Effect of rolling temperature on the deformation and recrystallization textures of warm-rolled steels

Warm-rolling trials were carried out on three interstitial-free (IF) steels (stabilized with either niobium or titanium), an extralow-carbon (ELC) steel, and an experimental low-carbon chromium (LC Cr) material at temperatures between 440 °C and 850 °C. The influence of rolling temperature on their as-rolled microstructures and deformation and recrystallization textures was investigated. Also, the effect of coiling simulation and degree of rolling reduction on the r values of some of these materials was examined. In-grain shear bands were evident in all as-rolled microstructures, but their sensitivity to deformation temperature varied between steels. Shear bands of moderate intensity were formed in the IF steels across all temperatures. In the ELC material, intense shear bands were formed at low rolling temperatures, but at higher temperatures, this intensity was drastically reduced. The development of shear bands at the higher rolling temperatures was significantly enhanced by alloying with chromium. The deformation textures produced were typical of rolled ferrite materials. The intensity of this texture increased markedly with temperature for the ELC grade. Conversely, the intensity of the recrystallization texture decreased with increasing temperature. The addition of chromium was found to strengthen the {111} component and, hence, the formability. The sharpness of both the deformation and recrystallization textures of the IF steels was relatively unaffected by rolling temperature. These differences are attributed to the intensity and frequency of shear-band formation and the dynamic strain-aging (DSA) behaviors of the various materials.     

Microstructural model for hot strip rolling of high-strength low-alloy steels

The microstructural evolution during hot-strip rolling has been investigated in four commercial high-strength low-alloy (HSLA) steels and compared to that of a plain, low-carbon steel. The recrystallization rates decrease as the Nb microalloying content increases, leading to an increased potential to accumulate retained strain during the final rolling passes. The final microstructure and properties of the hot band primarily depend on the austenite decomposition and precipitation during run-out table cooling and coiling. A combined transformation-ferrite-grain-size model, which was developed for plain, low-carbon steels, can be applied to HSLA steels with some minor modifications. The effect of rolling under no-recrystallization conditions (controlled rolling) on the transformation kinetics and ferrite grain refinement has been evaluated for the Nb-containing steels. Precipitation of carbides, nitrides, and/or carbonitrides takes place primarily during coiling, and particle coarsening controls the associated strengthening effect. The microstructural model has been verified by comparison to structures produced in industrial coil samples.     

Austenite formation during intercritical annealing

A systematic experimental study has been conducted on ferrite recrystallization and intercritical austenite formation for two low-carbon steels with chemical compositions typically used for dual-phase and transformation-induced plasticity (TRIP) steels. Different initial heating rates, holding temperatures, and times were applied to the materials to examine the ferrite recrystallization and austenite formation kinetics. An Avrami model was developed to describe the isothermal ferrite recrystallization behavior and was applied successfully to the nonisothermal conditions. It was found that the initial heating rate affects the isothermal austenite formation kinetics for both the hot-rolled and cold-rolled materials albeit the effect is more pronounced for the cold-rolled material. This can be attributed to the interaction between the ferrite recrystallization and austenite formation processes. Furthermore, it was found that the distribution of austenite phase is also affected by the ferrite recrystallization process. When ferrite recrystallization is completed before the austenite formation (i.e., under sufficiently slow heating rate conditions), austenite is to a large extent randomly distributed in the ferrite matrix. On the other hand, incomplete recrystallization of ferrite due to higher heating rates leads to the formation of banded austenite grains. It is proposed that this observation is characteristic of simultaneous recrystallization and austenite formation where moving ferrite grain boundaries do not provide suitable sites for austenite nucleation.     

Deep drawable ultra low carbon Ti IF steels hot rolled in the ferrite region

The control of the amount of solute carbon in ultra low carbon Ti IF steels during ferrite rolling and subsequent recrystallization is of prime importance for the development of an appropriate recrystallization texture and for the production of thin deep drawable hot strips. In the present work, the effect of the solute carbon content and the rolling conditions on the recrystallization texture after ferrite rolling and on the corresponding Lankford value was quantified. Therefore, ultra low carbon Ti IF steels with different sulphur and titanium contents were rolled in the ferrite region, in order to obtain a variation in solute carbon content (from 0 to about 10 ppm) at the ferritic rolling temperatures. It was shown that a deep drawing grade (r_mean> 1.4) can be obtained if the chemical composition of the steel guarantees a complete stabilisation of the solute carbon in the austenitic temperature region and if sufficient strain (85%) is given in the finishing train at temperatures lower than about 800°C. It can be concluded that the sulphur and titanium contents have to be chosen slightly higher in comparison to the conventional Ti IF steel grades used for cold rolling and annealing.     

Microstructures developed during thermomechanical treatment of HSLA steels

One purpose of thermomechanical treatment of steels, for example the control rolling of plate, is to produce the finest uniform microstructure in the product to optimize its strength and toughness. To achieve this end requires control of the structual changes that occur during reheating, high-temperature (roughing) deformation, lower temperature (finishing) deformation, and austenite transformation. A study has been made of the effects of the deformation processing variables on the microstructural changes that occur in high-strength low-alloy (HSLA) steels in the temperature ranges in which complete, partial, or no recrystallization occurs. The experimental technique comprised a sequence of plane-strain compressions of specimens being cooled at rates controlled to simulate the rate of cooling of slabs being rolled to plates. The results show that in the complete recrystallization range a fixed pass schedule refines the initial grain sizes in steels of a wide range of compositions and initial grain sizes to about the same final size; the final recrystallized grain size decreases with deformation temperature (within the complete recrystallization range), increasing strain rate, and increasing draft,i.e., with any deformation parameter that increases flow stress; the solute content and initial grain size, which are fixed by reheat temperature, control the temperature at which complete recrystallization stops during hot rolling. Deformation in the “partial recrystallization” range causes duplexing in the austenite that cannot be removed by subsequent rolling, consequently rolling in this range should be avoided. During finishing, the height of the flattened austenite grains decreases with increasing finishing reduction, but at a rate less than the rate of decrease of specimen height, indicating that some recovery is occuring. The grain diameter of the ferrite formed from fine, elongated austenite is about half the austenite grain-boundary separation (measured in the throughthickness direction) indicating that the elongated boundaries are the primary nucleation sites for ferrite.     

Influence of hot-rolling conditions on texture development in deep-drawing steels

As the textures of deep-drawing sheet steels are important for certain material properties the purpose of this paper was to take a closer look at the hot-rolling, cold-rolling and annealing textures of different deep-drawing steel grades. Several Al-killed mild steels and vacuum-degassed Ti-IF steels have been hot rolled in the mill varying the finishing temperature (FT). After coiling, cold rolling and short-time annealing the textures at different thickness levels have been measured by means of (110)-pole figures and orientation distribution functions (ODFs). For both steel groups the textures at the surface of the hot strip exhibit a more or less pronounced shear type character. Towards the mid-thickness level (with lower FT more clearly) typical (cold-) rolling textures exist characterized by a strong {001}<110> orientation and in the Ti-IF steels additionally by a significant {112}<110> orientation density. In the case of high FT cold rolling and annealing lead to favourable {111}-textures where deep-drawing application is concerned. For the Al-killed steels lowering FT results in diffuse recrystallization textures whereas in the Ti-IF steels a sharp texture with near {223}<582> orientations can be observed which have not been known for these steels before. The results prove that the hot strip textures can be of great importance for the resulting annealing textures and the according material properties.     

Plastic anisotropy of low-carbon,low-manganese steels containing niobium

The effect of niobium additions (up to 0.23 pct) on the plastic anisotropy of cold-rolled and annealed low-carbon, low-manganese steels has been studied. When hot-rolling conditions involving coiling temperatures below 1150°F were used, increased concentrations of niobium were deleterious to the development of high plastic anisotropy. Isothermal transformation studies and special hot-rolling studies showed that when transformation from austenite to ferrite and precipitation of carbonitrides occurred at high temperatures (\s>1350°F), excellent plastic anisotropy (?gn about 2) was obtained after cold rolling and annealing. The results are interpreted on the basis of an effect of critical particle sizes or dispersions on the selection of preferred orientations during recrystallization. From electron microscope studies, the critical carbonitride size and interparticle spacing, necessary for the development of high plastic anisotropy, was estimated to be 40 to 500Å and 0.04 to 0.5 μm, respectively. However, the presence of a large amount of niobium in solid solution in ferrite is apparently deleterious to the development of high plastic anistropy, even though carbonitrides having the critical size and spacing are present. On the basis of these observations and other published work, it is suggested that second-phase particles influence the development of plastic anisotropy in rimmed and aluminum-killed steels as well as in niobium-and titanium-containing steels. Thus, the degree of plastic anisotropy produced in these steels is influenced by textures developed during annealing, which in turn is dependent on the dispersion characteristics of the particular second-phase particles present.     

Crystallite orientation distribution analysis of the cold rolled and recrystallization textures in low-carbon steels

The development of the cold rolled and recrystallization textures in low-carbon rimmed and killed steels was investigated using the crystallite orientation distribution analyses. With increasing cold reduction low-carbon steels exhibit the simultaneous development of a partial <110> fiber axis parallel to the rolling direction and a <111> fiber axis parallel to the normal direction. The strongest individual texture component rotates from a {111} <110> at 60 pct cold reduction towards a {112} <110> at 80 pct. During the early stages of recrystallization the (110) and <111> fiber textures decrease in both the rimmed and killed steels. However, the decrease in the <111> fiber texture is greater in the rimmed than in the killed steel. With further recrystallization and grain growth this <111> fiber texture increases in both steels but to a greater extent in the killed steel. The strongest individual texture component after complete recrystallization is the {111} <110>, being ∼5.5 and ∼3.0 times random in the killed and rimmed steel, respectively.     

Thermomechanical processing of microalloyed powder forged steels and a cast vanadium steel

The effects of controlled rolling on transformation behavior of two powder forged (P/F) microalloyed vanadium steels and a cast microalloyed vanadium steel were investigated. Rolling was carried out in the austenitic range below the recrystallization temperature. Equiaxed grain structures were produced in specimens subjected to different reductions and different cooling rates. The ferrite grain size decreased with increasing deformation and cooling rate. Ferrite nucleated on second phase particles, deformation bands, and on elongated prior austenite grain boundaries; consequently a high fractional ferrite refinement was achieved. Deformation raised the ferrite transformation start temperature while the time to transformation from the roll finish temperature decreased. Cooling rates in the cast steel were higher than in P/F steels for all four cooling media used, and the transformation start temperatures of cast steels were lower than that of P/F steel. Intragranular ferrite nucleation, which played a vital role in grain refinement, increased with cooling rate. Fully bainitic microstructures were formed at higher cooling rates in the cast steel. In the P/F steels inclusions and incompletely closed pores served as sites for ferrite nucleation, often forming a ‘secondary’ ferrite. The rolling schedule reduced the size of large pores and particle surface inclusions and removed interconnected porosity in the P/F steels. Formerly Postgraduate Researcher in the Department of Metallurgy and Materials Science, UMIST/University of Manchester, United Kingdom     

降低武钢CSP低碳酸洗钢SAPH370屈强比的实践

屈强比偏高是CSP低碳产品的共性问题。为降低CSP低碳酸洗钢SAPH370的屈强比,采取了不同轧制工艺(终轧温度FT7、卷取温度CT和冷却方式)进行试验,对不同工艺下的低碳酸洗钢的力学性能、晶粒尺寸和相组成进行了对比分析。结果表明:SAPH370钢采用终轧温度(FT7)为860℃、卷取温度(CT)600℃、后段快速冷却的工艺,在满足强度要求的前提下,屈强比可降低到0.8以下。观察到铁素体晶粒粗化、珠光体弥散分布。分析表明:CSP采用后段快冷工艺与传统热连轧的两段冷却工艺相当,有利于获得合适的铁素体晶粒度和弥散分布的珠光体。     

Evolution of acicular ferritic microstructure in a titanium bearing HSLA steel

An attempt has been made to design and develop acicular ferritic steels by adopting three-stage controlled rolling, with titanium alone as microadditive. Process parameters such as finish rolling temperature (FRT), amount of deformation during the last pass and also the cooling rate after finishing have been altered to obtain different microstructural effects. Optical as well as transmission electron microscopy studies indicated that acicular ferritic microstructure can be engendered by applying a minimum deformation of 25 % at the finishing pass followed by water quenching. Decreasing the FRT from 850 °C, for water quenched steels deformed 33 %, resulted in the refinement of polygonal ferrite, whereas the reverse is the case with the aspect ratio of acicular ferrite. The volume fraction of acicular ferrite progressively increases as the FRT is lowered. Evaluation of tensile properties suggested that yield strength as high as ~ 600 MPa with an elongation of ~ 25 %, could be obtained by finishing at a lower temperature of 750 °C. An impact transition temperature (ITT) of -53 °C indicated that the steel possesses adequate toughness properties. As weldability is one of the criteria for structural applications, this aspect too has been investigated.     

Fiber texture and substructural features in the caliber-rolled low-carbon steels

Caliber rolling at the recrystallization temperatures of ferrite is a new process that was developed to fabricate an ultrafine-grained microstructure for low-carbon steels. In the present investigation, the electron backscattered diffraction (EBSD) measurement was carried out to characterize thoroughly the texture and substructural features in two caliber-rolled low-carbon steels, with special attention on the effects of a phosphorus addition to the steel and the annealing treatment after rolling. Finer ferrite grains appeared in the phosphorus-added steel under the same rolling condition. The phosphorus addition caused also the stronger <110>//rolling direction (RD) fiber texture in the caliber-rolled steel bars and, hence, showed a larger average Taylor factor than the steel without phosphorus. Microband features within the ultrafine ferrite grains were characterized with both transmission electron microscopy (TEM) observation and orientation-imaging micrograph (OIM) analysis. Nearly half of the low-angle boundaries, whose kernel average misorientation was larger than 0.8 deg, were found to have the planar character and were specifically parallel to the {110} or {112} planes. In the as-rolled condition, the total volume fraction of the low-angle boundaries was 0.3 and 0.23 in the steels with and without a phosphorus addition, respectively. More {112}-type planar boundaries were observed than the {110} type boundaries in both steels. Annealing treatment increased the volume fraction and changed the type of the low-angle boundaries in both steels. Using the model proposed by Peeters et al., the critical resolved shear stress (CRSS) of the 110 and 112 slip systems was calculated by considering the contributions of both the randomly distributed dislocations and the oriented planar boundaries. We concluded that the contribution of the planar low-angle boundaries to the total CRSS was less than 2 pct. The texture features and dislocation structure in the ultrafine-grained steels influenced the mechanical behavior to some extent, in addition to the refined ferrite grains. Phosphorus added to the steels showed a larger influence on the formation of texture and the retained dislocation structure, than that on the grain refinement in the caliber-rolling process.     

Effects of Different Modes of Hot Cross-Rolling in 7010 Aluminum Alloy: Part I. Evolution of Microstructure and Texture

The current study describes the evolution of microstructure and texture in an Al-Zn-Mg-Cu-Zr-based 7010 aluminum alloy during different modes of hot cross-rolling. Processing of materials involves three different types of cross-rolling. The development of texture in the one-step cross-rolled specimen can be described by a typical β-fiber having the maximum intensity near Copper (Cu) component. However, for the multi-step cross-rolled specimens, the as-rolled texture is mainly characterized by a strong rotated-Brass (Bs) component and a very weak rotated-cube component. Subsequent heat treatment leads to sharpening of the major texture component (i.e., rotated-Bs). Furthermore, the main texture components in all the specimens appear to be significantly rotated in a complex manner away from their ideal positions because of non-symmetric deformations in the two rolling directions. Detailed microstructural study indicates that dynamic recovery is the dominant restoration mechanism operating during the hot rolling. During subsequent heat treatment, static recovery dominates, while a combination of particle-stimulated nucleation (PSN) and strain-induced grain boundary migration (SIBM) causes partial recrystallization of the grain structure. The aforementioned restoration mechanisms play an important role in the development of texture components. The textural development in the current study could be attributed to the combined effects of (a) cross-rolling and inter-pass annealing that reduce the intensity of Cu component after each successive pass, (b) recrystallization resistance of Bs-oriented grains, (c) stability of Bs texture under cross-rolling, and (d) Zener pinning by Al₃Zr dispersoids.     

Quantitative Analysis of Micro‐Textures during Recrystallization in an Interstitial‐Free Steel

Two interstitial‐free steel samples were prepared by normal and by cross rolling. The effect of the resulting different deformation textures and microstructures on the subsequent recrystallization behavior was studied by micro‐texture analysis. The differences in recrystallization textures of the two differently rolled samples can be attributed to the microstructural differences in the as‐deformed state. The orientation distribution of the recrystallized grains forming at the early stages of the recrystallization dominated the final recrystallization textures, pointing to the importance of oriented nucleation in the formation of recrystallization textures of interstitial‐free steels.     

Neutron Scattering Study on Precipitation and Recrystallization Behaviours in P‐Free and P‐Alloyed IF‐Steel Sheets

In order to investigate the effect of coiling temperature and alloying of phosphorus (P) on the formation of precipitates and recrystallization behaviour, initial samples of P‐free and P‐alloyed IF‐steels were prepared by coiling at 580 and 720°C. Variations of precipitates and textures after coiling, cold rolling and recrystallization annealing were studied by means of small angle neutron scattering (SANS), neutron texture measurements and TEM observations. Hardness measurements and microstructure observations disclose that both coiling temperature and alloying of P affect softening of cold rolled samples. Coiling at a lower temperature increases the number of fine precipitates smaller than 10 nm in the hot band. Upon recrystallization annealing, precipitates ranging in size from 10 to 50 nm increase at the expense of fine particles smaller than 10 nm. Recrystallization behaviours in cold rolled IF‐steel samples were discussed by tackling the distribution of precipitates and the amount of solute P atoms in solution.     

控轧控冷工艺对汽车大梁板组织及冷弯性能的影响

在攀钢热轧板厂对汽车大梁板进行了控轧控冷试验,分析了终轧温度、轧后冷却方式以及卷取温度对汽车大梁板显微组织及冷弯性能的影响。结果表明：采用较低的终轧温度830 ℃、卷取温度600 ℃以及前段冷却的轧后冷却方式,汽车大梁板的铁素体晶粒细小均匀,珠光体分布弥散,并获得了良好的冷弯性能。     

Low-Temperature Toughening Mechanism in Thermomechanically Processed High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels were fabricated by varying thermomechanical processing conditions such as rolling and cooling conditions in the intercritical region, and the low-temperature toughening mechanism was investigated in terms of microstructure and the associated grain boundary characteristics. The steels acceleratedly cooled to relatively higher temperature had lower tensile strength than those acceleratedly cooled to room temperature due to the increased volume fraction of granular bainite or polygonal ferrite (PF) irrespective of rolling in the intercritical region, while the yield strength was dependent on intercritical rolling, and start and finish cooling temperatures, which affected the formation of PF and low-temperature transformation phases. The steel rolled in the intercritical region and cooled to 673 K (400 °C) provided the best combination of high yield strength and excellent low-temperature toughness because of the presence of fine PF and appropriate mixture of various low-temperature transformation phases such as granular bainite, degenerate upper bainite (DUB), lower bainite (LB), and lath martensite (LM). Despite the high yield strength, the improvement of low-temperature toughness could be explained by the reduction of overall effective grain size based on the electron backscattered diffraction (EBSD) analysis data, leading to the decrease in ductile-to-brittle transition temperature (DBTT).     

控轧控冷工艺对抗H2 S腐蚀X70管线钢组织和性能的影响

根据抗H2S腐蚀X70管线钢的使用特点,采用低碳、超低硫、超低磷、控制Mn含量的技术思路,以控制MnS夹杂物数量和形态、铸坯的枝晶偏析与中心偏析。通过实验室的控轧控冷试验,分析了不同终轧温度和终冷温度对组织和性能的影响,试验结果表明,终轧温度为820℃和840℃时,均可获得准多边形铁素体+粒状贝氏体组织,随着终轧温度的降低,晶粒细化;随着终冷温度的降低,粒状贝氏体含量增加。通过实验室研究结果,确定了工业生产方案,并完成了工业试制。试验结果表明,在820℃终轧,400℃卷取可以获得组织为准多边形铁素体+粒状贝氏体、综合性能优良的产品,其抗HIC敏感测试为0,抗SSCC性能未失效。     

Effect of thermomechanical processing on the retained austenite content in a Si-Mn transformation-induced-plasticity steel

The influence of hot deformation on the microstructure of a hot-rolled Si-Mn transformation-induced-plasticity (TRIP) steel was evaluated in an effort to better control retained austenite content. In this study, axial compressive strains varying in amounts from 0 to 60 pct were imposed in the austenite phase field, and effects on the formation of polygonal ferrite, bainite, and retained austenite were determined. In addition, modifications in simulated coiling temperature from 420 °C to 480 °C and cooling rates from the rolling temperature, between 10 °C/s and 35 °C/s, were assessed. Fast cooling rates, low coiling temperatures, and low degrees of hot deformation were generally found to decrease the amount of polygonal ferrite and increase retained austenite fraction. Unexpectedly, a sharp increase in polygonal ferrite content and decrease in retained austenite content occurred when the fastest cooling rate, 35 °C/s, was coupled with extensive hot deformation and high coiling temperatures. This effect is believed to be due to insufficient time for full recovery and recrystallization of the deformed austenite, even in the absence of intentional microalloying additions to control recrystallization kinetics. The resultant decrease in hardenability allowed the ferrite transformation to continue into the holding time at high (simulated) coiling temperatures.     

热轧钢板的织构

综述了热轧钢板织构的形成机制和形成特点,分析了化学成分、加热温度、终轧温度、轧制速度、轧后冷却速度以及润滑对热轧织构的影响规律,并描述了热轧织构对热轧板塑性应变比、屈服强度和韧性的影响特点,可为热轧钢板织构和性能的优化与控制提供依据。