| 其他摘要 | Grade Ⅲ rebar can be produced by water-cooling after final rolling, which can improve the mechanical property of rebar on the basis of low cost. However, the rebar produced by water-cooling process tends to rust during storage and transportation, which severely affects the appearance and surface quality of the rebar and brings economic losses to the rebar producer. Moreover, it will reduce the service life of constructions when using the rusted rebar in concrete. Therefore, the rusted rebar is usually requested to descale before it is embedded in concrete. And the descaling of rebar will increase the cost and the working procedure, which can limit the manufacture and use of water-cooled rebar. Therefore, improving the corrosion resistance of rebar, produced by water-cooling process, is the key to reduce the production and use cost and to prolong the service life of the constructions. Thus, this work is of great economic benefits and important social significance.
In this paper, the cooling process of producing hot-rolled rebar is improved. A new chemical cooling process is developed, in which a new type of chemical reagent FM is used in the cooling process of producing hot-rolled rebar. The corrosion resistance of rebar in atmosphere and concrete was improved through optimizing the composition and the structure of the scale. The main contents are as follows:
The compositions of chemical reagent FM were optimized, and the influences of the compositions of FM and the processing parameters in rolling and cooling processes on the scale quality were investigated. The results show that in order to produce the scale with better quality, the H2O2 content should be 10~16% and the pH value of the FM should be 7~9, and these two parameters have the corresponding relation with each other. Appling the FM-cooling process, the scale with good corrosion resistance was obtained when the finial rolling temperature is about 1000°C and the quenching time is 1s. However, it is disadvantageous to the scale quality when reducing the finial rolling temperature or increasing the quenching time.
The influences of the compositions and structure of FM-cooled rebar on the corrosion resistance were studied. The reasons causing the structural differences of three scales were discussed compared with the water-cooled and air-cooled rebar. The results show that if the Fe3O4 layer in scale is single, more compact and thicker, the corrosion resistance of the scale would be better. When cooled by FM, the relative content of Fe3O4 in scale is increased. The outer Fe3O4 layer becomes more compact and a thicker Fe3O4 layer is formed at the wüstite/steel interface. Moreover, the adhesiveness between the scale and steel is improved. Therefore, the favourable scale structure for the corrosion resistance was obtained.
The corrosion resistance of FM-cooled rebar in atmosphere is evaluated by a series of corrosion experiments comparing with water-cooled and air-cooled rebar. The results show that in atmospheric exposure test, water-cooled rebar rusted on the whole surface after 4 days exposure; however, FM-cooled rebar just rusted at some local ribbed regions after 1 month exposure. In wet/dry cyclic accelerating corrosion test and salt spray test, the corrosion resistance of FM-cooled rebar is also better than water-cooled and air-cooled rebar. In the environments simulating the acid rain atmospheric and coastal atmospheric corrosion, the anodic current density of the FM-cooled rebar electrode is less than that of the water-cooled and air-cooled rebar, and the charge transfer resistance of the FM-cooled rebar is four times of that of the water-cooled and twice of that of air-cooled rebar. These results indicate the corrosion resistance of rebar, produced by tempcore process, in atmospheric environment is improved significantly by FM-cooling.
The corrosion resistance of the FM-cooled rebar in concrete was evaluated compared with that of bare rebar, water-cooled rebar and air-cooled rebar using electrochemical impedance spectra method. The results show that in the dry/wet alternated corrosion test, FM-cooled rebar is the last one to initiate corrosion, and its corrosion evolution is also the slowest of the four kinds of rebar. After 14 cycles of accelerated corrosion tests, the corrosion degree of FM-cooled rebar is the slightest and most of the initial scale remains undamaged. This result proves that the FM-cooling can provide better corrosion resistance. In the long-term immersion test for more than one year, the ratio of corrosion rate of FM-cooled rebar, air-cooled rebar, bare rebar and water-cooled rebar is about 1.0: 5.6: 6.6: 20.2. It is obvious that applying FM-cooling can slow up the corrosion of rebar in concrete significantly. The corrosion evolution of rebar in concrete was divided into four stages. Initially, the rebar is in passive state. Then, the corrosion initiates due to the slight breakdown of the surface film, and the rate determining step of corrosion process is the charge transfer step. Then, the corrosion rate increases rapidly due to the expansion of breakdown area of the film, and some corrosion production accumulate on the anode surface, which induce that the diffusion step becomes the slow step of the electrode process. Finally, the whole passive film was destroyed, and a thick rust layer forms on the rebar surface. In this stage, the rate determining step of corrosion process is the diffusion process through a barrier layer and the corrosion rate of rebar keeps constant. Moreover, the corrosion type of four rebar is different. The local corrosion occurs to bare rebar and FM-cooled rebar at the initial stage of corrosion, but the uniform corrosion occurs to the water-cooled and air-cooled rebar.
The structure characteristics of scale are affected by the oxidation temperature, oxidation time, oxygen supplying quantity and cooling rate. In order to provide information for the establishment of FM cooling process, the temperature field of rebar in the cooling process is simulated using the finite elements method. The two-stage cooling scheme is proposed considering the effect of cooling rate on both the mechanical property and the corrosion resistance. The processing parameters of two-stage cooling were optimized by comparing with the processing parameters of water-cooling in steel mill. Furthermore, the effect of processing parameters of two-stage cooling on the mechanical property and the corrosion resistance is analyzed. The results show that applying the two-stage cooling scheme, the lower cooling rate of FM-cooling should be applied in the first stage to produce high quality scale and the higher cooling rate of water-cooling should be applied in the second stage to provide required mechanical property. Consequently, the overall property of rebar can be improved. Moreover, the decrease of rebar dimension and finishing rolling rate is favorable for the overall property. There is a critical value for the finishing temperature to acquire better overall property of the rebar. |
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