| Iron and steel are basic materials to create modern civilization and one of the important material bases of human advance. They are still the main materials in the world of today owing to their high strength, versatility and durability, and are the foundation of industrial development. It is the main objective for iron and steel industry in the21st century to increase strength, toughness, plasticity, processability and service life of iron and steel materials. The performance can be highly increased, the utilization ratio and recovery ratio of the resource can be effectively increased by using special technological process for the existing traditional iron and steel materials. At present, the performances of iron and steel materials are highly increased mainly by using combination of technical methods of purification, microalloying, controlled rolling and controlled cooling process etc..Low-carbon microalloy steel is a kind of low-carbon steel or ultra-low-carbon steel into which some microalloy elements such as Nb, V and Ti etc. are added. These elements, combined with carbon and nitrogen, form the carbides, nitrides and carbonitrides which play an important role in the precipitation of the steel as a second-phase. Combining with the controlled rolling and cooling technology to achieve high strength and high toughness, thus the comprehensive performances of the steel can be increased. Low-carbon microalloy steel is one of steels which has rapidly developed, has been widely used and has been paid more attention in recent years. How to improve its microstructure and increase its performance is the key problem which it faces. The tiny second-phase particles in low-carbon microalloy steel are almost precipitated from austenite below solidus, so the particles can not act as nucleating centers of austenite itself, thus they are of no effect on the refinement of the original austenite grains. And not only the production process is more complex, but its cost is higher. In order to overcome these disadvantages, a new technology of adding the second-phase particles into the molten steel to strengthen and toughen iron and steel materials is put forward. The refinement of the second-phase particles for steel includes nucleating refinement in freezing crystallization and recrystallizing refinement in the later rolling process. Compared with the method of endogenous precipitated second-phase particles, the added method can greatly decrease the uncertainty of the quantity and size of the endogenous particles, and it has more controllability. Moreover, the added method doesn’t need high purity in steel, so it can be conveniently used in the industrial scale production of steel materials.In this paper, a kind of low-carbon microalloy steel was used as the matrix into which1.3μm ZrC particles were added from outside by special technique during vacuum induction melting. The matrix’s main chemical composition (mass fraction,%) is0.054C,0.118V,0.070Nb,0.202Ti,0.018Si,0.346Mn,0.018P and0.01OS. Controlled rolling and controlled cooling processes were carried out after the cast ingot was heated for austenization. The test steels were studied contrastively before and after the ZrC particles were added, in order to investigate the effect of the added ZrC particles on the microstructures and the mechanical properties of the test steels. The mechanical properties of the test steels were characterized by tensile, impact and hardness tests. The microstructures of the test steels, the distribution and the morphology of the second-phase particles were systematically studied by metallographic microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). The chemical composition and quantity of the precipitated phase were determined by chemical phase analysis, and the grain size distribution of the precipitated phase was determined by small angle X-ray scattering. Thus the strengthening and toughening mechanisms of the test steels were discussed preliminarily. At last, the microstructures and the mechanical properties of the test steels after heat treatment were also investigated.The experimental results show that the grains of the test steels with added ZrC particles are refined notably, so the added ZrC particles have the effect of refining grains. The quantity of the added ZrC particles was optimized by single factor test. It is found that when the volume content of the added ZrC particles is1.1%, the test steel can obtain the minimum grains and the optimum comprehensive mechanical properties. And the grain size, yield strength, tensile strength, Vickers hardness, rate of elongationδs and impact toughnesses are5.5μm,517.5MPa,635MPa,214.0,20.66%and215.0J/cm2, respectively.The micro structure of the test steels is still consisted of ferrite and a tiny little pearlite, and no bainite and lath martensite are found after the ZrC particles were added. In the as-cast test steels, the ZrC particles disperse inhomogeneously and accumulate at the grain boundaries, so the particles inhibit the grain growth. This results in the grain refinement. Both the high temperature of austenitizing and great plastic deformation quantity during rolling promote the homogeneous distribution of the ZrC particles in the test steels. So the ZrC particles enter into the grains, and gradually become the deformation centers and recrystallization centers when dislocations detour the particles during rolling deformation, and then notably increase the nucleation rate in the area of deformation and dynamic recrystallization, and promote the grain refinement. Both the tensile fractures and impact fractures of the test steels exhibit the feature of typical ductile rupture, and the added ZrC particles can be found in the dimples of the fractures.The test steel added with ZrC particles of1.1%volume fraction was measured by chemical phase analysis and small angle X-ray scattering. The results show that the precipitated phase in the test steel is mainly carbide MC, its constitutional formula is (Nb0.121Ti0.528Zro.293V0.058)C, which belongs to face-centered cubic lattice. And there is no MC precipitated phase with size less than lOnm, the contents of10～18nm MC,18～36nm MC,36～96nm MC and96～430nm MC precipitated phases are2.3%,2.8%,13.5%and65.1%in the test steel respectively. Via calculation, the increments in strength for the3#test steel contributed to grain-refining strengthening, solution strengthening, precipitation strengthening and dislocation strengthening are234.6MPa,28.7MPa,61.0MPa and141.5MPa, respectively. Therefore, the effects of the added ZrC particles on the steel are to refine grains and increase dislocation density whereas solution strengthening and precipitation strengthening are not obvious relatively.After heat treatment, the optimized result is still obtained for the test steel added with ZrC particles of1.1%volume fraction. However, compared with the test steel before heat treatment, its comprehensive mechanical properties do not increase distinctly, and its microstructure doesn’t change. |
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