| As the concept of"Green Development"has gradually become a mainstream trend,the trade volume of Liquefied Natural Gas(LNG),one of the clean energy sources,has been increasing.9Ni steel is the key structural material for storing and transporting LNG,and its demand also increase accordingly.However,the nickel content in 9Ni steel is high(generally 9.0?9.5 wt.%)and the high price of nickel have led to excessively high production costs for enterprises.Studies have shown that every 1%Ni reduction in steel can reduce the cost by 5%.Therefore,the nickel saving and low cost of 9Ni steel are an important development direction.Different from the traditional"Trial and Error Method",scientific experiment design and performance research based on modern computing tools is a new research method.Based on this,this paper takes 9Ni steel as the research object,mechanical properties as the evaluation index,and uses JMat Pro software to optimize the composition.The changes in microstructure and properties of 9Ni steel before and after nickel saving are studied,and the feasibility of nickel reduction and low cost of 9Ni steel is discussed.The following conclusions are drawn:(1)The effects of alloying elements on the yield strength,tensile strength and other mechanical properties of 9Ni steel were systematically analyzed.The composition design of Ni,Si,Cr,Mo and Nb was carried out by orthogonal experiment.The experimental results were optimized by intuitive analysis,range analysis and variance analysis,and additional experiments were verified.Finally,the best chemical composition of nickel-saving 9Ni steel(1#)and standard 9Ni steel(2#)was designed.(2)With the help of JMatPro software,the equilibrium phase diagrams of the two components were studied.Except that the temperature of line A1 was slightly lower than that of line 2#,the phase transition temperatures of line 1#,line A3,Ms and Mf were slightly higher than those of line 2#.2#except Mo and Al elements,the change trend of other elements with temperature is almost consistent with 1#.The stress-strain curves and RTT curves of the two samples showed that there was no significant difference between the two samples.At the same time,the recrystallization activation energies of the two samples were calculated,with 1#as 621.6 k J/mol and 2#as 622.8 k J/mol.The results of cooling transformation and CCT curves showed that there is no pearlite transformation in the two samples.the critical quenching rate of 9Ni steel is between8.0?/s and 9.0?/s;when the cooling rate is between 10.0?30.0?/s,the increase rate of martensite content is the fastest,but the total content is not high enough;in order to obtain more martensite structure,it is recommended that the best cooling rate be controlled at 40.0?70.0?/s.(3)The morphology,quantity and size distribution of typical inclusions in the two samples were studied by scanning electron microscopy and inclusion analyzer.The results show that there are oxides and carbides in both samples.Although the total number of inclusions in 1#is more,they are all distributed in 20?m and below,while in2#only 83.67%of the inclusions are less than 20?m in size,but the total number of inclusions in 2#is less,and its unit area The number of inclusions is also less.A comparative study on the hardness and XRD of 9Ni steel shows that the hardness test value of 1#is generally lower than that of 2#.The reason is that the content of Mn,C and other elements that increase hardness in 1#is less,and the iron generated during the cooling process There are more elements;the reversal austenite content of 1#and 2#are close,1.9%and 2.2%,respectively.The micro-tensile test results show that the yield strength of 1#under different tensile rates is 550?590 MPa,the tensile strength is690?740 MPa,and the yield ratio is about 0.8.By observing the fracture morphology of the tensile sample,it is known that there are a few deep large dimples and a large number of small dimples on the cross section,and are inlaid with non-metallic inclusion particles. |
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