Effect of Zr-Ti combined deoxidation on the microstructures and mechanical properties of high-strength low-alloy steels were investigated by means of microstructure analysis, tensile- and Charpy impact-tests. Experimental results showed the addition of Zr-Ti based deoxidizing agent resulted in the formation of fine Zr-Ti complex oxides. Owing to abundant finely oxides dispersed in the matrix uniformly, the microstructures deoxidized using Zr-Ti was uniform and the strength was enhanced comparing against the steels deoxidized using Al based deoxidizer. The ductility and toughness of the thick steel plates deoxidized using Zr-Ti were also considerably improved, apparently due to the spheroidization and uniform dispersion of MnS.

Effect of Zr-Ti combined deoxidation on the microstructure and mechanical properties of high-strength low-alloy steels

Effect of heat input on impact toughness in transition temperature region of weld CGHAZ of a HY 85 steel

Recently there has been a renewed interest in the addition of zirconium to microalloyed steels. It has been used since the early 1920s, but has never been universally employed, as have niobium, titanium or vanadium. The functions of zirconium in steelmaking are associated with a strong chemical affinity, in decreasing order, for oxygen, nitrogen, sulphur and carbon. Historically, the main use of additions of zirconium to steel was for combination preferentially with sulphur, to avoid the formation of manganese sulphide, known to have a deleterious influence of the impact toughness of wrought and welded steel. Modern steelmaking techniques have also raised the possibility that zirconium additions can reduce the austenite grain size and increase dispersion strengthening, due to precipitation of zirconium carbonitrides, or in high nitrogen vanadium–zirconium steels, vanadium nitride. This review gathers information on the compounds of zirconium identified in steels together with crystallographic data...

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Role of zirconium in microalloyed steels: a review

Conventional transformation induced plasticity (TRIP) steel (0.17C-1.52Si-1.61Mn-0.03Al, wt%) was produced via strip casting technology simulated in the laboratory. Effects of holding temperature, holding time and cooling rate on ferrite formation were studied via analysis of the continuous cooling transformation diagram obtained here. A typical microstructure for conventional TRIP steels consisting of ~ 0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite was obtained. However, coarse prior austenite grain size of ~80 µm led to large polygonal ferrite grain size of ~17 µm, coarse second phase regions of ~21 µm size, small amount of retained austenite (0.02–0.045) and the presence of Widmanstatten ferrite. Optimisation of the microstructure-property relationship was reached via a variation in the isothermal bainite transformation temperature. The highest retained austenite fraction of 0.045±0.003 with medium carbon content of 1.23±0.01 wt% was obtained after holding at 400 °C, resulting in the highest ultimate tensile strength of 590±35 MPa and largest total elongation of 0.27±0.05. The presence of TRIP effect in the studied steel was revealed through the analysis of strain hardening exponent and modified Crussard–Jaoul model. Effect of processing parameters on retained austenite retention and stress-strain behaviour was discussed.

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Microstructures and mechanical properties of TRIP steel produced by strip casting simulated in the laboratory

The effect of Zr addition on the microstructure and impact toughness in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels subjected to 100 kJ cm-1 heat input was investigated. The second- phase particles were mainly Al–Ti complex oxides and (Ti,Nb)N precipitates in Zr-free steel, whereas lots of finer Zr–Al–Ti complex oxides and (Al,Ti,Nb)N precipitates were formed in Zr-bearing steel because of Zr addition. These finer oxides and precipitates effectively restricted the austenite grain growth by pinning effect during welding thermal cycle, and smaller and more uniform prior austenite grains were obtained in CGHAZ of Zr-bearing steel. Furthermore, more acicular ferrite grains nucleated on Zr–Al–Ti complex oxides, inducing formation of fine-grained microstructure in CGHAZ of Zr-bearing steel. The toughness improvement in CGHAZ of Zr-bearing steel with dimple fracture surface was attributed to the grain refinement by pinning effect and acicular ferrite formation.

Toughness improvement by Zr addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

Austenite grain sizes in the heat affected zone (HAZ) of a high heat input welded Zr-Ti bearing microalloyed steel are measured under different welding conditions simulated by a Gleeble-1500 thermal-mechanical simulator. The austenite grain growth is divided into two regimes in terms of temperature. When the temperature is lower than 1250 °C where the pinning effect of precipitates is strong, the austenite grain size increases slowly with increasing peak temperature, but it increases drastically when the temperature is higher than 1250 °C where the pinning effect of precipitates is weak. Based on the experimental measurements, an analytical model for predicting the austenite grain size in the heat affected zone is derived. Model predictions indicate that the initial grain size has little effect on the final one, and the grain growth depends mainly on heat input and peak temperature as well as growth activation energy and exponent. With the use of the model, the width of coarse grained heat affected zone (CGHAZ) for a thick plate is predicted.

Shen-hua Song

Papers

Austenite Grain Growth in Heat Affected Zone of Zr-Ti Bearing Microalloyed Steel