الفهرس 
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Abstract
Micro-alloying is playing an important role in the production of high-strength hot strip. It creates the prerequisite for thermo-mechanical processing, retarding or preventing the recrystallization of the hot-worked austenite and precipitation hardening. A part of the present study is aiming to adopt the addition of Nb as a micro-alloying element up to 0.1% for the production of steel with yield strengths up to 450 Mpa.
Experimental heats have been melted in 40kg vacuum induction furnace. The liquid steel was cast in a permanent Nodular C.I mould. Carbon was ranging from 0.044 to 0.050% and NB was ranging from 0-0.2%. The cast ingots were open die forged as a roughing step into slabs with 30mm thickness. Forging was carried out at 1200C-800C. The forged slabs were subjected to a flat hot rolling process on 6 consecutive passes at temperature ranging 1200c-800c some of the rolled strips were air cooled while the rest were directly water quenched.
A groung butt shielded metal are welding process has been done on the host `rolled strips. A six pass procedure was used. The welded strips were machined to tensile test specimens further specimens have been prepared to impact and bending tests.
It is concluded that, addition of NB raises the yield as well as the ultimate strength up to 0.09% NB.or the amout of deformation enhances the strenght. Over 0.04NB, the steel ductility is badly affected especially with both thin strips4mm and water quenching after deformation.YS/UTS ratio of the NB microalloyed steel satisfies 0.85 and 0.90.
The impact energy at room temperature is improved with the increase of NB up to 0.09% after which Nb is useless. However, the impact value decreases drastically with increase of the steel grain size. At subzero temperature, the impact resistance of steel decreases as the test temperature goes down. However down to 40c, the steel remains ductile snd there is no features indicating the transformation from ductile to brittle behavior. where all the fracture surface of the impact specimens showed dimpled ruptured.
The welded tensile specimens fracture outside the welding zone. The welded strips are satisfactory bent over two thicknesses. There are no cracks observed in the weldment either in the joint of HAZ. all mechanical properties of 0.05% c-0.04%Nb are much than that of the target steel.
furthermore, an industrial heat of 160 ton melt steel was subjected to extensive investugation, the heat was cast and directly rolled slabs by compact strip production CSP. the liquid steel contains 0.05 % and 0.04% Nb at 1544c. Cast has 52mm thickness.
Rolling was scheduled to get some coils with 10mm thickness while the others would have 6mm thickness. rolling was carried out on the 6 stand roll mill. some of the coils were late cooled while the rest of coils were early cooled.
A numerical model has been constructed to detect the qustenite grain size after hot deformation. A n analysis was conducted on pass by pass to evaluate the austenite grain size during hot strip rolling. This is achieved by using the available mill log data and the previous developed equations relating the rolling parameters to the grain size.