Sheet metal with variable mechanical properties over its thickness

The influence of one-sided accelerated cooling of A32 plate shipbuilding steel with thickness of 10·10–3 m on structure and mechanical properties was investigated. As a result of such cooling, continuous spectrum of microstructures from ferrite-bainite on the rapidly cooled surface to ferrite-perlite on the opposite surface is formed along the billet thickness. Therefore, over the billet thickness strength properties are reduced from rapidly cooled surface to the opposite one. Thus, the gradient of strength characteristics (hardness, yield strength and rupture strength) along the billet thickness is directed to rapidly cooled surface. For comparative analysis, other batches of billets were subjected to normalization and hardening with high tempering. The analysis of mechanical properties has shown that strength and plastic properties of the samples at unilateral accelerated cooling are at level of heat-strengthened state. Testing on impact strength of the samples with variable distribution of mechanical properties over their thickness has shown that the impact depends on correlation of gradient directions of strength properties and load application. In impact bending test at the temperature of –40 °C, if the direction of load application is opposite to gradient of strength properties, the impact work was more than 300 J (the sample did not collapse). At coincidence of directions of the gradient deformation resistance and load, energy of the blow was 262 J. Thus, if the direction of deformation resistance gradient coincides with the direction of external applied load, then it leads to an increase in plasticity of steel. It is shown that, knowing distribution of strength characteristics over the sample thickness, it is possible to calculate integral values of yield strength and rupture strength of the sample. Value of relative through-thickness elongation increases from the rapidly cooled surface to the opposite one. Integral elongation of the billet is less than the smallest relative through-thickness elongation. With changing thickness strength of the billet during bending, displacement of the neutral deformation line relative to the geometrically average line in the direction of the strength properties gradient is inevitable. The position of neutral line of deformation during bending is proposed to be determined by the value of experimental integral yield strength (rupture strength).