Microalloyed steel

About: Microalloyed steel is a research topic. Over the lifetime, 2183 publications have been published within this topic receiving 33586 citations.

Tensile Properties of Simulated Thin Slab Cast and Direct Rolled Low-carbon Steel Microalloyed with Nb, V and Ti

Abstract: Microalloying additions are made to low carbon steels for a number of purposes. In the conventional controlled rolling of reheated slabs, an important function of niobium is to control recrystallisation in the austenite phase field in order to refine the ferrite grain size of the controlled rolled product. A second benefit is that niobium makes a contribution to the strength of the product through precipitation in the ferrite. Vanadium carbides and nitrides are too soluble in austenite to have a significant effect on grain refinement, when added to a Nb-bearing steel, but may augment precipitation strengthening, particularly at raised nitrogen levels. Titanium is generally added to restrain grain growth in the austenite on reheating and in the heat affected zone of welds, but is also capable of restraining recrystallisation in the austenite during controlled rolling, and of contributing to tensile strength. In conventional processing of continuously cast thick slab, the steel is reheated to 1 200–1 250°C for up to 8 hr before rolling. This conditions the austenite by partial homogenisation and solution of the microalloying elements. The austenite grain size after soaking and at the start of the rolling process is then usually smaller than it was in the original casting, being determined by the dispersion of any undissolved microalloy nitrocarbides and undissolved inclusions. In recent years, however, the practice of hot direct rolling has been developed, in which the as-cast austenite is rolled directly, without the intermediate stages of cooling to a low temperature, reheating and soaking. The austenite then has the coarse-grained, dendritic and segregated microstructure of the original casting prior to the commencement of hot rolling. Although hot direct rolling has been practised using conventional thick slab, its most practical application is in the production of hot rolled strip from thinner slab in minimills. With this newer technology, slab in thicknesses down to 50 mm is continuously cast and directly hot rolled within, typically, 15–20 min after casting; the only “heat treatment” between casting and rolling is temperature equalisation at approximately 1 100–1 150°C during that time interval. Both niobium and titanium segregate to the liquid phase during freezing, to form eutectic carbonitrides or nitrides on solidification. In the absence of a prolonged soaking period at high temperature, it might be expected that the presence of such precipitates in the austenite at the commencement of direct hot rolling might reduce the effectiveness of the microalloying additions, compared to their effect in conventional processing. The purpose of this research, therefore, was to investigate the tensile properties of hot direct rolled low carbon steels microalloyed with niobium, the composition being varied to allow eutectic NbCN to form over part of the range. The effect of including V and Ti additions over part of the composition range was also investigated.

Influence of phosphorus and boron on hydrogen embrittlement susceptibility of high strength low alloy steel

Abstract: Hydrogen-charged miniaturized Charpy-type specimens were subjected to three-point bending tests to investigate an influence of phosphorus and boron on the hydrogen embrittlement (HE) susceptibility of high strength low alloy (HSLA) steels for bolts. The tests were carried out under wide variety of deformation rate to examine an effect of deformation rate on the susceptibility also. The experimental results revealed that the HE susceptibility increased with decreasing deformation rate. This dependence of susceptibility on deformation rate seemed to be associated with velocity relations between the deformation rate and the diffusion rate of hydrogen. The susceptibility was more pronounced by the addition of phosphorus, even though the phosphorus segregation was not sufficient by itself to cause temper embrittlement. On the other hand, boron had almost no influence by itself on the susceptibility. A cyclic voltammetry and thermal desorption spectroscopic analysis were conducted for understanding the change in HE susceptibility with phosphorus and/or boron from the point of view of hydrogen adsorption/absorption characteristics, respectively. However, it was hardly observed that those elements influenced the hydrogen content and the trapping site of hydrogen in the steel under the present hydrogen charging condition. Consequently, it was considered that the increase in HE susceptibility with phosphorus mainly resulted from the reduction in grain boundary strength due to phosphorous, rather than the variation in hydrogen adsorption/absorption characteristics.

Properties and homogeneity of 550-MPa grade TMCP steel for ship hull

Abstract: Ultra low carbon steels by the thermal mechanical control process (TMCP) with less Ni, Cr, and Mo contents have been developed for 550 MPa grade heavy gauge ship hulls and offshore structures. The relationships among microstructures, process, and properties of the studied steel have been investigated. A series of accurate control technologies have been developed for this kind of steel. Cu microalloying and TMCP+relaxation precipitation control (RPC)+accelerated cooling process were employed to optimize the mechanical properties and ensure the homogeneity of the 80-mm thick plate. The microstructures of thin plates slightly changed from surface to center, but the microstructures of the heavy gauge plate (80 mm) changed notably. Adopting the simple composition, it can meet the requirement of thin plates by adopting a few microalloys. As for thick plates (80 mm), a little higher Cu and Ni contents should be adopted. These steels can meet the needs without tempering. By these ways, the properties of the steels can be optimized, and the cost can be decreased notably.

Effect of thermomechanical processing on the microstructure and mechanical properties of Nb–Ti microalloyed steel

Abstract: Based on analysis of austenite deformation behaviour during thermomechanical processing of Nb–Ti microalloyed steel, the rolling schedules were designed to produce (i) recrystallized austenite, (ii) unrecrystallized austenite, and (iii) ferrite-pearlite. The effects of austenite conditioning on the final ferrite-pearlite microstructure and mechanical properties of steel were investigated. To rationalise the variation in final ferrite grain size with different thermomechanical processing schedules, it is necessary to consider the ferrite grain growth in addition, to the density of ferrite nucleation sites. Mechanical properties were the means to evaluate the variation in austenite solutioning and deformation conditions introduced into individual applied rolling schedules. The benefit of tensile tests, especially yield strength and ductility values, in determining the optimum deformation schedule and coiling condition for given steel is demonstrated.