1 Sheet Metal Research Institute, Shougang Research Institute of Technology, Beijing 100043, China 2 Beijing Key Laboratory of Green Recyclable Process for Iron & Steel Production, Shougang Group, Beijing 100043, China;
Cite this article:
Yajun HUI,Hui PAN,Wenyuan LI,Kun LIU,Bin CHEN,Yang CUI. Study on Heating Schedule of 1000 MPa Grade Nb-Ti Microalloyed Ultra-High Strength Steel. Acta Metall Sin, 2017, 53(2): 129-139.
In the production process of ultra-high strength steel, heating schedule of casting slab is one of primary controlling parameters in hot rolling techniques. Heating temperature and holding time affect the prior austenite grain size and the solution of microalloyed elements directly, which both affect austenite recrystallization, precipitation and mechanical properties. A plenty of researches have been made to get better understanding and controlling the austenite grain size or precipitation behavior during austenitizing process over the past half a century, but it lacks systematic researches. Hence it is very important to confirm a reasonable heating schedule. In this work, the austenite grain coarsening behavior and microalloying carbonitrides dissolving behavior in 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel during isothermal holding at different temperatures were studied by OM, TEM and EDS. The results showed that the precipitates in the slab can be obviously classified into three kinds by their size and shape. The average dimension of the bigger cubic precipitates is over 1 μm, and that of the smaller spherical, ellipsoid or cubic precipitates is below 500 nm or less. EDS results showed that the bigger cubic precipitates are TiN, and the smaller spherical, ellipsoid or cubic precipitates are mainly composed of Nb, Ti composite precipitates and a bit of TiS or Ti (C, S). With the increasing of holding temperature, increase of the original austenite grain size showed a monotonous increase trend, and the austenite grain grows rapidly when the heating temperature exceeds 1200 ℃; the amount of precipitates decreased and their size increased, the atomic ratio of Ti/Nb increased gradually, and EDS results showed all the precipitates contain Nb, Ti elements. With the increasing of holding time, the average austenite grain grew up in parabolic law, the amount of small sized spherical and ellipsoid precipitates dissolved gradually, and that of the large sized cubic precipitates increased gradually and their edges become blurred. The effects of heating temperature and holding time on austenite grain size and precipitation behavior were considered synthetically, the heating temperature at 1250 ℃ by holding 80 min will be more appropriate for the 1000 MPa grade Nb, Ti microalloyed ultra-high strength steel.
Fig.1 Effects of heating temperature on the austenite grain size and grade of 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel
Fig.2 OM images of 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel at 950 ℃ (a), 1050 ℃ (b), 1150 ℃ (c), 1200 ℃ (d), 1250 ℃ (e) and 1300 ℃ (f) by holding 40 min
Fig.3 Average austenite grain size and grade of 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel at 1150 and 1250 ℃ by holding different times
Fig.4 OM images of 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel at 1150 ℃ by holding 10 min (a), 40 min (b), 80 min (c) and 120 min (d)
Fig.5 OM images of 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel at 1250 ℃ by holding 10 min (a), 40 min (b), 80 min (c) and 120 min (d)
Fig.6 TEM image (a) and EDS results of precipitates 1 (b), 2 (c) and 3 (d) showed by arrows in original specimen of 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel
Fig.7 TEM images and corresponding EDS results (insets) of precipitates (showed by arrows) in 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel at 1300 ℃ (a), 1250 ℃ (b), 1200 ℃ (c), 1150 ℃ (d), 1050 ℃ (e) and 950 ℃ (f) by holding 40 min
Fig.8 Size distribution of precipitates in 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel at different heating temperatures by holding 40 min
Fig.9 TEM images of precipitates of 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel at 1150 ℃ by holding 10 min (a), 40 min (b), 80 min (c) and 120 min (d)
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