Effects of Niobium Addition on Microstructures and Formability in Si‐Al‐Mn TRIP Cold‐rolled Steels

The effects of Nb addition on microstructures and formability in Si‐Al‐Mn TRIP cold‐rolled steels were investigated. These steels were intercritical annealed at 770 °C for 5 min, and isothermally treated at 400 °C for 3 min. Microstructural observation, tensile tests and forming limit diagram (FLD) tests were conducted, and the changes of retained austenite volume fraction as a function of tensile strain were measured by using an X‐ray diffractometer. The results showed that Nb addition makes grain size refined, the volume fraction of ferrite increase and that of bainite decrease, however, obviously it does not affect the volume fraction and carbon content of retained austenite. The Nb addition increased the stability of retained austenite owing to grain refinement. With Nb addition, increase in strength, ductility, strain hardening exponent and formability could be achieved simultaneously. These findings indicate that Nb addition can be a new direction of microalloying design for the low carbon TRIP steels with excellent formability and high stability of retained austenite.     

Laboratory Investigations on Copper‐alloyed Interstitial Free Steel – Part II: Effect of Coiling Temperature

The effect of coiling temperature on the annealing behaviour of copper‐alloyed interstitial free steel has been studied during batch and continuous annealing. The batch annealing kinetics undergoes a severe sluggishness in the so‐called industrial low temperature coiling condition, while retardation is less with high temperature coiling. The mechanism is believed to be the particle pinning effect exerted by peak‐aged or over‐aged copper precipitates. Room and high temperature coiled materials show similar strength and ductility after batch annealing. However, the texture development is different in the two cases, which leads to a variation in deep drawability. Copper precipitation has been observed to give rise to an unusual trend in strain hardening with the progress of batch annealing. The strength and formability parameters of the continuous annealed copper‐alloyed interstitial free steel do not depend on the coiling temperature due to dissolution of copper precipitates of the hot rolled material during continuous annealing. Irrespective of the coiling temperature continuous annealed copper‐alloyed interstitial free steels are as good as conventional interstitial free steels particularly in formability parameters.     

1750mm冷连轧精冲钢的连续退火工艺

连续退火工艺制度是精冲钢冷连轧生产过程中的重要环节,对其成品组织性能有着极其重要的影响。在实验室进行轧制润滑试验研究,设计了多种连续退火工艺方案,采用CAS- 300连续退火试验机模拟连续退火试验,确定了C15/2精冲钢的再结晶温度为550℃。针对不同规格冷连轧精冲带钢,通过拉伸试验和显微组织鉴定,给出了适用于冷连轧生产实践的连续退火工艺制度。结果表明,退火后试样的屈服强度控制在(310±20)MPa,抗拉强度小于450MPa,伸长率大于30%,获得了良好综合力学性能的精冲钢组织。该研究完全满足工业生产实践的需求,对精冲钢冷连轧生产退火工艺研究具有重要意义。     

Gas‐pressure Forming of an AlMg‐Alloy Sheet at Elevated Temperatures

Forming of automotive leightweight parts using aluminium offers numerous advantages. Compared to other wrought aluminium alloys, in particular AlMg‐alloys generally show a good formability which is favourable for the production of complex parts. However, forming of Mg‐containing alloys at room temperature leads to yielding patterns preventing their implementation for class‐A‐surface applications. Furthermore, the formability of steel still exceeds that of AlMg‐alloys at room temperature. Thus, in the present study, sheet metal forming is applied at a temperature range that is typical for warm forming. It is supposed to profit from the advantages of warm forming like high achievable strains and improved surface quality of the formed part, while not having the disadvantages of long production times and high energy consumption, which is correlated with superplastic forming. Applying fluid‐based sheet metal forming in this paper, nitrogen is used as fluid working medium to satisfy the demand on high temperature resistance. Concerning the blank material used, formability of Mg‐containing aluminium alloys shows strong strain rate sensitivity at elevated temperatures. To figure out the optimal strain rates for this particular process, a control system for forming processes is developed within the scope of this paper. Additionally, FE‐simulations are carried out and adapted to the experiment, based on the generated process data. FE‐investigations include forming of domes (bulging) as well as shape‐defined forming, having the objective to increase formability in critical form elements by applying optimal strain rates. Here, a closed‐loop process control for gas‐pressure forming at elevated temperatures is to be developed in the next stages of the project.     

Low‐alloyed TRIP‐Steels with Optimized Strength,Forming and Welding Properties

To obtain the superior strength‐ductility‐balance of TRIP‐grades, a special chemical composition in combination with well adapted processing parameters are a prerequisite. Despite of their excellent formability performance in terms of drawability as characterized by high n‐ and elongation values, compared to mild steels TRIP‐grades are challenging in the press and the body shops. The high strength level in combination with the high work hardening of TRIP‐grades result in higher levels of spring back compared to mild steels and higher press forces are required. Furthermore, a higher sensitivity to failure for sharp bending radii and a deterioration of the formability of punched edges is reported for TRIP‐grades. While spring back can only be minimized by advanced forming processes supported by new simulation techniques with improved ability to predict spring back, the sensitivity to failure under special forming conditions can be influenced by optimizing microstructural features. Contrary to the forming behaviour, which is influenced significantly by the microstructure, the weldability is mainly governed by the chemical composition and the surface condition of the material. The high carbon content of TRIP‐grades compared to mild steels results in a higher hardening potential after welding. Additionally, a fracture behaviour untypical for mild steels after destructive testing of spot welds is sometimes observed for TRIP‐grades, which is assessed critically by some OEMs. In this work, after a discussion of the processing conditions, possibilities are demonstrated to improve the forming behaviour by an optimization of the microstructure and the spot weldability by adapting the chemical composition of low‐alloyed TRIP grades. First very promising results for TRIP‐grades with a minimum tensile strength level of 700 MPa are discussed.     

Effects of Process Parameters prior to Annealing on the Formability of Two Cold Rolled Dual Phase Steels

This study concerns the effects of coiling temperature after hot rolling and the degree of reduction during cold rolling on formability‐related properties of high strength cold rolled dual phase (DP) steels. The effect of coiling temperature on the final structure and properties of two cold rolled and annealed DP‐steels is investigated. Further, the effect of cold rolling reduction and its impact on the final properties of the material is studied. Aspects of the impact of the different process parameters on the ferrite to austenite and austenite to martensite transformation are discussed based on results from production scale experiments, tensile testing and metallographic examinations of the materials.     

Advanced Cold Rolled Steels for Automotive Applications

Advanced high‐strength steels offer a great potential for the further development of automobile bodies‐in‐white due to their combined mechanical properties of high formability and strength. They represent the first choice in material selection for strength and crash‐relevant parts with challenging geometries. The intensive development of multiphase steels by ThyssenKrupp Steel has led to hot dip galvanizing concepts with an outstanding forming potential. Hot rolled, hot dip galvanized complex‐phase steels are currently produced in addition to cold rolled dual phase (DP) and retained austenite (RA) or transformation induced plasticity (TRIP) steels. New continuously annealed grades of steel are being developed with tensile strength levels of up to 1000 MPa in combination with sufficient ductility for the high demands of structural automobile components. These steels make use of the classic advantages of microalloying as well as the principles of DP steels and RA / TRIP steels. Further improvement of properties will be reached by the new class of high manganese alloyed steels.     

Deformation Inhomogeneity and Texture Development brought about in Steel Strip by Multi‐Step Cold Rolling

Predicting the microstructure obtained in cold‐rolled strips is of major importance with regard to optimizing the production of deep‐drawing steels with high and defined mechanical and technological properties. By using the hydrodynamic rolling theory of A. Kneschke, it has been possible to determine the effects of the most important rolling parameters and also of the rolling strategy on the velocity field and on the in‐homogeneous distribution of deformation over the strip thickness. Rolling on experimental laboratory facilities has provided the opportunity to observe the texture evolution. The tests were carried out on two deep‐drawing steel grades IF and ULC. The initial thickness of the strip samples was 3.0 mm. The textures were determined at 3 locations over the strip thickness by means of pole figure measurements according to the X‐ray diffraction method.     

精轧温度对含铌Cr17铁素体不锈钢组织、织构及成形性的影响

 以铌稳定化的Cr17铁素体不锈钢为试验材料,系统研究了精轧温度,即高温精轧和低温精轧,对组织、织构和成形性能的影响。2种不同工艺的热轧板经相同的热带退火、冷轧及退火处理后,分别采用金相显微镜及X射线衍射技术观察2种工艺条件下的组织和织构演变。研究结果表明：与高温精轧相比,低温精轧有利于得到细小及均匀的冷轧退火组织;有利于冷轧退火板形成较强的γ纤维再结晶织构,并消除γ纤维再结晶织构的偏转。因此,低温精轧能够显著提高冷轧退火板的成形性能。     

Formability of High Strength Dual‐phase Steels

The application of ferritic‐martensitic dual‐phase (DP) steels has become an increasing trend in the automotive industry due to the possibility to achieve significant weight reduction and fuel efficiency with improved crash performance while keeping the manufacturing costs at affordable levels. In order to meet the different design requirements of individual auto‐body components, a wide variety of DP grades exhibiting different strength and ductility levels is currently industrially produced. Despite the numerous studies on the relationship between the mechanical properties and the microstructural characteristics of DP steels over the last decades, it is still a challenge to increase their formability at a constant strength level (or equivalently increasing the strength while maintaining a high ductility). One of the possibilities to increase strength is grain refinement. Ultrafine‐grained ferritic‐martensitic microstructures were produced by intercritical annealing of a cold‐rolled, pre‐processed dual‐phase steel. Ferrite mean grain sizes in the order of ~ 1.5 μm were obtained. The mechanical properties of these steels are studied, revealing the beneficial effect of grain refinement. Ultimate tensile strength above 800 MPa is achievable, while reaching remarkable high uniform and total elongations, which are only slightly affected by the martensite volume fraction. Moreover, the yield to tensile strength ratio can be adjusted between 0.4 and 0.5. Light and electron microscopy investigations, fracture profile and fracture surface analyses, hole expansion tests and additional ultramicrohardness measurements are used for the interpretation of the results and for the correlation of the mechanical properties and the formability characteristics with the microstructure of the steel.     

Improving the r‐value of warm rolled and annealed low carbon steel by adding chromium

The r‐value of warm rolled and annealed low carbon steel sheet is prohibitively low for many forming applications. It can be improved by removing carbon from solution but this requires costly vacuum degassing as well as titanium and/or niobium addition. The present work explores the cheaper possibility of using Cr addition to improve the r‐value and hence drawability of warm rolled and annealed steel strip. The addition of Cr was found to increase the degree of in‐grain shear band formation occurring during rolling. This can be explained by the effect that Cr has on the strain rate sensitivity of the flow stress. When the Cr added grades were annealed the r‐value was increased by up to 0.8. The maximum r‐value seen here was 1.7. The optimum rolling conditions comprised an average warm rolling temperature of 675°C and a total rolling reduction of 80%. The optimum annealing condition was a simulated batch annealing cycle. More work is required to understand the complex annealing phenomena displayed by these steels.     

Fatigue Behavior of Dual‐Phase and TWIP Steels for Lightweight Automotive Structures

Car bodies are increasingly made with high‐strength steels, for both lightweighting and safety purposes. Steel sheets, made by continuous casting, hot rolling, cold rolling, and continuous heat treating, are used to deep draw the car body parts, which are then joined by resistance spot welding (RSW). Two high‐strength automotive steels, with similar tensile strength, are studied here. The low alloy, dual‐phase steel consist of ferrite and martensite, obtained by an intercritical heat treatment, followed by fast cooling. The innovative, high‐Mn TWIP steel exhibits a promising combination of strength and toughness, arising from the austenitic structure, strengthened by C, and from the twinning induced plasticity effect. Tensile specimens are fatigue tested at room temperature with zero load ratio, both in the as‐fabricated (unnotched) condition and after the RSW of an homologous sheet square. Moreover, pre‐cracked compact tension specimens are tested with load ratio 0.1 to determine the fatigue crack growth behavior. These results are completed with crystallographic, microstructural, tensile, and fractographic examinations, and the influence of the microstructure and of the welding process is discussed.     

The Effect of Recrystallization on Interphase Precipitation in Hot-Rolled Steel Sheet

Hot-rolled steel products with high strength and good formability are in demand for automobile body parts, particularly steels which can reduce weight without sacrificing vehicle safety. Recent studies have suggested that interphase precipitation (IP) hardening is a promising approach for obtaining excellent high strength and superior formability from low-alloy steels. However, the effects of hot rolling conditions and alloying elements on IP hardening have not been clearly determined. In this study, we sought to clarify the above effects by analyzing the recrystallization behavior during hot rolling. As a result of sample testing and analysis, it was determined that the recrystallization which occurs during hot rolling plays a critical role in enhancing the IP hardening of low-alloy steels.     

Development of high strength low alloy TRIP‐aided steels with annealed martensite matrix

Formable high‐strength low‐alloy TRIP‐aided sheet steels with annealed martensite matrix or TRIP‐aided annealed martensitic steel were developed for automotive applications. The steels possessed a large amount of plate‐like retained austenite along annealed martensite lath boundary, the stability of which against the strain‐induced transformation was higher than that of the conventional TRIP‐aided dual‐phase steel with polygonal ferrite matrix. In a tensile strength range between 600 and 1000 MPa, the TRIP‐aided annealed martensitic steels exhibited superior large elongation and reduction of area. In addition, the steels possessed the same excellent stretch‐flangeability and bendability as TRIP‐aided bainitic steel with bainitic ferrite matrix. These properties were discussed by matrix structure, a strength ratio of second phase to matrix, retained austenite stability, internal stress in matrix and so on.     

Thixo‐Forging and Thixo‐Joining of an Integrated Product

In an effort to investigate thixo‐joining of aluminium alloy AISi7Mg with bolts of different metals, a series of experiments was carried out. An aluminium master part of high geometric complexity was thixo‐forged. Then thixo‐joining experiments were conducted. While the thixo‐forging of the aluminium master part being completed, bolts of brass, copper, plain carbon steel and stainless steel were integrated into the thixo‐forged part in one step. Evaluations of the produced parts were performed with X‐ray inspection, microstructure and segregation analyses. The experimental results confirm that the aluminium alloy A356 has good formability in semi‐solid state and that the thixo‐joining of the metal bolts with the aluminium part in one step is feasible.     

CSP低碳钢板的组织和性能

对采用EAF-CSP工艺生产的ZJ330低碳钢热轧板进行了组织、性能和夹杂物分析。结果表明：成品板的晶粒细小、均匀、强度较高、拉伸试样的断口为韧性断口；EBSD分析表明：成品板组织中铁素体晶粒间基本为大角度晶界，择优取向不显著。由于薄板坯连铸时的凝固和冷却速度快，钢水洁净度高，使得夹杂物含量少、尺寸小、钢板的伸长率高。     

Work Hardening Behavior of Ultrafine Grained Duplex Medium‐Mn Steels Processed by ART‐Annealing

The ultrafine grained duplex steels were fabricated by austenite reverted transformation annealing of the medium manganese steels after quenching or cold rolling. The microstructures were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and X‐ray diffraction (XRD). The mechanical properties were determined by uniaxial tensile test. It was demonstrated that both the quenched and cold rolled structures were transformed into ultrafine grained duplex structures with large fractioned austenite by ART‐annealing. Long time annealing is essential to obtain the large fractioned austenite in quenched steel, but only short time annealing is needed to get large fractioned austenite in the cold rolled sheet. The mechanical examinations indicated that ART‐annealing results in the superhigh tensile elongation (>40%) and superhigh strength (1000 MPa) in quenched steels after long time annealing but in cold rolled steels after short time annealing. Based on the analysis on the work hardening behaviors of these ART‐annealed steels, the abnormal work hardening rate was presented and analyzed. The substantially enhanced ductility was attributed to the Lüders band propagation of the ferrite matrix and/or the TRIP effects of the large fractioned austenite. At last the dynamic phase natures of both fraction and stress was proposed to interpret the abnormal hardening behaviors and the “S” shaped stress–strain curves.     

Improvement of weldability of TRIP steels by use of in‐situ pre‐ and post‐heat treatments

Low alloy TRIP‐aided steels are very interesting for the automotive industry as they combine both a high strength and an excellent formability. Though the actually developed TRIP steels can be considered as low alloyed when compared to the first generations of steels exhibiting TRIP effect, due to their chemical composition, they still exhibit a quite high carbon equivalent. This is particularly detrimental for the weldability of those materials. After solidification, welds are very hard and can show a brittle behaviour. The hardness of the heat affected zone of the welds can even exceed 500HV and cold cracking phenomena is prone to occur. In the automotive industry, spot welding is the main joining process. During spot welding of TRIP steels, the interface between the plates can act like a notch and promote fracture of the weld. This is particularly dangerous when brittle welds are submitted to peel stresses. The aim of the paper is to demonstrate that a careful choice of the process parameters can significantly improve the resistance of the welds. The selection of the welding cycle parameters is far from being an easy task as many different parameters are involved. Therefore, a design of experiment methodology (DOE) was chosen to optimise the welding cycle for a cold‐rolled TRIP steel with a tensile strength above 700 MPa. Mechanical properties of the welds were significantly improved by use of pre‐ and post‐heat treatments. Those improved welding cycles were realised without excessive extension of the total weld cycle on a conventional spot welding machine. This means that the optimised welds can be obtained in the existing production lines without any additional investment or significant decrease in productivity.     

Effect of hot‐rolling conditions on the tensile properties of multiphase steels exhibiting a TRIP effect

TRIP‐assisted multiphase steels have been thoroughly studied in the cold‐rolled and annealed state. The effects of hot‐rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP‐assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low‐carbon steel (mass content of 0.15 %) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium‐carbon steel (mass content of 0.4 %) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non‐recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low‐carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work‐hardening capabilities. In the case of the medium‐carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low‐carbon steel, the improvement of the work‐hardening behaviour was attributed to an Interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix.