Control Of Oxide Scale On Hot-rolled Steels For Cookware And Development Of Pickling-free Process

As a traditional cooker,iron pot is favored by Chinese people because of its good thermal conductivity,uniform heating,health benefits and low cost.At the same time,the iron pot with oxide scale on its surface has a high corrosion resistance,which is not easy to cause surface corrosion during long-distance transportation,and it can avoid sticking on the pot when cooking,thus the market demand is huge.However,due to the unreasonable control of the thickness and structure of oxide scale on the surface of steels for cookware produced by the original process,a large number of oxide scale falls off during the deep processing,which destroys the production environment of the iron pot,reduces the surface quality of the product,fails to meet the use requirements of downstream users,and weakens the market competitiveness.Therefore,it is urgent for iron and steel manufacturers to develop a production method to control the surface quality of hot-rolled pickling free steel for cookware.In this paper,GWYG steel was used as the target steel to study the high temperature oxidation behavior of experimental steel,the growth and evolution of oxide scale during the heating process,the high-temperature deformation behavior of oxide scale under different hot rolling conditions and the evolution law of oxide scale structure under different cooling processes.The main conclusions of this paper are as follows:(1)The oxidation kinetics experiment of the target steel in dry air at 950 to 1200℃ was studied by means of high temperature thermogravimetry,and the oxidation kinetics curves of the experimental steel was obtained.The oxidation weight gain curves in the early stage approximated a straight line.With the extension of oxidation time,the oxidation weight gain curves obeyed the parabolic law.By examining the microscopic superficial and cross-sectional morphologies of the oxide scale,it was found that the thickness of oxide scale and the evolution of the size of oxide oxide particles were related to the oxidation temperature.(2)The evolution of oxide scale on the surface of the target steel was observed in situ.It was found that the oxide nucleation first occurred at the grain boundary and the surface of the protruding crystal at low temperature.With the increase of the oxidation temperature and the prolongation of the time,the oxide particles underwent three growth modes:loose growth,polymerization growth and longitudinal uneven growth.It was found that there was no FeO below 570℃ by analyzing the types of oxide scale during the heating process.Due to the different diffusion rates of Fe ions in the three oxides,the structure of oxide scale was composed of the thinnest Fe2O3 in the outer layer,the thicker Fe3O4 in the middle layer,and the thickest FeO in the inner layer.(3)The effects of different hot-rolling conditions on the high-temperature deformation behavior of oxide scale were studied.The results showed that temperature affects the plasticity of oxide scale,and oxide scale had a large deformation capacity at high temperatures.The rolling reduction affected the interface roughness between oxide scale and the substrate,the greater the rolling reduction,the greater the interface roughness.When the rolling reduction was less than 10%,it was beneficial for oxide scale to coordinately deform with the substrate.(4)The structure evolution law of oxide scale was studied under isothermal conditions,it was found that the FeO layer would undergo eutectoid transformation,and its isothermal transformation diagram followed the "C" curve law.The temperature range of the eutectoid transition was 350 to 500℃,among which 450℃ was the nose tip temperature of the eutectoid transition.The process of eutectoid transformation could be divided into three stages,which were the incubation stage,the acceleration stage and the stagnation stage.(5)The continuous cooling experiment showed that the factors affecting the structure transformation of oxide scale include coiling temperature and cooling speed.With the change of coiling temperature and cooling rate,the structure transformation law of oxide scale was similar to "C" curve.The small cooling rate promoted the nucleation and growth of eutectoid structure,and the amount of eutectoid transformation increases.When coiling at high temperature,the pre-eutectoid structure was the main structure of oxide scale.(6)Based on the analysis of the on-site production process of the target steel,and combined with the theoretical research results of the laboratory,an optimization control strategy was proposed.The results of industrial trial production showed that,on the premise of ensuring the mechanical properties of steel,the thickness of oxide scale was reduced by 2.47～3.57μm,and the structure of oxide scale was mainly composed of dense Fe3O4,which solved the problem of oxide scale falling off during deep processing and met the requirements of downstream users.