Effect of Impact Toughness Anisotropy on Brittle Fracture Resistance Characteristics of High-Strength Steels Subjected t
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EFFECT OF IMPACT TOUGHNESS ANISOTROPY ON BRITTLE FRACTURE RESISTANCE CHARACTERISTICS OF HIGH-STRENGTH STEELS SUBJECTED TO THERMOMECHANICAL TREATMENT V. M. Goritskii,1 G. R. Shneiderov,2 and O. V. Goritskii3
UDC 621.789:669.14
The structure and mechanical properties under tension and impact bending of eleven batches of highstrength manganese-containing steels (Mn-steels) containing 0.09 to 0.14 wt.% Ti subjected to thermomechanical rolling were studied in the temperature range from –60 to + 20 °С. It was found that the values of the coefficient of impact toughness anisotropy in the range of Ka = 2.0–4.9 increase at higher titanium content and decrease at higher aluminum content. The difference between ductile-to-brittle transition temperatures T50 and T34 for longitudinal and transverse KCV samples increases at higher contents of titanium, aluminum, sulfur, and carbon. This effect is caused by an earlier nucleation and growth of large dimples of ductile fracture around sizable inclusions preferentially located in the direction of rolling. Keywords: thermomechanical rolling, high-strength steel, impact toughness anisotropy, carbonitrides, structure, fracture, delamination.
According to the results of numerous studies [1–6], the optimal combination of strength, cold-temperature resistance, ductility and other performance characteristics during the process of thermomechanical rolling (TMR) is achieved by optimizing the chemical composition of steels and process parameters of sheet rolling and cooling. Thermomechanical rolling requires strict monitoring of such parameters of hot deformation (which affect the process efficiency) as deformation temperature, rate, degree, number of passes, and scheme (method), as well as duration of austenitic state of steel after deformation [1, 6]. Considering the specifics of structure formation, there is a high probability of developing undesirable anisotropy of impact toughness during thermomechanical rolling [7, 8]. The authors of Ref. [7, 8] report the results of studies performed using 14 grades (30 melts) of commercial construction steels (sheet thickness — from 4 to 40 mm) having ferrite-pearlite structure, which show that the coefficient of impact toughness anisotropy Ka = KCVlg/KCVtr (KCVlg and KCVtr are the impact toughness values of the longitudinal and transverse samples, respectively, determined at the same test temperature) depends on a number of factors. The tested steel samples differed considerably with respect to the content of sulfur (from 0.002 to 0.050 wt.%) and carbon (from 0.08 to 0.20 wt.%), as well as hardness (from 110 to 268 HV10 ). Based on the results of the correlation analysis, it was found [7] that the Ka values increase at the higher contents of sulfur (due to sulfide and oxysulfide formation) and carbon (due to pearlite formation), and decrease at the higher thicknesses of rolled products. The Ka values are practically independent of the ferrite grain size, structural banding, and hardness of ferrite-pearlite steel. CJSC “Melnikov Central Research and
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