Comparison Between Different Processing Schedules for the Development of Ultrafine-Grained Dual-Phase Steel

A comparative study was carried out on the development of ultrafine-grained dual-phase (DP) (ferrite–martensite) structures in a low-carbon microalloyed steel processed using two thermomechanical processing routes, (i) intercritical deformation and (ii) warm-deformation and intercritical annealing. The samples were deformed using Gleeble3500^® simulator, maintaining a constant total strain (ε = 1) and strain rate ( $ \dot \varepsilon $  = 1/s). Evolution of microstructure and micro-texture was investigated by SEM, TEM, and EBSD. Ultrafine-grained DP structures could be formed by careful selection of deformation temperature, T _def (for intercritical deformation) or annealing temperature, T _anneal (for warm-deformation and annealing). Overall, the ferrite grain sizes ranged from 1.5 to 4.0 μm, and the sizes and fractions of the uniformly distributed fine-martensitic islands ranged from 1.5 to 3.0 μm and 15 to 45 pct, respectively. Dynamic strain-induced austenite-to-ferrite transformation followed by continuous (dynamic) recrystallization of the ferrite dictated the grain refinement during intercritical deformation, while, continuous (static) recrystallization by pronounced recovery dictated the grain refinement during the warm-deformation and the annealing. Regarding intercritical deformation, the samples cooled to T _def indicated finer grain size compared with the samples heated to T _def, which are explained in terms of the effects of strain partitioning on the ferrite and the heating during deformation. Alpha-fiber components dominated the texture in all the samples, and the fraction of high-angle boundaries (with >15 deg misorientation) increased with the increasing T _def or T _anneal, depending on the processing schedule. Fine carbide particles, microalloyed precipitates and austenitic islands played important roles in defining the mechanism of grain refinement that involved retarding conventional ferrite recrystallization and ferrite grain growth. With regard to the intercritical deformation, warm-deformation followed by annealing is a simpler process to control in the rolling mill; however, the need for high-power rolling mill and controlled annealing facility imposes industrial challenges.     

罩式退火铌微合金化汽车钢的强化机理

 通过比较相同冷轧与罩式退火工艺下Mn-Si系和铌微合金化2种汽车用低合金高强钢的显微组织与力学性能,研究微量铌在冷轧罩式退火低合金高强钢中的强化机理。利用OM、SEM、TEM和拉伸试验机分别对2种钢的显微组织与力学性能进行了表征。对比分析表明：相对热轧板来说,2种钢冷轧退火板的铁素体晶粒和第二相析出物的尺寸都有所长大,导致了强度降低。相对Mn-Si钢而言,铌微合金化钢热轧板和冷轧退火板中的铁素体晶粒和第二相析出物尺寸更细小,细小第二相析出物的数量也更多,在相同的伸长率水平下明显提高了强度。冷轧罩式退火板的强化机理分析表明,铌微合金化低合金高强钢的主要强化方式是细晶强化和NbC的沉淀强化;研究认为添加质量分数为0.025%的铌时细晶强化更强烈。     

Nb对C38+N2非调质钢组织和力学性能的影响

针对曲轴用新型铁素体+珠光体型非调质钢C38+N2锻件组织中铁素体含量往往难以达到用户要求的问题,研究了微量Nb元素(质量分数为0.022％)对其锻造后组织和力学性能的影响规律.结果表明,C38+N2钢中添加微量Nb能够细化晶粒和锻后组织,显著地提高组织中的铁素体含量,模拟曲轴锻造后组织中的铁素体体积分数可达到20％,...     

The Effect of Cooling Rate,and Cool Deformation Through Strain-Induced Transformation,on Microstructural Evolution and Mechanical Properties of Microalloyed Steels

In this article, a detailed study was conducted to evaluate the microstructural evolution and mechanical properties of microalloyed steels processed by thermomechanical schedules incorporating cool deformation. Cool deformation was incorporated into a full scale simulation of hot rolling, and the effect of prior austenite conditioning on the cool deformability of microalloyed steels was investigated. As well, the effect of varying cooling rate, from the end of the finishing stage to the cool deformation temperature, 673 K (400 °C), on mechanical properties and microstructural evolution was studied. Transmission electron microscopy (TEM) analysis, in particular for Nb containing steels, was also conducted for the precipitation evaluation. Results show that cool deformation greatly improves the strength of microalloyed steels. Of the several mechanisms identified, such as work hardening, precipitation, grain refinement, and strain-induced transformation (SIT) of retained austenite, SIT was proposed, for the first time in microalloyed steels, to be a significant factor for strengthening due to the deformation in ferrite. Results also show that the effect of precipitation in ferrite for the Nb bearing steels is greatly overshadowed by SIT at room temperature.     

Nb微合金钢连续冷却过程中铁素体晶粒尺寸的预测

基于相变动力学和热力学原理,讨论了合金元素Nb对铁素体相变的影响,建立了连续冷却过程中Nb微合金钢铁素体晶粒尺寸预测模型,模拟了应变量、轧制温度、冷却速度和固溶Nb含量对铁素体晶粒尺寸的影响,并将模型计算结果和试验结果进行比较,两者吻合良好,表明该模型能够用来预测Nb微合金钢连续冷却过程中铁素体晶粒尺寸。     

Effect of thermomechanical processing on structure and properties of low carbon steels

The microstructure of one plain carbon and two microalloyed steels has been refined by two different thermomechanical processing schedules in a laboratory rolling mill. The factors controlling the structural refinement have been investigated. The results indicate that a combination of single/two stage rolling prior to reaustenitization treatment and finish rolling in the (α + γ) region followed by controlled cooling improves the mechanical properties by refining the ferrite grain size.     

Ni含量对控轧控冷船舶用Nb-Ti微合金化NiCr钢组织和性能的影响

研究了0.31%Ni和0.88%Ni二种控轧控冷Nb-Ti微合金化NiCr钢的组织和性能。结果表明,船舶用钢控轧控冷获得粒状贝氏体、上贝氏体、针状铁素体、多边形铁素体及少量珠光体等组成的复合组织。控轧控冷造成铁素体晶粒尺寸细化,细小M-A岛增多。二种钢均获得较高的抗拉强度、屈服强度、伸长率和硬度,0.88Ni-0.32Cr钢性能优于0.31Ni-0.33Cr钢。船舶用钢-80℃试样纵向冲击功都在200J以上,0.88Ni-0.32Cr钢甚至超过了300J。该钢中最佳的Ni含量为0.88%Ni。由于控轧控冷造成了铁素体细晶强化、M-A岛复合强化、析出强化和位错强化,合金元素镍有效的提高了船舶钢的低温冲击韧性。     

Influence of rolling temperature on the austenite structure and properties of low-carbon microalloyed steel

In laboratory conditions, the influence of the temperature interval in the finishing stage of controlled rolling on the structure of hot-deformed austenite, the final microstructure, and the properties of low-carbon microalloyed steel is studied. Experiments show that reducing that temperature interval within the range T _nr-A _r3 reduces the grain size in the direction transverse to rolling and increases the number of deformational twins within the hot-deformed austenite grains. The effect is to improve the grain size and uniformity of the ferrite-bainite microstructure and hence to improve the mechanical properties of the thick sheet produced.     

N含量对Nb微合金化HRB400E钢筋组织与性能的影响

摘要：为了更好地发挥N元素在Nb微合金化钢筋中的作用,降低生产成本以及为钢筋成分设计提供理论依据。利用金相显微镜、扫描电镜、透射电镜、力学试验机对不同N含量的Nb微合金化高强抗震钢筋进行显微组织表征及力学性能测试,探究N含量对Nb微合金化钢筋的组织与力学性能的影响。研究表明,轧制过程中的奥氏体组织,随着N含量的增加,平均奥氏体晶粒有所减小;最终组织为铁素体和珠光体,随着N含量的增加,铁素体平均晶粒尺寸而减小,片层状珠光体的连续性增加,片层间距减小;析出相Nb(C,N)随着N含量的增大,沉淀析出的第二项颗粒体积分数增大,颗粒尺寸随之减小;在力学性能方面,屈服强度逐渐增加,抗拉强度先增加后减少。     

Effect of N content on microstructure and properties of Nb microalloyed HRB400E rebar

The purpose of this study is to better play the role of N in Nb microalloyed rebars, in order to reduce production costs and provide a theoretical basis for the design of rebars. Using metallographic microscope, scanning electron microscope, transmission electron microscope, and mechanical testing machine to carry out microstructure characterization and mechanical performance test of different N content of Nb microalloyed high strength seismic rebars, and explore the effect of N content on the structure and mechanics of Nb microalloyed high strength anti seismic rebars. The results show that the austenite structure during the rolling process decreases with the increase of N content; the final structure is ferrite and pearlite. As the N content increases, the average ferrite grain size and the interlamellar spacing decreases, but the continuity of lamellar pearlite increases. In Nb microalloyed high strength anti seismic rebars, the precipitated phase is Nb(C,N). With the increase of N content, the number of precipitated phase and the second particle volume fraction of precipitation increases, but the particle size decreases accordingly. In terms of mechanical properties, the yield strength gradually increases, and the tensile strength shows a phenomenon of increasing first and then decreasing.     

轧后冷却工艺对V-N-Cr微合金化Q550E高强钢组织性能的影响

对低碳V-N-Cr微合金化钢进行了控轧控冷实验,终冷后采用了随炉冷、保温毡缓冷、空冷3种冷却制度,并对3种不同冷却制度钢板进行了显微组织、综合力学性能和断口形貌的分析。研究表明,空冷钢板显微组织为细小多边形铁素体及针状铁素体复相组织,铁素体晶粒尺寸5～8μm,针状铁素体由交织的板条组成,宽度1～3μm。在随炉冷及保温毡缓冷时,由于冷却速率缓慢,多边形铁素体及针状铁素体发生了回火,并析出细小弥散的碳化物。3种冷却条件下,屈服强度均≥585 MPa,抗拉强度≥694 MPa,延伸率≥27%,而且1/2试样-60℃冲击功≥36 J,综合力学性能优于Q550F级国标要求。细晶强化、析出强化、组织强化为本钢种的主要强化方式,冲击断口均由韧窝组成,呈现韧性断裂模式,控轧控冷引起的晶粒细化及针状铁素体的形成有效阻碍解理裂纹的扩展,从而增强低温韧性。     

Improving properties of a low-carbon microalloyed steel by means of accelerated cooling of dynamically recrystallized austenite

Ferrite formation from the ultra-fine dynamically recrystallized austenite (d_γ < 5 μm) was investigated on a microalloyed steel with 0.11%C. Hot rolling conditions were simulated by the hot deformation simulator Wumsi. Due to accelerated cooling a corresponding fine homogeneous ferrite grain of d_α < 2 μm was achieved with a pearlite free acicular microstructure after a cooling rate of more than 20 K/s. Excellent mechanical properties (2.0% proof stress of over 700 MPa and impact transition temperature of -180°C) were obtained, superior to those after thermomechanical processing of the same steel without dynamic recrystallization of the low-temperature austenite.     

热变形和加速冷却对低碳微合金钢组织的影响

利用Gleeble-1500热模拟实验机研究了热变形和加速冷却工艺对3种低碳微合金钢组织演变的影响,结果表明,在再结晶或未再结晶温区实施1道次变形的晶粒细化效果不如2道次形次的效果明显,在再结晶和再结晶温区实施4道次变形可以得到更细的组织,配合较高的冷却速度可以形成部分针状铁素体组织。随着冷却速度的提高,组织变得更细,并且针状铁素体的数量增加。在相近的变形和冷却条件下,碳、锰含量较高的试样具有更细的组织。     

Influence of microalloying on the mechanical properties of medium carbon forging steels after a newly designed post forging treatment

For more than twenty years the classical quench and tempering of medium carbon Cr-alloyed steels has been substituted in the production of drop-forged parts for the automotive industry by a direct continuous cooling of less expensive V-microalloyed steels with lower carbon content. However, this simplified treatment has serious limitations concerning the yield strength and ductility if compared with the properties after quench and tempering. On a group of such V-bearing steels additionally microalloyed with Ti and Nb and with different N contents, an alternative two-step-cooling (TSC) strategy after forging, combined with an additional annealing (AN), has been applied. This new post forging treatment results in a significant improvement of the final mechanical properties. The paper is focused on the particular contributions of a different microalloying in the optimized deformation schedules to improve mechanical properties after TSC + AN. The aim of this additional microalloying is to achieve a more homogeneous distribution of ferrite in the final multi-phase microstructure due to a proper austenite conditioning as well as to make a full use of the strengthening potential of vanadium in these forging steel grades.     

冷却速率对高铌微合金钢组织的影响

 采用热模拟和组织显微分析方法研究了不同冷却速率对高铌微合金钢组织的影响。结果表明，铌含量高有利于在极低的冷却速率下获得针状铁素体组织，提高冷却速率则能进一步促进针状铁素体转变，并使铁素体板条得到细化。同时，固溶铌有利于针状铁素体的形成，而铌的沉淀析出则不利于针状铁素体的获得。     

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.     

Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Although there has been much research regarding the effect of austenite deformation on accelerated cooled microstructures in microalloyed steels, there is still a lack of accurate data on boundary densities and effective grain sizes. Previous results observed from optical micrographs are not accurate enough, because, for displacive transformation products, a substantial part of the boundaries have disorientation angles below 15 deg. Therefore, in this research, a niobium microalloyed steel was used and electron backscattering diffraction mappings were performed on all of the transformed microstructures to obtain accurate results on boundary densities and grain refinement. It was found that with strain rising from 0 to 0.5, a transition from bainitic ferrite to acicular ferrite occurs and the effective grain size reduces from 5.7 to 3.1 μm. When further increasing strain from 0.5 to 0.7, dynamic recrystallization was triggered and postdynamic softening occurred during the accelerated cooling, leading to an inhomogeneous and coarse transformed microstructure. In the entire strain range, the density changes of boundaries with different disorientation angles are distinct, due to different boundary formation mechanisms. Finally, the controversial influence of austenite deformation on effective grain size of low-temperature transformation products was argued to be related to the differences in transformation conditions and final microstructures.

     

Microstructural evolution during the austenite-to-ferrite transformation from deformed austenite

It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures below the nonrecrystallization temperature (T _nr ). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation. In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined T _nr for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence the mechanical properties of the steel.     

Processing of Bimodal Grain-Sized Ultrafine-Grained Dual Phase Microalloyed V-Nb Steel with 1370 MPa Strength and 16 pct Uniform Elongation Through Warm Rolling and Intercritical Annealing

Ultrafine-grained dual phase microalloyed V-Nb steel with ultimate tensile strength of 1371 MPa and uniform elongation of 16 pct characterized by bimodal ferrite grain structure was obtained through warm rolling and subsequent intercritical annealing. The bimodal ferrite grain structure with uniform dispersion of Nb/V carbides and strong γ-fiber texture promoted high strain hardening rate and high uniform elongation and high strength is attributed to ultrafine-grained ferrite and martensite.     

Effect of simulated thermal cycles on the microstructure of the heat-affected zone in HSLA-80 and HSLA-100 steel plates

The influence of weld thermal simulation on the transformation kinetics and heat-affected zone (HAZ) microstructure of two high-strength low-alloy (HSLA) steels, HSLA-80 and HSLA-100, has been investigated. Heat inputs of 10 kJ/cm (fast cooling) and 40 kJ/cm (slow cooling) were used to generate single-pass thermal cycles with peak temperatures in the range of 750 °C to 1400 °C. The prior-austenite grain size is found to grow rapidly beyond 1100 °C in both the steels, primarily with the dissolution of niobium carbonitride (Nb(CN)) precipitates. Dilatation studies on HSLA-80 steel indicate transformation start temperatures (T _s ) of 550 °C to 560 °C while cooling from a peak temperature (T _p ) of 1000 °C. Transmission electron microscopy studies show here the presence of accicular ferrite in the HAZ. The T _s value is lowered to 470 °C and below when cooled from a peak temperature of 1200 °C and beyond, with almost complete transformation to lath martensite. In HSLA-100 steel, the T _s value for accicular ferrite is found to be 470 °C to 490 °C when cooled from a peak temperature of 1000 °C, but is lowered below 450 °C when cooled from 1200 °C and beyond, with correspondingly higher austenite grain sizes. The transformation kinetics appears to be relatively faster in the fine-grained austenite than in the coarse-grained austenite, where the niobium is in complete solid solution. A mixed microstructure consisting of accicular ferrite and lath martensite is observed for practically all HAZ treatments. The coarse-grained HAZ (CGHAZ) of HSLA-80 steel shows a higher volume fraction of lath martensite in the final microstructure and is harder than the CGHAZ of HSLA-100 steel.