1、铜陵学院期末论文1Influence 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 combinati
2、on 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 t
3、hat 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% . A
4、nd springback defect did not happ -en in the samples.Key words: high streng th steel; lightw eight ; hot forming ; martensiteAs 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 i
5、ndustry. 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 t
6、hinner 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 ve
7、hicles, 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 vehi
8、cle 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
9、 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 . Comparedwith original automobile pearlite steel 2
10、, 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
11、 press forming at high 程艳. 热压成形技术对汽车高强度钢性能影响（翻译）2temperature 3- 5 , and the HSS with both excellent mechanical properties and light weight will be obtainedafter being formed and quenched 6- 8 . The application of hot-formed thinner HSS plates will becoman important measure to realize vehicle light w
12、eight.1 Experimental SetupIn 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 sa
13、mple, 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 experimen22M
14、nB5 C Mn Cr Si B P S AlMinimum 0.220 1.200 0.110 0.002 0.002 - - 0.020Maximum 0.250 1.400 0.200 0.005 0.005 0.020 0.005 0.050Actual 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 heat
15、ed 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 appe
16、ar 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 specimen2 Results and DiscussionMechanical 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, respecti
17、vely) 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
18、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 stren
19、gth were twice bet ter than those of 铜陵学院期末论文3samples 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 ingGen
20、erally , 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 strength
21、s 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 temperatureFrom Fig 3, it is obvious that the value of tensile strength Rm only reaches 900 MPa at 750 ; the optimal va
22、lue 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 程艳. 热压成形技术对汽车高强度钢性能影响（翻译）4750 . At this moment , austenite has ap
23、peared 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
24、 ( 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,
25、 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
26、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
27、,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 sampl
28、e 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, thetoughness of which is better than martensite,
29、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 stre
30、ngth, 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
31、,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 th
32、e 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 cro
33、ss-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 microstru
34、cture character, and it is a theoretical basis to design the hot-forming process for HSS in the article.铜陵学院期末论文5The 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 i
35、n 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
36、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
37、 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. Whileaustenitizing temperature is decreased by water-coo
38、ling 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 para
39、meters 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 curveFi
40、g 4 Curves of stress-strain and test force displacement for stretching testIn 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
41、sample after quenching at cooling rate of no less than 30/ s is show n in Fig 6. 程艳. 热压成形技术对汽车高强度钢性能影响（翻译）6Fig5 Picture of hot forming and cold forming vehicle partsIn Fig6 ( a) , the main micro structur e of initial sample, w hich has not been hot formed and water-cooling quenched, is composed offe
42、rrite, 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 stres
43、s 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
44、 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 ve
45、hicle 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 produ
46、ction(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铜陵学院期末论文73 Conclusions1) In the production line, as HSS is heated rapidly to
47、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
48、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 tha
49、n 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 Schiel 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
