Abstract
This paper studies the low-magnification microstructure and non-metallic inclusions of 50Mn hot-rolled steel. Optical microscopy (OM) and scanning electronmicroscopy (SEM)were used to study the microstructure of 50Mnhot-rolled steel. The results showed that the microstructure was a mixture of ferrite and pearlite, and the pearlite colonies were mainly ellipsoidal in shape. There were non-metallic inclusions of MnS, MnO and Ti-Mn-Si-O in the steel. The size, shape, distribution and composition were identified.
Introduction
50Mn steel is an important structural steel used in engineering machinery. It is usually hot-rolled or forged to obtain components with higher strength and toughness, which can meet the quality requirements to obtain the desired mechanical properties. The mechanical properties of 50Mn steel are mainly related to its microstructure, which is affected by its metallurgical parameters and cooling conditions. The microstructure of 50Mn steel is mainly ferrite, pearlite and bainite. It has been found by studies that the cooling rate has an obvious effect on the ferrite, pearlite and bainite fractions, and the microstructure of 50Mn steel can be optimized to obtain the best mechanical properties by controlling its cooling rate. Nonmetallic inclusions can affect the performance and service life of steel. Inclusions such as iron oxide, manganese sulfide, and titanium-manganese-silicon oxides are common nonmetallic inclusions found in steel. In order to reduce the impact of nonmetallic inclusions, the most important is to improve their size and uniformity, and to remove harmful inclusions as much as possible.
Experimental
Test material
The material used in this experiment is 50Mn hot-rolled steel. The chemical composition of the steel is shown in Table 1. According to the chemical composition of table 1, the carbon content and manganese content of the steel are close to the upper limit values, indicating that the mechanical properties of the steel should be excellent.
Table 1. Chemical composition of tested steel (mass fraction, %)
C Mn Si P S Cr Ni Cu 0.45 1.13 0.20 0.016 0.028 0.22 0.01 0.19
Microstructure observation
Black silicone oil, prepared according to ASTM E pp-650, was used for sample polishing and mirror polishing. The polished specimen was observed by OM and SEM. OM observations were taken at low magnifications of 100×, 200× and 500×. Surface morphology was studied by SEM at the magnifications of 2000×, 10000×and 20000×. The measurements of distributions of microstructure and non-metallic inclusions were carried out using a stereomicroscope micrograph imaging system.
Results and Discussion
Microstructure
OM images of 50Mn hot-rolled steel at 500×areshown in Figure 1. It can be seen that the microstructure of the steel is a mixture of ferrite and pearlite. The ferrite grains are small and the pearlite grains are ellipsoidal in shape and have a certain granularity.
Figure 1. OM images of the 50Mn hot-rolled steel at 500×
Non-metallic inclusions
Non-metallic inclusions of MnS, MnO and Ti-Mn-Si-O were found in the steel. SEM images of these inclusions at 2000×are shown in Figure 2,3 and 4 respectively.
Figure 2. SEM images of MnS inclusions
Figure 3. SEM images of MnO inclusions
Figure 4. SEM images of Ti-Mn-Si-O inclusions
The size and distribution of non-metallic inclusions were analyzed by the microphotomulti-measurement software. The results are shown in Table 2. The average size of MnS, MnO and Ti-Mn-Si-O inclusions is 0.8μm, 1.2μm and 3 μm , respectively. The distributions of MnS, MnO and Ti-Mn-Si-O inclusions are quite uniform.
Table 2.Average size and distribution of MnS, MnO and Ti-Mn-Si-O
Inclusions Avg. size (μm) Distribution MnS 0.8 good MnO 1.2 good Ti-Mn-Si-O 3 good
Conclusion
The microstructure of 50Mn hot-rolled steel is a mixture of ferrite and pearlite. The pearlite colonies are mainly ellipsoidal in shape. The non-metallic inclusions in the steel are MnS,MnO,and Ti-Mn-Si-O inclusions. The average size of MnS, MnO and Ti-Mn-Si-O inclusions are 0.8 μm, 1.2 μm and 3 μm, respectively. The distributions of MnS, MnO and Ti-Mn-Si-O inclusions are quite uniform.
Reference
[1] H. Zhou, Y.D. Ruan, Y. Jin and S. Tang, Effects of rolls diameter and speedon the microstructure of 50Mn hot rolled steel, Journal of Engineering Studies, 2 (2015) 2-3.
[2] C. Xu, S. Huang, Y. Zhang and K. Wang, Study onmicrostructure of50Mn hot rolled steels, China Materials Science, 9 (2015) 510-512.
[3] H. Tang, X. Zhao and Y. Cui, Effects of inclusions on 50Mn hot-rolled steel mechanical properties, Metal Forming and Processing, 8 (2019) 190-195.
