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Influence of Hot Press Forming Techniques on Properties of Vehicle High Strength Steels ( Scho ol of Automotive Engineering , State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology , Dalian 116024, Liaoning, China) Abstract: Based on the combination of materials science and mechanicalengineering ,hotpress forming process of the vehicle high strength steels was analyzed. The hot forming processinclud -ed: heating alloy srapidly to austenite micr ostructures, stamping and cooling timely,maintaining pressur eand quenching . The results showed that most of austenite micr ostructure w as changed into uniform mar tensite by the hot press form ing while the samples were heatedat 900 。C and quenched. The optimal tensile strength and yield streng th were up to 1530 MPa and 1000 MPa, respectively, and the shape deformation reached about 23% . And springback defect did not happ -en in the samples. Key words: high streng th steel; lightw eight ; hot forming ; martensite As an effective economical energy measure, the lightw eight dev elo pment dir ection of automo -bile has become one of the most important research subjects in the automotive industry. There are three major ways to achieve automobile light weight : optimizing vehicle frames and struc- tures; making vehicle bodyor f rame of new and alternativ ematerials to reduce the vehicle mass ( The high and ultra high strength steel can be used as alternative materials because of its thinner thickness) ; adopting advanced manufacturing techniques for the sake of automobile light wei- ght , such as thickness-gradient high strength steel (HSS) or metal based compound plates by con -tinuous pressing or hot press forming [ 1] . Although HSS has been applied in some domestic top grade vehicles, the key producing technologies have always been dominated by foreign compan- ies, such as Acelor Company, so as to raise the product cost obviously. By domestic self-designed hot press forming techniques and water-cooling mould, the automo bile HSS can be produced to subst itute foreign vehicle parts. In general, with the enhancement of steel blank，s mechanical strength, its formability is worsened dramatically. It is difficult to apply the traditional cold stamping technolog y into the f ield of pressing HSS. Thus, the hot stamping technology of martensit icsteel blank is applied as a new technology , which combines metal thermoplast ic forming metho d and water-cooling mould quenching principle. In this paper, boro n steel blank was formed and water-cooling mould was quenched simultane ously during the process of hot stamping . Compared with original automobile pearlite steel[ 2] , the automobile HSS obtained by advanced hot press forming technique can reduce about 30% of the total vehicle mass and achieve complex g eomet ries, high security and mechanical st reng th. The r easo n is that austenite microst ructure with optimal plast icity and ductility can be obtained by hot press forming at high temperature[ 3- 5] , and the HSS with both excellent mechanical properties and light weight will be obtained after being formed and quenched[ 6- 8] . The application of hot-formed thinner HSS plates will becoman important measure to realize vehicle light weight. 1 Experimental Setup In order to form HSS at high temperature, and to avoid cracks and springback, the sam -ples need rapid heating and transform completely into stabl eaustenite microst ructure. And then, samples are pressed and cooled in self-made water-cooling mould.For the obtained HS -S sample, its shape-freezing character or no spring back defect is an obvious advantage, and most of microst ructure in the sample is martensite. The thickness of sample is 1.6 mm, and the main elements of HSS in this experiment are show n in Table 1. Table 1 Main elements of material in the experimen 22MnB5 C Mn Cr Si B P S Al Minimum 0.220 1.200 0.110 0.002 0.002 - - 0.020 Maximum 0.250 1.400 0.200 0.005 0.005 0.020 0.005 0.050 Actual ex perimental procedure included: 1) set different heat t reatment temper atures in ther ange of750 to 1 000℃; 2) put the sample into the heat treated furnace to be heated for 4 min at a certain temperature; 3) remove it by mechanical hand and put it into the hot forming moulds to be pressed quickly ;4) simultaneously, it was water-cooled at about 30℃/s in the mound. The mechanical properties of sample were analyzed by tensile test system and the microstructure appear ance was analyzed by metal lographic analysis device. The shape and size of test sample are show n in Fig. 1. Fig 1 The shape and size of specimen 2 Results and Discussion Mechanical propert ies of HSS ( boron steels)with different thicknesses ( 1.0mm, 1.6mm, 2.0mm,2.5 mm, 3.0 mm and 4.0 mm, respectively) were checked (GBT 16865-1997 was consulted, and samples were selected along 0℃, 45℃ and 90℃ rolling direction respec -tively ) . The unidirectional tensile tests (based on the metal tensile test ing standard of GBT228-2002 ) were finished. Compared with USIBOR1500, the values of basic mechanical properties for HSS w ith dif ferent thicknesses in the experiment are shown in Fig 2. Fig 2 shows that after water-cooling quenching , the tensile strength and yield strength of samples ( except the one w ith thickness of 4.0 mm )reached 1 500 MPa and 1 000 MPa, respect ively. The values of the strength were twice bet ter than those of samples before quenching , and nearly the same to those of the plates of thickness 1.75 mm from Acelor Company ( USIBOR1500 shown in Fig 1) . Fig2 Tensile and yield strength of high strength steels with different thicknesses before and after quench –ing Generally , hot press forming of samples is operated above transition temperature of martensite micro structure. The heating temperature in this experiment was in the range of 750 to 1000 ℃ because it took 3 s or so for the samples to be delivered in the air. And then, based on analyzing tensile strengths Rm of samples after hot-forming at different temperatur -es and quenching , the optimal temperature can be found. It is shown in Fig3. Fig3 Curve of tensile strength vs preheating temperature From Fig 3, it is obvious that the value of tensile strength Rm only reaches 900 MPa at 750℃ ; the optimal value is 1530 MPa at 900℃ , and the value will fall as temperature is set above 900℃ . Based on analy zing microstructure and Fe-Fe3 C phase diagram, samples lay in the transition region of ferrite austenite microstr ucture coexistence at 750℃ . At this moment , austenite has appeared in microstructure of samples, and it can be transformed into martensite microstructure through water-cooling. So the mechanical properties, such as tensile strength and yield strength, will be improved. That is to say ,tensile strength of samples is a little hig her than that of original ones ( Rm is 600 MPa or so) . The content of austenite becomes larger as temperature is raised,and the tensile str ength will be improved gradually .As far as 22MnB5 steel is concerned, the austenitizing temperature is about 880℃ . As Fig3 shows, if samples are heated rapidly to 900℃ and air cooled for 3, austenite microstr uctures are obtained completely . Then samples are hot formed and water-cooling quenched, the fraction of martensite microstructure in samples is more than 95% , so the curve shows a peak. How ever, as temperature exceeds 900℃ , because superheat degree is too large, microg rains grow so large that the tensile strength will decrease. Thus high tem- perature austenite microstructure (obtained as samples w ere heated rapidly) and grain refinement are the main factors to determine the mechanical properties of high strength steel -s. In this paper, different from that in the lab,the interact ion mechanisms of molding and w ater-cooling system on samples produced in the production line can objectively show the manufacturing properties and microst ructure character of products in mass. A s far as the samples are concerned, A is the initial and untreated sample; B is the sample which was heated at 900℃ for 4 min; C is the sample after heat treatment and water-coo ling quenching. The deformation of A, B and C are 32% , 24% and 6% or so, respectively . Generally , A is composed of main pearlite and a small amount of ferrite, the toughness of which is better than martensite, so its deformation is relatively better. B is com -posed with the high-temperature transitional microstructure of austenite, whose toughness is also better than martensite, and deformation is larger than the latter. C is composed of over 95% martensite and little austensite. Owing to its higher strength, toughness and plasticity of martensite are lower, that is to say , deformation of C is the lowest In Fig 4, when the sample was heated for 4 min and stretched at 900℃ , stress-strain curve and testforce displacement curve were obtained respect ively. From Fig4 ( a) , after being heated up to 900℃,the microst ructure of sample has been completely turned into austenite. T he value in the elastic deformation stage of curve w ill tend towards the yield point , following the axial test force gradually being increased. That is to say, the obvious plastic deformation of sample will beg in after the yield point .When it is continuously stretched till the peak point of curve, the necking of sample will occur. Passing the peak, the st ress-strain relat ionship will become more complex . From Fig 4 ( b) , after the corresponding peak, the test force will be reduced, along with the decreasing cross-sectional area of sample till the f racture. It can be seen that the appropriate toughness and plastic deformation proper ties of austenitizing sample at 900℃ will help HSS be hot- formed to complicate vehicle parts. It is an effective measure to form HSS with room-temperature martensite microstructure character, and it is a theoretical basis to design the hot-forming process for HSS in the article. The vehicle hot forming parts and the original cold forming parts are practically contrasted. There areobvious differences both in the springback defect and in the formability, as shown in Fig5. From Fig5, it shows that the hot-forming parts havehig her accuracy, almost no shape distortion, and no springback defect . But the cold-forming parts will exhibit deformation defects, crimping,large spring back and twisted grooves obviously,which can destroy the yield of products seriouslyw hich can destroy the yield of products seriously .Therefore, instead of tradit ional cold forming , the vehicle-high strength steels which are produced by hot forming have become an inevitable trend. In addition, the compositions of samples are shown inTable 1, based on not only the contribution for formability and microst ructure, but also the cost .For example, component boron as a component of sample can reduce the energy-gradient on the grain boundary because it is easily adsorbed on grain boundary to fill the defect of lower energy. While austenitizing temperature is decreased by water-cooling system, ?-phase ferrite is easily to be nucleated on the grain boundaries. But the nucleation and growth of ferrite and bainite will become slower because of the low erenergy gradient on the grain boundaries, and are beneficial to make austenite stable; if the co ntent of boronor processing parameters are unsuitable, component boron would be precipitated to super saturation on the grain boundaries and become the new nucleus of precipitating phase which makes ener gy gradient larger, causing the harden ability of samples to fall. ( a) Stressst rain curve; ( b) Test force displacement curve Fig 4 Curves of stress-strain and test force displacement for stretching test In the production line, the precipitation and growth of mixed phase will be prohibited effectively by controlling temperature and heating rate. The sample is heated to 900℃ and held for 4 min. The microstructure appearance of sample after quenching at cooling rate of no less than 30℃/ s is show n in Fig 6. Fig5 Picture of hot forming and cold forming vehicle parts In Fig6 ( a) , the main micro structur e of initial sample, w hich has not been hot formed and water-cooling quenched, is composed offerrite, pearlite and a small amount of carbide. Its tensile strength Rm and yield strength are only 653MPa and 500MPa, respectively . Fig6 ( b) shows that most microstructure of sample after quenching is strip-shapemartensite, the content of which is over 95% , and there are no cracks and other stress defects. The reason is that the sample was evenly heated and water-cooled during the whole process; based on “C” curve, even and close-row lath martensite microsructure obtained is also due to the optimal water-cooling rate, so the content of residual phase is very little; in addition, the complete close-row microstructure shows that residual stress ( including thermal stress and phase transformation stress, etc. )has been released completely, and there is no microgap in the micrograins so as to benef it sample for higher security and better mechanical propert ies. T he domestic research of vehicle HSS is mostly limited to do in the lab, but advanced automated manufacturing technologies are difficult to be realized in the lab. In this paper ,the properties’ targets of HSS produced by practical production line are satisfactory, and the technical process also meets the demands of mass production (a) Original HSS microstructure before hot forming and quenching; (b) Obtained HSS microstructure after hot forming and quenching. Fig6 Microstructure appearance of HSS sample bef ore and after hot forming and quenching 3 Conclusions 1) In the production line, as HSS is heated rapidly to 900℃ and held for 4 min, the tensile strength can reach the optimal value of 1530 MPa.If temperature is too low , austenite transformation will be incomplete; on the contrary , if temperature is too high, micrograin will grow too large. Both of them will reduce the tensile strength. 2) T hanks to the appropriate toughness and plastic deformation properties of austenitizing HSS at high temperature, 22MnB5 steels ( HSS) can be favorably hot formed into complex and accurate automotive parts. 3) T he optimal water-cooling rate during quenching can make HSS achieve the ideal microstructure of more than 95% martensite and a very small amount of residual austenite, and help stress-relieving procedure accomplish effectively. It is also the guarantee for HSS parts to possess high strength and no defects, such as cracks and crimping. References: [ 1] Schieβl G, Pos schn T , Heller T , etal. Manufacturing a Roof Frame From Ultra High Strength Steel Materials by Hot Stamping [ C] IDDRG In ternational Deep Drawing Research Group 2004 Conference. Sindelfingen: [ s. n. ] , 2004: 158. [ 2] TANG Zhiyong, J IANG Haitao, TANG Di, etal. Study on the Continuous Cooling Transformati on of Austenite of 27MnC rB5 Steels [ J ] . Hot Working Technology, 2007, 36( 20) : 41. [ 3] FAN Junf eng, CHEN Ming. A Study on the Road of Vehicle Lightw eight in Chin a [ J] .Casting2006, 55( 10) : 995 ( in Chinese) . [ 4] CHEN He-qin g, PENG C hengyun, WEI Liangqing. High Strength Steels and Applicati on of Them to Vehicle Manufacturing [ J ] . Mould and Die Project, 2007 ( 8) : 88 ( in Chinese) . [ 5] LIN Jianping, WANG Liying, TIAN Haob in, etal. Research and Devel opment of the Hot Press Form -ing of Ultra High Strength Steel [ J] . Metal Casting Forgin g Welding Technology, 2008, 37( 21) : 140 ( in Chinese) . [ 6] XING Zhongwen, BAO Jun, YANG Yuying, etal. Hot Press Forming Experiment al Research on the Quenchenable Boron St eel [ J] . Materials Science and Technology, 2008, 16( 2) : 172. [ 7] Marion Merklein , Jrg en Lecher, Vera Gdel, et al. Mech anical Properties and Plastic Anisotropy of the Quenchenable High Strength Steel 22MnB5 at Elevated Temperatures [ J ] . Key Engineering Materials, 2007, 344: 79. [ 8] Geigera M, Merkleinb M, H off C. Basic Investigations on the Hot Stamping Steel 22MnB5 [ J] . Advanced Materials Research, 2005, 6( 8) : 795. 熱壓成形技術(shù)對(duì)汽車高強(qiáng)度鋼性能影響 常英，孟召喚，梁穎，李曉東，馬寧，胡平 （學(xué)院汽車工程國(guó)家重點(diǎn)實(shí)驗(yàn)室，工業(yè)裝備結(jié)構(gòu)分析，大連理工大學(xué)，遼寧，大連，116024） 摘要：基于材料科學(xué)和機(jī)械工程的結(jié)合上，車高強(qiáng)度鋼熱沖壓成型過程進(jìn)行了分析。熱成型工藝包括：快速加熱合金，奧氏體微觀結(jié)構(gòu)，沖壓和及時(shí)冷卻，保持壓力和淬火。結(jié)果表明，對(duì)樣品進(jìn)行淬火的熱壓成形，加熱至900℃時(shí)，大部分奧氏體微觀結(jié)構(gòu)改變成均勻的馬氏體。最佳的拉伸強(qiáng)度和屈服強(qiáng)度分別為1530 MPa和1000MPa的，均達(dá)到23％左右的形狀變形。樣品沒有發(fā)生過回彈缺陷。 關(guān)鍵詞：高強(qiáng)度鋼；重量輕；熱成型；馬氏體 0 引言 作為一種有效的經(jīng)濟(jì)的能源措施，輕巧的汽車發(fā)展方向，已成為汽車行業(yè)最重要的研究課題之一。實(shí)現(xiàn)汽車輕量化的主要途徑有三個(gè)：優(yōu)化汽車框架和結(jié)構(gòu)，使車輛的車身或者車架的，新的和替代材料，降低整車質(zhì)量（高和超高強(qiáng)度鋼，可作為替代材料，因?yàn)樗暮穸雀。囕p量化，如厚度梯度高強(qiáng)度鋼（HSS）或金屬系化合物板通過連續(xù)沖壓或熱壓成形[1]為了采用先進(jìn)的制造技術(shù)。HSS已經(jīng)應(yīng)用在國(guó)內(nèi)一些高檔車，關(guān)鍵生產(chǎn)技術(shù)一直占主導(dǎo)地位的外國(guó)公司，如Acelor公司，從而顯著提高了產(chǎn)品成本。由國(guó)內(nèi)自行設(shè)計(jì)的熱壓成型技術(shù)和水冷卻模具，汽車HSS可以生產(chǎn)替代國(guó)外汽車零部件。 在一般情況下，隨著鋼質(zhì)坯件的機(jī)械強(qiáng)度的增強(qiáng)，其可塑性急劇惡化。這是很難適用于傳統(tǒng)的冷沖壓技術(shù)進(jìn)入該領(lǐng)域取代HSS。同時(shí)，填補(bǔ)了馬氏體鋼應(yīng)用空白，熱沖壓技術(shù)作為一項(xiàng)新技術(shù)，它結(jié)合了金屬熱塑性成型法和水冷卻模具淬火原則。在本文中，形成硼鋼空白和水冷卻用模具驟冷的過程期間同時(shí)燙印。相對(duì)于原汽車珠光體鋼[2]，汽車HSS通過以下方式獲得先進(jìn)的熱壓成形技術(shù)可以減少車輛的總質(zhì)量的30％左右，實(shí)現(xiàn)復(fù)雜的幾何形狀，高安全性和機(jī)械強(qiáng)度。其原因是最佳的塑性和延展性的奧氏體顯微組織可以通過高溫下[3 - 5熱壓成形方式獲得，同時(shí)形成后和驟冷的[6 - 8]條件將得到具有優(yōu)異機(jī)械性能、重量輕的HSS將。為實(shí)現(xiàn)車輛的重量輕，熱成型更薄的HSS板的應(yīng)用將成為一個(gè)重要的措施。 1實(shí)驗(yàn)裝置 另外，為了在高溫下形成高速鋼，以避免裂紋和回彈，樣品需要快速加熱和完全變換成穩(wěn)定的奧氏體組織。然后，樣品被壓在自制的水冷卻模具中冷卻，對(duì)于得到的HSS樣本，其形狀凍結(jié)字符或沒有回彈缺陷是一個(gè)明顯的優(yōu)點(diǎn)，并且大部分樣品中的顯微組織為馬氏體。樣品的厚度是1.6毫米，在HSS這個(gè)實(shí)驗(yàn)中的主要元素，示于表1。 表1的實(shí)驗(yàn)技術(shù)中的材料的主要要素 22MnB5 C Mn Cr Si B P S Al 最低限度 0.220 1.200 0.110 0.002 0.002 - - 0.020 最高限度 0.250 1.400 0.200 0.005 0.005 0.020 0.005 0.050 實(shí)際實(shí)驗(yàn)步驟包括：1）設(shè)置不同的熱處理溫度的范圍為750至1 000℃；2）把熱處理過的樣品放入爐中，在一定的溫度下加熱4分鐘；3）刪除它由機(jī)械手并把它變成熱成形模具，快速按下；4）同時(shí)，在約30℃/ s的冷卻水在土堆，通過拉伸試驗(yàn)系統(tǒng)進(jìn)行分析的樣品的機(jī)械性能和由金屬金相圖片分析裝置分析的顯微組織的外觀。試驗(yàn)樣品的形狀和尺寸示于圖1。 2結(jié)果與討論 硼鋼（HSS）的機(jī)械性能不同厚度（1.0毫米，1.6毫米，2.0毫米，2.5毫米，3.0毫米和4.0毫米，分別）進(jìn)行了檢查（GBT16865-1997征求意見，樣本選取沿0℃，45℃和90℃軋制方向分別）。單向拉伸試驗(yàn)（金屬拉伸試驗(yàn)的標(biāo)準(zhǔn)GBT228-2002）的基礎(chǔ)上被完成。相比與USIBOR1500，HSS具有不同厚度的實(shí)驗(yàn)中基本力學(xué)性質(zhì)的值如圖2所示。 單位：mm 圖1形狀和尺寸試樣 圖2示出了樣品（除了用厚度為4.0毫米的一個(gè)）的拉伸強(qiáng)度和屈服強(qiáng)度，水冷淬火后，分別達(dá)到1500 MPa和1 000兆帕。淬火前的強(qiáng)度的值的兩倍優(yōu)于那些樣本，和幾乎相同的那些板的厚度1.75毫米從Acelor公司（USIBOR1500在圖1所示）。 鋼板厚度/mm 淬火后的拉伸強(qiáng)度 淬火后的屈服強(qiáng)度 淬火前的拉伸強(qiáng)度 屈服強(qiáng)度淬火前 工程應(yīng)力/MPa 圖2不同厚度的高強(qiáng)度鋼淬火后的抗拉強(qiáng)度和屈服強(qiáng)度 通常，熱壓成形的樣品被操作化轉(zhuǎn)變溫度以上的馬氏體組織。本實(shí)驗(yàn)中的加熱溫度的范圍是在750?1000℃，因?yàn)樗诳諝庵械臉悠芬桓读? s左右。然后，根據(jù)分析的樣品室的拉伸強(qiáng)度，熱成形后在不同的溫度和淬火，最適溫度可以發(fā)現(xiàn)，如圖3。 從圖3，這是明顯的價(jià)值達(dá)到900兆帕，抗拉強(qiáng)度Rm在750℃的最優(yōu)值在900℃，為1530兆帕，當(dāng)溫度高于900℃，該值將下降。在結(jié)構(gòu)的Fe-Fe3C相圖分析的基礎(chǔ)上，在750℃時(shí)，樣品處于鐵素體的奧氏體組織共存的過渡區(qū)。此時(shí)，奧氏體顯微組織的樣品中出現(xiàn)，并通過水冷卻，它可以轉(zhuǎn)化為馬氏體組織。因此，機(jī)械性能，如拉伸強(qiáng)度和屈服強(qiáng)度，將得到改善。也就是說，樣品的拉伸強(qiáng)度是一個(gè)小較高她比原有的（Rm是600兆帕斯卡或左右）。奧氏體的含量變大，隨著溫度的升高，和拉伸強(qiáng)度將逐漸提高。至于22MnB5鋼而言，奧氏體化溫度為約880℃。正如圖3所示，如果樣品迅速被加熱到900℃，空氣冷卻3，奧氏體的微觀結(jié)構(gòu)得到完全。然后，樣品是熱的形成和水冷卻的淬火，馬氏體組織樣品中的餾分是95％以上，所以該曲線示出了峰值。然而，當(dāng)溫度超過900℃，因?yàn)檫^熱度太大，微米晶粒長(zhǎng)得這么大的拉伸強(qiáng)度將降低。因此，高溫奧氏體組織樣品被加熱迅速獲得晶粒細(xì)化，以確定高強(qiáng)度鋼的力學(xué)性能的主要因素。不同于在實(shí)驗(yàn)室中，在本文中，成型和水冷卻系統(tǒng)的生產(chǎn)線中產(chǎn)生的樣品的相互作用機(jī)制可以客觀地顯示字符的質(zhì)量的產(chǎn)品的制造性能和微觀結(jié)構(gòu)。 溫度/℃ 圖3拉伸強(qiáng)度與預(yù)熱溫度曲線 至于樣品而言，A是初始的和未經(jīng)處理的樣品; B是在900℃加熱4分鐘的樣品，C是熱處理后的試樣和水冷卻的淬火。的A，B和C的變形，分別為32％，24％和6％左右。一般而言，A是由主珠光體和少量的鐵素體，這是優(yōu)于馬氏體的韌性，因此，其變形是相對(duì)較好的。B由與高溫的過渡奧氏體微觀結(jié)構(gòu)，其韌性也優(yōu)于馬氏體，和變形是大于后者。 C是組成超過95％的馬氏體和小奧氏體。由于其較高的強(qiáng)度，韌性和可塑性的馬氏體是較低的，這就是說，變形C是最低的，在圖4中，當(dāng)把樣品加熱4分鐘，拉伸在900℃，應(yīng)力 - 應(yīng)變曲線和testforce位移分別獲得曲線。 位移/mm （a）應(yīng)力 - 應(yīng)變曲線 （b）試驗(yàn)力 - 位移曲線 圖4應(yīng)力 - 應(yīng)變曲線和拉伸試驗(yàn)的試驗(yàn)力位移 從圖4（a）后，加熱至900℃時(shí)，樣品的微觀結(jié)構(gòu)已經(jīng)被完全變成奧氏體。曲線的彈性變形階段中的值將趨于屈服點(diǎn)，之后逐漸增大的軸向試驗(yàn)力。這就是說，將開始明顯的塑性變形的樣品后的屈服點(diǎn)。當(dāng)它被連續(xù)地拉伸，直到曲線的峰值點(diǎn)，縮頸的樣品會(huì)發(fā)生。通過高峰，應(yīng)力 - 應(yīng)變關(guān)系將變得更加復(fù)雜。從圖4（b）中，相應(yīng)的峰值后，試驗(yàn)力將降低，隨著樣品直到斷裂的減少的橫截面積。適當(dāng)?shù)捻g性及塑性變形奧氏體化的樣品，在900℃的適當(dāng)?shù)年P(guān)系可以看出，將有助于HSS是熱形成為復(fù)雜的汽車零件。這是一個(gè)有效的措施，構(gòu)成高速鋼與室溫馬氏體字符的，這本文對(duì)于HSS熱成型設(shè)計(jì)過程的一個(gè)理論基礎(chǔ)。 汽車熱成型零件和原來(lái)的冷成型件的實(shí)際對(duì)比。無(wú)論是在回彈缺陷和在成形性有明顯的差別，如在圖5-1所示。 從圖5-1，它表明，熱成型件具有更高的精度，形狀幾乎沒有失真，無(wú)回彈缺陷。但冷成型件出現(xiàn)變形缺陷，壓接，大的回彈和扭曲溝明顯，可以摧毀收益率的產(chǎn)品嚴(yán)重的產(chǎn)品嚴(yán)重破壞的產(chǎn)量，因此，同傳統(tǒng)的冷成型不同，車高強(qiáng)度鋼所生產(chǎn)的熱成型已成為一種必然的趨勢(shì)。此外，不僅成形性和微觀結(jié)構(gòu)的貢獻(xiàn)的基礎(chǔ)上，而且在成本上。樣品的組合物如表1所示。例如，組分硼作為樣本的一個(gè)組成部分，可以減少能量的晶界上的梯度，因?yàn)樗苋菀孜皆诰Ы缰?，以填補(bǔ)較低能量的缺陷。雖然水冷系統(tǒng)，一個(gè)相鐵素體的奧氏體化溫度下降很容易在晶界上成核。但是，鐵素體和貝氏體的成核和生長(zhǎng)將變得更慢，因?yàn)樵诰Ы缟系妮^低的能量梯度的，并且是有益的，使奧氏體穩(wěn)定，如果硼或處理參數(shù)的內(nèi)容是不適合的，將沉淀成分硼超飽和在晶界上，成為新的沉淀相，這使得能量梯度放大的核，導(dǎo)致硬化樣品的能力下降。 在生產(chǎn)線中，混合相的析出和生長(zhǎng)將有效地被禁止，通過控制溫度和加熱速率。樣品被加熱至900℃，保持4分鐘。淬火后的樣品的外觀，在不低于30℃/ s的冷卻速率的微觀結(jié)構(gòu)是在圖6所示。 原來(lái)的冷成型零件 熱成型部件 圖5-1熱成型和冷成型汽車零部件圖片 初始樣品的主要微結(jié)構(gòu)，還沒有得到熱成形和冷卻水驟冷，在圖6（a）中，組成的鐵素體，珠光體和少量的碳化物。其抗拉強(qiáng)度Rm和屈服強(qiáng)度分別只有653兆帕和500兆帕。如圖6（b）表示，大部分樣品的顯微組織的淬火后的馬氏