温轧温度对中碳钢组织与力学性能的影响

采用透射电镜(TEM)、扫描电镜(SEM)、室温拉伸等手段,研究了650~730 ℃温轧温度对0.46%C中碳钢的组织演变及力学性能的影响。结果表明,经90%的轧制变形,试验钢铁素体晶内引入大量位错,渗碳体片层产生应力集中导致层片状渗碳体弯曲、扭折、碎化为颗粒状。随着温轧温度的降低,位错增殖明显,渗碳体球化率增加,分布越来越均匀,抗拉强度和伸长率整体上升。当温轧温度为650 ℃时,渗碳体球化最好,抗拉强度877 MPa,断后伸长率16.0%,综合力学性能最好。拉伸断口结果表明,随着温轧温度的降低,试验钢的断裂机制由韧-脆混合断裂转变为韧性断裂,塑性提高。     

Influence of slab reheating practice on the microstructural evolution and toughness behavior of quenched and tempered plates of medium carbon microalloyed steel

In the current study, granular bainite was found to be the major component in the microstructure of air cooled 80 mm thick plates of medium carbon microalloyed steel. The second constituent in this granular bainite was identified as cementite. It was further observed that (1) ferrite lath size and (2) amount of cementite in granular bainite varied with slab reheating time before plate rolling. Smaller ferrite laths and a lesser amount of cementite were found in the plate processed with the longer slab reheating time of 26 h. Contrary to this, very large sized ferrite laths and a larger population of cementite were formed in the plate processed with the shorter slab reheating time of 4 h. Subsequent quenching and tempering of these plates favored the formation of lower bainite and tempered martensite in the plate with 26 h slab reheating time. On the other hand, upper bainite and coarser cementite were formed after the quenching and tempering of the plate with 4 h of slab reheating time. The influence of different microstructures, formed due to varied slab reheating time, on the toughness property of tempered plates was evaluated under different test conditions. In tensile test and fracture toughness testing of thinner specimens, a ductile mode of fracture was observed, irrespective of varied microstructures in the tempered plates. However, in the three-point bend test of full thickness specimens, the mode of fracture was ductile in the tempered plate with 26 h slab reheating time, while the tempered plate from the slab with 4 h reheating time gave rise to a predominantly brittle mode of fracture. These observations showed that the toughness property of these tempered plates was sensitive to the microstructure only under the specific condition, which prevailed during the three-point bend test of full thickness specimens. Under this condition, coarse cementite and upper bainite became prone to cracking resulting in a lower toughness of the tempered plate associated with lower slab reheating time.     

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生产检验中发现DC01钢断后伸长率不合格。介绍了DC01钢的生产工艺流程,对断后伸长率不合格的DC01钢试样进行化学成分、夹杂物和金相组织分析。结果表明:DC01断后伸长率不合格的主要原因是金相组织中渗碳体含量多,且呈链状和聚集状,钢中夹杂物也会引起断后伸长率不合格,提出了解决问题的措施。     

热轧变形对快凝Al－Fe－V－Si粉热挤合金断裂行为的影响

研究了不同热变形条件下用快餐Ａｌ—Ｆｅ—Ｖ—Ｓｉ合金粉挤压成形样品的拉伸断裂行为．结果表明，在沿轧制方向拉伸的试样中，与板面平行的颗粒界面（ＬＴ界面）处形成的微裂纹不影响主裂纹的扩展，沿与板材纵截面平行的颗粒界面（ＬＳ界面）的开裂可能导致试样反常断裂，沿与板材横截面平行的颗粒界面（ＴＳ界面）则易成为主裂纹的低能扩展路径，使试样沿颗粒断裂．热轧变形使粉末ＬＳ，ＴＳ界面的强度提高快于ＬＴ界面，导致热轧变形至一定程度（３０％变形量）时试样的断裂方式改变．断裂强度与延伸率的不同变化规律与断裂方式的改变及其后的形变硬化有关．     

大型精轧支撑辊疲劳断裂失效分析

某单位1580生产线精轧支撑辊在轧制过程中突然发生断裂,为查明失效原因,利用光学显微镜、拉伸、冲击试验机以及扫描电镜对轧辊材料显微组织、力学性能、断口形貌、起裂源位置进行了综合分析。结果表明,精轧支撑辊芯部材料为球墨铸铁,其组织主要由珠光体、棒状渗碳体、球状石墨及聚集在石墨周围的牛眼状铁素体构成。组织洁净度较差,夹杂物主要为球状Al₂O₃-MgO-SiO₂、Al₂O₃-MgO、Al₂O₃氧化物,尺寸集中在10～15 μm。支撑辊断口呈现典型旋转弯曲疲劳断口特征,裂纹源区位于轧辊内部且为多源起裂,断口内部呈现典型“鱼眼”特征,“鱼眼”区中最大夹杂物直径3 mm,为球状Al₂O₃-MgO-SiO₂。裂纹源最大拉应力30 MPa。裂纹在夹杂物周围萌生时其裂纹尖端最大应力强度因子为1.385 MPa·m^1/2,大于内部起裂裂纹扩展门槛值,因此裂纹在夹杂物与基体界面处形核。裂纹扩展至“鱼眼”区之外时,裂纹在应力作用下快速扩展并汇聚形成一条长裂纹,最终导致轧辊发生瞬断。     

热轧变形对快凝Al—Fe—V—Si粉热挤合金断裂行为的影响

研究了不同热变形条件下用快凝Ａｌ－Ｆｅ－Ｖ－Ｓｉ合金粉挤压成形样品的拉伸断裂行为。结果表明，在沿轧制方向拉伸的试样中，与板面平行的颗粒界面处形成的微裂纹不影响主裂纹的扩展，沿与板材纵截面平行的颗粒界面的开裂可能导致试样反常断裂，沿与板材横截面平行的颗粒界面则易成为主裂纹的低能扩展路径，使试样沿颗粒断裂。     

Precipitation of paraequilibrium cementite: Experiments,and thermodynamic and kinetic modeling

The precipitation of cementite prior to the precipitation of the strengthening M₂C phase is investigated using two model ultra-high strength (UHS) steels. The structure, microstructure and chemical composition of cementite are studied by analytical electron microscopy techniques. The structure of cementite precipitated during early stages of tempering at 755 and 783 K was confirmed by convergent beam electron diffraction. In an alloy containing 0.16 mass% C, the cementite particles were primarily plate shaped and interlath type, whereas in an alloy containing 0.247 mass% C both inter- and intralath particles were observed. Consistent with the earlier studies on tempering of Fe-C martensite, lattice imaging of cementite suggests microsyntactic intergrowth of M₅C₂ (Hägg carbide). Quantification of the substitutional elements in cementite confirms its paraequilibrium state with ferrite at the very early stage of tempering. Computational thermodynamic and kinetic tools, Thermo-Calc and dictra (diffusion controlled transformation) software, respectively, are used to model the precipitaton of paraequilibrium cementite in several multicomponent alloys. A thermodynamic model parameter describing the effect of Si on the stability of cementite is proposed. The model parameter is consistent with the following results: (a) that Si does not partition to cementite in Fe–Si–C and Co–Si–C alloys under orthoequilibrium conditions, and (b) there is a large driving force for the precipitation of paraequilibrum cementite in an Fe–0.41C–3Mn–2Si alloy where it has been experimentally verified. The nucleation driving forces for the precipitation of paraequilibrium cementite, and the two-phase (ferrite and cementite) paraequilibrium boundaries for multicomponent alloys are calculated using the Thero–Calc software systems. The results of growth simulations of cementite under paraequilibrium condition in multicomponent systems using the dictra software are also presented.     

TEM STUDY OF AUSTENITE FORMATION IN A LOW CARBON STEEL

<正> 低碳铁素体加马氏体双相钢的强度和塑性取决于马氏体的数量和分布,这与亚温区加热过程中奥氏体形成的特点有关。由于奥氏体的形成是在高温下进行,速度快,难于直接获得有关形核的信息。本文研究低碳钢中奥氏体形成的部位,以及预先冷轧的影响。 试验用低碳1.5Mn钢,其化学成分为(wt-％):C 0.08,Mn 1.45,si 0.21,Al 0.045,N 0.005。经高频感应电炉熔炼成45kg的钢锭后,热轧成2.5mm厚的板材。在真空炉中经过1200℃均匀化退火24h。然后,在盐炉中900℃加热15min后空冷,获得铁素体加珠光体型的正火原始组织。另将一部分正火的坯料进行厚度压下量     

共析珠光体钢在冷轧过程中的组织变化

用SEM、TEM及XRD等方法研究了共析钢中珠光体经40％-90％的冷轧变形后所产生的显微组织变化．接渗碳体形态特点,变形珠光体组织可分为以下3种类型：(1)不规则弯曲片层型,即变形后的渗碳体与轧制面呈大角度偏离且不规则弯曲的珠光体．(2)带有剪切带的粗大片层型,即被渗碳体剪切带分开且变形轻微的珠光体．(3)精细片层型,即与轧制方向平行排列、片间距细小且渗碳体严重变形的珠光体．精细片层区域的比例随着轧制压下率的提高而增大．此外,重度冷轧变形还引起渗碳体严重塑性变形和部分溶解．     

热轧工艺参数对72LXA钢盘条组织与性能的影响

研究了吐丝温度及控制冷却制度对72LXA盘条组织和性能的影响。结果表明,在相同的冷却制度下,提高吐丝温度,组织中索氏体化率增加和先析铁素体含量减少,材料强度提高,先析铁素体和索氏体化率是影响材料力学性能的重要因素;加大珠光体形成后的冷却能力,可以抑制片层渗碳体的溶解;通过辊道速度和冷却风量的适当配合,可以提高盘条的通条性能。获得了强度高于1050 MPa的盘条,表明确定的热轧工艺可以满足盘条的力学性能要求。     

冷轧退火对过共析珠光体钢组织和力学性能的影响

采用SEM、TEM、XRD及拉伸实验等方法研究了原始组织为层片状珠光体的过共析钢重度冷轧及退火后的微观组织及性能指标。结果表明:在重度冷轧及退火工艺下制备出了晶粒尺度均在亚微米量级的超微细复相组织(铁素体+渗碳体),其中等轴铁素体晶粒直径为0.5～0.8μm,晶界上钉扎着平均粒径为0.2～0.4μm球化完全的渗碳体颗粒,而晶界内则出现少量粒径为30～50 nm的渗碳体颗粒;在同一实验条件下,增大变形量、增加退火温度以及保温时间的延长,都能导致渗碳体球化程度的增加;与此同时,复相组织的强度指标显著下降,但其塑性指标明显上升;拉伸断口形貌特征也由脆性断裂(冷轧变形)转变为韧性断裂(退火态)。     

CRYSTALLOGRAPHIC ORIENTATION AND ELEMENTAL SEGREGATION OF FLAKES IN STEELS

利用电子光学仪器(扫描电镜、透射电镜、电子探针和Auger能谱仪)对钢中白点的晶体取向和元素偏析进行了综合研究。结果表明:白点断口以晶内的准解理和沿晶的波状撕裂条纹的混合断裂形态为特征;白点断裂面的晶体学取向主要是沿铁素体{112}晶面的准解理断裂,显徽空隙内枝晶表面为{111}自由面;白点断口上有P和Cr,Mo,Mn的正偏析,其中以P最为明显。     

Structural features of the behavior of a high-carbon pearlitic steel upon cyclic loading

The structural evolution upon high-cycle fatigue (tension with the magnitude of stress in a cycle below the macroscopic yield stress) of the hypereutectoid steel U10 (1.03 wt % C), in which pearlite of different morphology (fine-lamellar, coarse-lamellar, and partially spheroidized pearlite) was formed, has been investigated by scanning and transmission electron microscopy. Based on the fractographic analysis, features of fracture of these structural states have been considered. At a significant distance (10 mm) from the fatigue fracture, features of structural transformations caused by cyclic loading have been revealed. It has been shown that upon high-cycle fatigue in the steel U10 with structures of lamellar and partially spheroidized pearlite, substantial structural changes occur, namely, fragmentation and partial dissolution of cementite plates, and in fine pearlite, additionally, spheroidizing of cementite and polygonization of the ferritic component are observed. A dependence of the character of fracture on the type of structure formed upon fatigue loading has been established. In the steel with a nonequilibrium structure of unannealed fine pearlite, an enhanced elasticity modulus, as compared to other more stable structures (coarse-lamellar, annealed fine, and spheroidized pearlite), and a reduction in the magnitude of the elasticity modulus under the action of cyclic loading have been found. It has been established that the structural changes in fine pearlite of laboratory specimens of the steel U10 upon cyclic tension are qualitatively similar to those in a railroad wheel of the steel 65G under the service conditions.     

Structural aspect of delamination crack formation during the HTMT of steels with a ferritic structure

The fine structure, texture, and fracture of the ferritic 08Kh18T1 steel subjected to repeated hot rolling at temperatures of 800–1100°C and reductions of 35, 50, and 65% have been studied. When significant strains are reached, a layered subgrain structure forms in the steel, and {100} and {111} planes are predominantly located in the rolling plane. The mechanisms of delamination crack formation during rolling under HTMT conditions are considered. The development of a crack in a material with a layered structure is analyzed theoretically.     

Effect of cold rolling on properties and microstructures of dispersion strengthened copper alloys

Mechanical properties and microstructures of unidirectionally and tandem rolled alumina dispersion strengthened copper（ADSC） alloys under different conditions were investigated by tensile test, optical microscopy（OM）, transmission electron microscopy（TEM） and scanning electron microscopy（SEM）. For unidirectionally rolled ADSC alloys, their strengths and elongations in the longitudinal direction are higher than those in the transverse direction under both cold rolling and annealing conditions. Once fracture appears in their longitudinal stress--strain curves, sudden reduction of overall stress level before complete fracture can be observed in the transverse tensile curves. The anisotropy of mechanical properties for the ADSC alloy can be greatly improved by tandem cold rolling. And no sudden reduction of overall stress level appears in the stress--strain curves for tandem rolled ADSC alloys. The differences of their microstructures and tensile fractures were analyzed. In order to compare the differences of tensile fracture mechanism in different directions, longitudinal and transverse fracture models for unidirectionally rolled ADSC alloys were also introduced.     

轧前深冷处理对AZ31镁合金显微组织和力学性能的影响

本文通过对铸态AZ31镁合金经不同条件的深冷处理继而热轧,分析其强度、韧性、延展性效应及其规律。深冷实验条件分别为-60℃/12h、-120℃/12h、-180℃/12h、-60℃/2h和-180℃/2h。结果表明：AZ31经适宜的深冷处理,晶粒得到细化,强度、韧性及延展性得到有效提高;与未经深冷处理试样相比,深冷处理时间为2h的两种试样中都出现大量孪晶;经-60℃/12h深冷处理后再进行轧制试样的显微组织最为细小均匀,屈服强度提高了25.8%,而延伸率则更是提高了4倍,由3.06%提高到了12.31%;深冷处理后的AZ31镁合金断口呈现出脆性断裂和塑性断裂集合的复合性断裂特征。     

Subcritical Spheroidization of Medium-Carbon 50CrV4 Steel

An investigation on subcritical spheroidization anneal of the cold-rolled 50CrV4 steel at 720?°C has been carried out. During spheroidization anneal, the lamellar cementite was gradually broken down and changed to spherical shape. With prolonging of soaking time, the amount of lamellar cementite decreases gradually, and that of the spheroidized cementite particle increases gradually. The relationship of the spheroidization ratio versus soaking time for the steel can be described by a typically sigmoid curve. Additionally, the cold rolling deformation accelerates the breakup of lamellar cementite and the formation of spheroidal cementite particles during spheroidization anneal of the steel. The more severe the deformation is, the more rapidly the spheroidization occurs. From the results of tensile and hardness test, the yield strength, ultimate tensile strength, and hardness decrease and the percentage elongation to failure increases rapidly during the first 2?h of spheroidization. Between 2 and 8 h, the yield strength, ultimate tensile strength, hardness nearly keep a constant, which are independent of the soaking time, whereas the percentage elongation to failure firstly increases and then decreases with prolonging of soaking time.     

UNHEALED POROSITIES AND CRYSTALLINE STEPS IN FLAKES

在结构钢的白点断口中,用立体显微镜有时可以观察到沿轧制方向的暗线条;用扫描电子显微镜进行高倍观察则为未轧合的疏松残留下来的长条形空隙,内壁光滑,保留有露头枝晶、结晶台阶、显微孔洞等结晶形貌。这种空隙很可能是白点断口的一种断裂源。     

轧后冷却路径对中碳钢扩孔性能的影响

利用超快速冷却技术,通过控制轧后冷却路径,调整不同的终轧温度和终冷温度,研究了冷却路径对热轧后经退火处理的中碳钢组织和扩孔性能的影响.结果表明,通过后续退火处理可以在铁素体基体上形成弥散分布的球化渗碳体组织;随着终轧温度和终冷温度的降低,退火处理后的渗碳体更加细小,且弥散程度提高,在扩孔实验中,当切向延伸率达到材料成形极限时,裂纹优先在冲孔的边缘出现,裂纹主要通过微孔集聚的方式形成;均匀细化的铁素体和球化的渗碳体组织能够明显提高实验钢的延伸率,有效阻止相邻微孔聚合,从而提高材料的扩孔性能.     

扁平型铁基非晶粉末的取向对吸波材料电磁性能的影响

采用水雾化法和球磨处理工艺制备了扁平形Fe74Cr2Mo2Sn2P10Si4B4C2粉末,通过涂布工艺和非涂布工艺分别制备了扁平形粉末取向分布和随机分布的吸波材料,探讨了取向对其微波电磁性能的影响。结果表明：取向分布的吸波材料具有更高的磁导率和更低的介电常数,在10 MHz下,对取向分布的吸波材料,μ＇=20.1,ε＇=110,对随机分布的吸波材料,μ＇=14.4,ε＇=194。同时建立了取向度与磁导率的关系式,并与实验结果定量一致。