非调质钢中MnS分布对轧材横向塑性的影响

为了探究实际生产中非调质钢轧材横向塑性波动较大的原因,利用电子显微镜对比研究了连铸坯、轧材和拉伸试样断口中MnS夹杂物形貌及其演变行为,并对轧材横纵截面MnS夹杂物的尺寸、数量以及分布特征进行定量化表征,发现MnS夹杂物的分布特征是影响轧材横向塑性的关键。MnS夹杂物聚集分布,拉伸试样在受力过程中大量微裂纹在局部同时萌生,导致试样提前断裂,形成木质状的断口形貌,轧材表现出较差的横向塑性。当MnS夹杂物分布较为均匀时,拉伸试样受力相对均匀,应力集中小,轧材横向塑性较好。轧材中MnS夹杂物的分布行为主要取决于铸态下MnS夹杂物的形貌。减少铸态下II类树枝状MnS数量,是改善最终轧材横向塑性的关键。     

硫化物变性处理45MnVS非调质钢的高周疲劳性能

 长条状的硫化物夹杂往往会导致热轧/锻造含硫钢的力学性能呈现出明显的各向异性。为了明确硫化物变性处理铁素体-珠光体型非调质钢的疲劳性能各向异性,采用轴向力控制高频疲劳试验机(应力比R=-1)研究了工业生产的45MnVS非调质钢锻态及调质态的高周疲劳断裂行为。结果表明,试验料纵向样中的MnS夹杂分布较为均匀,多呈短棒状或纺锤形,平均长宽比为3.4±1.7。与未变性处理的含硫非调质钢相比,试验料塑性和韧性的各向异性得到显著降低。锻态与调质态横向样的疲劳性能略低于纵向样,调质态样的疲劳极限比(0.52~0.54)明显高于锻态样(0.46)。在ΔK值大于约35 MPa·m1/2时,横向样的疲劳裂纹扩展速率略大于纵向样。疲劳断口分析表明,2种状态横向样的疲劳裂纹均主要起源于钢中条棒状MnS夹杂,且调质态样受影响的程度更大。上述结果表明,试验钢硫化物变性处理后的疲劳性能各向异性很小,锻态组织的各向异性程度略低于调质态组织,但后者具有更为优异的疲劳性能。     

碳化物对粉末冶金刀具钢断裂机理的影响

通过对M390粉末冶金刀具钢材料进行扫描电子显微组织观察、X射线衍射分析以及原位拉伸试验，研究了M390粉末冶金刀具钢微观组织结构和拉伸断裂行为，并分析了碳化物在粉末冶金刀具钢断裂过程中的作用。研究发现：M390粉末冶金刀具钢的显微组织是由基体和分布在基体上的碳化物两部分组成，其中基体组织为铁素体，碳化物则包括富铬等合金元素的M₇C₃和M₂₃C₆型碳化物，这些碳化物相严重影响M390粉末冶金刀具钢的断裂。随着外加载荷增加，分布在基体上的碳化物相提前开裂，形成裂纹源，裂纹呈现穿过或绕过碳化物的形式扩展，在进一步加载的情况下，裂纹处形成应力集中，导致基体发生脆性断裂。碳化物相成为M390粉末冶金刀具钢断裂过程的薄弱环节。试样断口呈现脆性断裂特征，并包含少量韧窝，且韧窝中存在第二相质点，即碳化物。     

含硫非调质钢中硫化锰夹杂物的控制

在非调质钢中加入硫元素用于改善钢材切削性能，但硫与锰结合形成MnS夹杂严重影响钢材的横向冲击性能，因此含硫钢中的MnS夹杂物的控制是该类钢种生产的主要难题之一。对含硫非调质钢中MnS进行研究发现，在铸坯的柱状晶区域，MnS夹杂物分布均匀、量多细小；在等轴晶区域，MnS夹杂物分布不均匀，量少粗大；MnS超标比例为45%。通过在连铸过程中降低拉速和过热度、调整冷却参数，控制铸坯冷却速度，MnS夹杂显著细化，主要以2.5～3.5为主；在热处理过程中，随着加热温度由800℃提高到1 200℃,MnS分裂球化逐渐明显。调整连铸和加热工艺参数后，MnS超标比例降至10%,铸坯不合格率大大降低。     

钛对非调质钢韧性的影响

探讨了Ti对非调质钢的显微组织和变形过程的影响。测试了钢在平面应变状态下的断裂韧性,计算了冲击和拉伸状态下的裂纹形核能和扩展能。用SEM研究了断口形貌。结果表明:在钢中加入Ti可使先共析铁素体量增多;在冲击状态下,加入Ti对韧性影响不大;V钢和V+Ti钢均为准解理断裂;但在静态拉伸时,V+Ti钢为剪切断裂,V钢为准解理断裂。     

碲处理改善汽车曲轴用非调质钢38MnVS6硫化物形貌的研究

为研究碲元素对汽车曲轴用非调质钢38MnVS6硫化物形貌的影响,在冶炼过程中喂入70 m的Mn-Te线,加入0.012%Te,通过金相显微镜及扫描电镜对Φ380 mm连铸圆坯及Φ105 mm热轧圆钢的非金属夹杂物进行研究。研究发现,硫化物的形态在连铸坯横向与纵向的形态相同;MnS与MnTe形成共晶化合物,MnTe的存在改变了MnS的析出形态,可以使硫化物形态向短杆甚至椭球态转变,整体长度尺寸变小,平均长度由45μm缩短为15μm;加入0.012%Te后,MnTe与MnS两种夹杂物发生固溶,轧材夹杂物与铸坯相似,而未Te处理的圆钢硫化物随轧制的方向被明显的拉长。     

中高硫钢中硫化锰夹杂物控制技术

 近年来,随着制造业的不断升级发展,对节能环保的要求越来越高,对于一些用于复杂零部件制造的钢种,为了降低在制造加工过程中的能耗,通常向钢中加入易切削元素(硫、碲、铅)来改善其切削加工性能。添加一定的硫是目前最常用的改善手段。硫在钢中主要以MnS形式存在,其形貌及分布控制水平对钢材力学性能有重要影响。对于中高硫钢,硫化锰属于塑性夹杂,在析出过程中易发生聚集长大,并且容易在轧制过程中沿拉轧方向变形,成为大尺寸长条状,这类大尺寸MnS会严重破坏材料的横向性能。为保证中高硫钢的钢材性能,需要对钢中MnS夹杂的形貌及分布进行控制,目标是避免大尺寸MnS的产生,尽可能得到细小、均匀分布的纺锤状MnS夹杂。MnS夹杂控制是一个系统的问题,必须联系整个工艺流程进行。总结了部分合金元素、工艺参数对MnS夹杂析出的影响规律,并综述了近年来在整个生产流程中的MnS夹杂控制实践,包括精炼过程的改性处理、复合析出控制,凝固过程控制和控轧控冷控制,并指出,对于如非调质易切削钢等中高硫钢中的MnS形貌及分布控制,如何将实验室研究成果落实于工业生产是广大研究者未来共同努力的方向。     

B级船板钢形变断裂过程的原位研究

 在扫描电子显微镜下观察分析了B级船板钢试样在原位拉伸过程中的断裂行为以及低温脆性断口中二次裂纹的扩展。结果表明：在拉伸变形过程中,微裂纹首先在试样缺口处形成,然后沿铁素体+珠光体界面扩展。加载过程中,多边形铁素体发生塑性变形,裂纹在基体内以“Z”字型扩展。在低温脆性断裂区,二次裂纹以穿晶方式通过铁素体基体,在扩展到珠光体区域时有时沿铁素体和珠光体的界面扩展,有时穿过珠光体区域扩展。     

低硅含铝TRIP钢断裂机理及其残余奥氏体的稳定性

TRIP钢在原位拉伸过程中应力迫使残余奥氏体向马氏体转变,在此过程中伴随着应力松弛,断裂被延迟。整个变形过程中,试样加载后,先在铁素体内部产生滑移带,滑移带的方向与拉应力的方向约成45°;TRIP钢裂纹源首先在V型缺口与夹杂物处产生,然后扩展并连接,且裂纹的走向经常产生转折,即残余奥氏体转变为马氏体裂纹的尖端被钝化。利用EBSD技术,分析了未变形与断裂后钢板残余奥氏体的分布与稳定性,发现小尺寸和分布在铁素体内部的残余奥氏体比较稳定。     

新型奥氏体耐热钢C-HRA-5原位拉伸断裂机理

 断裂机理是材料力学性能研究的主要方向之一。为了阐明C-HRA-5钢单轴拉伸断裂机理,通过原位SEM观察拉伸全过程,对该耐热钢内部微裂纹萌生—扩展—断裂进行了深入分析。结果表明,C-HRA-5钢在室温和700 ℃下都表现出典型的韧性断裂特征。材料断裂主要受到M₂₃C₆相和MX相的影响,微裂纹由M₂₃C₆相聚集的三叉晶界处萌生。室温时,微裂纹与晶内MX相处裂纹相连,形成穿晶裂纹;高温时,微裂纹在剪应力的作用下形成局部穿晶裂纹,导致材料最终失效。     

Effects of Inclusions on Delayed Fracture Properties of Three TWinning Induced Plasticity (TWIP) Steels

In the present study, delayed fracture properties of a high-Mn TWinning Induced Plasticity (TWIP) steel and two Al-added TWIP steels were examined by dipping tests of cup specimens in the boiled water, after which the microcrack formation behavior was analyzed. The TWIP steels contained a small amount of elongated MnS inclusions, spherical-shaped AlN particles, and submicron-sized (Fe,Mn)₃C carbides. Since MnS inclusions worked as crack initiation sites, longitudinal cracks were formed along the cup forming direction mostly by MnS inclusions. These cracks were readily grown when high tensile residual stresses affected the cracking or hydrogen atoms were gathered inside cracks, which resulted in the delayed fracture. In the Al-added steels, MnS inclusions acted as crack initiation and propagation sites during cup forming or boiled-water dipping test, but residual stresses applied to MnS might be low for the crack initiation and growth. Thus, longitudinal cracks formed by MnS inclusions did not work much for delayed fracture. AlN particles present in the Al-added steels hardly acted as crack initiation or growth sites for the delayed fracture because of their spherical shape.     

C-Mn中厚钢板单向拉伸断后伸长率不合格原因分析

采用光学显微镜、扫描电子显微镜、能谱仪分析C-Mn中厚钢板单向拉伸断口形貌、组织、夹杂物、晶界及晶内成分。结果表明,断面微观形貌均为韧窝+解理特征。解理特征出现在断口中心部位,随解理特征减少,伸长率提高;断面均可见相互平行的裂隙,裂隙中存在MnS夹杂。MnS夹杂级数降低,伸长率也随之增大;伸长率不合格钢板的晶粒大小不均匀。据此判定,MnS夹杂和晶粒均匀性是影响伸长率的主要原因。     

Cause Analysis of Low Tensile Plasticity in Normal Direction for Hot-Rolled Thick Plate of HSLA Steel

To investigate the cause for the low tensile plasticity in normal direction of high-strength low-alloy steel thick plate, tensile fracture observation, inclusion statistic, and microstructure observation of thick plate and solidification structure observation of continuous casting slab are carried out. The experimental results reveal that many MnS inclusions are only distributed on the platform of the tensile fracture and they only appear in the center of thick plate together with martensite and severe macrosegregation of elements C and Mn. It is concluded that the aggregated large-sized MnS inclusions are the direct cause for reducing the tensile plasticity. Stress concentration on a single MnS inclusion during tensile testing can occur and cause a limited brittle fracture area. Aggregation of MnS inclusions creates a large brittle fracture area, which significantly reduces the tensile plasticity. The root cause of the aggregation of MnS inclusions is the well-developed columnar crystals of continuous casting slab. The well-developed columnar crystals push high concentration of solutes into the residual liquid-phase zone in the center of continuous casting slab and create severe central macrosegregation which eventually lead to the appearance of aggregated large-sized MnS inclusions with martensite in the center of thick plate.     

中厚钢板冷弯成型时脆性开裂原因分析

通过对 0 9CuPCrNi钢板冷弯开裂断面扫描电镜观察 ,结合开裂部位金相组织、夹杂、磷偏析等的综合分析 ,认为该钢冷弯开裂属偏析带脆性开裂。钢中存在的较强的磷偏析、层片状分布的MnS夹杂及表面微裂纹三者综合导致了弯背的脆性开裂。     

Inclusion-controlled fatigue properties of 1800 MPA-class spring steels

Fatigue tests were conducted on a series of 1800 MPa-class spring steels whose fatigue properties were inclusion controlled. The fatigue tests were conducted on billets and on hot-rolled bars, taking into account the elongation of the oxide-type composite inclusions that were deformed during hot rolling, i.e., controlled inclusions. Anisotropy of the fatigue properties due to the slender shape of the elongated inclusions was also discussed. Fatigue tests were then conducted for both the rolling direction (RD) and transverse direction (TD) in the case of the billets. The fatigue test results in the RD showed a slight difference between billets and bars. The inclusions that were deformed during hot rolling were sufficiently elongated, even for the billet specimens, and differences in the effective inclusion sizes between the billets and their hot-rolled bars for the RD were small. However, there were marked differences in fatigue strength between the RD and TD in the billet specimens: the fatigue strength was almost half in the TD due to the presence of fish-eye fractures originating in large and slender MnS inclusions. In these fatigue tests, the two types of deformable inclusions revealed remarkably different effects on fatigue strength: the deformable oxide-type inclusions never caused fish-eye fracture, although the MnS inclusions found in the billets were extremely detrimental to fatigue strength when stress was applied in the TD.     

高强度船板拉伸断口分层原因分析

针对高强度船板拉伸试样断口出现分层的问题,对典型断口分层试样和断口合格试样取样进行金相检验及扫描电镜能谱分析,结果表明,异常断口试样夹杂物级别高,试样心部组织晶粒粗大,带状组织明显且有裂纹,分析认为,2.0级以上的硅酸盐夹杂和MnS夹杂、28μm宽的珠光体带及Ti、Nb元素的富集是导致拉伸裂纹进而出现断口分层的主要原因。     

Indentation loading studies of acoustic emission from temper and hydrogen embrittled A533B steel

Isothermal tempering at 500 °C (within the region rendering low alloy steels susceptible to reversible temper embrittlement) induced acoustic emission activity in A533B steel during indentation loading. Samples, when sectioned, were found to contain small (∼10 μm long) MnS inclusions, some of which had debonded from the matrix material when they were near the indentations. Hydrogen charging prior to testing greatly enhanced the acoustic emission activity. It also resulted in the formation of small (∼20 to 200 μm) microcracks in samples tempered at 500 °C. These microcracks, when examined by optical metallography, appear to have propagated along prior austenite grain boundaries, consistent with fractographic observations of temper embrittlement in other low alloy steels. Many were nucleated by MnS inclusion debonding and all were confined to within a few hundred micrometers of the sample surface and within two or three indenter diameters from the indent. It is proposed that trace impurities (P, As, Sb, Sn) diffuse during the 500 °C temper to both the MnS inclusion interfaces and the prior austenite grain boundaries, reducing local cohesive strength. The tensile field created by the indenter debonds inclusions to form crack nuclei. Moderate acoustic emission results. In the absence of hydrogen these void nuclei may grow but do not coalesce to form observable cracks. The prior austenite grain boundaries, which in contrast to the dispersed inclusions can provide continuous crack paths, are not sufficiently temper embrittled to fracture without the assistance of hydrogen at these stresses. Hydrogen charging induces a high hydrogen concentration in a surface layer of the sample. This reduces further the grain boundary cohesion, and cracks initiated at inclusions are able to propagate along continuous grain boundary paths, generating additional energetic acoustic emission signals. This process can continue after unloading the indenter due to hydrogen diffusion to the residual stress field.