INFLUENCE OF MANUFACTURING CONDITIONS ON THE MECHANICAL PROPERTIES OF MICROALLOY STEELS Mihaela Taca1, Dan M. Constantinescu2, Florin Baciu2, Victor Geanta3, Dana D. Daisa1, Radu Ştefănoiu3 S.C. METAV Research and Development, str. C.A. Rosetti nr. 31, Bucharest, ROMANIA. E-mail:, taca@metav-cd.ro daisa@metav-cd.ro University POLITEHNICA of Bucharest, Department of Strength of Materials, Splaiul Independentei nr. 313, 060042 Bucharest, ROMANIA. E-mails: dan.constantinescu@upb.ro, florin.baciu@upb.ro University POLITEHNICA of Bucharest, Department of Metallic Materials, Splaiul Independentei nr. 313, 060042 Bucharest, ROMANIA. E-mails: victor.geanta@yahoo.com , radu.stefanoiu@upb.ro ABSTRACT Microalloyed steels are currently the most used steels in the automotive industry due to their high mechanical resistance in as normalised condition. The use of these steels is still limited because of their relatively low toughness resistance. The toughness improvement can be realised by combining the effect of microalloying and manufacturing process. The effect of thermomechanical processing on the mechanical properties for several batches of microalloyed steels having different content of Nb, V and C was analysed. EXPERIMENTAL Materials: The composition of the seven batches of steel as well as their mechanical characteristics in as cast condition are presented in Table 1. Process Parameters These batches were annealed and forged with two reduction degrees. Bars were cooled in still and compressed air with three cooling speeds 75-95 oC/min, 250-270 oC/min,110-130 oC/min. The parameters of the forging process are presented in table 2 Mechanical testing Three tensile tests were done for each bar, following the standard procedures; obtained average values are presented in Tab. 3. Table 2 Process parameters Bar Forging temp oC Red. degree % Speed of cooling oC/min 1.1 890 49 75-95 1.2.1 950 87 250-270 1.2.2 830 110-130 2.1 850 2.2.1 920 2.2.2 870 3.1 800 3.2.1 3.2.2 4.1 780 4.2.1 4.2.2 5.1 5.2.1 885 5.2.2 6.1 6.2.1 900 6.2.2 860 7.1 875 7.2.1 880 7.2.2 Table 1 Chemical composition of the experimental batches and mechanical properties in as cast condition Batch C % Nb V UTS MPa YS A5 1 0.34 0.130 0.148 642 618 1.5 2 0.258 0.151 0.141 602 580 0.4 3 0.207 0.194 0.143 532 4 0.305 0.145 0.124 608 581 5 0.286 0.144 0.162 632 531 1.9 6 0.203 640 521 2.2 7 0.300 0.201 680 545 2.4 Table 3 Mechanical characteristics of the forged samples Fig. 1 Variation of normalised specific strain energy Bar Specific strain energy J/cm3 Young’s modulud MPa UTS YS A5 % 4.* 69.87 216519 1146 718 7.05 4.2.1 65.21 213458 1457 892 5.46 4.2.2 74.93 207816 1179 666 7.52 5.1* 57.85 219921 1151 802 5.74 5.2.1 69.29 204860 1333 844 6.11 5.2.2 70.43 210286 1196 668 6.1 41.04 209181 1098 784 4.23 6.2.1 73.37 218850 1415 879 6.13 6.2.2 76.39 212435 1162 688 7.65 7.1 60.22 226562 1248 814 5.72 7.2.1 72.58 211760 1513 938 5.77 7.2.2 81.72 207969 1170 665 8.18 RESULTS The characteristic curves have different shapes depending on the forging and cooling parameters. In average the elongation at failure is between 5-8%, beeing greater for bars notated generically K.2.2 with a reduction degree of 87%, and cooling speed 110-130 oC/min. The average elongation at failure is with 1-2% greater then for the bars K.2.1 which were forged with the same reduction degree but cooled with a greater speed of 250-270oC/min The ultimate tensile strength and the conventional yield limit is greater for the bars K.2.1 than for the bars K.2.2. Additionally, bars K.2.1 have a greater necking after the maximum force is attained. A measure of material's toughness is the specific strain energy measured in J/cm3. For batches 4-7, having as reference bars K.1 for each batch (section 30x30 mm), the strain energy of bars K.2.1 and K.2.2 is normalized to K.1 and shown in Fig. 1. Thus is put into evidence the influence of the cooling process for each batch. CONCLUSIONS As it results from Fig. 1, batch 6 has, in overall, a better performance concerning the increased toughness for both speeds of cooling. However, in terms of absolute values, batch 7 is also a good candidate to be considered for further testing and analysis as the reference value 7.1 of the specific strain energy is higher than for bar 6.1. The UTS values of bars 6.2.1 and 7.2.1 are quite high, as well as the YS, indicating that a higher speed of cooling is increasing these values but is decreasing the elongation at failure. As a additional comment, the measured micro-hardness for the materials with the same chemical composition is greater for the increased reduction degree and speed of cooling. * Average of two values Aknowledgements This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, Project number PN-II-PT-PCCA-2011-3.2-1018, contract 170 /2012.