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1、Influence of Hot Press Forming Techniques on Properties of Vehicle High Strength Steels 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 srapi
2、dly 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 stren
3、gth 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 ; martensiteAs an effective economical energy measure, the lightw ei
4、ght 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
5、 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 compou
6、nd 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-designe
7、d 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 tec
8、hnolog 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 moul
9、d was quenched simultane ously during the process of hot stamping . Comparedwith 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
10、 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 obtainedafter being formed and quenched 6- 8 . The applicatio
11、n of hot-formed thinner HSS plates will becoman important measure to realize vehicle light weight.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 the
12、n, 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
13、 this experiment are show n in Table 1.Table 1 Main elements of material in the experimen22MnB5CMnCrSiBPSAlMinimum0.2201.2000.1100.0020.002-0.020Maximum0.2501.4000.2000.0050.0050.0200.0050.050Actual ex perimental procedure included: 1) set different heat t reatment temper atures in ther ange of750 t
14、o 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 samp
15、le 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 specimen2 Results and DiscussionMechanical propert ies of HSS ( boron steels)with different th
16、icknesses ( 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
17、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 )rea
18、ched 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 steel
19、s with different thicknesses before and after quench ingGenerally , 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 delivere
20、d 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 temperatureFrom Fig 3, it is obvious that the value of tens
21、ile 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 .
22、 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
23、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 ra
24、pidly 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
25、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 pap
26、er, 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 an
27、d 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, thetoughness of
28、 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 au
29、stensite. 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 (
30、 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 deformat
31、ion 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 redu
32、ced, 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 roo
33、m-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 th
34、e 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 yiel
35、d 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
36、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. Whileaustenitizing temperature
37、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 o
38、f 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 f
39、orce displacement curveFig 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 micr
40、ostructure 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 partsIn Fig6 ( a) , the main micro structur e of initial sample, w hich has not been hot formed and water-cooling quenched, is composed offer
41、rite, 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
42、 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
43、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 veh
44、icle 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 produc
45、tion(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 quenching3 Conclusions 1) In the production line, as HSS is heated rapidly to 900 and h
46、eld 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 appropria
47、te 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% mar
48、tensite 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 Fr
49、ame 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