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).

1. Maksimov A.B. Sposob uprochneniya listovogo prokata [The method of sheet metal hardening]. Patent Ua no. 75518. Byulleten̕ izobretenii. 2006, no. 4. (In Russ.).

2. Maximov A.B., Gadeyev A.V. Thermally reinforced steel flat pro- duct. International Robotics & Automation Journal. 2018, vol. 5, no. 4, pp. 343–345.

3. Maksimov A.B. Features of heterogeneous material destruction. Novye materialy i tekhnologii v metallurgii i mashinostroenii. 2012, no. 2, pp. 130–131. (In Russ.).

4. Maksimov A.B., Gulyaev M.V. Surface hardening of steels for mining equipment. Aktual’nye problemy v mashinostroenii. 2015, no. 2, pp. 370–375. (In Russ.).

5. Gol’dshtein M.I., Grachev S.V., Veksler Yu.G. Spetsial’nye stali [Special steels]. Мoscow: Metallurgiya, 1985, 408 p. (In Russ.).

6. Feodos’ev V.I. Soprotivlenie materialov. Uchebnik dlya vuzov [Strength of materials. Textbook for universities]. Мoscow: МGТU, 2000, 592 p. (In Russ.).

7. Danchenko V.N. Metal forming. Dnepropetrovsk: NMetAU, 2007, 183 p.

8. Gasko M., Rosenberg G. Correlation between hardness and tensile properties in ultra – high strength dual phase steel – short communication. Materials Engineering. 2011, no. 18, pp. 155–159.

9. Nobuhito Fujeta, Kazukisa Kusumi, Tochmasa Tomokijo. Present situation and future trend of ultra – high strength steel sheets of auto –body. Nippon Steel technical reports. 2013, no. 103, pp. 99–103.

10. Douthwaite R.M. Relationship between the hardness, flow stress, and grain size of metals. Journal of the Iron and Steel Institute. March, 1970, pp. 265–269.

11. Farrell K., Loh B.T.L. Hardness-flow stress-grain-size relationships in iron. Journal of the Iron and Steel Institute. November, 1971, pp. 915–917.

12. Chukin M.V., Poletskov P.P., Gushchina M.S., Berezhnaya G.A. Determination of mechanical properties of high-strength and super- strength steels by hardness. Proizvodstvo prokata. 2016, no. 12, pp. 37–42. (In Russ.).

13. Fridman A.V. Mekhanicheskie svoistva metallov. Ch. 1: Deformatsiya i razrushenie [Mechanical properties of metals. Part 1. Deformation and destruction]. Мoscow: Mashinostroenie, 1972, 472 p. (In Russ.).

14. Atsushi Monden, Kei Miyanishi, Shingo Yamasaki Tatsuro Ochi. Development of middle – carbon steel bars and wire rods for cold forging. Nippon Steel Technical Reports. 2013, no. 103, pp. 127–132.

15. Liu D.L., Huo X.D., Wang Y.L., Sun X.W. Aspects of microstruc- ture in low carbon steels produced by the CSP process. J. Univ. Sci. Technol. B. 2003, vol. 4, no. 10, pp. 1–6.

16. Potapova L.B. Mekhanika materialov pri slozhnom napryazhennom sostoyanii. Kak prognoziruyut predel’nye napryazheniya [Mechanics of materials in complex stress state. How to predict limit stress- es]. Мoscow: Mashinostroenie-1, 2005, 244 p. (In Russ.).

17. Finkel’ V.M. Fizicheskie osnovy tormozheniya razrusheniya [Physical basis of the destruction braking]. Мoscow: Metallurgiya, 1977, 60 p. (In Russ.).

18. Knott J.F. Fundamentals of fracture mechanics. London: Butter-worths, 1973. (Russ.ed.: Knott J.F. Osnovy mekhaniki razrusheniya. Мoscow: Metallurgiya, 1978, 256 p.).

19. Fikhtengol’ts G.M. Kurs differentsial’nogo i integral’nogo ischisleniya. T. 2 [Course of differential and integral calculus. Vol. 2]. St. Petersburg: Lan̕, 2018, 612 p. (In Russ.).

20. Wang Su-Fen, Peng-Yan, Li Zhi-Jie. Work – hardening and deformation mechanism of cold rolled low carbon steel. Research Journal of Applied Sciences, Engineering and Technology. 2013, vol. 3, no. 5, pp. 823–828.

21. ASM Handbook. Vol. 1: Properties and Selection: Irons, Steels and High­Performance Alloys. ASM Handbook Committee, pp. 673–688.

22. Vasil’ev G.G., Elagina T.V. On feasibility of considering ratio of yield strength to temporal resistance when choosing pipes for cons. Truboprovodnyi transport. Teoriya i praktika. 2013, no. 5 (39), pp. 34–38. (In Russ.).