Study On Strengthening Mechanism Of Mo And Replacement Of Mo In Fire-resistant Steel

With the increasing market demand and the increasingly high demand for qualityconstruction, there is an urgent need to develop a low-cost fire-resistant steel with low Moaddition and excellent strength. In the present investigation, a series of simplifiedFe-Mo-C steels with Mo addition from0to0.9wt.%have been prepared. Two heattreatments were carried out on Fe-Mo-C steels for obtaining either ferrite microstructuresor ferrite-bainite microstructures to separate the high-temperature strengtheningmechanisms of Mo in fire-resistant steel. And quantitative analysis is made to reveal thedominant high-temperature strengthening mechanism of Mo addition. The results in thiswork proposed a new design of the low-Mo high-strength fire-resistant steels based on acombination of bainite strengthening and the precipitation strengthening of Nb, V, and Ti.Thus, a low-Mo (about0.15wt.%) fire-resistant steel with high strength (yield strength:488.7-508.9MPa) and low yield ratio (0.634) has been successfully developed. Comparewith conventional fire-resistant steel, the Mo content was reduced more than70%, thealloy cost was reduced about1550yuan per ton of steel, and the yield strength wasincreased more than42%. Moreover, the results also established a relation between Mocontent, bainite volume fraction, the additive amount of microalloying elements addition,and high-temperature yield strength in the development of a series of low-costfire-resistant steels with UTS from490to790MPa. The main achievements are expressedbelow.Mo distribution in different microstructures of Fe-Mo-C steels is identified by usingelectron probe micro-analysis (EPMA) and transmission electron microscope (TEM). Theresults show that almost all Mo atoms are in solid-solution in ferrite microstructure whenMo content is less than0.3wt.%, and the precipitates Mo2C were formed in ferritemicrostructure when Mo more than or equal to0.3wt.%. Moreover, in bainite and pearlite microstructures, Mo atoms are first in solid-solution in the ferrite matrix, and the alloycementites consisting of Mo, Fe, and C were formed when the ferrite matrix Mo-saturated.Quantitative analysis indicates that the dominant high-temperature strengtheningmechanisms of Mo in fire-resistant steel are solid-solution strengthening (dominant whenMo less than0.3wt.%) and precipitation strengthening (both dominant when Mo morethan or equal to0.3wt.%), and these strengthening effects improve the yield strength at600oC by a significant147.6MPa/wt.%when Mo less than or equal to0.5wt.%(σss+σppt=147.6CMo+54.76), but these strengthening effects become relatively weak whenMo content is more than or equal to0.5wt.%.Bainite strengthening plays an important role in improving the high-temperaturestrength of fire-resistant steel, and it improve the yield strength at600oC by a significant1.655MPa/vol.%(σBs=8.833+1.655fB).Ferrite grain size has less effect on high-temperature strength of fire-resistant steel.The yield strength at600oC only increases about4.3MPa with the variation of the ferritegrain size from18.2to26.3μm.Results also provide that a low-Mo (Mo less than0.3wt.%) fire-resistant steels withexcellent strength can be designed by bainite strengthening and the precipitationstrengthening of Nb, V, and Ti.Two low-cost high-strength (Q345and Q500) fire-resistant steels with two Moaddition levels (about0.15and0.3wt.%) have been successfully developed by controllingcooling rate and microalloying with Nb, V, and Ti in combination. Two low-costhigh-strength fire-resistant steels have low yield ratio (below0.634), good ductility(elongation: more than22.3%and the charpy energy at0oC: above35J), and highstrength. The room-temperature yield strength of Q500class fire-resistant steels withabout0.15wt.%Mo addition are488.7-508.9MPa. It is possible to obtain two-thirds ofroom-temperature yield strength at600oC in two low-cost high-strength fire-resistantsteels.The results in this work verified that the quantitative expressions relating Mo content,bainite volume fraction, and the high-temperature yield strength of fire-resistant steel arereliable. Moreover, the results also proposed an expression (YS(600oC)=σ0+147.6CMo+1.655fB+296.27f ppt) relating Mo content (CMo, wt.%), bainite volume fraction (fB, vol.%), the amount of microalloying elements addition (precipitate volume fraction fppt,vol.%), and high-temperature yield strength (YS(600oC)) in the development of a seriesof low-cost fire-resistant steels with ultimate tensile strength from490to790MPa.There exist two stages of precipitation strengthening by microalloying with Nb on thehigh-temperature strength of fire-resistant steel. First, the dislocation tangle anddislocation density of fire-resistant steel increases during hot rolling due to the pinningeffect of Nb(C, N) precipitates, and the strength of ferrite matrix remarkably improves.Second, the precipitates have effects on retarding the climb motion and recovery ofdislocation during tensile tests, especially during high-temperature tensile tests. Thus, thehigh-temperature strength of steels shows a remarkable improvement。...