How To Improve The Duplex Stainless Steel Process?
1) Alloying elements and cooling rate
Experiments and theoretical calculations show that the critical cooling rate required to obtain the dual-phase structure after heating in the critical zone has a certain relationship with the manganese content in the steel. Based on the alloying elements present in the root steel, the critical cooling rate required to obtain the dual-phase structure can be estimated, which provides a basis for selecting an appropriate cooling method during the production of heat-treating dual-phase steel.
When the chemical composition of the steel is constant, the cooling rate should be as low as possible on the premise of ensuring the dual-phase structure, so that the carbon in the ferrite has sufficient time to diffuse into the austenite, thereby reducing the dual-phase structure. Yield strength of steel, improve ductility of dual-phase steel. If the content of alloying elements in the steel is higher than 4, the critical cooling rate is too high, and the ferrite contains high solid solution carbon after cooling, which is not conducive to obtaining dual-phase steel with excellent performance. In order to reduce the critical cooling rate, or in the production process of dual-phase steel, add a supplementary tempering process to reduce the solid solution carbon in ferrite and improve the performance of dual-phase steel. If the steel contains strong carbide formers, when estimating the critical cooling rate, consideration should be given to the hardenability and favorable effect of these elements on the austenite formed when the critical region is heated. Carbide particles of V and Ti can pass through The pinning effect of the phase interface improves the hardenability of austenite and reduces the critical cooling rate.
2) Two-stage cooling process
When the content of alloying elements in the steel is low, the cooling rate is slow to obtain ferrite and pearlite structure; when the cooling rate is fast, the solid solution carbon retained in the ferrite is high, which is not conducive to reducing the yield strength and improving the ductility. . Two-stage cooling can improve the properties of dual-phase steels, that is, slow cooling from the critical zone heating temperature to a certain temperature, and then fast cooling. Slow cooling can enrich the carbon in ferrite to untransformed austenite. Rapid cooling can avoid isothermal decomposition of untransformed austenite and ensure the desired dual-phase structure and properties. For example, 0.08%C-1.4%Mn steel, from 800℃; the mechanical properties of heating to water cooling are: σ0.2=365PMa, σb=700MPa, σ0.2/σb=0.52, eu=18%, et=21%. If a two-stage cooling process is adopted, that is, at 800°C; after heating, air-cooled to 600°C; and then water-cooled, its performance is: σ0.2=280MPa, σb=600MPa, σ0.2/σb=0.47, eu=21% , et=29%. The two-stage cooling reduces the yield strength and increases the ductility of the dual-phase steel.
3) Influence of coil temperature after hot rolling of duplex steel plate
For a given composition of steel, the hardenability of austenite when heated in the critical region can be corrected by hot rolling the steel sheet at high temperature. High temperature coiling can significantly enrich alloying elements such as carbon and manganese in the second group (pearlite or bainite). It is beneficial to improve the comprehensive properties of dual-phase steel during subsequent critical zone treatment. Taking the test results of 0.049%C-1.99%Mn-0.028%Al-0.0019%N steel as an example, two processes are used: one is ordinary rolling process, the final rolling temperature is 900 °C; → the oil is cooled to 600 °C; Coiling→Blow cooling to room temperature→Cold rolling 70%→Continuous annealing. The analysis results of the distribution of carbon and manganese in the two coiling processes show that the high temperature coiling can significantly enrich the carbon and manganese in the second phase, while the ordinary rolling process has basically no enrichment trend of manganese.
The technology of using high temperature coiling to correct the hardenability of steel with lower alloy content in the subsequent critical zone treatment, and to reduce the yield strength of heat-treated dual-phase steel and improve its ductility, has been used in relevant factories for heat-treatment of dual-phase steel. Production, the obtained heat-treated dual-phase steel plate has good comprehensive properties, the properties of each part of the plate are uniform, and the longitudinal and transverse properties are consistent. For example, 0.09%C-0.44Si-1.54%Mn-0.023%Al steel.