Welding Physical Metallurgy Of RPC Ultrafine Microstructure Steel

Steel with high strength and excellent toughness can be made by fining the microstructure. But the grain coarsen of ultrafine microstructure at welding situation may hinder their application. This thesis is focused on the welding physical metallurgy of new ultrafine microstructure steel, which was processed by RPC (Relaxation-Precipitation-Controlling transformation) technique. Firstly, the phase transformation kinetics of HAZ microstructure was investigated. A SH-CCT curve was gotten for this RPC ultrafine microstructure steel. It supplies plenty of information on the physical metallurgy, and is a base for the designing of welding process.When cooling time t8/5 (the cooling time from 800℃to 500℃) was less than seven seconds, martensite was gotten in the HAZ. If it changed from 7s to 500s, intermediate temperature phase transformation happened in the coarse grained region of the HAZ. Bainite was obtained within this range and martensite islands appeared as t8/5 rose. As t8/5 was greater than 500s, a little amount of allotriomorphic ferrite came into existing along the boundaries of austenite grains. During the RPC processing, a lot of micro-precipitates and sub-grain boundaries were produced. They supplied plenty of nucleuses and boundaries which were beneficial to the austenite growth. The transformation time from ultrafine microstructure to austenite was shorter than that of the general steel during the heating in welding. The effects of Mo, B on the continuous cooling transformation diagram were studied and the results showed that they decreased the phase transformation temperature greatly during the cooling process in welding. Comparing with general high strength low alloy steels, the grain coarsening tendency in the HAZ of RPC ultrafine microstructure steel was relatively weak. The average grain diameter of austenite in the coarse grained region was changed from 19μm to 45μm, as t8/5 from 5s to 200s. Even if t8/5 was 2500s, the average grain size was 65μm. With a relatively higher heat input such as t8/5 over 15 seconds, there were two softened regions in the HAZ of RPC ultrafine microstructure steel. One of them was located at the fine-grained region and another was at the coarse-grained HAZ. Comparing with the coarse grained HAZ, the fine-grained region had a higher softening level and could not be avoided even in the welding joints made by laser welding or electron beam welding process. TEM analysis discovered that a great amount of fine ferrite grains appeared in the fine-grained region and resulted in softening at this area. Ultra-low carbon bainite or granular bainite became the main microstructure in the coarse-grained HAZ. The wide bainite plates and dissolving of Nb(C, N),ε-Cu made this region have lower strength than the base metal. By controlling of the heat input, the softening of coarse grained region could be prevented. In order to overcome a low toughness in the coarse-grained HAZ, the effect of oxide metallurgy on HAZ mechanical properties and microstructure was studied on the weld metal after rolling. In the cooling process of welding, a great amount of ferrite cells nucleated at the boundaries of these inclusions. Some of these ferrite cells grew becoming acicular ferrite directly while the others stopped developing. Some acicular ferrite nucleated at these small ferrite cells. The CCT diagram showed that the acicular ferrite began to transform from austenite between the temperature 510~530℃. The average grain diameter was 3~4μm. This kind of steel produced by the simulated oxide metallurgy had high toughness in the weld HAZ as the cooling time t8/5 changed from 6s to 200s. The absorbed energy of Charpy impact specimens was above 150J at -20℃. In the joints of laser welding, electron beam welding and MAG welding, the coarse grained region in the weld HAZ of oxide metallurgy steel also had high toughness. Laser welding was used for the research of RPC ultrafine microstructure steel. The strength of the laser welding joint could be as the same as that of the base metal while the soften region existed; the coarse grained region of the HAZ had a high toughness; the bainite microstructure were gained both in the weld metal and the coarse grained region of the HAZ. By using electron beam welding, a penetrated weld on plates with 11mmthickness could be produced, the strength of the welding joint was similar to that of the base metal, but softening of the fine grained region still existed. It must be pointed out that the weld metal of high energy welding processes has poor toughness in the joints of 800MPa grade ultrfine microstructure steel. Only the oixde metallurgy steel can get high toughness in the weld metal. A high strength MAG welding wire has been developed for this RPC ultrafine microstructure steel. The welding deposit has 840MPa tensile strength combined with 128J Charpy impact energy at -40℃. It is not reported yet that a welding wire with this chemical composition has such a high strength and toughness. The joints by this MAG welding wire have a high strength and the weld metal has excellent toughness. Plenty of acicular ferrite has been obtained in the weld metal.