【正文】 e strength of up to 1500 MPa, which is veri?ed in different works using tensile tests (Naderi, 2007) and hardness. Corresponding author at: Department of Mining and Metallurgy, AmirKabir University of Technology (Tehran Polytechnic), PO Box 158754413, 424 Hafez Ave.,Tehran, Iran. Tel.: +98 21 64542978。 fax: +98 21 66405846.Email addresses: mnaderi@, mmzz52@ (M. Naderi).09240136/$ – see front matter 169。 2011 Elsevier . All rights reserved.doi:(2007) represented an ef?cient methodology which made it possible to optimize the geometrical design of the cooling ducts for a given set of boundary conditions and parameters. The objective was to quench the hot part effectively and at a constant rate and to provide a cooling rate of at least 27 K/s while marten site was formed. The method was successfully applied for a test tool. Kolleck et al. (2009) developed a twostep inductive heating system as an effective concept to decrease energy consumption of conventional heating methods. Tremendous reduction of heating time and consequently lower investment costs as well as reduced ?oor space for the heating device were some results of using thenew presented technology. Bariani et al. (2008) presented an innovative experimental procedure based on Nakazima test for evaluating the formability limits of high strength steels during hot stamping. The procedure generated formability data suitable for an FE modeling of the hot stamping process. They provided the formability data in the formof binations of strains that caused the onset of necking and fracture for given temperatures and average strain rates in the metastable austenitic phase. Xing et al. (2009) set up a material model under hot stamping condition of quenchable steel, based on the experimental data of mechanical and physical properties. They also simulated the whole hot stamping process by ABAQUS.M. Naderi et al. / Journal of Materials Processing Technology 211 (2011) 1117–1125The main objective of the present research is focused on investigation into microstructure and mechanical properties of different nonboron alloyed steel grades after being hot stamped. This objective was followed by selection of four different highstrength uncoated carbon steel sheets. Microstructural evaluation, lateral and surface hardness measurements and tensile tests after hot stamping were performed and considered.2. Materials and methods. Chemical positionThe investigated materials were different nonboron alloyed Fig. 1. An overview of the hot stamping process sequence (Altan, 2007). steel grades with carbon contents between and wt%.The chemical analyses as well as carbon equivalent values of theinvestigated steels are given in Table 1. Carbon equivalent (Ceq) of investigated steels was calculated according to the equation presented by Patchett (2003) for carbon steels. SteelsA, B, C and D show an increasing trend in their carbon equivalent value in order.As seen in Table 1, carbon equivalent ranges from in steelA to in steelD. Accordingly, all the investigated steels are listed in the low carbon steel grades.. CCT diagramsThe coolingpositiontemperature (CCT) diagrams of the investigated steels were obtained from a reference booklet collected by Dilatometer Laboratory of Ferrous Metallurgy DepartmentRWTH Aachen University (2007). All the contained diagrams were taken by means of dilatometry experiments, metallographic investigations and hardness measurements. The CCT diagrams of steel gradesB and D are represented in Fig. 2. The important parameters derived from CCT diagrams are marten site start temperature, Ms, critical time period and critical cooling rate to have fully martens tic microstructure. In Table 2, critical parameters derived from CCT diagrams of investigated steels are presented.. Experimental apparatus. Temperature evaluationDifferent austenization temperatures (between 870 and 970 ?C) and soaking times (between 10 and 20 min) were examined. Based on the resulted microstructures and hardness pro?les, the optimum austenization temperature and soaking time for each grade was selected. The mould assembly included water or nitrogencooled punch and a noncooled die. The cooling system was settled just inside the punch so that quenching was started as soon as forming began (Fig. 3).Descriptions of hot stamping facilities and methods used in experiments such as press, furnace and recording temperature evaluation in the blank during hot stamping process are explained in the . thesis of Naderi (2007), Naderi (2007) studied the effect of hot stamping process on microstructural and mechanical properties of Bbearing and nonboron alloyed steels and presented an innovative method to carry out metallographic analysis by application of lateral and surface hardness maps. Temperature evolution of blank, die and punch during press hardening was recorded digitally using a HOFFINGER BALDWIN MESSTECHNIK instrument. For each steel three Pt/Pt–Rh10% thermocouples were used. One thermocouple was soldered to the die, 10 mm beneath the contact surface and the other was soldered to the punch, 10 mm above the contact surface. The third thermocouples Fig. 4. Schematic representation of arrangement of three Pt/Pt–Rh10% thermocouples soldered to tools in order to monitor temperature evaluation of the tools. Ple was soldered to the blank 20 mm far from the edge of the blank. Every second, the temperature was recorded. In Fig. 4, schematic arrangement of the thermocouples is represented. It should be pointed that different regions of blanks do not experience the same cooling regime because of their location.Something that results in inhomogeneity of microstructure and correspondingly hardness pro?les (more can be found in SectionFig. 5 illustrates temperature evolution of the blank of steelC, die and punch during hot stamping process using water as coolant.Temperatures of punch and die were 25 ? C at the