JP2004124221A - Steel plate of excellent hardenability after hot working, and method for using the same - Google Patents
Steel plate of excellent hardenability after hot working, and method for using the same Download PDFInfo
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Abstract
Description
【0001】
【発明の属する技術分野】
本発明は、自動車部品の構造部材に使用されるような強度が必要とされる部材に関し、特に熱間成形後の硬化能および衝撃特性に優れた鋼板に関するものである。
【0002】
【従来の技術】
地球環境問題に端を発する自動車の燃費向上対策の一つとして車体の軽量化が進められており、自動車に使用される鋼板をできるだけ高強度化することが必要となる。しかし、自動車の軽量化のために一般に鋼板を高強度化していくと伸びやr値が低下し、成形性および形状凍結性が劣化していく。
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を図る技術が、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℃の温度域で保持・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いる。
【0003】
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可能性がある。
一方、より高強度を得る目的で、成形後に高温のオーステナイト単相域に加熱し、その後の冷却過程で硬質の相に変態させる技術が特許文献3に提案されている。さらに、成形性および焼入れ性に優れた薄鋼板の製造方法が特許文献4に提案されている。
【0004】
【引用文献】
(1)特許文献1 (特開2000−234153号公報)
(2)特許文献2 (特開2000−87183号公報)
(3)特許文献3 (特開2000−38640号公報)
(4)特許文献4 (特開2000−339025号公報)
【0005】
【発明が解決しようとする課題】
このように、これまでに開示されている技術を用い、熱間成形直後に高強度となる熱間プレスに適した鋼板を製造することは困難である。本発明は、上記課題を解決するためになされたものであり、熱間成形後にHv350以上の高い硬度を得ることができる熱間成形後硬化能および衝撃特性に優れた鋼板を提供することを目的とする。
【0006】
【課題を解決するための手段】
本発明の要旨は、
(1)質量%で、C:0.10〜0.20%未満、Si:0.01〜1.0%、Mn:0.3〜2.0%、Al:0.01〜0.50%、Ti:0.005〜0.05%、B:0.0005〜0.005%、N:0.001〜0.010%、P:0.03%以下、S:0.02%以下、O:0.015%以下、残部がFeおよび不可避的不純物および/又は付随的成分よりなり、(1)式及び(2)式を満足することを特徴とする熱間成形加工後の硬化能に優れた鋼板。
Ti/47.88−N/14.01≧0 ・・・(1)式
(0.06+0.4×%C)×(1+0.64×%Si)×(1+4.1×%Mn)×(1+2.33×%Cr)×(1+3.14×%Mo)×{1+1.5×(0.9−%C)×%B2 }≧1.0 ・・・(2)式
【0007】
(2)付随的成分として質量%で、Cr:0.01〜1%、Mo:0.005〜1%の1種あるいは2種を含有することを特徴とする前記(1)に記載の高温成形後硬化能に優れた鋼板。
(3)付随的成分として質量%で、Nb:0.005〜5%、V:0.01〜0.5%の1種あるいは2種を含有することを特徴とする前記(1)または(2)に記載の高温成形後硬化能に優れた鋼板。
【0008】
(4)付随的成分として質量%で、Ni:0.005〜1%、Cu:0.01〜1%の1種あるいは2種を含有することを特徴とする前記(1)〜(3)に記載の高温成形後硬化能に優れた鋼板。
(5)前記(1)〜(4)に記載の鋼板をAc3 変態点以上のオーステナイト領域に加熱後、Ar3 変態点以上の温度で成形加工を開始し、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ことを特徴とする熱間成形加工後の硬化能に優れた鋼板の使用方法にある。
【0009】
【発明の実施の形態】
本発明では、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。また、熱間成形の方法としては、Ac3 変態点以上のオーステナイト領域に加熱後、Ar3 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方法である。
【0010】
次に、鋼板の化学成分について説明する。
Cは、基地中に固溶あるいは炭化物として析出し、鋼の強度を増加させる元素であり、また、セメンタイト、パーライト、ベイナイト、マルテンサイト等の硬質な第2相として析出し、高強度化と一様伸びの向上に寄与する。強度向上のために0.10%以上のCが必要であるが、C含有量が0.20%を超えると、焼入れまま状態での靱性が低下するため、Cは0.10〜0.20%の範囲に規定した。尚、強度と靱性のバランスを考慮すると、好ましい範囲は0.15〜0.20%である。
【0011】
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0.01%のSiが必要であるが、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0.01〜1%の範囲に規定した。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。
Mnは、強度および焼入れ性を向上させる元素であり、0.3%未満では焼入れ時の強度を十分に得られず、また、2.0%を超えて添加しても効果が飽和するとともに靱性が劣化するため、Mnは0.3〜2.0%の範囲に規定した。尚、強度と靱性のバランスを考慮すると、好ましい範囲は、0.5〜1.5%である。
【0012】
Alは、溶鋼の脱酸材として使われる必要な元素であり、またNを固定する元素でもあり、その量は結晶粒径や機械的性質に大きな影響を及ぼす。このような効果を有するためには0.01%以上の含有量が必要であるが、0.1%を超えると非金属介在物が多くなり製品に表面疵が発生しやすくなる。このため、Alは0.01〜0.1%の範囲に規定した。
Tiは、B添加による焼入れ性を安定かつ効果的に向上させるために作用するが、0.005%未満およびTi/47.88−N/14.01≧0式を満足しない範囲では効果が期待できず、0.05%超ではTiの窒化物が多く生成して、靱性が劣化する傾向があるため、Tiは0.005〜0.05%の範囲に規定した。
【0013】
Bは、微量添加で鋼材の焼入れ性を大幅に向上させる元素であり、また、粒界強化およびM23(C,B)6 などとして析出強化の効果もある。添加量が0.0005%未満では焼入れ性に効果が期待できず、また、0.005%を超えると粗大なB含有相を生成する傾向があり、また、脆化が起こりやすくなる。このため、Bは0.0005〜0.005%の範囲に規定した。
Nは窒化物または炭窒化物を析出させ、強度を高める重要な元素の一つである。0.001%以上の添加により効果を発揮するが、0.01%を超えると窒化物の粗大化および固溶Nによる時効硬化により、靱性が劣化する傾向がみられる。このため、Nは0.001〜0.01%の範囲に規定した。
【0014】
Pは、溶接割れ性および靱性に悪影響を及ぼす元素であるため、Pは0.03%以下に規制した。なお、好ましくは、0.02%以下である。
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、靱性劣化,異方性および再熱割れ感受性の増大の原因となる。このため、Sは0.02%以下に規定した。なお、好ましくは、0.01%以下である。
Oは、靱性に悪影響を及ぼす酸化物の生成の原因となるとともに、疲労破壊の起点となる酸化物を生成するため、上限を0.015%に規定した。
【0015】
これらに加えて、以下の目的で下記付随的成分を添加することができる。
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。0.01%未満ではこれらの効果が十分期待できにくく、また、1%を超えると降伏強度が過度に上昇する傾向であるため、Crは0.01〜1%の範囲が望ましい。より好ましくは0.05〜1%である。
Moは、焼入れ性を向上させる元素であり、また固溶強化をもたらす元素であるとともに、マトリックス中のM23C6 型炭化物を安定化させる元素である。0.005%未満ではこの効果が十分期待できにくく、1%を超えると降伏強度が過度に上昇し、また靱性を劣化させる傾向であるため、Moは0.005〜1%の範囲が好ましい。
【0016】
Nbは、炭窒化物を形成し、強度を向上させる元素であるが、0.5%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傾向である。0.005%未満では強度向上の効果が発揮されにくいため、Nbは0.005〜0.5%の範囲が好ましい。
Vは、炭窒化物を形成し、強度を向上させる元素であるが、0.5%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傾向である。0.01%未満では強度向上の効果が発揮されにくいため、Vは0.01〜0.5%の範囲が好ましい。
【0017】
Niは、強度および靱性を向上させる元素であるが、1%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傾向である。0.005%未満では強度および靱性の向上効果が発揮されにくいため、Niは0.005〜1%の範囲が望ましい。より望ましくは0.01〜1%である。
Cuは、強度を向上させる元素であるが、1%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傾向である。0.01%未満では強度向上の効果が発揮されにくいため、Cuは0.01〜1%の範囲が好ましい。
【0018】
尚、上記の付随的成分は例であって、付随的成分は上記に限定されるものではない。
下式にしたがう値は、熱間成形後の硬さに影響し、その値が1.0未満では必要硬さが得られないため、その下限を1.0に規定した。
(0.06+0.4×%C)×(1+0.64×%Si)×(1+4.1× %Mn)×(1+2.33×%Cr)×(1+3.14×%Mo)×{1+1.5×(0.9−%C)×%B2 }≧1.0
【0019】
次に、熱間成形方法について説明する。
マルテンサイト変態させるためには、加熱時にオーステナイト組織になっている必要があるため、熱間成形前の加熱温度をオーステナイト領域となるAc3 変態点以上とした。熱間成形前の加熱方法は、炉加熱、誘導加熱、通電加熱等のいずれの加熱方法でもよいが、成形対象部分がほぼ均一な温度となっていることが望ましい。また、熱間成形開始温度がAr3 変態点よりも低くなると、フェライト相が出現するため、マルテンサイト変態後の強度が低下する。このため、熱間成形開始温度はAr3 変態点以上とすることが好ましい。
【0020】
【実施例】
表1の組成をも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、熱間圧延にて仕上温度850℃、巻取温度600℃で板厚4mmの熱延鋼板とした。また、一部の熱延鋼板を冷間圧延により板厚1.2mmの冷延鋼板とした。炉加熱によりAc3 点以上である950℃のオーステナイト領域に加熱した後、Ac3 点以上である900℃から水冷式金型を有するプレス機にてハットフォーム成形加工を行った。成形時間を約1秒とし、成形完了10秒間はプレス金型をそのままの状態にして金型による冷却を行った。また、10秒後の鋼板温度を測定した。成形された鋼板について、冷延鋼板の圧延方向に垂直な断面をビッカース硬度計にて硬度測定を実施し、更に光学顕微鏡にて金属組織を観察し、マルテンサイト率を測定した。また更に、板厚4mmの熱延鋼板を炉加熱によりAc3 点以上である950℃のオーステナイト領域に加熱した後、900℃から水冷した素材を用いて衝撃試験を実施した。その結果を表2に示す。
【0021】
【表1】
【0022】
【表2】
【0023】
表2に示した本発明例No.1〜8は、マルテンサイト率を90%以上とすることで熱間成形後の硬さがHv350以上、−60℃の衝撃値が90J/cm2 以上あり、形状凍結性も良く、自動車の構造部材として必要な特性を満足している。それに比較し、本発明の範囲を外れた比較例では、焼入れ硬さ、衝撃値および形状凍結性が劣化している。
【0024】
比較例No.16,19,21,24は、式Ti/47.88−N/14.01≧0を満足していないため、焼入れ性が不足し、焼入れ硬さを満足していない例である。
比較例No.9,11,13,14,15,17,18,19,20,21,22,25は、式(0.06+0.4×%C)×(1+0.64×%Si)×(1+4.1×%Mn)×(1+2.33×%Cr)×(1+3.14×%Mo)×{1+1.5×(0.9−%C)×%B2 }≧1.0を満足していないため、焼入れ性が不足し、焼入れ硬さを満足していない例である。
【0025】
比較例No.9は、C量が規定値より少ないために、焼入れ硬さを満足していない例であり、比較例No.10は、C量が規定値を超えているために靱性が低下した例である。
比較例No.11は、Si量が規定値より少ないために、焼入れ硬さを満足していない例であり、比較例No.12は、Si量が規定値を超えているために、衝撃値が低下した例である。
比較例No.13は、Mn量が規定値より少ないために焼入れ硬さを満足していない例であり、比較例No.14は、Mn量が規定値を超えているために、衝撃値が低下した例である。
【0026】
比較例No.15はP量が、比較例No.16はS量が、それぞれ規定値を超えているために、衝撃値が劣化した例であり、
比較例No.17は、Al量が規定値より少ないために脱酸が不十分で靱性が低下した例であり、比較例No.18は、Al量が規定値を超えているために、非金属介在物が多くなり衝撃値が低下した例である。
比較例No.19は、Ti量が規定値より少ないためにNを固定することができずにBによる焼入れ性が低下した例であり、比較例No.20は、Ti量が規定値を超えているために、Tiの窒化物が多く生成し衝撃値が低下した例である。
【0027】
比較例No.21は、B量が規定値より少ないために焼入れ性が低下した例であり、比較例No.22は、B量が規定値を超えているために、粗大なB含有相を生成し衝撃値が低下した例である。
比較例No.23は、N量が規定値より少ないために焼入れ硬さがが低下した例であり、比較例No.24は、Nが量規定値を超えているために、窒化物の粗大化および固溶窒素による時効硬化により衝撃値が低下した例である。
また、比較例No.20は、O量が規定値を超えているために酸化物が多く生成し、衝撃特性が劣化した例である。
【0028】
【発明の効果】
本発明鋼は、自動車部品の構造部材に使用され、熱間成形後の硬化能が高く高強度となる鋼板であり、また衝撃特性および形状凍結性にも優れており、加工工程の省略化に貢献することが可能である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a member that requires strength such as that used for a structural member of automobile parts, and more particularly, to a steel plate having excellent curability and impact properties after hot forming.
[0002]
[Prior art]
As one of the measures to improve the fuel efficiency of automobiles that originated from global environmental problems, the weight reduction of the vehicle body has been promoted, and it is necessary to increase the strength of steel plates used in automobiles as much as possible. However, in general, when the strength of a steel plate is increased in order to reduce the weight of an automobile, the elongation and the r value decrease, and the formability and the shape freezeability deteriorate.
In order to solve such a problem, Japanese Patent Application Laid-Open No. H10-228561 discloses a technique for forming the article warmly and using the heat at that time to increase the strength. This technique aims to appropriately control the components in the steel, hold and form in a temperature range of 200 to 850 ° C., and increase the strength using precipitation strengthening in this temperature range.
[0003]
Patent Document 2 proposes a high-strength steel sheet that has a low yield strength during warm pressing and a high yield strength at room temperature for the purpose of improving press forming accuracy. However, these techniques may limit the strength that can be obtained.
On the other hand, Patent Document 3 proposes a technique for heating to a high temperature austenite single phase region after molding and transforming to a hard phase in the subsequent cooling process for the purpose of obtaining higher strength. Furthermore, Patent Document 4 proposes a method for producing a thin steel plate having excellent formability and hardenability.
[0004]
[Cited document]
(1) Patent Document 1 (Japanese Patent Laid-Open No. 2000-234153)
(2) Patent Document 2 (Japanese Patent Laid-Open No. 2000-87183)
(3) Patent Document 3 (Japanese Patent Laid-Open No. 2000-38640)
(4) Patent Document 4 (Japanese Patent Laid-Open No. 2000-339025)
[0005]
[Problems to be solved by the invention]
As described above, it is difficult to manufacture a steel sheet suitable for hot pressing, which has high strength immediately after hot forming, using the techniques disclosed so far. The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a steel sheet excellent in post-hot-forming curability and impact properties that can obtain high hardness of Hv 350 or higher after hot forming. And
[0006]
[Means for Solving the Problems]
The gist of the present invention is as follows.
(1) By mass%, C: 0.10 to less than 0.20%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, Al: 0.01 to 0.50 %, Ti: 0.005-0.05%, B: 0.0005-0.005%, N: 0.001-0.010%, P: 0.03% or less, S: 0.02% or less , O: 0.015% or less, the balance being Fe and inevitable impurities and / or incidental components, satisfying the formulas (1) and (2) Excellent steel plate.
Ti / 47.88-N / 14.01 ≧ 0 (1) Formula (0.06 + 0.4 ×% C) × (1 + 0.64 ×% Si) × (1 + 4.1 ×% Mn) × ( 1 + 2.33 ×% Cr) × (1 + 3.14 ×% Mo) × {1 + 1.5 × (0.9−% C) ×% B 2 } ≧ 1.0 (2) Formula
(2) The high temperature according to (1) above, wherein the incidental component contains one or two of Cr: 0.01 to 1% and Mo: 0.005 to 1% in mass%. Steel sheet with excellent post-forming hardenability.
(3) The above-mentioned (1) or (1), wherein the incidental component contains one or two of Nb: 0.005 to 5% and V: 0.01 to 0.5% by mass%. A steel plate having excellent post-high temperature forming hardenability as described in 2).
[0008]
(4) The above-mentioned (1) to (3), wherein the incidental component contains one or two of Ni: 0.005 to 1% and Cu: 0.01 to 1% in mass%. A steel sheet having excellent post-high temperature forming hardenability described in 1.
(5) After heating the steel sheet described in (1) to (4) above to the austenite region above the Ac 3 transformation point, the forming process is started at a temperature above the Ar 3 transformation point, and heat is removed with a mold simultaneously with the processing. It is in the usage method of the steel plate excellent in the hardenability after the hot forming process characterized by carrying out rapid cooling by carrying out, and making it martensite transform and harden.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, a hot rolled material or a cold rolled material having a specific chemical composition is used. Also, as a hot forming method, after heating to the austenite region above the Ac 3 transformation point, molding processing (for example, press working) is started at a temperature above the Ar 3 transformation point, and heat is removed with a mold simultaneously with the processing. This is a method of rapidly cooling and transforming to martensite and curing.
[0010]
Next, chemical components of the steel plate will be described.
C is an element that precipitates as a solid solution or carbide in the matrix and increases the strength of the steel, and also precipitates as a hard second phase such as cementite, pearlite, bainite, martensite, etc. Contributes to the improvement of the growth. 0.10% or more of C is necessary for improving the strength, but if the C content exceeds 0.20%, the toughness in the as-quenched state is lowered, so C is 0.10 to 0.20. In the range of%. In consideration of the balance between strength and toughness, the preferred range is 0.15 to 0.20%.
[0011]
Si is a solid solution strengthened alloy element, and 0.01% Si is necessary to ensure strength. However, if it exceeds 1%, a problem of surface scale occurs. For this reason, Si was prescribed | regulated in 0.01 to 1% of range. 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%.
Mn is an element that improves strength and hardenability. If it is less than 0.3%, sufficient strength at the time of quenching cannot be obtained, and even if added over 2.0%, the effect is saturated and toughness is achieved. Therefore, Mn was specified in the range of 0.3 to 2.0%. In consideration of the balance between strength and toughness, the preferred range is 0.5 to 1.5%.
[0012]
Al is a necessary element used as a deoxidizer for molten steel, and is also an element that fixes N, and its amount has a great influence on the crystal grain size and mechanical properties. In order to have such an effect, a content of 0.01% or more is necessary. However, if it exceeds 0.1%, nonmetallic inclusions increase and surface defects are likely to occur in the product. For this reason, Al was specified in the range of 0.01 to 0.1%.
Ti acts to improve the hardenability by addition of B stably and effectively, but the effect is expected in the range of less than 0.005% and not satisfying the formula of Ti / 47.88-N / 14.01 ≧ 0. However, if it exceeds 0.05%, a large amount of nitride of Ti is generated and the toughness tends to deteriorate, so Ti was specified in the range of 0.005 to 0.05%.
[0013]
B is an element that greatly improves the hardenability of the steel material by adding a small amount, and also has the effect of strengthening the grain boundary and precipitation strengthening such as M 23 (C, B) 6 . If the added amount is less than 0.0005%, no effect on hardenability can be expected, and if it exceeds 0.005%, a coarse B-containing phase tends to be formed, and embrittlement tends to occur. For this reason, B was specified in the range of 0.0005 to 0.005%.
N is one of important elements for precipitating nitrides or carbonitrides and increasing the strength. The effect is exhibited by addition of 0.001% or more, but if it exceeds 0.01%, the toughness tends to deteriorate due to the coarsening of nitride and age hardening due to solute N. For this reason, N was specified in the range of 0.001 to 0.01%.
[0014]
Since P is an element that adversely affects weld cracking and toughness, P is regulated to 0.03% or less. In addition, Preferably, it is 0.02% or less.
S affects non-metallic inclusions in the steel and deteriorates workability, and causes toughness deterioration, anisotropy and reheat cracking sensitivity. For this reason, S was specified to 0.02% or less. In addition, Preferably, it is 0.01% or less.
O causes generation of oxides that adversely affect toughness, and also generates oxides that serve as starting points for fatigue fracture, so the upper limit was specified to be 0.015%.
[0015]
In addition to these, the following incidental components can be added for the following purposes.
Cr is an element that improves hardenability and has the effect of precipitating M 23 C 6 type carbide in the matrix, and has the effect of increasing the strength and miniaturizing the carbide. If it is less than 0.01%, these effects cannot be expected sufficiently, and if it exceeds 1%, the yield strength tends to increase excessively, so Cr is desirably in the range of 0.01 to 1%. More preferably, it is 0.05 to 1%.
Mo is an element that improves hardenability, an element that causes solid solution strengthening, and an element that stabilizes M 23 C 6 type carbide in the matrix. If it is less than 0.005%, this effect cannot be expected sufficiently, and if it exceeds 1%, the yield strength tends to increase excessively and the toughness tends to deteriorate, so Mo is preferably in the range of 0.005 to 1%.
[0016]
Nb is an element that forms carbonitrides and improves strength, but if added over 0.5%, the yield strength tends to increase excessively. If it is less than 0.005%, the effect of improving the strength is hardly exhibited, so Nb is preferably in the range of 0.005 to 0.5%.
V is an element that forms carbonitrides and improves strength, but if added over 0.5%, the yield strength tends to increase excessively. If it is less than 0.01%, the effect of improving the strength is hardly exhibited, so V is preferably in the range of 0.01 to 0.5%.
[0017]
Ni is an element that improves the strength and toughness, but if added over 1%, the yield strength tends to increase excessively. If it is less than 0.005%, the effect of improving strength and toughness is hardly exhibited, so Ni is preferably in the range of 0.005 to 1%. More desirably, the content is 0.01 to 1%.
Cu is an element that improves the strength, but if added over 1%, the yield strength tends to increase excessively. If it is less than 0.01%, the effect of improving the strength is hardly exhibited, so Cu is preferably in the range of 0.01 to 1%.
[0018]
In addition, said incidental component is an example and an incidental component is not limited above.
The value according to the following formula affects the hardness after hot forming, and if the value is less than 1.0, the required hardness cannot be obtained, so the lower limit is defined as 1.0.
(0.06 + 0.4 ×% C) × (1 + 0.64 ×% Si) × (1 + 4.1 ×% Mn) × (1 + 2.33 ×% Cr) × (1 + 3.14 ×% Mo) × {1 + 1. 5 × (0.9−% C) ×% B 2 } ≧ 1.0
[0019]
Next, the hot forming method will be described.
In order to make the martensitic transformation, it is necessary to have an austenite structure at the time of heating, so the heating temperature before hot forming is set to the Ac 3 transformation point or more which becomes the austenite region. The heating method before hot forming may be any heating method such as furnace heating, induction heating, and electric heating, but it is desirable that the part to be formed has a substantially uniform temperature. In addition, when the hot forming start temperature is lower than the Ar 3 transformation point, the ferrite phase appears, so that the strength after the martensitic transformation is lowered. For this reason, it is preferable that the hot forming start temperature be equal to or higher than the Ar 3 transformation point.
[0020]
【Example】
Cast into various steel slabs having the composition of Table 1. These slabs were heated to 1200 ° C., and hot rolled into hot rolled steel sheets having a finishing temperature of 850 ° C., a winding temperature of 600 ° C., and a plate thickness of 4 mm. Some hot-rolled steel sheets were made into cold-rolled steel sheets with a thickness of 1.2 mm by cold rolling. After heating to an austenite region of 950 ° C., which is Ac 3 point or higher, by furnace heating, hat foam molding was performed from 900 ° C., which is Ac 3 point or higher, with a press machine having a water-cooled mold. The molding time was about 1 second, and for 10 seconds after the molding was completed, the press mold was left as it was and cooling with the mold was performed. Further, the steel plate temperature after 10 seconds was measured. With respect to the formed steel sheet, the hardness of the cross section perpendicular to the rolling direction of the cold-rolled steel sheet was measured with a Vickers hardness meter, the metal structure was observed with an optical microscope, and the martensite ratio was measured. Furthermore, after a hot rolled steel sheet having a thickness of 4 mm was heated in an austenite region of 950 ° C., which is 3 points or more in Ac, by furnace heating, an impact test was performed using a material that was water-cooled from 900 ° C. The results are shown in Table 2.
[0021]
[Table 1]
[0022]
[Table 2]
[0023]
Invention Example No. 1 shown in Table 2. Nos. 1 to 8 have a martensite ratio of 90% or more, the hardness after hot forming is Hv 350 or more, the impact value at −60 ° C. is 90 J / cm 2 or more, the shape freezing property is good, and the structure of an automobile Satisfies the characteristics required as a member. In contrast, in the comparative examples that are out of the scope of the present invention, the quenching hardness, impact value, and shape freezing property are deteriorated.
[0024]
Comparative Example No. Nos. 16, 19, 21, and 24 are examples that do not satisfy the formula Ti / 47.88-N / 14.01 ≧ 0, so that the hardenability is insufficient and the quenching hardness is not satisfied.
Comparative Example No. 9, 11, 13, 14, 15, 17, 18, 19, 20, 21, 22, 25 are expressed by the formula (0.06 + 0.4 ×% C) × (1 + 0.64 ×% Si) × (1 + 4.1). ×% Mn) × (1 + 2.33 ×% Cr) × (1 + 3.14 ×% Mo) × {1 + 1.5 × (0.9−% C) ×% B 2 } ≧ 1.0 is not satisfied. Therefore, this is an example in which hardenability is insufficient and quenching hardness is not satisfied.
[0025]
Comparative Example No. No. 9 is an example in which the quenching hardness is not satisfied because the amount of C is less than the specified value. No. 10 is an example in which the toughness is lowered because the amount of C exceeds the specified value.
Comparative Example No. No. 11 is an example in which the quenching hardness is not satisfied because the Si amount is less than the specified value. No. 12 is an example in which the impact value decreased because the Si amount exceeded the specified value.
Comparative Example No. No. 13 is an example in which the quenching hardness is not satisfied because the amount of Mn is less than the specified value. No. 14 is an example in which the impact value decreased because the amount of Mn exceeded the specified value.
[0026]
Comparative Example No. No. 15 has a P amount of Comparative Example No. 16 is an example in which the impact value deteriorates because the S amount exceeds the specified value.
Comparative Example No. No. 17 is an example in which the amount of Al is less than the specified value, so that deoxidation is insufficient and toughness is reduced. 18 is an example in which the amount of Al exceeds the specified value, so that the non-metallic inclusions increase and the impact value decreases.
Comparative Example No. No. 19 is an example in which N was not able to be fixed because the Ti amount was less than the specified value, and the hardenability by B was lowered. No. 20 is an example in which the amount of Ti exceeds the specified value, so that a large amount of Ti nitride is generated and the impact value is lowered.
[0027]
Comparative Example No. No. 21 is an example in which the hardenability was lowered because the B amount was less than the specified value. No. 22 is an example in which since the amount of B exceeds the specified value, a coarse B-containing phase is generated and the impact value is lowered.
Comparative Example No. No. 23 is an example in which the quenching hardness was lowered because the N amount was less than the specified value. No. 24 is an example in which the impact value decreased due to the coarsening of the nitride and age hardening with solute nitrogen because N exceeded the specified amount.
Comparative Example No. No. 20 is an example in which a large amount of oxide is generated because the amount of O exceeds the specified value, and the impact characteristics are deteriorated.
[0028]
【The invention's effect】
The steel of the present invention is a steel plate that is used for structural parts of automobile parts and has high hardenability after hot forming and high strength, and also has excellent impact characteristics and shape freezing properties, thus eliminating the need for processing steps. It is possible to contribute.
Claims (5)
C :0.10〜0.20%未満、
Si:0.01〜1.0%、
Mn:0.3〜2.0%、
Al:0.01〜0.50%、
Ti:0.005〜0.05%、
B :0.0005〜0.005%、
N :0.001〜0.010%、
P :0.03%以下、
S :0.02%以下、
O :0.0015%以下、
残部がFeおよび不可避的不純物および/又は付随的成分よりなり、(1)式及び(2)式を満足することを特徴とする熱間成形加工後の硬化能に優れた鋼板。
Ti/47.88−N/14.01≧0 ・・・(1)式
(0.06+0.4×%C)×(1+0.64×%Si)×(1+4.1×%Mn)×(1+2.33×%Cr)×(1+3.14×%Mo)×{1+1.5×(0.9−%C)×%B2 }≧1.0 ・・・(2)式% By mass
C: 0.10 to less than 0.20%,
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.0%,
Al: 0.01 to 0.50%,
Ti: 0.005 to 0.05%,
B: 0.0005 to 0.005%,
N: 0.001 to 0.010%,
P: 0.03% or less,
S: 0.02% or less,
O: 0.0015% or less,
A steel plate excellent in curability after hot forming, characterized in that the balance consists of Fe and inevitable impurities and / or incidental components and satisfies the formulas (1) and (2).
Ti / 47.88-N / 14.01 ≧ 0 (1) Formula (0.06 + 0.4 ×% C) × (1 + 0.64 ×% Si) × (1 + 4.1 ×% Mn) × ( 1 + 2.33 ×% Cr) × (1 + 3.14 ×% Mo) × {1 + 1.5 × (0.9−% C) ×% B 2 } ≧ 1.0 (2)
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