A medium manganese steel designed for water quenching and partitioning

An aluminium-containing medium manganese steel has been designed to undergo intercritical annealing followed by quenching in water and subsequent partitioning. Water quenching, replacing the quenching temperature (QT) between 150 and 300°C in conventional quenching and partitioning steels, is therefore adopted in QP alloys, in order to guarantee the precise QT in practice. The low intercritical annealing temperature of 750°C refines both ferrite and prior austenite grains into submicron size. The large fraction of ultra-fine ferrite, as well as the transformation-induced plasticity effect of retained austenite, improves the overall ductility of this water-quenched and partitioned steel. The alloy has achieved excellent mechanical properties of 1130?MPa ultimate tensile strength combined with 19.2% total elongation.     

Properties assessment of the first industrial coils of low-density duplex δ-TRIP steel

A novel low-density steel with high aluminium content was, for the first time, manufactured as coils by the conventional industry process consisting of continuous casting followed by hot-rolling and cold-rolling. The duplex δ-TRIP microstructure, comprising a mixture of blocky retained austenite and both δ- and α-ferrite, was then produced by laboratory-scale heat treatments. The microstructure generation was achieved by an intercritical annealing followed by direct quenching, which was the same process as that for producing conventional dual-phase steels. The alloy exhibits quite interesting mechanical properties with a combination of 930?MPa tensile strength and of 21% total elongation, both in rolling and transverse directions. The formability of this novel alloy was assessed and discussed.     

前驱体对含Cu低碳钢I&Q&P处理后组织性能的影响

采用IQP工艺和EPMA、SEM和XRD等手段,研究了3种前驱体对含Cu低碳钢残余奥氏体含量及力学性能的影响。结果表明,双相区保温初期试验钢奥氏体长大由C配分控制,后期由合金元素Mn、Cu配分控制;双相区保温奥氏体化后,双相区配分后形成弥散分布的局部高浓度Mn、Cu区域仍保留富集效果,在随后的淬火-碳配分阶段易于形成残余奥氏体。经IQP处理后,前驱体为P+F的钢室温组织中马氏体板条较粗,原始奥氏体晶界并不明显;前驱体为F+M钢得到的马氏体板条有序细密;前驱体为M的钢室温组织中马氏体板条更加细密。其中,前驱体组织为M的钢中残余奥氏体量最高,延伸率为24.1%,强塑积可达25 338 MPa·%,综合性能最好。     

Intercritical annealing pre-treatment used in a hot-dip galvanised TRIP steel

An intercritical annealing pre-treatment was added before the conventional two-step heat treatment process, and the effect of the isothermal bainitic transformation (IBT) time on the steel's microstructure and mechanical properties were investigated. The microstructure was investigated using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, while the mechanical properties were evaluated using tensile testing. The microstructure of the three-step hot-dip galvanised transformation induced plasticity (TRIP) steel consists of ferrite, bainite, retained austenite, and martensite. The mechanical properties of the steel after the three-step heat treatment process are excellent, with a tensile strength above 770?MPa and elongation above 29%. The effect of IBT time on the mechanical properties was insignificant because the intercritical annealing pre-treatment increases the bainitic transformation rate.     

Prüfung der mechanischen Eigenschaften von “Quenching und Partitioning”‐Gefügen mit berührungsloser Dehnungsmessung

The “Quenching and Partitioning” (“Q&P”) concept was designed to fill the gap between the first and second generation of Advanced High Strength Steels (AHSS). It aims at a multiphase microstructure of retained austenite in a matrix of carbon depleted martensite. The martensitic components enhance the strength properties. The ductility is improved by the TRIP effect. This work investigates the “quenching and partitioning” response of a nickel and silicium alloyed TRIP steel. After “quenching and partitioning” processing the mechanical properties are evaluated by tensile testing. An adapted specimen geometry and the contact free measurement of the elongation by a laser speckle system are used. The mechanical properties of the “quenching and partitioning” microstructure are compared to the fully martensitic state and reviewed with respect to published data. Additional tests are stopped after a well defined plastic deformation. Subsequently the retained austenite fraction is measured magnetically in the test length. As a result the TRIP effect can be evaluated. The “quenching and partitioning” processing leads to tensile strengths of around 1300 MPa at elongations of more than 10 %. The martensitic microstructure exhibits a higher tensile strength and lower elongation values. The decreasing fraction of retained austenite with plastic deformation implies the TRIP effect. Comparable mechanical properties are reported in the published literature. The proposed method of annealing and adapted testing shows effective for the investigation of sophisticated heat treatment procedures.     

Microstructures and Mechanical Properties of Si-Al-Mn TRIP Steel with Niobium

Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP (transformation induced plasticity) steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10-2 to 10-4 s-1, the volume fraction of transformed martensite from retained austenite,as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.     

Microstructures and Mechanical Properties of Si-AI-Mn TRIP Steel with Niobium

Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP （transformation induced plasticity） steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10^-2 to 10^-4 s^-1, the volume fraction of transformed martensite from retained austenite, as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.     

低温临界区退火时间对22MnB5钢组织和性能的影响

为了研究22MnB5钢在退火过程中的组织演变规律,细化热冲压成形后马氏体板条束,通过扫描电镜(SEM)、能谱分析、电子背散射衍射(EBSD)分析技术和拉伸实验等方法,研究了不同低温临界区退火时间对22MnB5钢显微组织和力学性能的影响,并阐述了不均匀奥氏体在退火过程中的转变机制及合金元素对粒状珠光体形成的影响.研究表明,经低温临界区不同退火时间保温及随后等温处理后,得到不同的珠光体形态,在770℃保温0.5 h,并在700℃等温处理后,得到铁素体基体上分布颗粒状碳化物的粒状珠光体组织;随着临界区保温时间的延长,奥氏体转变逐渐均匀,使部分奥氏体在随后的等温过程中发生共析转变,得到多边形铁素体+片层状珠光体组织.粒状珠光体组织有利于细化淬火后的马氏体板条束,提高综合力学性能.     

Effect of Microstructure on the Wear Behaviour of a Dual Phase Steel

In this study wear behaviour of 0.1% C containing dual phase steels with three different microstructures have been examined. Intercritical annealing, step quenching and intermediate quenching heat treatments have been applied to the alloy in order to obtain different morphologies of ferrite and martensite. It has been observed that, intercritical annealing lead to highest strength but lowest wear resistance in this alloy. Step quenching suggested to have no beneficial effect on tensile properties but increased the wear resistance. Intermediate quenching was found to be the best heat‐treatment condition. The fibrous microstructure lead to the optimum tensile strength, ductility and wear resistance in this alloy.     

Q&P热处理工艺对含Cu TRIP钢组织和性能的影响

随着能源的短缺和环境污染的日益严重,汽车轻量化需求日益迫切,如何通过工艺及成分设计革新、获得兼具高强度和高塑性的钢板尤为重要.尝试将Cu作为合金元素加入TRIP钢中,采用淬火配分(QP)工艺对含Cu TRIP钢进行一步法和两步法热处理,通过拉伸试验、X射线衍射分析、扫描电镜、透射电镜等实验手段,对热处理后的组织及性能进行测试和观察,探究了不同热处理工艺对组织和性能的影响.研究结果表明:一步法处理后的显微组织为铁素体、马氏体和残余奥氏体,两步法处理后不仅包含上述3种组织,还含有贝氏体.一步法处理后,抗拉强度达2 200 MPa,拉伸延展率为15%,强塑积为33 GPa·%;两步法处理后综合力学性能优于一步法,在400℃等温5 min后,抗拉强度为1 300 MPa,拉伸延展率为43%,强塑积超过55 GPa·%.实验钢良好的综合力学性能得益于铁素体、马氏体/贝氏体和残余奥氏体的合理配比,变形过程中残余奥氏体的相变诱导塑性效应,以及马氏体位错与Cu粒子的交互作用.     

Deformation-induced martensitic transformation kinetics and correlative micromechanical behavior of medium-Mn transformation-induced plasticity steel

An in situ high-energy X-ray diffraction (HE-XRD) technique was mainly used to investigate the micromechanical behavior of medium-Mn Fe-0.12C-10.16Mn-1.87Al (in wt%) transformation-induced plasticity (TRIP) steel subjected to intercritical annealing at 625 °C, 650 °C, 675 °C and 700 °C for 1 h. As the intercritical annealing temperature increased, the volume fraction of retained austenite (RA) and ultimate tensile stress (UTS) increased, while the Lüders strain and yield stress (YS) decreased. The incremental work-hardening exponent of experimental steel increased with increasing intercritical annealing temperature. The overall trend of the transformation kinetics of the RA with respect to the true strain followed the sigmoidal shape predicted by the Olson and Cohen (OC) model. Load partitioning occurred among the ferrite, austenite and martensite immediately after entering the yielding stage. Because the stability of the RA decreased with increasing intercritical annealing temperature, the load undertaken by the martensite increased. The moderate transformation kinetics of the RA and effective load partitioning among constituent phases were found to contribute to a favorable combination of strength and ductility for this medium-Mn TRIP steel.     

Tensile properties of iron-based P/M steels with ferrite + martensite microstructure

The effect of intercritical heat treatments on the tensile properties of iron-based P/M steels was investigated. For this purpose, atomized iron powder (Ancorsteel 1000) was admixed with 0.3 wt.% graphite powder. Tensile test specimens were cold pressed at 700 MPa and sintered at 1120 °C for 30 min under pure argon gas atmosphere. After sintering, ∼20% pearlite volume fraction in a ferrite matrix was obtained. To produce coarse ferrite + martensite microstructures, the sintered specimens were intercritically annealed at 724 and 760 °C and quenched in water. To obtain fine ferrite + martensite microstructures, the sintered specimens were first austenitized at 890 °C and water-quenched to produce a fully martensitic structure. These specimens were then intercritically annealed at 724 and 760 °C and re-quenched. After the intercritical annealing at 724 and 760 °C and quenching, martensite volume fractions were ∼ 18% and 43%, respectively, in both the coarse- and fine-grained specimens. Although the intercritically annealed specimens exhibited higher yield and tensile strength than the as-sintered specimens, their elongation values were lower. Specimens with a fine ferrite + martensite microstructure showed high yield and tensile strength and ductility in comparison to specimens with a coarse ferrite + martensite microstructure. The strength values of specimens increased with increasing martensite volume fraction.     

Effects of carbon content on mechanical properties of 5%Mn steels exhibiting transformation induced plasticity

Abstract

A series of highly ductile, high strength steels exhibiting transformation induced plasticity due to retained austenite was developed by varying the carbon content in the range 0·01–0·4 wt-% in 5 wt-%Mn based steel. For up to 0·l%C the mechanical properties are insensitive to cooling rate after intercritical heating, but afurther increase in carbon content causes a large sensitivity to the cooling rate, owing to carbide precipitation occurring during slow cooling. By suppressing this carbide precipitation with water quenching after the intercritical holding, an excellent combination of tensile strength (1580 MN m^?2) and uniform elongation (21%) was attained at 0·3%C in this series.

MST/1964     

A medium-Mn steel processed by novel twin-roll strip casting route

A medium-Mn steel (Fe–0.3C–4Mn–1.82Al–0.6Si wt-%) was produced by a novel processing route involving twin-roll strip casting, hot rolling and intercritical annealing (IA). Macrosegregation was absent in the as-cast strip. The microstructure of the as-cast strip consisted of martensite and austenite (～10 vol.-%), and the solidification structure was characterised by dendritic structure. With an increase in IA temperature from 680 to 725 and to 755°C, austenite fraction in intercritically annealed steels was increased from 22 to 45% and then decreased to 27%. The 710°C intercritically annealed steel yielded excellent mechanical properties with a tensile strength of ～1007?MPa and total elongation of ～48%, achieved by a high volume fraction of austenite (～42%) with appropriate mechanical stability.     

Ferrite recrystallisation and intercritical annealing of cold-rolled low alloy medium carbon steel

Static recrystallisation of cold-rolled AISI 4130 medium carbon steel in the ferritic regime and its response to intercritical annealing treatment are studied. A fine and recrystallised microstructure with improved mechanical properties is obtained via subcritical annealing of cold-rolled sheet, where the subsequent intercritical annealing results in the enhancement of tensile strength via the formation of dual-phase microstructure. Intercritical annealing of the cold-rolled sheet is characterised by an initial drop in hardness due to recrystallisation and subsequent rise in hardness as a result of austenitisation. It is revealed that continuous martensite phase can result in a higher deformation resistance. Finally, the effects of intercritical annealing at temperatures below pearlite dissolution finish temperature (Ac_1f) are discussed.     

Improved properties of dual-phase steel via pre-intercritical annealing treatment and thermal cycling

The synergistic effects of pre-intercritical annealing treatments and multiple heating to the two-phase region (followed by quenching) on the microstructure and mechanical properties of dual-phase (DP) steel were studied. Intercritical annealing of ferritic-pearlitic, martensitic, and cold-rolled martensitic microstructures was investigated. The effects of tempering, austenitisation, grain growth, and recrystallisation were unravelled and an optimum holding time was characterised for each initial microstructure. Grain refinement by recrystallisation of cold-rolled martensitic microstructure resulted in the enhancement of tensile properties. Thermal cycling through multi-step intercritical annealing resulted in grain refinement and enhanced mechanical response. It was revealed that thermal cycling combined with the carefully controlled pre-intercritical microstructure could be simply used for the improvement of mechanical properties of DP steel.     

The effect of processing parameters on the microstructure and mechanical properties of low-Si transformation-induced plasticity steels

A base low Si, high-Al transformation-induced plasticity (TRIP) steel and one with 0.03Nb and 0.02Ti (wt%) additions were subjected to thermo-mechanical processing (TMP) and galvanising simulations. The microstructure and mechanical properties were analysed using a combination of optical and electron microscopy, X-ray diffraction and tensile testing and the results compared with those from intercritically annealed–galvanised steels. The addition of Nb and Ti results in microstructure refinement and an increase in the amount of the retained austenite after TMP which in turn, leads to increases in the tensile strength (~750 MPa) and the total elongation (TE) (~29 %). A deterioration in the volume fraction of retained austenite and the mechanical properties was noted in both steels after the additional galvanising simulation. For the base steel, all TMP and galvanised samples presented with continuous yielding during tensile testing. The Nb–Ti steel exhibited discontinuous yielding and extended Lüders banding when TMP was followed by a longer coiling time. Both steels returned discontinuous yielding after the intercritical annealing–galvanising treatment. The discontinuous yielding behaviour was associated with the much finer ferrite grain size in the intercritically annealed steels and the ageing processes that take place during galvanising.     

回火温度对两相区退火海工钢组织和性能的影响

对690 MPa级海工钢进行“淬火+两相区退火+回火”三步热处理,研究了回火温度对其组织和性能的影响、分析了力学性能变化与组织演变和残余奥氏体体积分数之间的关系。结果表明：回火后实验钢的显微组织为回火贝氏体/马氏体、临界铁素体和残余奥氏体的混合组织。随着回火温度的提高贝氏体/马氏体和临界铁素体逐渐分解成小尺寸晶粒,而残余奥氏体的体积分数逐渐增加;屈服强度由787 MPa降低到716 MPa,塑性和低温韧性明显增强,断后伸长率由20.30%增至29.24%,-40℃下的冲击功由77 J提升至150 J。残余奥氏体体积分数的增加引起裂纹扩展功增大,是低温韧性提高的主要原因。贝氏体/马氏体的分解和残余奥氏体的生成,引起组织细化、晶粒内低KAM值位错的比例逐渐提高和小角度晶界峰值的频率增大,使材料的塑性和韧性显著提高。     

The influence of cryogenic rolling on a transformation-induced plasticity steel

ABSTRACT

Here, a novel cryogenic rolling plus intercritical annealing process was applied to a transformation-induced plasticity (TRIP) steel with a low chemical composition of carbon and manganese. Compared with traditional cold rolling, obvious grain refinement was observed, due to a high amount of dislocations retained. In addition, austenite volume fraction was increased, because of a unique nucleation mechanism. Subjected to cryogenic rolling, strength and ductility were increased, due to the enhanced austenite stability, which provided continuous and active TRIP effect. Consequently, tensile strength was increased to 1030?MPa, and elongation was increased to 38.2%. Thus, a great mechanical combination was obtained in a steel with a relatively low chemical composition with carbon and manganese, only by cryogenic rolling process.     

Process control maps to design an ultra-high strength-ductile steel

Steel with 2.4–2.5 GPa tensile strength and elongation to fracture of 4.8–5.7%, is produced by designing a novel heat treatment identical to quenching and tempering, in less than a few minutes. Since addition of Si to Fe–Mn steel promotes the austenite stabilisation by carbon enrichment, the elongation to fracture of 0.6C–1.6Si–1.2Mn (wt-%) steel treated by different quenching and partitioning (Q&P) routes is improved. Results demonstrated by process control maps give a good overview of the final microconstituents. By using higher partitioning temperatures, the tempering of martensite, stabilisation of austenite and improvement of the mechanical properties, could effectively be accelerated. This approach results in significant time and cost reduction which makes this heat treatment attractive for industries.