Phase transformational kinetics and hardenability of low-carbon,boron-treated steels

The phase transformations and hardenability of 0.1 pct C boron-treated and boron-free steels containing Mn, Cr, Ni, or Cr plus Ni, and up to 1 pct Mo were studied. Continuous cooling transformation diagrams, hardenability characteristics, and diagrams of the ferrite start half-cooling time vs alloying were established. An unalloyed 0.1 pct C steel transforms diffusionally in the ferritic-pearlitic range when cooled from an austenitizing temperature, with a negligible contribution of the intermediate (bainitic) transformation occurring at very high rates of cooling. Molybdenum extends the range of the bainitic transformation and markedly delays the decomposition of austenite in the ferritic-pearlitic range. Boron treatment of the unalloyed (molybdenum-free) 0.1 pct C steel permits bainite formation over a wider range of fast cooling programs. At lower rates of cooling, the steel transforms diffusionally into ferrite and pearlite . Alloying additions of Mn, Cr, or Ni result in a slightly higher proportion of the bainitic transformation, which may occur over a wider range of cooling programs. When both nickel and chromium are present, a modest synergistic effect on the delay of the ferritic-pearlitic transformation may be noted. Introduction of molybdenum into all of the boron-treated 0.1 pct C steels strongly delays the decomposition of austenite into ferrite-pearlite structures and vastly expands the range of cooling programs that result in the formation of bainitic structures. In this important action, molybdenum is assisted to a smaller degree by alloying additions of manganese and chromium, and to a greater degree by nickel and chromium plus nickel. In all the steels studied, the alloying elements lower the temperatures of the bainitic transformation, thereby explaining, at least partly, the somewhat higher hardness for any specified cooling program. The observed beneficial effects of boron, molybdenum, and other alloying elements on the phase transformational behavior on continuous cooling are reflected in terms of higher hardenability.     

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.     

具有仿晶界型铁素体/粒状贝氏体复相组织的大截面钢筋

探索了仿晶界型铁素体／粒状贝氏体复相钢用于制造热轧低碳高强度可焊钢筋的可行性与工业化前景。对不同铬含量的空冷贝氏体钢筋的组织、强韧性及可焊性等进行了对比研究。结果表明，含有0．25％铬的Mn-Si—Cr系低碳贝氏体钢在轧后空冷条件下得到含有约26％(体积分数)的仿晶界型铁素体的粒状贝氏体复相钢，其抗拉强度、屈服强度和伸长率(δ6)分别达到830MPa、560MPa和18％，实现了较好的强韧性配合，并且具有较好的可焊性。存用干制造500MPa级大截面钢筋方面具有较大的市场化优势。     

Possibilities of influencing the γ-α-transformation products of hot-rolled pearlitic steels

Controlled alloying, optimum configuration of the rolling schedules, and a suitable cooling strategy make it possible to broadly influence the microstructure and mechanical properties of higher-carbon steels. Increasing the manganese content and microalloying with vanadium delay the diffusion-controlled γ-α-transformation, and the increase in the amount of pearlite improves strength properties. Lowering of the finishing temperature refines the austenite microstructure, and increases the level of residual strengthening that remains in the austenite. This changes the amount of ferrite and improves the resistance to brittle fracture. Applying a cooling interruption step in the ferrite formation temperature range also permits an additional variation of the amount of ferrite. The extent to which recalescence develops during the formation of pearlite can bring about significant changes in the pearlite morphology and hence influence strength and toughness properties. Accelerated cooling to a low coiling temperature, as a further process-related modification, promotes the formation of bainite, obtaining high strength properties without any mentionable loss of toughness.     

New Duplex Microstructure of Grain Boundary Allotriomorphic Ferrite/Granular Bainite

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Microstructure and properties of hot‐rolled high strength multiphase steels for automotive application

Effects of alloying with combinations of the elements Mo, Cr and B on the bainite transformation behaviour and microstructure of hot‐rolled high strength sheet steels microalloyed with mass contents of Ti and Nb, 0.05 or 0.15 % C and 1.5 % Mn have been studied. The relationships between microstructures formed in the steels coiled at various temperatures and their mechanical properties have been investigated. The 0.15 % C microalloyed steel alloyed with Mo,Cr and B with a complex bainitic microstructure was found to have distinctive high performance behaviour combining continuous yielding, high tensile strength and plasticity after coiling in a wide temperature region. The strain hardening of the micro‐constituents typical for the investigated steels has been analysed to have a better understanding of the mechanical properties of complex phase microstructures in low alloy ferrous alloys. It was found that bainitic ferrite with austenitemartensite islands as a second phase leads to high strength and adequate elongation. The features of the bainite formation in the Mo, Cr and B alloyed CMn steel microalloyed with Ti and Nb during slow cooling from temperatures between 650 and 550 °C was studied by dilatometry.     

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。     

Bainitic chromium-tungsten steels with 3 Pct chromium

Previous work on 3Cr-1.5MoV (nominally Fe-3Cr-2.5Mo-0.25V-0.1C), 2.25Cr-2W (Fe-2.25Cr-2W-0.1C), and 2.25Cr-2WV (Fe-2.25Cr-2W-0.25V-0.1C) steels indicated that the impact toughness of these steels depended on the microstructure of the bainite formed during continuous cooling from the austenitization temperature. Microstructures formed during continuous cooling can differ from classical upper and lower bainite formed during isothermal transformation. Two types of nonclassical microstructures were observed depending on the cooling rate: carbide-free acicular bainite at rapid cooling rates and granular bainite at slower cooling rates. The Charpy impact toughness of the acicular ferrite was considerably better than for the granular bainite. It was postulated that alloying to improve the hardenability of the steel would promote the formation of acicular bainite, just as increasing the cooling rate does. To test this, chromium and tungsten were added to the 2.25Cr-2W and 2.25Cr-2WV steel compositions to increase their hardenability. Charpy testing indicated that the new 3Cr-W and 3Cr-WV steels had improved impact toughness, as demonstrated by lower ductile-brittle transition temperatures and higher upper-shelf energies. This improvement occurred with less tempering than was necessary to achieve similar toughness for the 2.25Cr steels and for high-chromium (9 to 12 pct Cr) Cr-W and Cr-Mo steels.     

高碳贝氏体钢的低温转变组织与性能

摘要：采用光学与扫描电子显微镜、X射线衍射等手段研究了不同等温温度（300、250、200℃）对于高碳（质量分数0.79%）贝氏体钢低温转变样品的相含量、组织尺寸和力学性能的变化规律。结果表明,随贝氏体等温温度的降低,贝氏体最终转变量更高,贝氏体铁素体板条和薄膜状残余奥氏体宽度、块状残余奥氏体尺寸减小,抗拉强度升高,塑韧性降低。300℃的贝氏体抗拉强度为1525MPa,贝氏体铁素体宽度是116nm,而200℃的贝氏体铁素体板条尺寸达到62nm,抗拉强度达到1 928MPa。研究发现,在未充分转变的贝氏体样品中,尺寸大于4.7μm的块状残余奥氏体在冷却过程中易发生马氏体相变,而小于该尺寸的残余奥氏体比较稳定,可以保留到最终组织中。     

铬和铝元素对低碳贝氏体钢组织和性能影响

 低碳贝氏体钢通常需要添加一定量合金元素来提升性能,为了研究合金元素铬和铝在低碳贝氏体钢中的作用,以Fe-C-Si-Mn-Mo系贝氏体钢为基础,设计了单独添加铬元素和复合添加Cr+Al元素的3种低碳贝氏体钢,研究了铬和铝的添加对连续冷却处理低碳贝氏体钢显微组织、力学性能及贝氏体相变的影响规律。结果表明,连续冷却条件下,铬可以促进低碳贝氏体钢相变趋向于更低的温度区间进行,细化贝氏体组织,从而提高强度;铝可以促进贝氏体相变动力学,但对低碳贝氏体钢意义不大。同时,添加铝会使低碳贝氏体钢组织粗化,导致强度和伸长率同时下降。综合来看,复合添加铬和铝的优化效果不如单独添加铬,单独添加铬的低碳贝氏体钢强度达到1 623 MPa,伸长率为10.5%,结果可以为低碳贝氏体钢成分设计提供依据。     

层流冷却工艺对NM300TP热轧耐磨钢组织性能的影响

为探究NM300TP热轧耐磨板最佳冷却工艺,采用两段式冷却工艺,通过控制中冷温度和空冷时间,得到不同冷却工艺下的轧板.轧板具有贝氏体+铁素体+残余奥氏体的三相组织,无需轧后热处理便可获得良好的综合力学性能.研究结果表明,耐磨钢中各相含量与其力学性能有明显的对应关系,贝氏体越多,布氏硬度越大,抗拉强度越高,磨损失重越小,...     

Development of New Type Mn-Series Bainitic Steels in China

The alloying design idea,strengthening-toughening mechanism,microstructure,mechanical performances,development and application in China of new type Mn-series bainitic steels are introduced.Mn-series air-cooling bainitic steels including granular bainitic steels,FGBA /BG duplex steels,CFB/M duplex steels,medium carbon bainite/martensite steels,cast bainitic steels are presented.The invented idea mechanical performances,development and application of second generation of Mn-series bainitic steels,i.e.water quenching Mn-series bainitic steels invented by the authors newly are introduced.The water quenching Mn-series bainitic steels cover severe series steels containing ultra-low carbon,low-low carbon,medium-low carbon,and high-low carbon content etc,which can reduce the amount of alloying content,increase hardening capability and improve weldability.It should be pointed out that the application of both air cooling and water quenching Mn-series bainitic steels are complementary and mutually reinforcing,and the new type Mn-series bainitic steels can meet the performance requirements of most steels used in engineering structure.Some newest technologies of Mn-series bainitic steels in China are discussed in this paper.It is suggested that the significance of the development of the Mn-series bainitic steels can be summarized as:significantly reducing costs of both raw materials and production;good combination of strength and toughness;excellent weldability;simple procedure;large savings in energy resources and reduced environmental pollution.     

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.     

Effects of silicon content and intercritical annealing on the manganese partitioning in dual phase steels

 Steels of constant manganese and carbon contents with 0.34-2.26 wt. % silicon content were cast. The as-cast steels were then hot rolled at 1100°C in five passes to reduce the cast ingot thickness from 80 to 4 mm, air cooled to room temperature and cold rolled to 2 mm thickness. Dual-phase microstructures with different the volume fraction of martensite were obtained through the intercritical annealing of the steels at different temperatures for 15 min followed by water quenching. In addition of intercritical annealing temperature, silicon content also altered the volume fraction of martensite in dual-phase steels. The partitioning of manganese in dual-phase silicon steels were investigated using energy-dispersive X-ray spectrometry (EDS). The partitioning coefficient, defined as the ratio of the amounts of alloying element in the austenite to that in the adjacent ferrite, for manganese increased with increasing intercritical annealing temperature and silicon content of steels. It was also shown that the solubility of manganese in ferrite and austenite decreased with increasing intercritical temperature. The results were discussed by the diffusivity and the solubility of manganese in ferrite and austenite present in dual-phase silicon steels.     

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.     

Influence of V(C,N) Precipitates on Microstructure and Mechanical Properties of Continuous Cooled C-Mn-V

The effect of chemical composition and processing parameters on the formation of acicular ferrite and/or bainite has been investigated.In particular,this paper deals with the influence that N through its combination with V,as V(C,N) precipitates,has on the decomposition of austenite.Likewise,the intragranular nucleation potency of V(C,N) precipitates is analyzed through the continuous cooling transformation diagrams (CCT) of two C-Mn-V steels with different contents of N.Results reported in this work allow us to conclude that acicular ferrite can only be achieved alloying with vanadium and nitrogen,meanwhile bainite is promoted in steels with a low level of nitrogen.It is concluded that higher strength values are obtained in acicular ferrite than in bainitic steel but a similar brittle-ductile transition temperature (BDT),and lower values of impact absorbed energy (KV) has been recorded in nitrogen-rich steel.     

Phase transformational kinetics and hardenability of alloyed medium-carbon steels

The phase transformational kinetics and hardenability of 0.4 pct C steels were studied as influenced by alloying elements, singly and in combination. Sixteen series of steels, each containing up to about 0.75 pct Mo, were prepared by laboratory induction air melting. The base steels contained manganese, chromium, nickel, silicon, and combinations of these elements as alloy additions. Continuous cooling transformation diagrams, hardenability diagrams, and diagrams of the effects of alloying on the beginning of bainitic transformation and the beginning of ferritic transformation were established. The essential findings of the research program are summarized in this paper to establish a better understanding of alloy interactions and their effects on structure and properties. The effects of combinations of alloying elements in delaying the bainitic and ferritic-pearlitic transformations, and in increasing the hardenability cannot be predicted from the effects of each element alone. Furthermore, in most of the multi-alloy systems studied, molybdenum has been found to enhance the effectiveness of the other alloying elements present. Continuous cooling transformation diagrams clarify these relationships and together with the hardenability data provide a basis for predicting strength levels that can be obtained for steel products of varying section size. The 1.5 pct Si-1.5 pct Mn-1.4 pct Ni-0.7 pct Cr-Mo steels exhibit the maximum suppression of bainitic transformation and are closely followed by similar steels without nickel. The wide time range of the martensitic transformation in these steels qualifies them for consideration for air-hardenable constructional components of substantial section size. This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee.     

结构钢中的铌

自从20世纪70年代的Trans Alaska管线工程(TAPS)开始,铌被广泛用于管线钢生产,并因其具有最佳奥氏体调节的作用,激起含铌技术在整个TMCP钢中的应用。然而,近来钢材应用的发展不仅需要铁素体-珠光体型高强度结构钢,对贝氏体或者马氏体型高强度结构钢也提出了需求。在2002年,加Nb的TMCP钢的抗拉强度达到950MPa,首次用于神流川水电站工程建设。在该钢中,Nb是提高强度而不损害马氏体组织韧性的关键元素。另一方面,一些以铁素体(和贝氏体)为基的钢,比如海洋工程结构用钢和LPG运输船船体用钢,都添加Nb来降低碳含量提高低温韧性。结合工程实例,从母材钢板和焊接接头两个角度阐述了Nb冶金在结构钢中的种种应用。     

Creation of Air-Cooled Mn Series Bainitic Steels

The development and mechanical performances of new type air-cooled Mn series bainitic steels including granular bainitic steels, FGBA/BG duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitic steels invented by the authors are summarized. The novel series of bainitic steels are alloyed with Mn, and several series bainitic duplex microstructures can be easily obtained under the condition of air cooling through unique composition design. The invented idea, the principle of alloying design, the strengthening mechanism, and the evolution of the microstructure of new type air cooled Mn series bainitic steels are presented. Furthermore, the applications in different fields of these Mn series air cooled bainitic steels with different strength level are also introduced. It is suggested that the significance of the development of the air cooled Mn series bainitic steel can be summarized as follows： reducing costs of both raw materials and production; good combination of strength and toughness; self-hardening with high bainitic hardenahility by air cooling from hot working without additional quenching-tempering treatment or quenching procedure; large savings in energy resources; and reduced environmental pollution.     

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.