1、 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
2、 . 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
3、 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 eight dev elo pment dir ection of automo -bile has become one of the most important research subjec
4、ts 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 mate
5、rials 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
6、 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 su
7、bst 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
8、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 . Comparedwith original automob
9、ile 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 c
10、an 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 application of hot-formed thinner HSS plates will becoman important measure to realize vehicle light weight
11、.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 sample,
12、 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 experimen22MnB5CM
13、nCrSiBPSAlMinimum0.2201.2000.1100.002-0.020Maximum0.2501.4000.2000.0050.050Actual 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 heated for 4 min at a certain temperature; 3） remove i
14、t 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 d
15、evice.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, respectively） were checked （GBT 16865-1997 was consulted,
16、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
17、 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 befo
18、re 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 ingGenerally , hot press forming of samples is operated above tr
19、ansition 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
20、-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 value is 1530 MPa at 900 , and the value will fall as tempe
21、rature 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 micr
22、ostructure 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
23、 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 a
24、nd 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 aust
25、enite 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 produc
26、ed 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
27、-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, so its deformation is relatively better. B is com -posed with the high-temperature
28、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 , deformatio
29、n 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 valu
30、e 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
31、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 m
