Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength and Low-Yield Ratio Bainitic Steels

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar₁ [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar₁ [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.     

Effect of Boron on the Isothermal Bainite Transformation

The role of Boron on the isothermal bainitic transformation in low-C, lean-alloyed steel was investigated. B clearly affected both the transformation kinetics and the morphology of isothermally transformed bainite. The effect of B was more noticeable in the high-temperature range of the bainitic transformation. The microstructure of bainite formed at 773 K (500 °C) consisted of a bainitic ferrite matrix and the martensite/austenite constituent. While the martensite/austenite constituent had an elongated morphology in B-free steel, the martensite/austenite constituents in the B-added steel had a granular morphology. Two types of bainite unit nucleation were considered: the initial nuclei and the nuclei formed on previously formed units. Electron backscattered diffraction (EBSD) analysis showed that the initial bainitic ferrite nuclei were formed at austenite grain boundaries with a Kurdjumov-Sachs (K-S) crystallographic orientation relationship with respect to one of the neighboring austenite grains, revealing the importance of interfacial energy reduction in the nucleation stage. The nuclei of the bainite transformation in the B-added steel were confined to the austenite grain interior, and the bainitic ferrite nuclei had crystallographic orientations limited to K-S variants within the same Bain variant. The characteristic bainite microstructure in B-added steel is due to the inhibition of the bainitic ferrite nucleation at austenite grain boundaries.     

CCT Curves of low carbon Mn-Si steels and development of water cooled bainitic steels

CCT curves of Mn-Si steels with different manganese contents or carbon contents were determined. The results show that the transformation range of bainite can be separated from that of ferrite when the manganese content approaches a certain content,and the incubation period of ferrite increases more significantly than that of bainite transformation with the increase of carbon content in Mn-Si steels. Furthermore,water-cooled bainitic steels without adding expensive alloying element were developed. Granular bainite was obtained when a bar with diameter of 300mm was cooled by water,and a mixed microstructure of granular bainite and martensite was obtained in water-cooled plate with thickness of 40mm. The developed water-cooled bainitic steels containing no expensive alloying element showed a good combination of strength and toughness. The tensile strength,yield strength,and toughness (AKU at -20℃) of bar with diameter of 300mm after water cooling were higher than 850MPa,620MPa,and 65J,respectively,and those of plate with thickness of 40mm after water cooling were higher than 1000MPa,800MPa,and 50J,respectively.     

Detailed study of the transformation mechanisms in ferrous TRIP aided steels

Development of TRIP aided ferrous alloys is one answer to the demand for weight decrease in the automotive industry. The microstructure of hot rolled and cold rolled TRIP steels is quite complex and the optimisation of such steel products requires a detailed understanding of the mechanisms of phase transformation, during thermomechanical treatment as well as during mechanical testing or metal forming. We present in this paper the results obtained at Irsid concerning the study of austenite stabilisation through bainitic transformation during thermal treatment and its transformation into martensite during mechanical testing. First of all, the characterisation methods are presented. An effort has to be put on this point due to the refinement of the microstructure of TRIP steels, especially the size of austenite and martensite islands. Carbon replicas for the observation by means of transmission electron microscopy (TEM) are used to analyse the morphological features of the microstructure ‐ nature of the constituents, size and shape ‐ and the composition of cementite particles present in the steels. The mean value for this carbon content in retained austenite is deduced from X‐ray diffraction measurements. Then the kinetics of bainitic transformation are discussed as well as cementite precipitation. The typical composition of the steel studied is 0.5 % C, 1.5 % Mn. The use of 0.5 % C steels facilitates the study of bainitic transformation by avoiding the ferrite formation usually occurring in TRIP steels. Cementite nucleation appears at the ferrite/austenite interface without any partitionning of substitutional elements. To satisfy thermodynamic equilibrium at the interface, the silicon content on the cementite side is very low and high on the austenite side. Then, carbon diffusion towards austenite is delayed and, as a consequence, cementite growth is also delayed. As the diffusion kinetics are low at 400 °C, cementite keeps this “non partitioned” composition, even after 3 hours holding. At 500 °C, diffusion kinetics are higher and cementite composition approaches that predicted by equilibrium. Finally, the stability of retained austenite during mechanical testing is studied. Before and after mechanical testing the morphological characteristics of the microstructure (austenite island size and elongation) are analysed by TEM replicas and image analysis. There is a high density of very small austenite islands but they represent only a small fraction of the total retained austenite. These results confirm and quantify the size effect on austenite stabilisation during deformation.     

Morphology and properties of low-carbon bainite

Morphology of low-carbon bainite in commercial-grade high-tensile-strength steels in both isothermal transformation and continuous cooling transformation is lathlike ferrite elongated in the 〈11l〉_b direction. Based on carbide distribution, three types of bainites are classified: Type I, is carbide-free, Type II has fine carbide platelets lying between laths, and Type III has carbides parallel to a specific ferrite plane. At the initial stage of transformation, upper bainitic ferrite forms a subunit elongated in the [−101]f which is nearly parallel to the [lll]_b direction with the cross section a parallelogram shape. Coalescence of the subunit yields the lathlike bainite with the [−101]_f growth direction and the habit plane between (232)_f and (lll)_f. Cementite particles precipitate on the sidewise growth tips of the Type II bainitic ferrite subunit. This results in the cementite platelet aligning parallel to a specific ferrite plane in the laths after coalescence. These morphologies of bainites are the same in various kinds of low-carbon high-strength steels. The lowest brittle-ductile transition temperature and the highest strength were obtained either by Type III bainite or bainite/martensite duplex structure because of the crack path limited by fine unit microstructure. It should also be noted that the tempered duplex structure has higher strength than the tempered martensite in the tempering temperature range between 200 °C and 500 °C. In the case of controlled rolling, the accelerated cooling afterward produces a complex structure comprised of ferrite, cementite, and martensite as well as BI-type bainite. Type I bainite in this structure is refined by controlled rolling and plays a very important role in improving the strength and toughness of low-carbon steels. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Effect of Composition and Deformation on Coarse-Grained Austenite Transformation in Nb-Mo Microalloyed Steels

Thermomechanical processing of microalloyed steels containing niobium can be performed to obtain deformed austenite prior to transformation. Accelerated cooling can be employed to refine the final microstructure and, consequently, to improve both strength and toughness. This general rule is fulfilled if the transformation occurs on a quite homogeneous austenite microstructure. Nevertheless, the presence of coarse austenite grains before transformation in different industrial processes is a usual source of concern, and regarding toughness, the coarsest high-angle boundary units would determine its final value. Sets of deformation dilatometry tests were carried out using three 0.06 pct Nb microalloyed steels to evaluate the effect of Mo alloying additions (0, 0.16, and 0.31 pct Mo) on final transformation from both recrystallized and unrecrystallized coarse-grained austenite. Continuous cooling transformation (CCT) diagrams were created, and detailed microstructural characterization was achieved through the use of optical microscopy (OM), field emission gun scanning electron microscopy (FEGSEM), and electron backscattered diffraction (EBSD). The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling ranges to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformation, moving the CCT curves to higher temperatures and shorter times. However, an increase in the final heterogeneity was observed when BF packets were formed, creating coarse high-angle grain boundary units.     

等温淬火温度对超细贝氏体钢组织及耐磨性的影响

设计了一种0.7C的低合金超细贝氏体钢,并通过膨胀仪、二体磨损实验、光学显微镜、扫描电镜、X射线衍射、激光扫描共聚焦显微镜及能谱仪,研究了不同等温淬火温度对超细贝氏体钢的贝氏体相变动力学、微观组织以及干滑动摩擦耐磨性的影响,揭示超细贝氏体钢在二体磨损条件下的耐磨性能和磨损机理.研究结果表明,不同等温温度下的超细贝氏体钢都由片层状贝氏体铁素体和薄膜状以及块状的残留奥氏体组成;随着等温温度的升高,超细贝氏体的相变速率提高,相变孕育期及相变完成时间缩短,但贝氏体铁素体板条厚度增加,残留奥氏体含量增加,硬度值有所降低;超细贝氏体钢磨损面形貌以平直的犁沟为主,主要的磨损机理为显微切削;不同等温温度下所获得的超细贝氏体的耐磨性能都优于回火马氏体,且随着等温温度的降低,耐磨性能提高.其中在250℃等温所获得的超细贝氏体钢具有最优的耐磨性能,其相对耐磨性为回火马氏体的1.28倍.这主要与超细贝氏体钢中贝氏体铁素体板条的细化及磨损过程中残留奥氏体的形变诱导马氏体相变（TRIP）效应有关.      

Effects of B and Cu Addition and Cooling Rate on Microstructure and Mechanical Properties in Low-Carbon, High-Strength Bainitic Steels

The effects of B and Cu addition and cooling rate on microstructure and mechanical properties of low-carbon, high-strength bainitic steels were investigated in this study. The steel specimens were composed mostly of bainitic ferrite, together with small amounts of acicular ferrite, granular bainite, and martensite. The yield and tensile strengths of all the specimens were higher than 1000?MPa and 1150?MPa, respectively, whereas the upper shelf energy was higher than 160?J and energy transition temperature was lower than 208?K (?C65?°C) in most specimens. The slow-cooled specimens tended to have the lower strengths, higher elongation, and lower energy transition temperature than the fast-cooled specimens. The Charpy notch toughness was improved with increasing volume fraction of acicular ferrite because acicular ferrites favorably worked for Charpy notch toughness even when other low-toughness microstructures such as bainitic ferrite and martensite were mixed together. To develop high-strength bainitic steels with an excellent combination of strength and toughness, the formation of bainitic microstructures mixed with acicular ferrite was needed, and the formation of granular bainite was prevented.     

Morphology of bainite and widmanstätten ferrite

Morphology of bainite and Widmanstätten ferrite in various steels has been investigated by means of microstructural and surface relief observations. It was shown that upper and lower bainite should be classified by ferrite morphology,i.e., lathlike or platelike, and that the morphology of cementite precipitation cannot be the index for the classification. Widmanstätten ferrite formed in the upper C-nose where ferrite grain-boundary allotriomorphs nucleate exhibits quite similar appearance with bainitic ferrite that forms in the lower C-nose of bainitic reaction. The only difference between them exists in the fact that Widmanstätten ferrite laths grow in the temperature range where primary ferrite forms and often terminate at a grain boundary ferrite but that bainitic ferrite has its own C-curve at temperatures belowB _s and nucleates directly at an austenite grain boundary. The mechanisms for their formations are discussed.     

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.

     

Transformation Characteristics of Ferrite/Carbide Aggregate in Continuously Cooled, Low Carbon-Manganese Steels

Transformation characteristics and morphological features of ferrite/carbide aggregate (FCA) in low carbon-manganese steels have been investigated. Work shows that FCA has neither the lamellae structure of pearlite nor the lath structure of bainite and martensite. It consists of a fine dispersion of cementite particles in a smooth ferrite matrix. Carbide morphologies range from arrays of globular particles or short fibers to extended, branched, and densely interconnected fibers. Work demonstrates that FCA forms over similar cooling rate ranges to Widmanstätten ferrite. Rapid transformation of both phases occurs at temperatures between 798 K and 973 K (525 °C and 700 °C). FCA reaction is not simultaneous with Widmanstätten ferrite but occurs at temperatures intermediate between Widmanstätten ferrite and bainite. Austenite carbon content calculations verify that cementite precipitation is thermodynamically possible at FCA reaction temperatures without bainite formation. The pattern of precipitation is confirmed to be discontinuous. CCT diagrams have been constructed that incorporate FCA. At low steel manganese content, Widmanstätten ferrite and bainite bay sizes are significantly reduced so that large amounts of FCA are formed over a wide range of cooling rates.     

The Bainite reaction in Fe-Si-C Alloys: The primary stage

The morphology, crystallography, and substructure of bainitic ferrite formed in silicon alloyed high-carbon steels in the temperature range 290 to 380 °C have been studied. The bainite exhibits a plate shaped morphology, an irrational habit plane, and an irrational orientation relationship. The bainitic ferrite is heavily dislocated, while the surrounding austenite contains thin twins, the density of which is highest in the austenite between closely spaced ferrite plates. The bainite plates can cross these twins in such a manner that the twinned region remains in a crystallographic orientation, which is quite different from that of the other regions of the bainitic ferrite plates. Epsilon carbide subsequently precipitates on the austenite twin/bainitic ferrite boundaries. The bainitic ferrite shape strain direction and magnitude are estimated from displacements of austenite twins inherited in the ferrite. All results, including measurement of the austenite carbon content, are consistent with a shear mode of transformation. B.P.J. SANDVIK, formerly with Laboratory of Physical Metallurgy at Helsinki University of Technology, Finland and Ovako OyAb, Imatra, Finland     

70Si3MnCrMo钢中贝氏体及其回火稳定性

 对一种新型70Si3MnCrMo钢进行了等温和连续冷却贝氏体相变热处理。利用拉伸和冲击试验研究试验钢的力学行为,利用XRD、SEM和TEM等方法对试验钢进行了相组成分析和微观组织形貌观察。研究结果表明,试验钢经等温贝氏体相变,其最佳综合力学性能出现在200 ℃回火,强塑积为26.4 GPa·%。经连续冷却贝氏体相变,其最佳综合力学性能出现在300 ℃回火,强塑积达到28.6 GPa·%。回火温度较低的情况下,热处理后的组织为由贝氏体铁素体和残余奥氏体组成的无碳化物贝氏体组织,这种无碳化物贝氏体由超细贝氏体铁素体板条而获得超高强度,由一定量的高碳残余奥氏体来保证较高的塑性和韧性。试验钢经连续冷却贝氏体相变,其贝氏体铁素体板条中出现了超细亚单元,并且残余奥氏体呈薄膜状和小块状两种形态分布于贝氏体铁素体板条之间,这两种形态残余奥氏体的稳定性不同。拉伸试样在变形过程中残余奥氏体持续发生TRIP效应,直至全部残余奥氏体都发生转变生成应变诱发马氏体,从而使钢得到更好的强、塑性配合,表现出十分优异的综合性能。     

Characterization of Transformation Stasis in Low-Carbon Steels Microalloyed with B and Mo

In the present study, bainite transformation kinetics was examined in low C-Mn steels with the addition of small amounts of B and Mo. This addition delays the onset of the bainite transformation. Mo addition causes transformation stasis at temperatures between 873 K and 823 K (600 °C and 550 °C) just below the bainite-start (B _s) temperature, resulting from an incomplete bainite transformation. Post-stasis transformation after a prolonged hold proceeds by the formation of ferrite with a low dislocation density, and in Mo-containing alloys, often the formation of carbides. The volume fraction at which the transformation stops is higher for lower carbon contents and lower transformation temperatures. By contrast, at 773 K (500 °C), the bainite transformation accompanying cementite precipitation occurs regardless of microalloying and is completed after shorter hold times. EDX measurement performed on the Mo-added 0.15 pct C alloy with aberration-corrected STEM revealed that segregation at the bainite/austenite interphase boundary is small for Mn and negligible for Mo in the early stages of stasis, which does not support the incomplete transformation mechanism based on the solute drag theory for the alloys used.     

不同硼含量微合金钢的连续冷却相变行为

 利用热模拟试验技术对实验室制备的含硼微合金钢连续冷却转变形为进行了试验研究,利用光学显微镜研究冷却速度、变形对试验钢显微组织的影响,探讨了硼对转变行为的影响规律。结果表明：适量硼延缓多边形铁素体生成,有利于获得贝氏体组织;无硼及wB=00020%时,分别在1~25及05~25℃/s的冷速都能得到贝氏体组织;wB=00030%时,冷速在2℃/s 以上能得到贝氏体组织;与未变形相比,变形导致试验钢贝氏体冷速区间变窄。在同一冷速下,随硼含量增加贝氏体开始转变温度先降低再升高,显微硬度随硼含量增加先增加而后降低。     

B对X100/X120高强度管线钢连续冷却相变组织的影响

 利用热模拟技术（DIL805A热膨胀仪）和显微分析方法,对不同成分体系X100/X120高强度管线钢在连续冷却转变下的显微组织的变化规律进行了研究。研究结果表明,对于无B钢,随冷速增加,组织中依次出现多边形铁素体(PF)、粒状贝氏体(GB)、贝氏体铁素体(BF)和马氏体(M)。B元素的添加使得管线钢相变开始温度降低到500℃左右,抑制了多边形铁素体的形成,促进了贝氏体的形成。为了获得高级别管线钢X100的复相组织,无B钢的冷却速度应控制在20~30℃/s,而含B钢的冷速只需控制在5~15℃/s,简化了冷却工艺。     

Bainite in steels

The mechanism of the bainite transformation in steels is reviewed, beginning with a summary of the early research and finishing with an assessment of the transformation in the context of the other reactions which occur as austenite is cooled to temperatures where it is no longer the stable phase. The review includes a detailed account of the microstructure, chemistry, and crystallography of bainitic ferrite and of the variety of carbide precipitation reactions associated with the bainite transformation. This is followed by an assessment of the thermodynamic and kinetic characteristics of the reaction and by a consideration of the reverse transformation from bainite to austenite. It is argued that there are useful mechanistic distinctions to be made between the coherent growth of ferrite initially supersaturated with carbon (bainite), coherent growth of Widmanstätten ferrite under paraequilibrium conditions, and incoherent growth of ferrite under local equilibrium or paraequilibrium conditions. The nature of the so-called acicular ferrite is also discussed.     

Structure and Properties of a Low-Carbon, Microalloyed, Ultra-High-Strength Steel

The current study concerns the development of a low-carbon, microalloyed ultra-high-strength steel on a pilot scale. The continuous cooling transformation has been evaluated, and a flat top “C” curve with a mixed microstructure of bainite and martensite has been obtained at a lower transformation temperature. The steel has been processed thermomechanically, followed by air cooling and water quenching. In addition, a variation in microstructure and mechanical properties at different finish rolling temperatures has been studied. Although a mixture of granular bainite and bainitic ferrite with interlath and intralath precipitation of NbC/NbC(N)/TiC(N) particles are the characteristic microstructural feature of air-cooled steel, the lath martensitic structure along the fine NbC/NbC(N)/TiC(N) precipitate is obtained in case of a water-quenched steel. The high-strength value obtained in the current steel is caused by the accumulated contribution of fine-grained, pancaked austenite, highly dislocated fine lath martensitic structure along with the presence of tiny precipitates of microalloy carbide/carbonitride.     

块状组织细化对贝氏体钢断裂行为的影响

 中碳贝氏体钢由亚微米贝氏体铁素体板条和残余奥氏体组成,对韧性起主要作用的为残余奥氏体,通过细化块状组织能显著提高贝氏体钢的韧性。为了探究块状组织细化对断裂行为的影响,采用两步贝氏体等温工艺对中碳(碳质量分数为0.3%)贝氏体钢中块状组织进行细化,对拉伸和冲击性能及其裂纹扩展行为变化进行了研究。利用光学、扫描电子(SEM)、透射电子(TEM)显微镜、X射线衍射(XRD)等对试验钢的显微组织类型和尺寸、拉伸和冲击性能及断口形貌进行表征和分析。结果表明,与一步贝氏体工艺相比,两步贝氏体工艺中新形成的贝氏体铁素体分割细化块状马氏体+残余奥氏体,随着真应变的增加,加工硬化的效果更好;断裂形式为韧性断裂,且韧窝的数量、深度更优于一步贝氏体转变,塑韧性更佳。     

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.