1. 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. , University POLITEHNICA of Bucharest, Department of Strength of Materials, Splaiul Independentei nr. 313, Bucharest, ROMANIA. s: University POLITEHNICA of Bucharest, Department of Metallic Materials, Splaiul Independentei nr. 313, Bucharest, ROMANIA. s: ,
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 oC/min, oC/min, 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
1.2.2
830
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 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 oC/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 and 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 , contract 170 /2012.