｏ：０．０５〜０．５０％の１種または２種を含有することを特徴とする（１）に記載の高い靱性と高い加工性および成形性とを有し、さらに水素脆化起因による遅れ破壊特性に優れた高強度鋼板。
（３） さらに、ＳｉとＣｒの質量％で表される含有量が下記の式（２）を満たすことを特徴とする（１）または（２）に記載の高い靱性と高い加工性および成形性とを有し、さらに水素脆化起因による遅れ破壊特性に優れた鋼強度鋼板。
０．７＜Ｓｉ／Ｃｒ＜１．３ ・・・ （２）
（４） さらに、鋼板表面に、亜鉛めっき層またはアルミニウムめっき層を有することを特徴とする、（１）〜（３）のいずれかに記載の高い靱性と高い加工性および成形性とを有し、さらに水素脆化起因による遅れ破壊特性に優れた高強度鋼板。
In order to achieve the object as described above, the present inventors have invented the following high-strength steel sheets and methods for producing the same.
(1) By mass%
C: 0.20 to 0.35%,
Si: 0.05-1.00%,
Mn: 0.20 to 3.50%
P: 0.015% or less,
S: 0.05% or less,
Al: 0.005 to 0.300%,
Ti: 0.001 to 0.050%,
B: 0.0002 to 0.0050%,
N: 0.0040% or less,
0.001 to 3.000% in total of one or both of V and Cr,
0.005 to 2% in total of one or more of Sn, Ni and Cu,
Furthermore, it contains 0.0005 to 0.05% in total of one or more of Ca, Mg, Y, As, Sb, and REM, and consists of the balance Fe and inevitable impurities,
Satisfying the following formula ( 1) calculated from the content of the element,
After hot stamping, prior austenite grain size at 7μm least 20μm or less, comprising 97% or more of martensite, and a tensile strength of more than 1470 MPa, a high toughness and high processability and a formed shape of High strength steel plate with excellent delayed fracture characteristics due to hydrogen embrittlement.
{−1.6 × [C%] + 2.62} × {{ −2.04 × ([C%] − 0.08) 4 + 4.50 × ([C%] − 0.08) 3 −3.84 × ([C%] − 0.08) ) 2 + 1.87 × ([C%] − 0.08)} + {− 0.045 × [Si%] 2 + 0.277 × [Si%]} + {0.04 × [Mn%] 5 −0.31 × [Mn%] 4 + 0.93 × [Mn%] 3 −1.36 × [Mn%] 2 + 1.34 × [Mn%]} + {− 0.012 × [Ni, Cu, Sn%] 2 + 0.147 × [Ni, Cu, Sn%]} + {0.018 × [Cr , V %] 5 −0.142 × [Cr , V %] 4 + 0.431 × [Cr , V %] 3 −0.709 × [Cr , V %] 2 + 0.901 × [Cr , V %]} + {− 0.45 × [Mo%] 4 + 1.22 × [Mo%] 3 −1.43 × [Mo%] 2 + 1.26 × [Mo%]} + {− 0.94 × [Ti %] }} > 3.0 ・ ・ ・ (1)
Here, [Ni, Cu, Sn%] represents the total amount of Ni, Cu, Sn, and [Cr, V%] represents the total amount of Cr, V, respectively.
(2) Further, the steel contains one or two of Nb: 0.005 to 0.500% and Mo: 0.05 to 0.50% in mass% (1). A high-strength steel sheet having the high toughness, high workability and formability described in 1. and excellent delayed fracture characteristics due to hydrogen embrittlement.
(3) Furthermore, the content represented by the mass% of Si and Cr satisfies the following formula (2): high toughness and high workability and formability as described in (1) or (2) Steel strength steel plate with excellent delayed fracture characteristics due to hydrogen embrittlement.
0.7 <Si / Cr <1.3 (2)
(4) Furthermore, it has high toughness and high workability and formability according to any one of (1) to (3), wherein the steel plate surface has a zinc plating layer or an aluminum plating layer. In addition, a high-strength steel sheet with excellent delayed fracture characteristics due to hydrogen embrittlement.
本発明は、自動車部品の構造部材に適用され、ホットスタンプ後の焼き入れによる硬度のばらつきを低減させ、高強度で且つ靱性、および、遅れ破壊特性に優れ、表面メッキを施した場合、メッキを施さない場合共に、金型経内へスケールの剥離抑制に優れた部品を提供する事が出来、社会的貢献が大きいものである。 The present invention is applied to a structural member of an automobile part, reduces variation in hardness due to quenching after hot stamping, has high strength, excellent toughness, and delayed fracture characteristics. In both cases, it is possible to provide a part excellent in suppressing the peeling of the scale in the mold warp, which greatly contributes to society.
本発明においては、特定の化学組成を有する熱延素材あるいは冷延素材を用いるが、その熱延素材あるいは冷延素材を製造する手段は特に限定されない。また、熱間成形加工とは、Ac3 変態点以上のオーステナイト領域に加熱後、Ac3 変態点以上の温度で成形加工（例えばプレス加工）を開始し、加工と同時に金型で抜熱することにより急速冷却し、マルテンサイト変態させて硬化させる加工をいう。 In the present invention, a hot-rolled material or a cold-rolled material having a specific chemical composition is used, but the means for producing the hot-rolled material or the cold-rolled material is not particularly limited. Also, hot forming is rapid by heating to the austenite region above the Ac3 transformation point and then starting molding (for example, pressing) at a temperature above the Ac3 transformation point. It refers to a process of cooling and hardening by martensite transformation.
（鋼板の化学成分）
以下に本発明の化学成分、および、限定理由について説明する。なお、％は質量％を表す。
Ｃ：０．２０〜０．３５％、
Ｃは、本発明において重要な役割を果たす元素であり、とくに焼入れ後の強度に与える影響が大きい。したがって、１４７０ＭＰａ以上の強度を得るには０．２０％以上の添加が必要である。一方、０．３５％を超えると、衝撃変形時に破断が生じやすくなるとともに、溶接性の劣化と溶接部の強度が低下するため、これを上限とする。
(Chemical composition of steel sheet)
Below, the chemical component of this invention and the reason for limitation are demonstrated. In addition,% represents mass%.
C: 0.20 to 0.35%,
C is an element that plays an important role in the present invention, and has a great influence on the strength after quenching. Therefore, 0.20% or more of addition is necessary to obtain a strength of 1470 MPa or more. On the other hand, if it exceeds 0.35%, breakage is likely to occur at the time of impact deformation, and weldability is deteriorated and the strength of the welded portion is reduced.
Ｓｉ：０．０５〜１．００％
Ｓｉは、固溶強化型の合金元素であり、強度を確保するために０．０５％以上必要であるが、１％を超えると、表面スケールの問題が生じる。このため、Ｓｉは１％以下に規定した。また、鋼板表面にメッキ処理を行う場合は、Ｓｉの添加量が多いとメッキ性が劣化するため、上限を０．５％とすることが好ましい。また、Ｓｉの含有量が多いと、衝撃特性や延性が低下するため、この点からもＳｉの添加量を０．５％以下とすることが好ましい。なお、Ｓｉ含有量を低減するとシャルピー吸収エネルギーは向上し、同時に延性脆性遷移温度も低温化させることができるため、衝撃特性は向上する。このため、Ｓｉ含有量を０．２％未満に制限することがより好ましい。
Si: 0.05-1.00%
Si is a solid solution strengthening type alloy element and needs to be 0.05% or more in order to ensure strength, but if it exceeds 1%, a problem of surface scale occurs. For this reason, Si was specified to be 1% or less. In addition, when plating is performed on the surface of the steel sheet, if the amount of Si added is large, the plateability deteriorates, so the upper limit is preferably set to 0.5%. Moreover, since impact characteristics and ductility will fall when there is much content of Si, it is preferable also from this point that the addition amount of Si shall be 0.5% or less. If the Si content is reduced, the Charpy absorbed energy is improved, and at the same time, the ductile brittle transition temperature can be lowered, so that the impact characteristics are improved. For this reason, it is more preferable to limit the Si content to less than 0.2%.
Ｍｎ：０．２０〜３．５０％
Ｍｎは強度及び焼入れ性を向上させる元素であり、０．２０％未満では焼入れ時の強度を十分に得られず、また３．５０％を超えて添加しても効果が飽和し、中央偏析により材質が悪化するため、Ｍｎは０．２０〜３．５０％の範囲に規定した。
Mn: 0.20 to 3.50%
Mn is an element that improves strength and hardenability. If it is less than 0.20%, the strength at the time of quenching cannot be sufficiently obtained, and even if added over 3.50%, the effect is saturated, and due to central segregation. Since the material deteriorates, Mn is specified in the range of 0.20 to 3.50%.
Ｐ：０．０１５％以下
Ｐは、固溶強化元素であり、比較的安価に鋼板の強度を上げることができるが、粒界に偏析し、強度が高い場合には低温脆化が問題になることから、０．０１５％以下とする。一方、０．００１％よりも低くすることは脱Ｐコストを極端に高めるため好ましくないことから、好ましくはこれを下限とする。
P: 0.015% or less P is a solid solution strengthening element and can raise the strength of the steel sheet relatively inexpensively, but segregates at the grain boundary, and when the strength is high, low temperature embrittlement becomes a problem. Therefore, the content is made 0.015% or less. On the other hand, since lower than 0.001% is not preferable because the removal P cost is extremely increased, this is preferably set as the lower limit.
Ｓ：０．０５％以下
Ｓは鋼中の非金属介在物に影響し、加工性を劣化させるとともに、靭性劣化、異方性及び再熱割れ感受性の増大の原因となる。このためＳは０．０５％以下に規定した。より好ましくは０．０１％以下である。また、Ｓを０．００５％以下に規制することにより、衝撃特性が飛躍的に向上する。しかし、０．００１％未満とする場合には、脱硫コストの極端な上昇を招くため、好ましくはこれを下限とする。
S: 0.05% or less S affects non-metallic inclusions in the steel, which deteriorates workability and causes toughness deterioration, anisotropy and reheat cracking sensitivity. Therefore, S is specified to be 0.05% or less. More preferably, it is 0.01% or less. Moreover, by restricting S to 0.005% or less, impact characteristics are dramatically improved. However, when the content is less than 0.001%, the desulfurization cost is extremely increased, so this is preferably set as the lower limit.
Ａｌ：０．００５〜０．３００％
Ａｌは、脱酸のために添加されるものである。０．００５％未満では脱酸が不十分となり、鋼中に酸化物が多量に残存し、とくに局部変形能が劣化するとともに、特性バラツキも大きくなる。一方、０．３００％を超えて含有されると、鋼中にアルミナを主体とする酸化物が多く残存し、やはり局部変形能の劣化を招くため、好ましくない。
Al: 0.005 to 0.300%
Al is added for deoxidation. If it is less than 0.005%, deoxidation becomes insufficient, and a large amount of oxide remains in the steel, and in particular, the local deformability deteriorates and the characteristic variation also increases. On the other hand, if the content exceeds 0.300%, a large amount of oxide mainly composed of alumina remains in the steel, which also causes deterioration of local deformability, which is not preferable.
Ｔｉ：０．００１〜０．０５０％
Ｔｉも本発明においては重要な元素であり、ＴｉはＢの効果を有効に発揮させるため、Ｂと化合物を生成するＮを固着する目的で添加する。この効果を発揮させるためには、（Ｔｉ−３．４２×Ｎ）が０．００１％以上必要であるが、Ｔｉ量がむやみに増加するとＴｉと結合していないＣ量が減少し、冷却後に十分な強度が得られなくなるため、上限値を０．０５０％とする。
Ti: 0.001 to 0.050%
Ti is also an important element in the present invention, and Ti is added for the purpose of fixing B and N forming a compound in order to effectively exhibit the effect of B. In order to exhibit this effect, (Ti-3.42 × N) is required to be 0.001% or more. However, if the Ti amount increases excessively, the amount of C not bonded to Ti decreases, and after cooling. Since sufficient strength cannot be obtained, the upper limit is set to 0.050%.
Ｂ：０．０００２〜０．００５０％
Ｂはプレス成形中あるいはプレス成形後の冷却での焼入れ性を向上させるために添加するが、この効果を発揮させるためには０．０００２％以上の添加が必要である。しかしながら、この添加量がむやみに増加すると熱間での割れの懸念があることや、その効果が飽和するため、その上限は０．００５０％とする。
B: 0.0002 to 0.0050%
B is added in order to improve the hardenability during press molding or cooling after press molding, but 0.0002% or more must be added to exhibit this effect. However, if this amount increases excessively, there is a risk of hot cracking, and the effect is saturated, so the upper limit is made 0.0050%.
Ｎ：０．００４０％以下
Ｎも、極端に下げることはコストアップとなり好ましくないため、０．００１％を下限とする。一方、０．００４％を超えて含有されると、介在物を形成し、焼入れ後の靭性が劣化するため、これを上限とする。
N: 0.0040% or less N too low is not preferable because it increases the cost, so 0.001% is set as the lower limit. On the other hand, if the content exceeds 0.004%, inclusions are formed and the toughness after quenching deteriorates, so this is the upper limit.
Ｖ、Ｃｒのいずれか一方または両方の合計：０．００１〜３．０００％
Ｃｒは焼入れ性を向上させる元素であり、またマトリックス中へＭ２３Ｃ６型炭化物を析出させる効果を有し、強度を高めるとともに、炭化物を微細化する作用を有する。Ｖは、靭性確保の視点から、組織微細化のために添加される元素である。すなわち、鋼板をＡｃ３点以上に加熱した場合、微細な炭化物の形成により、再結晶及び粒成長を抑制してオーステナイト粒を細粒にするため、靭性を改善し、強度を向上させる。これらの合計含有量が０．００１％未満ではこれらの効果が十分期待できず、また３．００％を超えると降伏強度が過度に上昇する傾向にあるため、ＣｒとＶの合計で０．０１〜３．００％の範囲が望ましい。より望ましくは０．０５〜１％である。
Total of either one or both of V and Cr: 0.001 to 3.000%
Cr is an element that improves hardenability, and has the effect of precipitating M23C6 type carbide into the matrix, and has the effect of increasing the strength and miniaturizing the carbide. V is an element added for refining the structure from the viewpoint of securing toughness. That is, when the steel sheet is heated to the Ac3 point or higher, the formation of fine carbides suppresses recrystallization and grain growth and makes austenite grains fine, thereby improving toughness and increasing strength. If these total contents are less than 0.001%, these effects cannot be expected sufficiently, and if they exceed 3.00%, the yield strength tends to increase excessively, so the total of Cr and V is 0.01%. A range of ˜3.00% is desirable. More desirably, it is 0.05 to 1%.
Ｓｎ、Ｎｉ、Ｃｕの１種または２種以上の合計：０．００５〜２％
Ｓｎ、Ｎｉ、Ｃｕは鋼板表面近傍の酸化物に影響を及ぼし、衝撃特性と遅れ破壊特性を向上させると考えられるため、これらの１種または２種以上を合計で０．００５％以上添加する必要がある。しかし過度の添加は加工性を劣化させるため、その上限を２％に規制した。
Total of one or more of Sn, Ni and Cu: 0.005 to 2%
Sn, Ni and Cu affect the oxide near the surface of the steel sheet and are thought to improve impact characteristics and delayed fracture characteristics. Therefore, it is necessary to add one or more of these elements in a total amount of 0.005% or more. There is. However, excessive addition deteriorates workability, so the upper limit was regulated to 2%.
Ｃａ、Ｍｇ、Ｙ、Ａｓ、Ｓｂ、ＲＥＭの１種または２種以上の合計：０．０００５〜０．０５％
Ｃａ、Ｍｇ、Ｙ、Ａｓ、Ｓｂ、ＲＥＭは、主な硫化物であるＭｎＳの形状を変化させて衝撃特性と遅れ破壊特性を向上させると考えられるため、これらの１種または２種以上の合計が０．０００５％以上の添加が必要である。しかし過度の添加は加工性を劣化させるため、その上限を０．０５％以下に規制した。また、Ｃａを質量％で０．０００１％以上含有することで遅れ破壊特性の改善が顕著であることから、Ｃａは質量％で０．０００１％以上含有することが望ましい。
Total of one or more of Ca, Mg, Y, As, Sb, and REM: 0.0005 to 0.05%
Since Ca, Mg, Y, As, Sb, and REM are considered to improve the impact characteristics and delayed fracture characteristics by changing the shape of MnS, which is the main sulfide, the total of one or more of these Must be added in an amount of 0.0005% or more. However, excessive addition deteriorates workability, so the upper limit was regulated to 0.05% or less. Moreover, since the improvement of delayed fracture characteristics is remarkable when Ca is contained by 0.0001% or more by mass%, Ca is preferably contained by 0.0001% or more by mass%.
以上の元素に加えて、さらに、Ｎｂ、Ｍｏの１種また２種を含有することができる。
Ｎｂ：０．００５〜０．５００％
Ｎｂは、靭性確保の視点から、組織微細化のために添加される元素である。すなわち、鋼板をＡｃ３点以上に加熱した場合、微細な炭化物の形成により、再結晶及び粒成長を抑制してオーステナイト粒を細粒にするため、靭性を改善する効果があり、炭窒化物を形成し、強度を向上させ、さらに、遅れは基特性の向上に効果がある元素であるが、０．５００％を超えて添加すると、降伏強度の上昇が過度に大きくなる。０．００５％未満では強度向上の効果が発揮されにくいため、添加する場合は、Ｎｂは０．００５〜０．５００％の範囲が望ましい。
In addition to the above elements, one or two of Nb and Mo can be further contained.
Nb: 0.005 to 0.500%
Nb is an element added for refining the structure from the viewpoint of securing toughness. That is, when the steel sheet is heated to Ac3 point or higher, the formation of fine carbides suppresses recrystallization and grain growth and makes the austenite grains finer, which has the effect of improving toughness and forms carbonitrides. However, the delay is an element that is effective in improving the base characteristics, but if added over 0.500%, the yield strength increases excessively. If it is less than 0.005%, the effect of improving the strength is hardly exhibited. Therefore, when Nb is added, the Nb content is preferably in the range of 0.005 to 0.500%.
Ｍｏ：０．０５〜０．５０％
Ｍｏも０．０５％以上にすることで、粒界を強化する働きが顕著に表れ、最高加熱温度：−４０℃においてシャルピー衝撃値１０Ｊ／ｃｍ２以上とするためには、０．０５％以上の添加が必要である。一方、０．５％を超えて添加してもその効果が飽和するためこれを上限とする。またＴｉ、Ｎｂ及びＶと同様に、鋼板をＡｃ３点以上に加熱した場合、微細な炭化物の形成により、再結晶及び粒成長を抑制してオーステナイト粒を細粒にするため、靭性を改善する効果がある。そのため、０．０５％を下限とする。一方、0.5%を超えて添加してもその効果が飽和するばかりでなく、コストアップを招くことからこれを上限とする。
Mo: 0.05 to 0.50%
When Mo is made 0.05% or more, the function of strengthening the grain boundary appears remarkably, and in order to obtain a Charpy impact value of 10 J / cm 2 or more at the maximum heating temperature: −40 ° C., 0.05% or more. Must be added. On the other hand, even if added over 0.5%, the effect is saturated, so this is the upper limit. Similarly to Ti, Nb, and V, when the steel sheet is heated to Ac3 point or higher, the formation of fine carbides suppresses recrystallization and grain growth and makes the austenite grains fine, thus improving the toughness. There is. Therefore, 0.05% is set as the lower limit. On the other hand, adding over 0.5% not only saturates the effect but also increases the cost, so this is the upper limit.
（式（１））
Ｃ、Ｍｎ、Ｓｉ、Ｎｉ、Ｃｒ、Ｍｏ、Ｔｉ、は鋼板の焼き入れ性に大きな影響を与える。各元素の含有量と焼き入れ性に関する式を経験的に下記の（１）で示す式で表した。式（１）の値が３．０以上の鋼板において、ホットスタンプ後の強度は水焼き入れを行った時と同等の強度を示し、マルテンサイト分率は９７％以上となる。材料のばらつきを鑑みて望ましくは式（１）の値は３．１以上である。
(Formula (1))
C, Mn, Si, Ni, Cr, Mo, Ti have a great influence on the hardenability of the steel sheet. The expression regarding the content of each element and the hardenability was empirically expressed by the following expression (1). In a steel sheet having a value of formula (1) of 3.0 or more, the strength after hot stamping is equivalent to that when water quenching is performed, and the martensite fraction is 97% or more. In view of variations in materials, the value of equation (1) is desirably 3.1 or more.
{−1.6×[C%]＋2.62}×{{−2.04×([C%]−0.08)4＋4.50×([C%]−0.08)3−3.84×([C%]−0.08)2＋1.87×([C%]−0.08)}＋{−0.045×[Si%]2＋0.277×[Si%]}＋{0.04×[Mn%]5−0.31×[Mn%]4＋0.93×[Mn%]3−1.36×[Mn%]2＋1.34×[Mn%]}＋{−0.012×[Ni,Cu,Sn%]2＋0.147×[Ni,Cu,Sn%]}＋{0.018×[Cr,V%]5−0.142×[Cr,V%]4＋0.431×[Cr,V%]3−0.709×[Cr,V%]2＋0.901×[Cr,V%]}＋{−0.45×[Mo%]4＋1.22×[Mo%]3−1.43×[Mo%]2＋1.26×[Mo%]}＋{−0.94×[Ti%]}} ＞ ３．０ ・・・（１）
ここで、［元素％］は元素の質量％で表される含有量であり、[Ni,Cu,Sn%]はNi,Cu,Snの合計量、[Cr,V%]はCr,Vの合計量をそれぞれ示す。
{−1.6 × [C%] + 2.62} × {{ −2.04 × ([C%] − 0.08) 4 + 4.50 × ([C%] − 0.08) 3 −3.84 × ([C%] − 0.08) ) 2 + 1.87 × ([C%] − 0.08)} + {− 0.045 × [Si%] 2 + 0.277 × [Si%]} + {0.04 × [Mn%] 5 −0.31 × [Mn%] 4 + 0.93 × [Mn%] 3 −1.36 × [Mn%] 2 + 1.34 × [Mn%]} + {− 0.012 × [Ni, Cu, Sn%] 2 + 0.147 × [Ni, Cu, Sn%]} + {0.018 × [Cr , V %] 5 −0.142 × [Cr , V %] 4 + 0.431 × [Cr , V %] 3 −0.709 × [Cr , V %] 2 + 0.901 × [Cr , V %]} + {− 0.45 × [Mo%] 4 + 1.22 × [Mo%] 3 −1.43 × [Mo%] 2 + 1.26 × [Mo%]} + {− 0.94 × [Ti %] }} > 3.0 ・ ・ ・ (1)
Here, [Elemental%] is Ri content der represented by mass% of the elements, [Ni, Cu, Sn%] is Ni, Cu, the total amount of Sn, [Cr, V%] is Cr, V The total amount of each is shown .
（旧オーステナイト粒の粒径、マルテンサイト分率）
旧オーステナイト粒の粒径は焼き入れ性及び、靭性に大きな影響を与える因子であり、旧オーステナイト粒の粒径が２０μｍ以上の時に靭性は低下するため、旧オーステナイト粒径の上限は２０μｍとした。旧オーステナイト粒径が５μｍ以下でも式（１）を満たしている時焼き入れ性は良好である。
ホットスタンプ後の組織のマルテンサイト分率が９７％以上のとき水焼き入れと同等の強度となるため、マルテンサイト分率が９７％以上とした。
(Size of old austenite grains, martensite fraction)
The grain size of the prior austenite grains is a factor that greatly affects the hardenability and toughness. Since the toughness decreases when the grain size of the prior austenite grains is 20 μm or more, the upper limit of the prior austenite grain size is set to 20 μm. Even when the prior austenite grain size is 5 μm or less, the hardenability is good when the formula (1) is satisfied.
When the martensite fraction of the structure after hot stamping is 97% or more, the strength is equivalent to that of water quenching, so the martensite fraction is 97% or more.
（Ｓｉ／Ｃｒ比：０．７〜１．３）
ホットスタンプ前の鋼板加熱時に発生するスケールの鋼板密着性は重要な因子であり、Ｓｉ／Ｃｒ比が０．７〜１．３の時良好な密着性を示す。
(Si / Cr ratio: 0.7 to 1.3)
The steel sheet adhesion of the scale generated during the heating of the steel sheet before hot stamping is an important factor, and shows good adhesion when the Si / Cr ratio is 0.7 to 1.3.
（鋼板の製造方法）
本発明において圧延工程に先行する鋼の製造方法は特に限定するものではない。すなわち、高炉から出銑後に溶銑脱燐および溶銑脱硫等の溶銑予備処理を経て転炉による精錬を行うかもしくは、スクラップ等の冷鉄源を電炉等で溶解する工程に引き続き、各種の２次精練で目的の成分含有量になるように成分調整を行い、次いで通常の連続鋳造、インゴット法による鋳造の他、薄スラブ鋳造などの方法で鋳造すればよい。
また、連続鋳造方法もとくに規定されるものではなく、通常の連続鋳造方法やスラブ厚みが100mm以下の薄スラブ法によるものによっても、本発明における効果は何ら変わるものではない。
(Steel plate manufacturing method)
In the present invention, the method for producing steel preceding the rolling step is not particularly limited. In other words, after discharging from the blast furnace, refining with a converter through hot metal pretreatment such as hot metal dephosphorization and hot metal desulfurization, or various secondary refining following the process of melting a cold iron source such as scrap with an electric furnace, etc. Then, the components may be adjusted so as to achieve the desired component content, and then cast by a method such as thin continuous slab casting, in addition to normal continuous casting and casting by an ingot method.
Further, the continuous casting method is not particularly specified, and the effect in the present invention is not changed by the normal continuous casting method or the thin slab method having a slab thickness of 100 mm or less.
さらに、熱間圧延条件は通常実施される範囲でかまわない。すなわち、加熱温度は、その後に続く熱間圧延工程での圧延を可能とする変形抵抗が得られる条件であれば良い。また、仕上げ温度もＡｒ３点以上の温度域で実施すれば良く、その後に続く冷却条件もとくに規定する必要はなく、７５０℃以下の温度域で巻取を実施する。４００℃未満の温度で巻取ると熱延板強度が高くなり過ぎることから、これ以上の温度で巻取ることが望ましいが、一方、４００℃未満の温度で巻取った後に、軟質化を目的とした再加熱処理を実施してもかまわない。 Furthermore, the hot rolling conditions may be within the range that is usually carried out. That is, the heating temperature should just be the conditions from which the deformation resistance which enables the rolling in the subsequent hot rolling process is obtained. Further, the finishing temperature may be carried out in the temperature range of Ar3 or higher, and the subsequent cooling conditions need not be specified, and the winding is carried out in a temperature range of 750 ° C. or lower. Since the hot-rolled sheet strength becomes too high when it is wound at a temperature of less than 400 ° C, it is desirable to wind it at a temperature higher than this. On the other hand, after winding at a temperature of less than 400 ° C, the purpose is to soften it. The reheat treatment may be performed.
熱間圧延に続く冷間圧延条件、焼鈍条件及び必要に応じて行われるメッキ条件についても、とくに本発明においては規定されるものではなく、通常の範囲で実施すれば良い。すなわち、冷間圧延は、通常実施されている冷延圧下率の範囲で実施するものとし、具体的には、40〜80％で実施するものとする。メッキは熱間圧延ま、冷間圧延まま、あるいは再結晶焼鈍を実施した後に実施するものであるが、加熱条件や冷却条件はとくに規定されるものではない。さらに、メッキ種についてもZnあるいはAlがメッキされるが、とくにZnメッキについては合金化の有無については限定しない。また、Alメッキについてはメッキ中にSiを含んでも本発明に何ら影響を与えるものではない。
熱延板あるいは冷延・焼鈍後及びメッキ後の調質圧延についても、とくに規定するものではなく、形状を適切に調整するために実施するものである。
The cold rolling conditions following the hot rolling, the annealing conditions, and the plating conditions performed as necessary are not particularly defined in the present invention, and may be performed within a normal range. That is, the cold rolling is performed in the range of the cold rolling reduction that is normally performed, and specifically, it is performed at 40 to 80%. Plating is performed by hot rolling, cold rolling, or after recrystallization annealing, but heating conditions and cooling conditions are not particularly defined. Furthermore, Zn or Al is also plated as the plating type, but the presence or absence of alloying is not particularly limited for Zn plating. As for Al plating, the inclusion of Si in the plating does not affect the present invention.
The temper rolling after hot rolling or cold rolling / annealing and after plating is not particularly specified, but is performed to adjust the shape appropriately.
（鋼板の熱処理条件）
上記の条件で製造された熱延鋼板、冷延鋼板及びメッキ鋼板をホットスタンプするために加熱するに際し、加熱時間が１０ｓ未満では、加熱による炭化物の再溶解が不十分となり、強度を確保するための固溶Ｃ量が確保できない。一方、６０ｓより長くなると１５μｍ以下の旧オーステナイト粒径が得られない。また、旧オーステナイト粒径に対しては加熱温度の影響も大きく、９５０℃を超えるとやはり２０μｍ以下の旧オーステナイト粒径が得られない。一方、Ａｃ３点より低くなると部分的にオーステナイト化しない領域ができるため、当該部分ではマルテンサイトが形成されないことに起因し、十分な強度が得られないことが懸念される。さらに加熱に引き続き冷却するに際しても、Ａｒ３〜３００℃の温度域を、２００℃／ｓ以上の冷却速度で冷却を行わないとやはり強度が得られない。
(Heat treatment conditions for steel sheet)
When heating for hot stamping hot-rolled steel sheet, cold-rolled steel sheet and plated steel sheet manufactured under the above conditions, if the heating time is less than 10 s, remelting of carbide due to heating becomes insufficient, and strength is secured. The amount of solid solution C cannot be secured. On the other hand, if it is longer than 60 s, a prior austenite grain size of 15 μm or less cannot be obtained. Also, the influence of the heating temperature is large on the prior austenite particle size, and if it exceeds 950 ° C., the prior austenite particle size of 20 μm or less cannot be obtained. On the other hand, when the temperature is lower than the Ac3 point, a region that does not partially austenite is formed, and therefore, there is a concern that sufficient strength cannot be obtained due to the fact that martensite is not formed in the portion. Further, when the cooling is continued after the heating, the strength cannot be obtained unless the temperature range of Ar3 to 300 ° C. is cooled at a cooling rate of 200 ° C./s or more.
表１、２のＮｏ．１から５９の組成を持つ各種鋼スラブに鋳造した。これらのスラブを１２００℃に加熱し、熱間圧延にて仕上げ温度８７０℃、巻取り温度６００℃で板厚４ｍｍの熱延板とし、一部の熱延板を冷間圧延により板厚１．６ｍｍの冷延鋼板とした。
得られた冷延鋼板を加熱炉によりＡｃ３以上である、９３０℃のオーステナイト域に加熱後３００ｓｅｃ保持し、ホットプレスによる金型冷却を行った。成形時間を約１ｓｅｃとし、成形完了後１０秒間はプレス金型をそのままの状態にして金型冷却を行なった。
No. in Tables 1 and 2. Cast into various steel slabs having compositions from 1 to 59. These slabs are heated to 1200 ° C. and hot rolled to form a hot rolled sheet having a finishing temperature of 870 ° C. and a coiling temperature of 600 ° C. and a thickness of 4 mm. A 6 mm cold-rolled steel sheet was used.
The obtained cold-rolled steel sheet was heated in an austenite region of 930 ° C., which is Ac3 or higher, in a heating furnace and held for 300 seconds, and then mold cooling by hot pressing was performed. The molding time was set to about 1 sec, and the mold was cooled for 10 seconds with the press mold kept as it was after the completion of molding.
ホットスタンプ後のサンプルについて、光学顕微鏡を用いたマルテンサイトの分率測定を行い、表１、２にＭｓ分率として示すように、鋼板の組織はすべてのマルテンサイト組織であることが確認された。 The sample after hot stamping was subjected to martensite fraction measurement using an optical microscope, and as shown in Tables 1 and 2 as Ms fractions, it was confirmed that the structure of the steel sheet was all martensite structures. .
また、Ｎｏ．１からＮｏ．２４のホットスタンプ後のサンプルの引張り試験を行い、含有Ｃ量と引張強度との関係を調査した。引張試験は、ＪＩＳ Ｚ ２２０１に記載の５号試験片に加工し、ＪＩＳ Ｚ ２２４１に記載の試験方法に従って実施した。
引張強度の結果を鋼板のＣ量との関係で図１に示す。Ｎｏ．１〜８までの鋼（○印で示す）は、式（１）の左辺の値がいずれも３０未満であり、Ｎｏ．９〜１６までの鋼（黒丸印で示す）及びＮｏ．１７〜２４までの鋼（×印で示す）は、式（１）の左辺の値がいずれも３０以上の鋼である。
図１の結果より、本発明が対象としている１４７０ＭＰａ以上の強度を得るには、重量％で０．２０％以上のＣの添加と、且つ、成分元素の含有量が式（１）を満たすことが必要であることが知見された。
No. 1 to No. The sample after 24 hot stamps was subjected to a tensile test, and the relationship between the C content and the tensile strength was investigated. The tensile test was processed into a No. 5 test piece described in JIS Z 2201, and was performed according to the test method described in JIS Z 2241.
The result of the tensile strength is shown in FIG. 1 in relation to the C content of the steel plate. No. Steels Nos. 1 to 8 (indicated by circles) have values of less than 30 on the left side of the formula (1). Steel Nos. 9 to 16 (indicated by black circles) and No.
From the results shown in FIG. 1, in order to obtain the strength of 1470 MPa or more, which is the subject of the present invention, the addition of 0.20% by weight or more of C and the content of component elements satisfy the formula (1). Was found to be necessary.
実施例１で作成したＮｏ．２５から４０の冷延鋼板を、７８０℃、８３０℃、８７０℃、９１０℃、９５０℃の各種温度に加熱し、実施例１と同様にホットスタンプを行った。得られた鋼板について光学顕微鏡を用いて旧オーステナイト粒径を測定した。
ホットスタンプ前の加熱温度とホットスタンプ後の鋼板の旧オーステナイト粒径の関係を表３に示す。加熱温度を９５０℃以下とした場合、２０μｍ以下の旧オーステナイト粒径が得られることが知見された。しかし、Ａｃ３点より低い温度（７９０℃）での熱処理材では、フェライトが残存していることに起因し、強度が１２３７５ＭＰａと低く、１４７０ＭＰａ以上の強度が得られていないため、本発明の範囲外である。
No. 1 prepared in Example 1. 25 to 40 cold-rolled steel sheets were heated to various temperatures of 780 ° C., 830 ° C., 870 ° C., 910 ° C., and 950 ° C., and hot stamped in the same manner as in Example 1. About the obtained steel plate, the prior austenite particle size was measured using the optical microscope.
Table 3 shows the relationship between the heating temperature before hot stamping and the prior austenite grain size of the steel sheet after hot stamping. It has been found that when the heating temperature is 950 ° C. or less, a prior austenite particle size of 20 μm or less can be obtained. However, in the heat treated material at a temperature lower than the Ac3 point (790 ° C.), the strength is as low as 12375 MPa due to the remaining ferrite, and the strength of 1470 MPa or more is not obtained. It is.
実施例１で作成したＮｏ．２５から４０の組成を持つホットスタンプ後の鋼板に対し、低温脆性について、サブサイズのシャルピー衝撃試試験を−４０℃で行い、脆性破面率が５０％未満となった場合を合格（○）とし、５０％以上では不合格（×）とした。表４に試験結果を示す。
Ｍｏを含む組成は、９１０℃加熱では５０％未満の延性破面率を得ることができたが、９５０℃加熱では粒径の増大のため、靭性が低下し脆性破面率は５０％以上となった。Ｎｂを含む組成は、９１０℃と９５０℃加熱の両方で脆性破面率は５０％未満となった。
No. 1 prepared in Example 1. For steel sheets after hot stamping having a composition of 25 to 40, a sub-size Charpy impact test was conducted at -40 ° C. for low temperature brittleness, and the brittle fracture surface rate was less than 50% (O). In the case of 50% or more, it was judged as rejected (x). Table 4 shows the test results.
The composition containing Mo was able to obtain a ductile fracture surface ratio of less than 50% when heated at 910 ° C., but due to the increase in particle size when heated at 950 ° C., the toughness decreased and the brittle fracture surface ratio was 50% or more. became. The composition containing Nb had a brittle fracture surface ratio of less than 50% when heated at both 910 ° C. and 950 ° C.
実施例１で作成した表１、２のＮｏ．４１からＮｏ．５９組成を持つホットスタンプ後の鋼板に対し、遅れ破壊特性の評価を実施した。試験は図２に示すようなＶノッチを付与した試験片を使用し、室温にてチオシアン酸アンモニウム３ｇ／ｌを３％食塩水に溶かした水溶液に２４ｈ浸漬させ、破断の有無により判定した。
結果を、破断無し：○、破断有り：×として、表５に示す。
Nos. 1 and 2 in Tables 1 and 2 created in Example 1. 41 to No. Evaluation of delayed fracture characteristics was performed on the steel sheet after hot stamping having 59 composition. The test was performed using a test piece having a V-notch as shown in FIG. 2, immersed in an aqueous solution of 3 g / l of ammonium thiocyanate in 3% saline at room temperature for 24 hours, and judged by the presence or absence of breakage.
The results are shown in Table 5 with no break: ○ and with break: x.
実施例１で作成した表１、２のＮｏ．４５から５９の組成を持つ冷延鋼板について、溶融亜鉛に浸しめっきを施したサンプルと、めっきを施さないサンプルを準備し、これらのサンプルを９３０℃のオーステナイト域に加熱後３００ｓｅｃ保持し、鋼板温度を輻射温度計で測定し９００から６００℃まで空冷を行いホットプレスによる金型冷却を行った。成形時間を約１ｓｅｃとし、成形完了後１０秒間はプレス金型をそのままの状態にして金型冷却を行った。メッキの密着性はホットスタンプ後のメッキ表面のむら、われを調査し、割れ・むら無し：○、割れ・むら有：×と評価し、めっきを施さないサンプルは、金型内へのスケールの剥離の有無を調査し、剥離無し：○、剥離有：×と評価した。表６に得られた結果を示す。 Nos. 1 and 2 in Tables 1 and 2 created in Example 1. For cold-rolled steel sheets having a composition of 45 to 59, a sample immersed in molten zinc and plated and a sample not subjected to plating were prepared. These samples were heated in an austenite region at 930 ° C. and held for 300 seconds, and the steel plate temperature was Was measured with a radiation thermometer, air-cooled from 900 to 600 ° C., and the mold was cooled by hot pressing. The molding time was set to about 1 sec, and the mold was cooled for 10 seconds while the press mold was left as it was. The adhesion of the plating was investigated for unevenness and cracks on the plated surface after hot stamping, and evaluated as “no crack / unevenness: ○, crack / unevenness: x”. The presence or absence of peeling was investigated, and it was evaluated that there was no peeling: ○ and that there was peeling: x. Table 6 shows the results obtained.
本発明により、ホットスタンプを実施する際に加熱温度とその後の冷却条件により、１４７０ＭＰａ以上の強度と部材における延性の付与が可能となり、ホットスタンプ後の強度−延性バランスが優れかつ、遅れ破壊特性の優れた超高強度鋼板の製造が可能となる。 According to the present invention, when hot stamping is performed, it is possible to impart a strength of 1470 MPa or more and ductility in the member depending on the heating temperature and subsequent cooling conditions, and the strength-ductility balance after hot stamping is excellent, and delayed fracture characteristics An excellent ultra-high strength steel sheet can be manufactured.
Claims (4)
Ｃ：０．２０〜０．３５％、
Ｓｉ：０．０５〜１．００％、
Ｍｎ：０．２０〜３．５０％、
Ｐ：０．０１５％以下、
Ｓ：０．０５％以下、
Ａｌ：０．００５〜０．３００％、
Ｔｉ：０．００１〜０．０５０％、
Ｂ：０．０００２〜０．００５０％、
Ｎ：０．００４０％以下、
Ｖ、Ｃｒのいずれか一方または両方を合計で０．００１〜３．０００％、
Ｓｎ、Ｎｉ、Ｃｕの１種または２種以上を合計で０．００５〜２％、
さらにＣａ、Ｍｇ、Ｙ、Ａｓ、Ｓｂ、ＲＥＭの１種または２種以上を合計で０．０００５〜０．０５％を含有し、残部Fe及び不可避的不純物からなり、
前記元素の含有量から計算される下記の式（１）を満たし、
ホットスタンプ後において、旧オーステナイト粒径が７μｍ以上２０μｍ以下で、９７％以上のマルテンサイトを含み、引張強度が１４７０ＭＰａ以上であることを特徴とする、高い靱性と高い加工性および成形性とを有し、水素脆化起因による遅れ破壊特性に優れた高強度鋼板。
{−1.6×[C%]＋2.62}×{{−2.04×([C%]−0.08)4＋4.50×([C%]−0.08)3−3.84×([C%]−0.08)2＋1.87×([C%]−0.08)}＋{−0.045×[Si%]2＋0.277×[Si%]}＋{0.04×[Mn%]5−0.31×[Mn%]4＋0.93×[Mn%]3−1.36×[Mn%]2＋1.34×[Mn%]}＋{−0.012×[Ni,Cu,Sn%]2＋0.147×[Ni,Cu,Sn%]}＋{0.018×[Cr,V%]5−0.142×[Cr,V%]4＋0.431×[Cr,V%]3−0.709×[Cr,V%]2＋0.901×[Cr,V%]}＋{−0.45×[Mo%]4＋1.22×[Mo%]3−1.43×[Mo%]2＋1.26×[Mo%]}＋{−0.94×[Ti%]}} ＞ ３．０ ・・・（１）
ここで、[Ni,Cu,Sn%]はNi,Cu,Snの合計量、[Cr,V%]はCr,Vの合計量をそれぞれ示す。 % By mass
C: 0.20 to 0.35%,
Si: 0.05-1.00%,
Mn: 0.20 to 3.50%
P: 0.015% or less,
S: 0.05% or less,
Al: 0.005 to 0.300%,
Ti: 0.001 to 0.050%,
B: 0.0002 to 0.0050%,
N: 0.0040% or less,
0.001 to 3.000% in total of one or both of V and Cr,
0.005 to 2% in total of one or more of Sn, Ni and Cu,
Furthermore, it contains 0.0005 to 0.05% in total of one or more of Ca, Mg, Y, As, Sb, and REM, and consists of the balance Fe and inevitable impurities,
Satisfying the following formula ( 1) calculated from the content of the element,
After hot stamping, prior austenite grain size at 7μm least 20μm or less, comprising 97% or more of martensite, and a tensile strength of more than 1470 MPa, a high toughness and high processability and a formed shape of High strength steel plate with excellent delayed fracture characteristics due to hydrogen embrittlement.
{ −1.6 × [C%] + 2.62} × {{− 2.04 × ([C%] − 0.08) 4 + 4.50 × ([C%] − 0.08) 3 −3.84 × ([C%] − 0.08) ) 2 + 1.87 × ([C%] − 0.08)} + {− 0.045 × [Si%] 2 + 0.277 × [Si%]} + {0.04 × [Mn%] 5 −0.31 × [Mn%] 4 + 0.93 × [Mn%] 3 −1.36 × [Mn%] 2 + 1.34 × [Mn%]} + {− 0.012 × [Ni, Cu, Sn%] 2 + 0.147 × [Ni, Cu, Sn%]} + {0.018 × [Cr , V %] 5 −0.142 × [Cr , V %] 4 + 0.431 × [Cr , V %] 3 −0.709 × [Cr , V %] 2 + 0.901 × [Cr , V %]} + {− 0.45 × [Mo%] 4 + 1.22 × [Mo%] 3 −1.43 × [Mo%] 2 + 1.26 × [Mo%]} + {− 0.94 × [Ti %]}}> 3.0 ・ ・ ・ (1)
Here, [Ni, Cu, Sn%] represents the total amount of Ni, Cu, Sn, and [Cr, V%] represents the total amount of Cr, V, respectively.
０．７＜Ｓｉ／Ｃｒ＜１．３ ・・・ （２） Further, the content represented by mass% of Si and Cr satisfies the following formula (2), and has high toughness and high workability and formability according to claim 1, Steel strength steel plate with excellent delayed fracture characteristics due to hydrogen embrittlement.
0.7 <Si / Cr <1.3 (2)
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