When the 30MnSi steel is rolled at a high temperature, the surface of the steel is in contact with the external environment, and the chemical reaction occurs to produce oxides attached to the surface of the steel to form an oxide layer, also known as iron oxide. At present, it is recognized that the oxide layer is divided into three layers.
The inner layer attached to the surface of the matrix is FeO, which is loose and porous and easy to corrode, and the color is black. The middle is dense, magnetic black crystal Fe3O4; The outermost layer is reddish-brown Fe2O3, which is insoluble in acid.
Oxidation occurs in the cooling process of steel. Because the reaction starts from a higher temperature, FeO and Fe3O4 with lower oxygen content are first generated on the surface of iron, and further oxidation generates Fe2O3 on the surface. Fe3O4 is relatively dense, while Fe2O3 is brittle and easy to fall off. When the cooling rate is fast, FeO and Fe3O4 are fixed in a more stable phase before Fe2O3 is generated, so the Fe2O3 content in the oxide scale is less when the cooling rate is fast, and the surface quality is better than that when the cooling rate is slow. In addition, when cooling rapidly, the surface oxide scale tissue is not easy to grow, the scale is small, and the oxide scale tissue is dense.
When the cooling rate is slow, the tissue is fully expanded and connected with each other to form loose tissue, and the surface of the oxide scale with many holes is naturally loose. Low-carbon steel wire rods are generally cooled by slow cooling, and the oxide scale is thick. High carbon steel generally adopts enhanced cooling, and the oxide scale is thin.
The overall color of No. 1 30MnSi Steel Plate sample (FIG. 1) is blue. It can be seen from the photos taken by the body microscope that the scale oxide on the surface of sample No. 1 is relatively complete and smooth. The color of sample No. 2 (FIG. 2) is red. It can be seen from the photos taken by the body microscope that there are many missing iron oxide sheets on the surface of sample No. 2, and the surface is relatively rough.
FIG. 1 Surface macroscopic morphology of sample No.1
FIG. 2 Surface macroscopic morphology of sample No.2
FIG.3 and FIG.4 show the morphology of scale oxide on the surface of No. 1 and No. 2, respectively. It can be seen that the oxide layer on the surface of sample No. 1 is relatively complete, the thickness is basically uniform. And the gap between sample No. 1 and the steel matrix is small. The oxide layer on the surface of the No. 2 sample is intermittently disconnected, its thickness is inconsistent, and there is a gap between it and the steel matrix.
FIG. 3 Lateral microscopic morphology of sample No.1
FIG. 4 Lateral microscopic morphology of sample No.2
The scanned picture in FIG. 5 shows the steel surface morphology of sample No. 1. It can be seen from the left figure in Figure 5 that the surface of the oxide sheet of sample No. 1 is smooth with few defects. As can be seen from the figure on the right, the iron oxide sheet is micro block, the blocks are relatively close to each other, and the surface rust is less.
FIG.5 Surface microscopic morphology of sample No.1
FIG.6 shows the steel surface morphology of sample No. 2. From the left figure in FIG.6, it can be seen that there are many oxide sheets missing on the surface of sample No. 2, and the large sheets are directly exposed to the steel matrix after falling. It can be seen from the right picture that there are several cracks on the steel surface, and the exposed steel matrix has been corroded, and the steel surface has been basically rusted, and a furry layer of material has grown.
FIG.6 Surface microscopic morphology of sample No.2
The thickness of the oxide layer of sample No.1 is uniform and complete, which is consistent with the microscopic analysis results, and the average thickness of the oxide layer is 6.168 μm. The microstructure of the oxide layer is divided into two layers, and the inner layer is relatively dense, with a thickness of about 4.922μm. The outer layer is loose and about 1.296 μm thick. Moreover, there are small cracks between the loose layer and the compact layer.
The oxide layer on the surface of No. 2 sample is incomplete, with different thickness, and there are gaps between the surface and the matrix, and the average thickness of the oxide layer is 9.662 μm. The microstructure of the oxide layer is also divided into two layers, and the inner layer is relatively dense, with an average thickness of 4.579 μm. The outer layer is loose, with an average thickness of about 1.575 μm. Moreover, the gap between the surface oxide layer and the matrix is about 0.984 μm.
Element | Mass fraction /% |
O | 28.86 |
Fe | 71.17 |
Element | Mass fraction /% |
O | 32.54% |
Fe | 67.46% |
Element | Mass fraction /% |
O | 23.21 |
Si | 0.86 |
Fe | 75.94 |
Different methods of removing the oxide scale have different requirements on the surface of 30MnSi steel. If pickling is used, the thickness of the oxide scale should be as thin as possible and the structure should be controlled. If mechanical shell stripping is used, the oxide skin should have a certain thickness and quality, and a certain structure, so as to facilitate mechanical shell stripping.
Due to the sticky nature of FeO, it is difficult to remove from the steel matrix, so the bonding strength of the oxide layer and the steel matrix increases. Strengthened cooling after final rolling is conducive to the formation of a thick FeO layer, which can be well attached to the steel matrix and prevent the oxidation layer from peeling off.
In the cooling process after silking, the FeO layer can continue to oxidize to form a dense Fe3O4 layer, forming an oxide sheet protective film on the surface of the steel, which can prevent short-term atmospheric oxidation and corrosion.
However, according to the test results, the surface oxide layer is less than 10 μm, and the thickness of FeO is less than 8 μm. The FeO thickness of sample No. 2 is less than that of sample No. 1, and there is a gap between the oxide layer on the surface of sample No. 2 and the steel matrix. Due to these reasons, part of the oxide layer on the surface of sample No. 2 fell off, and the macroscopic state was reddish after rust.