Effect Of Mg/Zr Treatment On The Austenite Grain Growth In Ship Plate Steels

With the development of shipping building industry, the demand on high strength level ship plate steel is increasing. However, the high-performance ship plate steel is still in short supply. On the other hand, high heat input welding is widely used in the process of ship plate steel welding. As the temperature of welding heat affected zone is as high as 1350～1400℃, a big problem is that the austenite grains in welding heat affected zone grow up severely, exacerbating the performance of welding heat affected zone and also bringing the development of high strength ship plate steel serious test.In the present study, based on the FH40 level ship plate steel, some steels were refined in vacuum induction furnace with the capacity of 50kg by adding Mg, Zr and Mg-Zr. Then the test steels were all subjected to forging and rolling process. Austenitizing tests were performed under specific temperature and holding time, and the austenitic grain size of different test steels were investigated by using metallographic microscope observation. On the other hand, the austenitic grain growth activation energy and grain growth index were analyzed after fitting the experimental datas. The following conclusions were obtained:(1) In the process of isothermal austenitizing, single magnesium treatment can refine the austenitic grain size of ship plates steels, and increase the uniformity of grains. Mathematical regression analysis shows that magnesium treatment can increase the austenitic grain growth index, and affect the austenitic grain growth mechanism. However, with the addition of Mg increased from 0.024% to 0.072%, the change of austenite grain size is not obvious.(2) In the process of variable temperature austenitizing, in the range of 1000～1100℃, single magnesium treatment has no significant impact on the austenitic grain size. In the range of 1200～1300℃, slightly refinement austenite grain size is obtained by adding trace Mg. When the temperature exceeds 1350℃, even single Mg processing steel has a tendency of coarsening, but when heated to 1400℃, the trend of grain growth turns to slow.(3) In the process of isothermal austenitizing, under the condition of single zirconium treatment, single zirconium treatment can also reduce the austenitic grain size of ship plate steels, at the same time improved the uniformity of grains.(4) In the process of variable temperature austenitizing, in the range of 1000～1100℃, single zirconium treatment increases the austenitic grain size to a certain extent. In the range of 1000～1100℃, austenite grain size is more small than the benchmark steel, but when the temperature reaches 1350℃, the austenitic grain size of zirconium treatment steel becomes coarsening, when heated to 1400％,austenite grain size refines slightly.(5) On the condition of Mg-Zr composite treatment, when the heating temperature below 1350℃, regardless of isothermal or variable temperature growing up, the austenite grain size of experimental steel is slightly larger than the benchmark steel, but the uniformity of grains improves. When the temperature reaches 1400℃, the of trend grain growth slows, even refines after Mg-Zr composite addition. In addition, Mg-Zr composite treatment steel obtains a more refined rolling microstructure than benchmark steel.