Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： In accordance with the demand for reduced fuel consumption and CO2 emissions in automobiles, and with the increasing high demand for vehicle lightweight and safety, advanced high- strength steels (AHSS) have received more attentions in recent years. The recent trend for the development of AHSS has been concentrated on the complex microstructure with multiphase. Quenching and partitioning (Q&P) steel with carbon-enriched austenite within martensitic matrix as a competitive candidate of AHSS have been developed widely. It has high strength and good ductility depending on the multiphase microstructure. Therefore, the relationship of the mechanical property and the microstructure of the Q&P steels should be studied in detail. In the present work, the microstructure characterization and mechanical properties of the experimental steel treated by one step Q&P process were investigated, as well as the direct quenching and Q&T processes. The results show that the microstructure of the steel treated by one step Q&P process mainly consists of lath martensite, plate martensite and residual austenite films between martensite laths. With a increase in the holding time, the fraction of the plate martensite firstly increases and then reduces, while that of the retained austenite firstly increases and then becomes constant. The combination of strength and elongation of the steel processed by one step Q&P is much better than the one processed by the other two, that is to say, the former one can possess good strength and ductility at the same time. The product of tensile strength and elongation, the tensile strength and the elongation can achieve 21774.2 MPa�%, 1442 MPa and 15.1%, respectively. Along with the holding time increasing, tensile strength decreases but elongation rises and finally be stable.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： High strength low alloy steels utilize chemical composition design of low carbon content and are microalloyed with Nb, V and Ti, or other additions, such as Mo and B, etc. The increase of strength is attributed to grain refinement strengthening, solid-solution strengthening, dislocation strengthening and precipitation hardening. Moreover, the precipitation hardening attracts more and more attentions. However, the detailed results on the sheet spacing, inter-particle spacing, crystallography, composition and the nucleation site of the interphase precipitation carbides in Nb-Ti containing steels have not been reported as yet. In this work, the microstructure, mechanical properties and precipitation behaviors in a low carbon Nb-Ti microalloyed steel were investigated using the dilatometer and TEM. The results show that the interphase precipitation can be observed for different isothermal temperatures and the sheet spacing, inter-particle spacing and size of the interphase precipitation carbides are refined by lowing isothermal temperature. The characteristic sheets of interphase precipitation carbides were identified as planar and curved. The planar sheets of interphase precipitation carbides have been analyzed and found to be parallel with {011},{012},{013}and {035} planes of ferrite. Moreover, the interphase precipitation carbides which have been determined to be (Nb, Ti)C have a NaCl-type crystal structure with a lattice parameter of 0.434 nm and obey the Baker-Nutting (B-N) orientation relationship with respect to ferrite matrix. The contribution of the interphase precipitation hardening to the yield strength of the experiment steel has been estimated above 300 MPa, based on the Orowan mechanism.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： High strength low-alloy (HSLA) steel has been widely used in buildings, bridges, ships and automobiles because of the remarkable high strength and forming property. Conventional HSLA steels are strengthened by a combination of grain refinement, solid-solution strengthening and precipitation hardening, and the contribution of precipitation hardening is considered to be minor, since many of the alloying elements are added to HSLA steels in the past basically for the strengthening of grain refinement. However, in recent research, yield strengths up to 780 MPa have been achieved in Ti and Mo bearing HSLA sheet steels by producing microstructures that consist of a ferritic matrix with nanometer-sized carbides, and the precipitation strengthening has been estimated to be approximately 300 MPa. Nowadays, thermo mechanical controll process (TMCP) is widely used to process HSLA steels, the final temperature of ultra-fast cooling (UFC) plays a decisive role for microstructure evolution and precipitation behavior, and finally determines the mechanical properties of the steels. In this work, the effects of final temperature after UFC on microstructural evolution, precipitation behavior and micro-hardness of Nb-V-Ti bearing low alloy steel were studied by using the thermal mechanical simulator, OM, HRTEM and micro-hardness instrument. The results showed that the microstructure and nucleation sites of micro-alloy carbides changed with final temperature after UFC. The microstructure changed from bainite to pearlite and ferrite and the nucleation sites changed from bainite to ferrite with final cooling temperature increasing. The number density of the precipitates in ferrite matrix was greater than that in bainite. Furthermore, the number density of the nanometer sized carbides got the maximum values at 620 ℃. The aspect ratios of the precipitates were close to 1, which meat that the precipitation morphology close to spherical. The sizes of the carbides were all less than 10 nm and became smaller with the decrease of final cooling temperature. Through the calculation by Orowan mechanism, the contributions of the precipitation strengthening to yield strength could reach 25.6% at the final cooling temperature of 620 ℃.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： TiAl-Nb alloys have been determined as the advanced direction for the development of the high temperature TiAl alloys, so being one of the advanced materials for turbines of aircraft engines and gas-burning power-generation plants. However, highly-Nb addition can lead to the complex solidification behavior of TiAl-Nb alloy and multi- phase microstructure, which is important for the mechanical properties during the alloy design. Bridgman type directional solidification experiments were conducted for Ti-46Al-(8, 9, 10)Nb alloy. The effect of the growth rate and Nb content on the microstructure, phase transition and microsegregation was investigated, and finally the selection diagram of the phase transition and the microstructure of the directional solidified TiAl-Nb alloy were obtained. The results show that the planar-cellular-dendritic evolution of solid-liquid interface can be observed with the increase of the growth rate. Meanwhile the fully b phase solidification changes to the peritectic solidification with the increase of the growth rate, and correspondingly the final microstructure is composed of the a2/g lamellar structure and a multiphase microstructure (B2 phase, a2/g lamellar structure) respectively. The increase of the b-stabilizer Nb content can promote the fully b phase solidification and the formation of the multiphase microstructure (B2 phase, a2/g lamellar structure). The contribution of the growth rate and the Nb content to the phase transition and the microstructure is connected with the solute segregation (S- segregation, b- segregation) closely. The increase of the S-segregation amplitude can easily promote the peritectic reaction, which always leads to the highly solute segregation and the concentrated distribution of plenty of B2 phase in the core of the dendrite. b-segregation is the mainly origin of the B2 phase formation, in which the Nb enrichment in the retained b phase directly determines the morphology and the dimension of the B2 phase. Finally according to the selection diagram of the solidification process and the microstructure of the Ti-46Al alloy with the growth rate and the Nb content, the high Nb content and the low growth rate during fully b solidification should be selected for the prefer microstructure with the homogeneous distribution and the low solute segregation.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-10 Cooperative journals: 《金属学报》
Abstract：采用热膨胀实验研究了等温温度对Nb-Ti微合金钢组织性能及析出行为的影响规律，利用TEM对不同温度等温后试样的析出行为进行观察和分析。结果表明，不同温度等温后试样中均可观察到相间析出碳化物，且降低等温温度可显著细化相间析出碳化物所在面面间距、相间析出碳化物所在面内析出物间距及相间析出碳化物尺寸。具有层状分布特征的相间析出碳化物所在面呈现弯曲和平直两种形态，其中平直型相间析出碳化物所在面平行于铁素体的{011}, {012}, {013}和{035}面。经HRTEM确定相间析出碳化物具有NaCl型晶体结构，晶格常数为0.434 nm，且与铁素体基体符合Baker–Nutting (B–N)关系，经EDS分析确定相间析出碳化物为(Nb, Ti)C。利用Orowan机制对析出强化量进行估算，结果表明不同等温温度下析出强化量均大于300 MPa。
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