Effect of Primary and Secondary Particles on Grain Sizein a Fe-20mass% Cr Alloy Deoxidised with Ti and Zr

摘要

Because of the high and volatile Ni price in recent times, it becomesmore important to develop ferritic stainless steels with low level of Ni. Here,it is known that these steel grades usually contain oversized grains, which asa consequence leads to poor mechanical properties. One way to deal withthis problem is to control the microstructure and the size of grains in ferriticstainless steels. This would also make ferritic stainless steels morecompetitive in comparison to the more commonly used austenitic stainlesssteels. This study focuses on the grain refining effect of particles present in aferritic stainless steel. The particles were created by additions of Ti and Zr into a liquid Fe-20mass% Cr alloy, before the start of solidification. Aconstant O content (150 ppm) together with varying N contents (65, 248 and490 ppm) in the metal samples were used to vary the number, compositionand location of the precipitated particles. The grain sizes and particles werestudied in as-cast samples as well as for specimens heat treated for 60minutes at 1200 and 1400°C. It was found that the formation of particles isenhanced by an increased N content in the alloy. Based on SEMdeterminations, the precipitated particles were divided as primary (mainlyZr-oxides and Zr-nitrides) and secondary (mainly Ti-nitrides) particles andthe effect was studied for each of the types. An increased content of primaryparticles as “nucleators” for precipitation of α-ferrite during solidification ofthe melt lead to an increased formation of equiaxed small-size grains. Inaddition, an increased N content in the metal samples resulted in anincreased number of secondary particles, which are located near the grainboundaries. Therefore, the pinning effect of these particles on grain growthincreased at a holding temperature of 1200oC. However, most of the nitrideswere found to dissolve during heating and holding at a 1400 °C temperature.Thus, as a consequence, the pinning effect of these particles on grain growthdecreased rapidly with the holding time.