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Different combinations of normalizing and tempering were carried out to optimize the microstructure and enhance the high-temperature mechanical properties of HRB400FR fire-resistant steel bars. The results showed that with the increasing of the tempering temperature from 400 to 600°C, the steel bar’s hardness decreases linearly, mainly due to the formation of quasi-polygon ferrite and granular bainite. Besides, the reduced width and the dissolution of the lath bainite also undermine the performance of the tempered steel bars. The highest Vickers hardness of 380 HV is achieved when the steel is normalized at 950°C and then tempered at 400°C, mainly due to precipitation strengthening and bainite strengthening. The hardness of the test steel tempered at 600°C gives the lowest value, only 230 HV since the least amount of bainite is obtained. When the tempering temperature reaches 650°C, the hardness rises to 260 HV due to the formation of the lath bainite. The emergence of needle bainite generally reduces the matrix grain size, and the appearance of lath martensite refines the precipitated carbides.

Objective: Two kinds of fire-resistant steel with different Nb content (Nb-free and 0.03 wt.%) were prepared for studying the effects of Nb addition on the elevated-temperature strength of fire-resistant steel. Methods: Two stages of heat treatment were carried out on the steels to obtain different microstructures. Typical microstructures, dislocation, and precipitates morphology of steels were observed by SEM and TEM. The dislocation density was calculated by the X-ray data from the microstructures. High temperature and room temperature mechanical properties of steels were determined by tensile testing. Results: The results showed that the YS of N2-HR steel (addition of 0.03 wt.% Nb) at RT and 600 °C was higher than N1-HR steel (Nb-free) by about 81 and 30 MPa, respectively. This indicates that Nb is an alloying element as effective as Mo in increasing the elevated-temperature strength of fire-resistant steel. The dominant strengthening mechanisms of Nb addition on elevated-temperature yield strength are precipitation strengthening and bainite strengthening. Conclusions: Theoretical analysis shows that there are two precipitation strengthening stages in fire-resistant steel: (1) increasing dislocation density during hot rolling, and (2) blocking dislocation movement and recovery in tensile testing. The results also show that the effect of fine grain strengthening is not obvious at high temperature, but is obvious at room temperature. [ABSTRACT FROM AUTHOR]

Objective: Two kinds of fire-resistant steel with different Nb content (Nb-free and 0.03 wt.%) were prepared for studying the effects of Nb addition on the elevated-temperature strength of fire-resistant steel. Methods: Two stages of heat treatment were carried out on the steels to obtain different microstructures. Typical microstructures, dislocation, and precipitates morphology of steels were observed by SEM and TEM. The dislocation density was calculated by the X-ray data from the microstructures. High temperature and room temperature mechanical properties of steels were determined by tensile testing. Results: The results showed that the YS of N2-HR steel (addition of 0.03 wt.% Nb) at RT and 600 °C was higher than N1-HR steel (Nb-free) by about 81 and 30 MPa, respectively. This indicates that Nb is an alloying element as effective as Mo in increasing the elevated-temperature strength of fire-resistant steel. The dominant strengthening mechanisms of Nb addition on elevated-temperature yield strength are precipitation strengthening and bainite strengthening. Conclusions: Theoretical analysis shows that there are two precipitation strengthening stages in fire-resistant steel: (1) increasing dislocation density during hot rolling, and (2) blocking dislocation movement and recovery in tensile testing. The results also show that the effect of fine grain strengthening is not obvious at high temperature, but is obvious at room temperature.

A series of Fe-Mo-C steels with Mo addition from 0.1 to 0.8 wt.% has been prepared for studying the effect of Mo on the elevated-temperature strength of fire-resistant steel. Two heat treatments were performed for obtaining either ferrite microstructure or ferrite-bainite microstructure to study the contributions from two strengthening mechanisms with Mo addition, namely solid-solution strengthening and bainite strengthening. The results show that solid-solution strengthening is the predominant elevated-temperature strengthening mechanism of Mo in fire-resistant steel. This strengthening effect has a huge contribution in improving elevated-temperature strength when Mo content is below 0.5 wt.%, and the yield strength at 600 °C goes up by a significant 13.7 MPa per 0.1 wt.% Mo addition. However, it becomes relatively weak when Mo content is more than or equal to 0.5 wt.%. Moreover, results indicate that the elevated-temperature strength remarkably increases when the volume fraction of bainite is above 15%. Furthermore, it is found that the ferrite grain size has minor effect on elevated-temperature strength of fire-resistant steel. Results also provide fundamentals of designing low-cost fire-resistant steels with excellent strength. [ABSTRACT FROM AUTHOR]