RU2659542C2 - Super-strong high-manganese steel obtained by a combination of strengthening mechanisms - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of materials with ultrafine-grained (UFG) structure, namely, steels, that can be used in the automotive industry, nuclear power, in the development of microelectromechanical systems. Ultrafine-grained high-manganese steel has a yield strength of more than 2 GPa with a relative elongation of at least 5 %. Steel contains, as stabilizers of austenite, carbon in an amount of more than 0.5 % by weight, manganese is more than 15 % by weight and aluminum is not more than 2 % by weight, the rest is iron, while it has a structure consisting of equiaxial austenite grains smaller than 200 nm with predominantly large-angle boundaries, at that in the body of grains there are nano-twins up to 15 nm in thickness, and grain boundary segregations of atoms (C, Mn) are present at the grain boundaries.

EFFECT: ultrafine-grained high-manganese steel has increased strength properties due to the combination of hardening mechanisms.

1 cl, 5 dwg, 1 tbl

Description

The invention relates to the field of materials with ultrafine-grained (UFG) structure and increased mechanical properties, specifically to steels that can be used in many industries, in particular automotive, nuclear energy, in the development of microelectromechanical systems.

It is known that in determining the properties of a particular material, such as strength, ductility, fatigue, corrosion resistance, the microstructure plays a key role, which, depending on the processing method, can have different phase composition, grain size and shape, disorientation of their boundaries, dislocation density and other defects of the crystal lattice, etc. [Stremel M.A. The strength of alloys. M.: Metallurgy, 1982. Part 1: Defects of the lattice. 280 s .; Shtremel M.A. The strength of alloys. 4.2. Deformation. M., MISiS, 1997, 527 pp.]. The formation of ultrafine-grained structures containing predominantly high-angle boundaries makes it possible to achieve a unique combination of strength, ductility, and fatigue life in metals and alloys [R.Z. Valiev, I.V. Alexandrov. Volumetric nanostructured metallic materials. - M.: IKC "Akademkniga", 2007. - 398 p.].

Articles are known in which the results of studies of the structure of TWIP steel in samples subjected to intense plastic deformation by torsion under high pressure are published. So, in the article “Mariana S. Matoso, Roberto B. Figueiredo, Megumi Kawasaki, Dagoberto B. Santosa and Terence G. Langdond. Processing a twinning-induced plasticity steel by high-pressure torsion // Scripta Materialia 67 (2012) 649-652 ”it is shown that the structure is characterized by pronounced twinning in the early stages of deformation, martensite is present. In the article “H.N. An, Q.Y. Lin, G. Sha, M.X. Huang, S.P. Ringer, Y.T, Zhu, X.Z. Liao. Microstructural evolution and phase transformation in twinning-induced plasticity steel induced by high-pressure torsion // Acta Materialia 109 (2016) 300e313 ”shows the microstructural evolution and grinding of steel grains during deformation.

Known ultrafine-grained two-phase steel (CN 102618802, IPC C21D 1/26, C21D 8/02, published 01.08.2012), having a chemical composition in mass percent: (13.5 ~ 14.5)% Cr, (6, 1 ~ 6.9)% Ni, (2.3 ~ 2.7)% Mn, (0.33 ~ 0.37)% Si, (0.60 ~ 0.90)% copper, (0.01- 0.03)% C, (0.021 ~ 0.025)% B, (0.60 ~ 0.90)% Mo, P <0.02%, S <0.04%, the rest is Fe, with an ultrafine-grained microstructure consisting of the grains distributed in a random orientation, the grain diameter is 500 ~ 2000 nm, the microstructure of each grain is characterized by the presence of austenite and martensite, the yield strength at room temperature is 1100 ~ 1600 MPa, the tensile strength 1200 ~ 1850 MPa, elongated e from 10 to 20%.

In the known analogues, high steel strengths are not achieved.

The objective of the invention is the development of ultrafine-grained high-manganese steel with high strength properties due to the combination of hardening mechanisms.

The technical result is an increase in strength (compared with coarse-grained analogues and steels obtained by standard heat treatments) with a yield strength of more than 2 GPa and a relative elongation of at least 5%.

The problem is solved in that ultrafine-grained high-manganese steel, having a yield strength of more than 2 GPa with a relative elongation of at least 5%, contains as stabilizers austenite carbon in an amount of more than 0.5 wt.% And manganese more than 15 wt.%, Aluminum no more 2 wt.%, The rest is iron, while it has a structure consisting of equiaxed austenitic grains less than 200 nm in size, with predominantly higher-angle misorientations of the boundaries, with nanodoubles up to 15 nanometers thick in the body of grains, and ranitsah grains are present grain boundary segregation atoms (C, Mn).

The technical result is achieved due to the following.

The increase in steel strength is due, firstly, to a very small grain size (less than 200 nm) in the structure of the material, which ensures an increase in flow stress during plastic deformation according to the well-known Hall-Petch relation [Large plastic deformation and fracture of metals. Rybin V.V. M .: Metallurgy, 1986, 224 p.]. A significant increase in strength is also achieved by the fact that it is the large-angle grain boundaries in comparison with small-angle and special boundaries that provide the greatest contribution to hardening [R.Z. Valiev, I.V. Alexandrov. Volumetric nanostructured metallic materials. - M.: IKC "Akademkniga", 2007. - 398 p.]. The resulting steel nanostructure provides a high level of strength properties also due to the presence of nanodoubles up to 15 nanometers thick and grain-boundary atomic segregation (C, Mn) at grain boundaries. This is due to the fact that additional twin boundaries, as well as the presence of inhomogeneous segregation at the grain boundaries, are an obstacle to the movement of dislocations, therefore, additional stress is necessary for the generation and movement of dislocations, which increases the yield strength and, accordingly, strength.

The proposed combination of hardening mechanisms, namely, grinding grain to sizes less than 200 nm, the presence of nanodoubles and grain-boundary segregation of impurity atoms in the structure provides increased mechanical strength of ultrafine-grained high-manganese steel.

The structural changes of the material described above during processing are achieved by the features of steel production at the indicated temperature and speed conditions.

The invention is illustrated by the image of the microstructure of the inventive heavy-duty high-manganese steel after intense plastic deformation by torsion at 300 ° C, where:

in FIG. 1, 2 - bright-field image of the microstructure of steel, it is shown that the grain size is less than 200 nm, the grains are equiaxed, the structure is uniform;

in FIG. 2, in addition, the presence of twins in the structure is shown;

in FIG. 3 - a dark-field image of the microstructure of steel, a grain size of less than 200 nm is shown, the grains are equiaxed, the structure is uniform;

in FIG. 4, the dark-field image shows the presence of twins in the structure;

in FIG. Figure 5 shows the image obtained by atomic probe tomography, which is a 3D reconstruction of the distribution of atoms in ultrafine-grained high manganese steel, grain-boundary segregations of impurity atoms (C, Mn) are seen, marked by numbers # 1-4, which have a more saturated color.

Ultrafine-grained high manganese steel is prepared as follows.

A disk made of high manganese steel 0.6С-18Mn-2А1 with a diameter of 10 mm and a thickness of 2.5 mm is used as a blank. Carry out the treatment of intense plastic deformation by torsion on Bridgman strikers at a temperature of 300 ° C, hydrostatic pressure of 6 GPa, at a speed of 0.2 rpm, the total degree of deformation e = 6.5. The deformation is carried out first in a groove 0.9 mm deep in the amount of 9 revolutions, with the last (tenth) revolution, a 0.6 mm groove is used. The characteristics of the obtained steel and structural features are shown in the table.

The table shows that the steel obtained has improved strength characteristics while maintaining sufficient ductility. As a result of the formation of an ultrafine-grained structure with an austenitic grain size of 45 nm, twins with a thickness of 3 nm were observed inside the grains, and the grain boundaries were decorated with carbon and manganese segregations (see photo). As a result of the formation of such a structure due to the combination of several reinforcing mechanisms, the tensile strength increased to 2120 MPa.

Thus, the developed ultrafine-grained high-manganese steel has increased strength properties due to the combination of hardening mechanisms.

Claims (1)

Ultrafine-grained high-manganese steel with a yield strength of more than 2 GPa with a relative elongation of at least 5%, characterized in that it contains more than 0.5 wt.% Carbon, manganese more than 15 wt.% And aluminum no more than 2 as austenite stabilizers wt.%, the rest is iron, while it has a structure consisting of equiaxed austenitic grains smaller than 200 nm in size with predominantly higher-angle misorientations of the boundaries, with nanodoubles up to 15 nm thick present in the body of the grains, and at the grain boundaries tvuyut grain boundary segregation of the atoms (C, Mn).

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