Papers by Keyword: Grain Refinement Strengthening

Abstract: Grain refinement is attracting attention as a strengthening method which does not depend on the alloying elements added to steels. Many reports have described the manufacturing methods and mechanical properties of ultra-fine grained steels. In ultra-fine grained steels, it is well known that yielding stress drastically increases in accordance with the Hall-Petch relationship, while uniform elongation significantly decreases. These tendencies imply that grain size affects not only yielding but also work-hardening behavior. However, the influence of grain size on work-hardening behavior has not been clearly understood. Therefore, in this study, we investigated the work-hardening behavior during tensile deformation of 12Cr stainless steel with various grain sizes. Grain refining was conducted by cold-rolling of annealed and quenched steel specimens, followed by recrystallization annealing. The grain size of the specimens decreased as the cold-rolling reduction rate increased. The minimum grain size obtained by this method was approximately 5 μm. With decreasing grain size, 0.2% proof stress increased and the strain which reached the plastic instability condition decreased. In the session, we report the dislocation accumulation behavior estimated by grain hardness and XRD and the dynamic recovery behavior assessed by the Kocks-Mecking model.

Abstract: In ferritic stainless steels, the amount of Cr is moderately controlled to have good corrosion resistance in applied environment. However, it also affects the yield strength of ferritic stainless steels through solid solution strengthening and grain refinement strengthening. Until now, some researches have been performed using commercial stainless steels but the obtained results contain the effect of solute interstitials (C and N). In this paper, the influence of Cr on the above both strengthening mechanism was discussed by using interstitial free ferritic stainless steel in which carbon and nitrogen are completely fixed as Ti(C,N). A previous paper has reported that the addition of chromium gives different influences to the Hall-Petch coefficient depending on the amount of Cr. However, our research has reveals the fact that the change of Hall-Petch coefficient is not due to the effect of chromium but due to small amount of carbon which exists as an impurity in ferritic stainless steels. It was concluded that chromium itself does not give any influence to the Hall-Petch coefficient of ferritic iron.

Abstract: Yield strength of ferritic steel increases with grain refinement standing on the Hall-Petch relation. In low carbon ferritic steels, the following relation is established between yield strength σ_y and grain size d: σ_y [MPa]= 100+600/√d [μm]. The Hall-Petch coefficient of interstitial free steels is substantially small as 0.15MPa·√m but it can be greatly increased by the existence of small amount of solute carbon less than 60ppm. As for the effect of substitutional elements such as Cr and P, some papers reports fairly large influence to the Hall-Petch coefficient of ferritic iron. However, the effect of small amount of carbon is sometime neglected or not cleared on the evaluation of Hall-Petch coefficient in ferritic steels. In order to evaluate the effect of substitutional elements, the research should be performed using interstitial free steels to eliminate the influence of solute carbon and nitrogen. In this paper, Hall-Petch relation was examined in iron, Fe-Cr alloys and Fe-P alloys with 0.02-0.05mass% Ti and the following results were obtained: 1) The Hall-Petch coefficient of interstitial free iron is about 0.15MPa·√m. 2) Chromium does not give any influence to the Hall-Petch coefficient of ferritic iron, although the friction stress σ₀ is enhanced in proportional to chromium content (Δσ₀ [MPa]=7×(mass%Cr)). 3) Phosphorus does not affect the Hall-Petch coefficient of ferritic iron or reduce it somewhat but increases markedly the friction stress σ₀ (Δσ₀ [MPa]=250×(mass%P)^1/2). 4) Even under the co-existence of carbon with chromium and phosphorus, carbon dominantly works to increase the Hall-Petch coefficient of ferritic steels, but it is changeable due to the interaction between carbon and the other substitutional elements.

Abstract: Yielding of polycrystalline low carbon steel is characterized by a clear yield point followed by unstable Lüders deformation and such a yielding behavior is taken over to fine grained steel with the grain size of 1μm or less. Yield strength of ferritic steel is increased with grain refinement standing on the Hall-Petch relation. The following equation is realized up to 0.2μm grain size in the relation between yield strength y and grain size d: y [MPa]= 100+600×d[μm]-1/2. In low carbon steel, it might be concluded that the Hall-Petch coefficient (ky) is around 600MPa•μm1/2. However, the ky value of interstitial free steels is substantially small as 130-180MPa•μm1/2 and it can be greatly increased by a small amount of solute carbon less than 20ppm. It was also cleared that the disappearance of yield point by purifying is due to the decrease in the ky value. On the other hand, the ky value is changeable depending on heat treatment conditions such as cooling condition from an elevated temperature and aging treatment at 90°C. These results suggest the contribution of carbon segregation at grain boundary in terms of the change in the ky value. On the contrary, substitutional elements such as Cr and Si do not give large influence to the ky value in comparison with the effect by carbon.

Abstract: In IF steel without interstitials, the Hall-Petch coefficient (ky) is low as about 100 MPa･μm1/2 but it is enhnced by adding some alloying elements. Author et al. has already reported that a small amount of carbon increases ky greatly while nitrogen does not give influence so much. On the other hand, it is already known that phosphorus has a tendency to segregate at grain boundary of polycrystalline ferritic iron and that phosphorus also gives some influence to ky. However, the effect of carbon is not considered on the evaluation of ky in steels containing phosphorus. In this study, effect of phosphorus on the ky of ferritic iron was investigated by varying the grain size in Ti bearing IF-steel with different amount of phosphorus. Besides, the concentration of phosphorus and carbon at grain boundary was estimated by the grain boundary segregation model proposed by Seah et al. and the interaction between phosphorus and carbon was discussed in connection with the change of ky.

Abstract: Grain size dependence of yield strength was reviewed for polycrystalline ferritic iron and low carbon steel. Yielding of polycrystalline low carbon steels was characterized by a clear yield point (upper yield point) and such a yielding behavior is taken over to ultra fine grained steel with the grain size below 1m. Yield strength (y) of polycrystalline low carbon steels obeys the Hall-Petch relation: y[MPa]=+600×d[m]-1/2 . The Hall-Petch coefficient ky is around 600 MPa･m1/2 for the commercial low carbon steels but it is lowered to about 100 MPa･m1/2 for interstitial fee steel. Besides, it is known in industrial pure iron (Fe-30ppmC) that ky increases with aging at 363K. The value of ky is also increases with increasing the amount of solute carbon content. The ky is enlarged from 100 MPa･m1/2 to 550 MPa･m1/2 by adding 60ppm of solute carbon and then levels off at around 600 MPa･m1/2 in the carbon concentration region above 60ppm. On the other hand, nitrogen hardly influences the ky value. Difference between C and N in the contribution to ky is probably due to the difference in grain boundary segregation behavior. Macroscopic yielding of polycrystalline ferritic iron is reasonably explained by the Hall-Petch model considering dislocation pile-up against grain boundary and dislocation emission from the grain boundary where stress concentration has been generated by piled up dislocations. It is seemed that the segregated carbon stabilized the dislocation emission site at grain boundary and this leads to the increase in ky.

Abstract: Work hardening behavior and microstructure development during deformation by cold rolling were investigated in iron with different grain size. Grain refinement makes the introduction of dislocation easier. For instance, under the same deformation condition (5% reduction in thickness), dislocation density is the order of 1014m-2 in a coarse grained material (mean grain size; 20μm), while it reaches 7×1015m-2 in an ultrafine grained material (0.25μm). It is well known that the yield stress of metals is enlarged with an increase in dislocation density on the basis of the Bailey-Hirsch relationship. However, it should be noted that the ultrafine grained material never undergoes usual work hardening although the dislocation density is surely enhanced to around the order of 1016m-2: 0.2% proof stress is almost constant at 1.4 ~ 1.5GPa regardless of the amount of deformation. The dislocation density of 1016m-2 is thought to be the limit value which can be achieved by cold working of iron and the yield stress of iron with this dislocation density (ρ) is estimated at 1.1GPa from the Bailey-Hirsch relationship; σd [Pa] = 0.1×109 + 10 ρ1/2. On the other hand, yield stress of iron is enhanced by grain refinement on the basis of the Hall-Petch relationship; σgb [Pa] = 0.1×109 + 0.6×109 d-1/2 as to the grain size d [μm]. This equation indicates that the grain size of 0.35 μm gives the same yield stress as that estimated for the limit of dislocation strengthening (1.1GPa). As a result, it was concluded that work hardening can not take place in ultrafine grained iron with the grain size less than 0.35 μm because dislocation strengthening can not exceed the initial yield stress obtained by grain refinement strengthening.