Study On Microstructure Evolution,Precipitation Behavior And Mechanical Properties Of Super-ferritic Stainless Steels

Super-ferritic stainless steels（SFSS）have excellent corrosion resistance,nice mechanical properties and good thermal conductivity.The SFSS is a kind of potential low-cost material to replace the titanium and copper alloy,which are used for manufacturing the cooling tubes with seawater as the medium.However,the crack and embrittlement breakage phenomenon often appears during the process of production and service of SFSSs.For the purpose of solving the above problems,a typical grade of SFSS,i.e.27Cr-4Mo-2Ni-Nb-Ti alloy,was selected for the present research,and a systematic study was carried out focusing on the precipitation behavior,embrittlement mechanism,and microstructural evolution,as well as the influence of precipitation phases on the recovery and recrystallization behavior and their mechanical properties of super-ferritic stainless steels.The main innovative results are as follows:（1）The isothermal precipitation behavior of intermetallics and its influence on mechanical properties of SFSSs were analyzed.During the isothermal aging in the temperature range of 600-1000°C,there were three kinds of intermetallic phase,i.e.σ,χand Laves existing in the SFSSs.The TTP curves ofσ,χand Laves were all C-shaped,and their‘nose temperatures’located between 800-850°C.The Laves phase had the shortest incubation period,followed byχandσphase.The submicron Laves phase nucleated at dislocations,and the nanometerχphase preferentially precipitated at the grain boundary,while the micronσphase nucleated along theχphase near the grain boundary.With the prolongation of aging time,theσphase grew along the grain boundary,then grew from grain boundaries into grains,lastly causing the grain boundary to be occupied by the dendriticσphase.Besides,theχphase transformed intoσphase and the islandσphase started to precipitate in the inner of grains.When the annealing time is long enough,theσphase was also found distributed around the Ti N particles.Theσ,χand Laves precipitations deteriorated the ductility of the material and theσphase precipitation made the microhardness of the steels enhanced dramatically.The toughness of the materials was seriously deteriorated by both the bulkσphase and micron-sized Laves phase,while the nanometerχphase gave a less deterioration effect than that did by theσand Laves phase.The impact toughness of the material was more sensitive than the elongation to the intermetallic phase precipitation.Theσphase precipitation fraction can be well reflected by the impact toughness and microhardness values.There was a positive linear relationship between the microhardness and the fraction of precipitation.（2）The microcosmic mechanism both of the rolling deformation acceleration precipitation kinetics and the Laves phase precipitation temperature increasing were studied,then a process route namely the reheating after hot rolling was proposed to eliminate the acceleration precipitation tendency of the brittle phases induced by hot rolling deformation.The shear bands and the high strain regions around Ti N particles induced by the rolling process provided many additional nucleation sites for the intermetallics precipitation;the high density dislocations generated during the rolling process gave a channel for the fast diffusion of Cr and Mo elements,which ultimately enhanced the precipitation driving force,and synchronously caused the segregation of Nb and Mo elements at the interfaces during the process of high-temperature annealing.Both of the above mentioned reasons made the precipitation temperature of Laves increase to as high as1050°C.A reheating process was added behind the hot rolling process,thus the deformation energy storage was consumed in time,and the subgrain boundary was eliminated,thus the precipitation kinetics of the brittleσ-phase was effectively reduced.（3）It was found that the embrittlement induced by Laves phase precipitation was related to their dimension.During the furnace cooling of high-temperature annealing process,the micron level Laves phase precipitated both at the grain boundary and in the grains.These micron level Laves phase particles deteriorated the toughness of the material significantly and further caused the brittle fracture during the process of tension deformation.After annealing treatment,the rapid cooling can be used to eliminate the Laves phase precipitation and ensure the toughness of the material.During the recrystallization annealing process of cold rolling specimens,the nanoscale Laves phase precipitated along the grain boundary.These nanosize Laves phase particles do not deteriorate the plasticity of the material,but has the beneficial effect of optimizing the texture and refining the grain.（4）The relationship between the intermediate phase precipitation and the annealing texture was established.During the low/medium-temperature recovery annealing for hot rolled specimens,theχphases precipitated along the shear bands in<111>//ND orientation grains.These shear bandχphases particles hindered the recovery of shear bands during the prolonged aging.The<110>//RD and<001>//ND orientation grains recovered fast before Laves phase precipitaon,and many sub-grain boundaries were formed.Then,the Laves phases formed at dislocations and the sub-grain boundaries in<110>//RD and<001>//ND orientation grains.After that,the sub-grain boundaries Laves phase particles pinned the sub-grain boundary movement,then hindered the further growth of sub-grain associated with sub-grain boundary aggregation.At last,theγ-fiber texture was weakened,while theα-texture was enhanced.In the process of high temperature annealing for the hot rolled specimens characterized by sub-grain boundaries,the recrystallization nucleation mechanism was subgrain coalescence.During the annealing,the Laves phase precipitated at the sub-grain boundary in<111>//ND orientation grains,then hindered the recrystallization nucleation and grain growth.At last,the theγ-fiber texture was weakened.During the high-temperature annealing of cold rolled specimens,the recrystallization nucleation mechanism was mainly shear bands nucleation.The nano-scale Laves phases pinned the grain boundary and refined the recrystallization grains.With the increase of annealing temperature,the fraction of Laves phase decreased rapidly leading to the unpinning of recrystallized cores nucleated at the shear bands inγ-oriented grains,then the surrounding{111}<110>grains were rapidly swallowed by the orientation growth mechanism to form a single{111}<112>γ-fiber texture component.（5）A process route based on the low temperature solid solution treatment,was preliminarily designed to produce the super-ferritic stainless steel cold-rolled annealed sheets.The hot rolled sheets were firstly solution treated at a low temperature of 1050°C,and the nano-scale Laves phases was pre-precipitaed at sub-grain boundaries,after that,a cold rolling process with one stage or a two-stage cold rolling+intermediate annealing process can be used to obtain a cold rolled and annealed sheet with fine grains and excellent performances.The grain sizes of specimens after final recrystallization annealing using the above mentioned two routes were about 16.2μm and 22.4μm,respectively.The elongations after fracture are 27.4%and 24.4%,respectively.The corrosion rates measured in Fe Cl₃ acid solutio at 65±1℃for168 h are 0.033 mm/y and 0.022mm/y,respectively.