Effect of carbon on hot ductility of as-cast low alloy steels.

Abstract

SynopsisThe effect of C on hot ductility of low alloy steels has been studied in view of surface cracking of continuously cast (CC) slabs. As the ductility was not affected by C content in hot tensile test of the reheated specimens, the well-known C dependency of surface cracking susceptibility in CC slabs can be ascribed to the microstructural change during the solidification process. Austenite grain size of as-cast materials was found to depend largely on C content, i.e., the maximum grain size in O.1ON0.15 % C region. This can be explained by the higher austenite formation temperature in these C region. Austenite grain growth rapidly occurred after the complete transformation or solidification into r phase, as the strong pinning effect of the second phase such as o-ferrite or liquid phase on r grain boundary migration was relieved. Carbon dependency of r grain size became more marked with increasing cooling rate up to that of ordinary continuous casting.Such coarsening of r grains enhanced intergranular fracture, resulting in ductility loss inversely proportional to the r grain size. Uneven surface solidification in the mold due to the peritectic reaction will produce much coarse r structure because of the local delay of cooling. Surface cracking susceptibility will also be largely accelerated by this mechanism. Carbon range where surface cracking susceptibility was the largest varied with the chemical compositions. This shift can be explained in terms of the effect o f alloying elements such as Mn on the peritectic composition.

Austenite Grain Growth in Peritectic Solidified Carbon Steels Analyzed by Phase-Field Simulation

“…[12] The temperature dependence of M(T) is introduced in Eq. [3] and is given as M(T) = M 0 exp(−Q/(RT)) with a constant M 0 as commonly done in the experimental work. [7] If a continuous cooling process is considered, Eq.…”

“…Since the size of CCG is closely related to the occurrence of surface cracking and low ductility of the slabs, [1][2][3][4] the precise description and prediction of the as-cast γ grain size are quite important issues in the field of casting of steels.…”

Transition of solidification mode and the as-cast γ grain structure in hyperperitectic carbon steels

“…The γ grains significantly grow after the temperature decreases to a temperature for completion of transformation to γ single phase, T γ . When T γ depending on the steel composition becomes lower, the grain growth becomes less significant [3]. In continuous casting processes of peritectic carbon steels with carbon concentrations around the peritectic point, the as-cast γ grain structure consists of coarse columnar γ grains (CCG) near the slab surfaces.…”

Experimental Verification of a Critical Condition for the Formation of As-Cast Coarse Columnar Austenite Grain Structure in a Hyperperitectic Carbon Steel

“…This is one of typical carbon steels which have a strong tendency to exhibit the CCG structure in casting processes [3,5] . The equilibrium liquidus, T L , the peritectic reaction temperature T p and T γ of the steel are 1788 (1515), 1762 (1489) and 1749 K (1476 o C) according to the thermodynamic calculation [17] .…”

“…Control of the as-cast austenite (γ) structure in casting processes of peritectic solidified carbon steels is of great importance because the size and morphology of the γ grain structure determine the quality of the as-cast steels [1][2][3][4] . The peritectic solidified carbon steels especially having a composition around the peritectic point have a strong tendency to undergo the formation of coarse columnar γ grain (hereafter abbreviated as CCG) structure in continuous casting processes.…”