Tetsu-to-Hagane
Online ISSN : 1883-2954
Print ISSN : 0021-1575
ISSN-L : 0021-1575
The Effect of Temperature, Strain Rate, and Carbon Content on Hot Deformation of Carbon Steels
Taku SAKAIMasayuki OHASHI
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1981 Volume 67 Issue 11 Pages 2000-2009

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Abstract

High temperature tensile deformation of carbon steels containing from 0.036 to 1.09 wt% C was studied in the temperature range 873 to 1 373 K over a wide range of strain rates between 1 and 10-5 s-1. The shape of true stress-true strain (σ-ε) curves in the austenite (γ) range is expressed solely in terms of the first stress peak (σp) or Z in the following equation, and the relation is almost independent of C content. σp can be correlated with temperature (T) and strain rate (ε) by the following equation in the range of stresses below 110420 MPa;
Z=ε·exp(Q/RT)=A·σmp
in which A, m, and Q decrease with C content. The activation energies for deformation (Q) are nearly the same as those for self-diffusion. These results being almost the same as those of 0.16% C steel reported previously8)9), it is concluded that the high temperature deformation of carbon steels in the γ range is controlled by the dynamic recrystallization process assisted by the diffusion of vacancy.
σp in the γ range decreases with C content in the whole range of Z used. This solid solution softening is considered to be attributed to the enhanced dynamic recrystallization process caused by increased diffusivity of vacancy due to the addition of C in the γ range.
The flow stress maximum (α; ferrite) or the first stress peak (γ), σp, of mild steel (or iron) changes discontinuously near the A3 point, and σp at a Z is always larger in the γ range than in the α range. This is considered to be attributed to the differences of vacancy diffusivity and of the dynamic restoration process (i.e., recrystallization in the γ and recovery in the α) which may be caused by the difference in the stacking fault energy.
The flow stress in the initial work hardening region (σε) is larger in the γ range than in the α range in the lower Z, but σε in the both ranges can be expected to become equal with an increase in Z. This is caused by the difference of strain hardening behavior in the both ranges and the lower strain rate (or Z) dependence of σε in the γ range.

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© The Iron and Steel Institute of Japan
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