Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Low carbon and low alloy steels require good combination of strength and ductility to ensure safety and stability of structures. Heat treatment in intercritical area can not only produce multi-phase microstructure, but also lead to the redistribution of alloying elements in different phases. Multi-step intercritical heat treatment is favorable to obtain retained austenite that is stabilized by repeated enrichment of alloying elements in reversed austenite and nanometer-sized precipitate that are primarily formed during tempering. Excellent mechanical properties are contributed by transformation-induced-plasticity effect of retained austenite and precipitation hardening effect of nanometer-size precipitates. In this work, the microstructural evolution and relative mechanical properties were investigated in a low carbon low alloy steel processed by a three-step heat treatment, namely, intercritical annealing, intercritical tempering and tempering. The microstructure was a typical dual-phase microstructure consisting of intercritical ferrite and bainite/martensite after intercritical annealing, and primarily comprised of intercritical ferrite, tempered bainite/martensite and retained austenite after intercritical tempering. Retained austenite with volume fraction of 29% distributed at the ferrite/bainite (martensite) boundaries and betweent bainitic/martensitic laths. Retained austenite was stabilized by enrichment of C, Mn, Ni and Cu in reversed austenite during the reversion transformation process. NbC precipitates with average size of 10 nm was formed in ferrite matrix and bainite/martensite, while Cu-containing particles in size range of 10~30 nm precipitated in ferrite and retained austenite during intercritical tempering and tempering process. The morphology of NbC precipitates was spherical, elliptical and irregular, and copper precipitates were spherical. With the combination of transformation- induced- plasticity (TRIP) effect of retained austenite and precipitation hardening, the steel possessed outstanding mechanical properties: yield strength > 700 MPa, tensile strength > 900 MPa, uniform elongation > 20%, and total elongation > 30%.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：本文通过三步的连续热处理方式，即临界退火，临界回火以及回火工艺，研究了低碳低合金钢的组织演变与力学性能。结果表明，临界退火后的组织为板条状的临界铁素体及贝氏体的双相组织，经临界回火后，为临界铁素体，回火贝氏体以及残余奥氏体的多相组织。残余奥氏体呈粒状和条状，分布在铁素体/贝氏体相界面及贝氏体板条之间，含量高达29%，并在回火后保持稳定，主要通过C，Mn，Ni和Cu在逆转奥氏体中的富集来稳定。临界退火及回火过程中，NbC在铁素体及贝氏体中析出，呈球状或不规则形状，平均尺寸为10 nm；富Cu的析出相在临界回火及回火过程中形成，呈球状分布于铁素体及残余奥氏体中，尺寸在10～30 nm 之间。通过残余奥氏体的应变诱导塑性(TRIP效应)及纳米析出相的析出强化作用，实验钢具有优异的力学性能：屈服强度高于700 MPa,抗拉强度高于900 MPa,均匀延伸率高于20%,总延伸率高于30%。
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《材料研究学报》
Abstract： Five ferrite/bainite (F/B) multi-phase steels with different volume fractions of bainite were obtained by TMCP process. The strain hardening behavior of high deformability pipeline steel with F/B multi-phase was studied by the analysis of longitudinal mechanical properties and modified C-J analysis. The relationships between volume fraction of bainite and stress ratio as well as yield ratio were analyzed, and relevant mechanisms were illustrated by modified C-J analysis. The results show that the stage of elastic deformation of high deformability pipeline steel mainly corresponds to stage I in modified C-J analysis, and the stage of plastic deformation consists of stage II and stage III; and the stage near yield point (0.5% strain) can go across stage I and stage II. However, the strain hardening capability of each stage is obviously different from each other, and the strain hardening behavior is closely related to the volume fraction of bainite in F/B multi-phase steel. The optimal matching between strength and plasticity of pipeline steel can be achieved by controlling the microstructure suitably. The stress ratio of Rt1.5/Rt0.5 is appropriate to describe the strain hardening capability near the yield point, and the stress ratios of Rt2/Rt1 and Rt5/Rt1 are appropriate to represent the strain hardening capability of plastic deformation stage in X70 grade pipeline steel. The stress ratio of Rt2/Rt1 is suitable to characterize the strain hardening capability of plastic deformation stage in X80 grade pipeline steel.
Subjects: Materials Science >> Materials Science (General) submitted time 2017-03-31 Cooperative journals: 《金属学报》
Abstract：以Mn-Ni-Mo为主要合金体系，研制了K65热煨弯管用高强高韧埋弧焊丝。采用该焊丝制得的直缝管焊缝金属抗拉强度达741-768MPa，显微硬度为231-250HV10，-40℃冲击功均为90-185J；直缝管焊缝经热处理工序后，-40℃冲击功均值为65-124J，相比直缝管焊缝出现较大幅度的下降。本文通过OM、LePera、SEM（EBSD）及TEM对焊缝进行组织观察，研究焊缝中Mn-Ni-Mo含量对K65热煨弯管组织转变和低温韧性的影响，结果表明：直缝管焊缝中Mn、Ni的增加会促进针状铁素体的形成，适当增Mo降Mn/Ni能使焊缝达到最佳强韧性能；经过热处理后，焊缝中针状铁素含量降低，上贝氏体含量增加，以及大尺寸沿晶分布渗碳体是焊缝金属低温韧性下降的原因，但Mo含量为0.2%时仍能保证大角度晶界比例达67.1%，使焊缝金属的-40℃低温韧性达124J。
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