Keywords: ultra-fine grained steel: welding: grain growth: coarse weaving
China Library ClassificationNo.: TG40 1 Document IdentificationNo.: A DocumentNo.: 0253-360x (2001) 06-01-03.
Preface of 0
In the national major planning basic research project "the major basic research of new generation steel materials", the goal of doubling the strength and toughness of steel will be achieved through grain refinement. For ultra-fine grained steel, the deterioration of properties caused by grain coarsening in heat affected zone and the softening of heat affected zone caused by improper welding heat input will be the most important problems. It is necessary to study the influence of welding thermal cycle on the microstructure and properties of base metal, and to study the new welding technology and technology suitable for ultra-fine grain steel.
In its "super steel" planning, Japan regards the welding technology of super steel as one of the three major research topics, and puts welding in an extremely important position in the 800MPa high-strength project. South Korea also attaches great importance to the welding of ultra-fine grained steel in high-performance structural steel in the new century [3]. In order to make the welded joint have more than 90% parent metal properties (strength and toughness), comprehensive work has been carried out from three aspects: welding technology, welding materials and welding technology.
In this paper, the grain growth law of ultra-fine grained steel in welding heat affected zone is studied, the adaptability of pulsed MAG and laser welding to ultra-fine grained steel is studied, and the exploratory research on improving the properties of welded joints by special post-welding treatment technology is carried out.
1 Ultra-fine grained steel for test and its experimental study
The test material is SS400 hot rolled steel plate rolled by Baosteel 400MPa research group. The research goal of this material is to double the yield strength by grain refinement. The thickness of the plate is 3 mm, and its chemical composition and mechanical properties are shown in Table 1 and Table 2. The original ferrite size of the material is 6 ~ 8 μ m.
The law of grain growth in welding heat affected zone (HAZ) was studied by welding thermal simulation test, and the pulse MAG welding adaptability, microstructure of HAZ and mechanical properties of welded joints of 400MPa ultra-fine grain steel were studied.
Grain growth trend, microstructure and properties of ultra-fine grained steel in heat affected zone
In order to study the influence of welding thermal cycle on ultra-fine grained steel, Gleeble- 1500 welding thermal simulation testing machine was used to carry out welding thermal simulation test on the test materials. The test design is as follows. The peak heating temperature of (1) is fixed at Tp= 1350℃, and the cooling rate t8/5 is changed from 3 seconds to 24 seconds, so as to simulate the microstructure and properties of the coarse-grained zone in the heat affected zone under different welding heat input conditions. (2) The cooling rate is fixed at t8/5=5s, and the peak temperature Tp changes from 1400~650℃ to simulate the microstructure and properties of different parts of the welding heat affected zone under the same welding heat input condition. The results of welding thermal simulation test are shown in figure 1. Fig. 1a shows the influence of welding heat input on the original austenite grain size in the coarse-grained region. When the peak temperature is 1350℃, the original austenite grain size in the coarse-grained zone of the heat affected zone gradually increases with the increase of welding heat input, and when t8/5 is 20s, the austenite grain size reaches 170μm, which indicates the welding heat affected zone of ultra-fine grained steel. Fig. 1b shows the influence of peak temperature on the original austenite grain size when t8/5=5s. When Tp is between1100 ~1200℃, the austenite grain size obviously begins to coarsen, which can be used as the coarsening temperature of SS400 steel. When Tp> is at 1350℃, the austenite grains are not coarsened, but decreased, which may be due to the local melting at the austenite grain boundary, resulting in the decrease of grain size. Figures 1c and d show the microhardness measurement results. From the data in the above figure, it can be concluded that with the increase of t8/5, the hardness of the coarse-grained zone in the heat affected zone gradually decreases and tends to be stable, and the hardness reaches the maximum when t8/5=3s. When t8/5=5s, the microhardness increases with the increase of peak temperature Tp, and reaches the maximum when Tp= 1400℃. After welding thermal simulation at different peak temperatures of t8/5=5s, the hardness of the whole HAZ of SS400 steel is not lower than that of the base metal, so it can be predicted that the HAZ of SS400 steel will not soften when t8/5.
Figure 1 welding thermal simulation test results
Because ultra-fine grained steel mainly obtains fine grains under deformation conditions and cannot be recovered by heat treatment, the heat affected zone will soften after welding, especially when large heat is input. However, the influence of local softening on the overall strength of the joint is controlled by other factors, such as the width of local softening zone, plate thickness and weld strength matching. The tensile strength of SS400 joints under three specifications all broke at the base material, indicating that at least when T8/5