Study on Microstructure Transformation Law of Low Carbon Medium Manganese Seamless Steel Tube

This article studies the microstructure transformation law of low carbon medium manganese seamless steel tube, which plays an important role in deciding the performance index of steel tubes. The research includes the different heating, cooling, and holding processes of steel tubes and the corresponding microstructure transformation law. With the aid of optical microscopy and SEM, the microstructure transformation law of low carbon medium manganese seamless steel tube was obtained by experiment. The experimental results show that the microstructure of the steel tube is not uniform and changes with the heat treatment process. The carbides in the steel tubes are mainly distributed in the pearlite band, and the size and matrix are inversely proportional to the heating temperature and holding time.
    
Keywords: Low carbon medium manganese, seamless steel tube, microstructure transformation law.
    
Introduction  
Low carbon medium manganese seamless steel tubes are mainly used in the production of oil and gas pipelines and structural components, where they are characterized by relatively low prices, good weldability, and good plasticity. As an important part of steel tubes, microstructure plays an important role in determining the performance index of steel tubes. In this study, the microstructure transformation law of low carbon medium manganese seamless steel tube was studied under different heating and cooling conditions.
    
2 Experiment
    
2.1 Experimental materials
    
The experimental material was a common low-carbon medium-manganese seamless steel tube, with an outer diameter of 139mm and a wall thickness of 17mm. The chemical composition of the seamless steel tube is shown in Table 1.
    
Chemical Composition of Low Carbon Medium Manganese Seamless Steel Tube:
Element Content (%) C:0.18; Si:0.35; Mn:0.70; Cr:≤0.03; Cu:≤0.01; P:≤0.03; S:≤0.03; Als:0.02;
    
2.2 Experimental process
    
The microstructure of the seamless steel tube was analyzed by optical microscopy and SEM. The samples were cut by a vertical cutting machine, and the surfaces were polished. The heated samples were cooled in the furnace and then quenched. The holding time at different temperatures was controlled to obtain the microstructure transformation law of the steel tube.
    
Results and Discussion
    
The microstructure of the steel tube

Shows the microstructure of the steel tube. It can be seen that the microstructure of the steel tube is distributed in pearlite and lath martensite. At the same time, the size of the pearlite and the lath martensite varies with the different microstructures. The microstructures are distributed in the form of continuous bands, and under the magnified optical microscope, there are also some new microstructures of acicular ferrite and a small amount of carbides distributed in the pearlite band.
    
The microstructure transformation law of the steel tube
    
(1) The microstructure transformation law of the steel tube when heated to the phase transformation temperature.
    
The microstructure transformation law of the steel tube when heated to the phase transformation temperature
    
Shows the microstructure transformation law of the steel tube when heated to the phase transformation temperature. It can be seen that the microstructure is first distributed in the form of banded lath martensite, and then gradually changes to a lath martensite as the heating temperature increases. At the same time, with the decrease of the heating temperature, the martensite lath gradually changes to banded martensite.
    
(2) The microstructure transformation law of the steel tube when quenching and cooling.
    
The microstructure transformation law of the steel tube when quenching and cooling
    
Shows the microstructure transformation law of the steel tube when quenched and cooled. It can be seen that the size and shape of the carbides in the steel tube are inversely proportional to the cooling temperature. The low cooling temperature leads to the large carbides, while the high cooling temperature leads to the small carbides. At the same time, due to the high cooling rate and the small carbides, the matrix of the steel tube at the high cooling temperature is easier to quench than the low cooling temperature.
    
(3) The microstructure transformation law of the steel tube when holding at different temperatures.
    
The microstructure transformation law of the steel tube when holding at different temperatures
    
Shows the microstructure transformation law of the steel tube when held at different temperatures. It can be seen that the carbides in the steel colloid become smaller and the matrix becomes thicker as the holding time increases. At the same time, the size of the carbides is inversely proportional to the holding temperature, and the higher the holding temperature, the smaller the carbides are.
    

In this study, the microstructure transformation law of low carbon medium manganese seamless steel tube was studied. The experimental results show that the microstructure of the steel tube is mainly distributed in the form of continuous lath martensite and pearlite lath, and the size and matrix of the carbides are inversely proportional to the heating, cooling and holding temperature and time. These studies can provide some helpful reference for controlling the microstructure of steel tube and improving the performance of steel tubes.

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