文献外文.pdf

J Mater Sci Technol 2011 27 4 369 376 Semi hot Stamping as an Improved Process of Hot Stamping Malek Naderi1 Mostafa Ketabchi1 Mahmoud Abbasi1 and Wolfgang Bleck2 1 Department of Mining and Metallurgy Amirkabir University of Technology Tehran Iran 2 Department of Ferrous Metallurgy RWTH Aachen University Aachen Germany Manuscript received December 5 2010 in revised form February 11 2011 Reducing the forming load deletion of springback increasing the formability of sheets as well as producing high strength parts are the main reasons to apply hot stamping process Hot stamping process and 22MnB5 steels are the state of the art process and grades respectively however novel processes and steel grades are under considerations In the current research behavior of the steel grade MSW1200 blanks under semi and fully hot stamping processes was characterized During semi hot stamping process the blank was fi rstly heated to a temperature of about 650 C and then formed and quenched in the die assembly simultaneously Microstructure and mechanical properties of semi and fully hot stamped blanks were studied and the results were compared with those of normally water air quenched blanks The hot stamped blanks attained the strength values as high as water quenched blanks The highest ductility and consequently the best formability were achieved for the blank which had been semi hot stamped It was concluded that for the mentioned steel semi hot stamping process could be considered as an improved thermo mechanical process which not only guaranteed a high formability but also led to ultra high strength values KEY WORDS Semi hot stamping Hot stamping MSW1200 steel Mechanical properties 1 Introduction To optimize the fuel consumption of cars the weight reduction of cars is intensively required in the automobile industry Hence application of high strength steel sheets in automobile bodies would be a good solution to reduce the weight of cars 1 How ever the use of high strength steel sheets usually leads to some disadvantages like high impact on the tools reduced formability and the increased tendency to springback 2 To improve the formability charac teristic and reduce or avoid the springback of high strength steel sheets hot stamping process has been used widely During this process not only the fl ow stress is largely reduced by preheating the sheets but also due to simultaneous occurrence of forming and quenching forming load and springback decrease and formability increases 3 Hot stamping is de Correspondingauthor Tel 982164542949 Fax 98 21 66405846 E mail address m abbasi aut ac ir ab baci m M Abbasi fi ned as the process in which the blank is heated to the temperature of the austenite stabilization region i e about 900 C for defi nite time and then formed and quenched simultaneously 4 Many attempts have been done on the hot stamping process of steel sheets especially on quenchable boron alloyed steels It is re ported that boron enhances hardenability and retards and postpones thermally activated transformations e g ferrite and pearlite transformations 3 Merklein and Lechler 2 used 22MnB5 steel as material and studied its hot stamping process They showed that hot stamping process can result in production of com ponents with tensile strength more than 1500 MPa due to changes in microstructure Naderi et al 5 an alyzed the microstructural and mechanical properties of diff erent B bearing steels after being hot stamped Microstructures of the hot stamped blanks were com posed of martensite The resulted microstructures provided the yield strengths of 650 1370 MPa and the tensile strengths of 850 2000 MPa The disadvantage of the hot stamping is remarkable oxidation on the surface of products 6 Thesheets are heatedat 370M Naderi et al J Mater Sci Technol 2011 27 4 369 376 Table 1 Chemical composition of the investigated steel mass CSiMnCrNiAlTiCeq 0 140 121 710 550 060 020 0020 258 about 900 C to attain the martensitic transformation induced by the die quenching and the oxide scales are generated at this temperature The oxidation be comes remarkable due to contact with air after taking the sheet out of the furnace The shot blasting is used to remove the oxide scales of stamped products or the aluminum and zinc coated sheets are employed to prevent the oxidation during the hot stamping But the shot blasting and coating are costly Mori and Ito 7 evaluated the eff ect of the oxidation preventive oil on oxidation of quenchable steel sheets in a hot hat shaped bending experiment They found that the oxidation was prevented by using coating Schie l et al 8 analyzed corrosion behavior of coated and non coated steels after being hot stamped and notifi ed that high temperature austenization could damage the coatings Innovation of a stamping process that brings advantages of hot and cold stamping processes all together not only will result in low forming load and minimizing of springback but also may preserve the coating properties properly and save time and cost Semi hot stamping process is a reply to this request During the semi hot stamping the blank is heated for a while at the temperature of about 650 C and subsequently it is deformed and quenched simul taneously So study on the eff ects of semi hot stamp ing process on diff erent properties of sheet metals is worthy Mori et al 9 used a resistance heating to elevate the temperature of sheet during forming and studied the eff ects of warm stamping on the springback of ul tra high tensile strength steel sheets The springback in hat shaped bending of the high tensile strength steel sheets was eliminated by heating the sheets They found that the optimum heating temperature is around 600 C due to the small springback and ox idation and the increase in hardness Schie l et al 8 studied the corrosion behavior and mechanical prop erties of 22MnB5 CP W800 and MSW1200 steels af ter being hot stamped and semi hot stamped Prior to hot and semi hot stamping operations the blanks were heated to 950 and 650 C temperatures respec tively and then they were cooled in a closed die The material 22MnB5 reached component tensile strength levels over 1500 MPa at elongations of 5 8 while with non boron alloyed steels tensile strengths of only maximum 850 MPa were obtained at elongation of about 8 12 In the current research more details on semi hot stamping process along with MSW1200 steel grade characterizations were investigated MSW1200 is de veloped for cold forming and thus has high strength in the as delivered condition For better comparison and understanding cold and hot stamping processes using Fig 1 Microstructure of as received MSW1200 steel It consists of about 70 ferrite as well as 30 fi ne spheroidized pearlite water or nitrogen cooled punch were also performed 2 Experimental 2 1 As delivered condition The industrially processed un coated MSW1200 steel sheet with a thickness of 1 5 mm was provided Chemical composition of the studied steel as well as its corresponding carbon equivalent is given in Table 1 Carbon equivalent of investigated steel was calculated according to the equation presented by Patchett 10 for carbon steels As received microstructure of the steel is shown in Fig 1 As it is seen in Fig 1 most of the as received mi crostructure consisted of ferrite phase i e about 70 The yield and tensile strengths were about 400 MPa and 640 MPa respectively and the total elongation value A25 was about 26 2 2 Methods The eff ect of three processes i e cold stamping followed by quench hardening semi hot stamping and hot stamping on microstructure and mechanical prop erties of the studied steel was investigated Thermal schedules for each process as well as the resulted mi crostructure are given in Table 2 Geometry of blanks and description of tools including press die cooling system and temperature recording tools in addition to the condition of stamping are reported in literature 11 The mold assembly included water or nitrogen cooled punch and a non cooled die Mechanical properties of the deformed parts were determined by Vickers hardness test HV0 8 and standard tensile test as reported in literature 11 Surface hardness maps in addition to lateral hardness profi les were obtained to quantify and qualify material properties M Naderi et al J Mater Sci Technol 2011 27 4 369 376371 Table 2 Thermal schedules and the resulted microstructures after various stamping processes HeatingSoaking timeCoolingPhase fraction temperature C condition minMartensite Bainite Ferrite Pearlite Cold stamping 95010Air coolant 7030 Quench hardeningWater coolant85 15 Semi hot stamping6505Water cooled punch 6832 10Water cooled punch 6238 Hot stamping95010Water cooled punch42553 Nitrogen cooled punch60 536 53 Fig 2 The CCT diagram of the studied steel 2 3 CCT diagram The continuous cooling transformation CCT di agram Fig 2 was determined by means of dilatome try tests metallographic investigations and hardness measurements The circled numbers indicate the val ues of fi nal hardness given in the Vickers HV10scale The CCT diagram in Fig 2 indicates that the martensite start temperature Ms is about 400 C and cooling rates higher than 40 C s result in a fully martensitic microstructure 2 4 Temperature and force evaluation The temperature evolution of blanks die and punch during pointed processes was obtained using thermocouples set in proper positions of the blank and the tools Representations of non cooled die and cooling system designed for cooled punch as well as schematic demonstration of appropriate placement of Pt Pt Rh10 thermocouples for monitoring and recording the temperature evolution of the tools and blanks during the stamping processes are provided in Fig 3 The temperature and the force evolutions of the blank during hot stamping process after austeniza tion at 950 C for 10 min and in the condition of using water as coolant is represented in Fig 4 As it is ob served the cooling rate in the range of initial defor mation temperature 825 200 C was about 158 C s Fig 3 a Die assembly including cooling system in punch b position of Pt Pt Rh10 thermocouples for recording temperature evolution of tools and blanks during stamping processes 372M Naderi et al J Mater Sci Technol 2011 27 4 369 376 Fig 4 Temperature and the force evolutions during hot stamping process of MSW1200 steel Blank was austenized at 950 C for 10 min HS hot stamping b 950 C 10 min water quench Fig 6 Microstructure of MSW1200 steel after hot stamping a 950 C 10 min hot stamping WCP b 950 C 10 min hot stamping NCP WCP stands for water cooled punch NCP stands for nitrogen cooled punch M Naderi et al J Mater Sci Technol 2011 27 4 369 376373 Fig 7 Microstructure of MSW1200 steel after semi hot stamping a 650 C 5 min semi hot stamping b 650 C 10 min semi hot stamping The most noticeable microstructure belonged to semi hot stamped blanks It consisted of ferrite and pearlite having the morphology as shown in Fig 7 Before applying the stamping process the blanks were heated to a temperature of about 650 C It is predicted that low heating temperature of semi hot stamping process does not result in occurrence of any phase transformation Therefore minor discrepancies in volume fractions of ferrite and pearlite proportions in the microstructures of semi hot stamped blanks with respect to as received ones Table 2 might be related to minor image analysis errors during evalua tion Quantitative evaluations of the microstructures were conducted by using ImageJ program which was an image processor and analyzer developed at the National Institute of Health USA The kind of mi crostructure constituted in semi hot stamped blanks Fig 7 could be related to inherent characteristic of substances to decrease their interior energy At constant volume fraction of pearlite phase as nod ule size decreases surface energy of whole substance increases 12 As it was observed in Fig 1 as received microstructure of MSW1200 steel mostly included fer rite as well as fi ne pearlite nodules Heating the blank to a temperature of about 650 C for a suffi cient time brings the possibility of coalescence of very fi ne pearlite nodules Fig 7 in order to decrease the surface energy So large pearlite nodules as it is ob served in Fig 7 b form in the microstructure The presence of fi ne pearlite nodules in the microstructure of Fig 7 a indicates that fi ve minutes is not ade quate to complete the coarsening of the fi ne pearlite nodules Fast cooling due to the employment of water cooled punch during semi hot stamping resulted in non equilibrated ferrite grains with non ordered grain boundaries in both samples especially for the ten minutes heated specimen Fig 7 3 2 Hardness profi les Hardness profi les were obtained by using an in strument which not only measured the hardness va Fig 8 Lateral hardness profi les of MSW1200 steel along the half of the stamped blank lues linearly but also was able to determine the hard ness values for a predefi ned region More informa tion about this instrument can be found in literature 11 Lateral hardness profi les of blanks after being hot and semi hot stamped are represented in Fig 8 Hardness values of the hot stamped blanks samples A and B due to the presence of martensite and bainite phases were higher than those for semi hot stamped blanks samples C and D which consisted of ferrite and pearlite phases One important point in Fig 8 is the improvement in the homogeneity of hardness profi les by enhanc ing the heating time It is observed in Fig 8 that the non homogeneity of hardness in sample A is the most A suffi cient heating time for samples B and C resulted in a more homogeneous microstructure and correspondingly more homogeneity of hardness in semi hot and hot stamped blanks Unsuccessful aust enization treatment yields a non uniform distribution of carbon in the primary austenite and as a result in the obtained martensite 13 An increase in the carbon content of a local region of primary austenite increases the distortion of the body centered tetragonal BCT lattice during martensite formation and decreases the movement of dislocations in the region and as a result the hardness values increase 14 Therefore 374M Naderi et al J Mater Sci Technol 2011 27 4 369 376 Fig 9 Surface hardness map of MSW1200 steel a 950 C 10 min hot stamping WCP b 950 C 10 min hot stamping NCP NCP nitrogen cooled punch WCP water cooled punch Fig 10 Surface hardness map of MSW1200 steel a 650 C 5 min semi hot stamping WCP b 650 C 10 min semi hot stamping WCP WCP water cooled punch proper heat treatment prior to the semi hot and hot stamping processes of steel with non uniform initial microstructure should be an essential condition to im prove the hardness uniformity although due to the presence of various phases with diff erent hardness val ues beside each other the hardness non uniformity is inevitable Surface hardness profi les of hot stamped and semi hot stamped blanks are shown in Figs 9 and 10 respectively As it comes from Figs 9 and 10 the hot stamped blanks have hardness values greater than semi hot stamped ones Increasing the cooling rate employment of nitrogen as coolant instead of water during hot stamping process resulted in higher hard ness values Fig 9 The higher hardness limits rep resented in Fig 10 a with respect to the hardness limits in Fig 10 b might be originated from the de velopment of internal stresses in the microstructure due to little time of heating and correspondingly non uniform initial microstructure 15 The surface hardness maps can be a good sub stitution for metallographic analysis By using this technique hardness distribution and similarly phase distributions can be well studied The advantages of this technique over lateral hardness profi les metallo graphic investigations as well as more details about quantitative and qualitative measurement of diff erent phases are presented in literature 4 Fig 11 Flow of true stress in terms of true strain for MSW1200 steel in diff erent stamping conditions WQ water quenched AQ air quenched WCP water cooled punch NCP nitrogen cooled punch 3 3 Mechanical properties True stress strain curves of MSW1200 steel for dif ferent stamping conditions were determined Fig 11 Yield strength YS ultimate tensile strength UTS and total elongation A25 values related to diff erent blanking conditions are presented in Table 3 The highest strength values were related to the samples which had been cold stamped and quenched in water M Naderi et al J Mater Sci Technol 2011 27 4 369 376375 Table 3 Mechanical properties of MSW1200 steel grade after various types of stamping Heating temperatureSoaking timeCooling conditionYSUTSA25 C min MPa MPa Cold stamping 95010Air coolant54081014 quench hardeningWater coolant111014304 Semi hot stamping6505Water cooled punch44865025 6 10Water cooled punch40093020 Hot stamping95010Water cooled punch91613005 5 Nitrogen cooled punch93613302 5 As received 40064026 as well as the hot stamped samples however the sam ples which had been cold stamped and quenched in air along with the semi hot stamped samples represented the highest ductility values These were related to the microstructures of the studied samples As it was pointed before Table 2 the microstructure of the former sample contained mostly martensite while the latter consisted of ferrite pearlite phases Five minutes isothermal heating at 650 C prior to stamping did not aff ect the as delivered properties of the blanks however yield strength showed some incre ment The sample kept for 10 min at temperature of about 650 C before stamping obtained compromised values of high tensile strength and ductility High values of both tensile strength and ductility of a part bring the possibility of absorbing energy during the crash without being torn or fractured 16 It should be paid attention that in spite of the benefi ts pointed for semi hot stamping process one drawback of this process is the relatively low yield strength value of