Effect of Microadditives on Center Segregation and Mechanical Properties of High-Strength Low-Alloy Steels

Abstract

One of the main tasks of steelmaking is to obtain the necessary thermodynamic conditions that ensure fi ne and homogeneous solidifi ed microstructure. The relatively economical solution is to feed microadditives into liquid steel. The metallurgical considerations and techniques on the feeding of microadditives are described in the present work. The effect of small addition of reactive fi ne particles and ultra-high melting point powders on the control of non-metallic inclusions, slab center segregation, and mechanical properties, especially the Z-direction properties, was investigated. The results showed that the microadditives had composite effects on microalloying and modifi cation of inclusions. The segregation of elements in the slab center of high strength steels was remarkably reduced. The mechanical property along the Z-direction of steel plates was greatly improved.Introduction. The morphology, dimension, composition, and distribution of inclusions in steels have a remarkable infl uence on mechanical properties and microstructure of steels [1, 2]. There has been much attention focused on inclusions in steels as an effective method for grain refi nement. Feeding powders or cored wires into liquid steel is commonly practiced for microalloying and refi ning high strength steels. This is a relatively effi cient method to reduce the sulfi de content and signifi cantly modify the performance of inclusions in steels.Thermochemical data indicate that Ti, and Zr, and rare earths (RE) are strong deoxidation elements and their oxides are stable in liquid steels [3][4][5]. Meanwhile, extensive studies reported that Ti-bearing inclusions are the effective nucleating cores for acicular ferrite when the inclusion size is appropriate [6][7][8]. Many investigations demonstrated that the addition of Zr in steels has positive effects, such as inhibiting the austenite grain growth, controlling the sulfi de shape, as well as improving the impact toughness [9]. Cerium is also considered to be an extremely effective nucleation element and can modify the shape of inclusions [10,11], leading to improvement of the mechanical properties as well as the corrosion resistance of steel plates [12].However, less attention has been paid to the effect of composite microadditives on the improvement of center segregation and mechanical properties. The present work aims to investigate the effect of microadditives of Zr-Ti carbides and nitrides on the center segregation, microstructure, and mechanical properties of high-strength low-alloy steels.Experiments. Two high-strength low-alloy (HSLA) steel plates, Steel A treated by using microadditive feeding and Steel B treated by using conventional Si-Ca feeding, were made in an industrial scale. The refi ning process of both steels is identical. The basic chemical composition of these two steels before feeding is listed in Table 1. The type, size, and amount of microadditives are listed in Table 2. The feeding temperature of microadditives was 1530-1535°C.

Effect of Direct Powder Additions on the Solidification Structure and Microsegregation of 42CrMo4 Steel

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