Heating rate effects during non-isothermal annealing of AIK steel

The effects of heating rate on microstructural size and shape parameters during annealing of cold rolled aluminum killed steel strips have been examined under non-isothermal condition. It is shown that decrease in the heating rate results in accelerated grain growth behavior compared with the prediction by quasi-isothermal based kinetics. The {111} and {112} crystallographic orientations, which enhance the normal anisotropy and deep drawability of cold rolled annealed sheets, are found to exhibit a strong correlation with the grain shape anisotropy. This grain shape anisotropy itself is strongly dependent on heating rates. Lower heating rates result in higher aspect ratios and thus better drawability of the cold rolled sheets. A Hall-Petch type relationship is observed between grain size and hardness of the annealed samples.     

退火工艺对冷轧低碳热镀锌钢板组织均匀性的影响

通过模拟退火试验及工业试制,研究了退火工艺对冷轧低碳热镀锌钢板组织均匀性的影响,探讨了钢板出现混晶组织的原因,并制定了解决方案。结果表明:加热速率偏低是造成混晶组织的主要原因,随着加热速率的升高,组织细化且均匀性改善;在较低加热速率下,延长均热时间和提高退火温度在一定程度上加剧了组织的不均匀性。针对镀锌机组降速生产引起混晶的问题,结合模拟退火试验结果,降低退火温度可以改善降速导致的混晶问题,制定了降速生产的退火工艺并应用于实际生产。     

Grain growth phenomena and heat transport in non-oriented electrical steels

Heat transport phenomena and their influence on grain growth processes in non-oriented electrical steels are discussed. Special attention is given to the temperature distribution during the heating process. The temperature profile during the decarburization process enables clarification of the mechanism of grain boundary motion during final annealing. The kinetics of the microstructural evolution of non-oriented electrical steel is discussed in terms of the stored energy released and the temperature gradient. It is shown that small differences in the heating rate can leads to significant changes in the mechanism of grain boundary motion.     

Rapid annealing effects on microstructure, texture, and magnetic properties of non-oriented electrical steel

This paper presents a classic process-structure-properties approach for optimizing the magnetic properties of electrical steels. Cold-rolled non-oriented electrical steel (Fe; 0.001 wt% C; 0.2 wt.% Mn; 1.3 wt% Si) was subjected to extremely short 3–30 seconds annealing cycles in a range from 880 °C to 980 °C with a heating rate varying from 15 °C to 300 °C/sec. The resulting microstructure was studied by means of optical microscopy and X-ray orientation distribution function analysis. Recrystallized grains were refined with increased heating rate, caused by the nucleation rate increase, which is faster than the growth rate due to rapid heating. The optimal grain size of 60 to 80 mm in terms of magnetic properties was obtained by increasing the annealing temperature range to 920 °C to 940 °C with a higher heating rate of 300 °C/sec and an annealing time of 6 to 9 seconds. With the heating rate increase, the characteristic {111} recrystallization fiber of cold-rolled steel was depressed, but the beneficial {110}〈001〉 Goss texture component was significantly strengthened. The recrystallized grain size and texture were enhanced by rapid annealing, and, as a result, the magnetic properties of non-oriented electrical steel improved.     

TCS不锈钢复合热源焊接热影响区晶粒长大的预测

根据等温条件下的铁索体晶粒长大公式,推导出了焊接热影响区(HAZ)铁索体晶粒长大的动力学方程,并结合焊接热循环的数值分析结果对TCS不锈钢热影响区晶粒长大进行了预测.在此基础上,对复合焊新型焊接工艺条件下TCS不锈钢HAZ的晶粒长大情况进行了数值计算,得到了3种工艺参数下TCS不锈钢复合焊HAZ内的晶粒尺寸.基于TCS不锈钢复合焊接接头的金相图片,测量了HAZ内的平均晶粒尺寸,对数值模拟结果进行了验证.初步讨论了晶粒尺寸随热循环温度的长大情况和热循环温度差异对晶粒长大的影响规律.     

Kinetics of nonisothermal recrystallization

The kinetics of recrystallization from nonisothermal cycles is modeled using an extension of the Arrhenius relationship. This model is used to predict the annealing response for both isothermal and nonisothermal annealing cycles. A nondimensional parameter called the “annealing index” is based on time and temperature and is related to the extent of anneal through physical properties. The effect of heating and cooling rate also can be quantified because the entire thermal cycle is evaluated. Nonisothermal anneals with heating rates of 10 and 100 °C/s were evaluated (along with isothermal anneals) for colddrawn ETP copper wire. A numerical solution is set forth to evaluate nonisothermal cycles. Formerly Research Assistant, Rensselaer Polytechnic Institute, Troy, NY, USA.     

Correlation between interfacial segregation and surface-energy-induced selective grain growth in 3% silicon–iron alloy

Effects of final reduction and interfacial segregation of sulfur on surface-energy-induced selective grain growth have been investigated in 3% silicon–iron alloy strips with various bulk content of sulfur. Interfacial segregation kinetics of sulfur varies with annealing atmosphere: a convex profile under vacuum or hydrogen and a gradual increase under argon. This is because the segregated sulfur evaporates or gasifies to hydrogen sulfide during final vacuum or hydrogen annealing, resulting in a sulfur-depleted zone just below the strip surface. The surface-energy-induced selective growth of a grain at time t is determined by the concentration of segregated sulfur. The selective growth rate depends on the combined effect of the segregated sulfur and the final reduction that determines the average grain size. For obtaining (110)[001] Goss texture, the final reduction should, therefore, be controlled, depending on the bulk content of sulfur which influences directly the segregation kinetics of sulfur and thus the texture development.     

IF钢织构与晶界特征分布的研究

对IF钢板的罩式退火(BA)和连续退火(CA)工艺进行了模拟实验，并运用EBSD技术对两种IF钢板的织构、晶界特征分布及其与二次加工脆性之间的关系进行了研究．结果表明，两种不同退火工艺的IF钢板在织构和晶界特征分布上存在很大的差异：(1)连续退火IF钢板呈现强烈的{111}再结晶织构，它由{111}(110)和{111}(112)两类取向晶粒组成；罩式退火IF钢板的{111}再结晶织构相对较弱，且主要由{111}(110)组成；(2)连续退火IF钢板中含有较多的∑3，∑13，∑9和∑11重位晶界，而罩式退火的IF钢板中只含有∑3和∑13重位晶界；(3)连续退火IF钢板中低能晶界与高能随机晶界均匀分布，而罩式退火IF钢板中低能晶界成团集中分布，高能晶界分布在团簇周围构成粗大的网状，这是IF钢板产生二次加工脆性的重要原因。     

In situ measurement and modelling of austenite grain growth in a Ti/Nb microalloyed steel

Using a novel laser ultrasonics technique in situ measurements of austenite grain growth were conducted during continuous heating (10 °C s⁻¹) and subsequent isothermal holding at various temperatures in the range 950-1250 °C in a microalloyed linepipe steel. Based on the experimental results, a grain growth model was developed, which includes the pinning effect of precipitates present in the steel. Analyzing the grain growth behaviour and using the advanced thermo-kinetic software MatCalc, an approach was developed to estimate the initial distribution of precipitates in the as-received material and their dissolution kinetics. The evolution of the volume fractions and mean particle sizes of NbC and TiN leads to a time-dependent pinning pressure that is coupled with the proposed grain growth model to successfully describe the observed kinetics of austenite grain growth. The predictive capabilities of the model are illustrated by its application to independent grain growth data for rapid heat treatment cycles that are typical of the weld heat affected zone.     

The high temperature oxidation behaviour of austenitic stainless steels

High temperature annealing in a dynamic vacuum has been utilised to induce the growth of duplex oxide over the whole surface of stainless steel specimens. It is found that duplex oxide grows at a rate which does not obey a simple power law. The oxidation kinetics and oxide morphology have also been studied for a series of ternary austenitic alloys which cover a range of composition between 5 and 20% chromium. A model has been developed from the observations to describe the formation of duplex oxide and the subsequent formation of a “healing layer” which virtually causes the oxidation process to stop. This phase tends to form at grain boundaries and a relationship has been derived for the reaction kinetics which relates the reaction rate with grain size of the substrate.     

Grain growth simulation with the second phase particle pinning for heat-affected zone of microalloyed steel welds

The second phase particle dispersed in microaUoyed steel has different effects on grain growth depending on their size and volume fraction of the second phase particles which will change during welding thermal cycles.The particle coarsening and dissolution kinetics model was analyzed for continuous heating and cooling.In addition,based on experimental data,the coupled equation of grain growth was established by introdacing limited size of grain growth with the consideration of the second phase particles pinning effects.Using Monte Carlo method based on experimental data model,the grain growth simulation for heat-affected zone of microalloyed steel welds was achieved.The calculating results were well in agreement with that of experiments.     

Grain growth studies on nanocrystalline Ni powder

The microstructure of nanocrystalline Ni powder produced by ball-milling and its thermal stability were investigated by applying different methods of X-ray diffraction line-profile analysis: single-line analysis, whole powder-pattern modelling and the (modified) Warren–Averbach method were employed. The kinetics of grain growth were investigated by both ex-situ and in-situ X-ray diffraction measurements. With increasing milling time, the grain-size reduction is accompanied by a considerable narrowing of the size distribution and an increase in the microstrain. Upon annealing, initial, rapid grain growth occurs, accompanied by the (almost complete) annihilation of microstrain. For longer annealing times, the grain-growth kinetics depend on the initial microstructure: a smaller microstrain with a broad grain-size distribution leads to linear grain growth, followed by parabolic grain growth, whereas a larger microstrain with a narrow grain-size distribution leads to incessant linear grain growth. These effects have been shown to be incompatible with grain-boundary curvature driven growth. The observed kinetics are ascribed to the role of excess free volume at the grain boundaries of nanocrystalline material and the prevalence of an “abnormal grain-growth” mechanism.     

Induction Hardening 5150 Steel: Effects of Initial Microstructure and Heating Rate

Induction heating has permitted great progress in the surface hardening of a wide variety of steels, but results in a wide range of local thermal cycles. The metallurgical changes during rapid heating and cooling have not been sufficiently studied with respect to heating rate and prior microstructure. In the present investigation, induction dilatometry was performed on 5150 steel with ferrite-pearlite and tempered martensite initial microstructures to assess effects of experimentally controlled prior microstructure and heating rate on austenitization kinetics. Heating rates were varied from 0.3 to 300 °C/s to simulate industrial processes, and post-hardening metallography and hardness testing were performed. Results show that the transformation kinetics for prior ferrite-pearlite microstructures are significantly slower than for prior tempered martensite microstructures, although hardness is equivalent for a given thermal cycle. Metallographic evidence suggests significant remnant segregation of chromium in regions of pearlitic cementite (enriched); evidence of segregation was not observed metallographically for prior tempered martensite. Diffusion-based transformation simulations support observed ferrite-pearlite alloy segregation, suggest residual alloy segregation is possible for prior tempered martensite, and can be used to tailor austenitization thermal cycles to process requirements. Detailed time and temperature-dependent local microstructure development results from this study are directly applicable to practical induction hardening simulations.     

Kinetics of Austenite Grain Growth During Heating and Its Influence on Hot Deformation of LZ50 Steel

Grain growth behaviors of LZ50 have been systematically investigated for various temperatures and holding times. Quantitative evaluations of the grain growth kinetics over a wide range of temperature (950-1200 °C) and holding time (10-180 min) have been performed. With the holding time kept constant, the average austenite grain size has an exponential relationship with the heating temperature, while with the heating temperature kept constant, the relationship between the austenite average grain size and holding time is a parabolic curve approximately. The holding time dependence of average austenite grain size obeys the Beck’s equation. As the heating temperature increases, the time exponent for grain growth n increases from 0.21 to 0.39. On the basis of previous models and experimental results, taking the initial grain size into account, the mathematical model for austenite grain growth of LZ50 during isothermal heating and non-isothermal heating is proposed. The effects of initial austenite grain size on hot deformation behavior of LZ50 are analyzed through true stress-strain curves under different deformation conditions. Initial grain size has a slight effect on peak stress.     

Analysis of the growth of individual grains during recrystallization in pure nickel

The growth of individual grains during recrystallization in 96% cold-rolled pure nickel has been followed using electron backscatter pattern maps of the same surface area taken after each of several annealing steps. It was found that the growth is quite complex, with boundaries moving, stopping and moving again. The growth kinetics differ from grain to grain and, on average, cube-oriented grains grow the fastest. The growth of the grains has also been analyzed as a function of boundary misorientation. This analysis shows that there is no significant difference in misorientation distribution between boundaries that move and those that do not. This is contrary to the usual assumption that the boundary mobility and the migration rate depend on the misorientation across a boundary. This observation and the reasons for the faster growth of cube-oriented grains are discussed.     

冷轧变形量和退火制度对超快速加热下5083铝合金晶粒尺寸的影响

利用Gleeble-3500热模拟系统和电子背散射衍射(EBSD)技术对5083铝合金的超快速退火组织演变规律进行研究,探讨了快速加热速度、退火温度及冷轧变形量对5083铝合金晶粒尺寸的影响。结果表明,5083铝合金经80%的冷轧变形后分别以25、250、500℃/s的加热速度升温至450℃保温3s后以40℃/s冷却时,平均晶粒尺寸随加热速度的增加由7.43μm细化至4.98μm。5083铝合金经80%冷轧变形后在不同退火温度(350、400、420、450和500℃)下进行超快速退火(加热速度500℃/s,保温时间3 s,冷却速度40℃/s)后,所得晶粒尺寸先减小再增大,在420℃退火时,晶粒尺寸达到最小,为4.82μm。再结晶晶粒尺寸受晶界迁移速率和形核率的耦合作用,在350~420℃超快速退火时,由于快速加热使形核率急剧增大,而形核温度较低,使晶界迁移速率较小,导致晶界迁移速率小于形核率,因而再结晶晶粒尺寸由5.23μm细化至4.82μm;在420~500℃超快速退火时,形核温度变高,晶界迁移速率快速增大,则晶界迁移速率大于形核率,使合金晶粒由4.82μm粗化至6.20μm,420℃是5083铝合金晶界迁移速率和形核率之间竞争的一个临界点。5083铝合金经50%、60%、71.4%、80%和87.5%的冷轧变形后以500℃/s的超快速加热速度升温至450℃保温3 s后以40℃/s冷却,所得平均晶粒尺寸分别为7.94、6.82、6.03、4.98和4.84μm,随轧制变形量的增大晶粒尺寸减小,但是冷轧制变量达到80%以后再进行超快速退火晶粒尺寸减小不明显。     

Grain growth during the early stage of sintering of nanosized WC–Co powder

Rapid grain growth during the early stage of sintering has been found in many nano material systems including cemented tungsten carbide WC–Co. To date, however, there have been few reported studies in the literature that deal directly with the kinetics or the mechanisms of this part of grain growth. In this work, the grain growth of nanosized WC during the early stages of sintering was studied as a function of temperature and time. The effects of other influencing factors, such as the initial grain size, cobalt content, and the grain growth inhibitor VC, were investigated. The kinetics of the grain growth process was analyzed and the evolution of the morphology of WC grains during heating-up was studied using high resolution scanning electron microscopy. The results showed that the grain growth process consists of an initial stage rapid growth process which typically takes place during heat-up and the normal grain growth during isothermal holding. The initial rapid grain growth is at least partially attributed to the process of coalescence of grains via elimination common grain boundary. The preferred orientation between WC grains within the aggregates is considered a favorable condition for coalescence of grains, hence rapid grain growth. The solution–reprecipitation process is considered a mechanism of coalescence.     

快速再结晶退火过程金属晶粒度的计算

本文将D.L.Bourell的模型应用于LF21合金和BT9合金，经过计算分析证实，再结晶晶粒长大激活能Qg对再结晶退火温度和加热速度的变化不敏感，随后提出适用于快速再结晶退火晶粒度的计算式，LF21合金经快速退火处理实验后的晶粒度预测计算表明，该计算式的预测结果与实测值比较接近。     

Development of {110}?001? annealing texture in Al-free and 0.1 %Mn-added 3 %Si-Fe alloy strips containing 95 ppm sulfur

The solubility of sulfur is calculated in 0.1 %Mn-added 3 %Si-Fe alloys. The segregation kinetics of sulfur is compared in the alloy containing 95 ppm sulfur, depending on the annealing atmosphere. The effects of pre-annealing and annealing atmosphere on final annealing texture are investigated. Segregation behaviors of sulfur at free surfaces and grain boundaries are compared and, during the selective growth, the importance of the grain boundary concentration of sulfur is emphasized. Finally, a correlation between the development of the annealing texture and segregation kinetics of sulfur in the alloy strip is discussed.     

退火工艺对采暖系统用耐沟状腐蚀钢带性能的影响

通过对耐沟状腐蚀现象的研究,结合生产设备能力,设计了6种成分钢带和6种退火工艺。借助光学显微镜、扫描电镜、拉伸试验、中性盐雾试验等手段,研究了采暖系统用耐沟状腐蚀钢带生产过程中冷轧退火工艺对其性能的影响。结果发现:低于Ac₁温度(650、700 ℃)的退火不可能改变原来冷轧铁素体晶界的遗传结构,冷轧α相的再结晶长大不充分,碳化物的聚合和长大过程也不充分;当退火温度达到Ac₁左右(750 ℃),由于碳元素的固溶加剧,导致α相再结晶和析出物的聚合长大,得到极优的延展性,对深冲非常有利;退火温度位于Ac₁~Ac₃之间(850 ℃),发生α→γ相再结晶,可以取得最好的软化效果。对于含Ti的试验钢,随着退火温度的降低,其塑性应变比r值呈下降趋势。Cr的加入不利于耐沟状腐蚀性能,但是Al的加入有利于耐沟状腐蚀性能。采用两段式退火工艺进行工业化生产,可以得到外观和性能优良的采暖系统用耐沟状腐蚀产品。