Fatigue failure caused by crack propagation is one of the main failure modes of H13 die steel. Because H13 die steel is mainly used under operational conditions involving high-pressure cycling or high friction, its crack initiation and propagation behavior play a critical role in fatigue failure. However, to the best of the authors' knowledge, few reports on the direct observation of deformation and crack propagation behavior of H13 die steel exist, which limits the understanding of the fracture mechanism of H13 die steel. In this work, an in situ TEM tensile study combined with post-mortem EBSD analysis was employed to investigate the microstructure evolution and crack propagation behavior of rare earth (RE) H13 steel. Results indicate that the stress concentration at the grain boundaries and coarse granular inclusions in the tensile specimen were the main sources of crack initiation. After crack initiation and during the tensile process, many cracks converged into the main crack. The main crack propagated along the direction perpendicular to the tensile direction, exhibiting zigzag-shaped features. The stress distribution in the area near a crack in a specimen was heterogeneous; the length fraction of V1/V2 inter-variant boundaries in the relatively high-stress area increased from 56.5% to 58.8% compared with the relatively low-stress area, and the length fraction of V1/V3&V5 inter-variant boundaries increased from 16.3% to 21.6%. The increase in the length fraction of V1/V2 inter-variant boundaries indicated an increase in the twin martensite fraction, which effectively relieved the stress concentration at the grain boundaries and reduced crack initiation. During the tensile process, the austenite retained at the grain boundaries underwent stress-induced phase transformation and was partially transformed into V1/V2 and V1/V3&V5 variant pairs. The dislocation propagation in martensite contributed to dislocation pile-up at high-angle grain boundaries and carbide precipitation. Moreover, the dislocation pile-up at the high-angle grain boundaries promoted the stress-induced phase transformation of the retained austenite.

双相不锈钢兼具奥氏体和铁素体不锈钢的优良性能,多用于船舶、化工、核反应等领域。为了进一步提高S32550双相不锈钢的力学性能和抗腐蚀性能,采用真空感应炉成功冶炼了S32550双相不锈钢,并研究了有无添加稀土铈对其锻造、轧制后的微观组织、夹杂物形貌及冲击性能的影响。结果表明,添加稀土铈可以细化组织晶粒,使形状分布不均匀的铁素体组织与奥氏体组织均匀化;改善夹杂物形貌分布大小,对有害夹杂MnS进行改质,降低硫含量,使多余硫元素与铈反应形成Ce₂O₂S、Ce₂S₂夹杂弥散分布在钢中;另外,添加稀土铈可以提高S32550双相不锈钢在室温和低温(-40、-20 ℃)下的冲击韧性,在低温下可出现韧窝带,降低冷脆效应对钢材的危害。

With the increase of the strength of steel plate, the welding performance of the steel decreases sharply and the welding crack susceptibility increases. The properties of welding heat-affected zone of high strength steel lower with increasing the welding heat input. The present work aims to improve the toughness of welding heat-affected zone by rare earth oxide metallurgy technology. The effect of 5×10-6 and 23×10-6 rare earth Ce on the impact toughness, microstructure, austenite grains of heat-affected zone and fracture morphology of welded joint were studied. When the steel contains 5×10-6 rare earth Ce, the inclusions are MgO-Al2O3 spinels surrounded with a small amount of CeAlO3 inclusions. In the case of the steel with 23×10-6Ce, Ce can completely modify MgO-Al2O3 inclusions, resulting in the formation of (CeCa)S+MgO-Al2O3+MnS complex inclusions. The simulation welding of high strength steel was performed. The results show that the Charpy impact energy of heat-affected zone of the steel with 23×10-6Ce is higher at four different heat inputs, in comparison with the steel with 5×10-6Ce. The microstructure analysis shows that the fracture morphology of heat-affected zone of the steel with 23×10-6Ce appears dimples, which is an indication of a higher toughness. With increasing the heat input from 25 kJ/cm to 100 kJ/cm, the average grain size of the original austenite in the heat-affected zones of the steels with 5×10-6Ce and 23×10-6Ce was increased by 75.6% and 52.4%, respectively. It indicates that the growth of the original austenite grain during welding is suppressed with increasing the Ce content in the steel. Comparison of the microstructure shows that rare earth Ce can delay the formation of upper bainite structure in the heat-affected zone. Through the high temperature confocal microscope, it was observed that the rare earth inclusions pinned on the original austenite grain boundary, which can effectively restrain the grain growth during the welding process. It provides an evidence showing the mechanism of improvement in the heat-affected zone in the welding of the high strength steel by rare earth Ce. The present study demonstrates the rare earth oxide metallurgy can improve the weldability of the high strength steel.

As one of the energy-saving steels, non-quenched and tempered steel commonly requires the addition of alloying elements to improve its properties. The inclusions related to those elements have significant influence on the properties of the steels. In this work, the potentiodynamic polarization curves of C70S6 non-quenched and tempered steel and its Mg-Ca doped samples were measured by solution modification, and the samples before and after the test were characterized by SEM-EDS. The results show that the MnS inclusions in the samples act as active sites for pitting. The doped Ca and Mg elements make the distribution of inclusions more dispersed and the content of MnS in each inclusion lower, leading to the better pitting resistance of the doped specimens.

通过溶液调制的方法对C70S6非调质钢及其掺Ca、Mg样品进行了动电位极化曲线测试,并对测试前后的样品进行了SEM-EDS表征,揭示了夹杂物对非调质钢点蚀行为的影响。结果表明,MnS夹杂易作为活性位点诱发点蚀,而掺杂Ca、Mg元素使得夹杂物分布更弥散,MnS含量更低,相应的点蚀抗性也更高。

Fatigue fracture is the main failure forms of drill steel, and the hard oxide with large size is one of the main reasons for the fatigue fracture of drill steel. Therefore, the miniaturization and softening of inclusion can effectively improve the anti-fatigue performance of drill steel and prolong its service life. Rare earth elements have very good affinity with oxygen and sulfur in molten steel, and the hardness of resulting rare earth compounds is very low. In this work, the rare earth element cerium was added into drill steel to investigate the effect of Ce on the MgAl2O4 and sulfides. The composition, morphology, number, and size of inclusions in drill steel were analyzed by using SEM and EDS. The evolution process and modification mechanism of Ce on MgAl2O4 and sulfides were clarified by experimental results and calculated by thermodynamic software. The type of inclusions in drill steel without Ce addition is MgAl2O4 and (Ca, Mn)S. As the Ce content in drill steel reaches to 0.0078% (mass fraction), the type of inclusions changes to Ce-O and Ce-S. In addition, a few complex inclusions, mixture of Ce-O and MgO, were also found. The size of inclusions in drill steel decreases significantly as the oxides and sulfides were modified into Ce-O and Ce-S. The calculated results show that MgAl2O4 and (Ca, Mn)S in drill steel can be effectively modified into Ce-O and Ce-S as the Ce added into molten steel, and the modification sequence of Ce on the MgAl2O4 is as follows: MgAl2O4→CeAlO3+MgO→Ce2O3+MgO→Ce2O3. The content of Ce in drill steel has great influence on the type of inclusions. The modification mechanism of Ce on MgAl2O4 calculated by Factsage 6.3 agrees well with the experimental observations.

Non-oriented 6.5%Si electrical steel exhibits excellent high-frequency magnetic properties, such as low iron loss and near-zero magnetostriction. Moreover, the high Si content causes poor deformability due to the solution strengthening effect of Si and the resulting ordering transformations, delaying the commercial application. Strip casting is a near-net forming technology that directly produces thin strips from the melt and reduces the required rolling deformation for the fabrication of thin sheets. This technology could be a viable option for industrializing the production of non-oriented 6.5%Si electrical steel. Despite this, even in the strip casting process, this brittle material is prone to edge cracks. Thus, improving the intrinsic plasticity of strip casting non-oriented 6.5%Si electrical steel is necessary through chemical modification to eliminate the deforming defects. The effect of Ce on the ordered phase of the solidification microstructure, high-temperature tensile properties, and fracture mode of strip casting non-oriented 6.5%Si electrical steel was investigated in this work. The results showed that the addition of Ce introduced high-melting Ce2O2.5S and Ce4O4S3 during strip casting, which promoted heterogeneous nucleation and refined the solidification microstructure of the as-cast strip. The presence of Ce did not affect the ordering condition of the as-cast strip. In decreasing order, the tensile temperatures corresponding to the ordered degree of the as-cast strip were 650, 400, and 800oC. With the tensile temperature increasing, the yield and tensile strengths of the as-cast strip decreased, whereas the elongation gradually increased. When the tensile temperature exceeded 500oC, the purification and microstructure refining effects of Ce improved grain-boundary cohesion and prevented intergranular cracking. Moreover, the occurrences of dynamic recovery and recrystallization eventually made the as-cast strip doped with Ce fractured by dimples, leading to a considerably enhanced tensile ductility. According to the findings, the rare-earth treatment should be considered as an effective method for increasing the ductility of strip casting non-oriented 6.5%Si electrical steel.

Intra-granular Acicular Ferrite (IAF), as one of the most well-known desirable microstructure of ferrite with a chaotic crystallographic orientation, can not only refine the microstructure and retard the propagation of cleavage crack but also provide excellent combination of strength and toughness in steel. The effect of adding cerium on microstructure and controlling proper cerium-based inclusions in order to improve properties in low-carbon commercial steel (SS400) were investigated. The type of inclusions can be controlled by changing S/O ratio and Ce content. Without Ce modification, MnS is a dominate inclusion. After adding Ce, the stable inclusion phases change from AlCeO to CeOS. The optimum amount of cerium, 0.0235 wt.%, lead in proper grain refinement and formation of cerium oxide, oxy-sulfide and sulfide inclusions. Having a high amount of cerium results in increasing the number of inclusions significantly as a result it cannot be effective enough and the inclusions will act like barriers for others. It is found that the inclusions with a size of about 4∼7 μm can serve as heterogeneous nucleation sites for AF formation. Thermodynamic calculations have been applied to predict the inclusion formation in this molten steel as well, which show a good agreement with experimental one.

Ce microalloyed H13 hot die steel is widely used in manufacturing hot extrusion and die casting mold of magnesium-aluminium alloy because of its excellent combination of hot strength and impact toughness. The C content in Ce microalloyed H13 steel is approximately 4% (mass fraction), and the alloy elements content such as Cr, Mo, V et al are about 8%. Therefore, it is easy for primary carbide to precipitate during the solidification of the molten steel due to the segregation of alloy elements. Most researchers study the precipitation mechanism of primary carbide in the two-dimensional perspective. A few people are involved in the three-dimensional morphology of the primary carbide, especially the thermal stability of the primary carbide in the three-dimensional perspective in the Ce microalloyed H13 steel. Therefore, the precipitation mechanism and thermal stability of the primary carbide were systematically studied in this work. First, the SEM and inclusion automatic analysis system were used to analyze the morphology, number density and size of the inclusions in Ce microalloyed H13 steel. The three-dimensional morphology of the primary carbide in sample was observed after electrolyzed in a non-aqueous solution. The voltage was 20 V, the electrolysis time was about 3 min and the electrolyte was composed of 1% tetramethylammonium chloride, 10% acetylacetone, and 89% methanol (volume fraction). Three samples were heated to 1150, 1200 and 1250 ℃ for 1 h to investigate the thermal stability of the primary carbide. Finally, Factsage 7.2 software was used to calculate the precipitation mechanism and thermal stability of the primary carbide. Elemental Ce can effectively react with O, S, P and As elements to form the corresponding Ce-O, Ce-S and Ce-P-As inclusions. There is a huge difference between the two-dimensional and three-dimensional morphologies of the primary carbide, the two-dimensional morphology is strip and the three-dimensional morphology is irregular flake. Ti-V-rich carbide precipitates first, and then acts as the nucleation core of V-rich carbide. When the heating temperature reaches 1250 ℃, the V-rich carbide has completely dissolved, and the Ti-V-rich carbide begins to dissolve. The three-dimensional morphology of the wrapped Ti-V-rich carbide is completely exposed after the V-rich carbide disappears completely. The Ce-O inclusion is formed before solidification, and the primary carbide precipitates at the end of the solidification of molten steel. As the Ce content in molten steel increases, the stability diagram of Ce2O2S and Ce-S increases gradually. The precipitation temperature of Ti-V-rich carbide is approximately 1360 ℃, and the V-rich carbide starts to precipitate at about 1200 ℃. The calculated results are keeping well with the experimental observations. The damage of primary carbide in Ce microalloyed H13 steel can be partly reduced by higher heating temperature, but cannot be completely removed.