Thick plates have a wide range of applications. They can form both crosssections of load-bearing structural elements and dense, strong shells used for the
storage, handling, transportation, and processing of liquids, gases, and bulk materials.
In construction, thick plate rolled metal is primarily used in the production of welded
structural frames for buildings and structures. It is worth noting that the current
regulatory documentation requires the use of rolled metal of uniform thickness across
the cross-section of structural elements in the design of welded metal structures.
However, an analysis of the distribution of external loads across the cross-section of
structures shows that in some cases, plates of varying thickness should be used in the
production of structures.
Low-carbon microalloyed steel 10G2FB was selected as the material for the
study, with thicknesses of 16, 20, 30, 40, 50, 70, and 100 mm.
In accordance with the set objective, the study investigated properties that
characterize the kinetics of material failure, specifically the material's ability to resist
crack initiation and propagation. The experimental methods included static mechanical
testing (tensile testing) and dynamic mechanical testing (impact bending testing).
The study of the morphology of structural components and fracture surfaces was
conducted in stages:
1. First Stage: Examination of samples at low magnification (metallographic
analysis).
2. Second Stage: Examination of samples at medium and relatively high
magnification (scanning electron microscopy).
3. Third Stage: Examination of fracture surfaces (scanning electron microscopy).
Microstructural analysis revealed that the microstructural components of all
studied systems are ferrite and pearlite. The 16 mm thick steel exhibits a ferrite-pearlite
structure in a 70%-30% ratio, respectively. As the thickness increases, the percentage
of ferrite increases while the percentage of pearlite decreases. For the 100 mm thick
steel, the microstructure consists of 80% ferrite and 20% pearlite. Ferrite forms as
grains with a polyhedral shape, while pearlite colonies are located in segregation bands.
Analysis of the fine structure showed that for all thicknesses of rolled metal, the
grains of the ferritic phase have a regular polyhedral shape. Nucleation of new phases
most frequently occurs on the surface of austenite grains and inclusions. Subsequently,
cementite and ferrite grow gradually as roughly equiaxed formations. At a certain
point, cementite nuclei form along grain boundaries and grow as plates into the grain
interior. This process results in alternating recrystallization of cementite and ferrite,
which propagates along the boundaries. Pearlite colonies appear and continue to grow
until they come into contact with each other.
The dissertation demonstrates that as the thickness of the rolled metal increases,
there is a change in the shape of the cementite framework within pearlite colonies from
spherical to fan-like. This phenomenon is explained by corresponding changes in
cooling conditions with increasing thickness.
Based on the quantitative data obtained on the dependence of the percentage of
structural components on the thickness of the rolled metal, regression models were
developed. Analysis of these models shows that the observed dependencies are
nonlinear and can be described by logarithmic equations of the form Y = b0 +
b1хlog10(x).
Fractographic analysis of fracture surfaces revealed that for the 16 mm thickness,
fracture occurs due to shear failure, characterized by smooth surfaces and steps—signs
indicative of a quasi-cleavage mechanism. According to metallographic analysis, the
structure of the rolled metal of this thickness contains ferrite and pearlite. Therefore, it
can be assumed that the brittle fracture characteristics are associated with pearlite
colonies. At the initial stage of facet formation, quasi-cleavage shows signs of
predominantly crystalline separation, while in areas of microcrack coalescence, signs
of ductile fracture are observed.
