High Strength Hull Steel And Its Ultrafine Grain Strengthening Process

The rapid development of shipping industry, especially ship's tonnage become more and more big, the requirement of high strength hull structural steel has been a huge increase, which makes a good market prospect to research. This paper studies the micro-alloyed and grain refinement of high strength hull structural steel as viewed from strength and toughness mechanism beside describing the properties of high strength hull structural steel. In terms of characteristic of micro-alloyed elements, such as Ni, V, Ti, it can synthesize carbide nitrides and carbonitrides. The action of these compound is as follows, 1,obstruct original austenite grain to grow up 2,restrain recrystall during rolling 3,lead to precipitation strengthening in low temperature. The principle of steel strengthening is due to fine grains, precipitation strengthening and solution strengthening by adding alloyed elements. The grain refinement strengthening not only enhances strengthening of steel, but improves toughness and plastic.This paper analysis and summarizes the influence of impurity elements for mechanical properties. It mostly depends on chemical composition and mechanical tissues of steel, non-metallic impurity and gas, which destroy the continuity of base material of steel and make asymmetry in mechanical tissues, thereby reducing obdurability and fatigue of steel. The key point which influences mechanical properties is cleanness of steel. It is very important route to improve properties of steel that the technology is introduced such as pretreatment of molten iron and refinement outside furnace in smelt process.This paper analysis and studies chemical composition, microstructure, mechanical properties of high strength hull structural steel which is from Bao steel. All characteristic parameters of the trial-production are satisfactory to meet requirement of criterion or standard. It is well known that weld ability is very important to high strength hull structural steel. As can be seen from the results of testing, welded joint is of good synthesis mechanical properties.With the development of modern industry, ocean shipping makes material even more important than before. And the hull structural steel is also in face of huge development. The characteristic for new steel material in the future is of ultra-fine grain, high purity and high homogeneous distribution. The strength and longevity of service for steel doubled under no increase in cost of production , consuming less alloyed resource and energy and no decrease in toughness and plastic. The core theory and technology is to realize ultra-fine grain. Some methods leading to effective grain refinement of steel are introduced. This paper analysis super grain refinement structural steel through combining thermal mechanical rolling with re-crystallization treatment to micro-alloy steel. The toughness of steel is better than micro-alloyed with same composition. It play an important role in shipping and petroleum platform.