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Hot tensile tests were performed to examine the effect of boron on the hot ductility of Nb-Ti-microalloyed steels. The equilibrium precipitation in the steel was predicted by Thermo-Calc calculation. The microstructure, fracture surface and precipitates in the deformed steel were examined. The results show that boron addition is favorable to improving the hot ductility of Nb-Ti-microalloyed steel. This bene cial effect is caused by the soluble boron instead of coarse BN in the steel. The hot ductility of the steel decreases less from 1000 C with increasing boron addition. The hot ductility trough shifts toward lower temperatures because ferrite formation was restrained with increasing boron content of the steel. The formation of NbC, TiN and thin lm-like ferrite along austenite grain boundaries lead to the decrease in the hot ductility of the steel. Boron addition has negligible in uence on the precipitation temperature and amount of TiN and NbC precipitates in Nb-Ti-microalloyed steel. The amount of NbC precipitates is largest in the steel, followed by TiN and BN. The precipitation temperature of BN increases considerably with further increasing the boron content. The fracture mode of Nb-Ti-microalloyed steel tends to be more ductile with the increase in the boron content of the steel.

For three-dimensional radar, maintenance cost is high and maintenance time is difficult to be determined. What's more, model simulation computing is significantly complex. The subject of this paper is a new generation of meter wave active, phased array 3D radar. We put forth a concept that maintenance should be done after dividing into several regions to the asymmetrical distribution planar array antenna. First, a failure model of array elements is built to analyze the influence from the element to antenna. Second, the maintenance optimization model is established. Finally, computer simulations are conducted to verify the feasibility and effectiveness of the proposed model. Keywords: three dimensional radar; antenna array; maintenance optimization IntroductionThe three-dimensional radar(3D) has the feature of small bulk, light weight and simple structure.It plays an important role in the modern high tech war. It is of great significance to maintenance the 3D radar. As the main subsystem of the 3D radar, the optimization of array antenna's maintenance gains more attention from military and scientific research units. Generally, the antenna has a large number of array elements, and is designed with a redundant structure, so it can be disposed as a K/n(G) system [1-2]. At present, there exists three main problems in the maintenance optimization of the antenna. The first one is the high maintenance costs and uncertain maintenance time; the second one is that the subject of this paper is a new generation of meter wave active, phased array 3D radar, which is referred to hereafter simply as 3D radar. Compared with conventional radar, it has the asymmetrical distribution of the antenna elements due to the use of amplitude weighting [3]; the last one is the difficult simulation and calculation of the optimal models because of the huge number of array elements. In recent years, there are few reports about how to solve these problems. In literature [4] a condition-based optimal maintenance model is proposed by taking the antenna as a uniform distribution array. This is why we take meter wave active, phased array 3D radar for example. This solves the first problem. However, the other two problems are not considered in literature [4]. To solve these problems, this paper proposes a maintenance optimization model. Firstly, the antenna will be divided into several regions according to the influence rule of the elements to the antenna performance, and then the proposed problems can be solved.In order to study the relationship between elements' failure and antenna performance, some experts introduce the idea of density weighting matrix, and a series of research results are achieved [5][6][7]. These references consider that the effect from element in different positions on the antenna performance is the same. So they mainly study the effect of failure rate on antenna performance. However, the influence of element in different positions on antenna performance is not investigated. In

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