憑借持續(xù)創(chuàng)新和協(xié)作開發(fā)模式，鋼鐵作為交通行業(yè)的“主力”材料，地位仍不易撼動。
by Lindsay Brooke
2014 年，福特（Ford）隆重推出 F-150 鋁質(zhì)皮卡，整個汽車行業(yè)為之一震。在這之前不久，Ducker Worldwide 也曾發(fā)布了一項針對國際鋁業(yè)的研究，預(yù)測這種輕質(zhì)金屬材料將在下一個新車開發(fā)周期中在北美輕型卡車細(xì)分領(lǐng)域中占據(jù)主導(dǎo)地位，并表示屆時近七成新皮卡均將大量采用鋁材料。多重信號似乎均表明，一場鋁業(yè)革命正在汽車行業(yè)中醞釀。
然而，事情并未向預(yù)計的方向發(fā)展。五年過去了，全球市場中仍沒有任何一輛量產(chǎn)皮卡真正在駕駛室和底盤等主要位置大量采用鋁材。這種趨勢在全球主流車廠的新車發(fā)布中可見一斑：福特為全新 2018 款全尺寸 SUV 配備了鋁質(zhì)車身結(jié)構(gòu)，但 2019 款福特 Ranger 的主要架構(gòu)仍是鋼材；通用汽車 (GeneralMotors) 2019 款雪佛蘭（Chevrolet）、GMCSilverado、Sierra 1500 及一些高配車型均毫無例外地延續(xù)了公司專為皮卡和 SUV 車型打造的“混合材料戰(zhàn)略”；FCA 集團(tuán)旗下的 Ram 和 Jeep 品牌卡車也繼續(xù)采用鋼結(jié)構(gòu)。不過，新款 JL 系列 Jeep 牧馬人則改用鋁質(zhì)車門（和鉸鏈）、發(fā)動機(jī)罩、擋泥板和擋風(fēng)玻璃架（盡管仍不涉及駕駛室和底盤等主要位置），具體鋁材型號包括美鋁（Alcoa）新型 C6A1 高合金、6022 號合金和 A951 號合金。
事實上，牧馬人可以成功減重 200 磅（91 公斤）的一個原因，是采用了鎂材料的后擺門，也就是說FCA 為牧馬人重新啟用了 Pacifica 休旅車搭配的鎂材料升降門（成本并不低）。此外，這些卡車還有一個共同點，即全部采用了鋼質(zhì)梯形框架搭配部分鋁質(zhì)橫梁的設(shè)計，這也同時體現(xiàn)了經(jīng)過有限元方法優(yōu)化的高強(qiáng)度鋼材的極致性能。
對此，密歇根大學(xué)材料科學(xué)與工程系教授 Alan Taub 博士觀察到：“我們已經(jīng)從‘密集使用單一材料’的傳統(tǒng)材料觀轉(zhuǎn)變?yōu)?lsquo;將合適的材料，用合適的方法，應(yīng)用至合適的位置’的新思路。”
對此，初創(chuàng)公司 Rivian 的創(chuàng)新 R1T 電動皮卡就是一個很好的例子。據(jù)了解，這款電動皮卡預(yù)計將于2021 年問世，其主駕駛室經(jīng)過專門設(shè)計，采用了多種高強(qiáng)度鋼材搭配鋁質(zhì)封裝的設(shè)計，可顯著降低重量并優(yōu)化防撞性能。
事實上，除了一些經(jīng)濟(jì)型車型，這種混合材料應(yīng)用的思路也得到了眾多豪車制造商的認(rèn)可。幾個例子：特斯拉 Model3 和Model Y 均未延續(xù) Model S 和 Model X 密集使用鋁制材料的設(shè)計，反而應(yīng)用了相當(dāng)大比例的鋼材；奧迪（Audi）也不再一味推廣“鋁材料”戰(zhàn)略，轉(zhuǎn)而向“混合材料”的設(shè)計思路轉(zhuǎn)型（包括公司最新的 eTron 電動車也是如此），而這種轉(zhuǎn)型在汽車行業(yè)中并不少見。
鋼鐵市場發(fā)展研究所（Steel Market Development Institute）汽車市場副總裁Jody Hall 博士表示，“如今，一提到’汽車減重’，大環(huán)境追求的已經(jīng)不再是’密集使用鋁材料’，而是’靈活采用多種材料組合’ ——這就是汽車廠商向我們傳達(dá)的信息。”此外，Hall 和 Taub 博士均認(rèn)為，首先，鋼材較其他輕金屬和碳復(fù)合材料擁有得天獨厚的優(yōu)勢；其次，鋼鐵行業(yè)近年來也在積極研發(fā)更加結(jié)實的汽車專用鋼材。在此背景下，鋼材作為汽車車身結(jié)構(gòu)和底盤系統(tǒng)“主力”材料的地位無法撼動，至少未來十年的情況不會變化。
此外，我們也可以從圖片上看到，近些年來，強(qiáng)度更大、延展率更高的鋼種層出不窮，從大約 200 MPa到2000 MPa 不等，選擇范圍遠(yuǎn)遠(yuǎn)超過其他任何一種單一金屬材料。“要想獲得與新型鋼材類似的強(qiáng)度范圍，你只能選擇增強(qiáng)復(fù)合材料。”Hall 博士斷言，“但增強(qiáng)復(fù)合材料的成本卻是其他基礎(chǔ)材料的很多倍。事實上，從性價比來說，沒有任何滿足汽車行業(yè)要求的其他材料能夠與鋼材媲美。”
“第三代”材料閃電戰(zhàn)
對很多開發(fā)工程師而言，鋼材行業(yè)的吸引力在于這個行業(yè)不斷推陳出新的創(chuàng)新合作模式。事實上，這種合作創(chuàng)新始于 20 世紀(jì) 80 年代，當(dāng)時的鋼鐵制造商不得不聯(lián)合抵御塑料行業(yè)對汽車皮革的沖擊。“我們的合作是與整個鋼鐵行業(yè)展開的，而非某一家鋼鐵公司。”Hall 博士解釋說：“鋼鐵行業(yè)向我們展示他們的設(shè)計，而我們的作用是幫助他們以更具成本效益的方式讓這些設(shè)計成為現(xiàn)實。”
“當(dāng)然，這并不是說鋼鐵公司不會單獨與汽車制造商合作以滿足它們的特殊需求，只是說整個行業(yè)的廣泛參與已經(jīng)得到了汽車客戶的肯定，”Hall 博士表示，“我們都希望利用更少的資源，完成更大的目標(biāo)。因此，從調(diào)配資源和保證時效方面，行業(yè)合作是一種非常有效的方式。幾十年來，我們一直堅持這種工作方式，并從中持續(xù)受益。”
目前，市面上最先進(jìn)的高強(qiáng)度（AHSS）和超高強(qiáng)度（UHSS）鋼種可以提供 2000 MPa 的拉伸性能。這種所謂的“第三代”高強(qiáng)度鋼在 2018 年正式問世，主要供應(yīng)商包括 ArcelorMittal、AK Steel 和 Nucor。事實上，當(dāng)時，有一些原始設(shè)備制造商需要一種在保證同等強(qiáng)度的前提下，延展性更高的鋼材，第三代鋼種應(yīng)運而生。
“廠商這么要求部分是希望減少沖壓硬化鋼的用量。”Hall 博士解釋說，“具體來說，他們希望可以在不升級生產(chǎn)設(shè)施的前提下，在室溫環(huán)境將鋼材加工為一些幾何形狀更為復(fù)雜的零部件，這是目前的高強(qiáng)度鋼種無法滿足的。這些廠商希望降低沖壓生產(chǎn)線的成本。”
任何新材料在進(jìn)入汽車行業(yè)前均需等待一定的驗證和交付時間，因此第三代鋼種在汽車行業(yè)的大規(guī)模應(yīng)用預(yù)計將在 2023 年左右實現(xiàn)。專家指出，在未來，沖壓硬化鋼將允許車身工程師在保證高強(qiáng)度的前提下，打造幾何形狀更為復(fù)雜的零部件，因此并不會從汽車行業(yè)退出歷史舞臺。舉個例子，本田（Honda）已將這種沖壓硬化鋼應(yīng)用至旗下 NSX 超級跑車的 A 柱中，從而最大限度地縮小 A 柱的橫截面，為駕駛員提供不受遮擋的開闊視野。此外，RDX 車型的內(nèi)外門環(huán)中也采用了 1500-MPa 鋼。
在保證同等強(qiáng)度的前提下，鋁合金零部件的成形更加困難。Hall 博士表示，“事實上，零部件的幾何形狀對車輛性能有很大影響，而且也與車輛的整體減重息息相關(guān)，也就是說一種材料可以支持的幾何形狀越多，則降低材料用量的幾率就更大，因此也更容易實現(xiàn)減重。很顯然，這是鋁合金材料的弊端之一。”
許多人可能認(rèn)為，未來的汽車行業(yè)將是電動汽車的天下，且無論自動駕駛汽車還是非自動駕駛汽車均將主要采用鋁材和碳纖維材料，但事實上鋼材仍將在推進(jìn)汽車行業(yè)革新中發(fā)揮至關(guān)重要的作用。與同等尺寸的內(nèi)燃機(jī)車輛相比，電動汽車減重帶來的實際效益相對較小。
“傳統(tǒng)內(nèi)燃機(jī)車型的車身和底盤幾乎占車輛總重的一半，因此針對這些部位減重可以給車輛帶來相當(dāng)可觀的收益，比如大約0.2 mpg 的燃油經(jīng)濟(jì)性提升。”Hall 博士解釋道，“但減重對純電動汽車的幫助要小很多，收益完全無法覆蓋為減重本身支付的成本。”然而，一些一級供應(yīng)商向《汽車工程》分享，未來，鋁材料在電動汽車領(lǐng)域應(yīng)用的主要增長點將集中在電池組部分。
Hall 博士打趣到，“鋼材料在汽車行業(yè)的應(yīng)用已經(jīng)超過100 年了，所以大家可能都以為我們已經(jīng)沒什么可以繼續(xù)創(chuàng)新的地方了。”事實上，這種想法錯的離譜，這點工作在第一線的車身機(jī)構(gòu)工程師是最了解的。
Mobility’s longtime incumbent material maintains its star status for vehicle structures through constant innovation—and a collaborative development model.
by Lindsay Brooke
In 2014, just before Ford shook the industry with the introduction of its aluminum-intensive F-150, Ducker Worldwide released a study for the aluminum industry. The report predicted that the light metal would dominate the North American light-truck segment in the next new-model development cycle. Some seven out of ten pickups in the next round were going to be AL-intensive, the study opined. A tidal wave appeared to be building.
Five years later, not a single pickup has entered production with an AL-intensive cab and bed. While Ford changed over the body structures of its all-new 2018 large SUVs to aluminum, steel rules the midsized 2019 Ranger. In the enemy camps, the 2019 Chevrolet and GMC Silverado and Sierra 1500 and their brawnier HD cousins continue GM’s mixed-materials strategy for pickups and SUVs. FCA’s Ram and Jeep brands have stuck mainly with steel structures; the new JL-series Jeep Wrangler changed to aluminum doors (and hinges), hood, fenders and windshield frame, utilizing Alcoa’s new C6A1 high-form alloy and its 6022 and A951 alloys.
Also contributing to Wrangler’s shedding up to 200 lb (91 kg) is its magnesium rear swing gate—FCA’s reprise to the feathery (and not cheap) Mg lift gate used on the current Pacifica minivan. Underpinning each of these trucks are ladder frames representing the ultimate in finite-element-optimized high-strength steel, with a few AL cross members.
“We’ve migrated from the idea that we need a single- material-intensive vehicle” into an engineering mindset of “the right material, produced the right way and engineered into the right part of the vehicle,” observes Dr. Alan Taub, professor of Material Science & Engineering at the University of Michigan.
Case in PointDrive’s pioneering R1T electric pickup slated for 2021 features a main cab structure tailored for reduced mass and crash safety using various high-strength-steel grades, with aluminum closures.
Even the luxury carmakers are following subtotals abandoned the AL- intensive route used on its Models S and X in planning its high-volume, lower-priced Model 3 and Model Y, both of which have significant steel content. Audi has switched from an AL-intensive strategy on its unibodies to the mixed-materials pathway (even for its new eTron electric vehicle) that is becoming universal across the industry.
“The environment around mass reduction has changed from aluminum-intensive to mixed materials—that’s certainly what automakers are telling us,” notes Dr. Jody Hall, VP of the automotive market for the Steel Market Development Institute. She and Dr. Taub believe steel’s inherent value compared to light metals and carbon composites, along with the steel industry’s aggressive development of new, stronger grades suitable for vehicle use will continue its reign as the “core” material for body structures and chassis systems—for the next decade at least.
A look at the accompanying chart shows the growing number of steel grades based on their strength and elongation. Ranging from about 200 MPa to 2,000 MPa, the bandwidth is significantly wide for that of any single metal. “You’d have to use a reinforced composite to match steel’s strength range,” Dr. Hall asserted, “but the cost to achieve that changes to many times that of the base material. There’s no other material ‘system’ that delivers steel’s range of performance at such a high value.”
‘3rd-Gen’ material blitz
Steel’s appeal to development engineers has been the industry’s continuous innovation-by-collaboration model that began in the 1980s when steelmakers had to defend against a major assault on automotive skins by the plastics industry. “We work with them an industry instead of as individual steel companies,” Dr. Hall explained. “They show us their designs and we work with them to achieve them in a more cost-effective manner.
“That’s not saying individual steel companies don’t work with automakers on their needs, but when we do it as an industry, it’s really appreciated by the customers,” she said. “Because we’re all trying to do more with fewer resources. It’s a very efficient way of getting results, in terms of resources and timing. We’ve been working this way for decades and it continues to pay off.”
The most advanced high-strength (AHSS) and ultra-high-strength (UHSS) steel grades will offer 2,000 MPa tensile performance. The so-called “3rd Generation” AHSS was launched commercially in 2018, from major producers ArcelorMittal, AK Steel and Nucor. The products are a response to OEMs’ requests for more elongation/ductility for a given strength.
“Part of the reason for this request is they didn’t want to use as much press-hardened steel as they do now,” Dr. Hall explained. “They want to use their current infrastructure to stamp, at room temperature, the more complex geometries they couldn’t get with some of those AHSS. they’re looking at reducing cost in their stamping lines.”
Because of the lead times invalidating any new material for automotive use, the roll-out of the 3rd-Generation grades will happen gradually through 2023, when the new grades will enter production vehicle programs in significant volume. Experts note that press-hardened steel will remain available because it enables body engineers to achieve complex geometries with high strength. Honda has engineered it into the A-pillar design of its NSX supercar, in order to minimize the pillars’ cross-section and thus improve driver visibility, and uses 1,500-MPa grade for the inner and outer door rings of the RDX.
Aluminum alloys are lower in formability for the same strength levels; this hurts AL applications “because geometry adds a lot to the performance and can help with mass reduction—the more geometry you can get into a part enables thinner sections in many cases,” Dr. Hall said.
While many may think that future EVs, both autonomous and non-, will be constructed of aluminum and carbon fiber, steel has a major role to play in the propulsion revolution. In an EV, the actual impact of lightweighting actions is reduced compared with combustion-engine vehicles of comparable size.
“When you lightweight a conventional vehicle’s body and chassis components, which make up roughly half the vehicle’s mass, you achieve maybe a .2 mpg improvement; it’s incremental,” Dr. Hall explained. “If it’s a battery-electric vehicle, the improvement would be far less. The [body-structure weight-reduction] gains don’t justify the expense.” EV battery pack structures, however, are a major growth area for aluminum, Tier 1 suppliers tell AE.
“Everybody assumes that we’ve innovated as much as we can, because steel has been around in vehicles for well over 100 years,” Dr Hall quipped. Automotive body structure engineers know how wrong that assumption is.
Author: Lindsay Brooke
Source: Automotive Engineering
2019 款雪佛蘭Silverado 皮卡的駕駛艙是多種鋼材和鋁材混合應(yīng)用的典范。
過去十年中，先進(jìn)高強(qiáng)度鋼種家族出現(xiàn)了很多強(qiáng)度更高的新成員。
2019款本田謳歌的后方雙環(huán)負(fù)載傳遞路徑架構(gòu)主要采用了鋼材，RDX 的門環(huán)則采用了 1500 MPa AHSS 高強(qiáng)度鋼。