JPH05320749A - Production of ultrahigh strength steel - Google Patents
Production of ultrahigh strength steelInfo
- Publication number
- JPH05320749A JPH05320749A JP15268292A JP15268292A JPH05320749A JP H05320749 A JPH05320749 A JP H05320749A JP 15268292 A JP15268292 A JP 15268292A JP 15268292 A JP15268292 A JP 15268292A JP H05320749 A JPH05320749 A JP H05320749A
- Authority
- JP
- Japan
- Prior art keywords
- steel
- temperature
- bainite
- weight
- cooling
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Abstract
Description
【０００１】[0001]
【産業上の利用分野】本発明は，ベイナイトと残留オー
ステナイトを主相とする複合組織を有する,引張強さ１
６００〜２３００N/mm2(ニュートン/mm2) 級の超高強度
鋼の製造方法に関するものである。BACKGROUND OF THE INVENTION The present invention has a tensile strength of 1 having a composite structure containing bainite and retained austenite as main phases.
The present invention relates to a method for producing 600-2300 N / mm 2 (Newton / mm 2 ) grade ultra-high strength steel.
【０００２】[0002]
【従来の技術】高い強度が要求される機械構造部品用の
鋼材としては，従来より高炭素鋼を用いた焼入焼戻し材
や, ベイナイト鋼材が多く使用されてきた。しかし，こ
れらの鋼材は概して靭性には乏しく, 延性や耐衝撃性の
要求される部材に対して使用する場合には，その使用条
件が著しく制約されたり, 場合によっては靭性の欠如を
部材の寸法の増加で補うことが余儀なくされ，このため
に重量の増加を来していることが少なくなかった。2. Description of the Related Art As a steel material for machine structural parts requiring high strength, a quenching and tempering material using high carbon steel and a bainite steel material have been often used. However, these steel materials are generally poor in toughness, and when used for members that require ductility and impact resistance, the operating conditions are severely restricted, and in some cases, lack of toughness may cause dimensional deterioration of the members. It has been compelled to make up for the increase in weight, and this often results in an increase in weight.
【０００３】従来より，高炭素鋼を用いて高強度材を製
造する場合, １６００N/mm2以上の引張強さ (以下，Ｔ
Ｓと略称することがある）に強化しようとすれば，焼入
れ−焼戻し処理を行なうのが一般的であるが，この方法
では高い延性−靭性を得ることはできない。通常の焼入
れ焼戻し材が靭性に乏しいのは，金属組織がマルテンサ
イトやベイナイトを主体とする硬質の組織であることに
よる。Conventionally, when a high-strength material is manufactured using high-carbon steel, the tensile strength of 1600 N / mm 2 or more (hereinafter, T
If it is intended to strengthen the steel (sometimes abbreviated as S), quenching-tempering is generally performed, but this method cannot obtain high ductility-toughness. The reason that ordinary quenching and tempering materials have poor toughness is that the metallographic structure is a hard structure consisting mainly of martensite and bainite.
【０００４】この問題を解決すべく，特公昭58-42246号
公報において，ベイナイトと残留オーステナイトの混合
組織とすることにより高強度を保ちながら延性を改善す
る方法が提案された。従来のベイナイト鋼ではＴＳが１
２００N/mm2級では全伸びが約10％程度であるのに対
し，この方法によれば，同じ１２００N/mm2級で約30％
の全伸びが得られるものであった。In order to solve this problem, Japanese Patent Publication No. 58-42246 proposed a method of improving ductility while maintaining high strength by forming a mixed structure of bainite and retained austenite. TS is 1 for conventional bainite steel
In the 200N / mm 2 class, the total elongation is about 10%, whereas according to this method, it is about 30% in the same 1200N / mm 2 class.
Was obtained.
【０００５】また特開平3-215623号公報には，同じく金
属組織をベイナイトと残留オーステナイトの混合組織に
することによってＴＳが１５００N/mm2級で30％程度の
伸びを得る方法が示された。Further, Japanese Patent Laid-Open No. 3-215623 discloses a method in which TS has an elongation of about 30% at 1500 N / mm 2 grade by similarly making the metal structure a mixed structure of bainite and retained austenite.
【０００６】[0006]
【発明が解決しようとする課題】前記公報に提案された
方法では，高強度と言ってもＴＳは最大でも約１６００
N/mm2までである。例えば後者の公報の実施例では１６
４kgf/mm2のものが最大であり，これ以上のＴＳを得る
ことは困難であった。In the method proposed in the above-mentioned publication, even if the strength is high, the TS is about 1600 at the maximum.
Up to N / mm 2 . For example, in the embodiment of the latter publication, 16
The maximum was 4 kgf / mm 2 , and it was difficult to obtain a TS higher than this.
【０００７】本発明はこの問題点を解決し，ＴＳが１６
００〜２３００N/mm2級で延性の良好な超高強度鋼を得
ることを目的としたものである。The present invention solves this problem, and TS is 16
The purpose of the invention is to obtain an ultra-high strength steel having a ductility of 00 to 2300 N / mm 2 grade.
【０００８】[0008]
【課題を解決するための手段】本発明によれば，Ｃ：0.
4〜1.0 重量％, Ｓi：1.2〜3.0 重量％, Ｍn：0.3〜2.0
重量％, Ｃr：0.2〜1.5重量％, 残部Ｆeおよび不可避
的不純物元素からなる鋼を，さらにはこの鋼に0.05〜0.
5 重量％のＭo, 0.05〜0.5 重量％のＶまたは0.01〜0.5
重量％Ｎbのいずれか１種または２種以上を含有させた
鋼を，Ａc3変態点以上の温度に加熱して完全にオーステ
ナイト化したうえ，この温度からＴＴＴ線図のノーズを
通過する速度よりも大きな冷却速度でＭs点以上で３５
０℃未満の温度まで冷却し，この温度域に１０〜６０分
間恒温保持した後に室温まで空冷または空冷以上の冷却
速度で冷却することからなる，ベイナイトと残留オース
テナイトを主相とした複合組織を有し且つ引張強さが１
６００〜２３００N/mm2の超高強度鋼の製造方法を提供
する。According to the present invention, C: 0.
4 to 1.0% by weight, Si: 1.2 to 3.0% by weight, Mn: 0.3 to 2.0
%, Cr: 0.2 to 1.5% by weight, the balance Fe and the steel consisting of inevitable impurity elements, and 0.05 to 0.
5 wt% Mo, 0.05 to 0.5 wt% V or 0.01 to 0.5
Steel containing at least one of wt% Nb was heated to a temperature of Ac 3 transformation point or higher to completely austenite, and from this temperature, the rate of passing through the nose of the TTT diagram was determined. At a high cooling rate, 35 above the Ms point
It has a composite structure consisting of bainite and retained austenite as the main phase, which consists of cooling to a temperature below 0 ° C, holding the temperature in this temperature range for 10 to 60 minutes, and then cooling to room temperature by air cooling or at a cooling rate higher than air cooling. And has a tensile strength of 1
Provided is a method for producing an ultra high strength steel of 600 to 2300 N / mm 2 .
【０００９】[0009]
【作用】本発明に従う成分組成の鋼を下部ベイナイト領
域に恒温保持すると著しく微細な下部ベイナイトと残留
オーステナイトの混合組織が生成し，残留オーステナイ
トのＴＲＩＰ現象 (変態誘起塑性：Transformation Ind
uced-Plasticity)によってＴＳが１６００〜２３００N/
mm2級でありながら，伸び（Ｅl）が10〜20％の良好な強
度−延性バランスをもつ超高強度鋼が得られる。すなわ
ち，本発明法に従って製造された鋼が非常に優れた強靭
性を示すのは残留オーステナイトのＴＲＩＰ現象によ
る。When the steel of the composition according to the present invention is kept at a constant temperature in the lower bainite region, a remarkably fine mixed structure of the lower bainite and the retained austenite is formed, and the TRIP phenomenon of the retained austenite (transformation induced plasticity: Transformation Ind
uced-Plasticity) makes TS 1600-2300N /
It is possible to obtain an ultra-high strength steel having a good strength-ductility balance with an elongation (El) of 10 to 20% even though it is of the mm 2 class. That is, it is due to the TRIP phenomenon of retained austenite that the steel manufactured according to the method of the present invention exhibits extremely excellent toughness.
【００１０】本発明において，残留オーステナイトと下
部ベイナイトからなる複合組織が得られるのは，Ｓiの
作用が大きい。すなわちＳiを多量に含む炭素鋼をベイ
ナイト変態させると，Ｓiが炭化物の生成を抑制する作
用を有するので，未変態オーステナイト中にベイナイト
中の炭素原子が排出され，このために未変態オーステナ
イト中の炭素濃度が上昇し，マルテンサイト変態点 (Ｍ
s点) が室温以下に低下する。このため，鋼を室温まで
冷却してもマルテンサイトは生成せず，ベイナイトと残
留オーステナイトの混合組織が得られる。In the present invention, the effect of Si is large in that a composite structure composed of retained austenite and lower bainite can be obtained. That is, when a carbon steel containing a large amount of Si is transformed into bainite, Si has an action of suppressing the formation of carbides, and therefore carbon atoms in bainite are discharged into untransformed austenite, which results in carbon in untransformed austenite. As the concentration increases, the martensitic transformation point (M
s point) drops below room temperature. Therefore, martensite does not form even when the steel is cooled to room temperature, and a mixed structure of bainite and retained austenite is obtained.
【００１１】Ｓiを含まない鋼では，ベイナイト変態の
進行と同時に炭化物の析出を伴うので，未変態オーステ
ナイト中への炭素原子の濃縮は不充分で，残留オーステ
ナイトとベイナイトの混合組織を得ることができない。In the steel containing no Si, since the precipitation of carbides accompanies the progress of bainite transformation, the concentration of carbon atoms in untransformed austenite is insufficient, and a mixed structure of retained austenite and bainite cannot be obtained. ..
【００１２】一方, Ｓiは黒鉛化を助長する元素であり,
多量の炭素を含有する鋼の場合には軟化焼鈍時などに
黒鉛化を生ずる危険性が大きいので，これを抑制するた
めに黒鉛化抑止力の大きい元素を添加しなくてはならな
い。ただし, 黒鉛化抑止元素はベイナイト組織の靭性を
阻害するものであってはならない。On the other hand, Si is an element that promotes graphitization,
In the case of steel containing a large amount of carbon, there is a high risk that graphitization will occur during softening and annealing, so in order to suppress this, it is necessary to add an element with a large graphitization inhibiting power. However, the graphitization inhibiting element must not hinder the toughness of the bainite structure.
【００１３】また単純にＣ-Ｓi-Ｍnだけの化学成分から
なる鋼では，残留オーステナイトとベイナイトの混合組
織を得ることはできるが，ベイナイト変態の速度が速い
ために適切な残留オーステナイト量に制御することが難
しい。したがって適切な残留オーステナイト量を得るた
めにはベイナイト変態を抑制し，かつ延性に対して有効
な残留オーステナイトを生成するような適切なその他の
合金元素を添加しなければならない。[0013] Further, in the case of a steel having a chemical composition of only C-Si-Mn, it is possible to obtain a mixed structure of retained austenite and bainite, but since the bainite transformation speed is high, the amount of retained austenite is controlled to an appropriate amount. Difficult to do. Therefore, in order to obtain an appropriate amount of retained austenite, it is necessary to add other appropriate alloying elements that suppress bainite transformation and generate retained austenite effective for ductility.
【００１４】また, 下部ベイナイトと共存して残留オー
ステナイトが変態誘起塑性の現象を示すためには, 残留
オーステナイト中の炭素濃度は1.5％以上必要である
が，単純なＣ-Ｓi-Ｍn系の鋼を当該ベイナイトの恒温変
態温度領域に保持してもこのように高い炭素濃度を得る
ことはできない。Further, in order for retained austenite to exhibit a transformation-induced plasticity phenomenon coexisting with lower bainite, the carbon concentration in the retained austenite must be 1.5% or more, but a simple C-Si-Mn system steel is required. Is not maintained in such a constant temperature transformation temperature range of the bainite.
【００１５】前記の目的を達成するには，このようなこ
とを総合的に勘案して，鋼の成分組成を決めることが必
要なるが, 本発明者らは，これらの点に関する基礎的研
究の結果，Ｃ-Ｓi,Ｍnに加えてＣr,さらににＭo, Ｖ，
Ｎb等を適量添加した鋼を用いれば，黒鉛化抵抗を向上
させることができ, かつベイナイト変態処理時に微細な
下部ベイナイトが生成し，安定な残留オーステナイトと
微細な下部ベイナイトからなる複合組織が生成し，強度
−延性バランスの非常に優れたＴＳが１６００〜２３０
０N/mm2級の超高強度鋼が得られることを見い出した。In order to achieve the above-mentioned object, it is necessary to comprehensively consider such things and determine the component composition of the steel, but the present inventors have conducted basic research on these points. As a result, in addition to C-Si, Mn, Cr, and Mo, V,
If a steel with an appropriate amount of Nb, etc. is used, the graphitization resistance can be improved, and fine lower bainite is formed during the bainite transformation treatment, and a composite structure composed of stable retained austenite and fine lower bainite is formed. , TS with an excellent strength-ductility balance is 1600 to 230
It was found that 0N / mm 2 grade ultra high strength steel can be obtained.
【００１６】本発明に従う鋼の各成分の作用とその含有
量範囲の規制理由について以下に説明する。The action of each component of the steel according to the present invention and the reason for limiting the content range will be described below.
【００１７】Ｃは, オーステナイト安定化元素であり,
ベイナイト変態に不可欠な元素である。その添加量は，
最終的に生成する残留オーステナイト量に大きく影響
し，Ｃ添加量が0.4％未満では高強度を得ることはでき
ない。またＣ量が1.0％を超えると，生成する残留オー
ステナイト量が多すぎて，かえって強度が低下する。し
たがって, 適切な残留オーステナイト量を得るために
は，Ｃ量は0.4〜1.0％の範囲にする必要がある。本発明
鋼において，望ましい残留オーステナイト量は１５〜３
０容積％である。C is an austenite stabilizing element,
It is an essential element for bainite transformation. The amount added is
It has a great influence on the amount of retained austenite finally formed, and if the amount of C added is less than 0.4%, high strength cannot be obtained. On the other hand, if the amount of C exceeds 1.0%, the amount of retained austenite formed is too large and the strength is rather reduced. Therefore, in order to obtain an appropriate amount of retained austenite, the amount of C needs to be in the range of 0.4 to 1.0%. In the steel of the present invention, the desirable amount of retained austenite is 15 to 3
It is 0% by volume.
【００１８】Ｓiは，炭化物の生成を抑制する元素であ
り，Ｃ濃度が高い安定な残留オーステナイトを得るため
には不可欠な元素である。Ｓi量が1.2 ％未満では上記
の効果は希薄であり, 反対にＳi量が3.0 ％を超える
と，ベイナイト変態が著しく抑制されるばかりでなく,
熱間圧延−冷間圧延等の鋼の製造工程で著しい困難を伴
うようになる。従ってＳi量は1.2〜3.0％の範囲に限定
される。Si is an element that suppresses the formation of carbides and is an essential element for obtaining stable retained austenite having a high C concentration. If the Si content is less than 1.2%, the above effect is diminished. Conversely, if the Si content exceeds 3.0%, not only bainite transformation is significantly suppressed, but
In the steel manufacturing process such as hot rolling-cold rolling, there are significant difficulties. Therefore, the amount of Si is limited to the range of 1.2 to 3.0%.
【００１９】Ｍnはオーステナイト安定化元素であり,
焼入性を向上させることによってパーライト等の生成を
抑止する作用を供する。しかしＭn量が0.3％未満では焼
入性が不充分で，鋼の板厚が厚い場合には中心部の冷却
速度が遅いためにパーライトなどを生成することがあ
り，この場合には充分な残留オーステナイトが得られな
くなる。またＭn量が2.0％を超えるとベイナイト変態の
速度が遅くなってやはり充分な残留オーステナイトを得
られなくなる。このためＭn量は0.3〜2.0％に限定す
る。Mn is an austenite stabilizing element,
By improving the hardenability, it serves to suppress the formation of pearlite and the like. However, if the amount of Mn is less than 0.3%, the hardenability is insufficient, and if the steel plate is thick, pearlite, etc. may be generated due to the slow cooling rate of the central part. Austenite cannot be obtained. On the other hand, if the amount of Mn exceeds 2.0%, the rate of bainite transformation is slowed down and sufficient retained austenite cannot be obtained. Therefore, the amount of Mn is limited to 0.3 to 2.0%.
【００２０】Ｃrは，熱延材の軟化焼鈍中に起こる黒鉛
化を抑制するのに有効に作用する元素であり，またベイ
ナイト変態を遅らせて残留オーステナイトの得られる領
域を広げる作用を有する元素である。黒鉛化を防止する
ためにはＣr量は最低0.2％は必要であるが，1.5％を超
えて添加しても黒鉛化の抑止にはそれ以上の効果は望め
ないばかりか，軟化焼鈍時のセメンタイトの球状化を困
難にし，ベイナイト自体の塑性を劣化させる傾向がある
ためにＣr量は0.2〜1.5％に限定する。Cr is an element that effectively acts to suppress graphitization that occurs during softening annealing of a hot rolled material, and also has an action that delays bainite transformation and widens the region where retained austenite is obtained. .. The Cr content must be at least 0.2% to prevent graphitization, but addition of more than 1.5% is not expected to have any further effect on the suppression of graphitization, and also cementite during softening annealing. The Cr content is limited to 0.2 to 1.5% because it tends to make the spheroidization of spheroidizing difficult and deteriorates the plasticity of bainite itself.
【００２１】ＭoとＶは，ベイナイトの変態の形態を大
きく変える元素であり, 適量添加することによってベイ
ナイト組織を微細化させる作用を供し，これによりＴＳ
と靭性を高める効果を奏する。さらにＶは，鋼をオース
テナイト域に加熱した場合のオーステナイト粒径を微細
化する効果もあり, Ｖを適量添加すると, ベイナイト変
態を促進させることもできる。Mo and V are elements that greatly change the transformation morphology of bainite, and when added in an appropriate amount, they serve to refine the bainite structure, whereby TS
And has the effect of increasing toughness. Further, V also has the effect of refining the austenite grain size when the steel is heated to the austenite region, and the addition of an appropriate amount of V can accelerate the bainite transformation.
【００２２】Ｍoは0.05％未満の添加量ではベイナイト
の微細化効果は少なく, また0.5％を越えて添加しても
それ以上の微細化は望めず, かえって健全なベイナイト
の生成に障害をもたらすために0.05〜0.5％に限定する
必要がある。またＶは0.05％以下の添加ではベイナイト
の微細化効果は少なく, また0.50％を越えて添加しても
それ以上の効果は望めないばかりか，Ｍoの場合と同じ
くかえって健全なベイナイトの生成に障害となるために
0.05〜0.5％に限定する必要がある。If Mo is added in an amount less than 0.05%, the bainite refining effect is small, and if it is added in excess of 0.5%, further refining is not expected, and rather it causes an obstacle to the formation of sound bainite. It is necessary to limit it to 0.05-0.5%. When V is added in an amount of 0.05% or less, the bainite refining effect is small, and even if V is added in an amount of more than 0.50%, no further effect can be expected, and as with Mo, the formation of sound bainite is rather impaired. To become
It should be limited to 0.05-0.5%.
【００２３】Ｎbは，オーステナイト域に加熱した場合
のオーステナイト粒径を微細化する効果によりベイナイ
ト変態を促進し，かつ微細で靭性の高いベイナイトを生
成させる作用を有する元素である。しかし添加量が0.01
％未満ではオーステナイト粒径を微細化する効果は少な
く, ベイナイトの微細化には十分な効果を発揮しない
し，0.5％を越えて添加してもそれ以上の効果は望めな
いので0.01〜0.5％に限定する。Nb is an element which has the effect of promoting bainite transformation by the effect of refining the austenite grain size when heated to the austenite region, and of producing fine bainite with high toughness. However, the addition amount is 0.01
If it is less than 0.1%, the effect of refining the austenite grain size is small and it does not exert a sufficient effect on the refining of bainite. If it is added in excess of 0.5%, no further effect can be expected. limit.
【００２４】次に, 本発明法の熱処理条件について説明
する。Next, the heat treatment conditions of the method of the present invention will be described.
【００２５】本発明鋼の製造にあたっては，通常は，上
記の成分組成範囲の鋼を通常の熱間圧延, 軟化焼鈍, 冷
間圧延等を適宜施して鋼帯または鋼板を製造し，これを
ベイナイト変態処理に供する。黒鉛化傾向を抑制した本
発明鋼では通常の工程で製造する限りにおいては黒鉛化
を生ずる恐れは少ない。このベイナイト変態処理は，鋼
帯または鋼板に限らず，特殊な場合には，線材や条材に
も適用でき，また一次加工した材料にも適用可能であ
る。In the production of the steel according to the present invention, usually, steel having the above-mentioned composition range is subjected to usual hot rolling, softening annealing, cold rolling, etc. to produce a steel strip or a steel sheet, which is then bainite. Subject to transformation processing. With the steel of the present invention in which the graphitization tendency is suppressed, graphitization is less likely to occur as long as it is manufactured in a normal process. This bainite transformation process is applicable not only to steel strips or steel plates, but also to wire rods and strips in special cases, as well as to primary processed materials.
【００２６】本発明に従うベイナイト変態処理は，まず
鋼をＡc3点以上の温度域に加熱して完全にオーステナイ
ト化する。そして，この状態から，ＴＴＴ線図のノーズ
を通過する速度よりも大きい冷却速度でＭs点（マルテ
ンサイト変態開始点）以上，３５０℃未満の範囲の温度
まで冷却し，この温度域に10〜60分間恒温保持した後に
室温まで空冷または空冷以上の冷却速度で冷却する。In the bainite transformation treatment according to the present invention, first, the steel is heated to a temperature range of Ac 3 point or higher to be completely austenitized. Then, from this state, it is cooled to a temperature in the range of Ms point (martensitic transformation start point) or more and less than 350 ° C at a cooling rate higher than the rate of passing through the nose of the TTT diagram, and the temperature range is 10 to 60 After keeping the temperature constant for a minute, it is cooled to room temperature by air cooling or at a cooling rate higher than air cooling.
【００２７】ＴＴＴ線図のノーズを通過する速度よりも
大きい冷却速度とは, オーステナイト単相の組織からＭ
s点〜３５０℃未満の温度域に焼入れする際に, フエラ
イトやパーライトが生成しないような冷却速度のことで
ある。本発明の化学成分を有する鋼の場合では，５０℃
/sec以上の冷却速度があればフエライトやパーライトが
生成することはない。A cooling rate higher than the rate of passing through the nose of the TTT diagram means M from the structure of austenite single phase.
It is the cooling rate at which ferrite or pearlite does not form during quenching in the temperature range from s point to less than 350 ° C. In the case of the steel having the chemical composition of the present invention, 50 ° C
If the cooling rate is more than / sec, ferrite and perlite will not be generated.
【００２８】恒温処理温度はＭs点〜３５０℃未満とし
なければならない。３５０℃未満で恒温処理することに
より，ベイナイトは下部ベイナイトとして生成する。下
部ベイナイトは，この温度域以上で生成する上部ベイナ
イトに比べると，より針状の形態を有する点で区別がで
き硬さが一層高い。本発明鋼が超高強度と靭性を発揮す
るのは，この下部ベイナイトと残留オーステナイトの極
めて微細な混合組織を有し, 残留オーステナイトがＴＲ
ＩＰ現象を示すことにある。The isothermal treatment temperature must be set to the Ms point to less than 350 ° C. Bainite is generated as lower bainite by performing a constant temperature treatment at less than 350 ° C. The lower bainite is distinctive in that it has a more needle-like morphology and has a higher hardness than the upper bainite formed above this temperature range. The steel of the present invention exerts ultra-high strength and toughness because it has an extremely fine mixed structure of this lower bainite and retained austenite, and the retained austenite is TR
It is to show the IP phenomenon.
【００２９】ＴＲＩＰ現象は残留オーステナイトの安定
性, すなわち歪誘起変態の起こり易さに関わっており,
塑性変形中に適度に変態して歪の分散を起こさしめるよ
うな適度な安定性が必要になる。残留オーステナイトの
安定性は，主に炭素濃度によって決まる。通常，残留オ
ーステナイトとして存在し得るためには炭素濃度1.0〜
1.2％が必要であるが，ＴＲＩＰ現象を示す安定性を得
るには，より高濃度な炭素原子の濃縮が必要である。特
に本発明が目標とするような高強度鋼の場合には, 更に
高濃度で安定な残留オーステナイトが必要になる。それ
は以下の理由による。The TRIP phenomenon is related to the stability of retained austenite, that is, the susceptibility to strain-induced transformation,
Appropriate stability is required such that the plastic material is transformed during plastic deformation to disperse the strain. The stability of retained austenite is mainly determined by the carbon concentration. Normally, the carbon concentration of 1.0 ~ is required to exist as retained austenite.
1.2% is necessary, but a higher concentration of carbon atoms is necessary to obtain the stability indicating the TRIP phenomenon. Particularly in the case of the high strength steel targeted by the present invention, stable retained austenite with a higher concentration is required. The reason is as follows.
【００３０】恒温処理温度が350℃未満で生成した下部
ベイナイトは非常に硬度が高いが，残留オーステナイト
はそれ自体は比較的軟質であるために，歪は残留オース
テナイトに集中する。このため，ベイナイトが軟質な場
合に比べて歪誘起変態が一層起こりやすくなる。本発明
鋼のように１６００〜２３００N/mm2級の超高強度にお
いては，硬質な下部ベイナイトと共存してＴＲＩＰ現象
を示すためには残留オーステナイト中の炭素濃度は1.5
％以上必要となる。The lower bainite formed when the isothermal treatment temperature is lower than 350 ° C. has a very high hardness, but the retained austenite itself is relatively soft, so the strain concentrates on the retained austenite. Therefore, strain-induced transformation is more likely to occur than when bainite is soft. At the ultra high strength of 1600 to 2300 N / mm 2 grade like the steel of the present invention, in order to exhibit the TRIP phenomenon in coexistence with the hard lower bainite, the carbon concentration in the retained austenite is 1.5.
% Or more is required.
【００３１】本発明者らは, 恒温保持温度と残留オース
テナイト中の炭素濃度の関係について研究を行った結
果, 本発明鋼ではＣ-Ｓi-Ｍn 系にＣrさらにはＭo,Ｖ,
Ｎb等の炭化物形成元素を添加することにより，恒温保
持温度が３５０℃を下回ると炭素濃度が1.5％を超える
残留オーステナイトが得られることがわかった。すなわ
ち，本発明で規定する成分範囲の鋼を３５０℃未満の温
度で恒温保持を行うことにより，残留オーステナイトの
ＴＲＩＰ現象を利用した超高強度鋼が得られることがわ
かった。The present inventors have studied the relationship between the isothermal holding temperature and the carbon concentration in the retained austenite. As a result, in the present invention steel, C-Si-Mn system, Cr, Mo, V,
It was found that by adding a carbide-forming element such as Nb, retained austenite having a carbon concentration of more than 1.5% can be obtained when the isothermal holding temperature is lower than 350 ° C. That is, it was found that ultra-high strength steel utilizing the TRIP phenomenon of retained austenite can be obtained by keeping the steel having the composition range defined by the present invention at a temperature of less than 350 ° C.
【００３２】なお，恒温保持温度がＭs点より低いと焼
入マルテンサイトが生成し，ベイナイトと残留オーステ
ナイトの混合組織が生成しない。このために高延性を得
ることができない。一方, 保持温度が３５０℃以上では
１６００N/mm2以上の引張強さは得ることはできない。
したがって恒温保持する温度はＭs点以上，350℃未満と
する必要がある。When the isothermal holding temperature is lower than the Ms point, quenched martensite is produced and a mixed structure of bainite and retained austenite is not produced. Therefore, high ductility cannot be obtained. On the other hand, when the holding temperature is 350 ° C or higher, a tensile strength of 1600 N / mm 2 or higher cannot be obtained.
Therefore, the temperature to be kept constant must be above the Ms point and below 350 ° C.
【００３３】この恒温保持温度域に保持する時間は１０
〜６０分であればよい。本発明で規定する成分の鋼は，
当該温度域における保持時間が１０分未満ではベイナイ
ト変態量が不充分であり, 未変態オーステナイト中の炭
素濃度は1.5 ％に到達しないことがある。この場合には
高強度ではあるが延性に乏しくなる。一方，恒温変態時
間を６０分を超えて余り長くすると，残留オーステナイ
トが分解して炭化物が析出し，このために炭素濃度が再
び低下し，延性が低下するようになる。したがってＭs
点以上３５０℃未満の温度域に保持する時間は１０〜６
０分とするのがよい。The time for holding in this constant temperature holding temperature range is 10
It may be -60 minutes. The steel of the composition specified in the present invention is
If the holding time in the temperature range is less than 10 minutes, the amount of bainite transformation is insufficient, and the carbon concentration in untransformed austenite may not reach 1.5%. In this case, although the strength is high, the ductility becomes poor. On the other hand, if the isothermal transformation time exceeds 60 minutes and becomes too long, residual austenite is decomposed and carbides are precipitated, so that the carbon concentration is lowered again and the ductility is lowered. Therefore Ms
The time to keep in the temperature range above the point and below 350 ° C is 10-6
It is good to set it to 0 minutes.
【００３４】いったんこの恒温保持温度域に該時間保持
した後は，室温まで冷却するに際には，１℃/sec以上の
冷却速度であれば組織変化をきたすこともないので，空
冷以上の冷却速度であれば問題ない。After being kept in this constant temperature holding temperature range for the time, when cooling to room temperature, there is no change in the structure at a cooling rate of 1 ° C./sec or more, so cooling above air cooling is performed. There is no problem if it is speed.
【００３５】[0035]
【実施例】表１に供試鋼の化学成分値 (重量％) を示
す。これらのうちNo.Ａ，Ｂ，Ｃ，Ｄ，Ｅ，Ｆ，Ｇおよ
びＨは, いずれか一つの成分の含有量が本発明で規定す
る範囲を外れる比較鋼であり，No.Ｉ，Ｊ，Ｋ，Ｌ，Ｍ
およびＮは本発明の成分組成範囲内の鋼である。いずれ
も通常の熱間圧延，軟化焼鈍，冷間圧延を経て厚さ１mm
の鋼板とした。[Examples] Table 1 shows the chemical composition values (% by weight) of the test steels. Of these, Nos. A, B, C, D, E, F, G and H are comparative steels in which the content of any one of the components is out of the range specified in the present invention. K, L, M
And N are steels within the composition range of the present invention. All have a thickness of 1 mm after being subjected to normal hot rolling, softening annealing, and cold rolling.
Steel plate.
【００３６】各比較鋼Ａ〜Ｈと本発明鋼Ｉ〜Ｎを，それ
らのＡc3変態点以上の温度に加熱して完全にオーステナ
イト化したのち，ＴＴＴ線図のノーズを通過する速度よ
りも大きい冷却速度でＭs点〜350℃の範囲の温度まで冷
却し，この温度域に10〜60分間恒温保持した後に室温ま
で２℃／秒の冷却速度で冷却した。各供試鋼の熱処理条
件を表２の処理No.に示した。各々の処理No.の具体的な
条件は表３に示したものである。また本発明鋼Ｉ〜Ｊに
ついて，恒温処理条件が本発明で規定する範囲を外れる
熱処理を施した。その具体的な条件は表３の処理No.６
〜８に示した。Each of the comparative steels A to H and the steels of the present invention I to N was heated to a temperature not lower than the Ac 3 transformation point to completely austenite, and then was higher than the speed of passing through the nose of the TTT diagram. It was cooled to a temperature in the range of Ms point to 350 ° C. at a cooling rate, kept constant in this temperature range for 10 to 60 minutes, and then cooled to room temperature at a cooling rate of 2 ° C./sec. The heat treatment conditions of each test steel are shown in the treatment No. of Table 2. The specific conditions for each processing No. are shown in Table 3. Further, the present invention steels I to J were subjected to a heat treatment in which the constant temperature treatment conditions were out of the range specified in the present invention. The specific conditions are the processing No. 6 in Table 3.
~ 8.
【００３７】これらの処理を施した各鋼の機械的性質(J
IS 13B号に従う引張試験値) と, 残留オーステナイト量
および残留オーステナイト中の炭素濃度を測定し，その
結果を表２に示した。表２において，比較材とは前記比
較鋼に本発明で規定する範囲の熱処理を施した材料，本
発明材とは本発明に従う成分組成を有する鋼に本発明で
規定する範囲の熱処理を施した材料，そして比較例とは
本発明に従う成分組成を有する鋼に本発明の範囲外の条
件で熱処理を施したものである。Mechanical properties of each steel subjected to these treatments (J
The tensile test value according to IS 13B), the amount of retained austenite and the carbon concentration in the retained austenite were measured, and the results are shown in Table 2. In Table 2, the comparative material is a material obtained by subjecting the comparative steel to a heat treatment within the range specified in the present invention, and the material of the present invention is a steel having a chemical composition according to the present invention that is subjected to a heat treatment within the range specified in the present invention. The materials, and comparative examples, are steels having the composition according to the present invention, which are heat-treated under conditions outside the scope of the present invention.
【００３８】[0038]
【表１】 [Table 1]
【００３９】[0039]
【表２】 [Table 2]
【００４０】[0040]
【表３】 [Table 3]
【００４１】表２の結果から次のことがわかる。From the results shown in Table 2, the following can be seen.
【００４２】比較材Ａ２はＣrの含有量が低く且つＭo,
Ｖ, Ｎbも添加しない鋼を熱処理したものであるが，鋼
板製造時の軟化焼鈍時に黒鉛化を起こしてしまったの
で，充分にオーステナイト化ができず，このめたに強度
が低い。すなわち比較鋼Ａは黒鉛化抵抗が小さい。Comparative material A2 has a low Cr content and Mo,
Although the steel to which V and Nb were not added was heat-treated, it was not able to be austenitized sufficiently because it was graphitized during softening annealing during the production of steel sheet, and its strength was extremely low. That is, Comparative Steel A has low graphitization resistance.
【００４３】比較材Ｂ２は強度が低い。これは鋼中の炭
素量が低いことによる。Comparative material B2 has low strength. This is because the carbon content in steel is low.
【００４４】比較材Ｃ２は強度，延性（伸び）がともに
低い。これは, 鋼中の炭素量が過多であるために残留オ
ーステナイト量が多くなりすぎたからである。Comparative material C2 has low strength and ductility (elongation). This is because the amount of retained austenite was too large due to the excessive carbon content in the steel.
【００４５】比較材Ｄ５は延性が低い。これはＳi量が
過少であるためにベイナイト変態が速く進行し，30分間
の恒温保持によって残留オーステナイトが０％となった
からである。Comparative material D5 has low ductility. This is because the bainite transformation proceeded rapidly because the Si content was too small, and the retained austenite became 0% after the isothermal holding for 30 minutes.
【００４６】比較材Ｅ５も延性が低い。これはＭn量が
過多であるためにベイナイト変態が遅くなりすぎ，また
ベイナイトの形態も粗大なものとなったからである。Comparative material E5 also has low ductility. This is because the bainite transformation became too slow due to the excessive Mn content, and the bainite morphology became coarse.
【００４７】比較材Ｆ５はＣr量を本発明で規定するよ
り多量に含有させた鋼を熱処理したものであるが，延性
が低い。これはＣr量が過多であるためにベイナイト変
態が遅くなりすぎ，ベイナイト自身の靭性が低下したも
のである。Comparative material F5 is a heat-treated steel containing a larger amount of Cr than specified in the present invention, but has low ductility. This is because the bainite transformation was too slow due to the excessive Cr content, and the toughness of bainite itself was lowered.
【００４８】比較材Ｇ２はＶ量が過多であるために健全
なベイナイト組織が生成せず，靭性が低く。また比較材
Ｈ２もＭo量が過多であるために健全なベイナイト組織
が生成せず，靭性が低い。すなわち，硬さが高い割に伸
びが低く，耐力も低い。Since the comparative material G2 has an excessive V content, a healthy bainite structure is not formed and the toughness is low. In addition, the comparative material H2 also has a low toughness because a healthy bainite structure is not formed because the Mo content is excessive. That is, although the hardness is high, the elongation is low and the yield strength is also low.
【００４９】これに対し, 本発明材のＩ２，Ｉ４，Ｉ
５，Ｊ１，Ｊ２，Ｊ３，Ｋ２，Ｌ２，Ｍ２およびＮ２
は，いずれも引張強さ1600〜2300 N/mm2の超強度を示し
且つ伸びが１０％以上である。また0.2%耐力も 900 N/m
m2以上を示している。したがって本発明によれば，超高
強度を具備しながら延性に優れ，強度−延性バランスの
よい材料が得られたことがわかる。なお，本発明材の場
合には，いずれも残留オーステナイト中の炭素濃度は1.
5 ％以上となっており，また残留オーステナイト量もほ
ぼ１５％以上，３０％以下の適正範囲にある。On the other hand, the materials I2, I4 and I of the present invention are
5, J1, J2, J3, K2, L2, M2 and N2
Shows a super strength of tensile strength of 1600 to 2300 N / mm 2 and an elongation of 10% or more. The 0.2% proof stress is 900 N / m.
m 2 or more. Therefore, according to the present invention, it is understood that a material having excellent ductility while having an extremely high strength and having a good strength-ductility balance was obtained. In the case of the materials of the present invention, the carbon concentration in the retained austenite is 1.
It is 5% or more, and the amount of retained austenite is in the proper range of approximately 15% or more and 30% or less.
【００５０】しかし, 本発明で規定する化学成分値範囲
の鋼でも，比較例Ｉ８，Ｉ９，Ｊ６およびＪ７に見られ
るように，本発明で規定する熱処理条件が外れると超高
強度と靭性を同時に満足することはできない。However, even in the steel having the chemical composition value range defined by the present invention, as seen in Comparative Examples I8, I9, J6 and J7, when the heat treatment conditions defined by the present invention are deviated, ultra high strength and toughness are simultaneously obtained. I can't be satisfied.
【００５１】[0051]
【発明の効果】以上の実施例から明らかなように，本発
明によればＴＳが１６００〜２３００N/mm2で延性が優
れた超高強度鋼が得られる。As is apparent from the above examples, according to the present invention, an ultrahigh strength steel having a TS of 1600 to 2300 N / mm 2 and excellent ductility can be obtained.
Claims (5)
重量％, Ｍn：0.3〜2.0 重量％, Ｃr：0.2〜1.5 重量
％, 残部Ｆeおよび不可避的不純物元素からなる鋼を，
Ａc3変態点以上の温度に加熱して完全にオーステナイト
化したうえ，この温度からＴＴＴ線図のノーズを通過す
る速度よりも大きな冷却速度でＭs点以上で３５０℃未
満の温度まで冷却し，この温度域に１０〜６０分間恒温
保持した後に室温まで空冷または空冷以上の冷却速度で
冷却することからなる，ベイナイトと残留オーステナイ
トを主相とした複合組織を有し且つ引張強さが１６００
〜２３００N/mm2の超高強度鋼の製造方法。1. C: 0.4 to 1.0% by weight, Si: 1.2 to 3.0
%, Mn: 0.3 to 2.0% by weight, Cr: 0.2 to 1.5% by weight, the balance Fe and steel consisting of unavoidable impurity elements,
After heating to a temperature above the Ac 3 transformation point to completely austenite, cooling from this temperature to a temperature below 350 ° C above the Ms point at a cooling rate higher than the rate of passing through the nose of the TTT diagram. It has a composite structure consisting mainly of bainite and retained austenite, and has a tensile strength of 1600, which consists of holding the temperature in the temperature range for 10 to 60 minutes and then cooling it to room temperature by air cooling or at a cooling rate higher than air cooling.
~ 2300N / mm 2 Ultra high strength steel manufacturing method.
重量％, Ｍn：0.3〜2.0 重量％, Ｃr：0.2〜1.5 重量
％, さらに，0.05〜0.5 重量％のＭo, 0.05〜0.5 重量
％のＶまたは 0.01〜0.5 重量％Ｎbのいずれか１種また
は２種以上を含有し, 残部がＦeおよび不可避的不純物
元素からなる鋼を, Ａc3変態点以上の温度に加熱して完
全にオーステナイト化したうえ, この温度からＴＴＴ線
図のノーズを通過する速度よりも大きな冷却速度でＭs
点以上で３５０℃未満の温度まで冷却し，この温度域に
１０〜６０分間恒温保持した後に室温まで空冷または空
冷以上の冷却速度で冷却することからなる，ベイナイト
と残留オーステナイトを主相とした複合組織を有し且つ
引張強さが１６００〜２３００N/mm2の超高強度鋼の製
造方法。2. C: 0.4 to 1.0% by weight, Si: 1.2 to 3.0
%, Mn: 0.3 to 2.0% by weight, Cr: 0.2 to 1.5% by weight, further, 0.05 to 0.5% by weight Mo, 0.05 to 0.5% by weight V or 0.01 to 0.5% by weight Nb, either one or two. A steel containing more than one species and the balance consisting of Fe and unavoidable impurity elements is heated to a temperature above the Ac 3 transformation point to completely austenite, and from this temperature, the rate at which it passes through the nose of the TTT diagram is determined. Ms with a large cooling rate
A composite consisting of bainite and retained austenite as the main phase, which consists of cooling to a temperature below 350 ° C. at a temperature above the point, maintaining a constant temperature in this temperature range for 10 to 60 minutes, and then cooling to room temperature by air cooling or at a cooling rate above air cooling. A method for producing an ultra-high strength steel having a structure and a tensile strength of 1600 to 2300 N / mm 2 .
重量％以上である請求項１または２に記載の超高強度鋼
の製造方法。3. Retained austenite has a carbon concentration of 1.5.
The method for producing ultra-high strength steel according to claim 1 or 2, wherein the content is at least wt%.
積が１５〜３０容積％である請求項１，２または３に記
載の超高強度鋼の製造方法。4. The method for producing an ultrahigh strength steel according to claim 1, wherein the volume of the retained austenite is 15 to 30% by volume of the whole.
有するものである請求項１，２，３または４に記載の超
高強度鋼の製造方法。5. The method for producing an ultra high strength steel according to claim 1, 2, 3 or 4, wherein the ultra high strength steel has an elongation of 10% or more.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP15268292A JPH05320749A (en) | 1992-05-20 | 1992-05-20 | Production of ultrahigh strength steel |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP15268292A JPH05320749A (en) | 1992-05-20 | 1992-05-20 | Production of ultrahigh strength steel |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| JPH05320749A true JPH05320749A (en) | 1993-12-03 |
Family
ID=15545816
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP15268292A Pending JPH05320749A (en) | 1992-05-20 | 1992-05-20 | Production of ultrahigh strength steel |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JPH05320749A (en) |
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