JP4681492B2 - Steel plate hot pressing method and press-formed product - Google Patents
Steel plate hot pressing method and press-formed product Download PDFInfo
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本発明は、自動車部品の構造部材等、強度が必要とされる部材を、鋼板を熱間でプレス成形して製造する方法及びその方法によって成形されたプレス成形品に関する。 The present invention relates to a method of manufacturing a member that requires strength, such as a structural member of an automobile part, by hot pressing a steel plate and a press-formed product formed by the method.
近年、環境保護と地球温暖化を防止するため、化石燃料の消費を抑制する動きが強まっており、移動手段である自動車においても車体の軽量化による低燃費化が強く求められている。自動車車体の軽量化を達成するためには、従来に比べ高い機械強度を有する材料を使用することが必要となるが、高強度を有する鋼板は形状凍結性の低下によって、成形加工後の形状が劣化する傾向にあり、また成形性も軟鋼板より劣るため複雑な形状を有する部品の成形は困難である。これらの問題を解決する方法のひとつとして、鋼板を８００℃以上の高温に加熱し、プレスによる成形加工を行った後、冷却するいわゆる熱間プレス（ホットスタンプ）による成形方法が有望である。 In recent years, in order to protect the environment and prevent global warming, there has been an increasing movement to reduce the consumption of fossil fuels, and there is a strong demand for reducing fuel consumption by reducing the weight of automobile bodies even in automobiles that are moving means. In order to achieve weight reduction of the car body, it is necessary to use a material having higher mechanical strength than before, but a steel sheet having high strength has a shape after forming due to a decrease in shape freezing property. Since there is a tendency to deteriorate and the formability is also inferior to that of a mild steel plate, it is difficult to form a component having a complicated shape. As a method for solving these problems, a forming method by a so-called hot press (hot stamp) in which a steel plate is heated to a high temperature of 800 ° C. or higher, formed by pressing, and then cooled is promising.
熱間プレスは、組織がオーステナイト相になる温度（オーステナイト温度域という。）に加熱され、強度が低下して成形性が向上した鋼板をプレス成形し、プレス金型によって鋼板からの抜熱とプレス成形を同時に行うものである。これにより、熱間でのプレス成形後、急冷して成形品の組織をオーステナイトからマルテンサイトに変態させ、いわゆる焼入により高強度化が達成される。即ち、熱間プレスの金型は通常の冷間プレスで要求される成形機能のみならず、鋼板の組織制御、即ち材質作り込みの機能も要求されており、熱間プレスの成形技術は成形性確保と焼入性確保の両面から種々検討されている（例えば、特許文献１〜５）。 In hot pressing, a steel sheet heated to a temperature at which the structure becomes an austenite phase (referred to as an austenite temperature range) is pressed to reduce the strength and formability is improved. Molding is performed simultaneously. As a result, after hot press molding, it is rapidly cooled to transform the structure of the molded product from austenite to martensite, and high strength is achieved by so-called quenching. In other words, hot press molds are required not only for forming functions required in ordinary cold pressing, but also for controlling the structure of steel sheets, that is, for incorporating materials into materials. Various studies have been made in terms of both securing and hardenability (for example, Patent Documents 1 to 5).
特許文献１には、鋼板を加熱し、冷却速度を鋼板組成の焼入臨界速度以上としてマルテンサイト組織にすることが示されており、特許文献２には装置内で熱間成形及び焼入れする方法が提案されている。しかし、これらにはプレス成形の開始温度及び停止温度は示されていない。特許文献３にはプレス直前の鋼板を８５０℃以上にすることが示されているが、プレス前後での鋼板の冷却速度、プレス成形の停止温度については記載がない。また、特許文献４には金型のクリアランスを制御すること、加熱温度を８００〜１０００℃とすること、プレス成形開始温度を９００℃以上とすることは記載されているものの、プレス前後での鋼板の冷却速度、プレス成形の停止温度については記載がない。 Patent Document 1 discloses that a steel sheet is heated and the cooling rate is equal to or higher than the critical quenching speed of the steel sheet composition to form a martensite structure. Patent Document 2 discloses a method of hot forming and quenching in an apparatus. Has been proposed. However, they do not show the start and stop temperatures of press forming. Patent Document 3 discloses that the steel plate immediately before pressing is set to 850 ° C. or higher, but there is no description about the cooling rate of the steel plate before and after pressing and the stop temperature of press forming. Patent Document 4 describes that the clearance of the mold is controlled, the heating temperature is set to 800 to 1000 ° C., and the press molding start temperature is set to 900 ° C. or more. There is no description about the cooling rate and the stop temperature of press molding.
特許文献５には変態誘起塑性現象を利用するため、鋼板を臨界冷却速度以上の冷却速度で冷却し、成形温度範囲を（Ｍｓ点−８０℃）以上（Ｍｓ点＋１２０℃）以下とする熱間プレス技術が提案されている。しかし、特許文献５に記載された方法では、実施例に示されているように、引張試験では伸びが増加するかもしれないが、複雑形状となる部品のプレスでは、硬いマルテンサイト組織を含んで成形することになり、成形性を損なうことが懸念される。 In Patent Document 5, in order to use the transformation-induced plastic phenomenon, the steel sheet is cooled at a cooling rate equal to or higher than the critical cooling rate and the forming temperature range is (Ms point−80 ° C.) or more and (Ms point + 120 ° C.) or less. Press technology has been proposed. However, in the method described in Patent Document 5, as shown in the examples, the tensile test may increase the elongation, but the press of the component having a complicated shape includes a hard martensite structure. There is a concern that moldability will be impaired.
また、特許文献６、非特許文献１には、成形性を向上させるための温間加工が開示されているが、これらは金型等を加熱して加工するものであり、また、例えば特許文献７に記載されているように、潤滑剤の効果の確保、表面性状の劣化防止のため、加工前に鋼板をオーステナイト組織まで加熱することはない。そのため、鋼板の組織がオーステナイト相となるような高温から焼入れする熱間プレスとは異なり、部材を高強度化することができない。 In addition, Patent Document 6 and Non-Patent Document 1 disclose warm working for improving moldability, but these heat and process a mold or the like. As described in No. 7, the steel sheet is not heated to the austenite structure before processing in order to ensure the effect of the lubricant and prevent the deterioration of the surface properties. Therefore, unlike the hot press which quenches from the high temperature that the structure of a steel plate turns into an austenite phase, a member cannot be strengthened.
以上のように、上記の何れの従来技術にも、加熱後の鋼板を成形する温度と熱間プレスでの成形方法による成形性の差異については十分に言及されているとはいえない。
本発明は、ブランク押さえを要する絞り変形や張り出し変形にも適用可能な、鋼板を熱間プレスする技術及び熱間プレスによって製造された高強度のプレス成形品の提供を目的とするものである。 An object of the present invention is to provide a technique for hot pressing a steel sheet and a high-strength press-formed product manufactured by hot pressing, which can be applied to drawing deformation and overhang deformation that require blank pressing.
本発明者らは、中炭素薄鋼板の熱間プレス成形性に及ぼす成形温度、冷却速度の影響について検討を行い、加熱、冷却、プレス成形、更に冷却する際の、プレス成形開始温度、停止温度及びプレス成形時間を制御し、より低温で、かつ成形が終了するまでマルテンサイト変態させることなく、プレス成形を行うことにより、優れたプレス成形性を確保できるという知見を得た。本発明はこのような知見に基づいてなされたものであり、その要旨は以下の通りである。 The inventors have studied the influence of the forming temperature and the cooling rate on the hot press formability of the medium carbon thin steel sheet, and the press forming start temperature and stop temperature when heating, cooling, press forming, and further cooling are performed. In addition, the inventors have obtained knowledge that excellent press formability can be ensured by controlling the press molding time, performing the press molding at a lower temperature and without performing martensitic transformation until the molding is completed. This invention is made | formed based on such knowledge, The summary is as follows.
（１）質量％で、Ｃ≧０．１５％を含有する鋼板を８５０℃以上に加熱後、１０℃／ｓ以上の冷却速度で冷却しながら、プレス成形開始温度Ｔｐ［℃］を６００℃以下とし、プレス成形終了温度を前記鋼板のマルテンサイト変態開始温度Ｍｓ［℃］以上としてプレス成形し、その後３００℃以下まで１０℃／ｓ以上の冷却速度で冷却し、ビッカース硬さＨｖが４００以上のプレス成形品を得ることを特徴とする鋼板熱間プレス方法。 (1) A steel sheet containing C ≧ 0.15% by mass% is heated to 850 ° C. or higher and then cooled at a cooling rate of 10 ° C./s or higher, and the press forming start temperature Tp [° C.] is 600 ° C. or lower. And press forming the press forming end temperature as the martensite transformation start temperature Ms [° C.] or higher of the steel sheet, and then cooling to 300 ° C. or lower at a cooling rate of 10 ° C./s or higher, and the Vickers hardness Hv is 400 or higher. A method of hot pressing a steel sheet, wherein a press-formed product is obtained .
（２）プレス成形時間ｔｐ［ｓ］を２０ｓ以内としてプレス成形することを特徴とする上記（１）に記載の鋼板熱間プレス方法。 (2) The steel sheet hot pressing method according to (1) above, wherein press forming is performed with a press forming time tp [s] within 20 s.
（３）前記プレス成形開始温度Ｔｐ［℃］、前記鋼板のマルテンサイト変態開始温度Ｍｓ［℃］及び前記プレス成形時間ｔｐ［ｓ］が、ｔｐ≦（Ｔｐ−Ｍｓ）／１００を満足することを特徴とする上記（２）に記載の鋼板熱間プレス方法。 (3) the press forming start temperature Tp [℃], martensitic transformation start temperature Ms of the steel sheet [℃] and the press molding time tp [s] is, by satisfying the tp ≦ (Tp-Ms) / 100 The steel plate hot pressing method according to (2), which is characterized in that
（４）プレス成形前の加熱した前記鋼板を、気体及び液体の何れか一方又は双方を冷媒として冷却することを特徴とする上記（１）〜（３）の何れか１項に記載の鋼板熱間プレス方法。 (4) The heated the steel sheet before press-forming, the either one or both of the gas and liquid, characterized in that cooling as a refrigerant (1) to the steel plate heat according to any one of (3) Inter-press method.
（５）プレス成形前の加熱した前記鋼板をプレス装置の金型で冷却することを特徴とする上記（１）〜（４）の何れか１項に記載の鋼板熱間プレス方法。 (5) The steel sheet hot pressing method according to any one of (1) to (4) above, wherein the heated steel sheet before press forming is cooled by a die of a press device.
（６）ブランク押さえにより前記鋼板を拘束しながら、絞り変形、張り出し変形の一方又は双方を含むプレス成形を行うことを特徴とする上記（１）〜（５）の何れか１項に記載の鋼板熱間プレス方法。 (6) while restraining the steel sheet by the blank holding, squeezing deformation, the steel sheet according to any one of the above and performing the press forming includes one or both of the protruding deformation (1) to (5) Hot pressing method.
（７）プレス成形後の成形品を、気体及び液体の何れか一方又は双方を冷媒として冷却することを特徴とする上記（１）〜（６）の何れか１項に記載の鋼板熱間プレス方法。 (7) The hot-pressed steel sheet according to any one of (1) to (6) above, wherein the formed product after press forming is cooled using one or both of gas and liquid as a refrigerant. Method.
（８）プレス成形後の成形品をプレス装置の金型で冷却することを特徴とする上記（１）〜（７）の何れか１項に記載の鋼板熱間プレス方法。 (8) The steel sheet hot pressing method according to any one of (1) to (7) above, wherein the formed product after press forming is cooled by a die of a press device.
（９）上記（１）〜（８）の何れか１項に記載の方法で熱間プレスされたプレス成形品であることを特徴とするプレス成形品。 (9) A press-molded product which is a press-molded product hot-pressed by the method according to any one of (1) to (8) above.
本発明により、自動車部品の構造部材に代表されるような高強度部材の製造において、従来の熱間プレス法に比して、絞り変形や張り出し変形を必要とする複雑形状の成形品を製造することが可能になり、高強度のプレス成形品が得られ、工業的に価値の大きなものである。 According to the present invention, in the manufacture of a high-strength member represented by a structural member of an automobile part, a molded product having a complicated shape that requires a drawing deformation or an overhanging deformation as compared with the conventional hot pressing method is manufactured. Therefore, a high-strength press-molded product can be obtained, which is industrially valuable.
本発明者らは、鋼板、特に中炭素薄鋼板を素材としてブランク押さえを要する絞り変形や張り出し変形での熱間プレス成形を行う際、加熱炉から鋼板を抽出してプレス成形するまでの時間及び冷却方法を種々変えて試験を行った。その結果、加熱炉から鋼板を抽出した後、短時間でプレス成形するよりも、金型と接触させた状態で暫く時間をおいてプレス成形を行うと、成形高さが向上することを見出した。このプレス成形前の保持時間は、プレス成形時の鋼板の温度に影響を与えるため、プレス成形時の鋼板温度を従来よりも低くすると成形高さが向上するという結論が得られた。 When performing hot press forming in a drawing deformation or overhang deformation that requires blank pressing using a steel plate, particularly a medium carbon thin steel plate as a raw material, the present inventors extract the steel plate from the heating furnace and press-form it. The test was conducted with various cooling methods. As a result, after extracting the steel plate from the heating furnace, it was found that if the press forming is performed for a while in the state of being in contact with the mold, the forming height is improved rather than the press forming in a short time. . Since the holding time before press forming affects the temperature of the steel plate during press forming, it was concluded that the forming height is improved when the steel plate temperature during press forming is lower than the conventional one.
一方、加熱炉から鋼板を抽出した後、冷却速度が所定速度（後述する１０℃／ｓ）よりも遅い場合、例えばプレス成形前後に空冷すると、十分な焼入れ性が確保できないことから、成形性と共に焼入れ性を確保するためには、冷却速度を適正に制御する必要があることがわかった。更に、プレス成形を終了する前にマルテンサイト変態が生じると、成形性が著しく劣化するという知見を得た。 On the other hand, after extracting the steel plate from the heating furnace, if the cooling rate is slower than a predetermined rate (10 ° C./s to be described later), for example, if air cooling is performed before and after press molding, sufficient hardenability cannot be ensured. It was found that the cooling rate must be controlled appropriately to ensure hardenability. Furthermore, it has been found that if martensitic transformation occurs before the press molding is completed, the moldability is significantly deteriorated.
本発明は以上の検討によって得られた知見に基づくものであり、最大の特徴は、オーステナイト温度域まで加熱した鋼板に対して、冷却速度を制御しながら、プレス成形を行うようにし、プレス成形の開始時における鋼板の温度を６００℃以下（且つ、Ｍｓ点以上、以下同様である）とし、プレス成形の終了時における鋼板の温度をＭｓ点以上とすることである。 The present invention is based on the knowledge obtained by the above examination, and the greatest feature is that the steel sheet heated to the austenite temperature range is subjected to press forming while controlling the cooling rate, and The temperature of the steel plate at the start is set to 600 ° C. or lower (and Ms point or higher, the same applies hereinafter), and the temperature of the steel plate at the end of press forming is set to Ms point or higher.
即ち、本発明は、通常の熱間プレス温度よりも低温で鋼板のプレス成形を開始する点で従来の熱間プレス成形と相違し、プレス成形中に歪み誘起マルテンサイト変態を生じさせることなくプレス成形を終了させる点で、成形温度を（Ｍｓ点＋１２０℃）〜（Ｍｓ点−８０℃）の範囲とする熱間プレス成形とは相違する。また、鋼板の加熱温度をオーステナイト温度域として、加速冷却しながらプレス成形と焼入れを同時に行い、高強度成形品を得るという点において、従来の温間成形とは相違する。 That is, the present invention is different from conventional hot press forming in that it starts press forming of a steel sheet at a temperature lower than the normal hot press temperature, and presses without causing strain-induced martensitic transformation during press forming. This is different from hot press forming in which the forming temperature is in the range of (Ms point + 120 ° C.) to (Ms point−80 ° C.) in that the forming is finished. Moreover, it is different from conventional warm forming in that the heating temperature of the steel sheet is set to the austenite temperature range and press forming and quenching are simultaneously performed while accelerated cooling to obtain a high strength formed product.
本発明で規定したように、６００℃以下の温度域でプレス成形を開始し、Ｍｓ点以上の温度でプレス成形を終了させることで、成形品の側壁部分などに生じ易い破断を防止することができる。この破断は、絞り変形や張り出し変形などでブランク押さえを要する場合、即ち、プレス成形時にフランジ部の材料流入が抑制されるような成形で生じ易く、本発明は特にこのような変形での破断防止に効果を発揮する。 As defined in the present invention, by starting press molding at a temperature range of 600 ° C. or lower and ending press molding at a temperature equal to or higher than the Ms point, it is possible to prevent breakage that tends to occur on the side wall portion of the molded product. it can. This rupture is likely to occur when blank pressing is required due to squeeze deformation or overhang deformation, that is, molding that suppresses the material inflow of the flange portion during press molding, and the present invention particularly prevents rupture due to such deformation. To be effective.
高温域に比して６００℃以下の範囲でプレス成形を開始することで破断が抑制される理由は明確ではないが、熱間プレスにおける破断は、鋼板温度が不均一で、局所的に高温になり、強度が低下した部位で生じると考えられることから、６００℃以下の温度域では鋼板の強度が増しており、温度がやや不均一であっても、成形性の不均一性が顕著ではなくなるためであると推察される。 The reason why rupture is suppressed by starting press forming in the range of 600 ° C. or lower compared to the high temperature range is not clear, but rupture in hot pressing is due to uneven steel plate temperature and locally high temperature. Therefore, the strength of the steel sheet is increased in a temperature range of 600 ° C. or lower, and even if the temperature is slightly non-uniform, the non-uniformity of formability is not significant. This is presumed to be due to this.
また、Ｍｓ点以上の温度でプレス成形を終了させるのは、歪み誘起マルテンサイト変態を生じることなくプレス成形を終了させるためである。これは、プレス成形時にマルテンサイト変態が生じると、角筒絞り成形に代表されるような、フランジ部の材料流入が抑制されるプレス成形では、マルテンサイトを起点として破断が生じ易くなるためである。 The reason why the press molding is terminated at a temperature equal to or higher than the Ms point is to terminate the press molding without causing strain-induced martensitic transformation. This is because, when martensite transformation occurs during press molding, breakage tends to occur starting from martensite in press molding in which the material inflow of the flange portion is suppressed, as represented by rectangular tube drawing. .
次に、重要な条件は、加熱した鋼板及びプレス成形後の成形品を冷却する際の冷却速度である。加熱炉から抽出した鋼板を、プレス成形を行う直前まで１０℃／ｓ以上の冷却速度で冷却し、鋼板の温度がプレス成形中及びプレス成形後の温度から３００℃以下に到達するまでの冷却速度も１０℃／ｓ以上とすることが必要である。ただし、鋼板の温度がＡｒ３温度超の領域では冷却速度の影響は顕著ではないため、鋼板のＡｒ３温度以下から３００℃以下に到達するまでの冷却速度を１０℃／ｓ以上とすることが好ましい。この冷却速度が１０℃／ｓ未満であると、冷却中にオーステナイトからフェライトへの変態が起こり、十分に焼入れ強度が確保できない。プレス成形後の冷却停止温度は、マルテンサイト変態を完了させるために３００℃以下とし、室温まで冷却しても良い。冷却速度の上限は規定しないが、後述する金型による抜熱と冷媒の噴射を組み合わせても、５００℃／ｓより速くすることは困難である。 Next, an important condition is the cooling rate when cooling the heated steel sheet and the molded product after press forming. The steel sheet extracted from the heating furnace is cooled at a cooling rate of 10 ° C./s or more immediately before press forming, and the cooling rate until the temperature of the steel plate reaches 300 ° C. or less from the temperature during and after press forming. Needs to be 10 ° C./s or more. However, since the influence of the cooling rate is not significant in the region where the temperature of the steel plate exceeds the Ar 3 temperature, the cooling rate until reaching the 300 ° C. or lower from the Ar 3 temperature or lower of the steel plate may be 10 ° C./s or higher. preferable. When the cooling rate is less than 10 ° C./s, transformation from austenite to ferrite occurs during cooling, and sufficient quenching strength cannot be ensured. The cooling stop temperature after press molding may be 300 ° C. or lower in order to complete the martensitic transformation and may be cooled to room temperature. Although the upper limit of the cooling rate is not stipulated, it is difficult to make it faster than 500 ° C./s even if combined with heat removal by a mold to be described later and injection of refrigerant.
また、プレス成形時間ｔｐ［ｓ］は、２０ｓ以内とすることが好ましい。これは、１０℃／ｓ以上の冷却速度で冷却しながら、鋼板の温度が６００℃以下の状態でプレス成形を開始し、マルテンサイト変態を生じさせることなく、Ｍｓ点以上の温度でプレス成形を終了するための好ましい上限である。製造コストの観点から、プレス成形時間の上限は、５秒以下とすることが好ましい。プレス成形の下限時間は、特に規定しないが、０．０５秒未満では鋼板の少なくとも一部が破断するおそれがあるため、０．０５秒以上とすることが好ましく、安定してプレス成形するためには０．１秒以上とすることが更に好ましい。 Further, the press molding time tp [s] is preferably within 20 s. This is because, while cooling at a cooling rate of 10 ° C./s or higher, press forming is started at a temperature of the steel plate of 600 ° C. or lower, and press forming is performed at a temperature of the Ms point or higher without causing martensitic transformation. A preferred upper limit for termination. From the viewpoint of manufacturing cost, the upper limit of the press molding time is preferably 5 seconds or less. The lower limit time of press forming is not particularly specified, but if it is less than 0.05 seconds, at least a part of the steel sheet may be broken. Is more preferably 0.1 seconds or more.
また、プレス成形時間ｔｐ［ｓ］は、プレス成形開始温度Ｔｐ［℃］、鋼板のマルテンサイト変態温度Ｍｓ［℃］との関係が、ｔｐ≦（Ｔｐ−Ｍｓ）／１００を満足することが好ましい。これは、冷却速度を速くした際に、プレス成形中の歪み誘起マルテンサイト変態を防止するための好ましい条件であり、これにより、良好な成形性を確保して、高強度の成形品を得ることができる。 The press forming time tp [s] preferably satisfies the relationship tp ≦ (Tp−Ms) / 100 in relation to the press forming start temperature Tp [° C.] and the martensitic transformation temperature Ms [° C.] of the steel sheet. . This is a preferable condition for preventing strain-induced martensitic transformation during press molding when the cooling rate is increased, thereby ensuring good moldability and obtaining a high-strength molded product. Can do.
冷却速度を１０℃／ｓ以上とする場合において、例えば、板厚が１〜２ｍｍの薄鋼板の場合、特に７００℃以上の高温域では空冷しても良いが、７００℃未満の温度域では、水冷、ミスト冷却、空気による強制冷却など、液体、気体の一方又は双方を冷媒として用いて冷却することが好ましい。 In the case where the cooling rate is 10 ° C./s or more, for example, in the case of a thin steel plate having a thickness of 1 to 2 mm, air cooling may be performed particularly in a high temperature range of 700 ° C. or more, but in a temperature range of less than 700 ° C., It is preferable to cool by using one or both of liquid and gas as a refrigerant, such as water cooling, mist cooling, forced cooling with air, and the like.
通常、熱間プレスは金型による抜熱によって焼入れを達成することから、プレス成形後、プレス下死点でポンチを停止させ、鋼板温度の冷却速度を高める方法が用いられている。本発明では、プレス成形前の鋼板の冷却速度を１０℃／ｓ以上とすることが必要であるため、プレス成形後の金型による抜熱のみならず、プレス成形前にも金型で鋼板の抜熱を行うことが好ましい。 Usually, a hot press achieves quenching by removing heat with a mold, and therefore, after press forming, a method of stopping the punch at the press bottom dead center and increasing the cooling rate of the steel sheet temperature is used. In the present invention, since it is necessary to set the cooling rate of the steel plate before press forming to 10 ° C./s or more, not only the heat removal by the die after press forming but also the steel plate with the die before press forming. It is preferable to perform heat removal.
プレスの金型による抜熱とは、プレス装置の金型を構成するダイス、ポンチ、ポンチ押さえ、ブランク押さえなどを鋼板の表面に接触させて、鋼板の温度を低下させる方法である。鋼板は、下金型の上に放置して冷却しても良いが、更に上金型で挟むように接触させて冷却することが好ましい。 The heat removal by the press mold is a method of lowering the temperature of the steel sheet by bringing a die, a punch, a punch press, a blank press or the like constituting the press mold into contact with the surface of the steel sheet. The steel sheet may be cooled by leaving it on the lower mold, but it is preferable that the steel sheet is further cooled by being brought into contact with the upper mold.
これは、下金型をダイス、上金型をポンチ、ポンチ押さえ、ブランク押さえとする場合、鋼板をダイス上に設置し、プレス途上で、上金型、すなわちポンチ、ポンチ押さえ、ブランク押さえの何れか１つ以上、又は全てが素材鋼板の表面に接触した時点で停止させることで容易に達成される。プレス装置によっては、ポンチ、ポンチ押さえ、ブランク押さえが下金型、ダイスが上金型になるものもある。 This is because when the lower mold is a die and the upper mold is a punch, punch press, blank press, a steel plate is placed on the die, and any of the upper mold, that is, punch, punch press, blank press is in the process of pressing. It is easily achieved by stopping when one or more or all of them contact the surface of the material steel plate. Depending on the press device, there are punches, punch presses, and blank presses that serve as a lower die, and dies that serve as an upper die.
プレス成形後の冷却は、プレス下死点でポンチを停止させ、鋼板の温度が３００℃以下に到達するまで放置（下死点保持）すれば、１０℃／ｓ以上の冷却速度で冷却することができる。プレス成形後は、ポンチを退避させる。ここで、冷媒噴出機構を設けたポンチによって、液体、気体の一方又は双方を冷媒として用いて、鋼板を冷却することが好ましい。 Cooling after press forming should be stopped at a cooling rate of 10 ° C / s or more if the punch is stopped at the bottom dead center of the press and left until the temperature of the steel sheet reaches 300 ° C or lower (holding the bottom dead center). Can do. After press molding, the punch is retracted. Here, it is preferable to cool the steel sheet by using one or both of a liquid and a gas as a refrigerant by a punch provided with a refrigerant ejection mechanism.
プレス成形前後の冷却速度を高めるための冷媒は、気体、液体の一方又は双方を用いれば良い。この場合、冷却は、プレス装置とは別に冷却装置を設け、鋼板の表面に冷媒を噴射しても良く、冷却槽中に鋼板を浸漬しても良い。また、プレス装置の金型内に流路を設け、冷媒である水やガスなどを流し、鋼板表面の冷却と金型の冷却も同時に行うと、抜熱の効果が著しく向上する。 As the refrigerant for increasing the cooling rate before and after the press molding, one or both of gas and liquid may be used. In this case, for cooling, a cooling device may be provided separately from the press device, and the coolant may be injected onto the surface of the steel plate, or the steel plate may be immersed in the cooling bath. In addition, if a flow path is provided in the mold of the press device, and water or gas as a coolant is allowed to flow, and the cooling of the steel sheet surface and the mold are simultaneously performed, the heat removal effect is remarkably improved.
ここで、冷媒のうち、気体としては空気が代表的なものであり、ファンにより空気を吹き付ければ良く、また加熱炉の雰囲気と同様に窒素やアルゴンなどのガスを用いても良い。冷媒のうち、液体としては水が代表的なものであり、油などの有機溶液を用いても良い。更に空気と水を混合させて霧状に吹き付けるなど、気体と液体の混合物を冷媒として用いても良い。 Here, air is a typical gas among the refrigerants, and air may be blown by a fan, or a gas such as nitrogen or argon may be used similarly to the atmosphere of the heating furnace. Among the refrigerants, water is typical as the liquid, and an organic solution such as oil may be used. Furthermore, a mixture of gas and liquid may be used as the refrigerant, for example, air and water are mixed and sprayed in a mist form.
本発明の対象とする鋼板は、炭素を質量％で０．１５％以上を含有するものとする。炭素が０．１５％未満では冷却速度を高くしないと焼きが入り難くなり、焼入れ後の鋼板強度を確保できず、ビッカース硬さＨｖで４００以上とならないことがある。炭素の質量％の上限は、成形品の衝撃変形時の強度確保のため、０．８％以下であることが好ましい。 The steel plate which is the subject of the present invention contains 0.15% or more by mass of carbon. If the carbon content is less than 0.15%, it is difficult to quench unless the cooling rate is increased, the steel plate strength after quenching cannot be secured, and the Vickers hardness Hv may not be 400 or more. The upper limit of the mass% of carbon is preferably 0.8% or less in order to ensure the strength at the time of impact deformation of the molded product.
なお、炭素（Ｃ）以外の鋼組成は、質量％でＳｉが２％以下、Ｍｎが３％以下、ＰとＳが０．０２％以下の範囲で含有し、残部が鉄及び不可避的不純物からなることが好ましい。また、熱間プレス用鋼板としての焼入れ性をより安定に確保するために、添加されるＣｒやＭｏは２％以下、Ｂは０．００５％以下を含有しても良い。更には、Ｔｉ、Ｎｂ、Ｖをそれぞれ選択的に０．２％以下添加しても良い。 In addition, steel composition other than carbon (C) contains Si in an amount of 2% or less, Mn in 3% or less, P and S in a range of 0.02% or less, with the balance being iron and inevitable impurities. It is preferable to become. Moreover, in order to ensure the hardenability as a steel plate for hot press more stably, Cr and Mo to be added may contain 2% or less, and B may contain 0.005% or less. Furthermore, Ti, Nb, and V may be selectively added in an amount of 0.2% or less.
また、本発明の鋼板は板厚が１〜２ｍｍの冷延鋼板とすることが好ましいが、加熱時のスケール生成防止及びプレス成形後の耐食性の観点から、亜鉛やアルミニウムなどのめっきが施されためっき鋼板でも良く、更にはこのめっき層が鉄と合金化したものでも良い。 The steel sheet of the present invention is preferably a cold-rolled steel sheet having a thickness of 1 to 2 mm. However, from the viewpoint of preventing scale formation during heating and corrosion resistance after press forming, plating such as zinc or aluminum is applied. A plated steel sheet may be used, and further, this plated layer may be alloyed with iron.
このような鋼板を加熱する際、本発明ではその鋼板温度を８５０℃以上とする。これは８５０℃未満では加熱時間が短い場合、十分に金属組織がオーステナイト相に変態しておらず、焼入れ時に十分な強度が得られないためである。ここで、加熱時の鋼板温度を８５０℃未満として長時間加熱を行うことも可能であるが、この場合には、スケール生成を助長してしまうため、好ましくない。なお、鋼板温度が８５０℃以上であれば本発明では加熱時間は特に規定しない。しかし、長時間の加熱はスケール生成を助長することから通常、加熱炉内での保持時間を５分以内とすることが好ましい。なお、スケール生成を低減させる目的で加熱炉の雰囲気として窒素やアルゴンなどのガスを用いれば、加熱炉内での保持時間を長くすることも可能であるが、生産性を考慮すると１２０分を上限とすることが好ましい。また、鋼板の加熱は加熱炉で行えば良いが、通電加熱、誘導加熱で行っても良い。 When heating such a steel plate, the temperature of the steel plate is set to 850 ° C. or higher in the present invention. This is because if the heating time is short at less than 850 ° C., the metal structure is not sufficiently transformed into the austenite phase, and sufficient strength cannot be obtained during quenching. Here, it is possible to perform heating for a long time by setting the steel plate temperature at the time of heating to less than 850 ° C. However, in this case, scale generation is promoted, which is not preferable. In addition, if steel plate temperature is 850 degreeC or more, in this invention, a heating time will not be prescribed | regulated in particular. However, since heating for a long time promotes scale formation, it is usually preferable to keep the holding time in the heating furnace within 5 minutes. In addition, if a gas such as nitrogen or argon is used as the atmosphere of the heating furnace for the purpose of reducing scale generation, it is possible to lengthen the holding time in the heating furnace, but considering productivity, the upper limit is 120 minutes. It is preferable that Moreover, although heating of a steel plate should just be performed with a heating furnace, you may carry out by electrical heating and induction heating.
自動車の軽量化を達成するためには、従来に比べ高い機械強度を有する材料を使用することが必要であり、そのためには、成形品のビッカース硬さがＨｖ４００以上であることが好ましい。ビッカース硬さは、成形品側壁部の鋼板を切断し、樹脂埋め込み、研磨して、板厚方向１／４位置の断面を、ＪＩＳ Ｚ ２２４４に準拠して測定すれば良く、３点以上測定し、単純平均値を求める方法が好ましい。 In order to achieve a reduction in the weight of an automobile, it is necessary to use a material having higher mechanical strength than before, and for that purpose, it is preferable that the molded article has a Vickers hardness of Hv400 or more. Vickers hardness can be measured by cutting the steel plate on the side wall of the molded product, embedding resin, polishing, and measuring the cross section at 1/4 position in the plate thickness direction in accordance with JIS Z 2244. A method of obtaining a simple average value is preferable.
実施例により本発明をさらに説明する。質量％で、Ｃ：０．２２％、Ｓｉ：０．２％、Ｍｎ：１．４％、Ｃｒ：０．２％、Ｍｏ：０．１％、Ｔｉ：０．０５％、Ｂ：０．００２％の化学成分を有する鋼を鋳造し、１１５０℃〜１２５０℃の温度に再加熱後、熱延し、酸洗、冷間圧延を行い、焼鈍し、さらに圧下率０．８％の調質圧延を施して１．５ｍｍ厚の冷延鋼板を製造した。この鋼板のＭｓ点は、フォーマスター試験機を用い、冷却中の試験片の熱膨張変化を測定して求めた結果、４００℃であった。 The examples further illustrate the invention. In mass%, C: 0.22%, Si: 0.2%, Mn: 1.4%, Cr: 0.2%, Mo: 0.1%, Ti: 0.05%, B: 0.00. Cast steel with 002% chemical composition, reheat to 1150 ℃ ~ 1250 ℃, then hot-roll, pickling, cold rolling, annealing, tempering with 0.8% reduction A cold rolled steel sheet having a thickness of 1.5 mm was manufactured by rolling. The Ms point of this steel sheet was 400 ° C. as a result of measuring the thermal expansion change of the test piece during cooling using a Formaster testing machine.
この鋼板をブランクサイズ１５０ｍｍ角に切断したものを熱間プレスに供した。このプレス成形は、ブランク押さえを用いた、８０ｍｍ×８０ｍｍの角頭ポンチを下金型とする角筒成形であり、ポンチ押さえもポンチ底に対面する位置に配した。プレス成形高さは３５ｍｍとした。なお、ポンチ肩及びダイス肩は１０Ｒ、ポンチコーナーは１５Ｒである。 What cut this steel plate into the blank size 150 square mm was used for the hot press. This press molding was a square tube molding using a blank presser with a square head punch of 80 mm × 80 mm as a lower die, and the punch presser was also arranged at a position facing the punch bottom. The press molding height was 35 mm. The punch shoulder and the die shoulder are 10R, and the punch corner is 15R.
また、ブランク押さえ力は２トンとした。成形速度（プレス成形の際の鋼板の移動速度）は２０〜１５０ｍｍ／ｓとしたが、プレス成形前後での鋼板の冷却を金型抜熱で行う時には、適宜プレス成形の途中で一時停止させた。即ち、プレス成形前の金型抜熱では、ポンチとブランク押さえ上の鋼板にダイスとポンチ押さえが接触した時点で（プレス成形の直前）、またプレス成形後の金型抜熱では、プレス下死点で（プレス成形の直後）、所定の時間停止させた。ここで、所定の時間とは、プレス成形開始温度およびプレス成形終了温度が、上述した温度（プレス成形開始温度が６００℃以下の温度であり、プレス成形終了温度がＭｐ点以上の温度）に到達するまでの時間である。 The blank pressing force was 2 tons. The forming speed (moving speed of the steel plate during press forming) was 20 to 150 mm / s. However, when cooling the steel plate before and after press forming by heat removal from the mold, it was temporarily stopped during the press forming as appropriate. . That is, in the heat removal from the die before press forming, the die and the punch press come into contact with the steel plate on the punch and the blank press (immediately before press forming). At a point (immediately after the press molding), it was stopped for a predetermined time. Here, the predetermined time means that the press molding start temperature and the press molding end temperature reach the above-described temperatures (the press molding start temperature is a temperature of 600 ° C. or lower and the press molding end temperature is a temperature equal to or higher than the Mp point). It is time to do.
加熱炉により鋼板を９００℃で３分加熱した後抽出し、プレス上死点位置で鋼板をポンチとブランク押さえ上にセットし、表１に示すように冷却方法を種々組み合わせ、プレス成形前後の冷却速度とプレス成形温度を変えた成形を行った。冷却停止温度が１５℃であるＮｏ．２は、プレス成形後の冷却において、１５℃の水を冷媒として金型から噴射したもの（金型抜熱及び水噴射）であり、冷却停止温度が５℃であるＮｏ．７は、水温を５℃として水を噴射し、冷却したものである。Ｎｏ．１０、１１及び１４は、プレス成形前に空冷した例であるが、プレス成形開始温度の低下とともに冷却速度も小さくなっている。 Extraction after heating the steel plate at 900 ° C for 3 minutes in a heating furnace, setting the steel plate on the punch and blank presser at the top dead center position, and combining various cooling methods as shown in Table 1, cooling before and after press forming Molding was performed at different speeds and press molding temperatures. The cooling stop temperature is 15 ° C. No. 2 is the one in which water at 15 ° C. is injected from the mold as a refrigerant in the cooling after press molding (mold removal heat and water injection), and the cooling stop temperature is 5 ° C. No. 7 is water cooled by jetting water at a water temperature of 5 ° C. No. Nos. 10, 11 and 14 are examples of air cooling prior to press molding, but the cooling rate is reduced as the press molding start temperature decreases.
この冷却速度と成形温度は、予め、表１のそれぞれの冷却方法による冷却曲線を作成して算出した。この冷却曲線を基に、加熱炉から鋼板を抽出後、プレス成形までの冷却時間を調整し、プレス成形開始温度を制御した。なお、冷却曲線は、鋼板に熱電対を取り付け、表１のそれぞれの条件で冷却し、プレス成形を行わずに、温度の時間変化として測定した。 The cooling rate and the molding temperature were calculated in advance by preparing cooling curves according to the respective cooling methods shown in Table 1. Based on this cooling curve, after the steel plate was extracted from the heating furnace, the cooling time until press forming was adjusted, and the press forming start temperature was controlled. The cooling curve was measured as a change with time in temperature without attaching a thermocouple to the steel sheet, cooling under the conditions shown in Table 1, and performing press forming.
このようにして得られた成形品の外観を目視してプレス破断の有無を判断し、また成形品側壁部の鋼板を切断し、樹脂埋め込み、研磨して、断面硬さを荷重１０ｋｇｆ（９８．０７Ｎ）にてビッカース硬さ計でＪＩＳ Ｚ ２２４４に準拠し、測定した。結果を表１に併記した。 By visually observing the appearance of the molded product thus obtained, the presence or absence of press breakage is judged, and the steel plate on the side wall of the molded product is cut, resin-embedded, polished, and the cross-sectional hardness is set to 10 kgf (98. 07N) with a Vickers hardness meter according to JIS Z 2244. The results are also shown in Table 1.
Ｎｏ．１〜１０は、プレス成形前後の冷却方法を金型抜熱、水噴射、ミスト（水と空気の混合物）による冷却とし、冷却速度を１０℃／ｓ以上、プレス成形開始温度を６００℃以下、プレス成形終了温度をＭｓ点以上に制御し、本発明の範囲内の条件で熱間プレスを行ったものであり、外観上割れなく、また鋼板（成形品）のビッカース硬さＨｖが４００以上という高強度が達成されている。 No. 1-10 is the cooling method before and after the press molding is mold heat removal, water injection, cooling by mist (a mixture of water and air), the cooling rate is 10 ° C / s or more, the press molding start temperature is 600 ° C or less, The press forming end temperature is controlled to the Ms point or higher, and hot pressing is performed under the conditions within the scope of the present invention. The appearance is not cracked, and the Vickers hardness Hv of the steel sheet (formed product) is 400 or higher. High strength has been achieved.
これに対しＮｏ．１１〜１７は、本発明の範囲外で熱間プレスを行った比較例である。即ち、Ｎｏ．１１〜１３は、プレス成形開始温度が６００℃超であるために成形品に割れが生じていた。従来の熱間プレスにおける通常の条件であるＮｏ．１１では、割れが生じることから、本発明によれば、従来の熱間プレスでは割れが発生するような厳しい条件でも成形が可能であることが証明された。Ｎｏ．１２、１３は、プレス成形前に金型抜熱やミスト噴射で冷却速度を１０℃／ｓ以上に高めたにも拘らず、プレス成形開始温度が高いためにやはり割れが生じている。 In contrast, no. 11 to 17 are comparative examples in which hot pressing was performed outside the scope of the present invention. That is, no. In Nos. 11 to 13, since the press molding start temperature was higher than 600 ° C., the molded product was cracked. No. which is a normal condition in the conventional hot press. No. 11 shows that cracking occurs, and according to the present invention, it has been proved that molding can be performed under severe conditions in which cracking occurs in the conventional hot press. No. Nos. 12 and 13 are still cracked because the press molding start temperature is high even though the cooling rate is increased to 10 ° C./s or higher by heat removal from the mold or mist injection before press molding.
一方、Ｎｏ．１４、１５は、プレス成形開始温度は本発明要件を満たしているものの、プレス成形前又はプレス成形後の冷却速度が１０℃／ｓ未満のため十分な強度が得られていない（ビッカース硬さＨｖが４００よりも低くなっている）。即ち、Ｎｏ．１４は、プレス成形前に放冷（空冷）によってプレス成形開始温度を下げているために、プレス成形前の冷却速度が１０℃／ｓよりも低く、硬度が著しく低下し、Ｎｏ．１５は、プレス成形後に放冷（空冷）したために、プレス成形後の冷却速度が１０℃／ｓよりも低く、やはり硬度が低下している。 On the other hand, no. Nos. 14 and 15 show that the press forming start temperature satisfies the requirements of the present invention, but sufficient strength is not obtained because the cooling rate before press forming or after press forming is less than 10 ° C./s (Vickers hardness Hv). Is lower than 400). That is, no. No. 14, since the press molding start temperature was lowered by cooling (air cooling) before press molding, the cooling rate before press molding was lower than 10 ° C./s, and the hardness was remarkably reduced. Since No. 15 was allowed to cool after press molding (air cooling), the cooling rate after press molding was lower than 10 ° C./s, and the hardness was also lowered.
Ｎｏ．１６及びＮｏ．１７は、プレス成形前後の冷却速度が１０℃／ｓ以上であるが、プレス成形終了温度がＭｓ点（本実施例の場合では、４００℃）よりも低いため、マルテンサイト組織が生成して変形能が低下し、割れが発生した。
No. 16 and no. No. 17 has a cooling rate of 10 ° C./s or higher before and after the press molding, but the press molding end temperature is lower than the Ms point (400 ° C. in the present example), so that a martensite structure is generated and deformed. Performance declined and cracks occurred.
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