The tensile strength in the cured state of molding and core sands is one of the important characteristics of molding materials. However, there is a significant spread of values at determining the tensile strength, which indicates the deficiencies of the existing methodology. The standard sample for tensile strength is a "figure-of-eight" shape. When it is tested for tension, the stresses in the holdsite points under central loading are 2-2.5 times greater than in the neck; therefore 8-12 % of the samples are destroyed in the holdsite points, and not along the neck. In this article, it is proposed to change the design of the standard "figure-of-eight" sample -to make a recess on one side of the neck, while creating an eccentric loading of the sample. The conducted studies and modeling show that using eccentric loading of the sample, due to the manufacture of a one-sided recess in the neck of the sample with a depth of 2-3 mm and a radius of at least 4 mm, the spread of strength indicators on the samples is 2-3 times less than on the standard one, and therefore the validity of the indicators is much higher.

This paper presents an approach to predicting Continuous Cooling Transformation (CCT) diagrams of low-carbon steels using a mathematical model based on regression and classification. The method of digitizing CCT diagrams and its application taken from atlases and current scientific articles are given. The digitization method is based on reading the color value from the CCT diagram image and then converting the coordinates of the color position in accordance with the scale of the CCT diagram axes. It was assumed that CCT diagram consists of zones defined by the beginning and end of ferrite transformation, the beginning of pearlite transformation, the end of bainite transformation, and the beginning and end of martensite transformation. When developing a predictive mathematical model, an optimization algorithm was used to find a model with the best hyperparameters among classical machine learning models (k-Nearest Neighbors, Support Vector Machine, Linear/Logistic Regression) and based on decision trees (LightGBM, CatBoost). The model solves regression (temperature prediction) and classification (binary mask prediction) problems. The superimposition of a binary mask on the temperature vector made it possible to constrain the resulting phase transformation curve along the time axis. To build test CCT diagrams, a number of dilatometric studies of four steel grades were carried out. The new predictive approach made it possible to achieve satisfactory values of metrics on test CCT diagrams. The average absolute error did not exceed 20°C; the coefficient of determination was in the range of 0.55-0.86, but for the martensite transformation it took negative values, which can be explained by the initial approximation of the transformation by a polygonal chain; ROC AUC metric was at least 0.80.

The paper presents new high-strength steels for the components of heavy carrying and lifting machines, which operates in the Far North and Arctic areas. Combination of the main parameters of the developing steels leaves behind the existing global analogues and demonstrates high strength, ductility as well as cold resistance at the temperature down to -70 °С: tensile strength not less than 1,200 MPa, yield strength not less than 950 MPa, relative elongation not less than 10 %, hardness not less than 350 HBW, impact strength KСV -70 not less than 30 J/cm 2 . 4 steel chemical compositions on the base of С-Mn-Mo alloying system with additives of nickel and copper as well as microalloying elements, which were used together or separately, are examined in this research. Thermokinetic diagrams of decomposition of overcooled austenite were built and temperature-temporal conditions of forming of bainitemartensite structure were determined; these conditions are related for the new high-strength cold-resistant steels containing 0.17-0.21 % C; 0.70-1.30 % Mn; 0.25-0.35 % Si and 0.28-2.00 % (Ni+Cu+Mo). Influence of varying chemical composition on phase transformations and structure during continuous cooling of the examined steels was established and the heat treatment procedure for achievement of the required construction strength (combination of high tensile strength and low temperature impact strength) was suggested. As a result, it was determined that the most wide range of cooling rate values for obtaining of bainite-martensite structure (not less than 10 °С/с) and the optimal complex of strength and toughness-ductile properties are provided in the steel with the following chemical composition (mass. %): 0.21 С; 0.30 Si; 0.73 Mn; 0.017 Cr; 2.00 (Ni+Cu+Mo); 0.027 Ti; 0.003 B after quenching from 860 °С.