Microstructure and Transformation Characteristics of Acicular Ferrite in High Niobium-Bearing Microalloyed Steel

 The transformation behavior and microstructural characteristics of a low carbon high Nb-bearing microalloyed pipeline steel have been investigated by deformation dilatometry and microstructure observation. The continuous cooling transformation curves (CCT) of the tested steel was constructed. High Nb content and deformation enhancing the formation of acicular ferrite; the microstructures are range from PF, QF to AF with increasing cooling rates from 0.5 to 50℃/s and dominated by acicular ferrite in a broadened cooling rate higher than 5℃/s. The chaotic microstructure consists of non-equiaxed ferrite and interwoven ferrite laths distributed high density dislocations and sununits. The results of isothermal holding show that acicular ferrite microstructure is formed in region of 550-600℃. With the holding time or temperature increased, some low misorientations boundaries change to high misotrentationn as dislocations moving and grain boundaries coarsening.     

Chemistry and Process Optimization of V-Microalloyed N80-Class Seamless Casing Tube

The complex effects of different nitrogen (N) contents and thermal routines on the microstructure and mechanical properties of 33Mn2V steels for N80-Class seamless casing tube application were investigated using Gleeble simulation technique. The results showed that the N additions of 0.014%-0.021% in the steels for the in-line normalization process (ILNP) increased the strength while the toughness remained at a high level as compared with the steel with N content of 0.005%. It was also revealed that the N addition of 0.021% could enhance the performance combination of strength and toughness in the steels by using 700℃ as the cooling interrupted temperature (CIT) for the non in-line normalizing process (NILNP). It was further evidenced that the toughness was improved at expense of strength to some degree in all the steels by decreasing reheating temperature for the ILNP,while an increase of CIT for NILNP severely impaired the toughness and slightly improve the strength in the high-N steel with N content of 0.021%. This can be attributed to the dissolution and precipitation behavior of V(C,N). The optimization of V(C,N) precipitation could be achieved by enhancing N. The precipitation of V(C,N) in austenite was promoted by cooling to a certain temperature lower than the Ar1 for the ILNP. The V(C,N) particles formed in austenite contributed to grain refinement by the VN-induced nucleation of intragranular ferrite,but as a result the effect of precipitation on strengthening would become weaker due to a decrease of the precipitation of V in ferrite.     

Modeling for Formation of Proeutectoid Ferrite in Steel during Continuous Cooling

A novel model of the evolution of microstructure during continuous cooling with the formation of proeutectoid ferrite in steel was proposed from a Voronoi construction for the austenite grains, based on the Rappaz‘s integral nucleation model and the assumption that the ferrite nucleates at the edges of the original austenite grains and the successive growth of the ferrite grain is radial. The model can be used to calculate the fraction of ferrite as a function of time or temperature during continuous cooling, and to determine the microstructure of ferrite. The calculated results are in agreement with experimental results investigated in 0.38C-0.28Si-0.55Mn-0.92Cr-0.20Mo steel under continuous cooling using a Gleeble 1500 thermomechanical simulator.     

Effect of Mg Addition on Nucleation of Intra-granular Acicular Ferrite in Al-killed Low Carbon Steel

To verify the formation behaviors and mechanisms of intra-granular acicular ferrite( IAF) grains nucleated by Mg-Al-O in low carbon steel,the steels containing different Mg contents were refined in a vacuum induction furnace. The effect of Mg addition on the formation of IAF structure in Al-killed low carbon steel was investigated by optical microscope( OM) and scanning electron microscope with energy dispersive X-ray spectroscope( SEM-EDX). It reveals that the IAFs are only detected in Mg-added steels,and the volume fraction of IAF increases with the Mg concentration from 8 × 10~(-6) to 26 × 10~(-6). It shows that not only the MgO-Al_2O_3-MnS and MgO-Al_2O_3-P_2O_5 particles are the effective nucleation sites for IAF,but also the pure MgO·Al_2O_3 phase can promote the ferrite nucleation. A Mn-depletion zone( MDZ) is characterized adjacent to the MgO-Al_2O_3-MnS,which is believed to be one of the possible mechanisms to explain the IAF nucleation. The MDZ around the MgO-Al_2O_3-MnS inclusion would be induced by the Mn S precipitation on the inclusion. It seems that the ability of Mg-containing inclusions to induce the nucleation of ferrite might be attributed to a new mechanism,i. e.,the Prich zone formed on a few Mg-Al-O inclusions might be another factor for promoting the IAF formation.     

Effect of Boron on CGHAZ Microstructure and Toughness of High Strength Low Alloy Steels

Effect of boron on the microstructure and impact toughness in the coarse-grained heat-affected zone(CGHAZ)of two high strength low alloy steels,boron-free and boron-containing,was investigated by means of weld thermal simulation test.The result shows that,for the boron-free steel,a microstructure consisting of grain boundary ferrite degenerates pearlite and granular bainite for longer t8/5(the cooling time from 800 to 500 ℃),while lath bainite for shorter t8/5.For the boron-containing steel,granular bainite is dominant for a wide range of t8/5.Continuous cooling transformation(CCT)study on the CGHAZ indicates that the transformation start temperature decreases by about 50-100℃under different t8/5,for the boron-containing steel compared with the boron-free steel.The presence of boron suppresses the nucleation of ferrite at prior austenite grain boundaries and hence enlarges the range of t8/5for granular bainite transformation.However,the addition of boron deteriorates the impact toughness of CGHAZ,which may be due to a markedly increased fraction of martensite-austenite(M-A)constituents and decreased fraction of high angle grain boundaries.     

The Evolution of Ultrafine Grained Structure Formation Through Isothermal Static Phase Transformation

In the current study,a 0.3C-2Si-2Mn-0.28Mo (in wt%) steel with high hardenability was deformed at a relatively low temperature followed by isothermal static phase transformation.This novel thermomechanical processing made it possible to successfully produce an ultrafine ferrite grained structure (～2 μm) in the absence of both dynamic phase transformation and controlled cooling.The use of a model Ni-30Fe austenitic alloy showed that the low temperature deformation induced very fine intragranular defects throughout the microstructure,which would then act as fine spaced ferrite nucleation sites at an early stage of phase transformation.As a result,the coarsening of ferrite was extremely limited during isothermal phase transformation,resulting a very fine ferrite grained structure;even nanoscale in the region of the prior austenite grain boundary.     

Influence of on-line tempering parameters on microstructure of medium-carbon steel

A new process involving ultra-fast cooling(UFC)and on-line tempering(OLT)was proposed to displace austempering process,which usually implements in a salt/lead bath and brings out serious pollution in the industrial application.The optimization of the new process,involving the evolution of the microstructure of medium-carbon steel during various cooling paths,was studied.The results show that the cooling path affected the final microstructure in terms of the fraction of pearlite,grain size and distribution of cementite in pearlite.Increasing the cooling rate or decreasing the OLT temperature contributes to restraining the transformation from austenite to ferrite,and simultaneously retains more austenite for the transformation of pearlite.It is also noted that bainite was observed in the microstructure at the cooling rate of 45°C/s and the OLT temperature of 500°C.Through either increasing the cooling rate or decreasing the OLT temperature,the distribution of cementite in pearlite is more dispersed and grain is refined.Taking the possibility of industrial applications into account,the optimal process of cooling at 45°C/s followed by OLT at 600°C after hot rolling was determined,which achieves a microstructure containing nearly full pearlite with an average grain size of approximately 7μm and a homogeneously dispersed distribution of cementite in pearlite.     

The Cellular Automata Model of Electroslag Remelting Ingot Structure

Electroslag remelting (ESR) is an advanced process for the production of high quality steels. The microstructure of remelted steel which affect the mechanical properties and the performance of the ingot was determined by the technological parameters. A two-dimensional axisymmetric geometry which was established in this paper was divided into macro-grid finite element in order to compute temperature field; then the grid was divided into more detailed and uniform cells, and at last the continuous nucleation model based on the Gaussian distribution and KGT growth model was established for nucleation and growth calculations using cellular automaton method (CA) on the solidification of molten steel. The results show that: a vertical columnar grain zone and a inverted V-shaped columnar crystal zone appeared in the ESR ingot. In addition, the temperature field with different electrode melting rate and slag pool temperature parameters and the microstructure with different average nucleation under cooling and maximum grain density were studied in this paper. The simulation results agree well with the experimental results, so it is proved that the model and calculation method is reliable. To produce ideal solidified ingot and achieve the purpose of optimizing the production process, the production process was adjusted according to the simulation results.     

Effect of Cooling Rate on the Room Temperature Microstructure Following the Intercritical Deformation of Steel S460

The effect of final hot rolling in the intercritical (α+γ) region on microstructure and properties is very specific to the individual processing conditions and the chemical composition of a steel.S460 is a plate steel processed in this way.To reproduce at the laboratory scale,a multi-stage simulation was developed which included a high temperature austenite deformation and an isothermal hold.The effect of the applied cooling rate following intercritical deformation was investigated.At 1K/s (typical industrial cooling) the microstructure was similar to the reference sample,but included an intragranular ferrite fraction.This was due to differences in processing history,and considered to be linked to a larger prior austenite grain size.At an accelerated cooling rate (15K/s),acicular ferrite formed on shear bands within the strained austenite phase.EBSD scans have been completed to provide further information about the microstructures,with band contrast able to identify the pearlite phase at the slowest cooling rate.This is a starting point from which to focus on the ferrite morphologies.     

Effect of Mo on the continuous cooling transformation behavior of Nb-Ti micro-alloyed low carbon steel

The effect of molybdenum on the continuous cooling transformation behavior of the micro-alloyed low carbon steel containing niobium and titanium was investigated by a Gleeble 3800 thermo-mechanical simulator.The phase transformation temperature of the steel at various cooling rates was detected.The microstructure was observed by optical microscope(OM) and scanning electronic microscope(SEM),and its Vickers hardness was tested.Based on these,its dynamic continuous cooling transformation(CCT) diagrams were determined.The results show that the transformation temperature from deformed austenite to acicular ferrite(AF) is decreased when Mo is added,and the formation of pro-eutectoid ferrite(F) and pearlite(P) is either inhabited or postponed.Mo can also enlarge the range of the cooling rate in forming AF,and refine the microstructure effectively.     

20MnSi中夹杂物对针状铁素体形成的影响

 通过热模拟试验对20MnSi连续冷却过程中的相变规律进行了测定,通过电石、硅钙线脱氧及热处理得到了含有晶内针状铁素体试样,利用显微硬度仪对针状铁素体聚集区进行了显微硬度的测定,利用光学显微镜对晶内针状铁素体进行了形貌观察,利用扫描电镜和能谱仪对诱导针状铁素体生成的夹杂物的性质进行了分析。结果表明,20MnSi中可以形成晶内针状铁素体的冷却速度范围为5～20 ℃/s;能够诱发针状铁素体组织形核的夹杂物主要为MnS夹杂,其次为MnO·SiO2和MnS·SiO2夹杂,并且3类夹杂物的尺寸主要在小于3 μm的区间内;MnS夹杂促进针状铁素体形核是由应力-应变能和惰性界面能等原因共同造成的;高温加热和等温保温有利于使贫锰区减弱或消失,不利于针状铁素体的形成;高熔点夹杂物有利于诱导针状铁素体的形核,复合夹杂物和镶嵌存在的夹杂物可以为针状铁素体的形核提供多个合适的形核区,有利于促进多个针状铁素体的同时形核、长大。     

稀土处理C-Mn钢显微组织和夹杂物演化

对稀土处理C-Mn钢的夹杂物和显微组织进行分析,统计稀土处理C-Mn钢中针状铁素体形核核心尺寸,并将稀土处理钢在不同温度下淬火,研究稀土夹杂物生成和长大过程.实验结果表明:C-Mn钢加入少量稀土后钢中夹杂物从MnS＋硅铝酸盐夹杂转变为La₂O₂S＋LaAlO₃＋MnS＋硅铝酸盐夹杂,尺寸得到细化,显微组织也从马氏体＋贝氏体组织变成侧板条铁素体、针状铁素体和块状铁素体组织;稀土处理C-Mn钢中针状铁素体有效形核核心的尺寸集中在1~4μm,主要是在钢液中形成,冷却和凝固过程形成的数量较少;稀土夹杂物在钢液温度和冷却及凝固过程容易碰撞黏合长大,上浮从钢液中去除,MnS能在稀土夹杂物颗粒间析出.      

Al对稀土处理C-Mn钢连续冷却组织转变的影响

研究Al预脱氧对Ce处理钢夹杂物和显微组织的影响,利用热力学计算、带能谱的扫描电镜和DIL805A热膨胀仪检测进行了对比研究。得到如下结论:Ce处理后钢中的主要夹杂物从MnS转变为Ce2O2S+MnS,Al脱氧能使Ce处理钢中夹杂物转变为CeAlO3+Ce2S3+MnS。Ce处理C-Mn钢连续冷却过程有利于获得晶内铁素体的冷速为2~8℃/s。Al脱氧能改变诱导晶内铁素体形核的核心夹杂物种类,诱导铁素体形核的能力降低,且Al能够使Ce处理钢连续冷却组织转变(CCT)曲线向左上方移动,促进铁素体在晶界形核,不利于Ce处理后晶内铁素体的形成。Al脱氧Ce处理C-Mn钢在冷速为2~5℃/s时,由于夹杂物核心成分的改变与Al合金化作用导致晶内铁素体含量较未用Al脱氧Ce处理钢少。     

冷却速度对La脱氧钢中晶内铁素体形成的影响

采用光学显微镜、带能谱的扫描电子显微镜等分析技术观察了不同冷速下La脱氧钢的显微组织,并绘制了静态连续冷却转变图．实验结果发现,在4~10℃·s-1的冷速范围内,钢中会有大量晶内铁素体形成,其中在8℃·s-1冷却时可以获得大量细小且弥散分布的晶内针状铁素体．La脱氧钢中含La夹杂物与α-Fe相有较低错配度,其中La₂O₂S夹杂物与α—Fe相的品格错配度最小为0．2％,钢中含La夹杂物均可诱导晶内针状铁素体形核并可促进感生形核．进而细化组织．      

Effect of sulfur content on the microstructure and toughness of simulated Heat-Affected zone in Ti-Killed steels

Four Ti-killed steels were made to study the specific influence of sulfur on the inclusion, microstructure, and toughness of a simulated heat-affected zone (HAZ). The HAZ toughness was mainly determined by the volume fraction of intragranular acicular ferrite (IAF) which was closely related to the supercooling required to initiate austenite to ferrite transformation. The extent of supercooling was strongly influenced by the composition of grain boundary and inclusions. Sulfur addition up to 102 ppm caused a segregation of sulfur to the grain boundaries and a change of inclusion phase from predominantly Ti-oxides to Ti-oxysulphides and MnS. This behavior, in turn, suppressed the formation of polygonal ferrite and promoted the formation of IAF. Further addition of sulfur elevated transformation temperature and promoted the formation of polygonal ferrite due to the refinement of grain size and the increase of grain boundary associated inclusions. A methodology was proposed to evaluate the intragranular nucleation potential of inclusions, and the results showed that Ti-oxysulphides possessed better nucleation potential for IAF than Ti-oxides and MnS. With the lowest transformation temperature and most effective nuclei, the best HAZ toughness can be obtained at sulfur content of 102 ppm due to the achievement of the maximum volume fraction of IAF.     

冷却速率对Ti-Zr处理钢针状铁素体转变的影响

 大线能量焊接技术在低合金钢中的广泛使用降低了低合金钢的低温韧性,向低合金钢中添加适量的钛、锆元素可以促进针状铁素体形核,细化晶粒组织从而提高低合金钢的低温韧性。为了研究Ti-Zr处理钢中针状铁素体转变机理,使用25 kg真空感应炉中熔炼试验所需钢种,向低合金钢中添加了0.04%钛元素和0.014%锆元素;利用高温激光共聚焦显微镜原位观察了冷却速率对针状铁素体转变行为的变化;使用扫描电镜观察了Ti-Zr处理钢中的夹杂物成分和针状铁素体在夹杂物表面形核,使用光学显微镜观察不同冷速下的微观组织变化差异。试验发现,随着冷却速率从1增加至10 ℃/s,侧板铁素体的开始转变温度从770.2降低至632.4 ℃,针状铁素体的开始转变温度从731.5降低至612.6 ℃,针状铁素体的面积分数从47.91%增加至68.04%,针状铁素体与侧板铁素体的面积分数比值从1.34增加至3.54,针状铁素体与侧板铁素体的开始转变温度与结束转变温度差的比值从0.52增加至0.83,针状铁素体与侧板铁素体的面积分数之比与其转变温度区间之比存在正比例关系;不同冷却速率下,Ti-Zr处理钢中的主要夹杂物为ZrO₂-TiN-MnS,并且ZrO₂-TiN-MnS夹杂物可以有效促进针状铁素体形核;一方面是因为生成的贫锰区增加了铁素体转变驱动功;另一方面是因为ZrO₂-TiN-MnS夹杂物与铁素体有良好的晶格匹配关系降低了针状铁素体在夹杂物表面形核的界面能。     

Upgrade Rolling Based on Ultra Fast Cooling Technology for C-Mn Steel

By measuring the expansion curves of a C-Mn steel at different cooling rates by using an MMS-300 thermo- mechanical simulator, continuous cooling transformation curves were obtained. The new process ＂ultra fast cooling＋ laminar cooling＂ was simulated and the effects of ultra fast cooling ending temperature on microstructure had also been investigated. The hot rolling experiment was done by adopting ＂high temperature rolling-[-forepart ultra fast cooling＂ technologies at laboratory scale. The results revealed that ultra fast cooling can delay the decrease of disloca- tion density and refine ferrite grains. Diversity control of the microstructure and phase transformation strengthening can be realized by changing the ultra fast cooling ending temperature. With the decrease of ultra fast cooling ending temperature, the strength and toughness increase, but plasticity does not decrease obviously. The new technique can improve the yield strength by over 50 MPa. Therefore, the upgrade of mechanical properties of C-Mn steel can be realized by using ＂high temperature rolling＋ ultra fast cooling＋laminar cooling＂ technique. Compared with ＂low temperature rolling with large deformation degree＂ technique, this new technology can decrease the roiling force and in- crease the production efficiency.     

钛稀土复合处理对C--Mn钢粗晶热影响区组织及韧性的影响

利用Gleeble-1500D热模拟机进行焊接热影响区热循环模拟实验,研究了在焊接热输入为65 kJ·cm-1时稀土单独处理和钛稀土复合处理对C-Mn钢粗晶热影响区组织及冲击韧性的影响,并利用扫描电镜观察和分析了实验钢中的夹杂物和冲击断口形貌,利用光镜观察了热循环模拟后实验钢中的微观组织.实验结果表明:稀土单独处理和钛稀土复合处理的试样微观组织分别主要是晶界铁素体+块状铁素体+针状铁素体和晶界铁素体+晶内针状铁素体.经稀土单独处理的试样中夹杂物为La₂O₂S+锰铝硅酸盐+MnS复合夹杂;钛稀土复合处理的试样中的夹杂主要是La₂O₂S+TiO_x+锰铝硅酸盐+MnS复合夹杂.钛稀土复合处理钢中的复合夹杂更细小,有利于形成细小的晶内针状铁素体.钛稀土复合处理极大地改善了实验钢的焊接热影响区低温冲击韧性,比稀土单独处理对试样的冲击性能提升效果更好.      

稀土和Ca、Mg元素对高强度钢焊接热影响区组织和韧性的影响

研究了低合金高强度非调质中厚板钢中添加稀土(REM)和Ca、Mg微量元素对大线高能焊接热影响区(HAZ)显微组织微细化和晶内针状铁素体(IAF)形成的影响.结果表明,添加REM和Ca、Mg元素可在钢的HAZ中形成弥散稳定的氧硫化物(CeCa)2O2S和(CeMg)2O2,热轧奥氏体化(1450℃)和焊接热输入10 kJ/mm时都十分稳定,比传统采用TiN强化的钢具有更优良的低温韧性.有效地控制细小弥散的氧硫化合物,能获得适中的奥氏体有效晶粒尺寸和提供HAZ中形成晶内针状铁素体及稳定活性的形核位置.促进晶内铁素体协同形核生长,有效地使得HAZ组织微细化.