In-Situ High-Energy X-ray Diffraction Study of Austenite Decomposition During Rapid Cooling and Isothermal Holding in Two HSLA Steels

In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s⁻¹) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form.

     

Notch-Fatigue Properties of Advanced TRIP-Aided Bainitic Ferrite Steels

To develop a transformation-induced plasticity (TRIP)-aided bainitic ferrite steel (TBF steel) with high hardenability for a common rail of the next generation diesel engine, 0.2?pct C-1.5?pct Si-1.5?pct Mn-0.05?pct Nb TBF steels with different contents of Cr, Mo, and Ni were produced. The notch-fatigue strength of the TBF steels was investigated and was related to the microstructural and retained austenite characteristics. If Cr, Mo, and/or Ni were added to the base steel, then the steels achieved extremely higher notch-fatigue limits and lower notch sensitivity than base TBF steel and the conventional structural steels. This was mainly associated with (1) carbide-free and fine bainitic ferrite lath structure matrix without proeutectoid ferrite, (2) a large amount of fine metastable retained austenite, and (3) blocky martensite phase including retained austenite, which may suppress a fatigue crack initiation and propagation.     

2 GPa中碳中锰纳米贝氏体钢的相变和塑性机理

 高碳(质量分数为0.78%~0.98%)高硅(质量分数约为1.5%)钢采用低温贝氏体转变(通常为150~250 ℃),可获得不小于2.0 GPa超高强度,但塑性较低(通常不大于8.0%);同时需要非常长的贝氏体相变时间(通常不小于4 d)。采用降低碳含量(Fe-0.30C-1.5Si-1.5Ni)的成分设计,可以显著加速贝氏体相变(300 ℃等温0.5 d),获得优良强度(抗拉强度(1 138±6) MPa)和塑性(伸长率为18.5%±1.5%)匹配的性能;但很难达到超高强度(1 500 MPa)级别。参考高/中碳贝氏体钢的合金设计、显微组织和力学性能特点,采用“中碳、以铝代硅、以锰代镍”的合金成分(Fe-0.30C-1.2Al-5.0Mn)体系,在M_s(马氏体开始转变温度)温度(300 ℃)附近进行贝氏体相变,可以获得强度为2.0 GPa级((2 029±9) MPa),伸长率超过10.0%(11.5%±1.0%)的高塑性纳米贝氏体钢,同时贝氏体相变时间适中(等温2 d),合金制造成本低廉(镍质量分数约为0.5%)。Fe-0.30C-1.2Al-5.0Mn钢具有超高强度主要是由于硬相组织贝氏体铁素体和马氏体总体积分数为85.1%,其中贝氏体铁素体板条宽度为(85±30) nm。具有较高塑性主要是由于软相组织残留奥氏体的体积分数为14.9%,碳质量分数为1.12%,位于贝氏体铁素体板条之间的薄膜状残留奥氏体尺寸为(30±15) nm;同时碳、锰元素能够增加残留奥氏体稳定性,特别是相对于低锰含量,5%中锰元素对残留奥氏体有更显著的稳定性作用,使其在低应力作用下不容易发生相变,但在高应力过程中持续发生TRIP效应以提高塑性。     

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.     

Recent Development of Air-Cooled Bainitic Steels Containing Manganese

The superiorities of air-cooled bainitic steels were described. A series of air-cooled bainitic steels containing manganese were developed and presented, which include low carbon granular bainitic steels, low carbon grain-boundary allotriomorphic ferrite/granular bainite dual phase steels, medium and medium high carbon bainite/martensite dual phase steel, low carbon carbide free bainite/martensite dual phase steels and casting bainitic steels.The development of ultra-low carbon bainitic steels in China was also introduced.     

The Evolution of Ultrafine Grained Structure Formation Through Isothermal Static Phase Transformation

In the current study,a 0.3C-2Si-2Mn-0.28Mo (in wt%) steel with high hardenability was deformed at a relatively low temperature followed by isothermal static phase transformation.This novel thermomechanical processing made it possible to successfully produce an ultrafine ferrite grained structure (～2 μm) in the absence of both dynamic phase transformation and controlled cooling.The use of a model Ni-30Fe austenitic alloy showed that the low temperature deformation induced very fine intragranular defects throughout the microstructure,which would then act as fine spaced ferrite nucleation sites at an early stage of phase transformation.As a result,the coarsening of ferrite was extremely limited during isothermal phase transformation,resulting a very fine ferrite grained structure;even nanoscale in the region of the prior austenite grain boundary.     

纳米贝氏体的热力学分析及强韧化研究

针对目前高碳高硅低温贝氏体(纳米结构贝氏体)相变速度缓慢的现状,采用贝氏体相变热力学理论分析主要合金元素对低温贝氏体相变驱动力的影响,设计了新型纳米结构贝氏体钢成分0.83C-2.44Si-0.43Mn-0.73Al.利用膨胀仪研究该成分贝氏体钢在不同温度下的相变整体动力学,综合使用扫描电子显微镜、X射线衍射、电子背散射衍射等方法研究热处理工艺对实验钢组织和力学性能的影响.结果表明,350℃等温转变贝氏体的抗拉强度为1401 MPa,延伸率为42.21%,强塑积可达59136 MPa·%,在室温拉伸过程中发生明显的相变诱导塑性效应;230℃等温转变组织中贝氏体铁素体片层厚度小于100 nm,抗拉强度达2169 MPa.      

In-Situ Synchrotron Diffraction Studies on Transformation Strain Development in a High Strength Quenched and Tempered Structural Steel—Part I. Bainitic Transformation

In-situ phase transformation behavior of a high strength (830 MPa yield stress) quenched and tempered S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)) structural steel during continuous cooling under different mechanical loading conditions has been studied. Time-temperature-load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the phase fractions and lattice parameters of the phases during heating and cooling cycles under different loading conditions. In addition to the thermal expansion behavior, the effects of the applied stress on the elastic strains during the formation of bainite from austenite and the effect of carbon on the lattice parameter of bainitic ferrite were calculated. The results show that small tensile stresses applied at the transformation temperature do not change the kinetics of the phase transformation. The start temperature for the bainitic transformation decreases upon increasing the applied tensile stress. The elastic strains increase with increase in the applied tensile stress.     

Development of Non-Quenched Prehardened Steel for Large Section Plastic Mould

In order to meet the demand of prehardened steel for large section plastic mould and save energy, a non-quenched prehardened (NQP) steel is developed. The temperature field of a large block is researched by finite ele-ment method simulation and 9 test steels are designed in the laboratory. Their microstructures and hardness are in-vestigated when they are air cooled and control cooled at cooling rate similar to the simulation. The result shows that the hardness uniformity through section is closely correlated to bainitic hardenability for the NQP steel, and the hardness of one test steel (0.27C-1.95Mn-1.04Cr-0. 45Mo-0.1V) fluctuates between HRC 40 and 43 under both cooling conditions. The test steel has better machinability compared with C45 steel, and the NQP steel is produced successfully in the factory based on the laboratory results. Its microstructure is bainite, and it is distributed uniformly through the size of 460 mm×800 mm×3 200 mm.     

A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Plate Steel

Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.     

Anisotropy in Thermal Expansion of Bainitic Ferrite

The evolution of local d-spacings between lattice planes of bainitic ferrite in a high strength quenched and tempered structural steel, S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)), has been determined to calculate the thermal expansion behavior. For this purpose, in-situ continuous cooling tests have been carried out in a high-energy synchrotron X-ray diffractometer. The results indicate thermal anisotropy in the bainitic ferrite planes.     

Research status of low carbon high strength bainitic steel

The research status of low carbon Si- Mn bainitic steel at home and abroad in recent years was summarized. The mechanism of bainite transformation and the formation process were introduced. By analyzing the effects of alloying elements on the properties and microstructure of low carbon Si- Mn bainitic steel, the partitioning behavior of alloying element Mn in the process of dual phase insulation was discussed and the strengthening mechanism of low carbon bainitic steel was revealed. Finally, the relationship between the technology, organization and properties of low carbon high strength bainitic steel was elaborated and several kinds of preparation techniques which can obtain yield strength and elongation higher than 1000MPa and 15% respectively were introduced. On this basis, the main research directions of low carbon high strength bainitic steels were prospected.     

退火温度对0.2C-1.5Si-1.5Mn-(0.5Cu)TRIP 钢组织的影响

用彩色金相法、X 射线衍射、扫描电镜研究了成分(%)为 0.2C-1.5Si-1.5Mn和0.2C-1.5Si-1.5Mn-0.5Cu TRIP钢 750～780℃不同退火温度对组织的影响。结果表明,含0.5 %Cu TRIP钢750℃、760℃、780℃退火的残余奥氏体体积含量为15.2 %～17.2 % ,770℃退火为15.2 %～15.7% ;不含CuTRIP钢760℃和770℃退火的残余奥氏体体积含量分别为12.5 %～12.9%和14.6 %～15.1% ,低于750℃和780℃退火的奥氏体量分别为17.0 %～17.5%和16.0 %～16.4%。随退火温度由750℃提高至780℃ ,两种钢中的铁素体量由～70%降至～5 0% ,贝氏体量由～10%增至～30%     

Interrelations of cooling rate,microstructure, and mechanical properties in four HSLA steels

The present study was carried out on four steels containing 0.1 pct C-1.5 pct Mn-0.003 pct B* in common, with additions of 1 pct Cr, 0.5 pct Mo, 0.25 pct Mo + 1 pct Cr, 0.2 pct Ti + 1 pct Cr. They were designated, accordingly, as Cr, Mo, Mo-Cr, and Cr-Ti steels. All the steels exhibited a complete lath martensite microstructure with thin interlaths of retained austenite (≈0.05 pct) in the quenched condition. The normalized microstructures, granular bainite, contained massive areas of ferrite and granules of bainite laths. Both microconstituents contained a fine dispersion of cementite particles (size ≈50 Å) together with high dislocation densities. A mechanism explaining their for-mation has been given. The Cr steel, due to its low hardenability, showed in addition polygonal ferrite in the neighborhood of the so-called M-A constituent (twinned martensite and/or austenite). The annealed microstructure (using a cooling rate of 0.033 °C s^?1) of the Cr steel consisted of coarse ferrite-pearlite. Addition of 0.2 pct Ti to the Cr steel markedly refined the structure, whereas an addition of 0.25 pct Mo altered the microstructure to ferrite-lower bainite. In the 0.5 pct Mo steel, polygonal ferrite was found to be completely missing. The mechanical properties of the four steels after quenching, normalizing, and annealing were investigatedvia hardness and tensile test mea-surements. An empirical equation, relating the ultimate tensile strength to the steel composition, for steels that had granular bainite microstructures in the normalized condition, was proposed. The fracture surfaces exhibited cleavage and variable-size dimples depending on the microstructure and steel composition.     

Effect of phosphorus on the formation of retained austenite and mechanical properties in Si-containing low-carbon steel sheet

The effect of phosphorus and silicon on the formation of retained austenite has been investigated in a low-carbon steel cold rolled, intercritically annealed, and isothermally held in a temperature range of bainitic transformation followed by air cooling. The steel sheet containing phosphorus after final heat-treatment consisted of ferrite, retained austenite, and bainite or martensite. Phosphorus, especially in the presence of silicon, in steel was useful to assist the formation of retained austenite. Mechanical properties, such as tensile strength, uniform elongation, and the combination of tensile strength/ductility, were improved when phosphorus was increased up to 0.07 pct in 0.5 pct Si steel. This could be attributed to the strain-induced transformation of retained austenite during tensile deformation. Furthermore, two types of retained austenite were observed in P-containing steel. One is larger than about 1 μm in size and usually exists adjacent to bainite; the other one is of submicron size and usually exists in a ferrite matrix. High phosphorus content promotes the formation of stable (small size) austenites which are considered to be stabilized mainly by their small size effect and have a different formation mechanism from the coarser retained austenite in the lower P steels. The retained austenites of submicron size showed mechanical stability even after 10 pct deformation, suggesting that these small austenites have little effect on ductility. The 0.07 pct P-0.5 pct Si-1.5 pct Mn-0.12 pct C steel showed a high strength of 730 MPa and a total elongation of 36 pct.     

Constitutive Modeling of the Mechanical Properties of V-added Medium Manganese TRIP Steel

In this study, medium Mn transformation-induced plasticity steel with the composition Fe-0.08 pct C-6.15 pct Mn-1.5 pct Si-2.0 pct Al-0.08 pct V was investigated. After intercritical annealing at 1013 K (740 °C), the steel contained coarse-grained ferrite and two ultrafine-grained (UFG) phases: ferrite and retained austenite. The material did not deform by localized Lüders band propagation: it did not suffer from this major problem as most UFG steels do. Localization of plastic flow was shown to be suppressed because of a combination of factors, including a bimodal grain size distribution, a multiphase microstructure, the presence of nanosized vanadium carbide precipitates, and the occurrence of the deformation-induced martensitic transformation of retained austenite. A constitutive model incorporating these effects was developed. The model was used to identify the factors which can lead to a further improvement of the mechanical properties of the UFG medium Mn TRIP steels.     

钼微合金化低碳钢的显微组织与力学性能

借助物理模拟系统采用四种不同的多道次变形及控制冷却工艺,研究了成分为0.12C-0.78Si-1.42Mn-0.74Al-0.32Mo钢的显微组织和力学性能.结果显示:使用物理模拟系统进行高温区的多道次热连轧,并结合控制冷却处理,能够得到不同的复相组织（铁素体/贝氏体组织,贝氏体/马氏体组织）.依贝氏体含量和形态的不同,铁素体/贝氏体复相组织钢的屈服强度为388~558 MPa,抗拉强度为681~838 MPa,总延伸率为15%~27%;贝氏体/马氏体复相组织钢的屈服强度为746 MPa,抗拉强度为960 MPa,总延伸率为19%.      

Effect of alloying elements on the microstructure‐property relationship in thermomechanically processed C‐Mn‐Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe‐C‐Mn‐Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X‐ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise ~ 50 % of polygonal ferrite, 7 ‐12 % of retained austenite, non‐carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure‐property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C‐Si‐Mn‐Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb‐Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C‐Si‐Mn‐Nb steel leads to a good combination of strength (~ 940 MPa) and elongation (~ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ~7 ‐ 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb‐Mo‐Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb‐Mo‐Al steel appears to be more granular than in the Nb‐Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.     

超低碳高强度Q550D贝氏体钢的研制

利用超低碳和微合金化的成分设计,采用TMCP(Thermomechanical Controlled Proces)-T(Tempering)工艺,充分利用晶粒细化和针状铁素体与粒状贝氏体组织强化等手段,辅以回火处理工艺,在工业试制条件下得到韧性良好,屈服强度为550 MPa级的超低碳贝氏体钢。     

70kg级低碳贝氏体钢相变规律研究

利用MMS-200热模拟试验机和光学显微镜研究了70kg级低碳贝氏体钢板在不同终轧温度和冷却速度下的相变规律。结果表明,随冷却速度的增大,钢中依次出现多边形铁素体、珠光体、针状铁素体、粒状贝氏体、下贝氏体和马氏体组织,奥氏体向铁素体相变温度Ar3降低,晶粒细化。随着终轧温度的降低,铁素体诱导相变明显增加,铁素体晶粒细化。