M. Gaško, G. Rosenberg: Correlation between hardness and tensile properties 155 in ultra-high strength dual phase steels – short communication CORRELATION BETWEEN HARDNESS AND TENSILE PROPERTIES IN ULTRA-HIGH STRENGTH DUAL PHASE STEELS – SHORT COMMUNICATION Martin Gaško 1,*, Gejza Rosenberg 1 1Institute of materials research, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice - Slovakia *corresponding author: Tel.: +421 55 729 2473, e-mail: [email protected] Resume Article info The possibility to predict yield strength, strength limit, fatigue live Article history: estimation as well as other mechanical properties depending on values of Received 27 May 2011 materials hardness is commonly known and it is often used in practice. Accepted 28 July 2011 The main aim of this contribution is to review the possibilities of Online 3 November 2011 application of correlation relationships between hardness and ultimate tensile strength of steel sheets in various structural states. The Keywords: experiments were performed on advanced steels with structure which is Dual phase steel composed from ferrite and martensite (dual phase steels). Vickers hardness Mechanical properties Available online: http://fstroj.uniza.sk/PDF/2011/27-2011.pdf ISSN 1335-0803 1. Introduction HB for different materials [1-3]. For aluminium alloys, in comparison with steels, generally the Although, basically the hardness test of smaller values of coefficient k are observed the metal materials evaluates only surface (Fig.1). This can be well seen from results resistance of the material against the plastic mentioned in Fig. 1 based on work [3]. In this deformation, the hardness test is frequently used contribution the fact that the value of coefficient because it represents cheap non-destructive and k, in dependence on microstructure state (SDAS simple method for assessment of various secondary dendrite arm spacing), is in the range material properties like yield strength, tensile from 2.63 to 2.88 was found. At the steels, the strength, fatigue limit, but also distribution of coefficient k ranges the most frequently in the residual strains and, in a case of brittle materials, interval from 3.0 to 3.6. fracture toughness [2-4]. In order to determine the relationship between the ultimate tensile According to standard ČSN 420379, strength (UTS) and hardness (H) a number of which is replaced now by STN EN ISO 18265 , in relations were established [2-4]. In practice, the dependence on applied heat treatment or heat simplest equation is most often used: mechanical treatment, the influence of microstructure on correlation of TS-HB is UTS = H.k (1) reflected by the ratio of yield strength and where k is coefficient. In contribution [1], the tensile strength of steel. By this standard [2], for coefficient was in range from 3.38 to 3.55 for the ratio YS/TS in range from 0.5 to 0.9 the steel, from 3.48 to 3.21 for brass and from 2.86 coefficient k in the range from 3.54 to 3.21 (with to 3.63 for nodular iron (the hardness was increase the ratio YS/TS low value of k is measured according to Brinell, HB). Fig. 1 recommended to use) is recommended to use. In shows the progress of UTS in dependence on present, there exist a number of correlation Materials Engineering - Materiálové inžinierstvo 18 (2011) 155-159 156 M. Gaško, G. Rosenberg: Correlation between hardness and tensile properties in ultra-high strength dual phase steels – short communication UTS - HB 1400 1200 Steel [2] Al-Si-Mg (A356) 1000 Brass fine SDAS 800 Steel[1] 600 Cast iron UTS [MPa] Al-Si-Mg (nodular) 400 large SDAS 200 90 140 190 240 290 340 HB Fig. 1. Plot of ultimate tensile strength of various materials as a function of hardness [1-3] relationships HB – UTS a HB – YS type, where 2. Experimental material besides the empirical coefficients, many other For experiments five low carbon steels material characteristics are involved. For with carbon content C = 0.07 - 0.15 % and example, the equations by Cahoon et al. are manganese content Mn = 1.0 - 1.8 % were used. among the most known which include the strain All steels were processed with two modes of hardening exponent [3]. intercritical annealing consisting of heating on 750 °C or 800 °C (10 minutes hold) and The utilizing of the high strength steels consequential quenching in water. The volume constantly increases in the entire area of fraction of martensite was in range from 20 to 60 industry. The high demands on strength and %. plastic properties of the steel sheets for The tensile properties was measured on automotive industry intended for the autobody specimens, which were 120 mm long, and are placed. Among all advanced high strength 10 mm wide, with starting measured length steels the dual phase steels (DP) are most often L = 50 mm (thickness of specimens was in used for automotiv industry. By comparison to 0 range from s = 1.0 to s = 1.2 mm). In this study conventional steels, DP steels have significantly the correlation of relationship in form TS - HV, better combination values of strength versus YS - HV, YS/TS - HV5 as well as the ductility and a very good compressibility. The correlation between strain hardening exponent high plasticity of DP steels is given by and hardness: n - HV was examined. microstructure consisting of soft ferrite and hard martensite. In consequence of this, these steels are also known by low value of YS/TS ratio 3. Results and discussion (mostly YS/TS = 0.6 to 0.7, valid even for steels Among the all studied correlations, as with strength over the 1000 MPa) [5,6]. expected, the highest coefficient of correlation The main aim of this work was to find out was found between hardness and strength. how the individual composition of However, also in this case, the correlation 2 microstructure and low ratio YS/TS results in coefficient did not attain the value R = 0.9, value of coefficient k and, at the same time, with therefore the interdependence between strength which accuracy it is possible to predict YS and and hardness is relatively low. The results show TS of these steels, by the Vickers hardness that the prediction of UTS value based on the testing. measurement of values of HV5 could be loaded Materials Engineering - Materiálové inžinierstvo 18 (2011) 155-159 M. Gaško, G. Rosenberg: Correlation between hardness and tensile properties in ultra-high strength dual phase steels – short communication 157 by considerable error. The biggest scatter of data The slope in the established equations is observed in the range from 270 to 325 HV5. reaches more than three times higher values The anticipated strength (UTS calculated from (Fig. 3). It is likely that for the YS - HV5 equation on Fig. 2) is in comparison with the correlation using the equation in exponential strength based on tensile test measurement in the form would be more suitable. ± range UTS 125 MPa. Also in the relation YS / UTS = f (HV5), When we take into account all measured for the same hardness, it is possible to observe values mentioned in Fig.1, and the shape of the break. From the set of correlation correlation equation (1) then we detect that the equations it is clear that, for the hardness of coefficient k is in the range from 2.8 to 3.6. The about 330 units HV5, the relation shows a correlation coefficient for the relation YS = f minimum (Fig. 4). In practical terms we (HV5) is less than R 2 = 0.8. cannot speak about correlation of YS / UTS - This result clearly shows that the HV5, because correlation coefficient is low. prediction of the yield stress of steel through the The data in Fig. 4 show that in the range from measured values of hardness is loaded with 210 to 330 units of HV5 the values of ratio larger error as it was in the case of strength YS / UTS are in the range from 0.5 to 0.7, and prediction. Measured results in Fig. 3 indicate the hardness from 370 to 430 units that the relation YS = f (HV5) at hardness over of HV5 results in the ratio YS / UTS = 0.7 to the 325 units HV5 is steeper. 0.85. UTS - HV5 1400 1300 1200 y = 2,77x + 92,754 2 1100 R = 0,8786 1000 U T S 900 800 700 600 200 250 300 350 400 450 HV5 Fig. 2. Ultimate tensile strength as a function of hardness HV5 YS - HV5 1200 y = 2,5009x - 181,44 1000 y = 1,5703x + 76,165 R² = 0,7695 R² = 0,5694 800 Y S 600 y = 5,4035x - 1312,5 400 R² = 0,736 200 200 250 300 350 400 450 HV5 Fig. 3. Yield strength as a function of hardness HV5 Materials Engineering - Materiálové inžinierstvo 18 (2011) 155-159 158 M. Gaško, G. Rosenberg: Correlation between hardness and tensile properties in ultra-high strength dual phase steels – short communication YS/UTS - HV5 0,9 y = 0,0006x + 0,417 y = -0,0003x + 0,6852 0,8 2 R2 = 0,0562 R = 0,2303 0,7 0,6 y = 0,0027x - 0,3592 YS/U0,5 T S R2 = 0,5422 0,4 200 250 300 350 400 450 HV5 Fig. 4. The yield strength to tensile strength ratio, as a function of hardness (HV5) n - HV5 0,2 0,18 0,16 0,14 0,12n 0,1 0,08 y = -0,0003x + 0,2344 0,06 R² = 0,5518 0,04 200 250 300 350 400 450 HV5 Fig. 5. The strain hardening exponent as a function of hardness (HV5) Fig.