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。     

Nucleation of intragranular ferrite in Fe-Ni-P alloys

The nucleation of intragranular ferrite from austenite in Fe-Ni-P alloys was investigated in order to understand the development of the Widmanstätten pattern in iron meteorites. Alloys containing 5 to 10 wt pct Ni and 0 to 1 wt pct P were used to simulate iron meteorite compositions. In the isothermal and controlled cooling experiments the reaction path γ → α+ γ serves only to nucleate ferrite along austenite grain boundaries. It is necessary for (FeNi)₃P to be present within y grains in order to nucleate intragranular ferrite. The reaction path γ → γ+ phosphide → α + γ + phosphide yields rod shaped ferrite nuclei that bear a near Kurdjumov-Sachs orientation relationship with the surrounding matrix. The precipitation of ferrite, both along grain boundaries and within the austenite grains, is suppressed in the absence of P.     

Effect of Slab Subsurface Microstructure Evolution on Transverse Cracking of Microalloyed Steel during Continuous Casting

As to the continuous casting process of low carbon microalloyed steel, subsurface microstructure evolution plays an important role in the slab surface cracking. In order to study the effect of the slab subsurface microstructure evolution on the transverse cracking, three different secondary cooling patterns （i. e. , mild cooling, strong cooling and controlled cooling） were performed in the corresponding slab curved continuous caster. Based on the metallo- graphic results, three transformation regions were found to be formed with the evolution of microstructures at different depths in the slab subsurface. The three regions are strong cooling transformation （SCT） region, double phase transformation （DPT） region and mild cooling transformation （MCT） region, respectively. Meanwhile, it was also found that the crack index used for evaluating slab surface cracking susceptibility was decreased when the range of the DPT region was increased. This can be explained by the fact that the double phase transformation （austenite-ferrite-austenite） occurred resulting from thermal cycling in DPT region, which resulted in promoting the refinement of prior austenite grains and inhibiting the precipitation of film-like ferrite and chain-like precipitates. Under the con- trolled cooling pattern, the widely-distributed DPT region was formed in the range of 3.5--8.0 mm to the slab surface. And compared with other cooling patterns, the cracking susceptibility is lowest with a crack index of 0.4.     

超低碳钢在铁素体生长过程中硼分布的变化

通过对照显微结构与硼的自射线径迹显微照相（ＰＴＡ）,研究了一种超低碳含硼钢在铁素体生长过程中硼分布的变化,发现：在发生等温铁素体相变前,硼已偏聚在奥氏体晶界上;铁素体在奥氏体晶界形核;在有铁素体生成的奥氏体晶界上,硼偏聚减弱;沿晶界长出的小块铁素体中硼浓度明显高于奥氏体,但随铁素体长大,其硼含量逐渐与母相持平。这些现象表明,铁素体长大不受硼在奥氏体中扩散的控制     

不同终轧温度下36Mn2V钢的连续冷却转变规律

采用Gleeble-1500热模拟机,测定了36Mn2V钢经四种终轧温度变形后的连续冷却膨胀曲线,结合金相-硬度法,获得了该钢种的连续冷却转变曲线.结果表明:随冷却速度的增大,实验钢的γ/α相变开始温度逐渐降低,贝氏体相变开始温度先升高到一个平台,随冷却速度的进一步增加又降低,晶粒细化;随终轧温度的降低,实验钢的动态连续冷却转变曲线整体向左上方移动,网状铁素体和晶内铁素体明显减少,晶粒略有细化;经四种温度终轧后以3℃.s-1的冷速冷却到室温的四个试样中,唯独950℃终轧的试样中未观察到贝氏体.      

双相变回温温度对钢组织及其热塑性的影响

含铌钢连铸过程极易产生铸坯角部横裂纹。对连铸坯角部实施γ→α→γ双相变控冷工艺,可提高其组织的高温热塑性而减少裂纹产生。其中,α→γ相变阶段的回温温度是影响双相变控冷工艺实施效果的重要参数。通过Gleeble热模拟与金相观察、析出物透射以及断口扫描相结合的检测手段,研究分析了双相变过程回温温度对Q345D-Nb钢组织演变及其热塑性的影响规律。结果表明,回温温度为850℃时的奥氏体晶粒相比传统冷却工艺下的晶粒尺寸未产生细化,平均晶粒尺寸为502.2μm;回温温度升至900℃时,回温奥氏体出现了明显的混晶现象;当回温温度达到950℃时,晶粒细化至61.2μm;当回温温度达到1 000℃时,回温奥氏体晶粒出现了一定程度粗化,相比950℃回温温度下的奥氏体平均晶粒尺寸增加了38.07%。传统冷却工艺和不同回温温度时的双相变控冷工艺(回温温度为850、900、950、1 000℃),钢组织在700～900℃温度区内的断面收缩率最低值分别为29.6%、45.0%、56.3%、68.2%、63.2%。在传统冷却工艺下,钢组织在750℃时晶界铁素体膜的厚度为20～25μm,且碳氮化物呈大尺寸链状分布,...     

Effect of Cooling Rate on the Precipitation Behavior of Carbonitride in Microalloyed Steel Slab

In the continuous casting of the microalloyed steel, the slab surface transversal cracking could be prevented through the control of the slab surface microstructure, which correlates with the precipitation behavior of carbonitrides in the microalloyed steel. Therefore, the cooling rate is the key factor to determine the precipitation behavior of carbonitrides. This article used confocal laser scanning microscopy to study the effect of different cooling rates on the precipitation behavior of the carbonitrides in the microalloyed steel slab. When the cooling rate is less than 3 K·s⁻¹, the precipitates in the steel are coarse, growing out along the austenite grain boundaries, and form a chain-like distribution. These precipitates seriously reduced the hot ductility of slab. Quantitative study between the cooling rate and the precipitation behavior of carbonitrides in microalloyed steel also has been developed. The results of this study could be used to improve the understanding of the slab surface microstructure controlling to enhance the hot ductility of the slab and avoid the surface crack of the slab.     

不同实验条件对冷轧双相钢高温热塑性的影响及影响机制

 采用Gleeble 2000高温力学性能模拟实验机对不同冷却速率及不同拉伸速率下600 MPa级Al Mo系冷轧双相钢高温热塑性进行了研究。结果表明,随拉伸应变速率增大,双相钢的高温热塑性明显提高;降低冷却速率,能显著提高双相钢高温区(t＞1 100 ℃)的塑性性能。为了避免铸坯在连铸过程中产生表面裂纹,矫直温度应保证在1 050~1 150 ℃范围内,同时二次冷却应采用弱冷水制度,以降低冷却速率。金相观察发现,沿奥氏体晶界呈网状分布的铁素体薄膜是造成两相区塑性低谷的主要原因,而AlN、FeO等析出相致使奥氏体单相区脆化。     

Effect of temperature sequence during hot strip rolling on structure and properties of mild steels

Reduced temperatures can be applied for rolling of mild steels in wide hot strip mills. This has been investigated by laboratory and production trials with unalloyed and microalloyed deep drawing grades. A reduced slab reheating temperature leads to a grain refinement prior to rolling and to incomplete dissolution of precipitates. Decreased rolling temperature results in partial deformation in the ferrite region. The flow curves of mild steels show a relative minimum below γ/α transformation temperature, thus only low deformation forces are necessary. Ferrite rolling can develop coarse or partly recrystallized microstructures, depending on amount of strain and on coiling temperature. The yield strength, as a measure of cold formability, can be lowered compared to conventional rolling in the austenite region.     

22SiMn2TiB钢连铸坯横向断裂原因分析

对22SiMn2TiB钢连铸坯横向断裂原因进行分析。结果表明:铸坯柱状晶发达,晶间存在大量中间裂纹,且Ti、Mn元素偏聚于奥氏体晶界,使奥氏体晶界弱化。在矫直力、热应力作用下,中间裂纹沿奥氏体晶界扩展,导致连铸坯发生横向断裂。适当降低二次冷却比水量和中包过热度可获得良好的铸坯质量,有效解决22SiMn2TiB钢铸坯断裂问题。     

Analysis of thin-slab casting by the compact-strip process: Part I. Heat extraction and solidification

This article reports on an extensive experimental and modeling study undertaken to elucidate the thermal evolution of thin slabs during their passage through the mold and secondary cooling system of a compact-strip process (CSP) caster. In industrial trials covering a wide range of casting conditions, temperature measurements were carried out at (1) the copper plates of an operating mold and (2) the stainless steel frame of an operating grid. Separately, water-flux and heat-flux distributions generated by the several water and air-mist sprays produced by the different nozzles used in the process were determined in the laboratory. The analysis of these pieces of information, together with a detailed consideration of the geometry of the mold and the arrangement of the rolls and spray nozzles, were used to establish appropriate boundary conditions for a two-dimensional, curvilinear-coordinate, unsteady-state heat-conduction model for predicting the solidification rate of thin slabs. The predicted slab surface temperatures show very good agreement with corresponding measured values taken in plant tests at several locations along and across the secondary cooling system. The validation trials involved a wide range of low- and medium-carbon steel grades, casting speeds, slab widths, and secondary cooling strategies. The second part of this article combines the solidification model with a creep model of the shell to yield useful information about design parameters and casting conditions associated with undesirable bulging behavior of the slab after the last support roll, which causes stoppage of the process by slab clogging at the pinch rolls.     

Thermomechanical processing of microalloyed powder forged steels and a cast vanadium steel

The effects of controlled rolling on transformation behavior of two powder forged (P/F) microalloyed vanadium steels and a cast microalloyed vanadium steel were investigated. Rolling was carried out in the austenitic range below the recrystallization temperature. Equiaxed grain structures were produced in specimens subjected to different reductions and different cooling rates. The ferrite grain size decreased with increasing deformation and cooling rate. Ferrite nucleated on second phase particles, deformation bands, and on elongated prior austenite grain boundaries; consequently a high fractional ferrite refinement was achieved. Deformation raised the ferrite transformation start temperature while the time to transformation from the roll finish temperature decreased. Cooling rates in the cast steel were higher than in P/F steels for all four cooling media used, and the transformation start temperatures of cast steels were lower than that of P/F steel. Intragranular ferrite nucleation, which played a vital role in grain refinement, increased with cooling rate. Fully bainitic microstructures were formed at higher cooling rates in the cast steel. In the P/F steels inclusions and incompletely closed pores served as sites for ferrite nucleation, often forming a ‘secondary’ ferrite. The rolling schedule reduced the size of large pores and particle surface inclusions and removed interconnected porosity in the P/F steels. Formerly Postgraduate Researcher in the Department of Metallurgy and Materials Science, UMIST/University of Manchester, United Kingdom     

含镧高纯钢中的块状相变组织

 采用金相法和JEM 2010电镜研究了含镧高纯钢的相变。结果表明：固溶镧对γ→α相变影响显著,镧可细化铁素体晶粒。在冰盐水中淬火后,含镧高纯钢中γ→α多型性转变按块状相变方式进行,铁素体晶界不规则,呈块状,有时呈条片状或针状。     

Surface microstructure control of microalloyed steel during slab casting

Lots of work has been done to investigate slab surface microstructure evolution during continuous casting in order to improve hot ductility and avoid transverse cracks.The slab surface microstructure after continuous casting was characterized by optical microscopy, and the precipitation behavior was investigated by transmission electron microscopy.At the same time, the mechanical properties of the slabs were measured using a Gleeble 1500D thermal simulator and the transformation temperatures were examined by means of a thermal dilatometer.The experimental results show that homogeneous microstructure without film-like ferrites and chain-like precipitates at grain boundary can be obtained through surface intensive cooling and transverse cracks do not occur on the slab surface.For the ex-perimental steel, fine ferrite can form at slab surface when the water flow rate is larger than 1560 L/min at vertical section.As the distance to surface increases, microstructure turned to ferrite and pearlite. Moreover, nano-size carbonitrides precipitated in the ferrite grain and the size was larger at the junc-tion of the dislocations.The mechanical experiment results show that the hot ductility of the sample deformed at 650℃ was better than that of the sample deformed at 750℃.The reason is that film-like ferrite formed at the grain boundary in the sample deformed at 750℃.Thus, the slab must be cooled quickly below A r3 to prevent the occurrence of film-like ferrite and transverse cracks on the slab surface during casting.     

Effects of heating and cooling rates on δ↔γ phase transformations in duplex stainless steel by in situ observation

Ultra high temperature confocal laser scanning microscope is applied to observe the characteristics of the γ?δ phase transformations at different heating and cooling rates on the surface of duplex stainless steels. It is found that the migration of the δ/γ IB is always the main form of the phase transformation and leads to the continuous decline and final disappearance of the retained γ-phase during the γ→δ phase transformation at different heating rates. Heating rate does not radically later the mode of transformation under the current composition system. Interestingly, the δ-phase inside the γ-phase acts as the nucleation cores and grows more rapidly at the slow heating rate than at the rapid heating rate. During the δ→γ phase transformation, the γ-cells prefer to precipitate along the δ/δ grain boundaries with a flaky pattern, and their fronts are jagged in shape at the slow cooling rate, but in needle-like feature at the rapid cooling rate. The γ-cells also nucleate inside the δ-ferrite grains with a flaky pattern at the slow cooling rate and with a needle-like pattern at the rapid cooling rate. Furthermore, at different cooling rates, the growth speed in the longitudinal direction is always much faster than that in the lateral direction. More importantly, the effects of cooling rates on the precipitating morphologies of γ-phase are clarified based on the experimental results and the diffusion controlled growth theory.     

Effect of Cooling Rates on the Second‐Phase Precipitation and Proeutectoid Phase Transformation of a Nb–Ti Microalloyed Steel Slab

During the continuous casting of low‐carbon Nb–Ti microalloyed steel, control of the slab surface microstructure and the behavior of the second‐phase precipitation are significantly influenced by the cooling rate. Through confocal laser scanning microscopy, the effect of the cooling rate on the behavior of ferrite precipitation both at the grain boundary and within the austenite was observed in situ and analyzed. The relationship between the cooling rate and precipitation of the microalloying elements on the slab surface microstructure was further analyzed by transmission electron microscopy. The results showed that the effect of microalloying element precipitation on proeutectoid ferrite phase transformation is mainly manifested in two aspects: (i) the carbonitrides of microalloying elements act as inoculant particles to promote nucleation of the proeutectoid ferrite and (ii) the carbon near the grain boundary is depleted when the microalloying elements precipitate into carbonitrides, inducing a decrease in the local carbon concentration and promoting ferrite precipitation.     

50钢铸坯冷却过程断裂分析

分析了50钢铸坯异常组织以及在不同温度下的力学性能。分析结果表明,快速和不均匀的冷却将产生尖锐、突出的铁素体组织,弱化晶界,并产生较强的组织应力和热应力;在700、300℃附近分别存在高温脆性区和低温脆性区,在此区间移动铸坯会助长裂纹的产生。采用合适的堆冷工艺,提高热送热装率,可以有效减少铸坯断裂风险。     

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.     

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。     

Effect of prior austenitic grain boundary composition on temper brittleness in a Ni-Cr-Sb steel

Grain boundary segregation during temper embrittlement of an Sb-containing, Ni-Cr steel has been examined both by Auger electron analysis and by chemical analysis by neutron activation of residues of surface layers dissolved by etching intercrystalline fracture surfaces. No grain boundary segregation of either alloying additions or impurities was detected during austenitization or tempering. Redistribution of Cr, Ni, and Sb between carbide and ferrite was observed during tempering, but no grain boundary segregation was noted. Both Ni and Sb were observed to segregate to the boundaries during embrittling. The segregated Sb was shown to be uniformly distributed along the prior austenitic grain boundaries and to control the ductile brittle transition temperature of the alloy studied. Ni segregating to the prior austenitic boundaries during embrittling was shown to be localized in a phase other than the ferritic portions of the boundaries. A possible location was shown to be the ferritecarbide interfaces in the grain boundaries. Weakening of these normally tenacious carbide and ferrite interfaces could account for the change in mode of brittle failure from transcrystalline cleavage to intercrystalline along the prior austenitic grain boundaries that is observed in temper brittle steels.