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1、Journal of Materials Processing Technology 177 (2006) 452455 Investigation of the thermo-mechanical properties of hot stamping steels M. Merklein1, J. Lechler Department of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstrasse 11, 91058 Erlangen, Germany Abstract Within the inn

2、ovative hot forming process for sheet metals, called hot stamping, it is possible to combine forming and quenching in one process step. This affords the opportunity to manufacture components with complex geometric shapes, high strength and a minimum of springback which currently fi nd applications a

3、s crash relevant components in the automotive industry. As standard material for hot stamping the quenchenable high strength steel 22MnB5 is commonly used. With regard to the numerical modeling of the process, the knowledge of thermal and thermo-mechanical properties of the material is required. To

4、determine the thermo-mechanical material characteristics, the fl ow behavior of the steel 22MnB5 in the austenitic state has been investigated by conductive, hot tensile tests with a Gleeble 1500 system dependent on the timetemperature characteristic of the hot stamping process. 2006 Elsevier B.V. A

5、ll rights reserved. Keywords: Hot stamping; High strength steels; 22mnB5; Thermo-mechanical properties; Flow behavior 1. Introduction One of the most important challenges for the automotive industry in the upcoming years is to meet the demand of reduc- ing the fuel consumption with a contemporaneous

6、ly increase of the safety properties. This can be primarily realized by reducing the weight of body in white components by using thinner mate- rials with higher strength. Therefore more high and ultra-high strength steels are increasingly used in the automotive indus- try, due to their improved form

7、ing properties 1. For example withtheapplicationofthequenchenableultra-highstrengthsteel 22MnB5, complex crash relevant components like reinforce- ment parts, front bumpers, etc. with a fi nal strength of about 1500MPa 2 can be manufactured by simultaneously reducing the materials thickness. But the

8、 use of high strength steel usu- ally leads also to some disadvantages like a high impact on the tools, reduced formability and the tendency to springback. To improvetheformabilityofsuchmaterialsnewformingtechnolo- gies like for example the hot stamping process of quenchenable Correspondingauthorat:

9、DepartmentofManufacturingTechnology,Univer- sity of Erlangen-Nuremberg, Egerlandstrasse 11, D-91058 Erlangen, Germany. Tel.: +49 9131/85 28309; fax: +49 9131/930142. E-mail addresses: m.merkleinlft.uni-erlangen.de (M. Merklein), j.lechlerlft.uni-erlangen.de (J. Lechler). 1 Present address: Departmen

10、t of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstrasse 11, D-91058 Erlangen, Germany. Tel.: +49 9131/8527961; fax: +49 9131/930142. steels had been developed. Hot stamping is a non-isothermal forming process for sheet metals, where forming and quench- ing takes place in one

11、 combined process step. In Fig. 1 the hot stamping process is schematically illustrated. As-delivered the base material 22MnB5 has a ferritic-pearlitic microstruc- ture with a tensile strength of about 600MPa. After passing through the hot forming process, the component fi nally exhibits a martensit

12、ic microstructure with strength of about 1500MPa. A pre-condition for the desired fi nal high strength martensitic microstructure, is that the blank must be austenitized fi rst for about 510min in a furnace at about 900950C. After having achieved a homogeneous austenitic microstructure the blank is

13、transferred automatically to the water cooled die within three seconds, where forming and quenching takes place simultane- ously. Thereby advantage is taken of the reduced fl ow stress due the elevated temperature. Through the occurring contact of the hot blank with the cold die, high cooling rates

14、can be realized, and a non diffusional martensitic transformation occurs. With regard to a reliable process modeling, besides the tri- bological conditions and the mechanical characteristics like the Young modulus, the Poisson ratio, etc. the knowledge of ther- mal and thermo-mechanical material pro

15、perties, in dependency of the timetemperature characteristic of the hot forming pro- cess, is required. In the following sections experimental results of investigations on the thermo-mechanical fl ow properties of 22MnB5, according to the hot stamping process requirements andtheinfl uencingparameter

16、s,willbepresented.Duetothathot 0924-0136/$ see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.03.233 M. Merklein, J. Lechler / Journal of Materials Processing Technology 177 (2006) 452455453 Fig. 1. Schematic illustration of the direct hot stamping process. tensil

17、e tests have be performed with a modifi ed, servo hydraulic mechanical Gleeble 1500 testing system. 2. Materials and experimental proceeding 2.1. Material characteristics Intheautomotiveindustryfordirectandindirecthotstampingthequenchen- able, ultra-high strength steel 22MnB5 is commonly used. Withi

18、n the scope of this paper a cold-rolled strip with a material thickness of 1.75mm produced by Arcelor is used. The boron/manganese micro-alloyed steel, so-called USIBOR 1500P, exhibits a ferritic-pearlitic microstructure with a hardness of 171 HV10, a yield strength of 400MPa and tensile strength of

19、 approximately 600MPa 3. Regarding the base material fl ow properties in dependency of the rolling direc- tion and the strain rate, it should be referred to Merklein et al. 4. In 4 it is shown,thatUSIBOR1500Pexhibitsnosignifi cantsensitivityonthefl owbehav- iorwithrespecttotherollingdirectionandthed

20、eformationvelocityas-delivered. In order to prevent the blank from oxidation and decarburization during the heat treatment and the transfer from the furnace to the die, USIBOR 1500P is pre- coated with an aluminum-based layer. The thickness of the coating usually is between 23 and 32?m according to

21、the supplier. To achieve the required homo- geneous austenitic microstructure before quenching, according to 5, a furnace dwell time of at least 3.5min for a 1.75mm thick blank is essential. Follow- ing the continuous time temperature transformation (TTT) diagram in Fig. 2, a Fig. 2. TTT diagram of

22、USIBOR 1500P according to Arcelor 3. cooling rate of at least 27Ks1is essential for avoiding bainitic transformation and to achieve a full martensitic microstructure for hot stamped parts. 2.2. Experimental set-up and procedure According to the microstructural transformations during the hot stamping

23、 process, the temperature window for the actual forming process is limited to the austenitic phase of 22MnB5. Due to the martensite start temperature (MS) at approximately 400C and the transfer dependent cooling on air, the form- ing of the blank occurs usually between 850 and 400C. Pre-condition fo

24、r a numerical modeling of the hot forming process, the determination of material thermo-mechanical characteristics in dependency of the infl uencing parameters like temperature, heating and cooling rate, true strain and strain rate, etc. is essential. With conventional mechanical testing systems thi

25、s challenge is diffi - cult to meet. Therefore a Gleeble 1500 testing system had been modifi ed to be capable to reproduce the hot stamping process for characterizing the fl ow prop- erties of USIBOR 1500P in conductive hot tensile test, in dependency of the relevant process parameters. The adjusted

26、 servo hydraulic Gleeble 1500 system is schematically displayed in Fig. 3. For receiving more precise force or stress data, respectively, due to the reduced stress values at elevated temperatures, an external, more sensitive 50kN load cell, with an according self-constructed clamping device, was imp

27、lemented in the test chamber of the machine. In order to realize cooling rates higher than 27Ks1, two compressed-air nozzles have been integrated. As a consequence of these modifi cations hot tensile tests with timetemperature characteristics, that fi t to the hot stamping process, and cool- ing rat

28、es up to 8090Ks1can be achieved. The measuring of the specimen elongationswasrealizedusinganopticaldeformationsystem,ARAMIS(GOM, Germany). Within this work uniaxial, conductive hot tensile tests have been performed to determine the fl ow properties of USIBOR 1500P in dependency of rolling direction,

29、 temperature (500, 650, 700 and 800C) and strain rate (0.01, 0.1 and 1s1), according to the DIN EN10002 Part 5 guideline. Therefore the spec- imens have been imposed to the following thermo-mechanical test program: With regard to the recommendation of the steel supplier the test samples had beenheat

30、eduptoanaustenitizationtemperatureof950Cnotfasterthanapprox- imately 16Ks1. After leaving the specimen at 950C for 180s to guarantee a complete, homogenous austenitization 5, rapid cooling and stabilization at a temperature between 800 and 500 C for fi ve seconds took place. Afterwards thetensiletes

31、twascarriedoutunderisothermalconditions.Themeasurementof the temperature is realized using Ni/CrNi thermocouples spot welded onto the sample at half of the length (compare Fig. 3). The deformation of the specimens were detected using the optical measuring system ARAMIS. The specimens geometry follow

32、s the recommendation of EN482-2 6, pictures of the deforma- tion process have been taken with a frequency of 10Hz. For each investigated Fig. 3. Schematic sketch of the modifi ed test chamber of the servo hydraulic Gleeble 1500 mechanical system. 454M. Merklein, J. Lechler / Journal of Materials Pro

33、cessing Technology 177 (2006) 452455 parameter at least fi ve test runs had been carried out. For the calculation of the fl ow curves the essential stress and strain data had been received from the 50kN load cell and the Aramis system, respectively. The fi nal calculation of the true stress strain v

34、alues followed 7,8. 3. Experimental results 3.1. Infl uence of the rolling direction on the fl ow properties in the austenitic state The infl uence of the rolling direction on the fl ow behavior of 22MnB5 had been investigated for different temperatures in the austenitic state following the test pat

35、h mentioned in the section before. For the lower and upper test limit tem- peratures, 500 and 800 C, the impact of the rolling direction on fl ow properties ofUSIBOR1500P,isrepresentativelyshowninFig.4.Forbothtesttemperatures an exemplarily fl ow curve is illustrated for each of the three rolling di

36、rection 0, 45and 90, after rapid cooling with a cooling rate of approximately 80Ks1 and an exemplarily strain rate of 0.1s1. Furthermore for a test temperature of 650 C the fl ow curves in dependency of the rolling direction are shown. The good accordance of all curves shows, that the material exhib

37、its no dependency ontherollingdirectionintheausteniticphase.Furtherexperimentswithvarious strainandcoolingratesconfi rmedthisresults.Basedonthis,furthertensiletests have been carried out without taking the rolling direction in account. Thus only specimens with an orientation parallel to the rolling

38、direction had been used. 3.2. Infl uence of temperature on the fl ow properties in the austenitic state The infl uence of the temperature on the fl ow properties of the test material USIBOR1500Phasbeeninvestigatedfordifferenttemperaturesandstrainrates intheausteniticstateafterrapidcooling.Fig.5shows

39、thetemperature-sensitivity of the material. For various temperatures between 500 and 800C after rapid cooling,representativetruestressstraincurvesaredisplayedforanexemplarily strain rate of 1s1 . The fl ow curve characteristics show, that the temperature has a signifi cant infl uence on the forming

40、behavior of the quenchenable steel. Increasing the temperature leads to a signifi cant reduction of the fl ow stress and a decreasing work hardening exponent, resulting in a noticeable decrease of the slope of the true stressstrain curves. For lower strain rates like, e.g., of 0.1s1the material show

41、s the same dependency on temperature, but with addi- tional, simultaneously occurring dynamic annihilation and recovery processes at temperatures above 650C (compare Fig. 6) during deformation. This leads to an increasing tendency of the sheet metal to exhibit almost a plane fl ow curve characterist

42、ic after the initial strain hardening with increasing temperature. Due to the timetemperature dependency this effect is more apparent the higher the temperatureandthelowerthedeformationvelocityare.Accordingtotheresults Fig. 4. Dependency on rolling direction of USIBOR 1500P, sheet thickness t0=1.75m

43、m. Fig. 5. Infl uence of the temperature on the fl ow curve properties of USIBOR 1500P, strain rate d/dt=1s1. showninFigs.5and6,thestrainrateseemstoinfl uenceaswellthefl owproper- ties of 22MnB5 at elevated temperature besides the temperature, and has thus to be considered regarding the characteriza

44、tion of the materials forming behavior. 3.3. Infl uence of the strain rate on the fl ow properties in the austenitic state The dependency of the materials fl ow behavior on the strain rate has been investigated at three different strain rates 0.01, 0.1 and 1s1, after rapid cool- ing in the austeniti

45、c phase. Exemplarily for the sensitivity on this infl uencing parameter, in Fig. 7 the strain hardening function is shown in dependency of the various deformation velocities at a temperature of 650C. For each strain rate a representative fl ow curve is displayed. According to the curve characteristi

46、cs, it is obvious that the strain rate has an signifi cant impact on the forming behavior of USIBOR 1500P. Increasing the strain rate leads to appreciable increase of the stress level and the slope of the curve as a consequence of an enforced work hardeningofthematerial.Furtheritcouldbeseen,thatwith

47、adecreasingtesting velocity and thus with increasing deformation time, the fl ow curves exhibit a tendencytoachieveasteadystateaftertheinitialstrainhardening.Thisarisesin theapproachofanalmostasymptotictrendofthestrainhardeningfunctionwith progressive elongation. This effect can related to occurring

48、 diffusional depen- dent microstructural recovery processes balancing the strain hardening 9,10. For higher temperatures up to 800C, a comparable material sensitivity on the deformation velocity could be detected 4. Fig. 6. Infl uence of the temperature on the fl ow curve properties of USIBOR 1500P,

49、 strain rate d/dt=0.1s1. M. Merklein, J. Lechler / Journal of Materials Processing Technology 177 (2006) 452455455 Fig. 7. Infl uence of the strain rate on the fl ow properties of USIBOR 1500P at 650C. 4. Conclusion In this paper the thermo-mechanical fl ow properties of pre- coated 22MnB5 had been

50、investigated in dependency of the timetemperature characteristic of the hot stamping process. The data receiving from these tests are necessary regarding the numerical modeling of the materials forming behavior during the hot stamping process. The results in chapter three show, that for the mathemat

51、ically description of the materials fl ow behav- ior at elevated temperatures in the austenitic state, the rolling direction has not to be taken in account. In opposite the temper- atureandthestrainrateaswellhaveaninfl uenceontheforming behavior of the ultra-high strength steel 22MnB5. Increasing th

52、e temperature leads to signifi cant decrease of the fl ow stress values and the slope of the initial strain hardening. For the sen- sitivity of the materials forming properties regarding the strain rate,anincreaseofthedeformationvelocityleadstoasignifi cant increase of the stress levels and the work

53、 hardening. 5. Summary and outlook Within this work the thermo-mechanical fl ow properties of thequenchenableultra-highstrengthsteelUSIBOR1500Pman- ufactured by Arcelor was investigated. Hence a servo hydraulic Gleeble 1500 system was modifi ed to be capable to characterize the forming behaviour of

54、22MnB5 in the austenitic state, fol- lowing the timetemperature characteristics of the hot stamping process. Therefore conductive hot tensile test have been car- riedoutindependencyoftheinfl uencingparameterslikerolling direction, temperature and strain rate after rapid cooling down from950Ctoanisot

55、hermaltesttemperature.Theresultsshow, that the material exhibits a high sensitivity on temperature and strain rate. For rolling direction no signifi cant infl uence could be determined. Regarding the generation of a numerical model to describe the materials fl ow behavior, the temperature and the strain rate have to be considered as well. In future work the impact of the heating and the cooling rate onthefl owbehaviorwillbeinvestigatedwithregardtoareliable material model for the numerical process design. Additionally an experimental