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A total of 1 experimental steels with Nb content of 3, lower limit (0.2%), middle limit (0.4%) and upper limit (0.6%) were obtained. The chemical composition was shown in table 2. The test of steel smelting 25kg vacuum induction furnace, after casting forging with 15mm bar, and then 1250 c * 30min solid solution treatment, according to the S31042 steel in the use of temperature in ultra supercritical boiler, the temperature is 700 DEG C endurance test set. At the same time, the treated samples were treated with long time aging, the temperature was 700, and the time was 10, 100, 1000, 3000, 10000h, respectively. The samples of solid solution and aging treatment were observed by metallographic structure. The physical and chemical phase analysis of some specimens was carried out to determine the types and quantities of precipitates in steel.

Test results show that:

(1) after solution treatment, there is an insoluble precipitate in S31042 steel, Z and Nb (C and N) phases. With the increase of Nb content, the content of Z and Nb (C and N) increases obviously, and the size is larger.

(2) the main precipitation phases of S31042 steel at 700 DEG C aging are M23C6 phase, Z phase and MX phase, and there is a sigma phase after aging 10000H.

(3) the content of niobium has no obvious influence on the kinds of precipitates in aged S31042 steel. With the increase of Nb content, the number of M23C6 phases in steel is little changed, and the number of Z+MX phases increases remarkably.

(4) the best niobium content in S31042 steel is 0.40%, enduring strength limit of niobium content can not only provide the second phase to maintain a high enough, and can avoid the large size of Nb particles adverse effect on steel durable performance.