Microalloyed steel

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

The influence of microstructure on the tensile and fatigue behavior of SAE 6150 steel

Abstract: The tensile and reverse-bending fatigue behaviors of the SAE 6150 steel in the dual-phase (DP), fully martensitic, and tempered states, respectively, have been investigated using mechanical tests, scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) microscopy, and optical microscopy. Residual stresses, inherent microcracks, and retained austenite films in the martensitic steel, quenched from 900 °C, lead to the development of inferior tensile and fatigue strength. Tempering at 700°C relieves the residual stresses associated with martensite, causes the precipitation of microalloy carbides (MACs), and thus results in superior strength, increased fatigue resistance, and moderate ductility. The DP microstructure, consisting of martensite islets in a ferrite matrix, gives rise to a combination of good strength, excellent ductility, and commendable fatigue characteristics. MAC in the tempered steel and martensite islands in the DP variant enhance fatigue performance by causing crack tip deflection and concomitant crack path tortuosity. Strain incompatibility between martensite and ferrite in the DP steel, and cementite films and ferrite in the tempered variant are identified as fatigue crack initiation sites.

Mechanical properties and machinability of a high-strength, medium-carbon, microalloyed steel

Abstract: The development of a high-strength (tensile strength beyond 900 N/mm2), medium-carbon, vanadium microalloyed steel for hot-forged automotive components has been reviewed in the paper. The influence of different alloying elements was investigated. The most effective elements to increase the strength were chromium and manganese. In TEM investigation, it was found that, in comparison with the lower-strength melt, chromium plus manganese alloyed steel showed a high density of small V(C, N) precipitates. Mechanical properties were studied in tensile, impact, fatigue, and fracture mechanics tests. Most mechanical properties of the microalloyed and quenched and tempered steel were essentially equal. Exceptions were the lower impact strength and higher fatigue strength of the microalloyed steel. Possibilities to improve the impact strength of the microalloyed steel are being considered. Machinability was tested in different operations. Generally, the machinability of the high-strength microalloyed steel was comparable with quenched and tempered steels. However, a different behavior was found in deephole drilling. The use of calcium treatment to improve the machinability is discussed. The high-strength microalloyed grade developed can substitute for alloyed quenched and tempered steels in most components. When weight saving is desired, the possibility of substituting the high-strength grade for lower-strength microalloyed steels is considered.

Fracture toughness characterization of a weldment in a microalloyed steel using resistance curves

Abstract: Various weldment regions, e.g. the weld zone, heat-affected zone (HAZ) and parent metal, are characterized in terms of strength and fracture toughness parameters, J integral and crack-tip-opening displacement (CTOD) δ after manual metal arc weld to a microalloyed steel plate. The critical J values are obtained by three methods, namely the ASTM Standard E 813-81, the stretched-zone width measurement with superposition on the R curve and the proposed method based on the experimental data. On the contrary the critical δ values are determined by four methods: (i) British Standard 5762; (ii) superposition of the blunting line on the R curve; (iii) stretched-zone width measurement and superposition on the R curve; (iv) the proposed method. A linear relationship between the CTOD and stretched-zone width with a slope of 0.43 is obtained in the present study. The crack growth toughness values are also obtained from the R curves. The crack initiation toughness values obtained by the above methods are compared. The study focuses on the J - δ relationship for the weld metal and the HAZ. Quantitative microstructural analysis and fractographic studies have also been conducted to rationalize the observed toughness values in the weld metal, HAZ and the parent metal.

Effect of boron on the hot ductility of the Nb-microalloyed steel in austenite region

Abstract: Two grades of the Nb-microalloyed steel, one modified with B, were subjected toin situ melting and the thermal schedules experienced by the billet surface in the continuous casting process. The hot ductility was evaluated at various temperatures at the straightening stage of the process. It was found that addition of B improves the hot ductility in the austenite region. Such improvement could be due to fast precipitation at grain boundaries as well as depletion of precipitants and strengthening elements in the matrix.

Formation and Thermal Stability of Large Precipitates and Oxides in Titanium and Niobium Microalloyed Steel

Abstract: As-cast CC slabs of microalloyed steels are prone to surface and subsurface cracking. Precipitation phenomena initiated during solidification reduce ductility at high temperature. The unidirectional solidification unit is employed to simulate the solidification process during continuous casting. Precipitation behavior and thermal stability are systematically investigated. Samples of adding titanium and niobium to steels have been examined using field emission scanning electron microscope (FE-SEM), electron probe X-ray microanalyzer (EPMA), and transmission electron microscope (TEM). It has been found that the addition of titanium and niobium to high-strength low-alloyed (HSLA) steel resulted in undesirable large precipitation in the steels, i. e., precipitation of large precipitates with various morphologies. The composition of the large precipitates has been determined. The effect of cooling rate on (Ti, Nb), (C, N) precipitate formation is investigated. With increasing the cooling rate, titanium-rich (Ti, Nb) (C, N) precipitates are transformed to niobium-rich (Ti, Nb) (C,N) precipitates. The thermal stability of these large precipitates and oxides have been assessed by carrying out various heat treatments such as holding and quenching from temperature at 800 and 1 200 °C. It has been found that titanium-rich (Ti, Nb)(C, N) precipitate is stable at about 1 200 °C and niobium-rich (Ti, Nb)(C, N) precipitate is stable at about 800 %. After reheating at 1 200 °C for 1 h, (Ca, Mn)S and TiN are precipitated from Ca-Al oxide. However, during reheating at 800 °C for 1 h, Ca-Al-Ti oxide in specimens was stable. The thermodynamic calculation of simulating the thermal process is employed. The calculation results are in good agreement with the experimental results.