JPH1150186A - Hot-rolled steel plate with high strength and high workability, excellent in impact resistance and reduced in yield ratio - Google Patents
Hot-rolled steel plate with high strength and high workability, excellent in impact resistance and reduced in yield ratioInfo
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- JPH1150186A JPH1150186A JP14957298A JP14957298A JPH1150186A JP H1150186 A JPH1150186 A JP H1150186A JP 14957298 A JP14957298 A JP 14957298A JP 14957298 A JP14957298 A JP 14957298A JP H1150186 A JPH1150186 A JP H1150186A
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- phase
- mass
- strength
- martensite
- impact resistance
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Abstract
Description
【０００１】[0001]
【発明の属する技術分野】この発明は、自動車用鋼板と
しての用途に用いて好適な耐衝撃特性に優れた高強度高
加工性熱延鋼板に関するものである。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength and high-workability hot-rolled steel sheet excellent in impact resistance and suitable for use as a steel sheet for automobiles.
【０００２】[0002]
【従来の技術】自動車の軽量化が指向される中、成形性
に優れる高強度薄鋼板に対する要求が殊の外強くなって
いる。また、最近では、自動車の安全性も重視され、そ
のためには衝突時における安全性の目安となる耐衝撃特
性の向上も要求されている。さらに、経済性に対する配
慮も必要とされ、かかる経済性を考慮した場合には、冷
延鋼板に比べると熱延鋼板の方が有利である。2. Description of the Related Art With the aim of reducing the weight of automobiles, the demand for high-strength thin steel sheets having excellent formability has become particularly strong. In addition, recently, importance has been placed on the safety of automobiles, and for that purpose, an improvement in impact resistance, which is a measure of safety in a collision, is required. Furthermore, consideration for economic efficiency is also required, and in consideration of such economic efficiency, a hot-rolled steel sheet is more advantageous than a cold-rolled steel sheet.
【０００３】上記の現状を背景として、これまでにも種
々の高強度熱延鋼板が開発されている。例えば、特公平
６-41617号公報、特公平５-65566号公報および特公平５
-67682号公報には、高加工性高強度熱延鋼板として、残
留オーステナイト：５％以上を含むフェライト、ベイナ
イトおよび残留オーステナイトの組織になる鋼（以下、
ＴＲＩＰ鋼という）の製造方法が開示されている。しか
しながら、このＴＲＩＰ鋼は、伸びが高く、成形性は良
好ではある（TS×El≧ 24000 MPa・％）ものの、現在の
厳しい耐衝撃特性を満足するまでにはいかないところに
問題を残していた。また、プレス成形時における加工硬
化量（ＷＨ）およびその後の塗装焼付時における焼付硬
化量（ＢＨ）が、70 MPa程度と低いという問題もあっ
た。この加工・焼付硬化量（ＷＨ＋ＢＨ）が低いと、加
工−塗装焼付後における強度保証の面での不利が大き
い。[0003] Against the background of the above situation, various high-strength hot-rolled steel sheets have been developed. For example, Japanese Patent Publication No. 6-41617, Japanese Patent Publication No. 5-65566, and Japanese Patent Publication
JP-67682 discloses a high-workability, high-strength hot-rolled steel sheet containing a retained austenite: steel containing 5% or more of ferrite, bainite, and a retained austenite structure (hereinafter, referred to as a steel).
(Referred to as TRIP steel). However, although this TRIP steel has high elongation and good formability (TS × El ≧ 24000 MPa ·%), it still has a problem in that it cannot meet the current severe impact resistance. There is also a problem that the work hardening amount (WH) at the time of press molding and the baking hardening amount (BH) at the time of subsequent baking of paint are as low as about 70 MPa. If the amount of work and bake hardening (WH + BH) is low, there is a great disadvantage in terms of guaranteeing the strength after work-paint baking.
【０００４】一方、耐衝撃特性に優れた高強度熱延鋼板
としては、特開平９−111396号公報に開示されているよ
うに、フェライトとマルテンサイトの２相組織になるい
わゆるDual Phase鋼（以下ＤＰ鋼という）が開発されて
いる。しかしながら、このＤＰ鋼は、耐衝撃特性には優
れるものの、伸びが十分とはいえず、成形性の点に問題
を残していた。On the other hand, as a high-strength hot-rolled steel sheet having excellent impact resistance, as disclosed in Japanese Patent Application Laid-Open No. 9-111396, a so-called Dual Phase steel (hereinafter, referred to as a two-phase structure of ferrite and martensite) is disclosed. DP steel) has been developed. However, although this DP steel is excellent in impact resistance, it cannot be said that elongation is sufficient, and there is a problem in formability.
【０００５】[0005]
【発明が解決しようとする課題】上述したとおり、現在
までのところ、十分な成形性と厳しい安全性の両者を満
足する熱延鋼板は見当たらず、その開発が望まれてい
た。この発明は、上記の要望に有利に応えるもので、優
れた成形性と耐衝撃特性を兼ね備え（具体的には、強度
−伸びバランス（TS×El）が 24000 MPa・％以上、動的
ｎ値が0.35以上）、しかも降伏比が65％以下と低く、か
つ加工・焼付硬化量（ＷＨ＋ＢＨ）が 100 MPa以上と高
い、耐衝撃特性に優れかつ降伏比が低い高強度高加工性
熱延鋼板を提案することを目的とする。As described above, no hot-rolled steel sheet satisfying both sufficient formability and strict safety has been found so far, and its development has been desired. The present invention advantageously satisfies the above demands and has both excellent moldability and impact resistance (specifically, the strength-elongation balance (TS × El) is 24000 MPa ·% or more, and the dynamic n value Is 0.35 or more), and the yield ratio is as low as 65% or less, and the amount of work and bake hardening (WH + BH) is as high as 100 MPa or more. It is excellent in impact resistance and has a low yield ratio. The purpose is to propose.
【０００６】ここに、動的ｎ値とは、発明者らが耐衝撃
特性の指標として新たに見出したもので、この動的ｎ値
を用いることによって、耐衝撃特性を従来よりも一層的
確に評価することができる。すなわち、従来、耐衝突安
全性については、強度との関連で考察され、単に強度が
大きければ耐衝突安全性も高いとされてきたが、強度と
耐衝突安全性とは必ずしも一義的な関係にあるわけでは
ないことが判明した。そこで、この点につき、鋭意研究
を重ねた結果、耐衝突安全性を向上させる、つまり高速
での変形時（自動車の衝突時にはひずみ速度Here, the dynamic n value is newly found by the present inventors as an index of the impact resistance, and by using this dynamic n value, the impact resistance can be more accurately measured than before. Can be evaluated. In other words, in the past, collision safety was considered in relation to strength, and it was considered that the higher the strength, the higher the crash safety. However, the relationship between strength and collision safety is not necessarily unique. It turned out not to be the case. Therefore, as a result of diligent research on this point, it has been found that the collision safety is improved, that is, when the vehicle is deformed at high speed (the strain rate is
【外１】 が２×103/s まで増加）におけるエネルギーを、鋼板で
より多く吸収するためには、鋼板を[Outside 1] The energy in but increased to 2 × 10 3 / s), in order to absorb more with the steel sheet, the steel sheet
【外２】 の条件で引張変形させた時のｎ値（以下、動的ｎ値とい
う）を高くすることが有効であることが解明されたので
ある。ここでは、伸び10％における瞬間ｎ値を動的ｎ値
とする。なお、この動的ｎ値を高くすることは、高速変
形時における強度向上にも有効であることが併せて見出
された。[Outside 2] It has been clarified that it is effective to increase the n value (hereinafter referred to as dynamic n value) when tensile deformation is performed under the following conditions. Here, an instantaneous n value at an elongation of 10% is defined as a dynamic n value. In addition, it was also found that increasing the dynamic n value is effective for improving strength during high-speed deformation.
【０００７】[0007]
【課題を解決するための手段】以下、この発明の解明経
緯について説明する。さて、発明者らは、上記の目的を
達成すべく、まず従来鋼であるＴＲＩＰ鋼について、そ
の組織と特性との関係について調査した。その結果、Ｔ
ＲＩＰ鋼においては、成形性の向上に有利な残留オース
テナイトを十分な量得るためには、ベイナイト相を生成
させることが不可欠とされてきたが、このベイナイト相
が耐衝撃特性を劣化させる原因になっていることが判明
した。The details of the invention will be described below. By the way, the present inventors first investigated the relationship between the structure and properties of a conventional TRIP steel in order to achieve the above object. As a result, T
In the RIP steel, it has been indispensable to form a bainite phase in order to obtain a sufficient amount of retained austenite which is advantageous for improving formability. However, this bainite phase causes deterioration of impact resistance. Turned out to be.
【０００８】そこで、発明者らは、かようなベイナイト
相とくに炭化物の生成を抑制したところ、すなわち、主
相である初析フェライト以外の第２相を、従来のベイナ
イト＋残留オーステナイトから、針状フェライト＋マル
テンサイト＋残留オーステナイトの混合組織に変更した
ところ、所期した目的の達成に関し、望外の成果が得ら
れたのである。さらに、第２相の形成に際し、マルテン
サイトに対する針状フェライトの生成量を多めにする
と、降伏比が効果的に低減し、加工性の面でより有利に
なることも併せて見出した。この発明は、上記の知見に
立脚するものである。[0008] Then, the present inventors have suppressed the formation of such a bainite phase, particularly carbides, that is, the second phase other than the proeutectoid ferrite, which is the main phase, is converted from the conventional bainite + residual austenite into acicular phase. When the structure was changed to a mixed structure of ferrite + martensite + retained austenite, unexpected results were achieved in achieving the intended purpose. Furthermore, it has also been found that, when forming the second phase, if the amount of acicular ferrite with respect to martensite is increased, the yield ratio is effectively reduced, and the workability is further improved. The present invention is based on the above findings.
【０００９】すなわち、この発明は、Ｃ：0.05〜0.40ma
ss％、 Si：1.0 〜3.0 mass％、Mn：0.6 〜3.0 ma
ss％、 Cr：0.2 〜2.0 mass％を含有し、残部は実
質的にFeの組成になり、鋼組織が、初析フェライトを主
相として、マルテンサイト、針状フェライトおよび残留
オーステナイトからなる第２相を有し、かつ該第２相中
の針状フェライトとマルテンサイトが下記式の関係を満
足することを特徴とする耐衝撃特性に優れた高強度高加
工性熱延鋼板である。 記 2.0 ≦ＦA ／ＭA ≦ 20 ここで、ＦA : 第２相中における針状フェライトの面積
率（％） ＭA : 第２相中におけるマルテンサイトの面積率（％）That is, the present invention provides a method for producing C: 0.05 to 0.40 ma.
ss%, Si: 1.0 to 3.0 mass%, Mn: 0.6 to 3.0 ma
ss%, Cr: 0.2 to 2.0 mass%, the balance being substantially Fe, and the steel structure is composed of a second phase composed of martensite, acicular ferrite, and retained austenite with proeutectoid ferrite as a main phase. A high-strength, high-workability hot-rolled steel sheet having excellent impact resistance, characterized by having a phase and acicular ferrite and martensite in the second phase satisfy the following formula: Serial in 2.0 ≦ F A / M A ≦ 20 wherein, F A: the area ratio of the acicular ferrite in the second phase (%) M A: area ratio of martensite in the second phase (%)
【００１０】この発明では、鋼の成分組成につき、上記
した基本組成の他、オーステナイト生成元素として Ｐ：0.01〜0.2 mass％、 Al：0.01〜0.3 mass％ のうちから選んだ少なくとも一種を、またさらには強度
改善成分として Ti：0.005 〜0.25mass％、 Nb：0.003 〜0.1 mass％ のうちから選んだ少なくとも一種を含有させることもで
きる。[0010] In the present invention, in addition to the basic composition described above, at least one element selected from the group consisting of P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass% as an austenite-forming element, and furthermore, May contain at least one selected from the group consisting of Ti: 0.005 to 0.25 mass% and Nb: 0.003 to 0.1 mass% as a strength improving component.
【００１１】また、この発明においては、鋼組織中に占
める第２相の比率は３〜40％とすることが好ましい。In the present invention, the ratio of the second phase in the steel structure is preferably 3 to 40%.
【００１２】[0012]
【発明の実施の形態】以下、この発明を具体的に説明す
る。図１に、従来のＴＲＩＰ鋼の代表的な連続冷却変態
曲線図（ＣＣＴ図）を示す。同図に示したとおり、従来
のＴＲＩＰ鋼は、熱間圧延後、初析フェライト域に若干
保持して初析フェライト（ポリゴナルフェライトともい
う）を析出させ、同時に未変態オーステナイト相への固
溶炭素の濃縮を促進して、オーステナイトの安定度を増
したのち、ベイナイト域に導き、この領域を徐冷するこ
とによって、ベイナイト変態を生じさせつつ、所定量の
オーステナイトを残留させていた。しかしながら、この
ようにして製造されたＴＲＩＰ鋼は、強度および加工性
の面では優れるものの、十分な耐衝撃特性が得られない
ことは前述したとおりである。BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. FIG. 1 shows a typical continuous cooling transformation curve diagram (CCT diagram) of a conventional TRIP steel. As shown in the figure, after hot rolling, the conventional TRIP steel is slightly retained in the pro-eutectoid ferrite region to precipitate pro-eutectoid ferrite (also called polygonal ferrite), and at the same time, to form a solid solution in the untransformed austenite phase. After promoting the concentration of carbon to increase the stability of austenite, it was led to a bainite region, and the region was gradually cooled to cause a bainite transformation while leaving a predetermined amount of austenite. However, as described above, the TRIP steel manufactured as described above has excellent strength and workability but does not have sufficient impact resistance.
【００１３】そこで、発明者らは、ベイナイト変態を回
避すべく数多くの実験と検討を重ねた結果、(1) 鋼成分
としてCrを少量含有させると、上記ＣＣＴ図におけるベ
イナイト変態域のノーズが後退して、ベイナイトの析出
（特に炭化物の析出）が抑制され、代わりに針状フェラ
イト（アシキュラーフェライトともいう）が析出する、
(2) かようにして形成された、針状フェライト、残留オ
ーステナイトおよびマルテンサイトからなる第２相は、
成形性を阻害することなしに、耐衝撃特性を格段に向上
させることを究明したのである。The inventors have conducted numerous experiments and studies to avoid bainite transformation. As a result, (1) when a small amount of Cr is contained as a steel component, the nose of the bainite transformation region in the CCT diagram recedes. As a result, the precipitation of bainite (particularly the precipitation of carbides) is suppressed, and instead needle-like ferrite (also called acicular ferrite) precipitates,
(2) The thus formed second phase consisting of acicular ferrite, retained austenite and martensite is
They have sought to significantly improve the impact resistance without impairing the formability.
【００１４】図２に、この発明の成分系における代表的
ＣＣＴ図を示す。同図に示したとおり、Crを少量添加す
ることによってベイナイト変態域のノーズが後退し、代
わりに針状フェライト域が顕著に出現するので、この針
状フェライト域に短時間保持し、好ましくはその後に急
冷することによって、第２相を針状フェライト、残留オ
ーステナイトおよびマルテンサイトからなる混合組織と
することができ、かくして優れた成形性と耐衝撃特性と
を兼ね備えた熱延鋼板を得ることができたのである。FIG. 2 shows a representative CCT diagram for the component system of the present invention. As shown in the figure, by adding a small amount of Cr, the nose of the bainite transformation region recedes, and instead, a needle-like ferrite region appears remarkably. By rapidly cooling, the second phase can have a mixed structure composed of acicular ferrite, retained austenite and martensite, and thus a hot-rolled steel sheet having both excellent formability and impact resistance can be obtained. It was.
【００１５】ここに、針状フェライトとは、結晶粒の長
径が概ね10μm 以下、アスペクト比が１：1.5 以上、そ
してセメンタイト析出量が５％以下のものをいう。な
お、従来のＴＲＩＰ鋼のベイナイト中には、セメンタイ
トの析出が多く認められる（10％以上）ので、この発明
の針状フェライトとＴＲＩＰ鋼のベイナイトとは明確に
区別されるものである。Here, the acicular ferrite refers to a ferrite having a major axis of crystal grains of approximately 10 μm or less, an aspect ratio of 1: 1.5 or more, and a precipitation of cementite of 5% or less. In addition, since the precipitation of cementite is often observed in the bainite of the conventional TRIP steel (10% or more), the acicular ferrite of the present invention and the bainite of the TRIP steel are clearly distinguished.
【００１６】図３(a) に、この発明に従い得られる第２
相の特徴的な相構成を、また図３(b) には、従来のＴＲ
ＩＰ鋼の第２相の相構成を、それぞれ模式で示す。従来
のＴＲＩＰ鋼の第２相は、ベイナイト中に残留オーステ
ナイトが点在する相構成になっているのに対し、この発
明の第２相は、針状フェライトとマルテンサイトが層状
にならび、その界面（マルテンサイト側）に残留オース
テナイトが点在する形態になっている。このように、第
２相中に針状フェライトを析出させたことが、この発明
の特徴の一つであり、この針状フェライト相がTS×Elを
増加させると共に、動的ｎ値を向上させるものと考えら
れる。FIG. 3 (a) shows a second example obtained according to the present invention.
FIG. 3 (b) shows a conventional TR structure.
The phase structure of the second phase of the IP steel is schematically shown. Whereas the second phase of the conventional TRIP steel has a phase structure in which residual austenite is scattered in bainite, the second phase of the present invention is characterized in that acicular ferrite and martensite are layered and the interface thereof is formed. (Martensite side) in a form in which retained austenite is scattered. As described above, it is one of the features of the present invention that the needle-like ferrite is precipitated in the second phase. The needle-like ferrite phase increases TS × El and improves the dynamic n value. It is considered something.
【００１７】ところで、発明者らの実験によれば、図２
に示す冷却工程において、針状フェライト域での保持時
間を、ベイナイト変態が生じない範囲でできるだけ長く
し、針状フェライトの生成量を増大させたところ、降伏
比が効果的に低減することが判明した。According to the experiments conducted by the inventors, FIG.
In the cooling step shown in the figure, it was found that the retention time in the acicular ferrite region was made as long as possible without causing bainite transformation and the amount of acicular ferrite generated was increased, and the yield ratio was effectively reduced. did.
【００１８】すなわち、上記のようにして析出させた針
状フェライトとマルテンサイトの比が面積率で、次式 2.0 ≦ＦA ／ＭA ≦ 20 ここで、ＦA : 第２相中における針状フェライトの面積
率（％） ＭA : 第２相中におけるマルテンサイトの面積率（％） の関係を満足する範囲に制御することによって、降伏比
を65％以下まで低減することができたのである。[0018] That is, the ratio of acicular ferrite and martensite were precipitated as described above is an area ratio, wherein the following formula 2.0 ≦ F A / M A ≦ 20, F A: needle in the second phase By controlling the area ratio of ferrite (%) M A to the range that satisfies the relationship of the area ratio of martensite (%) in the second phase, the yield ratio could be reduced to 65% or less. .
【００１９】Ｃ：0.10mass％、Si：1.3 mass％、Mn：1.
5 mass％、Ｐ：0.01mass％、Ｓ：0.005 mass％、Al：0.
04mass％、Ｎ：0.003 mass％およびCr：0.60mass％を含
有し、残部は実質的にFeの組成になる鋼スラブを、1200
℃で１時間加熱後、粗圧延し、ついで仕上げ温度：850
℃で熱間仕上げ圧延を終了したのち、 100℃/sの速度で
650〜700 ℃まで冷却し、この温度域に５秒保持してか
ら、同じく 100℃/sの速度で 400〜500 ℃まで冷却し、
この温度に10〜120 分の種々の時間保持したのち、70℃
/hの速度で室温まで冷却した。得られた熱延板から、引
張試験片を切り出し、降伏強さ(YS)、引張強さ(TS)およ
び伸び(El)を求めた。また、断面組織をＳＥＭ観察する
と共に、第２相の針状フェライトとマルテンサイトの分
散状態を画像解析して、材質との関係について調査し
た。得られた結果を、整理して図４に示す。C: 0.10 mass%, Si: 1.3 mass%, Mn: 1.
5 mass%, P: 0.01 mass%, S: 0.005 mass%, Al: 0.
A steel slab containing 04 mass%, N: 0.003 mass% and Cr: 0.60 mass%, with the balance being substantially Fe
After heating at ℃ for 1 hour, rough rolling was performed, and then finishing temperature: 850
After finishing hot finishing rolling at 100 ° C, at a rate of 100 ° C / s
Cool to 650-700 ° C, keep it in this temperature range for 5 seconds, then cool to 400-500 ° C at the same rate of 100 ° C / s,
After holding at this temperature for various times of 10 to 120 minutes,
Cooled to room temperature at a rate of / h. A tensile test piece was cut out from the obtained hot-rolled sheet, and the yield strength (YS), tensile strength (TS) and elongation (El) were determined. In addition, the cross-sectional structure was observed by SEM, and the dispersed state of the second phase acicular ferrite and martensite was image-analyzed to investigate the relationship with the material. The results obtained are summarized and shown in FIG.
【００２０】同図に示したとおり、第２相中における針
状フェライトとマルテンサイトとの面積率比ＦA ／ＭA
が 2.0〜20の範囲を満足する場合に、TS×El≧ 24000 M
Pa・％、YR≦65％という優れた強度−伸びバランスおよ
び低降伏比を得ることができた。そこで、この発明で
は、第２相中における針状フェライトとマルテンサイト
の比を、面積率比で 2.0〜20の範囲に限定したのであ
る。なお、この発明において、面積率は、顕微鏡写真を
画像解析することによって算出した。As shown in the figure, the area ratio F A / M A between acicular ferrite and martensite in the second phase.
Satisfies the range of 2.0 to 20, TS × El ≧ 24000 M
Excellent strength-elongation balance and low yield ratio of Pa ·% and YR ≦ 65% were obtained. Therefore, in the present invention, the ratio of acicular ferrite to martensite in the second phase is limited to an area ratio of 2.0 to 20. In the present invention, the area ratio was calculated by image analysis of a micrograph.
【００２１】また、この発明において、上記した第２相
の鋼組織中に占める比率は３〜40％とすることが好まし
い。というのは、相比率が３％に満たないと十分な耐衝
撃特性が得られず、一方40％を超えると伸びひいては強
度−伸びバランスが低下するからである。より好ましい
比率は10〜30％である。In the present invention, the ratio of the above-mentioned second phase in the steel structure is preferably 3 to 40%. The reason is that if the phase ratio is less than 3%, sufficient impact resistance cannot be obtained, while if it exceeds 40%, the elongation and hence the strength-elongation balance are reduced. A more desirable ratio is 10 to 30%.
【００２２】なお、この発明において、鋼組織は全て、
主相である初析フェライトと、第２相であるマルテンサ
イト、針状フェライトおよび残留オーステナイトの混合
相からなっているとは限らず、ベイナイト相などが若干
析出する場合もあるが、かような第３相が混入しても、
その比率が第２相全体の10％以下であれば特性上何ら問
題はない。In the present invention, all the steel structures are
It does not always consist of a mixed phase of proeutectoid ferrite as a main phase and martensite, a needle-like ferrite and a retained austenite as a second phase, and a bainite phase and the like may be slightly precipitated. Even if the third phase is mixed,
If the ratio is 10% or less of the entire second phase, there is no problem in characteristics.
【００２３】次に、この発明において、鋼板の成分組成
を前記の範囲に限定した理由について説明する。 Ｃ：0.05〜0.40mass％ Ｃは、鋼の強化に有効に寄与するだけでなく、残留オー
ステナイトを得る上でも有用な元素である。しかしなが
ら、含有量が0.05mass％未満では、その効果に乏しく、
一方0.40mass％を超えると延性を低下させるので、Ｃ量
は0.05〜0.40mass％の範囲に限定した。Next, the reason why the composition of the steel sheet is limited to the above range in the present invention will be described. C: 0.05 to 0.40 mass% C is an element that not only effectively contributes to the strengthening of steel but is also useful in obtaining retained austenite. However, if the content is less than 0.05 mass%, the effect is poor,
On the other hand, if it exceeds 0.40 mass%, the ductility decreases, so the C content is limited to the range of 0.05 to 0.40 mass%.
【００２４】Si：1.0 〜3.0 mass％ Siは、残留オーステナイトの生成に不可欠な元素であ
り、そのためには少なくとも 1.0mass％の添加を必要と
するが、 3.0mass％を超える添加は、延性の低下を招く
だけでなく、スケール性状を低下させ表面品質上も問題
となるので、Si含有量は 1.0〜3.0 mass％の範囲に限定
した。Si: 1.0 to 3.0 mass% Si is an element indispensable for the formation of retained austenite. For this purpose, at least 1.0 mass% must be added. In addition, Si content is limited to the range of 1.0 to 3.0 mass%, because not only does this cause deterioration of the scale properties, but also poses a problem in surface quality.
【００２５】Mn：0.6 〜3.0 mass％ Mnは、鋼の強化元素として有用なだけでなく、残留オー
ステナイトを得る上でも有用な元素である。しかしなが
ら、含有量が 0.6mass％未満ではその効果に乏しく、一
方 3.0mass％を超えると延性の低下を招くので、Mn量は
0.6〜3.0 mass％の範囲に限定した。Mn: 0.6 to 3.0 mass% Mn is not only useful as a strengthening element for steel, but also useful for obtaining retained austenite. However, if the content is less than 0.6 mass%, the effect is poor, while if it exceeds 3.0 mass%, the ductility is reduced.
Limited to the range of 0.6 to 3.0 mass%.
【００２６】Cr：0.2 〜2.0 mass％ このCr添加は、この発明の特徴の一つである。Crを添加
することにより、前述したように、第２相が針状フェラ
イト化する。そのためには、0.2 mass％以上の添加が必
要であるが、 2.0mass％を超えて添加すると粗大なCr炭
化物が生成して延性が阻害され、強度−伸びバランスお
よび動的ｎ値とも劣化するので、Cr量は0.2 〜2.0 mass
％の範囲に限定した。好ましくは 0.3〜1.8 mass％であ
る。Cr: 0.2 to 2.0 mass% This addition of Cr is one of the features of the present invention. As described above, the addition of Cr causes the second phase to become acicular ferrite. For this purpose, it is necessary to add 0.2 mass% or more. However, if it exceeds 2.0 mass%, coarse Cr carbides are formed, ductility is inhibited, and the strength-elongation balance and the dynamic n value deteriorate. , Cr content is 0.2-2.0 mass
%. Preferably it is 0.3 to 1.8 mass%.
【００２７】図５および図６に、Cr量と強度−伸びバラ
ンスおよび動的ｎ値との関係について調べた結果をそれ
ぞれ示す。図５，６より明らかなように、Cr含有量が
0.2mass％以上、 2.0mass％以下の範囲で、TS×El≧240
00 (MPa・%)、動的ｎ値≧0.35の優れた加工性および耐
衝撃特性が得られている。FIGS. 5 and 6 show the results of investigation on the relationship between the Cr content and the strength-elongation balance and dynamic n value, respectively. As is clear from FIGS.
TS × El ≧ 240 in the range of 0.2 mass% or more and 2.0 mass% or less
Excellent workability and impact resistance of 00 (MPa ·%) and dynamic n value ≧ 0.35 are obtained.
【００２８】以上、基本成分について説明したが、この
発明では、オーステナイト生成元素としてＰやAl、また
強度改善成分としてTiやNbを、以下の範囲で適宜含有さ
せることができる。 Ｐ：0.01〜0.2 mass％ Ｐは、残留オーステナイト生成元素として有用である
が、含有量が0.01mass％に満たないとその添加効果に乏
しく、一方 0.2mass％を超えると耐二次加工性が劣化す
るので、添加する場合には0.01〜0.2 mass％の範囲とす
ることが望ましい。Although the basic components have been described above, in the present invention, P or Al as an austenite-forming element and Ti or Nb as a strength improving component can be appropriately contained in the following ranges. P: 0.01 to 0.2 mass% P is useful as a retained austenite-forming element, but if the content is less than 0.01 mass%, the effect of its addition is poor, while if it exceeds 0.2 mass%, the secondary workability deteriorates. Therefore, when it is added, it is desirable to set it in the range of 0.01 to 0.2 mass%.
【００２９】Al：0.01〜0.3 mass％ Alも、Ｐと同様、残留オーステナイト生成元素として有
用なものであるが、含有量が0.01mass％に満たないとそ
の添加効果に乏しく、一方 0.3mass％を超えると延性の
低下を招くので、添加する場合には0.01〜0.3 mass％の
範囲とすることが望ましい。Al: 0.01 to 0.3 mass% Al is also useful as a retained austenite forming element, like P, but if the content is less than 0.01 mass%, the effect of its addition is poor. If the amount exceeds the above range, the ductility is reduced. Therefore, when added, the content is preferably in the range of 0.01 to 0.3 mass%.
【００３０】Ti：0.005 〜0.25mass％、Nb：0.003 〜0.
1 mass％ TiおよびNbはいずれも、主相であるフェライトを細粒化
させることによって、強度の向上に有効に寄与するの
で、必要に応じて添加することができる。特にTiを含有
させると、針状フェライトのノーズが短時間側に移行
し、コイルミドル部と比較して冷却速度が速くなるコイ
ル端部においても十分針状フェライトが析出するので、
歩留りが向上する効果もある。しかしながら、含有量が
あまりに少ないとその添加効果に乏しく、一方過度の添
加は延性の低下を招くので、それぞれ上記の範囲で含有
させることが好ましい。Ti: 0.005 to 0.25 mass%, Nb: 0.003 to 0.2%
Since both 1 mass% Ti and Nb effectively contribute to the improvement of the strength by making ferrite as the main phase finer, they can be added as necessary. In particular, when Ti is included, the nose of the acicular ferrite shifts to a short time side, and the acicular ferrite is sufficiently precipitated even at the coil end where the cooling rate is faster than the coil middle part,
There is also an effect of improving the yield. However, if the content is too small, the effect of the addition is poor. On the other hand, excessive addition causes a decrease in ductility. Therefore, it is preferable that each content is in the above range.
【００３１】次に、この発明鋼の製造方法について具体
的に説明する。この発明鋼は、基本的に、第２相として
マルテンサイト、針状フェライトおよび残留オーステナ
イトからなる混合組織を形成させれば良いのであるか
ら、前掲図２に示した冷却曲線に沿って、冷却させれば
良い。そして、針状フェライト域での保持時間を、ベイ
ナイト変態が生じない範囲でできるだけ長くすることに
よって、針状フェライトの生成量を増大させてやれば良
く、かくして65％以下という優れた降伏比が得られるの
である。Next, the method for producing the steel of the present invention will be specifically described. Basically, the steel of the present invention only needs to form a mixed structure composed of martensite, acicular ferrite, and retained austenite as the second phase, so that the steel is cooled along the cooling curve shown in FIG. Just do it. Then, by increasing the holding time in the acicular ferrite region as long as possible within a range where bainite transformation does not occur, the amount of acicular ferrite generated can be increased, and thus an excellent yield ratio of 65% or less can be obtained. It is done.
【００３２】すなわち、 780〜980 ℃程度で熱間仕上げ
圧延後、 620〜780 ℃の初析フェライト域のノーズ近傍
まで冷却したのち、この温度域に１〜10秒程度保持（ま
たは緩冷却）することにより、主相である初析フェライ
トを析出させ、ついで 350〜500 ℃の針状フェライト域
まで冷却し、この領域で、少なくとも40分以上（ただし
ベイナイト変態が生じない時間）等温保持するかまたは
緩冷却後、好ましくは50℃/h以上の速度で室温まで冷却
することにより、針状フェライト、マルテンサイトおよ
び残留オーステナイトからなる第２相を形成させると共
に、第２相中における針状フェライトとマルテンサイト
の比を面積率比で 2.0〜20の範囲に制御するのである。That is, after hot finish rolling at about 780 to 980 ° C., the material is cooled to the vicinity of the nose of the pro-eutectoid ferrite region at 620 to 780 ° C., and then kept (or gently cooled) in this temperature range for about 1 to 10 seconds. As a result, proeutectoid ferrite, which is the main phase, is precipitated, and then cooled to a needle-like ferrite region at 350 to 500 ° C, and is kept isothermally in this region for at least 40 minutes (however, a time during which bainite transformation does not occur) or After slow cooling, cooling to room temperature, preferably at a rate of 50 ° C./h or more, forms a second phase composed of acicular ferrite, martensite, and retained austenite. The site ratio is controlled in the range of 2.0 to 20 in terms of area ratio.
【００３３】[0033]
実施例１ 表１示す種々の成分組成になる鋼スラブを、1200℃に加
熱後、粗圧延し、ついで仕上げ温度：860 ℃で熱間仕上
げ圧延を終了した後、80℃/sの速度で 700℃まで冷却
し、この温度に５秒保持してから、同じく80℃/sの速度
で 450℃まで冷却したのち、コイルに巻取り、巻取り
後、70〜90分間保持してから、70℃/hの速度で室温まで
冷却した。得られた熱延板から、引張試験片を切り出
し、それらの試験片について、ひずみ速度：２×10-2/s
の条件で引張試験を実施し、降伏強さ(YS)、引張強さ(T
S)および伸び(El)を求めた。また、ホプキンソンプレッ
シャーバー試験材（材料とプロセス vol.9 (1996)P.110
8〜1111）を用いて、ひずみ速度：２×103/s の条件で
引張試験を実施し、伸びが10％の時の瞬間ｎ値（動的ｎ
値）を求めた。さらに、プレス成形時における加工硬化
量（ＷＨ）およびその後の塗装焼付時(170℃）における
焼付硬化量（ＢＨ）についても測定した。なお、ＷＨ，
ＢＨは、ひずみ速度：２×10-2／ｓの引張試験機を用
い、図７により求めた。各熱延鋼板の鋼組織、TS×Elバ
ランス、YR、ＷＨ＋ＢＨおよび動的ｎ値ついて調べた結
果を整理して表２に示す。Example 1 Steel slabs having various component compositions shown in Table 1 were heated to 1200 ° C., rough-rolled, hot-finished at a finishing temperature of 860 ° C., and then heated at a rate of 80 ° C./s to 700 ° C. After cooling to 450 ° C at the same rate of 80 ° C / s, it is wound on a coil, held for 70 to 90 minutes, and then cooled to 70 ° C. Cooled to room temperature at a rate of / h. Tensile test pieces were cut out from the obtained hot-rolled sheet, and the strain rate of the test pieces was 2 × 10 −2 / s
Tensile test was conducted under the conditions described above, yield strength (YS) and tensile strength (T
S) and elongation (El) were determined. In addition, Hopkinson pressure bar test materials (Materials and Process vol.9 (1996) P.110
8-1111), a tensile test was performed under the conditions of a strain rate: 2 × 10 3 / s, and the instantaneous n value when the elongation was 10% (dynamic n
Value). Further, the work hardening amount (WH) at the time of press molding and the bake hardening amount (BH) at the time of subsequent coating baking (170 ° C.) were also measured. WH,
BH was determined according to FIG. 7 using a tensile tester having a strain rate of 2 × 10 −2 / s. Table 2 summarizes the results of a study on the steel structure, TS × El balance, YR, WH + BH, and dynamic n value of each hot-rolled steel sheet.
【００３４】[0034]
【表１】 [Table 1]
【００３５】[0035]
【表２】 [Table 2]
【００３６】表２に示したとおり、この発明に従い、第
２相として、マルテンサイト、針状フェライトおよび残
留オーステナイトの混合組織を形成させ、かつ第２相中
の針状フェライトとマルテンサイトとの面積率比ＦA ／
ＭA を 2.0〜20の範囲に制御したものはいずれも、TS×
El≧ 24000 MPa・％の優れた強度−伸びバランスとYR≦
65％の低降伏比、さらには動的ｎ値≧0.35という優れた
耐衝撃特性およびＷＨ＋ＢＨ≧ 100 MPa・％という高い
加工・焼付硬化量を得ることができた。As shown in Table 2, according to the present invention, a mixed structure of martensite, acicular ferrite and retained austenite was formed as the second phase, and the area of the acicular ferrite and martensite in the second phase was determined. Rate ratio F A /
All of the products in which M A was controlled in the range of 2.0 to 20 were TS ×
El ≧ 24000 MPa ・% Excellent strength-elongation balance and YR ≦
It was possible to obtain a low yield ratio of 65%, an excellent impact resistance such as a dynamic n value ≧ 0.35, and a high processing and baking hardening amount of WH + BH ≧ 100 MPa ·%.
【００３７】[0037]
【発明の効果】かくして、この発明に従い、主相を初析
フェライトとし、かつ第２相をマルテンサイト、針状フ
ェライトおよび残留オーステナイトの混合組織にすると
共に、第２相中の針状フェライトとマルテンサイトとの
面積率比ＦA ／ＭA を 2.0〜20の範囲に制御することに
より、優れた成形性と耐衝撃特性とを兼ね備えた熱延鋼
板を得ることができる。Thus, according to the present invention, the main phase is proeutectoid ferrite, the second phase is a mixed structure of martensite, acicular ferrite and retained austenite, and the acicular ferrite and the martensite in the second phase are mixed. By controlling the area ratio F A / M A to the site in the range of 2.0 to 20, a hot-rolled steel sheet having both excellent formability and impact resistance can be obtained.
【図１】従来のＴＲＩＰ鋼の代表的な連続冷却変態曲線
図（ＣＣＴ図）である。FIG. 1 is a typical continuous cooling transformation diagram (CCT diagram) of a conventional TRIP steel.
【図２】この発明の成分系における代表的連続冷却変態
曲線図（ＣＣＴ図）である。FIG. 2 is a typical continuous cooling transformation curve (CCT diagram) in the component system of the present invention.
【図３】(a) この発明に従い得られる第２相の特徴的な
相構成および(b) 従来のＴＲＩＰ鋼の第２相の相構成を
示す模式図である。FIG. 3 is a schematic diagram showing (a) a characteristic phase structure of a second phase obtained according to the present invention and (b) a phase structure of a second phase of a conventional TRIP steel.
【図４】ＦA ／ＭA 比と降伏強さ(YS)、引張強さ(TS)、
伸び(El)、降伏比（YR) および強度−伸びバランス（TS
×El）との関係を示すグラフである。[Figure 4] F A / M A ratio and the yield strength (YS), tensile strength (TS),
Elongation (El), yield ratio (YR) and strength-elongation balance (TS
XEl).
【図５】Cr量と強度−伸びバランスとの関係を示すグラ
フである。FIG. 5 is a graph showing the relationship between the Cr content and the strength-elongation balance.
【図６】Cr量と動的ｎ値との関係を示すグラフである。FIG. 6 is a graph showing a relationship between a Cr amount and a dynamic n value.
【図７】加工硬化量（ＷＨ）および焼付硬化量（ＢＨ）
の説明図である。FIG. 7: Work hardening amount (WH) and bake hardening amount (BH)
FIG.
───────────────────────────────────────────────────── フロントページの続き (72)発明者 高木 周作 千葉県千葉市中央区川崎町１番地 川崎製 鉄株式会社技術研究所内 (72)発明者 古君 修 千葉県千葉市中央区川崎町１番地 川崎製 鉄株式会社技術研究所内 (72)発明者 小原 隆史 千葉県千葉市中央区川崎町１番地 川崎製 鉄株式会社技術研究所内 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shusaku Takagi 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Corporation (72) Inventor Osamu Furukuni 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corporation Technical Research Institute (72) Inventor Takashi Ohara 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corporation Technical Research Center
Claims (4)
3.0 mass％、 Mn：0.6 〜3.0 mass％、 Cr：0.2 〜2.0 mass％ を含有し、残部は実質的にFeの組成になり、鋼組織が、
初析フェライトを主相として、マルテンサイト、針状フ
ェライトおよび残留オーステナイトからなる第２相を有
し、かつ該第２相中の針状フェライトとマルテンサイト
が下記式の関係を満足することを特徴とする耐衝撃特性
に優れた高強度高加工性熱延鋼板。 記 2.0 ≦ＦA ／ＭA ≦ 20 ここで、ＦA : 第２相中における針状フェライトの面積
率（％） ＭA : 第２相中におけるマルテンサイトの面積率（％）(1) C: 0.05 to 0.40 mass%, Si: 1.0 to
3.0 mass%, Mn: 0.6-3.0 mass%, Cr: 0.2-2.0 mass%, the balance is substantially Fe composition, and the steel structure is
It has a second phase composed of martensite, acicular ferrite, and retained austenite with proeutectoid ferrite as a main phase, and the acicular ferrite and martensite in the second phase satisfy the following formula. High-strength, high-workability hot-rolled steel sheet with excellent impact resistance. Serial in 2.0 ≦ F A / M A ≦ 20 wherein, F A: the area ratio of the acicular ferrite in the second phase (%) M A: area ratio of martensite in the second phase (%)
ことを特徴とする耐衝撃特性に優れた高強度高加工性熱
延鋼板。2. The impact resistance according to claim 1, wherein the steel composition further comprises at least one selected from the group consisting of P: 0.01 to 0.2 mass% and Al: 0.01 to 0.3 mass%. High strength, high workability hot rolled steel sheet with excellent properties.
さらに Ti：0.005 〜0.25mass％、 Nb：0.003 〜0.1 mass％ のうちから選んだ少なくとも一種を含有する組成になる
ことを特徴とする耐衝撃特性に優れた高強度高加工性熱
延鋼板。3. The steel composition according to claim 1, wherein the steel composition is
Further, a high-strength, high-workability hot-rolled steel sheet having excellent impact resistance, characterized in that the composition contains at least one selected from the group consisting of 0.005 to 0.25 mass% Ti and 0.003 to 0.1 mass% Nb.
中に占める第２相の比率が３〜40％であることを特徴と
する耐衝撃特性に優れた高強度高加工性熱延鋼板。4. The high-strength and high-workability hot-rolled steel sheet according to claim 1, wherein the proportion of the second phase in the steel structure is 3 to 40%. .
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP14957298A JP3724193B2 (en) | 1997-06-06 | 1998-05-29 | High-strength, high-workability hot-rolled steel sheet with excellent impact resistance and low yield ratio |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP9-149299 | 1997-06-06 | ||
| JP14929997 | 1997-06-06 | ||
| JP14957298A JP3724193B2 (en) | 1997-06-06 | 1998-05-29 | High-strength, high-workability hot-rolled steel sheet with excellent impact resistance and low yield ratio |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH1150186A true JPH1150186A (en) | 1999-02-23 |
| JP3724193B2 JP3724193B2 (en) | 2005-12-07 |
Family
ID=26479234
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP14957298A Expired - Fee Related JP3724193B2 (en) | 1997-06-06 | 1998-05-29 | High-strength, high-workability hot-rolled steel sheet with excellent impact resistance and low yield ratio |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP3724193B2 (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2001355042A (en) * | 2000-04-10 | 2001-12-25 | Kawasaki Steel Corp | Hot dip galvanized steel sheet excellent in press formability and strain age hardening characteristic and its production method |
| EP1195447A1 (en) * | 2000-04-07 | 2002-04-10 | Kawasaki Steel Corporation | Hot rolled steel plate, cold rolled steel plate and hot dip galvanized steel plate being excellent in strain aging hardening characteristics, and method for their production |
| JP2012126974A (en) * | 2010-12-16 | 2012-07-05 | Usui Kokusai Sangyo Kaisha Ltd | High strength steel artifact superior in notch fatigue strength, and manufacturing method therefor |
| JP6569842B1 (en) * | 2018-12-11 | 2019-09-04 | 日本製鉄株式会社 | High-strength steel sheet excellent in formability, toughness, and weldability, and manufacturing method thereof |
-
1998
- 1998-05-29 JP JP14957298A patent/JP3724193B2/en not_active Expired - Fee Related
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1195447A1 (en) * | 2000-04-07 | 2002-04-10 | Kawasaki Steel Corporation | Hot rolled steel plate, cold rolled steel plate and hot dip galvanized steel plate being excellent in strain aging hardening characteristics, and method for their production |
| EP1195447A4 (en) * | 2000-04-07 | 2003-05-02 | Kawasaki Steel Co | Hot rolled steel plate, cold rolled steel plate and hot dip galvanized steel plate being excellent in strain aging hardening characteristics, and method for their production |
| JP2001355042A (en) * | 2000-04-10 | 2001-12-25 | Kawasaki Steel Corp | Hot dip galvanized steel sheet excellent in press formability and strain age hardening characteristic and its production method |
| JP2012126974A (en) * | 2010-12-16 | 2012-07-05 | Usui Kokusai Sangyo Kaisha Ltd | High strength steel artifact superior in notch fatigue strength, and manufacturing method therefor |
| JP6569842B1 (en) * | 2018-12-11 | 2019-09-04 | 日本製鉄株式会社 | High-strength steel sheet excellent in formability, toughness, and weldability, and manufacturing method thereof |
| WO2020121417A1 (en) * | 2018-12-11 | 2020-06-18 | 日本製鉄株式会社 | High-strength steel plate having excellent formability, toughness and weldability, and production method of same |
| CN113166865A (en) * | 2018-12-11 | 2021-07-23 | 日本制铁株式会社 | High-strength steel sheet having excellent formability, toughness, and weldability, and method for producing same |
| CN113166865B (en) * | 2018-12-11 | 2022-07-12 | 日本制铁株式会社 | High-strength steel sheet having excellent formability, toughness, and weldability, and method for producing same |
Also Published As
| Publication number | Publication date |
|---|---|
| JP3724193B2 (en) | 2005-12-07 |
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