The Research On The Key Issues About Rolling Process Of T91 Steel

T91 ferritic heat-resistant steel is widely used in the supercritical power plant boiler heat pipes today for its excellent high-temperature, long-lasting performance, creep resistance, low thermal expansion coefficient, low production costs and other excellent performance. With the difference of conventional heat-resistant steel, it has different production processes. In the hot rolling process of T91 steel, the more demanding processing requirements, we must develop a reasonable process and strictly enforced.In this paper, the large section casting steel is used as the main target and we research the key issues about rolling process of T91 steel. By the research on recrystallization behavior of T91, heating, rolling and cooling after rolling,we provide technical support for the rolling process of large section of T91 casting steel billet. The main conclusions are:(1) When the strain rate is lower than 0.2S-1 and the temperature is higher than 1000℃, the true stress- true strain curves of T91 steel has a characteristics of bimodal. Apparent dynamic recrystallization occurs in steel, and it belong to a continuous dynamic recrystallization. When other deformation conditions remain unchanged, the lower the strain rate, the higher the deformation temperature, the more prone to dynamic recrystallization.(2) Because of its low thermal conductivity, cracts and other defects are easy to appear for the large center stress on the heating process, we should strictly control the heating system. The higher the heating speed is, the shorter the heating time is, but the greater the stress is. 125~175℃/h is the best heating system.For Φ380mm of T91 steel round billet, when the preheating temperature is 600℃,the total heating time is 11.4h, axial stress of center is 180 MPa.When the preheating temperature rise 50℃, the time required to achieve the effect of shortening the soaking approximately 0.2h, axial stress of center increased by about 24 MPa; For Φ450mm of T91 steel round billet, when the preheating temperature is 600℃,the total heating time is 12.5h, axial stress of center is 219 MPa. When the preheating temperature rise 50℃, the time required to achieve the effect of shortening the soaking approximately 0.3h, axial stress of center increased by about 28 MPa.(3) By simulating seven passes and nine passes rolling,we found that the total strain of the center of seven passes rolling was 1.74,higher than that of nine passes rolling of 1.70. The deformation coefficient of seven passes rolling was larger than that of nine passes rolling. From those we knew that seven passes rolling is better than nine passes rolling on the deformation permeability. Accompanied by dynamic recrystallization in center occurs more easily,seven passes rolling is more pronounced than nine passes rolling on grain refinement, deformation homogenization and cardiac tissue improvement.(4) T91 billet occurs only Ferritic and martensitic transformation during cooling process. The critical cooling rate of martensitic transformation of continuous casting billet of T91 is 8℃/min and the critical cooling rate of ferritic transformation of continuous casting billet of T91 is 2℃/min. While the critical cooling rate of martensitic transformation of rolling billet of T91 is 10℃/min and the critical cooling rate of ferritic transformation of rolling billet of T91 is 3℃/min.