Mechanisms of Static Strain Aging in B.C.C. Fe-Mn-N Alloys - A Study by Impulse Internal Friction

DoctorBake hardenable (BH) steel has an initially low yield strength (YS) and good formability, allowing for more complex forming operations required during fabrication of press-formed parts. This type of steel is used mainly as large size sheet designed for external panels, for which an excellent shape stability in terms of dent-resistance is required after deep drawing. Recently, it was reported that the Fe-Mn-N alloys showed a substantial increase in both the yield strength (YS) and the ultimate tensile strength (UTS) after pre-straining and aging at 170 degreeC for 20 min. Furthermore, the strain aging response of the YS and UTS increased with the amount of the solute N. This property was achieved by precisely adjusting the content of solute N in the steel and refining the grain size using an appropriate cooling after hot rolling. These steel sheets have press-formability similar to that of a conventional steel sheet, i.e. low YS and good formability. A substantial increase in UTS can be obtained after the bake hardening process. The application of theses steels is, therefore, expected to contribute to an enhancement of the critical mechanical properties of automotive steels, e.g. crashworthiness, which are essential requirements for the critical safety parts as the structural components of the car body such as cross members and pillars, through the paint baking process. It is widely accepted that the strain aging in steel is related to the concentration and mobility of interstitial impurities and to the presence and mobility of dislocations. The aging process occurs at room temperature (RT) or during heating at temperatures above RT due to the high diffusivity of C and N in b.c.c. Fe alloy. The strain aging of the type of steel used in the present work results in an increase of both YS and UTS. It should be noted that at levels as low as 1 wt-ppm of C and N, significant aging effects can be detectable, and thus, a good measure associated with the interstitial impurities is required for quality control of the strain aging properties. The challenge is to accurately measure very low concentrations of the mobile interstitial atoms. In addition, the mechanism, resulting in a substantial increase in UTS after the bake hardening process of Fe-Mn-N alloys is not fully understood due to a lack of reliable experimental data.The internal friction (IF) technique is unique in its capability to quantify the concentration of mobile interstitial C or N in b.c.c. Fe and alloys. The variation of mobile interstitial populations with important metallurgical process parameters, such as the initial composition, the type and degree of cold work, the annealing temperature and the aging time, can be also monitored by the IF techniques. The presence of substitutional solutes such as Mn atoms has a pronounced influence on the Snoek effect associated with interstitial atoms. In the Fe-N system, two N Snoek peaks (h1 and h2) occur with the small amounts of Mn addition. This results in a significant peak broadening of the Snoek peak. In addition to the compositional analysis of the mobile interstitials, the IF technique provides structural information about the steel in terms of the dislocations and their interaction with the interstitials by the analysis of the dislocation-related damping peaks. An in-depth investigation of the dislocation-related damping peaks such as the Snoek-Köster peak (SK), the Bordoni peak and the dislocation-enhanced Snoek peak (DESP) was carried out for Fe-Mn-N alloys by correlating the evolution of the respective relaxation processes with the static strain aging property. The IF analysis method could be used as an efficient and direct method to measure the strain aging response. Experimental evidence for the occurrence of a transition between the Bordoni relaxation and the SK relaxation in Fe-Mn-N alloys was identified. The present work shows that the Bordoni peak can arise in b.c.c. Fe-Mn-N alloys, provided that the alloy contains a low enough amount of interstitial N atoms, so that the motion of the dislocations can remain free of the influence of the point defects after deformation. The relaxation mechanism transition from the Bordoni relaxation to the SK relaxation was observed. The appropriate deconvolution method for the Snoek relaxation peaks and the complex dislocation related relaxation peaks makes it possible to measure directly the static strain aging response of the Fe-Mn-N alloys since the intensity of the SK relaxation is indicative of the degree of the Cottrell atmosphere formation, which is the main mechanism governing the static strain aging. The present study revealed that the significant increase of the UTS by the paint baking in Fe-Mn-N alloys could be attributed to two factors: (a) the strengthening by the additional stress required for the unpinning of dislocations pinned by Cottrell atmospheres and (b) the strengthening resulting from the residual interstitial N content after the paint baking which acts as a solid solution hardening agent, governing the later stage of the plastic deformation