Effect of Excess Phase Precipitation on Strengthening of Structural Steels Prepared by Hot Stamping
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EFFECT OF EXCESS PHASE PRECIPITATION ON STRENGTHENING OF STRUCTURAL STEELS PREPARED BY HOT STAMPING A. V. Koldaev,1 F. V. Arifulov,2 A. I. Zaitsev,3 N. A. Arutyunyan,4 and N. M. Alexandrova5
UDC 669.14.018.298
The contribution of nano-sized precipitates to hardening metal products prepared by hot stamping from microalloyed niobium and titanium steels is evaluated. In order to calculate the characteristics of carbide precipitates (quantity, size), kinetics are modeled for their precipitation from austenite. It is shown that the increase in yield strength due to microalloying steel with niobium is greater than with titanium, and it increases with increase in their content. With a concentration of 0.1% each of Nb and Ti, the contribution to hardening steel with 0.3% C reaches 186 and 129 MPa, respectively. Keywords: hot stamping, excess phase precipitation kinetics, phase precipitation, modeling, precipitation hardening, niobium carbide, titanium carbide.
An increase in the demand for operating efficiency and safety of new engineering objects in transport, machine building, and construction with a reduction in production expenditure stimulates introduction of new technology for preparing ultra-high strength steel assemblies, components, and structures. In view of this use of hot stamping is currently expanding. This nonisothermal method for forming metal sheet with which treatment of a workpiece after austenitizing is subjected to hot stamping combined with hardening. A metal workpiece has a ferrite-pearlite structure with quite low strength properties. After hot stamping the metal of a finished component has a martensitic structure and its strength increases by several factors to values exceeding 2000 MPa [1, 2]. In order to prepare a high-strength martensitic structure as a result of hot stamping an important condition is adequate steel hardenability, which as a rule is achieved due to the alloy system. As alloying elements apart from manganese, nickel, chromium, and molybdenum, in order to improve hardenability boron is used whose effect develops with addition of hundredths and thousandths of a per cent [3]. In spite of significant development of high-stamping technology, starting from the instant of use of this method and up to the present time, the most extensive steels used are alloyed with manganese and boron grades 20MnB5, 22MnB5, 28MnB5, 33MnB5, 37MnB4, and 38MnB5. The mechanism of improving hardenability has been developed in the case when boron is in solid solution and segregates over austenite grain boundaries, although at austenitizing temperatures there is formation of boron nitride (BN) or carboboride of complex composition. Microalloying with niobium and titanium is used in order to prevent formation of these unfavorable phases [3–5]. In this case it is 1 2 3 4 5
FGUP I. P. Bardin TsNIIchermet, Moscow, Russia; e-mail: [email protected].
FGUP I. P. Bardin TsNIIchermet, Moscow, Russia; e-mail: [email protected].
FGUP I. P. Bardin TsNIIchermet, Moscow, Russia, M. V. Lomonosov Moscow State
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