Controlled Rolling and Controlled Cooling Technology of Ultra-High Strength Steel with 700 Mpa Grade

With Gleeble-1500 system, the influences of rolling temperature, finishing tempera ture and cooling rate on the mechanical properties of two ultra-high strength steels were analyzed. The microstructure of the hot rolled specimens was observed by optical microscope, TEM and SEM. The TRIP of HSLA steels was studied. The results show that the yield stress of 700 MPa can be reached for two steels. The controlled rolling and controlled cooling technology has different effects on two sleds, but it is rational to adopt finishing temperature 800℃ for both of them. The microstructure of the steels is mainly bainite, and the influence factors ofmechanical properties are the size of bainite, and the size, distribution, composition and morphology of secondary phases. The deformation of high molybdenum steels at a high temperature with a high cooling rate would promote TRIP.     

水电站用低碳贝氏体钢07MnCrMoVR的组织性能研究

摘要：采用不同的宽展比对水电站用低碳贝氏体钢07MnCrMoVR进行了轧制,对回火前后试验钢的微观组织形貌进行了观察,并对力学性能进行了检验,同时利用EDS能谱分析了回火过程中碳化物析出行为。结果表明：采用较小的宽展比能提高粗轧纵轧阶段的单道次压下率以及变形区系数,有效地破碎奥氏体再结晶晶粒,轧制后获得细小的粒状贝氏体组织,高温回火后析出大量的渗碳体和合金碳化物均匀弥散地分布在贝氏体铁素体基体上。随着回火温度的提高,试验钢强度性能呈现先升高再降低的现象,伸长率和低温冲击韧性持续升高。     

Micro‐scale Strain Distribution in Hot‐worked Duplex Stainless Steel

A modified microgrid technique has been applied to a laboratory‐made duplex stainless steel, to experimentally simulate the local state of deformation of the austenite‐ferrite microstructure of low‐alloy steels subject to intercritical deformation. A sample containing such a microgrid was deformed by plane strain compression at high temperature under conditions representative of hot rolling processes. The distortion of the microgrid after hot deformation revealed, in a quantifiable manner, the plastic flow of both phases and different deformation features. The micro‐strain distributions measured can be used to validate the models predicting the hot deformation of low alloyed C‐Mn steels during intercritical rolling.     

高钢级低温管线钢的韧化机制及控制技术

针对高钢级管线钢在低温服役环境条件下韧性断裂止裂控制难题,分析了高钢级管线钢的低温断裂机理,明确了影响DWTT性能的两个重要因素。通过热模拟实验,讨论了提高DWTT性能的控制技术。采用粗轧再结晶区低温段大变形,增加粗轧阶段总累积变形量,及将精轧温度降低到临近Ar3的温度范围内,并施加足够的累积变形量,可获得一定数量的超细晶多边形铁素体,将板材心部的平均有效晶粒尺寸细化到2.5μm,使-60℃下DWTT断口剪切面积达到100%,为大口径高钢级管线钢的研发生产提供理论依据和技术参考。     

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.     

锆含量对高强韧Ti-Zr微合金化钢奥氏体组织的影响

为了满足各行业对钢铁材料提出的更高强度、更好韧性的要求,在钢中加入适量的微合金化元素并结合合理的控扎控冷工艺,有利于在高温轧制阶段获得细小均匀的奥氏体再结晶晶粒,这是提高钢材强度及韧性的有效途径之一。通过Gleeble-3800型热模拟试验机,对两种不同锆含量的低碳微合金Ti-Zr钢进行多道次压缩变形试验,模拟实际轧制情况,研究试验钢在不同锆含量和不同变形方式(等温变形和变温变形)下的热变形行为,并结合组织观察分析讨论了锆含量和变形条件对试验钢奥氏体组织细化行为和析出行为的影响。结果表明,变形温度的升高可以降低高锆钢各道次的流变应力。变温变形条件下,锆含量的升高会提高试验钢各道次的流变应力;奥氏体再结晶晶粒会随着锆含量的升高和变形温度的降低而发生细化,采用变温变形方式比等温变形方式更有利于得到细小的奥氏体晶粒,高锆钢在1 050℃→1 25℃→1 000℃变温变形后得到了最小的奥氏体平均晶粒尺寸(为8.2μm);锆含量升高会提高试验钢中析出相的数量,变形方式对析出相数量的影响不大,变形温度的升高会使析出相发生粗化。锆含量增加所导致的形变诱导析出相的增多,以及变温变形过程中温度的降低,起...     

改善轧材内裂纹缺陷的研究

圆棒轧材在生产过程中经常出现内裂纹,本文研究了变形程度和变形区力学条件对内裂纹的影响.在生产过程中进行合理控制可有效减少内裂纹.     

Influence of Temperature Evolution on Precipitation Behaviour of Second Phase Particles in Grain‐oriented Electrical Steels

A model concerning dynamic precipitation of second phase particles during hot deformation of conventional grain‐oriented electrical steels is proposed. The model involves the solid solubility of the elements forming MnS, the corresponding nucleation driving force, as well as the evolution of dislocation densities during hot deformation, which supplies sites for heterogeneous nucleation of MnS particles. The model describes the dynamic process in which temperature keeps decreasing and dislocation density keeps increasing. The influences of reheating temperature, initial rolling temperature, finishing temperature and deformation duration on the precipitation behaviour are involved. Calculations have been conducted based on the model, to simulate the precipitation behaviour of MnS particles during hot rolling of grain‐orientated electrical steels under different parameters. The results indicate that hot rolling is very important for dispersive precipitation of MnS particles. The increase of particle density is promoted by proper increase of reheating temperature or proper reduction of initial and finishing rolling temperature, in which the driving force and dislocation density are increased. The simulation results agree basically with the experimental observations in hot strip of a grain oriented electrical steel.     

Ferritic rolling to produce soft deep‐drawable thin hot strips

In a conventional production of deep‐drawable sheets of steel, a hot rolling in austenite and a cold rolling at room temperature together with a subsequent recrystallization annealing are applied with a cold strip as a final product. As a cost saving replacement for this, a thin‐gauge hot strip with a required deep‐drawability can be employed. A conventionally (in austenite) hot rolled strip is hardly applicable for this because of its unsatisfactory texture as well as because of the technical difficulties to produce hot strips with sufficiently small thicknesses. As a promising realization of cost saving thin‐gauge deep‐drawable hot strips of steel, a warm rolling can be applied with reduced finish rolling temperatures. In this practice the finish rolling is shifted down into the temperature region of ferrite. In the present work extensive laboratory tests on IF‐ and ELC‐steel were carried out by using the hot deformation simulator Wumsi. By the measurements of the texture development as well as by the computing of r‐values, the process parameters of ferritic rolling and a subsequent direct recrystallization in the coil of these steels could be optimized.     

宝钢1580热轧IF钢铁素体轧制工艺探讨

介绍了宝钢1580热轧铁素体轧制镀锡板用钢T-2.5CA的工艺参数制定及实际轧制效果。通过对其他IF钢实际轧制过程变形抗力与变形温度变化情况的分析,制定了合适T-2.5CA铁素体轧制温度制度和变形制度,并在实际轧制过程中取得良好效果,为在1580热轧进一步推广铁素体轧制提供了参考。     

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.     

精冲带钢冷连轧生产过程的卷取分析

实现厚规格精冲带钢的顺利卷取是精冲钢冷连轧生产工艺成功应用的关键。利用4辊可逆冷轧机进行轧制试验,在MMS-200热模拟试验机上测定精冲钢的应力-应变曲线,建立精冲钢冷轧终态变形抗力模型。针对1750mm冷连轧机组配置的卡罗塞尔卷取机,进行精冲带钢卷取过程的机理分析和卷取状态的受力分析,为精冲钢冷连轧极限厚度卷取能力的分析和计算提供理论支撑。在此基础上,完成了不同摩擦条件下多规格精冲钢冷连轧生产卷取机的极限厚度计算,并给出相应的安全卷取厚度窗口。该研究适合于工业生产实践,对精冲钢冷连轧生产工艺研究具有重要意义。     

Cold-rolling technologies of advanced high strength steels in Ansteel

Low - carbon becomes a high - frequency and fashionable word which gets the greatest concern in the world.Low - carbon refers to a minimal output of greenhouse gas emissions into the biosphere, specifically refers to the greenhouse gas carbon dioxide.To reduce energy consumption of automobile,more and more high strength steels are used by vehicle companies.To meet the request of vehicle companies, various high strength steels are developed in steel companies all over the world.Ansteel can provide,ultra-low carbon bake hardening(BH) steels,dual phase(DP) steels and transformation - induced plasticity (TRIP) steels with the grades of under 780 MPa,up to now.AHSS steels have much different composition, microstructure and strenght than conventional vehicle steels,so there are some trouble in producing in cold rolling mills,for example,difficult to join,poor thickness and flatness,accurate temperature and velocity control and so on.To reduce the opportunity of strip breakage,we have done many welding experiments and special research.Now,DP and TRIP steels can be continuously produced in Ansteel.To assure thickness and flatness of strips,we optimized the hot rolling parameter to get low deformation resistance,optimized rolling oil to get fit frictional coefficient and fix on the targat rolling curve.To get more accurate and repetitive results over the production shifts,the Mathematical Model(MM) is used in Ansteel.The MM analyses information transmitted by various sensors and transmitters,compares the collected data with the chosen parameters and adjusts the settings of the various pieces of equipment to hone on the parameter setpoints.At present,the key production technologies of AHSS were grasped by cold rolling mill Ansteel,and Ansteel is the first company to apply the TRIP with the grade of 780 MPa.     

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.     

Analysis of DWTT property of thick X70 pipeline steel

The microstructures of X70 pipeline steels with different thickness specifications and different rolling reduction ratios were analyzed by metallographic microscope, scanning electron microscope, and electron backscatter diffraction techniques, and the original austenite microstructures were compared. The effect of compression ratio on the original austenite structure was studied. The results show that the large compression ratio in the rolling process is more conducive to refine austenite grains and improve the low- temperature toughness of thick X70 pipeline steel. However, due to the fact that the temperature during the finish rolling process is too low to permeate the center of the deformation, it is necessary to further use the low- temperature stage of the rough rolling process to perform deformation infiltration. Ultra- rapid cold cooling after rolling can improve the DWTT performance of the steel, but it is not sufficient to compensate for the fluctuation of DWTT performance caused by insufficient early austenite grain refinement.     

Effect of the finish rolling and cooling conditions on the properties of microalloyed plate steels

The processing parameters of finish rolling and cooling of three HSLA steels were varied in a wide range applying the plane strain hot compression simulation test on the hot deformation simulator (Wumsi). Evaluating the flow curves of the deformation steps, some metallurgical phenomena in austenite during hot deformation could be determined. The results of material testing after the simulation experiments exhibit a strong correlation between the finish rolling and cooling conditions and the final mechanical properties of the steels tested. As a way to increase strength, lowering finishing temperatures and/or accelerating cooling after finish rolling proved to be most effective. To improve notch toughness, on the other hand, a high total finishing strain should be applied as well as low finishing temperatures, however, above A_r3.     

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.     

Deep‐drawing quality of cold rolled sheet made of ferritically rolled hot strip

In the so‐called ferritic rolling the finishing is shifted down into the temperature region of ferrite, which enables a production of thinner hot strips (compared to the conventional hot rolling in austenite) with a changed texture development. The present study is focused on the effect of the process parameters of such initial warm rolled hot strips on the texture formation and hence on the deep‐drawing quality of the final cold strips. A special attention is given to the transmission of the hot strip texture to the cold strip. The investigation was carried out on commercial IF‐ and ELC‐steels by a laboratory simulation with the hot deformation simulator Wumsi (rolling simulated by the plane strain compression test). It was possible to optimize cold rolling and subsequent annealing by means of the measurements of the texture development during the ferritic (warm) deformation as well as the parameters of the processing route of such special production of cold strips by means of the calculation of r‐values. If compared with the conventional processing route, a considerable improvement of deep‐drawability with a diminished earing is achievable especially with the IF‐steel.     

Effects of Cold Rolling Reduction on Retained Austenite Fraction and Mechanical Properties of High-Si TRIP Steel

 Transformation-induced plasticity-aided steel ［TRIP steel (0. 4C-1. 5Si-1. 5Mn)］ was rolled at room temperature to different thickness reductions (0, 4%, 10%, 20%, 40%, and 60%). The mechanical properties, microstructure and austenite fractions of the rolled samples were measured by tensile test, electron back scattered diffraction (EBSD) and X-ray diffraction (XRD) for each rolling. The deformation behavior was studied based on the analysis of the mechanical properties and microstructure of steel after tensile deformation, aiming at understanding the effects of cold rolling reduction on the decay behavior of the austenite and the change of mechanical properties of the TRIP steels. It was found that increasing rolling reduction increases the yield stress gradually but decreases the total elongation significantly. It is very interesting that after 10% rolling reduction the yield stress is about 1000 MPa but still with 20% total elongation, which gives an excellent combination of yield strength and ductility. Based on the XRD results, it was revealed that in both rolling and tension the austenite volume fraction monotonically decayed with the increase of rolling strain, but the decaying rate is faster in tension than in rolling, which may be ascribed to the higher temperature in rolled specimens than in the tensioned ones during deformation. Experimental results and theoretical reasoning indicate that the decreasing trend of austenite volume fraction with strain can be formulated by a unique equation.     

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。