SS CAMPOS, EV MORALES, and H.-J. KESTENBACH where pct Mn, pct Si, and pct Nf are the weight percentages of manganese, silicon, and free nitrogen dissolved in ferrite, Precipitation hardening by fine carbonitride particles has and d is the ferrite grain size in millimeters. Due to the long been recognized as an important mechanism for the microalloy addition, Nf was assumed to be negligible. Yield strengthening of microalloyed steels.[1] In the past, several strength predictions from Eq.[1] are compared to tensile important review articles have emphasized that a significant test results in Table III. The difference between calculated strengthening effect could only be expected from very fine and measured strength is usually attributed to some addicarbonitride particles, which had precipitated semicoher- tional strengthening mechanism such as carbonitride precipiently in the ferrite phase.[2, 3] Such precipitation should be tation [8] or substructure strengthening.[9] The important point particularly effective in the case of hot strip products, where in Table III is the very large additional strengthening of 177 a combination of shorter rolling times, higher finish rolling MPa in the case of the Nb-Ti steel, which was reduced to temperatures, and accelerated cooling rates after rolling 69 MPa after normalizing. As mentioned previously, norshould cause a larger amount of microalloy elements to malizing did not reduce the yield strength of the Nb steel, remain in solution prior to coiling.[4] However, no ferrite- and the additional strengthening contribution in this steel nucleated carbonitride particles were found in the authors’ remained at around 60 MPa, a level very close to the 69 laboratory during a recent study of a commercial Nb-Ti MPa exhibited by the Nb-Ti steel after normalizing. microalloyed steel, which had been processed under indus- Transmission electron microscopy (TEM) was used to trial conditions on a hot strip mill.[5] It has been argued for investigate the possible sources of additional strengthening a long time that the loss of strength, which the as-rolled hot mechanisms. In order to check for carbonitride precipitation, strip tends to suffer during normalizing, can be taken as a a total of 20 ferrite grains were examined in each steel. strong indication for the presence of such fine carbonitride Fine carbonitride precipitation was identified in all of them particles, which, during austenitizing, would lose their coher-(Figure 2), but orientation relationships determined from ence with the surrounding ferrite lattice.[6] Significantly, no electron diffraction [5, 8] showed that these particles had nucleloss of strength was observed for the commercial hot strip ated in austenite. In addition, carbonitride distributions as steel after normalizing.[5] On the other hand, the yield shown in Figure 2 appeared to be very similar to the disstrength of this 0.02 pct Nb and 0.06 pct Ti containing steel tributions observed in the previous steel.[5] In that case, had been rather low (332 MPa), suggesting that perhaps quantitative metallography and the application of the the thermomechanical processing conditions had not been Orowan–Ashby model of precipitation strengthening had very effective. indicated a strengthening contribution of about 60 to 80 New results were now obtained from another commercial MPa for carbonitride particles formed in austenite, in good Nb-Ti microalloyed hot strip steel, which reached a yield agreement with the additional strengthening shown in Table strength of 534 MPa and, according to expectations, lost III for the Nb steel and also for the Nb-Ti steel after part of that strength during normalizing. A Nb steel, which normalizing. only reached 310 MPa and maintained …