Microstructure and precipitation in as cast low carbon Nb–V–Ti microalloyed medium thin slab

Abstract

The solidification structure, austenite and precipitates in a quenched compact strip processing (CSP) medium thin slab (170 mm thick) of Nb–V–Ti microalloyed steel have been studied. It was found that secondary dendrite arm spacing and austenite grain size are slightly larger than that of similar steel produced by CSP thin slab. This is partially attributed to the slower cooling rate caused by the increased slab thickness. On the other hand, the formation of carbonitride during solidification reduces the width of secondary dendrite arm spacing, while TiN particles and alloying elements in solution may inhibit the growth of austenite grain during solidification and subsequent cooling. In addition to the semidendritic, larger cubic and fine cubic precipitates, which can be observed in CSP thin slab, dendritic precipitates were also found in CSP medium thin slab.     

GCr15轴承钢连铸冷却速率下凝固原位观察

GCr15轴承钢在连铸凝固过程中的组织生长与溶质偏析是碳化物液析的重要诱因,成为产品质量提升的关键.为此,针对国内某钢厂240 mm×240 mm GCr15轴承钢的连铸过程,选取方坯表面下方40、80和120 mm位置处的坯样为研究对象,首先建立二维凝固传热模型,结合红外测温试验,求解它们在糊状区的平均冷却速率,然后...     

Simulation of austenite decomposition in continuous cooling conditions: a cellular automata-finite element modelling

Transformation of austenite to ferrite under continuous cooling condition was investigated. The heat conduction problem was managed by finite element method while two-dimensional cellular automata modeling was simultaneously performed to predict the progress of austenite decomposition using a two-step algorithm to reduce surface-to-volume ratio. Continuous cooling experiments on low carbon steel were made and the ferrite structure was determined and compared with the simulation data. The predicted and the experimental results demonstrated an acceptable consistency and the activation energy for ferrite growth was determined as 171 kJ/mole. The rate of ferrite transformation increased under examined continuous cooling conditions owing to higher nucleation rate. Moreover, the initial austenite grain size has shown a significant impact on the rate of transformation e.g. in air-cooled samples as the austenite grain size decreased from 24 to 34 µm, the mean ferrite grain size decreased about 8 µm.     

微合金钢连铸坯表面裂纹敏感性预测模型

为了从凝固及相变特性角度解决微合金钢连铸坯表面裂纹问题,建立了与合金化相关联的初始凝固包晶反应度模型、奥氏体晶粒长大模型、铁素体转变量模型以及碳氮化物的析出模型。结合铸坯实际冷却条件,进一步建立了包晶反应度预测、初生奥氏体晶粒长大、铁素体转变、析出相析出等对铸坯表面裂纹敏感性的预测模型。针对某J55钢连铸板坯,奥氏体晶粒尺寸超过1 mm、铁素体析出量为10%、二次相析出量增加时,横裂纹敏感性最大。表面裂纹敏感性预测模型有助于实现基于成分微调和组织调控的微合金钢连铸、热装等生产过程表面裂纹控制技术。     

冷却速率对薄带连铸低碳钢微观组织的影响

摘要：为了探究冷却速率对薄带连铸低碳钢微观组织的影响,采用相变仪设备对铸带重新加热并进行不同冷却速率冷却,采用光学显微镜、场发射电子探针和电子背散射衍射等手段对微观组织和针状铁素体形核所利用的夹杂物进行了分析。结果表明,铸带经1200℃保温3min后,原奥氏体晶粒尺寸约为150～650μm,可以满足晶内针状铁素体形核对原奥氏体晶粒尺寸的需要。在2～5℃/s的冷却速率范围内,试样中得到了大量的针状铁素体组织,冷却速率为2℃/s的试样中大角度晶界所占比例约为60%;当冷却速率大于20℃/s,针状铁素体的形成受到抑制。铸带中针状铁素体形核所利用的夹杂物是Ti-Al-Si-Mn-O+MnS复合夹杂物。     

Austenite Grain Growth and the Surface Quality of Continuously Cast Steel

Austenite grain growth does not only play an important role in determining the mechanical properties of steel, but certain surface defects encountered in the continuous casting industry have also been attributed to the formation of large austenite grains. Earlier research has seen innovative experimentation, the development of metallographic techniques to determine austenite grain size and the building of mathematical models to simulate the conditions pertaining to austenite grain growth during the continuous casting of steel. Oscillation marks and depressions in the meniscus region of the continuously casting mold lead to retarded cooling of the strand surface, which in turn results in the formation of coarse austenite grains, but little is known about the mechanism and rate of formation of these large austenite grains. Relevant earlier research will be briefly reviewed to put into context our recent in situ observations of the delta-ferrite to austenite phase transition. We have confirmed earlier evidence that very large delta-ferrite grains are formed very quickly in the single-phase region and that these large delta-ferrite grains are transformed to large austenite grains at low cooling rates. At the higher cooling rates relevant to the early stages of the solidification of steel in a continuously cast mold, delta-ferrite transforms to austenite by an apparently massive type of transformation mechanism. Large austenite grains then form very quickly from this massive type of microstructure and on further cooling, austenite transforms to thin ferrite allotriomorphs on austenite grain boundaries, followed by Widmanstätten plate growth, with almost no regard to the cooling rate. This observation is important because it is now well established that the presence of a thin ferrite film on austenite grain boundaries is the main cause of reduction in hot ductility. Moreover, this reduction in ductility is exacerbated by the presence of large austenite grains.     

Influence of casting size and graphite nodule refinement on fracture toughness of austempered ductile iron

Casting size affects the solidification cooling rate and microstructure of casting materials. Graphite nodules existing in the structure of ductile iron are an inherent and inert second phase that cannot be modified in subsequent heat-treatment processing. The matrix and the fineness of the second phase undoubtedly have some impact on the fracture toughness of the as-cast material, as does the subsequent heat treatment, as it alters the microstructure. This research applied austempering heat treatment to ductile iron of different section sizes and graphite nodule finenesses. The influence of these variables on the plane strain fracture toughness (K _IC ) of the castings so treated was compared to that of the as-cast state. Metallography, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.     

Study on Austenite Grain Growth Law of Ultra-High Strength Hot-Stamped 22MnB5 Steel During Thin Slab Casting and Rolling Process

Clarifying the austenite grain growth law in the thin slab casting and rolling (TSCR) process can provide theoretical guidance for the control of austenite grain in the slab. Starting with the austenite nucleation during solidification process, the growth law of austenite grains is methodically studied throughout the TSCR continuous casting and soaking process. The results show that the austenite growth is not interrupted during the TSCR continuous casting and soaking process. The austenite grain growth in the continuous casting process accounts for more than 70% of the total growth. The growth rate of austenite in the continuous casting cooling process is always faster than that when reheated to this temperature. Compared with the holding temperature and holding time, the final size of austenite grains in the TSCR process slab is most affected by the continuous casting cooling rate. In addition, compared with the traditional process, the growth rate of austenite in TSCR process is faster at the end of soaking.     

Microstructures of Austenitic Stainless Steel Produced by Twin-Roll Strip Caster

 The microstructures of austenitic stainless steel strip were studied using color metallographic method and electron probe micro analysis (EPMA). In the cast strips, there are three kinds of solidification structures: fine cellular dendrite in the surface layer, equiaxed grains in the center and fine dendrite between them. The solidification mode in the surface layer is the primary austenite AF mode because of extremely high cooling rate, with the retained ferrite located around the primary cellular austenite. In the fine dendrite zone, the solidification mode of molten stainless steel changes to FA mode and the residual ferrite with fish-bone morphology is located at the core of the dendrite. The retained ferrite of equiaxed grains in the center is located in the center of broken primary ferrite dendrite with vermicular morphology.     

Hot ductility trough elimination through single cycle of intense cooling and reheating for microalloyed steel casting

Surface transverse cracking, especially corner cracking, is prone to generate in continuously cast slabs of microalloyed steels. The method of surface structure control (SSC) was supposed to the best way to avoid the detrimental defects. However, the mechanism of improving hot ductility by SSC and the specific parameters to control the process are still unclear for the reasonable adoption in production. In the present work, the impact of cooling rate, holding temperature and holding time on austenite decomposition, and the austenite grain size before and after intense cooling were investigated by thermal simulation method. With the increase of cooling rate, it is observed that the phase is transformed from austenite?→?grain boundary film-like alltromorph ferrite?→?Widmanstätten ferrite plates (or intragranular ferrite plates)?→?bainite+martensite. Mostly important, the film-like ferrite can be eliminated through intense cooling and the following reheating, but the austenite grain size is not observed to be refined through the single γ?→?α?→?γ cycle. Even though, the reduction of area (RA) is improved drastically to over 70% in the third ductility trough, whereas the RA value is just 相似文献   

High temperature grain growth during slab reheating of oriented 3 pct Si-Fe made using continuous casting

Studies were made into the process behind the excessive grain growth which is observed in continuous cast slabs of both regular and high permeability oriented 3 Pct Si-Fe during reheating from 1230 °C to 1400 °C. These large grains are undesirable because of the greater difficulty incurred in obtaining the suitably uniform and fine primary grain size desired prior to the final high temperature anneal during which the (110) [001] texture is developed. It was found that the driving force for the growth is the subgrain structure which develops due to the strains of solidification and cooling during continuous casting; however, the temperature at which growth initiates is related to the austenite-ferrite phase relationship. The grain growth begins when the austenite which forms during slab reheating decomposes to form highly perfect ferrite which then grows by consuming the strained preexisting (as-cast) ferrite matrix. Data summarizing studies into the energy storage and recrystallization processes which occur with the use of slab breakdown (or prerolling) prior to reheating from 1230° to 1400 °C are also discussed. This paper is based on a presentation made at the symposium “Physical Metallurgy of Electrical Steels” held at the 1985 annual AIME meeting in New York on February 24–28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee.     

Solidification mechanism of martensitic stainless steel

Abstract

In an unidirectional solidification experiment, an 8 kg stainless steel ingot with the composition 0·25%C, 17%Cr, and 1%Mn was solidified under continuous casting conditions. The dwell time of primary cooling was varied, followed by secondary spray cooling. Metallographic investigation, heat transfer, and segregation were carried out to study the solidification mechanism. The partition ratio of the elements present in ferrite and in austenite (martensite) was determined. It was indicated that the solidification follows: L → L + δ → L + δ + γ → δ + γ + carbides. Under high cooling rates γ austenite solidifies as a leading phase. The beginning of spray cooling has the main effect in controlling the obtained microstructures. Carbide thickening is observed in the rapidly cooled zone between the ferrite and the martensitic matrix. Tempered martensite increases by lowering the cooling rate, which gives more time for carbide dissolution and for carbon to diffuse into the ferrite, eventually increasing the austenite (martensite) fraction in the final matrix at the expense of ferrite.     

Effects of Widmanstätten ferrite on the mechanical properties of a 0.2 pct C-0.7 pct Mn steel

Laboratory melted and rolled C-Mn steel plates were austenitized at either 925 °C or 1150 °C to produce nominal austenite grain sizes of 60 and 200 μm, resspectively. The plates were then cooled at rates in the range of about 2 °C/min to 400 °C/min to produce mixed polygonal ferrite/Widmanst?tten ferrite/pearlite microstructures. The percentage of Widmanst?tten structure (a Widmanst?tten ferrite/pearlite aggregate) increases with increasing prior austenite grain size and cooling rate. Both yield strength and impact toughness increase with decreasing austenite grain size and increasing cooling rate. This simultaneous improvement in strength and toughness is attributed to overall refinement of both the polygonal ferrite and Widmanst?tten structure. Both yield and tensile strength increase with an increase in the volume fraction of Widmanst?tten ferrite and a reduction in ferrite grain size. In contrast, the toughness level achieved in these polygonal ferrite/Widmanst?tten ferrite/pearlite microstructures depends largely on the ferrite grain size; the finer the grain size, the better the toughness.     

冷却条件对EH40钢板坯表层奥氏体晶粒长大的影响

铸坯表层异常长大的奥氏体晶粒是产生横裂纹的重要原因之一,研究冷却过程对其生长行为的影响对科学制定连铸工艺、降低铸坯裂纹敏感性有重要意义。采用原创连铸坯凝固过程热模拟方法,再现了EH40低碳船板钢板坯的凝固过程,观察在传统板坯连铸条件下,2种结晶器冷却强度对铸坯表层奥氏体晶粒长大行为的影响。结果表明,在结晶器冷却阶段,热模拟坯表层5 mm的绝大多数奥氏体晶粒短轴尺寸均不超过0.5 mm,但已出现粗大晶粒,且强冷条件下奥氏体晶粒尺寸平均值和极大值均更大,分别为弱冷条件下的2.5倍和2.0倍。在足辊区到矫直点区间,表层奥氏体晶粒生长非常缓慢,平均尺寸仍未超过0.5 mm。矫直点处,结晶器强冷热模拟坯表层20 mm的晶粒短轴最大尺寸为2.2 mm,为弱冷条件下的1.7倍。综上,奥氏体晶粒在连铸不同阶段表现为不同的生长行为,且采用结晶器弱冷更有利于EH40钢板坯获得相对细小的表层奥氏体晶粒。     

Microstructural origin of the skeletal ferrite morphology of austenitic stainless steel welds

Scanning transmission electron microscopy (STEM) was conducted on welds exhibiting a variety of skeletal, or vermicular ferrite morphologies in addition to one lathy ferrite morphology. These ferrite morphologies result from primary ferrite solidification followed by a solid state transformation upon cooling. During cooling, a large fraction of the ferrite transforms to austenite leaving a variety of ferrite morphologies. Comparison of composition profiles and alloy partitioning showed both the skeletal and lathy ferrite structures result from a diffusion controlled solid state transformation. However, the overall measured composition profiles of the weld structure are a result of partitioning during both solidification and the subsequent solid state transformation.     

304奥氏体不锈钢亚快速凝固组织演化和形成机理

通过感应炉熔化的304钢(/%:0.053C、0.55Si、1.50Mn、0.030P、0.002S、17.02Cr、8.01 Ni、0.50Cu、0.08Mo)直接浇铸在水冷铜模上得到厚7 mm直径25 mm的圆形试样,研究了Cr、当量/Ni当量和1.5～1 000℃/s的冷却速率对奥氏体不锈钢铸态凝固组织形态和分布的影响。结果表明,随冷却速率增加至75～90℃/s,该钢的凝固模式由FA(铁素体-奥氏体)模式向AF(奥氏体-铁素体)模式转变,初生相由枝晶铁素体转变成枝晶奥氏体,但冷却为～1 000℃/s时,观察到块状铁素体组织,并且枝晶状奥氏体转变成胞状奥氏体。     

Interrupted and Isothermal Solidification Studies of Low and Medium Carbon Steels

Low and medium carbon steels experience multiple phase transformations during solidification and subsequent cooling. The sequence, extent, and nature of the different transformations have a significant bearing on the microstructural evolution that occurs in the steel. The change in microstructure with temperature is very important, since it may influence the hot ductility of the steel during casting and/or rolling and the subsequent response of the material to thermoprocessing. The aim of this investigation was to gain a better understanding of the development of the as-cast structure in low and medium carbon steels. Of particular interest is the origin of the large austenite grains frequently associated with poor hot ductility. Interrupted and isothermal solidification experiments were therefore conducted to study the nonequilibrium and near-equilibrium structures which form at different stages of the freezing process. The results of the investigation established delta-ferrite as the primary solidifying phase in low carbon steels. Austenite forms as the secondary phase by nucleation at the solidification (delta-ferrite) boundaries. While excessive austenite grain coarsening is suppressed by the coexistence of the second phases delta-ferrite or liquid, this suppression occurs over only a limited temperature range, just below the peritectic temperature. Subsequent cooling leads to very large austenite grains, ranging up to 5 mm in diameter, in steels of low carbon content. N.S. POTTORE, formerly with the Basic Metals Processing Research Institute, Department of Materials Science and Engineering, University of Pittsburgh     

高碳马氏体不锈钢8 Cr13 MoV钢铸态组织及碳化物

利用Thermo-Calc软件对8Cr13MoV马氏体不锈钢的凝固过程进行计算,利用光学显微镜、扫描电子显微镜和X射线衍射分析仪对铸态组织和碳化物形貌以及类型进行观察与分析,利用Gleeble热模拟试验机测定材料的静态连续冷却转变曲线.结果表明,8Cr13MoV在平衡凝固条件下组织为铁素体和M₂₃C₆型碳化物,而在实际的凝固条件下,组织为铁素体、马氏体、残余奥氏体、M₇C₃型和M₂₃C₆型碳化物,由于偏析导致最终组织中碳化物以M₇C₃型为主,少量M₂₃C₆以薄片或树枝状分布在晶界上.由于较高的C和Cr含量,以0.1℃·s-1的冷却速率冷却时,奥氏体也会发生马氏体转变.      

铝对Cr8WMo2V2SiNb冷作模具钢组织及力学性能的影响

采用金相、扫面电镜等分析手段,研究了铝对Cr8WMo2V2SiNb钢组织和力学性能的影响。研究结果表明:铝能够细化铸态组织;细化碳化物颗粒尺寸;提高临界点温度,使淬火组织中容易出现铁素体组织;细化奥氏体晶粒尺寸;促进残余奥氏体向马氏体转变,提高回火硬度;在韧性损失不大的情况下提高钢材的抗弯强度和耐磨性。