Abstract
Reasonable control of the niobium (Nb) content in steel slabs is of great significance for improving the surface quality of the slabs. In this study, a Gleeble-3500 thermal simulation testing machine was used to test the high-temperature mechanical properties of steel slabs with four different Nb contents (A: 0.006%, B: 0.031%, C: 0.050%, D: 0.065%) and to analyze the effect of Nb on the high-temperature performance of the steel slabs. Optical microscopy and transmission electron microscopy (TEM) were used to study the microstructure of the casting slabs with different Nb contents and the precipitation in the samples after heat treatment at different temperatures. The results of the study show that the microstructure of the cast slab is mainly composed of ferrite and pearlite. The distribution of each of the two structures was found to become more uniform and a significant improvement in yield strength was observed with increasing Nb content. After heat treatment, the precipitates in the high-rise structural steel were mainly square and star shaped Nb(C,N) precipitates. Increasing the Nb content was found to increase the amount of precipitates in steel. Beyond a threshold, increasing the Nb content did not increase the amount of precipitates. However, the size of the precipitates increased.
Highlights
High-rise structural steel is a type of low-alloy high-strength steel, which is the most produced and consumed type of steel in China [1,2,3,4,5]
In the cooling process of continuous casting billets and rolling materials, these precipitates can play a role of precipitation strengthening and improving the strength of high-rise structural steel on one hand, and on the other hand, they can pin the austenite grains in the welding heat-affected zone in the subsequent welding of steel to prevent the coarsening of the structure or affecting the welding performance
For the continuous casting of thick slabs of high-rise structural steel, the surface temperature of most wide side parts of the cast slab is maintained at 1000–1100 ◦C for a long time, while the temperature of the corners of the cast slab is maintained at 800–900 ◦C, due to the two-dimensional heat dissipation, which corresponds exactly to the precipitation temperature of Nb-containing carbonitrides
Summary
High-rise structural steel is a type of low-alloy high-strength steel, which is the most produced and consumed type of steel in China [1,2,3,4,5] It has good comprehensive mechanical properties, low-temperature performance, welding performance, and plasticity. For the continuous casting of thick slabs of high-rise structural steel, the surface temperature of most wide side parts of the cast slab is maintained at 1000–1100 ◦C for a long time, while the temperature of the corners of the cast slab is maintained at 800–900 ◦C, due to the two-dimensional heat dissipation, which corresponds exactly to the precipitation temperature of Nb-containing carbonitrides. This research intends to start from the laboratory smelting of steels with different Nb contents through to systematically observing and analyzing the microstructure, high-temperature mechanical properties, and Nb-containing precipitate characteristics of high-rise structural steel under different Nb content as-cast conditions, in order to determine the best Nb content range in high-grade high-rise structural steel, providing a basis for the smelting process of high-rise structural steel and improving product quality
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