Dissolving of Nb and Ti carbonitride precipitates in microalloyed steels

The dissolving behaviour of Nb and Ti carbonitride precipitates in microalloyed steels during isothermal holding at 1300℃ was investigated by Transmission electron microscopy (TEM) and energy dispersion x-ray spectrum (EDX). It was found that all precipitates in Nb-Ti microalloyed steel are (Nb, Ti)(C, N). With holding time increasing, the atomic ratio of Nb/Ti in precipitates decrease gradually. These precipitates still existe even after holding for 48 h at 1300℃while Nb(C, N) precipitates dissolve away in Nb microalloyed steel only after 4 h at the same temperature. These results show that formation and thermostability of precipitates are considerably influenced by interaction between Nb and Ti.     

Nature of large （Ti, Nb）（C, N） particles precipitated during the solidification of Ti, Nb HSLA steel

To investigate the microsegregation phenomena and complex （Ti, Nb）（C, N） precipitation behavior during continuous casting, a unidirectional solidification unit was employed to simulate the solidification process. The samples of Ti, Nb-addition steels after unidirectional solidification were examined using field emission scanning electron microscope （FE-SEM） and electron probe X-ray microanalyzer （EPMA）. In such specimens, dendrite structure and mushy zone can be detected along the solidification direction. It shows that the addition of titanium, niobium to high-strength low-alloyed （HSLA） steel results in undesirable （Ti, Nb）（C, N） precipitation because of microsegregation. The effect of cooling rate on （Ti, Nb）（C, N） precipitation was investigated. The composition of large precipitates was determined using FE-SEM with EDS. Large （Ti, Nb）（C, N） precipitates could be divided into three kinds according to the composition and morphology. With the cooling rate increasing, Ti-rich （Ti, Nb）（C, N） precipitates are transformed to Nb-rich （Ti, Nb）（C, N） precipitates.     

热送热装工艺对X80管线钢析出行为的影响

为了研究热送热装X80管线钢铸坯析出行为，使用了析出物定量分析、TEM和EDS测试方法，分析研究了析出物的成分、形貌、分布、质量分数以及频度分布情况.结果表明：两种工艺制度下，铸坯中析出物主要在奥氏体晶界析出，尺寸较大.室温装炉铸坯析出物在冷却和加热保温过程中发生析出、回溶和再析出行为，枝晶状的析出物大部分回溶并重新析出为两类碳氮化物，即富Ti和富Nb的（Ti，Nb）（C，N）碳氮化物.1100℃热装态的铸坯在加热和保温过程中将主要发生析出行为，析出颗粒的总质量分数要低于室温装炉条件，并且铸坯中固溶Nb的质量分数要远高于室温装炉条件.     

Relaxation of Deformed Austenite and Refinement of Bainite in a Nb-Containing Microalloyed Steel

By means of stress relaxation and strain-induced precipitation in deformed austenite, bainite with micron sheaves size was ob-tained in Nb-containing steel. Microstructures of deformed samples isothermally relaxed for various time followed by cooling in water were examined. Stress relaxation test and transmission electron microscopy (TEM) were employed to detect precipitation of microalloy elements, such as Nb, Ti, during isothermal holding after deformation. All the samples were constituted by lath-like bainite along with acicular ferrite, but the size of bainitic sheaves and the amount of acicular ferrite were changed with relaxation time. To achieve optimum refinement, relaxation should be confined in the stage when the precipitates have sufficiently grown and started to coarsen. The sample having not undergone relaxation does not exhibit obvious refinement despite of its higher dislocation density. These results indicate that relaxation promotes bainite to refine, which is because deformed austenitic grains are divided by dislocation walls formed during rela-xation and acicular ferrite formed before bainitic transformation.     

Effect of Nb on the Microstructure and Properties of Cold Rolled and Annealed Mo Microalloyed Bainitic Steel

Two kinds of low carbon bainitic steels,Nb-free Mo bearing and Nb + Mo addition steels,were cold rolled and annealed to investigate the effect of micro-alloying element Nb on the microstructure and properties of Mo microalloyed low carbon high strength bainitic steel. No precipitates were observed in Nb-free Mo bearing steel,whereas,two types of precipitates,i.e.,Nb( C,N) and composite( Nb,Mo)( C,N),were observed in the Nb + Mo microalloyed steel,resulting in precipitation strengthening. The strength of Mo bearing steel was improved by addition of Nb under the same annealing conditions. The grain size of Nb addition steel was almost the same as Nb-free steel. Unlike the obvious grain refinement and precipitation strengthening in hot rolling,the increase in yield strength of Nb addition steels in cold rolling and annealing mainly results from the precipitation strengthening,while the effect of grain refinement strengthening can be almost ignored.     

Process Modeling and Control Simulation for a 500W@4.5K Helium Refrigerator

In order to investigate dynamic behaviors of a 500W@ 4.5K helium refrigerator,the process simulator has been completed. The cryogenic process model is described and main components are customized. The realtime communication between the process model and the control system has been achieved. Compared with the preliminary experimental data,the errors of temperatures during the process of 300 K-80 K are less than 10%.The process model is validated to predict the cool-down process very well. The controller parameters are tuned in simulation and applied to the actual refrigerator suitably. Based on the dynamic simulation,the operation of Joule-Thomson( JT) by pass valve has been optimized. And the cool- down process from 300 K to 4. 5 K has been simulated under the control programs. Simulation results indicate that this dynamic simulator based on actual control architecture is available to process control and operation optimization for the helium refrigerators.     

钒和钼对微合金化贝氏体钢第二相析出的影响

利用热模拟试验机Gleeble-3800,热模拟试验研究了V、Mo、V＋Mo等元素、加热温度、保温时间对微合金化贝氏体钢中第二相析出的影响.结果表明,Nb、V、Ti、Mo的复合加入有利于第二相的析出,加热温度和保温时间对析出量的影响较小.     

Precipitation behaviors of X80 acicular ferrite pipeline steel

The precipitation behaviors of X80 acicular ferrite pipeline steel were investigated by using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The results show that dendritic precipitates in the as-cast steel slabs precipitate mainly in grain boundaries, and these dendritic precipitates dissolve and re-precipitate to two kinds of carbonitrides: Ti- and Nb-rich (Ti, Nb)(C, N) carbonitrides during reheating. Four types of precipitates mainly exist in the hot rolled plate: Ti-ri...     

Composition Optimization of Nb-Ti Microalloyed High Strength Steel

Four Nb-Ti microalloyed steels were refined and rolled to study the composition optimization of Nb-Ti microalloyed steels. The effects of Nb and Ti on the microstructures, precipitates and properties of Nb-Ti microalloyed steel were investigated. The results showed that an increase in Ti content resulted in the appearance of many fine precipitates leading to a strong precipitation strengthening effect. Hence, the yield strength increased. Besides, the increased strength by the combined increase of Nb and Ti was similar to that observed for the increase in Ti content alone. This increase in strength was attributed widely to the increase in the Ti content alone rather than Nb. Moreover, the increase in Nb content beyond 0.036 wt% exerted no significant effect on the strength of Ti-Nb microalloyed steels, in which more Ti could be added to further improve the strength of steels.     

Effects of Ti addition on low carbon hot strips produced by CSP process

Large quantity of fine Ti（C,N） particles, 15-30 nm in size, were observed in low carbon hot strips added to a small amount of Ti and produced by CSP process. The results showed that the precipitation of Ti（C,N） mostly took place during soaking and hot rolling, which is significantly different from that in the conventional production. These fine Ti carbonitride particles could be very effective on the austenite grain refinement by hindering grain growth of recrystallized austenite. Their precipitation behavior was discussed and compared with that of the steels produced in the conventional production.     

X80管线钢中氮化物和碳化物析出热力学分析

基于规则熔体理论和平衡反应基础,对连铸工序X80管线钢中碳化物、氮化物析出进行热力学分析,研究Ti、Nb及Al的碳化物、氮化物析出规律。结果表明,固液两相区,TiC0.005N0.995和TiN分别在1785K和1784K时开始析出;奥氏体相区,碳化物、氮化物析出先后顺序为NbC0.64N0.36、NbC、NbN、AlN、TiC,相应析出温度为1457、1404、1354、1267、1226K;γ→α相变过程中,TiC发生相间析出,Nb主要以NbC形式析出,AlN析出量较少。     

Precipitation characteristic of high strength steels microalloyed with titanium produced by compact strip production

Transmission electron microscopy (TEM) and physics-chemical phase analysis were employed to investigate the precipitates in high strength steels microalloyed with Ti produced by compact strip production (CSP). It was seen that precipitates in Ti microalloyed steels mainly included TiN, Ti4C2S2, and TiC. The size of TiN particles varied from 50 to 500 nm, and they could precipitate during or before soaking. The Ti4C2S2 with the size of 40-100 nm might precipitate before rolling, and the TiC particles with the size of 5-50 nm precipitated heterogeneously. High Ti content would lead to the presence of bigger TiC particles that precipitated in austenite, and by contrast, TiC particles that precipitated in ferrite and the transformation of austenite to ferrite was smaller. They were less than 30 nm and mainly responsible for precipitate strengthening. It should be noted that the TiC particles in higher Ti content were generally smaller than those in the steel with a lower Ti content.     

Precipitation and Hetero-nucleation Effect of V(C,N)in V-Microalloyed Steel

The precipitation behavior of V(C,N) in steels microalloyed with vanadium was researched using a thermal simulator during single-pass deformation at 800-750 ℃.The V(C,N) precipitates and its nucleation effect on ferrite were investigated by TEM and EDS.The experimental results show that there are two remarkable heterogeneous nucleation effects of V(C,N) particles precipitated before y→αphase change:primary reason is that high coherency between V(C,N) and ferrite promotes V(C,N) to become a nucleating center of intragranular ferrite;secondary reason is that the coarsening of V(C,N) causes locally solute-poor region in austenite,thus expedites the nucleation of intragranular ferrites further.Furthermore,the relationship between the size and shape of V(C,N)was studied,and identification method was provided for distinguishing interphase precipitation and general precipitation to avoid erroneous judgment and misguide.     

17-4PH不锈钢时效析出相的形成过程

对17-4PH(0Cr17Ni4Cu4Nb)马氏体不锈钢进行了3种不同条件下的时效处理。利用透射电子显微镜(TEM)技术详细分析了17-4PH不锈钢时效处理后析出相的形成过程。实验结果表明,时效条件为442℃,8h空冷时,在马氏体内部析出弥散的、尺寸小于10nm的ε-Cu、Nb6C(N)5和Nb2C(N)型亚稳相颗粒;时效条件为465℃,4h空冷时,析出尺寸为10～15nmε-Cu和稳定的NbC(N)相;时效条件为480℃,1h空冷时,析出尺寸为20～25mm的ε-Cu+NbC(N)相,且析出相NbC(N)与马氏体为共格析出,取向关系为NbC[001]//α-Fe[001]。     

Change in dislocation configuration of deformed Fe-Ni-Nb-Ti-C-B alloy during stress relaxation

Transmission electron microscopy (TEM) was applied to investigate the evolution of dislocation configuration and strain induced precipitation behavior during relaxation process after deformation in Fe-Ni-Nb-Ti-C-B alloy. Experimental results indicate that the dislocation density is very high and distribute randomly before relaxation. As the relaxation time increasing, dislocation cells will form gradually by polygonization. The strain induced precipitation retards the progress. In the final relaxation stage, most dislocations get rid of pinning of precipitates and the cells have developed into subgrains with large size.     

Influence of secondary carbide precipitation and transformation on abrasion resistance of a 3Cr15Mo1V1.5 white iron

The relationship between the secondary carbide precipitation and transformation of the 3Cr15Mo1V1.5 white iron and abrasion resistance was investigated by using optical microscope （OM）, transmission electron microscopy （TEM） and X-ray diffrac- tion （XRD）. The results show that the properties of secondary carbides precipitated at holding stage play an important role in the abrasion resistance. After certain holding time at 833 K subcritical treatment, the grainy （Fe, Cr）23C6 carbide precipitated and the fresh martensite transformed at the holding stage for 3Cr15Mo1V1.5 white iron improve the bulk hardness and abrasion resistance of the alloy. Prolonging holding time, MoC and （Cr, V）2C precipitations cause the secondary hardening peak and the corresponding better abrasion resistance. Finally, granular （Fe, Cr）23C6 carbide in situ transforms into laminar M3C carbide and the matrix structure transforms into pearlitic matrix. These changes weaken hardness and abrasion resistance of the alloy sharply.     

Ti(C,N)基金属陶瓷与3Cr13不锈钢的真空钎焊

采用AgCuZnNi钎料对Ti(C,N)基金属陶瓷和3Cr13不锈钢进行真空钎焊。研究了钎焊温度和钎焊时间对钎焊接头剪切强度的影响,通过SEM和EDS对Ti(C,N)基金属陶瓷/3Cr13不锈钢接头的显微组织、元素分布及断口形貌进行分析。研究表明:随着钎焊温度和钎焊时间的增加,钎焊接头的剪切强度先增大后减小。在钎焊温度为820℃,钎焊时间为10 min的工艺条件下,钎焊接头的结合强度达到最大,其剪切强度为154 MPa。Ti(C,N)基金属陶瓷/3Cr13不锈钢焊缝由Ag基固溶体和Cu基固溶体构成。Ag基固溶体主要集中在焊缝的中部,而Cu基固溶体大多数分布在焊缝两侧与母材结合。Ti(C,N)基金属陶瓷/3Cr13不锈钢接头断裂主要发生在Ti(C,N)基金属陶瓷侧,其断裂方式为脆性断裂。     

Interfacial microstructure and properties of Ti(C,N)/Ni bonded by transient liquid-phase diffusion

Effects of the main process parameters(temperature and time) on microstructure and properties of Ti(C, N)/Ni interface bonded by (Cu+Nb) interlayer in a vacuum diffusion bonding device were investigated. The interfacial microstructures consisted initially of Ni₃Nb metallic compound and eutectic of Ni₃Nb+CuNi_SS, and finally transformed to (Ti, Nb) (C, N)+Ni₃Nb near Ti (C, N) and NiCu_SS+Ni₃Nb near Ni when diffusion bonding temperature was 1 523–1 573 K. It was clear that Cu was a constituent in the transient liquid phase (TLP) into which Ni was dissolved by forming Cu-Ni transition liquid. Nb was dissolved in Cu-Ni transition liquid rapidly. Ti (C, N) conld be wetted by resultant Ni-Nb-Cu transient liquid phase which was followed by a little (Ti, Nb) (C, N) solid solution formed at interface. This increased the interface combining capability. Ultimately the interface shear strength was able to reach 140 MPa. The theoretle analysis and experimental results show that the growth of interfacial reaction layer Ni₃Nb accords with parabola law, and the activation energy of diffusion reaction is 115.0±0.5 kJ/mol, while the diffusion reaction speed constant is 12.53 mm/s^1/2. Funded by the National Natural Science Foundation of China (No. 50074017/E0408)     

Effect of Titanium Content on Microstructure and Wear Resistance of Hardfacing Alloy

The hardfacing alloys with different concentrations of titanium were deposited on carbon steel substrates by shielded metal arc welding, and the effect of titanium content on the microstructure characteristics of the hardfacing alloys was investigated. The wear resistance test of the hardfacing alloys was carried out by using a slurry rubber wheel abrasion test machine, and the wear behaviour was also studied. The results indicate that the addition of titanium can effectively promote the precipitation of the complex carbides of Nb and Ti due to the prior precipitation of titanium carbide which acts as nucleation sites for complex carbides. With the increase of titanium content, the wear resistance of the hardfacing alloys is increased gradually resulting from the refinement of microstructure and dispersive distribution of fine carbide precipitates. And the wear mechanism is mainly minimum plastic deformation with interrupted grooves due to the strengthening and protecting effects of carbide precipitates.     

Dynamic recrystallization behaviour of Nb-Ti microalloyed steels

The dynamic recrystallization （DRX） behavior of Nb-Ti microalloyed steels was investigated by isothermal single compression tests in the temperature range of 900-1 150 ℃ at constant strain rates of 0.1-5 s^-1. DRX was retarded effectively at low temperature due to the onset of dynamic precipitation of Nb and Ti carbonitrides, resulting in higher values of the peak strain. An expression was developed for the activation energy of deformation as a function of the contents of Nb and Ti in solution as well as other alloying elements. A new value of corrective factor was determined and applied to quantify the retardation produced by increase in the amount of Nb and Ti dissolved at the reheating temperature. The ratio of critical strain to peak strain decreases with increasing equivalent Nb content. In addition, the effects of Ti content and deformation conditions on DRX kinetics and steady state grain size were determined. Finally, the kinetics of dynamic precipitation was determined and effect of dynamic precipitation on the onset of DRX was clarified based on the comparison between precipitate pinning force and recrystallization driving force.