Nb(C,N) Precipitation and Austenite Recrystallization

The influence of boron addition, amount of deformation, and solution heat-treatment temperature on the precipitation and recrystallization behaviors of a family of high-strength low-alloy (HSLA) steels was studied. A stress relaxation technique was employed to detect the occurrence of austenite recrystallization and to determine the precipitation start(P _s) and finish(P _f) times. After preheating to 1100 °C or 1200 °C for 30 minutes, the specimens were cooled to test temperatures between 800 °C and 1000 °C. They were subsequently deformed to true strains of 5 or 25 pct and subjected to stress relaxation. The advent of recrystallization produced a sharp increase in relaxation rate, while the occurrence of carbonitride precipitation led to the appear- ance of a stress plateau. The results indicate that the presence of boron (1) accelerates carbo- nitride precipitation and (2) retards austenite recrystallization when present in combination with Nb. The precipitation-time-temperature (PTT) diagrams determined in this investigation are C-shaped for both the B-free as well as the B-modified Nb steels. These data were analyzed in terms of the classical theory of nucleation, on the basis of which it is demonstrated that the acceleration of the nucleation kinetics of precipitation can be attributed to the segregation of boron and of boron-vacancy complexes to dislocations and grain boundaries, as well as to the faster diffusion of Nb in the presence of boron.     

低碳钒氮微合金钢中V(C,N)在奥氏体中的析出动力学

控制VN在奥氏体中的有效析出是利用VN诱导晶内铁素体细化铁素体晶粒的关键技术。采用应力松弛方法研究了低碳钒氮微合金钢中V(C,N)在奥氏体区的等温析出行为。结果表明：试验钢的析出-温度-时间曲线(PTT)呈典型的C形,本试验条件下析出开始时间最短的“鼻子”温度为870℃左右。钢中的碳、氮含量以及变形量对PTT曲线有较大影响,它们增加均使C曲线向左移,特别是氮含量对V(C,N)析出的影响最显著。在碳含量约为0．10％的试验钢中,当氮含量从0．0036％增加到0．0140％时,可使870℃的析出开始时间从400s缩短到70s左右。     

碳氮化钒在奥氏体中析出的动力学模型

 采用应力松弛方法研究了钒氮微合金钢中V(C,N)在奥氏体区的等温析出行为,证实了动力学计算模型的模拟结果。结果表明,试验钢的析出 温度 时间(PTT)曲线呈典型的“C”形,本试验条件下析出开始时间最短的“鼻子”温度为850 ℃左右,与模型计算得到的820 ℃相差很小。钢中氮含量对PTT曲线形状有较大影响,随着氮含量的增加C曲线向左移动,这与模型计算结果非常吻合。     

Numerical Analysis of the Nb(C,N) Precipitation Kinetics in Microalloyed Steels

Precipitation kinetics of Nb(C,N) in microalloyed steels is crucial for the achievement of favoured steel properties. Therefore, numerous experimental studies have been performed in the past and various theoretical models have been developed to describe Nb(C,N) precipitation. However, the experimental data is sometimes contradictory and even the thermodynamic data for NbC solubility in austenite have a large scatter. In this paper, experimental results on the Nb(C,N) and NbV(C,N) precipitation kinetics in deformed and undeformed austenite are reviewed. Based on these data and with the precipitation kinetics module of the software package MatCalc, computer simulations are performed. The predicted interfacial energy of precipitates is adjusted to match the observed kinetics. A comparison between experimental information and simulation, i.e. time ‐ temperature ‐ precipitation (TTP) diagrams, is drawn and discussed. The results of the computer simulations using modified interfacial energies are in good agreement with the experiments.     

Equilibrium Modeling of (Nb,Ti, V)(C,N) Precipitation in Austenite of Microalloyed Steels

A theoretical model was proposed to predict complex equilibrium precipitation in Nb–Ti–V bearing microalloyed steel. It is assumed that the complex precipitate with B1 type consists of six kinds of binary compound, namely, NbC, NbN, TiC, TiN, VC, and VN. The composition of multicomponent precipitates and solute concentration in the matrix can be computed by the developed model which is based on the solubility product. The equilibrium volume fraction of the second phase particles at a given temperature can also be predicted easily. Comparison with other approaches in previous research was made. Good match demonstrated the model's validation and reliability.     

Dissolution of Nb(C,N) in mild steel during annealing

A formula for the solubility products of Nb and C in mild steel containing small amount of Ti which has been annealed in the austenitic region for 90s is presented. This solubility product gives closer agreement than that calculated from Smith's formula based on long-term annealing. Fine ferrite grains were achieved by dissolving Nb(C,N) into the matrix during annealing due to the increased supercooling during the austenite → ferrite transformation.     

铌微合金化HSLA钢铁素体中Nb(C,N)析出强化的研究进展

介绍了含微量铌的高强度低合金(HSLA)钢铁素体中Nb(C,N)析出的数学模型,以及计算经900～1200℃奥氏体化处理后的铁素体中NbC形核半径、形核率和形核长大率。一些研究者的试验结果表明,Nb的碳氮化物在铁素体上的析出需要较高的位错密度,与奥氏体中形成的NbC相比较,铁素体中形成的NbC相对较少,在680℃卷取钢板的抗拉强度较未经卷取直接空冷至室温的钢板抗拉强度有明显提高。     

Hardening Through an Ultrafine Carbide Precipitation in Austenite of a Low-Carbon Steel Containing Titanium and Tungsten

Metallurgical and Materials Transactions A - In this study, a significant hardening effect was obtained in a low-carbon steel containing Ti and W by performing a 20 pct hot compressive...     

Modeling of Strain-Induced Precipitation Kinetics and Evolution of Austenite Grains in Nb Microalloyed Steels

Considering the effect of strain and chemical composition onprecipitation behavior, new models for the start and end time of Nb（C,N） precipitation in austenite under the conditions of different temperatures and strains have been investigated for Nb microalloyed steel. The value of n in the precipitation kinetic equation has been determined by using the available experimental data in literature, which indicated that n is a constant and independent of temperature. The values of the start and end time of the predicted precipitation are compared with the experimental values. Calculated results are in good agreement with the experimental results. Also, the evolution of austenite grains before ferrite transformation is simulated by taking the effect of precipitation into consideration. The measured austenite grain size is in good agreement with predicted one prior to ferrite transformation.     

Recrystallization, Precipitation Behaviors, and Refinement of Austenite Grains in High Mn, High Nb Steel

Through a series of experiments conducted on three kinds of high Mn steels with different Nb content, including stress relaxation tests, physical metallurgical modeling, and observation of prior austenite grains and precipitates, the effect of Nb on recrystallization and precipitation behaviors were investigated. The results indicate the existence of a novel deformation temperature range for grain refinement resulting from complete static recrystallization (SRX) in high Mn, high Nb steel, whereas slow SRX kinetics can be accelerated by a finer initial grain size. In this deformation temperature range, the effect of precipitation is too weak to prohibit SRX nucleation efficiently, but solute drag is still large enough to slow down growth rate. As a consequence, shorter incubation and homogeneous recrystallized nucleation can be realized at relative low temperature, and the coarsening rate of grains is much slower because of the high solute drag effect in the rolling of low C high Mn, high Nb line pipe steel.     

Nb(C，N)作为取向硅钢中抑制剂的可行性

在铌微合金高强度钢中,主要利用铌来提高奥氏体的再结晶温度,从而实现控制轧制和控制冷却;同时,利用铌的析出进行强化来提高铌微合金钢的强度和韧性。结合取向硅钢抑制剂的特点和要求,分析了Nb(C,N)作为抑制剂的可行性。由于Nb(C,N)在钢中的固溶温度低,析出颗粒尺寸小,Nb(C,N)具备作为取向硅钢抑制剂的基本特征和优势,故其可作为取向硅钢中抑制剂。     

Reverse Austenite Transformation and Grain Growth in a Low-Carbon Steel

Metallurgical and Materials Transactions A - The mechanisms controlling the reverse austenite transformation and the subsequent grain growth are examined in a low-carbon steel during slow...     

连铸过程中铸坯应变对碳氮化铌析出行为的影响

采用应变诱导析出模型分析了连铸过程中铸坯应变对碳氮化铌析出行为的影响。结果表明:拉速变化对碳氮化铌的最快析出时间有一定的影响,在同样的应变及应变温度下,拉速越快碳氮化铌最快析出时间越短;在连铸过程的矫直段及弯曲段,由于铸坯应变及应变速率较小,在研究连铸过程碳氮化铌析出行为时铸坯应变对碳氮化铌析出行为的影响可以忽略。     

钒氮钢中铁素体等温形核规律的试验研究

采用Geeble 3500热模拟试验机，对比研究了钒氮钢、钒钢的等温相变特征。结果表明，在相变过程中，钒氮钢中铁素体形核总量和晶内铁素体形核数量都明显高于钒钢，分析认为，这与钒氮钢中V（C，N）的析出造成了铁素体形核位置的改变，同时也增加了形核位置密度有关，尝试用一种新的形核方式对此进行了描述。     

Influence of Deformation on the Precipitation Behavior of Nb(CN) in Austenite and Ferrite

The evolution of Nb precipitates in a low-alloyed steel at 973 K (700 °C) as a function of strain and subsequent dwell time is studied via atom-probe tomography (APT) and transmission electron microscopy. The results show that the volume fraction of the precipitates increases with increasing accumulated deformation because deformation-induced dislocations act as nucleation sites. The chemistry of these precipitates which are Nb carbonitrides changes with the dwell time after the deformation step. With increasing time, the C fraction increases. The precipitation analysis by APT in the austenite and the ferrite reveals that precipitates in the ferrite are larger and exhibit a higher C fraction compared to the precipitates in the austenite after the same thermo-mechanical treatment. The investigations also show that the volume fraction of Nb carbonitrides in the ferrite is higher than in the austenite.     

Morphology and Precipitation Kinetics of MnS in Low-Carbon Steel During Thin Slab Continuous Casting Process

The morphology of manganese sulfide formed during thin slab continuous casting process in low-carbon steel produced by compact strip production （CSP） technique was investigated. Using transmission electron microscopy analysis, it was seen that a majority of manganese sulfides precipitated at austenite grain boundaries, the morphologies of which were spherical or close to the spherical shape and the size of MnS precipitates ranged from 30 nm to 100 nm. A mathematical model of the manganese sulfide precipitation in this process was developed based on classical nucleation theory. Under the given conditions, the starting and finishing precipitation temperatures of MnS in the continuous casting thin slab of the studied low-carbon steel are 1 189 ℃ and 1 171 ℃, respectively, and the average diameter of MnS precipitates is about 48 nm within this precipitation temperature range. The influences of chemical components and thermo-mechanical processing conditions on the precipitation behavior of MnS in the same process were also discussed.     

低碳铌微合金钢在海水中的腐蚀规律

 铌钢主要用于大型船舶和海上工程建设，所以铌钢的耐海水腐蚀性能非常重要。研究了铌钢和不含铌钢在海水中不同腐蚀阶段的交流阻抗谱和腐蚀电位，获得了腐蚀过程的电荷传递电阻。研究结果表明：铌钢在海水腐蚀过程中的电荷传递电阻随腐蚀时间的延长而增大，其耐海水腐蚀性能优于不含铌钢。     

低碳含钒钢组织变化及V(C,N)析出规律

通过热模拟试验,研究了0．058％C-0．92％Mn-0．51％V钢在变形后等温过程中的组织变化规律及显微硬度变化规律,并用透射电镜研究了V(C,N)的沉淀析出规律。结果表明,变形后在650、700、750℃等温1130s时可获得单相铁素体组织,从而基本消除珠光体组织。等温转变过程中,铁素体先是主要沿奥氏体晶界形核,随后逐渐变为晶内形核。V(C,N)的沉淀析出存在两种形式：相间析出和随机析出。V(C,N)的相间析出消除了C元素在奥氏体中的富集,是获得单相铁素体组织的重要条件。     

碳和氮含量对V(C,N)在形变奥氏体中析出的影响

采用Thermo-Calc和经典动力学理论对2组碳、氮含量的中碳V-N微合金化钢分别进行了热力学和动力学分析,研究了V(C,N)在形变奥氏体中的析出规律.结果表明:在1000～800℃温度区间变形时,形变储能△G的存在导致V(C,N)的形变诱导析出;当氮的质量分数增加0.005%时,V(C,N)的析出开始时间(5%析出...