Microalloyed steel

About: Microalloyed steel is a research topic. Over the lifetime, 2183 publications have been published within this topic receiving 33586 citations.

Effect of nitrogen on the microstructures and mechanical properties in simulated CGHAZ of vanadium microalloyed steel varied with different heat inputs

Abstract: The microstructures and mechanical properties of simulated weld coarse heat affected zone (CGHAZ) of two kinds of vanadium microalloyed steel, i.e. V–Ti and V–N–Ti steel, had been studied using the welding thermal simulation method on a Gleeble-3800 thermal simulator, under different welding heat input. The yield strength of simulated CGHAZ decreases with heat input increasing. The 50% fraction appearance transition temperature (50% FATT) of simulated CGHAZ in V–N–Ti steel decreased at first, and then increased. While for the V–Ti steel, 50% FATT increased with t 8/5 increasing. The V–N–Ti steel had a better combination of strength and toughness of simulated CGHAZ compared with V–Ti steel. Increasing nitrogen cannot only refine the austenite grain size, but also simulate V-rich cap part forming on the TiN core part which can be as intragranular ferrite nucleation positions, resulting in refining the effective grain size and raising cleavage fracture stress of simulated CGHAZ. The detrimental effect of the free N on toughness can be remedied by austenite grain and effective grain size refinement.

Influence of Al and Nb on optimum Ti/N ratio in controlling austenite grain growth at reheating temperatures

Abstract: On the basis of steel 38MnSiVS5 a high number of castings were manufactured varying only the Ti and Al contents and obtaining ingots in which the Ti/Al ratio was different in each case. The specimens will be heated in air furnace to different austenitization temperatures, between 900°C and 1 200°C, and subsequently quenched in water. The most important achievement would be that Al plays a harmful role as an austenite grain controller at high temperatures (>1 050°C), as a consequence of the quick dissolution of AIN particles and therefore the reduction in inhibition forces at the grain boundaries which are intercepted by them. The best austenite grain control was shown by steel with contents of Ti=0.044, Al=0.009, N=0.0131 (mass%) and a Ti/N ratio of 3.36. But the interpolation of the results allows it to be concluded that the best Ti/N ratio would be close to 2.5. It was also found that the control of austenite grain size improved with an addition of Nb. In paralell, a study of precipitate sizes was carried out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

Investigation of hot ductility in Al-killed boron steels

Abstract: The influence of boron to nitrogen ratio, strain rate and cooling rate on hot ductility of aluminium-killed, low carbon, boron microalloyed steel was investigated. Hot tensile testing was performed on steel samples reheated in argon to 1300 °C, cooled at rates of 0.3, 1.2 and 3.0 °C s −1 to temperatures in the range 750–1050 °C, and then strained to failure at initial strain rates of 1 × 10 −4 or 1 × 10 −3 s −1 . It was found that the steel with a B:N ratio of 0.19 showed deep hot ductility troughs for all tested conditions; the steel with a B:N ratio of 0.47 showed a deep ductility trough for a high cooling rate of 3.0 °C s −1 and the steel with a near-stoichiometric B:N ratio of 0.75 showed no ductility troughs for the tested conditions. The ductility troughs extended from ∼900 °C (near the Ae 3 temperature) to ∼1000 or 1050 °C in the single-phase austenite region. The proposed mechanism of hot ductility improvement with increase in B:N ratio in these steels is that the B removes N from solution, thus reducing the strain-induced precipitation of AlN. Additionally, BN co-precipitates with sulphides, preventing precipitation of fine MnS, CuS and FeS, and forming large, complex precipitates that have no effect on hot ductility.

Effects of Sulfur Content and Sulfide-forming Elements Addition on Impact Properties of Ferrite–Pearlitic Microalloyed Steels

Abstract: Longitudinal Charpy impact value of ferrite–pearlitic microalloyed steel was improved by large addition of S (0.05–0.1mass%), however, transverse Charpy impact value was rather deteriorated. This study focused on the sulfide shape, which have large effects on impact properties. The elements of Ca, Mg, Ti, and Zr were added to ferrite–pearlitic microalloyed steels with 0.1 mass% S for sulfide shape control, and their impact properties were compared. The addition of Ca or Mg to the steels improved the transverse Charpy impact value without changing the longitudinal Charpy impact value, while the addition of Ti or Zr to the steels resulted in deteriorating significantly both longitudinal and transverse Charpy impact values. The crack initiation energy of the Ca or Mg bearing steels improves by enlargement of the minute dimple area ratio in the ductile fracture surface because the elongation of sulfides in the hot forging process are controlled by the addition of Ca or Mg. On the other hand, the reason why the Charpy impact value of Ti or Zr bearing steels are deteriorated is that (Ti, V)C or plate type sulfides promote cleavage fracture and then raise their ductile-to-brittle transition temperature.