Study On Structure, Mechanical Attrition Pickling And Acid Free Scale Removal Of Oxide Scale Formed On Hot-rolled Steels

Structure and phase component of oxide scales formed on BRC3,QstE380TM,B510L,SPHC,SS400 hot-rolled steel strips were studied. The results showed that oxide scales formed on BRC3 steel consisted of outmost Fe2O3 layer,intermediate Fe3O4 layer and inner FeO layer. The thickenss ratios of Fe3O4 and FeO layers is 1:3. The oxide scales of QstE380TM,B510L,SPHC,SS400 hot-rolled steel strips contained a mixture of magnetite and iron-magnetite eutectoid with average thickenss about 6-7μm.Researches on conventional pickling showed that pickling kinetic curves of oxide scales formed on BRC3,B510L,SS400,SPHC steel strips obeyed Johnson-Mehl equation. Pickling rate of oxide scale formed on BRC3 steel was highest, followed by that of B510L and SS400, where the lowest rate of SPHC. Comparing with conventional pickling, pickling time can be remarkably reduced by adopting mechanical attrition pickling, together with favorite size of glass ball. Best effects can be achieved when vertical mechanical attrition pickling was applied. Pickling time can be further shortened by vertical-horizontal combining mechanical attrition. Kinetic curve of mechanical pickling of oxide scales is generally linear During hydrogen reduction of oxide scales on BRC3 steel strips, the composition and structure of oxide scale remain unchanged, when temperature was 500℃or below. When temperature was up to 600℃, a large amount of Fe3O4 precipitates within original FeO layer.When temperature was 700℃, all oxides was transformed into FeO completely. During hydrogen reduction, when temperature was 400℃, only a little amount of Fe3O4 layer was reduced. While temperature reached 500℃, oxide scale was completely reduced by hydrogen gas. But the effect of hydrogen gas reduction became less when temperature was 600 or 700℃. During reduction of oxide scale of QstE380TM steel strips, when temperature was 500℃or below, the composition and structure of oxide scale remained unchanged. When temperature was up to 600℃, a large amount of Fe3O4 /Fe were transformed into FeO. When temperature was 700℃, all oxides were transformed into FeO completely. During hydrogen reduction, when temperature was 400℃, only a little amount of oxide scale was reduced. While temperature reached 500℃, oxide scale was completely reduced. But the effect of hydrogen gas reduction became less when temperature is 600 or 700℃During high-temperature CO reduction, all iron oxides of scales on BRC3 hot-rolled steel strips were entirely transformed into wustite, which was then reduced by carbon monoxide. After reduction, the oxide scale comprised mostly white porous iron and a small amount of unreduced grey wustite. It was found that the effect of reduction of oxide scale was the best under conditions that the temperature was 750℃,carbon monoxide flow was 3L/min and reaction time was 180s. And the effect of reduction increased with the increase of carbon monoxide flow at the same reaction time and temperature.When heated up to 730°C, both magnetite and eutectoid of oxide scales on QstE380TM were transformed entirely into wustite, which was reduced into iron subsequently by carbon monoxide. Under the condition that temperature was 730℃,CO flow was 3L/min and reaction time was 180s,reduced oxide scales comprised mostly white porous iron and small amount of unreduced wustite. Cementite also appeared in oxide scales after reduction. It was found that the effect of CO reduction was the best when the temperature was 750℃,carbon monoxide flow was 3L/min and reaction time was 180s.