Influence of Thermo‐Mechanical Processing Parameters and Chemical Composition on Bake Hardening Ability of Hot Rolled Martensitic Steels

In recent years, the increasing demand for advanced high‐strength steels (AHSS) has mainly been driven by the automotive industry and the need to reduce weight and to improve safety. Besides good ductility and high strength, AHSS have a high bake hardening and ageing effect, giving additional contribution to the strength of structural parts, subjected to the paint baking process. This paper investigates their bake hardening behaviour in dependence of hot rolling parameters and chemical composition, however, focussing on martensitic steels. Tosimulate the finishing steps of the hot rolling process with slight changes in reduction and temperature and their influence on the final mechanical properties of hot rolled martensitic steels, different thermo‐mechanical paths were applied. The increase in strength due to bake hardening was determined for different thermo‐mechanical schedules. Additionally, samples of different chemical compositions within the characteristic industrial tolerance range were studied under variation of pre‐load conditions, simulating the thermo‐mechanical hot rolling process. The samples were then subjected to bake hardening to study the varying chemical composition on this effect. Furthermore, the local use of bake hardening and ageing in hot rolled multiphase steels was investigated. It could be shown that characteristic values integrally describing the ageing effect, depend on the deformation path and the degree of pre‐strain, as well as on temperature and duration of the subsequent heat treatment. This partial ageing is stable and has a potential to be used for local strengthening of the steels.     

Influence of Pre‐Straining and Bake Hardening on the Strain Rate Sensitivity of Automotive Sheet Steels

The present investigation deals with the influence of pre‐straining with or without bake hardening on the strain rate sensitivity of automotive sheet steels in typical crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 0.005‐1000 s^?1 and in the temperature range 233‐373K. A bake hardening heat treatment at 170 °C for 20 min without pre‐straining does not influence the m‐value in comparison to the base material condition. A small pre‐straining near plane strain condition, as commonly found in outer door panels, or a 10% uniaxial, plane strain and biaxial pre‐straining, as typically used in formed automotive crash components, without bake hardening does not affect the m‐value of sheet steels in comparison to the base material condition. Uniaxial 2% to 10% pre‐straining, longitudinal or transverse to rolling direction with subsequent bake hardening, does not clearly change the m‐value in comparison to the base material condition either. Small differences in the strain rate sensitivity behaviour are rather attributed to experimental scattering without real physical background.     

In‐situ Synchrotron XRD Analysis of the Effect of Static Strain Aging on the Elasto‐plastic Transition in CMnSi TRIP Steel

In situ synchrotron X‐ray diffraction was used to investigate the martensitic transformation kinetics, lattice straining and diffraction peak broadening in cold‐rolled TRIP steel during tensile testing. Direct evidence of stress‐strain partitioning between different phases, dislocation pinning and differences in yielding behaviour of the different phases were clearly observed. The TRIP steel was subjected to a bake‐hardening treatment and a pronounced static strain aging effect was observed. In the present work, the martensitic transformation kinetics and the elastic micro‐strain evolution for both ferrite and retained austenite during the elasto‐plastic transition are reported with an emphasis on bake‐hardening with and without pre‐straining.     

Local Use of Ageing Effects in Multiphase Steels for Designing Local Properties of Constructional Elements

This paper describes processes leading to local bake hardening (BH) effects in multiphase steels. The investigations are part of a comprehensive project which investigates the influence of the deformation path and of the temperature and duration of thermal treatments on strengthening in modern multiphase steels, in regard to both local and bulk properties of steel structures. Dual phase (DP), retained austenite steels (TRIP) ‐ both hot and cold rolled ‐ and complex phase (CP) steel are investigated to examine the effect of thermomechanical processing parameters on local bake hardening ability. For this purpose two ways to achieve a local BH effect, i.e. local deformation and local heat treatment, are studied, as well as ageing stability of the adjusted strength. Hardness increased after local deformation through bending as result of work hardening and bake hardening effect. The local heat treatment leads to an improvement of mechanical properties (hardness and strength) and to local strengthening of material. The stability of the local bake hardening effect could be confirmed.     

各向同性钢与烘烤硬化钢的烘烤硬化性和抗凹陷性

 研究了烘烤温度、烘烤时间和预拉伸应变量对罩式炉退火工艺生产的各向同性钢的烘烤硬化性的影响，并与力学性能相当的冷轧烘烤硬化钢进行了对比。结果表明，在不同的烘烤条件下，各向同性钢的烘烤硬化值均低于同样强度级别的烘烤硬化钢。抗凹陷性测试结果表明，各向同性钢的抗凹陷性低于烘烤硬化钢。汽车外板实物分析结果表明，两种钢冲压成形后再烘烤，其屈服强度几乎没有提高。     

烘烤硬化高强深冲冷轧汽车板BH340的研制开发

介绍了烘烤硬化高强深冲冷轧汽车钢板BH340的开发.试验证明,鞍钢生产的烘烤硬化冷轧钢板BH340是具有适中烘烤硬化值(BH值)及良好成形性的汽车用钢,能够满足汽车实际生产工业的要求.     

Effect of Overaging on Solute Distributions and Bake Hardening Phenomenon in Bake Hardening Steels

 Specimens of two different kinds of bake hardening steels (BH-Mn and BH-P) were prepared and treated with different annealing processes (water quenching and overaging). A novel technique of three dimensional atom probe was used to investigate solute distributions in these steels. The results indicate that C concentration decreases, whereas V increases during overaging in both bake hardening steels. The conclusion that no vanadium carbides precipitate during the overaging is therefore originally obtained by microanalysis in bake hardening steels. Moreover, bake hardening values of all the specimens were tested by tensile experiments with 2% pre-deformation. However, those of overaged specimens were further measured with higher levels of pre-deformation because no bake hardening phenomenon was present at 2% pre-deformation. As the pre-deformation increases from 2% to 6% and 8%, both overaged steels show bake hardening values, and the value data are almost the same.     

Influence of Carbon on Mechanical Properties of Cu bearing Extra Low Carbon Steel Sheets

This paper examines the effect of carbon (C) on solid solution strengthening, bake hardening and anti‐strain aging property for copper (Cu) bearing extra low carbon (ELC) steel sheets. For this purpose, five ELC steels that contain different content of C were selected. We have investigated the effect of C on mechanical properties and microstructures for the continuous annealed ELC steel sheets. Mechanical properties and microstructures were analyzed as well using uni‐axial tensile test and electron back‐scattered diffraction (EBSD) technique following pilot rolling and continuous annealing. It has been found that the addition of C increases the solid solution strengthening as well as 19.9MPa per 0.0010wt%C in yield strength. What is more, the addition of C increases the bake hardenability (BH) as well as 18.7MPa per 0.0010wt%C. In addition, the addition of C delays the recrystallization during continuous annealing process. From an industrial standpoint, it is possible to control both a stable anti‐strain aging property and high bake hardenability for the ELC steel sheets without Ti and Nb addition.     

Study of Bake Hardening Effect on Laser Welded Hot Rolled Ferrite‐Bainite Dual Phase Steels

The influence of bake hardening (BH) effect on laser welded hot rolled ferrite‐bainite dual phase steel (FBS) with respect to the process conditions was investigated. The samples were firstly laser welded and then pre‐strained. Pre‐strained the samples with defined degrees of deformation and a subsequent aging treatment leads to enhanced hardness and strengthening for both conditions. The microstructure of the fusion zone (FZ) and heat affected zone (HAZ) was studied. A high volume fraction of martensite could be observed in the FZ as well as in the HAZ. Both steels exhibited a clear BH effect in both, the as‐received and the laser welded conditions. The BH effect is more pronounced in the pre‐strained laser welded condition.     

Some Strengthening Methods for Austenitic Stainless Steels

Austenitic stainless steels possessing good corrosion resistance have recently found growing applications as a constructional material. In this instance, increasing strength properties, which are typically quite low, is of great interest. Due to the low stacking fault energy, strain hardening of alloyed austenite is efficient for increasing tensile strength without impairing ductility seriously. In addition, certain grades are unstable, so that cold working creates strain‐induced martensite that enhances strengthening. Grain size refinement to micrometer scale or even finer can also increase the yield strength, still providing good ductility. In the present paper dislocation and phase transformation strengthening and thereby properties achievable in temper rolled austenitic stainless steels are discussed. Strengthening by the reversion annealing is also described and excellent results achievable are shown. Finally, the effect of bake hardening through the static strain ageing is presented. Long‐term research work in various projects indicates that the current knowledge of strengthening of austenitic stainless steels is close to the industrial utilisation.     

The bake hardening behaviour of electro‐galvanized cold rolled CMnSi and CMnAlSi TRIP steels

Cold rolled, intercritically annealed TRIP steels will very likely be used as electrogalvanized sheet steel for passenger safety‐related automotive body parts. This implies that their properties during and after manufacturing, i.e. in the deformed and paint‐baked state, must be known to evaluate their actual in‐service properties. The ageing behaviour of electrogalvanized TRIP steels and the effect of controlled ageing of the complex microstructure by paint baking on the mechanical properties was therefore investigated. Electroplating tests showed that the H uptake during pretreatment and electrogalvanization is limited and that the paintability of the electroplated sheet steel is unaffected even after deformation induced transformation of the austenite. In addition, a strong bake hardening effect was found in the case of the CMnAlSi TRIP steel. Internal friction measurements indicated that a damping peak for interstitial C could be obtained but that the free C content was less than 1 ppm prior to paint baking. A BH mechanism for TRIP steels is proposed.     

Effect of Bake‐Hardening on the Structure‐Property Relationship of Multiphase Steels for the Automotive Industry

The effect of a bake‐hardening (BH) treatment on the microstructure and mechanical properties has been studied in C‐Mn‐Si TRansformation Induced Plasticity (TRIP) and Dual Phase (DP) steels after: (i) thermomechanical processing (TMP) and (ii) intercritical annealing (IA). The steels were characterized using X‐ray diffraction, transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). All steels showed high BH response. However, the DP and TRIP steels after IA/BH showed the appearance of upper and lower yield points, while the stress‐strain behavior of the TRIP steel after TMP/BH was still continuous. This was due to the higher volume fraction of bainite and more stable retained austenite in the TMP/BH steel, the formation of plastic deformation zones with high dislocation density around the “as‐quenched” martensite and “TRIP” martensite in the IA/BH DP steel and IA/BH TRIP steel, respectively.     

不同成分体系对超低碳烘烤硬化钢性能的影响

研究了不同成分体系超低碳烘烤硬化钢的性能特征,生产结果表明:Ti-BH钢屈服强度和BH值较低,伸长率和"值富裕量较大,并且BH值波动较大.通过对不同成分体系钢板的微合金化原理进行分析,确定了不同成分体系烘烤硬化钢性能不同的原因,并据此将烘烤硬化钢的成分体系由Ti-ULC改为Nb+Ti复合体系.更改后的烘烤硬化钢性能指标...     

Microstructure and Properties of Ti and Ti+Nb Ultra-Low Carbon Bake Hardening Steels

 Hot rolling, cold rolling and continuous annealing processes of Ti bearing and Ti+Nb stabilized ultra low carbon bake hardening(ULC-BH) steels were experimentally studied. The microstructure and texture evolution, as well as the morphology, size and distribution of the second phase precipitates during the hot rolling, cold rolling and continuous annealing were also analyzed. The results showed that the size of NbC precipitates in Ti+Nb stabilized ULC-BH steel was smaller than that of TiC precipitates in Ti bearing ULC-BH steel, this caused the average grain size of Ti+Nb stabilized ULC-BH steel to be finer than that of Ti bearing ULC-BH steel, for the yield strength, the former was higher than the latter, but for the r value which reflecting the deep drawing performance, the former was lower than the latter.     

Bake Hardening of Laser Welded Dual Phase Steel

The influence of bake hardening on the mechanical properties of laser welded dual phase steel was investigated. A remarkalbe increase of the hardness in the zone influenced by laser welding was observed. The fusion zone had a low carbon lath martensite microstructure. The laser weld region had a higher interstitial carbon content than the base material. The dual phase steel exhibited a clear bake hardening effect in both the as‐received and the laser‐welded conditions. The bake hardening effect is more pronounced in the prestrained laser welded condition. A pronounced decrease of the ductility was observed for prestrained laser welded DP steel.     

平整对罩式退火生产BH钢板力学性能和自然时效性能的影响

研究了平整对罩式退火生产的BH钢板力学性能和自然时效性能的影响。结果表明，平整对BH钢的BH性能有显著影响，平整延伸率在1．0％～2．0％之间时BH值有最大值；平整使得BH钢的屈服强度先下降而后上升，屈服强度最小时的平整延伸率消除了钢板单向拉伸时的屈服点延伸现象；足够的平整延伸率是BH钢抗自然时效性能的有效保证。实验结果在工业生产中得到了应用。     

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.     

Strengthening Mechanism of a New 700 MPa Hot Rolled High Strength Steel

The microstructural evolution in a 700 MPa hot rolled high strength steel was analyzed in terms of strengthening mechanisms.The results show that the hot rolled sheet steel has yield strength of 710 MPa with good elongation and toughness.The strength of the developed 700 MPa hot rolled high strength steel is derived from the cumulative contribution of fine grain size,dislocation hardening and precipitation hardening.The fine grain strengthening and precipitation hardening are the dominant factors responsible for such high strength,and the amount of precipitation hardening is two or four times higher than that of conventional microalloyed hot rolled sheet steels reported in the past.Good toughness is due to refinement of ferrite grain size.     

连续退火工艺对超低碳烘烤硬化钢烘烤硬化性能的影响

应用连续退火模拟机研究了连续退火工艺各个参数对超低碳烘烤硬化钢板烘烤硬化性能的影响。试验结果表明:提高退火温度可增加NbC的回溶量,而且晶粒尺寸增大使碳原子的晶界偏聚量减少,从而提高了钢板的烘烤硬化性能;提高缓冷段冷速能减少NbC析出,有利于烘烤硬化性能的提高;快冷段冷速和过时效温度对烘烤硬化性能均无明显影响;提高过时效后冷速使碳原子向晶界偏聚的时间缩短,对于提高烘烤硬化性能也是有利的。     

Study of Bake Hardenability and Strengthening Mechanism of Hot Rolled Low Carbon Steel

The controlled rolling and controlled cooling,bake hardening experiments have been carried out for the test steel,mechanical property test of the sample and microstructure analysis have been made by tensile testing machine,optical microscope,TEM and X-ray diffractometer.The results show that fine and uniform microstructures can achieve much higher BH and BHT values;With the increase of pre-strain there is a trend of increase first and then decrease in the BH and BHT values;bake hardening mechanism mainly consists of Cottrell atmosphere strengthening,second phase strengthening,dislocation strengthening.