Effect of the crystallographic texture on the anisotropy of fracture characteristics of control-rolled low-alloy steel

Abstract

C; 1.58 Mn; 0.31 Si; 0.13 Cr; 0.05 Ni; 0.06 V; 0.02 A1; 0.02 P; 0.009 S; 0.05 Cu; 0.04 Nb; 0.03 wt.% Mo), produced by control rolling technology was examined in relation to the direction of propagation of the main crack in impact bend tests. The microstructure of the examined rolled stock consists of ferrite-pearlite bands with the ferrite grain size 8-10, no more than 0.01 vol. % of nonmetallic inclusions, and no more than 20% pearlite. The variation of the trajectory of the main crack was reproduced by cutting out impact bend tests specimens in different directions in the rolling plane (RP) - from the rolling direction (RD) to the transverse direction (TD). The notch (V-Sharpy) was made perpendicular to the RP (Fig. la) and in RP (Fig. 2a). The tests were carried out in the temperature range 20 ...-196°C. A DRON-3 diffractometer was used for layer-by-layer x ray examination by the Schultz method [5] in every 0.0625 h (h = 20 mm is sheet thickness) from the surface to the center of the sheet in the direction of the thickness in radiation of K s of molybdenum. The required depth was obtained by chemical etching. We constructed straight pole figures (PF) { 110} and {200} and determine the pole density Plakl in the direction normal to the RP and in the directions coinciding with the directions in which the specimens were cut out. These data were used to construct inverse pole figures. To examine the effect of plastic deformation, formed during impact bend testing on the texture formed in the surface layer of the fracture surface we took inverse pole figures of these surfaces. A reference specimen was in the form of a textureless specimen of fine shavings of the examined steel after vacuum annealing for 1 hour at 700°C. Characteristics of the fracture surfaces were examined on optical macro- and electron microfractographs of the fracture surfaces. The distribution of the pole densities of the crystallographic directions coinciding with the directions in which the specimens were cut out is not equally probable, according to inverse pole figures (Fig. lb). Crystallographic directions of the type {110} coincide in most cases with the RD. In the direction displaced in relation to the RD by 45 ° there are mainly crystallographic directions of the type { 100), and the TD is characterized by a scatter of the directions from (110) to (111). The results of layer-by-layer examination of the texture in the rolling plane (Fig. 2a) indicate that the type of texture over the entire surface of the sheet in the sections parallel to the RP is almost identical and can be described by the main orientation {001 } (110) and additional orientations { 112} { 110) and { 111} ( 110-112). At the same time, the pole density changes in the thickness of the sheet. There is a general tendency for an increase of the intensity of the main orientation on the surface at the center of the thickness. The minimum intensity of the main textural component {001} {110) occurs at a distance of (1/16)h from the surface and equals -4 in relation to the intensity of the textureless specimen. The maximum density of the given main texture component is recorded in the layer situated at the distance (3/8)h from the surface of the sheet and equals -6 in relation to the intensity of the textureless specimen. In the middle section, the intensity of the component {001} (110) is slightly smaller and reaches - 5 in fractions of the intensity of the textureless specimen. The maxima of the intensity of the remaining texture components are detected in the middle section of the sheet and equal to -3.0 and 2.8 for, respectively, the