Effect of manganese on the annealing texture and strain ratio of low-carbon steels

A correlation between the plastic strain ratio, or γ value, and the manganese content of laboratory-prepared, low-carbon steels has been established. The \(\bar r\) value decreases consistently when manganese is increased from 0.05 to 0.56 pct. Excellent \(\bar r\) values are associated with manganese contents of 0.1 pct or less. The desirable (111) fiber texture in the low-manganese steels is obtained through recrystallization without appreciable grain growth. It is believed that a very low content of manganese in the steel, or its consequential effects in connection with nonmetallic impurities, may have influenced significantly the characteristics of recovery, hence the nucleation of recrystallized grains, of the various deformation texture components. X-ray line sharpening of several reflections during the early stages of annealing appears to be consistent with this interpretation.     

Effect of hot-rolling texture on the plastic strain ratio of low-carbon steels

The effect of hot-rolling texture on the plastic strain ratio of a continuously cast low-carbon steel and a vacuum-melted low-carbon iron has been investigated as a function of the temperature of hot rolling. Normal cold rolling and a simulated box anneal, in dry hydrogen to decarburize, were employed subsequently. The microstructure and texture at every stage of the process was examined, and the plastic strain ratio (R value) of the final strip was determined. Results indicate that •R varies strongly with the hot-rolling temperature. OptimumR values were obtained when the hot-rolling temperature was 925° to 950°C.     

Delta-ferrite recovery structures in low-carbon steels

The development of delta-ferrite recovery substructures in low-carbon steels has been observed in-situ utilizing laser scanning confocal microscopy (LSCM). Well-developed sub-boundaries with interfacial energies much smaller than that of delta-ferrite grain boundaries formed following transformation from austenite to delta-ferrite on heating. It is proposed that transformation stresses associated with the austenite to delta-ferrite phase transformation generate dislocations that subsequently recover into sub-boundaries by a process of polygonization. Experimental evidence in support of this proposal was found in a ferritic stainless steel. Thermal cycling through the high-temperature delta-ferrite/austenite/delta-ferrite phase transformation leads to the development of a well-defined recovery substructure, which, in turn, modifies the low-temperature austenite decomposition product from Widmanstätten to polygonal ferrite, with a commensurate change in hardness.     

On the structure and properties of high-nitrogen low-carbon non-austenitic steels

Phase transformations, when cooling and heating non-austenitic high-nitrogen low-carbon steels containing chromium and other alloying elements, as well as structure and mechanical properties of these steels were analyzed. It was confirmed that these steels have high temperature chromium diffusion controlled pearlitic type transformation and martensitic type transformation. Experimental high nitrogen steels after quenching and tempering provide mechanical properties of about the same level as high strength commercial alloyed steels. Features of nitrogen as an alloying element in steels discussed allow the supposition of a possible reduction of the consumption of nickel, manganese, molybdenum or tungsten in high strength alloyed steels.     

Effect of manganese on annealing texture,plastic anisotropy,and mechanical properties of low-carbon steels containing 0.067 Pct phosphorus

To establish the range of manganese content in phosphorus-containing low-carbon steels that will provide superiorr _m andAr values in cold-rolled sheets, the effects of manganese on annealing texture, plastic anisotropy, and mechanical properties of steels containing 0.067 pct P were studied. Both vacuum and air-melted laboratory heats were investigated. Results show that highr _m and low Ar values, a desirable combination for deepdrawing applications, can be obtained with manganese contents up to 0.25 pct, when the hot-rolled band is cold-rolled 80 pct, and annealed at 710 or 780° (1310 or 1435∮F) for 20 h. Annealing at the higher temperature developed better plastic anisotropy than did annealing at the lower temperature. Ther _m values of the air-melted steels were superior to those of the vacuum-melted steels. It is believed that complex interactions of manganese with other elements in the steel, such as sulfur and oxygen, and possibly carbon, influenced the annealing behavior of the steels.     

Structure of continuously cooled low-carbon vanadium steels

The structures of four 0.15 pct carbon steels containing vanadium, nitrogen, and aluminum separately and together were studied systematically, with the help of transmission electron microscopy, by cooling suitable steels at four different rates ranging from 120 °C/min to 3.6 °C/ min from temperatures giving a common austenite grain size of 35 μm. Except for the steel containing only vanadium and that containing only aluminum and nitrogen cooled at the fastest rate used, the observed microstructures were all essentially mixtures of polygonal ferrite and expected amounts for pearlite. For all the steels studied, except the one containing aluminum and nitrogen, it was found that general precipitation was more common than interphase precipitation, although the extent of the latter increased at lower cooling rates. Moreover, in some cases, both general and interphase precipitation were present in the same area. The presence of aluminum was observed to enhance the formation of interphase precipitates at all cooling rates, and the spacing between parallel rows of precipitates increased as the cooling rate was decreased. The dislocation density was high at all cooling rates in all the steels, but it was found to decrease with decreasing cooling rates. Very fine precipitates were found in all the steels, except the steel containing aluminum and nitrogen. At the fast cooling rates, the segregation of vanadium and interstitial elements, which led to locally lower transformation temperatures and higher supersaturations, resulted in clusters of fine particles of vanadium carbonitride, V(C, N). At the slower cooling rates, all the steels showed severe heterogeneity in precipitate morphology which was more pronounced in the steel containing aluminum and nitrogen, while a needlelike morphology of V(C, N) precipitate was occasionally found in steels containing either vanadium and nitrogen or vanadium, nitrogen, and aluminum. As the cooling rate decreased, particle coarsening and growth occurred, causing a reduction in the number of particles/unit area. The coarsening rate of V(C,N) in the presence of aluminum is considerably lower than that of vanadium carbide, VC, or of V(C, N) in the absence of aluminum. Because of the unfavorable precipitation kinetics, any aluminum nitride (A1N) formed during cooling did not nucleate separately but was deposited on the pre-existing A1N particles, thus causing them to be coarsened very rapidly with decreasing cooling rate. Formerly with the Department of Metallurgy, The University of Sheffield, Sheffield, England     

The melting and dissolution of low-carbon steels in iron-carbon melts

Direct experimental proof is presented in the paper for the role played by the mass transfer of carbon in accelerating or facilitating the melting and dissolution of pure iron specimens in iron-carbon melts. It is shown that pure iron may readily melt in iron-carbon melts even under conditions where the temperature of the molten phase is considerably below the melting point of the pure iron or low-carbon specimen. A mathematical interpretation is developed for these experimental results that includes the mass transfer of carbon and the unsteady state heat transfer within a moving boundary system. The results of the analysis were found to agree with the experimental data thus providing a basis for further calculations aimed at predicting the melting of scrap in the basic oxygen furnace. These calculations show that scrap melting, facilitated by carbon diffusion from the melt to the scrap surface, begins very early during the process and that melting is retarded and even terminated during the blow when the bath has insufficient superheat to provide the necessary sensible and latent heat required for melting. It follows therefore that the rate of scrap melting in steelmaking processes is accelerated if the removal of carbon in the bath is retarded or if the temperature of the bath is increased rapidly in order to maintain a high level of superheat during the refining process.     

Transformation during the isothermal deformation of low-carbon Nb-B steels

The transformation behavior during isothermal deformation of four steels containing different microalloying additions was investigated by means of the “strain-rate change” technique. The flow curves obtained at temperatures ranging from 620 °C to 850 °C, and the associated microstructures, indicate that the transformation in the Mo-Nb-B and Mo-B steels is of the austenite-to-bainite type. Here, dramatic increases in flow stress are observed at lower temperatures. By contrast, the transformation in the Nb-15B and Nb-64B steels is basically of the austenite-to-ferrite type; in these two grades, the flow stress increases observed are attributable to strengthening by NbC precipitation. Large intergranular and intragranular Fe₂₃(C,B)₆ particles were found in the Nb-64B steel samples deformed to ɛ=0.1 after holding for 60 seconds at 800°C. These large precipitates are considered to be responsible for accelerating the transformation in the Nb-64B steel by reducing the concentration of boron atoms available for boundary segregation and by acting as nucleation sites for the formation of polygonal ferrite. The flow curves of the Mo-Nb-B steel exhibit distinct serrations, indicating that a displacive mechanism is involved in the γ-to-B transformation.     

Study of the mechanical characteristics of low-carbon plate steels

The Azovstal’ Metallurgical Combine has conducted tests of the production of plates of low-carbon (0.06–0.08% C) low-alloy steels 06GBD and 06G2B. The plates, ranging up to 45 mm in thickness and having yield points ≥390 and ≥440 N/mm², respectively, are intended for use in structures employed for critical applications. Both steels are microalloyed with niobium, vanadium, molybdenum, and additions of copper. It is shown that in the quenched-and-tempered state the two steels fully conform to the specifications. The plates are characterized by excellent cold resistance, impact toughness, and weldability (C_e = 0.34–0.37 for steel 06GBD and C_e = 0.38–0.41 for steel 06G2B) and satisfactory values of the ratio σ _y/σ _u (≤0.90 for plates with a thickness ≤20 mm and ≤0.85 for plates with a thickness >20 mm). The construction of serial curves describing impact work and the percentage of the ductile component in the fracture of Charpy specimens prepared from plates of steel 06GBD showed that the plates have a high resistance to brittle fracture (impact work was at least 145 J at ?70°C, and the percentage of ductile fracture was 100% for 14-mm-thick plates and 80% for 32-mm-thick plates).     

Martensite and retained austenite in hot-rolled,low-carbon bainitic steels

The microstructure and occurrence of a microconstituent, consisting of martensite and retained austenite in hot-rolled plates of low-carbon bainitic steels was studied by electron microscopy and microprobe analysis. The results of the studies showed that the formation of the martensite-austenite constituent is controlled by the composition of the steel and by the cooling rate of the plates following hot-rolling. The mechanisms involved in the formation of the martensiteaustenite constituent are discussed.     

低碳多相钢的组织调控与力学性能

采用优化后的临界区再加热-淬火中温等温(T1、T2)热处理工艺,对具有不同前躯体组织的(0.22/0.17)C-(1.91/1.85)Mn-(1.32/0.94)Si两类热轧6 mm钢板分别进行处理,获得了具有铁素体、贝氏体、马氏体以及弥散分布于原奥氏体晶界、相界等处的残余奥氏体所构成的多相组织.利用扫描电镜、X射线衍射以及电子背散射衍射分析技术等对不同热处理阶段钢的微观组织进行了表征.结果证实,采用不同的前躯体组织设计可以很好地调控临界区再加热逆转变奥氏体的组织形貌、比例以及碳含量,进而通过后续处理来实现对钢中多相组织的调控.前躯体为马氏体的0.22C钢,经T1工艺后获得了以针状铁素体为基体的多相组织,其强塑积超过了30 GPa·%;前躯体为铁素体+马氏体的0.17C钢经T2工艺后获得了以块状铁素体为基体的多相组织,其强塑积超过了27 GPa·%.     

低碳Nb-Ti二元微合金钢析出过程的演变

建立规则溶液亚点阵模型计算了不同温度(1073~1523 K)下低碳Nb-Ti二元微合金钢(Nb质量分数为0.023%,Ti质量分数为0.012%)中碳氮化物析出相的平衡摩尔分数、化学驱动力和各组元摩尔分数,对微合金钢中析出粒子演变规律进行研究,并利用透射电镜观察及能谱分析验证这种析出模式.计算结果表明,1523 K下析出粒子化学式组成为(Nb_0.15Ti_0.85)(C_0.16N_0.84),由富Ti的析出物逐渐过渡至Nb-Ti均匀析出,析出粒子演变顺序为(Nb_0.15Ti_0.85)(C_0.16N_0.84)、(Nb_xTi_1-x)(C_yN_1-y)和(Nb_0.5Ti_0.5)(C_0.56N_0.44),与实验结果符合较好.随着温度降低,Ti/Nb质量比逐渐减小,得到的TiC比NbC更难溶.对均匀形核及位错处形核的临界核心尺寸和相对形核速率进行计算,得到最大形核率即可获得最细小第二相尺寸的温度.     

低碳含铌钢粗大奥氏体的静态再结晶

利用Gleeble1500热模拟机研究了Nb质量分数0.089%的高温热机械轧制钢(4^# Nb钢)在粗大奥氏体晶粒条件下的静态再结晶规律.作为对比,同时研究了一种铌质量分数0.049%的低碳含铌钢(2^# Nb钢)的静态再结晶规律.实验结果表明:4^# Nb钢的静态再结晶动力学过程比2^# Nb钢慢,在高温时差异较小;随着形变温度的降低,4^# Nb钢的静态再结晶动力学被极大地延迟.根据实验数据建立了静态再结晶模型,该模型对轧制工艺的制定有一定的指导意义.     

Microstructure and texture evolution under strain-path changes in low-carbon interstitial-free steel

The transmission electron microscopy (TEM) microstructure of a low-carbon Ti-killed interstitial-free (IF) steel has been examined after simple-shear/simple-shear and uniaxial-tension/simple-shear strain-path changes, in connection with the crystallographic orientation of the grains. The results are discussed in the context of the inter-relation between the microstructure and texture evolutions and their joint influence on the mechanical behavior. A partial disappearance of prestrain microstructures is shown to cause the stagnation of work hardening at earlier stages of reversed loading during Bauschinger simple-shear sequences. Under progressing reversed deformation, a fragmentation of the grains of unstable orientations still slows down the work-hardening rate. A strong localization of plastic flow within microbands following an orthogonal strain-path change is shown to occur within the grains containing well-developed prestrain dislocation boundaries and belonging to certain orientation groups.     

Development of ferrite rolling textures in low- and extra low-carbon steels

The texture changes that affect the main ideal orientations displayed by rolled low-and extra low-carbon steels are examined. Predictions are made regarding the stability of these components using a rate-dependent theory for mixed 112 (111) and 110 (111) slip. Both full constraint (Taylor) and relaxed constraint (lath and pancake) grain interaction models are employed. It is shown that the experimentally observed texture changes can be reproduced by adopting the deformation modes which require the least plastic work. The orientation dependences of the preferred deformation modes are described together with the relative stabilities of the expected end textures.     

Characterization of the nucleation and growth behavior of copper precipitates in low-carbon steels

The nucleation and growth behavior of copper precipitates in ferrite was investigated both theoretically and experimentally for two low-carbon steels with and without niobium additions in samples cooled directly to the desired aging temperature from the austenitizing temperature. Theoretical nucleation and growth rate models were constructed using calculated thermodynamic data in conjunction with classical theories. The maximum nucleation and growth rates for Cu were experimentally determined to be 8.0 × 10²¹ nuclei/m³ s at 612 °C and 0.12 nm/s at 682 °C, respectively. Using an experimentally determined “effective” activation energy for the diffusion of copper, the theoretical nucleation rate curve compared very well with the hardness data for the first 5 minutes of aging. The growth behavior of the Cu precipitates was investigated through use of a conventional transmission electron microscope (TEM) for samples directly aged at 550 °C. For aging times up to 21 hours, the average precipitate size scaled with a time dependence of t ^1/2.