Influence of Al on the Microstructural Evolution and Mechanical Behavior of Low-Carbon,Manganese Transformation-Induced-Plasticity Steel

Microstructural design with an Al addition is suggested for low-carbon, manganese transformation-induced-plasticity (Mn TRIP) steel for application in the continuous-annealing process. With an Al content of 1 mass pct, the competition between the recrystallization of the cold-rolled microstructure and the austenite formation cannot be avoided during intercritical annealing, and the recrystallization of the deformed matrix does not proceed effectively. The addition of 3 mass pct Al, however, allows nearly complete recrystallization of the deformed microstructure by providing a dual-phase cold-rolled structure consisting of ferrite and martensite and by suppressing excessive austenite formation at a higher annealing temperature. An optimized annealing condition results in the room-temperature stability of the intercritical austenite in Mn TRIP steel containing 3 mass pct Al, permitting persistent transformation to martensite during tensile deformation. The alloy presents an excellent strength-ductility balance combining a tensile strength of approximately 1 GPa with a total elongation over 25 pct, which is comparable to that of Mn TRIP steel subjected to batch-type annealing.     

C和Mn元素配分行为对冷轧中锰TRIP钢组织性能的影响

采用冷轧+两相区温轧退火（CR+WR+IA）热处理工艺,研究了两相区退火时间对超细晶铁素体与奥氏体中组织形貌演变、C和Mn元素配分行为以及力学性能的影响。结果表明,冷轧试验钢经两相区形变退火处理后,获得了由铁素体、残余奥氏体或新生马氏体组成的超细晶复相组织。在645℃随退火时间的延长,形变马氏体向逆相变奥氏体配分的C、Mn元素增多,C、Mn元素富集位置增加,同时富Mn区形变马氏体回复再结晶现象明显;伴随少量碳化物溶解,试验钢的屈服强度由741持续降低到325MPa。两相区退火10min时,试验钢力学性能最佳,此时抗拉强度达到最大值1141MPa,断后伸长率及均匀伸长率分别为236%和181%,强塑积达到26928MPa·%。     

Influence of intercritical annealing temperature on microstructure and mechanical properties of medium Mn TRIP steel

The transformation, microstructure and mechanical properties of the 0. 2C- 5Mn TRIP steel after intercritical annealing were investigated using dilatometer, scanning electronic microscopy (SEM), transmission electron microscopy(TEM), X- ray diffraction (XRD), and tensile testing machine. The phase transformation thermodynamics of the investigated steel after intercritical annealing was calculated by Factsage software and the characteristics of the transformation were discussed. The results show that the reversed austenite content increases with the increasing of the intercritical annealing temperature, the carbon content in reversed austenite firstly increases and then decreases, manganese content in reversed austenite decreases, which results in the decreasing of the thermal stability of reversed austenite. When the intercritical annealing temperature is 700??, an obvious martensitic transformation occurs during the cooling process. With the increasing of intercritical annealing temperature, cementite is gradually dissolved, but it cannot be completely dissolved due to the short transformation time. When the intercritical annealing temperature is 600-675??, the microstructure after intercritical annealing consists of ferrite, cementite and retained austenite. When the intercritical annealing temperature is 700??, the microstructure after intercritical annealing consists of ferrite, retained austenite, martensite and a small amount of undissolved cementite. The engineering stress and strain curves of the investigated steel are significantly changed with increasing intercritical annealing temperature. At the same time, the optimal mechanical properties with tensile strength of 1138MPa and total elongation of 23% can be obtained after annealed at 675?? for 3min.     

780MPa级热镀锌用TRIP钢退火工艺及组织演变

研究了热镀锌用高强TRIP钢的退火工艺对性能的影响和组织演变规律.结果表明:实验用钢可获得780.00MPa以上的抗拉强度和24.00%以上的断后延伸率;两相区加热温度和贝氏体保温时间对钢的力学性能具有显著影响,两相区加热温度为850℃,贝氏体保温时间为30s时,实验用钢能获得最佳的综合力学性能;在贝氏体中温相变后,仍有部分亚稳奥氏体(碳含量较低)在后续冷却过程中发生马氏体相变,从而导致钢退火后的微观组织由铁素体、贝氏体、残余奥氏体和马氏体组成.     

Microstructure and Mechanical Properties of TRIP Steel with Annealed Martensite

The microstructure and mechanical properties of cold rolled TRIP steel containing C 0.2, Si 0.5, Mn 1.5, A1 1.3, and Nb-kV 0.13 （mass%） with annealed martensite （TAM steel） were investigated using optical microscopy, field emission gun scanning electronic microscope （FEG SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and mechanical testing. The mierostructure of the TAM steel mainly consisted of polygonal ferrite, bainite, annealed martensite and retained austenite. The martensite after annealing did not spheroidize, which consisted of annealed lath martensite structure and interlath second phase. Compared with the traditional TRIP steel with polygonal ferrite matrix （TPF steel）, the TAM steel has more excellent elongation rate over 32%. The TAM steel also has better strain hardening behavior than the TPF steel. The excellent elongation and strain harden- ing behavior of TAM steel result from high retained austenite stability of the TAM steel, which is attributed to its fine distribution and medium strength ratio of second phase to matrix.     

Mechanical Property and Microstructural Characterization of C-Mn-Al-Si Hot Dip Galvanizing TRIP Steel

 Mechanical properties and microstructure in high strength hot dip galvanizing TRIP steel were investigated by optical microscope (OM), transmission electron microscope (TEM), X-ray diffraction (XRD), dilatometry and mechanical testing. On the heat treatment process of different intercritical annealing (IA) temperatures, isothermal bainitic transformation (IBT) temperatures and IBT time, this steel shows excellent mechanical properties with tensile strength over 780 MPa and elongation more than 22%. IBT time is a crucial factor in determining the mechanical properties as it confirms the bainite transformation process, as well as the microstructure of the steel. The microstructure of the hot dip galvanizing TRIP steel consisted of ferrite, bainite, retained austenite and martensite during the short IBT time. The contents of ferrite, bainite, retained austenite and martensite with different IBT time were calculated. The results showed that when IBT time increased from 20 to 60 s, the volume of bainite increased from 14.31% to 16.95% and the volume of retained austenite increased from 13.64% to 16.28%; meanwhile, the volume of martensite decreased from 7.18% to 1.89%. Both the transformation induced plasticity of retained austenite and the hardening of martensite are effective, especially, the latter plays a dominant role in the steel containing 7.18% martensite which shows similar strength characteristics as dual-phase steel, but a better elongation. When martensite volume decreases to 1.89%, the steel shows typical mechanical properties of TRIP, as so small amount of martensite has no obvious effect on the mechanical properties.     

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.     

Ultrafine Grained Duplex Structure Developed by ART-annealing in Cold Rolled Medium-Mn Steels简

The microstructural evolutions of the cold rolled Fe-0.1C-5Mn steel during intercritical annealing were ex- amined using combined advanced techniques. It was demonstrated that intercritical annealing results in an ultrafine granular ferrite and austenite duplex structure in cold rolled 0.1C-5Mn steel. The strong partitioning of manganese and carbon elements from ferrite to austenite was found during intercritical annealing by scanning transmission elec- tron microscopy （STEM） and X-ray diffraction （XRD）. Strong effects of boundary characters on the austenite for- mation were indicated by austenite fast nucleation and growth in the high angle boundaries but sluggish nucleation and growth in the low angle boundaries. The ultrafine grained duplex structure in 0.1C-5Mn was resulted from the the sluggish Mn-diffusion and the extra high Gibbs free energy of ferrite phase. Based on the analysis of the micro- structure evolution, it was pointed out that the intercritical annealing of the medium Mn steels could be applied to fabricate an ultrafine duplex grained microstructure, which would be a promising approach to develop the 3rd genera- tion austomobile steels with excellent combination of strength and ductility.     

退火温度对退火马氏体基TRIP钢显微组织和力学性能的影响

将C-Si-Mn系TRIP钢通过完全淬火和两相区退火相结合的工艺,得到一种以退火马氏体为基体的TRIP钢（简称TAM钢）,并对比分析了TAM钢在不同温度退火后的显微组织和力学性能.结果表明,TAM钢经退火后的显微组织特征为精细规整的板条退火马氏体基体、片状残余奥氏体和贝氏体/马氏体组成的混合组织.这种组织降低了基体的硬度以及基体和第二相之间的强度比,减少了基体的位错密度.随着退火温度的提高,退火马氏体基体的板条形态逐渐消失,新生马氏体/贝氏体的团状混合组织逐渐增多.当退火温度为780℃时,综合力学性能优异,抗拉强度为1130 MPa,延伸率可达20%,强塑积为22600 MPa·%.当退火温度较低时,残余奥氏体主要以片状存在于退火马氏体板条间,有利于TRIP效应的发生.     

低碳中锰热轧TRIP钢退火工艺及组织演变

研究了第三代高强度高塑性TRIP钢的退火工艺对性能的影响和组织演变规律.热轧后形成的原始马氏体与临界退火时形成的残余奥氏体使TRIP钢具有良好的强度和塑性.结果表明:实验用钢可获得1000MPa以上的抗拉强度和30%以上的断后延伸率,且强塑积>30 Gpa·%;退火温度和保温时间对钢的力学性能具有显著影响,热轧TRIP钢临界退火温度为630℃,保温时间18 h时,实验用钢能获得最佳的综合力学性能.     

Enhancement of the mechanical properties of a low-carbon, low-silicon steel by formation of a multiphased microstructure containing retained Austenite

Dual-phase and transformation-induced plasticity (TRIP)-assisted multiphase steels are related families of high-strength formable steels exhibiting excellent mechanical characteristics. This study shows how a ferrite-bainite-martensite microstructure containing retained austenite can improve the mechanical properties of a cold-rolled low-carbon, low-silicon steel. Such a multiphased microstructure is obtained by a heat treatment involving intercritical annealing followed by a bainite transformation tempering. Depending on the heat-treatment parameters, the samples present a variety of microstructures. Due to the presence of retained austenite, some samples exhibit a TRIP effect not anticipated with such a low silicon content. A composite strengthening effect also results from the simultaneous presence of a ductile ferrite matrix with bainite and martensite as hard second phases. A true stress at maximum load of 800 MPa and a true uniform strain of 0.18 can be obtained by forming a ferrite-bainite-martensite microstructure containing up to 10 pct of retained austenite. These properties correspond to a favorable evolution of work hardening during plastic deformation.     

热镀锌工艺对无Si含P的TRIP钢力学性能影响

利用光学显微镜、透射电镜、X射线衍射和拉伸试验等方法,分析测试了热镀锌工艺对无Si含P的TRIP钢力学性能和微观组织的影响.结果表明:实验用钢可获得780 MPa以上的抗拉强度和24%以上的断后延伸率.在热镀锌工艺中,两相区加热温度和贝氏体等温温度对钢的力学性能影响较小,而贝氏体等温时间的影响最为显著.当贝氏体等温时间由20 s增加到60 s时,实验用钢的屈服强度上升了65 MPa,抗拉强度下降了45 MPa,延伸率大幅度增加,从23.01%增加到27.56%,出现最佳的综合力学性能.无Si含P热镀锌TRIP钢的微观组织由铁素体、贝氏体、残余奥氏体和马氏体组成,随着贝氏体等温时间的减少,钢中残余奥氏体含量和稳定性降低,相应地,马氏体含量明显增加,实验用钢从典型的TRIP钢力学特征慢慢转变为与双相钢相似的力学特征.     

低成本工程机械用Q690E级高强钢的热处理工艺与工业化

  以低成本、高附加值Q690E级调质板开发为目标,研究了亚温热处理与调质热处理工艺参数对试验钢显微组织与力学性能的影响。结果表明：在进行810℃亚温淬火处理的前躯体中存在大块状的铁素体时,易导致试验钢的低温冲击韧性恶化;以板条马氏体为前躯体经相同亚温淬火后,显微结构为更加细小的马氏体和以条状形态呈平行趋势分布在马氏体之间的铁素体两相混合组织,试验钢的-40℃冲击功值高达247J,但强度较低;常规调质热处理后的试验钢具有良好的综合力学性能,采用修正后的工艺参数,工业试制6~60mm规格产品的强韧性能均明显超过相关标准要求。     

Development of ultra-fine grained dual phase microalloyed steels through severe cold rolling and intercritical annealing

A Nb-microalloyed structural steel with ferrite-pearlite microstructure was subjected to cold rolling and intercritical annealing to produce ultra-fine grained dual phase microstructure. Optical and transmission electron microscopy techniques were employed to characterise the microstructure. Initial results showed that the intercritical annealing (at 790°C for 90s) of samples rolled to a true strain of 2.4 resulted in a significant grain refinement from the average initial grain size of 20 μm to 1–2 microns. The microstructure primarily consisted of UFG ferrite matrix with homogeneously distributed islands of plate martensite with volume fraction of 27%.     

Effect of Intercritical Annealing Time on the Microstructure and Mechanical Properties of Dual-Phase Steel Processed by Large-Strain Asymmetric Cold-Rolling

Herein, the effect of intercritical annealing time on the microstructure and mechanical properties of dual-phase steel processed by large-strain asymmetric cold-rolling is studied. It is observed that the martensite islands are uniformly distributed in the ferrite phase in the microstructures of dual-phase steels due to performing the asymmetric cold-rolling before intercritical annealing treatment. As the intercritical annealing time increases up to 10 min, the fraction of martensite increases. By increasing the holding time and fraction of martensite, the carbon content of the martensite phase is decreased. The short-term intercritical annealing eliminates the yield point phenomenon. However, intercritical annealing at 860 °C for 20 min leads to the reoccurrence of a yield point phenomenon. Increasing the intercritical annealing time to 10 min improves the yield strength to 505 MPa and ultimate tensile strength to 834 MPa. However, the strength decreases sharply after the holding time of 20 min. There is a perfect linear relationship between the mechanical properties and the fraction of martensite. Ductile failure is observed at the center of the fracture surfaces of dual-phase steels while shear failure occurs at the edges of the fracture surfaces.     

临界热处理对中碳 Q&P 钢组织与性能的影响

摘要：对中碳钢采用Q&P（淬火 碳分配）和I&QP（临界热处理,淬火 碳分配）热处理工艺,通过对试样的显微组织,残余奥氏体的体积分数及其碳含量,硬度及其拉伸性能进行分析,研究了临界加热对中碳Q&P钢组织和性能的影响。实验结果表明,经临界热处理的Q&P钢组织中,除了马氏体和残余奥氏体,还存在部分铁素体,同时残余奥氏体的体积分数较少,马氏体板条更加细小。在相同的碳分配时间下,I&QP试样的硬度和抗拉强度都比Q&P试样低,但由于I&QP试样中软相铁素体的存在以及残余奥氏体能发挥更好的TRIP效应,使得临界热处理的实验钢的伸长率更高,加工硬化指数增加,强塑积更大。     

Microstructure Characterization and Mechanical Properties of TRIP-aided Steel under Rapid Heating for Different Holding Time

In order to develop a comprehensive understanding about the effect of different holding time under rapid heating on the microstructural evolution and mechanical properties of transformation-induced plasticity (TRIP)steel, continuous annealing process simulations were performed using a thermal system with resistance heating method. The morphology and distribution of all phases present in the microstructure and the mechanical properties of TRIP steel were revealed.It appeared that the final tensile strength of the TRIP steel increased and retained austenite car-bon content decreased with increasing holding time.An overlap between ferrite recrystallization and austenitization was observed during intercritical holding.In addition,the work hardening of the samples was evaluated by calculat-ing the instantaneous n value as a function of the true strain.The difference in work hardening behavior corresponds to the rate of the retained austenite transformation during straining,which can be attributed to the carbon content and the morphology of the retained austenite.     

不同热处理工艺对Nb-Mo中锰钢组织和力学性能的影响

摘要：探讨了不同热处理工艺对Fe-0.25C-3.98Mn-1.22Al-0.20Si-0.19Mo-0.03Nb中锰钢组织演变与力学性能的影响。研究发现,与临界退火和淬火配分（IA & QP）工艺相比,奥氏体逆转变（ART）工艺处理后的实验钢获得了更为优异的力学性能;采用ART工艺经过680℃临界退火5h后的实验钢展现出了最佳的力学性能,即抗拉强度为830MPa,伸长率为48.9%,强度与塑性的乘积达到40.6GPa·%;ART工艺实验钢因长时间退火而促进了Mn向奥氏体中富集,有利于奥氏体含量及稳定性增加,在应变中后期可展现更为广泛的TRIP效应,更有利于获得优异的力学性能。另外,微合金元素Nb和Mo在中锰钢主要起析出强化和细晶强化的作用。     

Localized Deformation in Multiphase,Ultra-Fine-Grained 6 Pct Mn Transformation-Induced Plasticity Steel

Multiphase, ultra-fine-grained transformation-induced plasticity (MP UFG TRIP) steel containing 6 mass pct Mn was obtained by cold rolling and intercritical annealing of an initially fully martensitic microstructure. UFG microstructures with an average grain size less than 300 nm were obtained. The amount of austenite in the microstructures, speculated to be formed by diffusionless transformation, was controlled by changing the intercritical temperature. The tensile properties were strongly influenced by the volume amount and the stability of the reversely transformed austenite. The MP UFG TRIP steel was characterized by pronounced localization of the deformation. The deformation band properties were analyzed in detail.     

Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel

Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental temperatures between 848 K and 948 K (575 °C and 675 °C) for 1 week to enrich austenite in manganese. The resulting microstructures are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between annealing conditions help explain the observed strain hardening behaviors.