Effect of in-situ nanoparticles on the mechanical properties and hydrogen embrittlement of high-strength steel

We investigated the critical influence of in-situ nanoparticles on the mechanical properties and hydrogen embrittlement(HE) of high-strength steel.The results reveal that the mechanical strength and elongation of quenched and tempered steel(919 MPa yield strength,17.11% elongation) are greater than those of hot-rolled steel(690 MPa yield strength, 16.81% elongation) due to the strengthening effect of insitu Ti_3O_5–Nb(C,N) nanoparticles.In addition, the HE susceptibility is substantially mitigated to 55.52%, approximately 30% lower than that of steels without in-situ nanoparticles(84.04%), which we attribute to the heterogeneous nucleation of the Ti_3O_5 nanoparticles increasing the density of the carbides.Compared with hard TiN inclusions, the spherical and soft Al_2O_3–MnS core–shell inclusions that nucleate on in-situ Al_2O_3 particles could also suppress HE.In-situ nanoparticles generated by the regional trace-element supply have strong potential for the development of high-strength and hydrogen-resistant steels.     

Comparison between the surface defects caused by Al2O3 and TiN inclusions in interstitial-free steel auto sheets

Al₂O₃ and TiN inclusions in interstitial-free (IF) steel deteriorate the properties of the steel. To decrease the defects of cold-rolled sheet, it is important to clearly distinguish between the degrees of damage caused by these two inclusions on the surface quality of the steel. In this study, a nanoindenter was used to test the mechanical properties of the inclusions, and the distribution and size of the inclusions were obtained by scanning electron microscopy (SEM). It was found that when only mechanical properties are considered, TiN inclusions are more likely to cause defects than Al₂O₃ inclusions of the same size during the rolling process. However, Al₂O₃ inclusions are generally more inclined to cause defects in the rolling process than TiN inclusions because of their distribution characteristic in the thickness direction. The precipitation of Al₂O₃ and TiN was obtained through thermodynamical calculations. The growth laws of inclusions at different cooling rates were calculated by solidification and segregation models. The results show that the precipitation regularity is closely related to the distribution law of the inclusions in IF slabs along the thickness direction.     

Effect of magnesium addition on inclusions in H13 die steel

The effect of magnesium addition on the number, morphology, composition, size, and density of inclusions in H13 die steel was studied. The results show that the total oxygen content in the steel can be significantly decreased to 0.0008wt%. Al₂O₃ and MnS inclusions are changed into nearly spherical MgO·Al₂O₃ spinel and spherical MgO·MgS inclusions, respectively. The number of inclusions larger than 1 μm decreases and the number of inclusions smaller than 1 μm increases with increasing magnesium content. V(N,C) precipitates around MgO·Al₂O₃ and MgO·MgS inclusions during solidification of liquid steel. The densities of MgO·Al₂O₃ spinel inclusions are lower than that of alumina inclusions. With increasing magnesium content in the Mg-containing inclusions, the density of inclusions decreases, leading to the improvement of inclusion removal efficiency.     

AISI 321不锈钢钢钢包精炼过程中钙含量对夹杂物的影响

The effect of three heat processes with different calcium contents on the evolution of inclusions during the ladle furnace refining process of AISI 321 stainless steel was investigated. The size, morphology, and composition of the inclusions were analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. After the addition of aluminum and titanium, the primary oxide in the AISI 321 stainless steel was an Al₂O₃–MgO–TiO_x complex oxide, in which the mass ratio of Al₂O₃/MgO was highly consistent with spinel (MgO·Al₂O₃). After calcium treatment, the calcium content in the oxide increased significantly. Thermodynamic calculations show that when the Ti content was 0.2wt%, the Al and Ca contents were less than 0.10wt% and 0.0005wt%, respectively, which was beneficial for the formation of liquid inclusions in molten steel. Moreover, the modification mechanism of calcium on TiN-wrapped oxides in combination with temperature changes was discussed.     

Inclusion variations and calcium treatment optimization in pipeline steel production

SiCa line and SiCaBaFe alloy were injected into liquid pipeline steel at the end of LF refining as calcium treatment, and samples were taken from the ladles, mould, and slabs. Analysis of Ca content and inclusions shows that Ca content in steel decreases obviously in the following process after calcium treatment; the compositions, morphology, and sizes of inclusions also vary much in the production; primary inclusions in the ladles prior to calcium treatment are mainly Al₂O₃ inclusions, but they turn to fine irregular CaS-CaO-Al₂O₃ compound inclusions after the treatment, then become fine globular CaO-Al₂O₃ inclusions in the mould, and finally change to a few larger irregular CaS-CaO-Al₂O₃ complex inclusions in the slabs. Thermodynamic study reveals that inclusion variations are related with the preferential reactions among Ca, Al₂O₃, and S and the precipitation of S in CaO-Al₂O₃ inclusions with high sulfur capacity. New evaluation standards for calcium treatment in high-grade pipeline steel were put forward according to the inclusion variations and requirements of pipeline steel on inclusion controlling, and the calcium process was studied and optimized.     

18wt%Cr–8wt%Ni不锈钢中增塑性MnO–Al2O3–SiO2基非金属夹杂物的演变及其浸泡性能

The properties of MnO–Al₂O₃–SiO₂-based plasticized inclusion are likely to change during soaking  process due to its low melting point. In this study, the evolution of the MnO–Al₂O₃–SiO₂-based inclusion of 18wt%Cr?8wt%Ni stainless steel under isothermal soaking process at 1250°C for different times was investigated by laboratory-scale experiments and thermodynamic analysis. The results showed that the inclusion population density increased at the first stage and then decreased while their average size first decreased and then increased. In addition, almost no Cr₂O₃-concentrated regions existed within the inclusion before soaking, but more and more Cr₂O₃ precipitates were formed during soaking. Furthermore, the plasticity of the inclusion deteriorated due to a decrease in the amount of liquid phase and an increase in the high-melting-point-phase MnO–Cr₂O₃ spinel after the soaking process. In-situ observations by high-temperature confocal laser scanning microscopy (CLSM) confirmed that liquid phases were produced in the inclusions and the inclusions grew rather quickly during the soaking process. Both the experimental results and thermodynamic analysis conclude that there are three routes for inclusion evolution during the soaking process. In particular, Ostwald ripening plays an important role in the inclusion evolution, i.e., MnO–Al₂O₃–SiO₂-based inclusions grow by absorbing the newly precipitated smaller-size MnO–Cr₂O₃ inclusions.     

Effect of interaction of refractories with Ni-based superalloy on inclusions during vacuum induction melting

This study documents laboratory-scale observation of the interactions between the Ni-based superalloy FGH4096 and refractories. Three different crucibles were tested—MgO, Al₂O₃, and MgO–spinel. We studied the variations in the compositions of the inclusions and the alloy–crucible interface with the reaction time using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction. The results showed that the MgO and MgO–spinel crucibles form MgO-containing inclusions (Al–Mg oxides and Al–Mg–Ti oxides), whereas the inclusions formed when using the Al₂O₃ crucible are Al₂O₃ and Al–Ti oxides. We observed a new MgAl₂O₄ phase at the inner wall of the MgO crucible, with the alloy melted in the MgO crucible exhibiting fewer inclusions. No new phase occurred at the inner wall of the Al₂O₃ crucible. We discuss the mechanism of interaction between the refractories and the Ni-based superalloy. Physical erosion was found to predominate in the Al₂O₃ crucible, whereas dissolution and chemical reactions dominated in the MgO crucible. No reaction was observed between three crucibles and the Ti of the melt although the Ti content (3.8wt%) was higher than that of Al (2.1wt%).     

Novel mechanism for the modification of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-based inclusions in ultra-low carbon Al-killed steel considering the effects of magnesium and calcium

Many researchers have explored the inclusion modification mechanism to improve non-metallic inclusion modifications in steelmaking. In this study, two types of industrial trials on inclusion modifications in liquid steel were conducted using ultra-low-carbon Al-killed steel with different Mg and Ca contents to verify the effects of Ca and Mg contents on the modification mechanism of Al₂O₃-based inclusions during secondary refining. The results showed that Al₂O₃-based inclusions can be modified into liquid calcium aluminate or a multi-component inclusion with the addition of a suitable amount of Ca. In addition,[Mg] in liquid steel can further reduce CaO in liquid calcium aluminate to drive its evolution into CaO-MgO-Al₂O₃ multi-component inclusions. Thermodynamic analysis confirmed that the reaction between[Mg] and CaO in liquid calcium aluminate occurs when the MgO content of liquid calcium aluminate is less than 3wt% and the temperature is higher than 1843 K.     

Characteristics of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, MnS,and TiN inclusions in the remelting process of bearing steel

The Al₂O₃, MnS, and TiN inclusions in bearing steel will deteriorate the steel's mechanical properties. Therefore, elucidating detailed characteristics of these inclusions in consumable electrode during the electroslag remelting process is important for achieving a subsequently clean ingot. In this study, a confocal scanning violet laser microscope was used to simulate the remelting process and observe, in real time, the behaviors of inclusions. The obtained images show that, after the temperature exceeded the steel solidus temperature, MnS and TiN inclusions in the specimen began to dissolve. Higher temperatures led to faster dissolution, and the inclusions disappeared before the steel was fully liquid. In the case of an observed Al₂O₃ inclusion, its shape changed from angular to a smooth ellipsoid in the region where the solid and liquid coexisted and it began to dissolve as the temperature continued to increase. This dissolution was driven by the difference in oxygen potential between the inclusion and the liquid steel.     

Effects of Nb on the microstructure and mechanical properties of 38MnB5 steel

The effects of the microalloying element niobium (Nb) on the microstructure and mechanical properties of the hot stamped steel 38MnB5 were investigated. The impact of Nb addition on the microstructure was studied through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The experimental results indicated that the microstructures of the steel containing Nb were finer than those of the steel without Nb. Moreover, Nb mainly presented as a second-phase particle in 38MnB5 steel, and the particles included Nb carbonitrides. In addition, the tensile strength and elongation of the hot rolled and hot stamped steels were also measured, and they demonstrated that the appropriate addition of Nb was beneficial to the mechanical properties of 38MnB5. Under the same conditions, the tensile strength of 38MnB5Nb was higher than that of 38MnB5, which increased from 2011 to 2179 MPa. The yield strength also increased from 1316 to 1476 MPa, and the elongation increased from 5.92% to 6.64%. Overall, Nb had a positive effect on the performance of the hot stamped steel.     

屈服强度800 MPa的低碳微合金钢

研究了不同成分特征的含锰含碳微合金钢通过控轧控冷后回火的组织与力学性能,Mn含量为1．7％－2．3％的试验钢轧后经快冷和中温回火获得了高强度和一定的塑性、韧性,其中含0．06％,2．3％Mn,0．21％Al和微量Nb,Ti,B的快冷回火钢屈服强度（σ0．2）超过800MPa,延伸率大于4％。室温冲击韧性134－174．5J／cm^2,－40℃冲击韧性为103－110J／cm^2,具有由变形奥氏体转变成的细小贝氏体组织。     

Mo含量对Ti–Mo系铁素体钢热稳定性影响

The effects of tempering holding time at 700°C on the morphology, mechanical properties, and behavior of nanoparticles in Ti–Mo ferritic steel with different Mo contents were analyzed using scanning electron microscopy and transmission electron microscopy. The equilibrium solid solution amounts of Mo, Ti, and C in ferritic steel at various temperatures were calculated, and changes in the sizes of nanoparticles over time at different Mo contents were analyzed. The experimental results and theoretical calculations were in good agreement with each other and showed that the size of nanoparticles in middle Mo content nano-ferrite (MNF) steel changed the least during aging. High Mo contents inhibited the maturation and growth of nanoparticles, but no obvious inhibitory effect was observed when the Mo content exceeded 0.37wt%. The tensile strength and yield strength continuously decreased with the tempering time. Analysis of the strengthening and toughening mechanisms showed that the different mechanical properties among the three different Mo content experiment steels were mainly determined by grain refinement strengthening (the difference range was 30–40 MPa) and precipitation strengthening (the difference range was 78–127 MPa). MNF steel displayed an ideal chemical ratio and the highest thermodynamic stability, whereas low Mo content nano-ferrite (LNF) steel and high Mo content nano-ferrite (HNF) steel displayed relatively similar thermodynamic stabilities.     

基于原位合金化的SiCp/Al复合材料等离子弧焊接

为了抑制SiCp/Al基复合材料在焊接过程中的界面反应,补充烧损元素,同时原位产生新的增强颗粒,分别以Al-Ti-Si和Al-Ti-Mg两种药芯焊丝作为填充材料,向熔池中直接添加Al,Si,Ti和Mg等金属元素,用氩氮混合等离子气对SiCp/Al基复合材料进行等离子弧原位焊接.对比分析了两种药芯材料对焊缝组织和性能的影响.结果表明:以两种药芯焊丝作为原位反应填充材料进行等离子弧原位焊接时,均可以有效抑制针状脆生相Al4C3的生成,形成了稳定熔池,得到了以TiC,TiN,AlN,Ti5 Si3,MgAl2O4和细小棒状的Al3Ti等新生增强相的焊缝;焊缝组织致密结合良好,最大抗拉强度分别为232和196 MPa.     

高品质GCr15轴承钢二次精炼过程中夹杂物的演变规律

采用FE-SEM/EDS研究了转炉流程生产的GCr15轴承钢LF、RH精炼过程中夹杂物的演变规律,分析了其演变机理。结果表明:钢中复合夹杂物的演变规律可归纳为:Al2O3→MgO·Al2O3→(CaO-MgO-Al2O3-(CaS))复合氧化物夹杂和Al2O3→(Al2O3-MnS)→(Al2O3-MnS-Ti(C,N))复合氧硫碳氮物夹杂2种方式。LF精炼过程脱硫作用明显,钢中的硫化物夹杂数量大幅减少。LF精炼初期钢中主要是MnS、Al2O3、TiN的单相夹杂物。LF精炼结束后钢中的夹杂物演变为Al2O3为核心外包氧化物及MnS、TiN、Ti(C,N)、CaS的复合夹杂物。精炼渣中的CaO和耐火材料中的MgO经还原后与钢中溶解氧反应导致LF精炼结束时D类夹杂物增加。RH及软吹处理进一步强化了去除钢中的硫化物,但D类及其与A、T类复合的夹杂物含量增加。在LF阶段,夹杂物尺寸主要集中在1~3μm范围内,到RH阶段,夹杂物尺寸则主要集中分布在小于1μm的粒度范围。最大夹杂物尺寸由10.79μm降到5.68μm,单位面积夹杂个数由372个/mm2降到258个/mm2。RH及软吹处理有效地降低了钢中大于3μm的夹杂物。     

CSP流程钛微合金化高强钢的第二相粒子析出行为

采用光学显微镜、扫描电镜、透射电镜、物理化学相分析等方法并结合热力学计算,分析了CSP工艺生产的钛微合金化高强钢的析出物特征及析出规律.研究发现：屈服强度700 MPa级高强钢中存在大量球形的纳米级TiC和Ti( C,N)粒子及少量不规则形状、100 nm以上的Ti4 C2 S2粒子,TiN在连轧前完成析出,TiC主要在卷取和空冷时析出.不含钼钢和含钼钢(0.1% Mo)中MC相的质量分数为0.049%和0.043%,由于钼的加入,含钼钢中Ti的析出量较少,但析出粒子更为细小,并定量得到了不含钼钢和含钼钢的析出强化效果分别为126 MPa和128 MPa.     

冷却工艺对低碳Ti微合金化热轧超高强钢组织性能的影响

对实验钢采用低碳高Ti微合金化的成分设计,进行了控轧控冷实验,通过控制不同的冷速和卷取温度,研究了过冷度和原子扩散速率对钢组织演变及(Ti,Mo)C粒子的析出行为的影响.研究结果表明,冷速为30℃/s,卷取温度为420℃时,实验钢屈服强度大于690MPa,抗拉强度为820MPa,断后伸长率达18%,并具有良好的低温冲击韧性.显微组织性能研究表明,多边形铁素体、针状铁素体、细小M/A岛及弥散的(Ti,Mo)C析出粒子的混合组织可实现强度和韧性的良好匹配.     

合金元素及热处理工艺对1000MPa级TRIP钢性能的影响

为了获得强度高(1 000 MPa)、塑性好的相变诱发塑性(TRIP)钢,通过拉伸实验、SEM、TEM和XRD等方法研究了合金元素及贝氏体等温温度对实验钢的影响.结果表明:随着等温温度的升高,强度先下降后上升.延伸率与强塑积都随着温度的升高先上升,在400℃出现峰值后下降.等温温度低于320℃或高于480℃时,抗拉强度最高,达到1 000 MPa以上,强塑积达到22 080 MPa.%;400℃等温时,延伸率和强塑积最高,分别达到31%和27 150 MPa.%.Al部分取代Si后,实验钢强度下降.再加入0.5%Cu,强度明显上升,延伸率下降,强塑积达到25 085 MPa.%.说明在一定热处...     

Effects of aluminum and titanium on the microstructure of ODS steels fabricated by hot pressing

Three oxide-dispersion-strengthened (ODS) steels with compositions of Fe-14Cr-2W-0.2V-0.07Ta-0.3Y₂O₃ (wt%, so as the follows) (14Y), Fe-14Cr-2W-0.2V-0.07Ta-1Al-0.3Y₂O₃ (14YAl), and Fe-14Cr-2W-0.2V-0.07Ta-0.3Ti-0.3 Y₂O₃ (14YTi) were fabricated by hot pressing. Transmission electron microscopy (TEM) was used to characterize the microstructures and nanoparticles of these ODS steels. According to the TEM results, 14Y, 14YAl, and 14YTi ODS steels present similar bimodal structures containing both large and small grains. The addition of Al or Ti has no obvious effect on the microstructure of the steels. The spatial and size distribution of the nanoparticles was also analyzed. The results indicate that the average size of nanoparticles in the 14YTi ODS steel is smaller than that in the 14YAl ODS steel. Nanoparticles such as Y₂O₃, Y₃Al₅O₁₂ and YAlO₃, and Y₂Ti₂O₇ were identified in the 14Y, 14YAl, and 14YTi ODS steels, respectively.     

Effects of overaging temperature on the microstructure and properties of 600 MPa cold-rolled dual-phase steel

C–Mn steels prepared by annealing at 800°C for 120 s and overaging at 250–400°C were subjected to pre-straining (2%) and baking treatments (170°C for 20 min) to measure their bake-hardening (BH2) values. The effects of overaging temperature on the microstructure, mechanical properties, and BH2 behavior of 600 MPa cold-rolled dual-phase (DP) steel were investigated by optical microscopy, scanning electron microscopy, and tensile tests. The results indicated that the martensite morphology exhibited less variation when the DP steel was overaged at 250–350°C. However, when the DP steel was overaged at 400°C, numerous non-martensite and carbide particles formed and yield-point elongation was observed in the tensile curve. When the overaging temperature was increased from 250 to 400°C, the yield strength increased from 272 to 317 MPa, the tensile strength decreased from 643 to 574 MPa, and the elongation increased from 27.8% to 30.6%. Furthermore, with an increase in overaging temperature from 250 to 400°C, the BH2 value initially increases and then decreases. The maximum BH2 value of 83 MPa was observed for the specimen overaged at 350°C.