Bake Hardening of Hot Rolled Multiphase Steels under Biaxial Pre‐strained Conditions

Multiphase steels show a strong bake hardening effect being of importance for shaping of car body structural parts. The raised yield strength is exploited for improved crash resistance. Especially the automotive industry has a growing interest in using this effect. Normally the bake hardening effect is examined in tensile tests whereas under industrial conditions shaping of structural parts shows a wide spread of stress strain behaviour, from uniaxial conditions over plain strain to biaxial ones. So it is obvious that the bake hardening behaviour of a material cannot be described with results of the uniaxial tension test only. To give a first answer to this question, the dependence of the bake hardening effect on different biaxial prestrains was investigated for several hot rolled multiphase steels using various baking temperatures and holding times whereas the bake hardening effect under uniaxial prestrain had already been examined in [5]. Considering the choices to generate biaxial strain, a Marciniak forming tool with a diameter of 250 mm mounted on a 2500 kN hydraulic press was chosen. For control of plastic deformation and adjustment the non‐contact measuring system ARGUS, was used. To reduce the quantity of experiments “Design of Experiments” and statistical methods were applied for a martensitic steel, a dual phase steel, a complex phase steel, a ferrite‐bainite steel, and a retained austenite steel known as TRIP, all in hot rolled condition. As a result, a formula for yield stress, tensile strength and residual deformability was developed. Furthermore, a method was found to predict easily whether a steel under investigation is qualified for additional experiments in regard to bake hardening or more exactly its response to different baking temperatures and holding times.     

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.     

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.     

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.     

管线钢的化学成分和性能分析

论述了管线钢的化学成分设计、力学性能和工艺性能要求。分析了钢中碳、锰、铌、钒和钛等元素对管线钢组织和性能的影响,以及硫、磷、氧等杂质元素对钢的性能影响。     

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.     

ULC BH钢常规轧制和铁素体区轧制组织与性能比较

在实验室完成超低烘烤硬化钢的常规奥氏体区和铁素体区润滑轧制,观察了组织,比较了在2种不同轧制制度下的组织差别;并通过MTS810拉伸实验机的综合性能测试,比较了2种制度下的性能。可以看出,在铁素体区轧制除值外,其它性能均能满足要求。作者认为,造成 值降低的主要原因是铁素体区的固溶碳原子较多所致,但具体原因还有待进一步研究。但是,通过铁素体区轧制并结合合理的退火制度,可生产出满足要求的超低碳烘烤硬化钢。     

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.     

Mechanical Properties and Bake Hardening Behaviour of two Cold Rolled Multiphase Sheet Steels subjected to CGL Heat Treatment Simulation

In recent years, multiphase steels have become a material of choice for use in the car manufacturing industry owing to their excellent mechanical properties. It is anticipated that in the years to come these steels will show the highest increase in usage. A particular aspect of their potential is that multiphase steels often show good bake hardening (BH) properties. The main factors that govern the microstructures and the properties of these steels are the chemical composition and the production process parameters. In this work two commercial cold rolled sheet steels with different carbon content were investigated. In order to produce dual phase (DP) steels with a ferrite‐martensite microstructure, the as‐received material was subjected to heat treatment simulating continuous galvanising line (CGL) cycles with an overageing zone before the zinc pot. After a first CGL cycle predominantly ferritic microstructures with small amounts of martensite, pearlite and retained austenite were obtained, which resulted in deviations from typical DP properties, e.g. in the occurrence of discontinuous yielding. A higher line speed led to improved mechanical properties. BH prestrain was varied between 0 and 10%. While only very little bake hardening was observed without prestrain, with increasing prestrain the amount of BH was evolving quickly towards larger values of more than 60 MPa. Generally, the BH values were somewhat larger for higher carbon content. Finally, an optimised CGL cycle was simulated at laboratory scale with changes in the process parameters. Thus, characteristic DP microstructures resulting in desired mechanical properties were obtained. For these optimised conditions, BH₂ values in excess of 60 MPa were achieved for both steels investigated.     

Ultrafine Grained Steels with Improved Ductility Fabricated by Repetitive Thermo‐Mechanical Processing

Ultrafine grained (UFG) steels with bimodal grained microstructure consisting of ultrafine grains in the size of 400 nm and micrometer‐sized grains were obtained by repetitive thermo‐mechanical processing. The microstructure evolution and mechanical properties were studied. It was revealed that the formation of the bimodal grained microstructure is attributed to the combination of continuous and discontinuous recrystallization. Compared with the uniform UFG steel with average grain size of about 400 nm, the ductility was remarkably improved by introducing micrometer‐sized grains into the UFG microstructure, providing better strength‐ductility balance.     

Analysis of Microstructure and Mechanical Properties of Different Hot Stamped B‐bearing Steels

Hot stamping is a technique to produce ultra high strength automobile components. The common material used in hot stamping process is coated and/or uncoated 22MnB5 boron alloyed steel. Ferritic‐pearlitic microstructure in as‐delivered sheets is transformed to fully lath martensitic after hot stamping. In the present research, hot stamping under water or nitrogen cooling media was investigated using different boron alloyed steel grades. Microstructural analyses, linear and surface hardness profiling as well as tensile tests of hot stamped samples were performed. Various microstructures of fully bainitic and/or fully martensitic were produced. The resulting microstructures provided yield strengths of 650–1370 MPa and tensile strengths of 850–2000 MPa. There is an optimum carbon equivalent content for which the highest formability index value, UTS × A₂₅, is achieved. Using a nitrogen cooled punch resulted in higher yield strength without significant changes in ultimate tensile strength. It is concluded that a wide range of B‐bearing steels having an extended carbon equivalent range with an acceptable formability index value can be used by increasing the cooling rate in the die assembly.     

Comparison of Work Hardening Behaviour of Ferritic‐Bainitic and Ferritic‐Martensitic Dual Phase Steels

In this research, the stress‐strain curves of two types of dual phase steels, namely ferritic‐bainitic and ferritic‐martensitic steels with 0.16%C and 1.2% Mn have been obtained using tensile tests. Both steels were intercritically annealed under different conditions and the ferritic bainitic steels subsequently quenched in a salt bath, while the ferritic martensitic steels were water quenched. The stress‐strain data of the specimens were checked using Hollomon's equation. The results showed that both types of dual phase steels had two stages of work hardening and each stage had a different work hardening exponent. The effects of volume fraction of hard phases (bainite and martensite) on ultimate tensile strength, total elongation and work hardening exponent were also investigated. The results indicated that with increasing volume fraction of hard phase the UTS was increased whereas the work hardening exponent and total elongation were decreased.     

控轧控冷时中碳钢的力学性能与组织参量之间的关系

本文对微合金化中碳L45钢进行了控制轧制和控制冷却试验;测定了各项力学性能及先共析铁素体量f_a、铁素体尺寸d_a、铁素体自由程λ_a和珠光体片层间距S_o;采用对大量实验数据进行数理统计的方法,研究了控轧控冷微合金化中碳钢力学性能与组织参量之间的关系,得到了一些表达其定量关系的估计式,探讨了控轧控冷中碳钢的强韧化机制。     

化学成分对热轧带钢氧化铁皮生长速率的影响

通过氧化模拟实验,观测了试样加热到不同温度时的氧化铁皮起皮情况,绘制了氧化动力学曲线,研究了高温下钢板在不同硅、磷质量分数时对氧化铁皮生长速率的影响。实验结果表明,在1100 ℃且经30 s氧化后,除含硅较低的钢种C外,其它钢种的表面都出现了独立的小气泡;经120 s氧化后各钢种的表面都完全形成了氧化膜。在较高温度(＞1000 ℃)时增硅会促进氧化铁皮的起皮;w（P）≤0005％时,无论在任何温度下都抑制了氧化铁皮起皮。     

Strain‐Rate Sensitivity in Hot Forming of Steels – Influence of Microstructure,Temperature and Chemical Composition

A methodology to determine the strain‐rate sensitivity index was developed, based on rolling of a set of samples with the same draught but different speed at defined temperatures. It was shown that initial grain size has nearly negligible influence on the investigated variable, in contrast to phase composition whose influence is very considerable. Combined influence of strain rate and temperature on deformation resistance of various types of steel was studied. For a selected group of steels a universal equation was set up, which described, with a good accuracy, impact of reciprocal temperature and chemical composition (expressed simply by nickel equivalent) on strain‐rate sensitivity in hot state.