Phase Transformation Of Precipitates At Intermediate Temperature In Two Types Of Low-carbon Martensitic Alloy Steel And Its Effect On Mechanical Properties

Vigorously developing ultra-supercritical coal-fired thermal power and advanced automobile manufacturing technology,as much as possible to improve the efficiency of power plants and achieve lightweight vehicles,while reducing manufacturing costs,is currently the most practical and effective way to reduce CO₂ emissions.9-12%Cr and 4-12%Mn low-carbon martensitic steels are often used as key candidate materials for thermal power units and advanced high-strength automobiles,the improvement of former's steam parameters(temperature and pressure)is mainly limited by the coarsening of subgrains and precipitation of harmful phases during service,the improvement of latter's strong plastic product mainly depends on the coupling effect of metastable austenite and precipitates during thermomechanical treatment.The stability of subgrains in low-carbon martensitic heat-resistant steel during service at intermediate temperature(500-650?)is mainly controlled by the precipitates at the grain boundaries,phase transformation and mutual coupling of various types of precipitates will seriously affect the stability of subgrains and creep properties.The stability of metastable austenite during the thermomechanical treatment of medium-manganese advanced automotive steel at intermediate temperature(450-700?)is affected by the precipitates,phase transformation of various types of precipitates and their coupling with the martensite transformation/austenite reverse transformation will significantly affect the TRIP effect,strength and plasticity.Therefore,the properties of these two types of low-carbon martensitic steels are closely related to phase transformation of various types of precipitates at intermediate temperature.The systematic study of mechanisms of phase transformation under intermediate temperature and designing the corresponding microstructure,are the key to breaking through the bottleneck of"steam parameters"and"strong plastic product".In this paper,two typical representatives of low-carbon martensite steel such as10.5%Cr heat-resistant steel(X12Cr Mo WVNb N10-1-1)and 9%Mn medium manganese steel(Fe-0.1C-9.12Mn-3.18Ni-1.31Al),were selected as the research object.Phase transformation behavior of precipitates at intermediate temperature in these two types of low-carbon martensitic steels were systematically studied.The interaction mechanism of various types of phase transformation were revealed,and the phase transformation of precipitates at intermediate temperature were used to tailor the stability of metastable austenite and TRIP effect in medium manganese steel.Based on the experimental results,the relationship between phase transformation of precipitates and mechanical properties was established.The main results are as follows:Long-term aging at 650?for 10.5%Cr heat-resistant steel to study the effect of precipitation and dissolution of precipitation phase on phase transformation and mechanical properties.(1)Before aging at 650?for 18000h,it was found that Laves phase tends to grow by directly swallowing the adjacent M₂₃C₆,moreover,the swallowing process preferentially initiates along M₂₃C₆/ferrite interfaces and then gradually extends to the center of M₂₃C₆,which is mainly due to a strong segregation of Si and P at M₂₃C₆/ferrite interfaces.The swallowing growth mechanism will induce formation of large and irregular Laves phase.(2)After aging at 650°C for 33500h,deterioration of plasticity is dominated by severely inhomogeneous and large size of Laves phase caused by two different nucleation and growth mechanisms and high coarsening rate(?32.0 nm/h^1/3).(3)After aging at 650°C for 40500h,M₂(C,N)carbonitride gradually dissolves and induces the formation of a large number of nano-sized MX carbonitride.The increase in the volume fraction of MX from 40500 h to 49500 h not only contributes to the stability of the strength through the pinning effect and precipitation strengthening but also benefits the ductility by suppressing the coarsening of the Laves phase through the strong competition for Si between the Laves phase and MX.An unusual combination of stable strength and stable ductility in the later stages of long-term aging at 650°C(approximately 9000 h)was achieved.The 9%Mn medium Mn steel was cold rolled first and then partitioned and tempered at intermediate temperature to study the effect of Ni Al phase precipitation on phase transformation and mechanical properties.It was found that some Ni Al-type nanoparticles retained on phase interfaces in the first step of partitioning treatment,could effectively hinder the interface migration from austenite to ferrite by the pinning effect during the second step of tempering,which was beneficial for stabilizing the austenite.In addition,more Ni Al-type nanoparticles were introduced in the tempering,a harder martensite matrix was obtained,which could“shield”the austenite during deformation and induce a lower transformation rate of austenite.Based on the dual effects of Ni Al nanoprecipitation on the austenite stability,an excellent combination of strength(yield strength:1108 MPa/ultimate tensile strength:1241 MPa)and ductility(total elongation:46.4%)was achieved in the PT₆₃₀ specimen,moreover,appearance of Lüders band was suppressed.The 9%Mn medium Mn steel was cold rolled first and then pre-tempered and partitioned at intermediate temperature to study the effect of precipitation and dissolution of metastable Mn-rich M₁₂C on phase transformation and mechanical properties.Utilizing precipitation and dissolution of intragranular Mn-rich M₁₂C carbide to induce the formation of fine austenite in martensite grains,which is rich in Mn and C and has a gradient distribution of Mn elements.Therefore,austenite nucleated on the basis of Mn-rich M₁₂C not only has higher mechanical stability,but also higher chemical stability,which is conducive to improving work hardening rate and ductility.These phase transformation behaviors and their interaction mechanisms at intermediate temperature in two types of low-carbon martensitic alloy steels not only provide a guidance for martensitic heat-resistant steels to regain strength and ductility in the later stages of long-term service,but also make them useful in medium-manganese steels.It also makes it possible to tailor the austenite stability and optimize the TRIP effect by using the precipitated phase in medium manganese steel.