| In view of excellent corrosion properties and reasonable price,super ferritic stainless steels(SFSSs)have been widely applied to chemical industry for heat exchange and condensing equipment.However,annealing process is indispensable to the production of these stainless steel products,during which harmful phase precipitated,Therefore,how to obtain fine recrystallization structure and avoid brittle phase precipitation through reasonable annealing process is particularly important.In this paper,27Cr-4Mo-2Ni SFSS hot rolled plate produced from Taiyuan Iron & Steel Group co.LTD was used as raw material for solution annealing,cold rolling and annealing procedure.Optical microscope,SEM and TEM were adopted for structure observation and analysis.Electronic universal testing machine and Vickers Hardness tester were employed for mechanical properties measurement.Effects of different cooling manner on the microstructure evolution and mechanical properties of hot rolled 27Cr-4Mo-2Ni steel after solid solution treatment were studied,and the effects of cold-rolled reduction and annealing temperature on microstructure evolution and mechanical properties of 27Cr-4Mo-2Ni steel were also studied.Moreover,precipitation and recrystallization kinetics of cold-rolled samples annealed at high temperature were calculated in this paper.The main conclusions were drawing as follows:(1)The 27Cr-4Mo-2Ni hot-rolled steels was recrystallized after solution treatment at 1100 ? for 15 min.The grain sizes of WQ?AC and FC samples after solution treatment at 1100 ? reached 62.1±1.7 ?m,75.4±2.2 ?m and 77.9±3.9 ?m,respectively.The work hardening caused by hot rolling was completely eliminated after solution treatment.Only Ti N and Nb(C,N)particles were found in the water-cooled and air-cooled samples after solid solution,and a large number of Laves phases were found in the furnace-cooled samples except for Ti N and Nb(C,N).The tensile strength,yield strength and Vickers hardness increased from 590.0 MPa to 732.5 MPa,480.0 MPa to 632.5 MPa and 253.0 HV to 295.8 HV respectively with the decrement of cooling rates,while the total elongation decreased from 28.6% to 21.6%.The Laves phase produced during furnace cooling increases the strength and hardness of the sample while deteriorates the plasticity.(2)The grains of solution-annealed 27Cr-4Mo-2Ni steel sample changed from equiaxed to strip-shaped elongated along rolling direction after cold rolling.With the increase of the deformation amount,the degree of grain deformation gradually increased.The cold rolled samples showed excellent tensile strength(> 960.0 MPa),yield strength(> 900.0 MPa),but the elongation was extremely low(<1.2%).The Laves and sigma phases precipitated after cold-rolled 27Cr-4Mo-2Ni annealing at 950-1100 ?,the former precipitated during 950-1050 ? with precipitation time less than 2 min and distributed in grain boundary or sub-grain boundary with short incubation period,while the latter precipitated during 950-1000 ? with precipitation time more than 15 min and distribute around Ti N particles or grain boundary.The precipitation kinetics calculation indicated that the sigma precipitation kinetics at 950 ? is obviously faster than that at 1000 ?,and the content of sigma phase increases gradually with annealing time extension.In addition,the precipitation kinetics of sigma phase can be accelerated by the increase of cold rolling reduction.The precipitation of sigma phase can significantly improve the hardness of cold-rolled sheet,while the plasticity of the steel was deteriorated seriously,resulting in brittle fracture.Most of grains in cold-rolled samples remained elongated after 950 ? annealing.The recrystallization degree of cold-rolled samples gradually increased annealing temperature,and then basically recrystallized after annealing at 1050 ? for 2 min.The recrystallization kinetics were promoted by increasing the annealing temperature or cold rolling reduction.In addition,increasing cold rolling reduction is beneficial for the refinement of recrystallization grains.After annealing at 950 ? for 1h,the cold-rolled samples showed strong ?-fiber texture and weak ?-fiber texture.As the annealing temperature increased,the ?-fiber texture gradually weakened while the ?-fiber texture strengthened.When the annealing temperature reached 1050 ? for 1h,the samples possess a single ?-fiber recrystallization texture,which is beneficial to improve the formability of ferritic stainless steel.By comprehensively comparing the different cooling methods after solution treatment and the mechanical properties of cold-rolled sheets under different annealing processes,it can be concluded that water quenching is suitable for hot rolled plates after solution treatment.Its Vickers hardness,tensile strength,yield strength and elongation are 253 HV,590 MPa,480 MPa and 28.6% respectively.The best annealing process for cold-rolled sheet is: the annealing temperature range is 1050-1100 ?,the annealing time is 2 min,and the cooling method is water cooling.Under this annealing process,the 27Cr-4Mo-2Ni stainless steel cold-rolled sheet has the best strength(590-615 MPa/460-475 MPa),hardness(254-263 HV)and plasticity(20.6-23.1%). |
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