As-cast Austenite Grain Research Articles

Microstructural Features and Formation Processes of As-cast Austenite Grain Structures in Hypoperitectic Carbon Steels

Microstructural Features and Formation Processes of As-cast Austenite Grain Structures in Hypoperitectic Carbon Steels

As-cast Austenite Grain Structure in Al Added 0.2 wt% Carbon Steel

Effects of Al addition on as-cast γ-austenite grain structure in 0.2wt%C–0.035wt%P steel with Al concentration ranging from 0.04 to 1.04 wt% were studied by means of furnace cooling and casting experiments. In the furnace cooling experiment with a cooling rate of 0.03°C/s, the as-cast γ grain structure consisted of equiaxed grains and the γ grain size was not affected by the increase in Al concentration up to 0.54 wt%. In the casting experiment of the sample with 0.04 wt% Al, on the other hand, the as-cast γ grain structure consisted of Coarse Columnar Grain (CCG), Fine Columnar Grain (FCG) and Equiaxed Grain (EG) regions, sequentially, from the mold side to the center of the ingot. The increase in Al concentration leads to increase in the fraction of FCG region at the expense of both CCG and EG regions. Even in the samples with high Al concentrations, AlN particles were rarely found and also Al segregation did not occur substantially. Instead, P segregated in interdendritic regions. The concentration of the segregated P increased from the CCG region to the FCG region. It was suggested based on a thermodynamic calculation that the segregation of P is enhanced by Al addition and the high P concentration stabilizes the high temperature phase such as liquid or δ-ferrite, depending on Al concentration at lower temperatures. This stabilized high temperature phase is considered to retard the γ grain boundary migration. Therefore, the increase of FCG region and the decreases of CCG and EG regions due to Al addition should be attributable to pinning effect of the stabilized high temperature phase.

Refinement of As-cast Austenite Grain Structure in Hyperperitectic Carbon Steel Based on Stabilization of High Temperature Phase during Peritectic Transformation

Refinement of As-cast Austenite Grain Structure in Hyperperitectic Carbon Steel Based on Stabilization of High Temperature Phase during Peritectic Transformation

Effects of Al and P Additions on As-cast Austenite Grain Structure in 0.2 mass% Carbon Steel

Effects of addition of P and simultaneous additions of Al and P on as-cast γ grain structures of 0.2 mass% C steel have been investigated by means of permanent mold casting. The as-cast γ grain structure consists of Coarse Columnar Grain (CCG), Fine Columnar Grain (FCG) and Coarse Equiaxed Grain (CEG) regions from the mold side to center of the ingot. The single addition of P increases the FCG region in which short axis diameter of the columnar γ grain is comparable to the primary dendrite arm spacing. The simultaneous additions of Al and P also lead to refinement of the structure and, importantly, the complete refinement of the as-cast γ grain structure, viz., the structure without CCG and CEG regions was obtained even with a small amount of P addition when Al was added. The EPMA analysis showed that the refinement is associated with P segregation at interdendritic regions which is enhanced by Al addition. From thermodynamic calculation, it was demonstrated that high P concentrations stabilize δ and liquid phases at lower temperatures and produce the pinning effect on the growth of γ grains at interdendritic regions.

Effects of Ti Addition on As-cast γ Grain Structure in Hyper-peritectic Carbon Steel

Effects of titanium addition on as-cast austenite grain size in 0.2 mass% carbon steel cooled at 0.03°C/s has been studied. The average austenite grain size was reduced from 4.3 to 1.9 mm by increasing the titanium concentration from 0 to 0.13 mass%. Two different shapes of Ti(C, N) particles were observed in the solidified samples; one was a facet shape which were dispersed over the whole observation area and the other was a filmy shape, most of which were located on the austenite grain boundary. Thermodynamic calculations and EPMA analyses showed that the facet shaped Ti(C, N) particles crystallize at relatively high temperatures, while the filmy shaped ones form from the last solidifying liquid in the inter-dendritic regions at lower temperatures. The latter Ti(C, N) is considered to provide the grain refinement effect of austenite grain structure. The grain refinement effects of titanium are further discussed by comparing the results of 0.2 mass% carbon steel with those of 0.45 mass % one, which supports the substantial effect of the filmy shaped Ti(C, N) on the as-cast austenite grain structure.

Refinement of As-cast Austenite Grain in Carbon Steel by Addition of Titanium

Effects of Ti addition on as-cast austenite structure of S45C steel have been investigated by means of furnace cooling experiment at a cooling rate of 0.03°C/s, focusing on the Ti addition ranging from 0 to 0.5 mol%. The Ti addition reduces average austenite grain diameter down to a size comparable to secondary dendrite arm spacing. In samples with the Ti addition, the austenite grain boundary is located at inter-dendritic position where Ti(C,N) particles exist and the refinement of austenite grain structure is ascribable to pinning effect of the Ti(C,N) particle formed in L+γ-Fe+Ti(C,N) phase field. The increment of Ti addition does not substantially change the size of Ti(C,N) particle but increases the number of the Ti(C,N) particles, leading to further refinement of the austenite grains.

チタンの添加による炭素鋼の鋳造オーステナイト結晶粒の微細化

Effects of Ti addition on as-cast austenite grain size in S45C carbon steel have been investigated in range of Ti addition between 0 and 0.5 mol%. The cooling rate was set to be 0.03°C/s. It is shown that the average austenite grain diameter decreases from 1900 to 250 μm as the Ti addition increases from 0 to 0.5 mol%. The microstructural observation revealed that the motion of austenite grain boundary is inhibited at inter-dendritic position, which is ascribable to pinning effect of Ti(C,N) particle. The CALPHAD analysis showed that this particle crystallizes mainly in L+γ−Fe+Ti(C,N) phase field. As increasing Ti addition, the size of Ti(C,N) particle does not substantially change, however, the number of the Ti(C,N) particles increases, leading to further refinement of the austenite grains.

炭素鋼の柱状オーステナイト結晶粒に及ぼすチタンとホウ素添加の影響

We performed casting experiment of S45C steel, focusing on the effect of addition of Ti and B on as-cast austenite grain structure. Without the addition of these elements, the austenite structure consists of columnar grains over the whole sample in the present casting conditions. The addition of Ti and B leads to the formation of equiaxed austenite grains and, importantly, fully equiaxed austenite structure was observed in the sample with 0.2 mol% Ti and 0.4 mol % B. The microstructural observations indicated that this behavior of austenite structure stems from the columnar-to-equiaxed transition of ferrite dendrite structure. The addition of these elements, furthermore, leads to refinement of the columnar austenite grains.

Austenite Grain Refining of As-cast Bloom Surface by Reduction of Oscillation Mark Depth

Austenite grain refining in the surface layer of as-cast bloom is effective for the reduction of surface cracks in steel production. This study was carried out to clarify the influence of cooling rate on the as-cast austenite grain size and its growth mechanism. Depth and interval of oscillation marks were reduced and the austenite grains at the bloom surface were refined under the oscillation conditions of shorter stroke and higher frequency. The average cooling rates were estimated to be from 6 to 16 K/s in austenite phase temperature and the rates were related to characteristics of oscillation mark. It was found that the austenite grain growth direction did not vary even when characteristics of oscillation mark changed. The austenite grain size below the surface layer was also determined by austenite grain size at the bloom surface.

ニアネットシェイプCCにおける鋳造γ粒径の予測

ニアネットシェイプCCにおける鋳造γ粒径の予測