Major progress in low alloy steel

- Apr 20, 2019-

Since the 1970s, the development of low-alloy high-strength steel in the world has entered a new era. Based on the control of rolling technology and metallurgy of microalloying, modern low-alloy high-strength steel, ie microalloying, has been formed. New concept of steel. In the 1980s, a variety of industries involving a wide range of industrial fields and specialized materials, reached the peak with the achievements of metallurgical process technology. In the four-in-one relationship of chemical composition-process-organization-performance of steel, the dominant position of steel microstructure and micro-fine structure is highlighted for the first time. It also shows that the basic research of low-alloy steel has matured and unprecedented. The new concept of alloy design.

Firstly, significant progress has been made in the study of the dissolution-deposition behavior of microalloying elements, especially Nb, V, Ti, and Al. The formation of carbides and nitrides of these elements and their quantity, size, and distribution depend on cooling. Deformation temperature and deformation of the process, and the existence of carbon and nitride in the heating process and its characteristics are manifested in the secondary hardening of tempering, the recrystallization refinement of normalized grains, and the control of grain size under the thermal cycle of welding. 3 main aspects.

Second, attach importance to the development of Nb-containing microalloyed steel, Nb-V and Nb-Ti composite microalloyed steel. According to statistics, it has almost 75% of the newly developed microalloyed steel grades and the total output of microalloyed steel in the past 20 years. 60%. In recent years, it has been noticed that the trace amount of Ti (≤0.015%) is very beneficial. The micro-treatment of Ti not only changes the morphology of sulfide in steel, but also TiO2 or Ti2O3 becomes the particle of nucleation of ferrite grains in austenite grains. Nb-Ti composite microalloying constitutes the metallurgical foundation of ultra deep drawing automotive sheet IF steel, and also significantly improves the crack sensitivity of Nb steel continuous casting.

Thirdly, the Hall-Petch relationship of low carbon steel strengthening is systematically summarized, and the principle of accelerated cooling is studied in more depth. People are very interested in the application of a staged accelerated cooling process. The accelerated cooling in the early stage is used to suppress the transformation of ferrite. The purpose of accelerated cooling in the later stage is to control the grain size and fine structure of the medium and low temperature products, so as to achieve a wide range. The internal strength of the steel is adjusted to match the strength/toughness.