Effect of Controlled Cooling After Hot Rolling on Mechanical Properties of Hot Rolled TRIP Steel

A three-step cooling pattern on the runout table（ROT）was conducted for the hot rolled TRIP steel.Microstructural evolution during thermomechanical controlled processing（TMCP）was investigated.Processing condition of controlled cooling on a ROT in the laboratory rolling mill was discussed.The results indicated that the microstructure containing polygonal ferrite,granular bainite and a significant amount of the stable retained austenite can be obtained through three-step cooling on the ROT after hot rolling.TMCP led to ferrite grain refinement.Controlled cooling after hot rolling resulted in the stability of the remaining austenite and a satisfactory TRIP effect.Excellent mechanical properties were obtained through TMCP for the hot rolled TRIP steel.     

Low Cost High Performance Steel Plate With Superior HAZ Toughness for Large Heat Input Welding

In this paper, a laboratory study has been made to develop low cost high performance steel plates with superior HAZ toughness for large heat input welding. Simulated results show that the absorbed impact energy of heat-affected zone (HAZ) at -20℃reaches above 200J when large heat inputs of 100 to 400kJ/cm were applied, suggestive of superior HAZ toughness for large heat input welding of developed steel plate. The microstructures in HAZ are transformed from mainly fine ferrite and bainite at 100kJ/cm, through an intermediate stage of ferrite, bainite and pearlite at 200 and 300kJ/cm, to nearly fine ferrite and pearlite at 400kJ/cm. The prior austenite grain size and ferrite grain size in HAZ are controlled to ～50 and ～20μm, respectively. The high HAZ toughness is due to the inhibition of prior austenite grain size at high temperatures and the formation of beneficial microstructures to HAZ toughness during continuous cooling.     

微合金化控轧控冷钢筋纵向金相组织研究

 对微合金化控轧控冷钢筋的纵向金相组织进行了研究,并分析了不同成分试验钢纵向“条带”组织的差异及形成原因。研究结果表明：偏析元素（P、Si、Mn等）在轧制过程中沿轧制方向呈条状分布,是20MnSi、20MnSiV钢产生带状组织的原因。铌及其碳氮化物的溶质拖曳和“钉扎”作用,使20MnSiNb钢的奥氏体未再结晶轧制温度提高到1050℃,在冷却过程中,先共析铁素体在形变奥氏体晶界和内部变形带均匀析出,随后沿形变奥氏体晶界（在先共析铁素体与奥氏体的界面上）生成珠光体带,最后在形变奥氏体晶粒内部形成贝氏体条。研究条件下优势形核点的排序为：形变奥氏体晶界和形变奥氏体晶内变形带、偏析元素和夹杂、再结晶奥氏体晶界。     

Effects of thermomechanical processing on the microstructure and mechanical properties of a Ti-V-N steel

Based on studies of austenite deformation behavior and continuous-cooling-transformation behavior of a Ti-V microalloyed steel by cam plastometer and quench-deformation dilatometer, respectively, plate rolling schedules were designed to produce (i) recrystallized austenite, (ii) unrecrystallized austenite, (iii) deformed ferrite + unrecrystallized austenite. The effects of austenite condition and cooling rate on the final microstructure and mechanical properties were investigated. To rationalize the variation in final ferrite grain size with different thermomechanical processing schedules, it is necessary to consider the kinetics of ferrite grain growth in addition to the density of ferrite nucleation sites. The benefit of dilatometer studies in determining the optimum deformation schedule and cooling rate for a given steel is domonstrated. A wide range of tensile and impact properties results from the different microstructures studied. Yield strength is increased by increasing the amount of deformed ferrite, bainite, or martensite, and by decreasing the ferrite grain size. Impact toughness is most strongly influenced by ferrite grain size and occurrence of rolling plane delaminations. B. Dogan, Formerly with CANMET, Ottawa, Canada,     

Effect of Thermo‐Mechanical Process on Phase Transformation Behaviors of V–Ti–N Microalloyed Steel

Thermo‐mechanical simulation tests were performed on V–Ti–N microalloyed steel under three hot working conditions by using Gleeble‐3800 thermo‐mechanical simulator to study the effects of hot deformation and post‐deformation holding process on the continuous cooling transformation behaviors of overcooled austenite. The continuous cooling transformation diagrams (CCT diagrams) were determined by thermal dilation method and metallographic method. The effects of the hot deformation, post‐deformation holding, and cooling rate on the microstructure evolution were analyzed. The results show that deformation promotes ferrite and pearlite transformation. In addition, deformation leads to an increase in bainite start temperature, which becomes more markedly with the increase in cooling rate. The post‐deformation holding process is much favorable to promote carbonitride precipitation of the microalloying elements, which contributes to ferrite nucleation and smaller austenite grains. As a result, an increase in ferrite quantity and a decrease in ferrite grain size can be observed. And further more, the post‐deformation holding process reduces the effect of hot deformation on the bainite start temperature.     

A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Plate Steel

Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.     

Effect of Boron on the Isothermal Bainite Transformation

The role of Boron on the isothermal bainitic transformation in low-C, lean-alloyed steel was investigated. B clearly affected both the transformation kinetics and the morphology of isothermally transformed bainite. The effect of B was more noticeable in the high-temperature range of the bainitic transformation. The microstructure of bainite formed at 773 K (500 °C) consisted of a bainitic ferrite matrix and the martensite/austenite constituent. While the martensite/austenite constituent had an elongated morphology in B-free steel, the martensite/austenite constituents in the B-added steel had a granular morphology. Two types of bainite unit nucleation were considered: the initial nuclei and the nuclei formed on previously formed units. Electron backscattered diffraction (EBSD) analysis showed that the initial bainitic ferrite nuclei were formed at austenite grain boundaries with a Kurdjumov-Sachs (K-S) crystallographic orientation relationship with respect to one of the neighboring austenite grains, revealing the importance of interfacial energy reduction in the nucleation stage. The nuclei of the bainite transformation in the B-added steel were confined to the austenite grain interior, and the bainitic ferrite nuclei had crystallographic orientations limited to K-S variants within the same Bain variant. The characteristic bainite microstructure in B-added steel is due to the inhibition of the bainitic ferrite nucleation at austenite grain boundaries.     

热轧态与回火态 AH80 DB 低碳贝氏体钢组织与冲击韧性

采用光学显微镜、扫描电子显微镜和透射电子显微镜对热轧态和回火态AH80DB低碳贝氏体钢的显微组织、马氏体/奥氏体（M/A）岛、第二相的析出行为以及晶界取向差、有效晶粒尺寸进行研究,揭示回火后低碳贝氏体钢冲击韧性得到改善的原因.结果表明:两种试样的组织均由板条状贝氏体、粒状贝氏体和针状铁素体组成,其中回火态试样中针状铁素体组织较多.热轧态钢中存在较大尺寸M/A岛且呈方向性分布,大角度晶界比例占17.33%,有效晶粒尺寸为3.57μm;而回火态钢中M/A岛的尺寸较小,大角度晶界比例增加3.43%,有效晶粒尺寸减小0.56μm.热轧态钢中析出相主要是（Nb,Ti）C,尺寸在50~150 nm之间,回火态试样中析出较多细小的球状（Nb,Ti）C析出相,尺寸在10 nm左右.      

Effect of Austenite Deformation on Continuous Cooling Transformation Microstructures for 22CrSH Gear Steel

 The effect of compressive deformation of austenite on continuous cooling transformation microstructures for 22CrSH gear steel has been investigated using a Gleeble 1500 thermal simulator. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, and leads to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The grain boundary allotriomorphic ferrite occupies the austenite grain surfaces when the prior deformation takes place or the cooling rate is decreased, which causes a transition from bainite to acicular ferrite. The deformation enhances the stability of transformation from austenite to acicular ferrite, which results in an increase of M/A constituent.     

Effect of Cooling Method on Microstructure and Mechanical Properties of Hot-Rolled C-Si-Mn TRIP Steel

 The controlled cooling technology following hot rolling process is a vital factor that affects the final microstructure and mechanical properties of the hot-rolled transformation induced plasticity (TRIP) steels. In the present study, low alloy C-Si-Mn TRIP steel was successfully fabricated by hot rolling process with a 450 hot rolling mill. To maximize the volume fraction and stability of retained austenite of the steel, two different cooling methods (air-cooling and ultra-fast cooling “AC-UFC” and ultra-fast cooling, air-cooling and ultra-fast cooling “UFC-AC-UFC”) were conducted. The effects of the cooling method on the microstructure of hot-rolled TRIP steel were investigated via optical microscope, transmission electron microscope and conversion electron Mssbauer spectroscope. The mechanical properties of the steel were also evaluated by conventional tensile test. The results indicated that ferrite and bainite in the microstructure were refined with the cooling method of UFC-AC-UFC. The morphology of retained austenite was also changed from small islands distributing in bainite district (obtained with AC-UFC) to granular shape locating at the triple junction of the ferrite grain boundaries (obtained with UFC-AC-UFC). As a result, the TRIP steel with a content of retained austenite of 1152%, total elongation of 32% and product of tensile strength and total elongation of 27552 MPa·% was obtained.     

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.     

控轧控冷TRIP钢的微观相组成及其与力学性能的关系

采用电子背散射衍射技术等实验方法,研究了控轧控冷工艺制备的铌钒微合金化C-Mn-Si系热轧TRIP钢的显微组织及相组成,并分析了与其对应的力学性能.奥氏体轧制过程中的热变形及随后的冷却工艺对最终各相组织的形貌、大小和分布都有直接影响,并决定TRIP钢最终的力学性能.对TRIP钢卷取温度的模拟结果显示,与450和350℃模拟卷取温度相比,400℃模拟卷取温度能使该钢获得更好的综合力学性能.      

Modeling of Austenite Decomposition in Low Si-Mn TRIP Steel During Cooling

Transformation behavior in low carbon Si Mn TRIP steel was investigated by means of microstructural ob servation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.     

相变时间对CSP热轧TRIP钢组织和性能的影响

利用OM、SEM、XRD、EBSD和室温拉伸试验机等研究了CSP热轧TRIP钢中间缓冷时间及贝氏体等温时间对组织和力学性能的影响。结果表明,随着中间缓冷时间的延长,试验钢中的铁素体和残余奥氏体体积分数增加,贝氏体体积分数减少;抗拉强度基本不变,屈服强度逐渐降低,断后伸长率和强塑积变化不明显。中间缓冷时间为6 s时,可满足CSP产线的要求。对贝氏体相变时间的研究表明,当等温时间为15 min时,试验钢中的残余奥氏体主要分布于铁素体/铁素体界面、铁素体/贝氏体界面以及贝氏体中,体积分数约为7.1%,表现出良好的TRIP效应。其抗拉强度、屈服强度、断后伸长率和强塑积分别达到744.0 MPa、522.5 MPa、29.3%和21.8 GPa·%,力学性能最优。当等温时间延长至50 min时,试验钢中的贝氏体含量增加,残余奥氏体体积分数减少至2.7%,强塑积明显下降。     

Predicting the ferrite grain size in low-alloy steel

Mathematical models are proposed for calculating the onset temperature of ferrite transformation and the ferrite grain size as a function of the cooling conditions and the initial austenite grain size. The models are calibrated on the basis of experimental data for steels with ferrite-pearlite microstructure. The models are integrated in HSMM software, with considerable gain in computational power when considering the mechanical properties of steel rolled on the 2000 mill at OAO Severstal??.     

New Duplex Microstructure of Grain Boundary Allotriomorphic Ferrite/Granular Bainite

ｈｅｍｏｓｔｐｏｐｕｌａｒｍｉｃｒｏｓｔｒｕｃｔｕｒｅｉｎｃｕｒｒｅｎｔｓｔｒｕｃｔｕｒａｌｓｔｅｅｌｓｉｓｓｔｉｌｌａｍｉｘｔｕｒｅｏｆｆｅｒｒｉｔｅａｎｄｐｅａｒｌｉｔｅ .Ｍｉｃｒｏａｌｌｏｙｉｎｇ (ａｎｄｔｈｅａｓｓｏｃｉａｔｅｄｔｈｅｒｍｏ ｍｅｃｈａｎｉｃａｌｐｒｏｃｅｓｓ)ａｎｄｓｅｃｏｎｄ ｐｈａｓｅｓｔｒｅｎｇｔｈｅｎｉｎｇａｒｅｕｓｕａｌｌｙａｐｐｌｉｅｄｔｏｏｂｔａｉｎｈｉｇｈｓｔｒｅｎｇｔｈａｎｄｔｏｕｇｈｎｅｓｓ.Ｓｉｎｃｅｔｈｅｓｔｒｕｃｔｕｒｅｓｔｒｅｎｇｔｈｏｆｇｒａｎ…     

TMCP工艺对低碳Ni-Nb钢组织转变和力学性能的影响

 为了研究TMCP工艺对低碳Ni-Nb钢显微组织转变类型和晶粒尺寸的影响规律,研究了不同TMCP工艺下的显微组织特征及其对力学性能的作用机理。结果表明,在未变形轧制情况下,当冷却速度小于5 ℃/s时,显微组织为铁素体和珠光体,铁素体晶粒尺寸随着冷却速度的增大而减小;在变形轧制情况下,随着冷却速度的增加,组织中的铁素体晶粒尺寸明显减小;当冷却速度增大到5 ℃/s时,微观组织中出现了大量粒状贝氏体。试制钢板试验表明,当冷却速度为4 ℃/s时,试验钢的组织为准多边形铁素体,可以有效提高钢的低温韧性;当冷却速度达到6 ℃/s时,试验钢微观组织中出现大量粒状贝氏体,明显降低钢的低温韧性。     

微合金化热轧TRIP钢的工艺模拟

 借助MMS-300热模拟试验机研究了控轧温度区间、终冷温度、贝氏体区等温处理以及冷却路径对微合金化热轧TRIP钢组织演变规律的影响。结果表明,随着控轧温度区间“下调”,组织中的铁素体晶粒越来越细小,铁素体量逐渐增加,残余奥氏体量则先增加后减少。终冷温度升高时,组织中的残余奥氏体量也呈现出先增加后减少的变化趋势,而贝氏体温度范围等温时间的延长使残余奥氏体量增加。相对于“缓冷+快冷”,轧后采用“快冷+缓冷+超快冷”冷却路径更有助于铁素体晶粒的细化和奥氏体的残留。在“快冷+缓冷+超快冷”冷却路径下,当控轧温度区间为900~840℃,缓冷温度范围为710~680℃,贝氏体等温处理制度为450℃×5min时,组织中的残余奥氏体量达到最高值113%。     

终轧温度对低硅含磷系TRIP钢组织性能的影响

通过实验室热轧试验,研究了不同终轧温度下低硅含磷系热轧TRIP钢的组织特征及性能特点。结果表明：终轧温度由900℃降低到790℃,铁素体体积分数增加,贝氏体体积分数降低,残余奥氏体体积分数变化不太明显;终轧温度900和820℃时,得到贝氏体为基体的室温组织;终轧温度降低到790℃时,低温变形促进了奥氏体到铁素体的相变率...     

X80抗大变形管线钢的生产工艺与组织性能研究

设计了X80级抗大变形管线钢的合金成分,采用TMCP热轧及轧后两阶段控制冷却技术,完成了试验室制备并实现了工业试制。利用SEM、TEM和拉伸、冲击实验等分析方法研究了不同冷却条件对组织与性能的影响。研究结果表明,采用空冷+水冷两阶段冷却工艺后可得到铁素体+贝氏体双相组织的X80抗大变形管线钢,当加速冷却中终冷温度为450℃、冷却速度为20℃/s时,组织中铁素体与贝氏体相得到最佳配比,M/A相尺寸约为1μm,组织均匀性良好,试样得到最优的强度塑性匹配。