Inclusion evolution behaviors, in terms of composition, size, and number density, and associated influence on the microstructures of the as-cast slabs, rolled plates, and simulated welded samples of plain EH36 and EH36-Mg shipbuilding steels have been systematically investigated. The results indicate that the inclusions in the as-cast plain EH36 are almost Al-Ca-S-O-(Mn) complex oxides with sizes ranging from 1.0 to 2.0 μm. After Mg addition, a large amount of individually fine MnS precipitates and Mg-containing Ti-Al-Mg-O-(Mn-S) complex inclusions are generated, which significantly refine the microstructure and are conducive to the nucleation of acicular ferrite in the rolled and welded sample. Moreover, after rolling and welding thermal simulation, the number of individual MnS decreases gradually due to its precipitation on the surface of Ti-Al-Mg-O oxides.

An Integrated Study on the Evolution of Inclusions in EH36 Shipbuilding Steel with Mg Addition: From Casting to Welding

We elucidate here the contribution of intragranular acicular ferrite (IAF) on impending crack initiation and propagation in the heat-affected-zone (HAZ) of low carbon steels based on Instrumented Charpy impact test. It was observed that both crack initiation energy and crack propagation energy of the steel in the presence of IAF were higher than steel that did not contain IAF, where the former situation was observed to have a large ductile fracture zone as compared to the latter. An analysis of the statistical distribution of voids in the cross-section of impact fracture indicated that IAFs increase the length of plastic zone at the tip of the main crack and impede crack initiation and enhance crack initial energy because of refined prior austenite grain and superior deformability. Furthermore, EBSD analysis of secondary cracks indicated that IAF impedes crack propagation and increases crack propagation energy by forming high angle grain boundaries with surrounding bainite and its own superior deformability.

The contribution of intragranular acicular ferrite microstructural constituent on impact toughness and impeding crack initiation and propagation in the heat-affected zone (HAZ) of low-carbon steels

Acicular ferrite is a microstructure nucleating intergranularly on non-metallic inclusions and forming an arrangement of fine, interlocking grains. This structure is known to improve steel properties, especially steel toughness, essentially. The formation of acicular ferrite is mainly affected by steel composition, cooling rate, inclusion landscape and austenite grain size. In recent decades, extensive research has been conducted to investigate these factors. The present paper provides an overview of the impact of published results and the state of knowledge regarding acicular ferrite formation. Special attention is paid to the effect of carbon, manganese and titanium addition to steel, as well as the optimum size, number and composition of non-metallic inclusions. In addition, the reactions during the nucleation and growth of acicular ferrite needles are briefly addressed. Further, characteristics of acicular ferrite and bainite are summarized, which should help to distinguish these similar structures.

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Acicular Ferrite Formation and Its Influencing Factors - A Review

Continuous cooling and isothermal transformation behavior of a Ti–Zr deoxidized microalloyed steel was studied and compared with that of C–Mn steel, in conjunction with the evolution of microstructure and measurement of toughness of simulated high heat input welding coarse grain heat-affected zone (CGHAZ). The study indicates that the inclusions in Ti–Zr steel were primarily composite inclusions (ZrO 2 ·MnS) with high density of inclusions of size less than 1 μm. Polygonal ferrite, acicular ferrite, multi-directional bainite, and segmented martensite were the typical microstructures obtained with increase in cooling rate. Each of these phases was analyzed with decreasing isothermal temperature. Intragranular ferrite nucleation was significantly promoted by ZrO 2 ·MnS inclusions. Inclusions of smaller size were more effective with decreasing temperature such that greater fraction of ferrite plates nucleated on one inclusion. Mn-depletion is the operating mechanism and precipitation of TiN was complementary. The simulated CGHAZ was characterized by interlock acicular ferrite microstructure and high impact toughness such that the dimple fracture and fine cleavage facets contributed to high impact energy.

Transformation behavior of a Ti–Zr deoxidized steel: Microstructure and toughness of simulated coarse grain heat affected zone

The effect of Mg content on the microstructure and toughness of the heat-affected zone (HAZ) of steel plates after high heat input welding was investigated by means of welding thermal simulation test and in situ observation through high-temperature laser scanning confocal microscopy. It was found that with the increase of Mg content in the steel, the former austenite grain sizes were greatly decreased and the mainly microstructural constituents in HAZ were changed from the brittle constituents of Widmanstatten ferrite, ferrite side plate and upper bainite to the ductile constituents of intragranular acicular ferrite and polygonal ferrite. The proportion of grain boundary ferrite was decreased greatly with the further addition of Mg from 27 to 99 ppm. As a result, the HAZ toughness after welding with heat input of 400 kJ cm−1 is increased with increasing Mg content in the steel plate.

Effect of Mg Content on the Microstructure and Toughness of Heat-Affected Zone of Steel Plate after High Heat Input Welding

The effect of Mg content on the evolution behavior of inclusions in the steel plates with Mg deoxidation was investigated based on the experimental studies and thermodynamic calculations. Simulating welding process and Charpy impact test were also carried out to evaluate the Heat Affected Zone (HAZ) toughness of steel plates. The typical inclusions found in the steels were the complex inclusions comprising central oxide and peripheral MnS. With the increase of Mg content in the steel samples from 0 to 27, 38 and 99 ppm, the phase of central oxide inclusions changed sequentially from Al2O3 to (Mg-Ti-O), and MgO, which is consistent with the thermodynamic calculation results. Furthermore, the size of inclusions decreases and the number density of inclusions increases with the increase of Mg content. The HAZ toughness of steel plates after welding with heat input of 400 kJ cm−1 has been greatly improved by inclusion control with Mg deoxidation.

Improvement of HAZ Toughness of Steel Plate for High Heat Input Welding by Inclusion Control with Mg Deoxidation

To study the effect of Mg addition on inhibiting weld heat affected zones (HAZ) austenite grain growth of Ti-bearing low carbon steels, two steels with and without Mg treated were prepared using a laboratory vacuum. The welding testing was simulated by Gleeble 3500 thermomechanical simulator. The performance of HAZ was investigated that the toughness was improved from 33 to 185 J by adding 0. 005% Mg (in mass percent) to the steel, and the fracture mechanism changed from cleavage fracture to toughness fracture. Through in-situ observation by a confocal scanning laser microscope, a significant result was found that the austenite grain of the steel with Mg treated was still keeping fine-grained structure after holding at 1 400 ’C and lasting for 300 s. This inhibition of austenite grain growth was mainly attributed to the formation of pinning particles after the addition of Mg. The obtained results propose a potential method for improving HAZ toughness of structure steels.

Effect of Mg Addition on Inhibiting Austenite Grain Growth in Heat Affected Zones of Ti-Bearing Low Carbon Steels

The effects of Mg content, inclusion size, and austenite grain size on the intragranular acicular ferrite (IAF) nucleation in heat-affected zone of steel plate after high-heat-input welding of 400 kJ/cm were investigated by welding simulation and observation using a scanning electron microscope equipped with an energy dispersive spectrometer and an optical microscope. The IAFs are observed in steel with Mg addition, and the volume fraction of IAF is as high as 55.4% in the steel containing 0.0027 mass% Mg. The MgO–Al2O3–Ti2O3–MnS inclusions with size around 2 μm are effective nucleation sites for IAF, whereas Al2O3–MnS inclusions are impotent to nucleate the acicular ferrite. The prior-austenite grain (PAG) size distribution in low Mg steel is similar to that in steel without Mg addition. The austenite grain with size about 200 μm is favorable for the IAF formation. In the steel with high Mg content of 0.0099%, the growth of PAG is greatly inhibited, and PAG sizes are smaller than 100 μm. Therefore, the nucleation of IAF can hardly be observed.

Effect of Mg addition on formation of intragranular acicular ferrite in heat-affected zone of steel plate after high-heat-input welding

In the present study, the effect of cooling conditions on the evolution of non-metallic inclusions in high manganese TWIP steels was investigated based on experiments and thermodynamic calculations. In addition, the formation and growth behavior of AlN inclusions during solidification under different cooling conditions were analyzed with the help of thermodynamics and dynamics. The inclusions formed in the high manganese TWIP steels are classified into nine types: (1) AlN; (2) MgO; (3) CaS; (4) MgAl2O4; (5) AlN + MgO; (6) MgO + MgS; (7) MgO + MgS + CaS; (8) MgO + CaS; (9) MgAl2O4 + MgS. With the increase in the cooling rate, the volume fraction and area ratio of inclusions are almost constant; the size of inclusions decreases and the number density of inclusions increases in the steels. The thermodynamic results of inclusion types calculated with FactSage are consistent with the observed results. With increasing cooling rate, the diameter of AlN decreases. When the cooling rate increases from 0.75 to 4.83 K s−1, the measured average diameter of AlN decreases from 4.49 to 2.42 μm. Under the high cooling rate of 4.83 K s−1, the calculated diameter of AlN reaches 3.59 μm at the end of solidification. However, the calculated diameter of AlN increases to approximately 5.93 μm at the end of solidification under the low cooling rate of 0.75 K s−1. The calculated diameter of AlN decreases with increasing cooling rate. The theoretical calculation results of the change in diameter of AlN under the different cooling rates have the same trend with the observed results. The existences of inclusions in the steels, especially AlN which average sizes are 2.42 and 4.49 μm, respectively, are not considered to have obvious influences on the hot ductility.

Ruizhi Wang

Papers

Effect of Mg Content on the Microstructure and Toughness of Heat-Affected Zone of Steel Plate after High Heat Input Welding

Improvement of HAZ Toughness of Steel Plate for High Heat Input Welding by Inclusion Control with Mg Deoxidation

Effect of Mg Addition on Inhibiting Austenite Grain Growth in Heat Affected Zones of Ti-Bearing Low Carbon Steels

Effect of Mg addition on formation of intragranular acicular ferrite in heat-affected zone of steel plate after high-heat-input welding

The Effect of Cooling Conditions on the Evolution of Non-metallic Inclusions in High Manganese TWIP Steels