Secondary hardened bainite

Abstract

The effect of Mo additions on the development of bainitic ferrite in hot rolled low carbon (0·05 wt-%C) Nb containing steel strips has been studied. The steel strips were fabricated by a combined process of controlled rolling and accelerated cooling. Microstructural characterisation and mechanical testing for the corresponding strips were investigated. The results indicated that a small amount of Mo addition (0·1–0·3 wt-%) causes the production of a high volume fraction of bainite, which undergoes significant secondary hardening after tempering treatment at 600°C for 1 h. It is noticeable that the secondary hardening effect provides an additional way to significantly increase the strength of low carbon Nb–Mo containing bainitic steels.     

Mechanical properties and hot-rolled microstructures of a low carbon bainitic steel with Cu-P alloying

A low carbon bainitic steel with Cu-P alloying was developed. The new steel aims to meet the demand of high strength, high toughness and resistance to chloride ion corrosion for the components used in the environment of sea water and oceanic atmosphere. Mechanical properties of the steel were tested and strengthening and toughening mechanisms were analyzed by comparing hot-rolled microstructures of the low carbon bainitic steels with and without Cu-P alloying. The results show that Cu-P alloying provided strong solution strengthening with weak effect on ductility. The toughness loss caused by Cu-P alloying could be balanced by increasing the amount of martensite/remained austenite (M/A island) at lower finishing temperature. The static recovery process during rolling interval was delayed by the interaction of phosphorous, copper atoms with dislocations, which was favorable to the formation of bainitic plates. Super-fine Nb(C, N) particles precipitated on dislocations had coherency with bainite ferrite at 830 °C finishing temperature. Raising finishing temperature to 880 °C, Nb(C, N) particles were prone to coarsening and losing coherency. It was also found that no accurate lattice match relationship among retained austenite, martensite and bainite in granular bainitic microstructure.     

The effect of alloying elements on the structure and mechanical properties of ultra low carbon bainitic steels

Microalloying with Nb and B leads to a granular bainite microstructure which is composed of a bainitic ferrite matrix and a uniformly distributed martensite/austenite-constituent in the as-rolled condition. Due to this transformation strengthening mechanism, high strength and toughness could be achieved even though the C content was extremely low. It was found that both dissolved Nb in austenite and free B are prerequisites for granular bainite formation. Furthermore, there is a critical B content to achieve the complete bainitic transformation strengthening effect. The critical B content increases with C content. C thus diminishes the effect of B in promoting bainite transformation, due to the formation of boron carbides or the depletion of dissolved Nb in austenite. The effect of Mn, Mo and Ni on the decomposition of austenite is similar. A parameterMneq which relates the effect of these alloying elements on the Bs temperature was derived. It was confirmed that the strength of bainitic steels is inversely proportional to theBs temperature.     

Study on carbide-bearing and carbide-free bainitic steels and their wear resistance

Carbide-free and carbide-bearing bainitic steels have been obtained. The relationship between the bainitic microstructure and wear resistance has been studied. Results show that carbide-free upper and lower bainitic microstructures obtained in the steel with Si?+?Al mainly consist of bainitic ferrite and retained austenite. Carbide-bearing upper and lower bainitic microstructures obtained in the steel without Si?+?Al consist of bainitic ferrite, carbide and trace amounts of retained austenite. The carbide-free bainite exhibits higher strength and toughness than carbide-bearing bainite, especially the toughness. Under lower wear loading, carbide-bearing lower bainite (LB) exhibits higher wear resistance. Under higher wear loading, carbide-free LB exhibits higher wear resistance, which results from the improved surface hardness due to strain-induced martensitic transformation from the retained austenite.     

Effect of B and B + Nb on the bainitic transformation in low carbon steels

Development of new, advanced high and ultra-high strength bainitic steels requires the selection of the optimum balance of bainite promoting elements allowing the production of the desired bainitic microstructure over a wide range of cooling rates. The addition of boron or a combined addition of boron and niobium is well known to retard strongly the polygonal ferrite formation but very little knowledge has been acquired on the bainitic transformation. Therefore, the purpose of this study is to investigate the influence of boron and boron plus niobium on the bainite transformation kinetics, microstructural evolution and mechanical properties in a low carbon steel (Fe-0.05C-1.49Mn-0.30Si). Isothermal and continuous cooling transformation diagrams were determined and followed by a detailed quantitative characterisation of the bainite microstructure and morphology using complementary advanced metallographic techniques (FEG-SEM-EBSD, SIMS and TEM). The relationship between microstructure and hardness has been evaluated. Finally, results of SIMS and TEM analyses coupled with microstructural investigations enable to propose a mechanism to explain the effect of the synergy between boron and niobium on the bainitic transformation and the resultant microstructure.     

Development and study of high-strength low-Mo fire-resistant steel

Microstructures and properties of hot-rolled fire-resistant steels with two Mo addition levels (about 0.15 and 0.3 wt.%) have been investigated. The results show that a low-Mo (about 0.15 wt.%) fire-resistant steel with high strength (ultimate tensile strength: 770.3 MPa, yield strength: 488.7 MPa) and low yield ratio (0.634) has been successfully developed by microalloying Nb, V, and Ti and controlling cooling rate in combination. It is possible to obtain two-thirds of room-temperature yield strength at 600 °C in high-strength low-Mo (about 0.15 wt.%) fire-resistant steel. The analysis indicates that the bainite strengthening and precipitation strengthening are both important in improving the high-temperature strength of fire-resistant steel, and these strengthening effects effectively replace the strengthening effects of Mo. The present results also show that two different components in bainitic microstructure, discontinuous cementites or retained austenite, have similar effects on high-temperature strength of fire-resistant steel.     

Mechanical properties of a carbide-free bainitic cast steel with dispersed free ferrite

ABSTRACT

A high silicon, medium carbon cast steel was designed and heat-treated in order to develop microstructures composed of carbide-free bainite and small amounts of free ferrite, with the aim of obtaining high strength cast steels with improved ductility. Because of microsegregation, it was observed that ferrite present in partially austenitised samples is mostly present at the highly alloyed zones, creating an interconnected network even for low proportions of this phase. Despite the coarse solidification structure and marked microsegregation in the cast steel, the mechanical properties obtained for both fully bainitic and bainitic-ferritic microstructures largely satisfy the minimum standard requirements for high strength cast steels and are similar to those reported for wrought steels of similar microstructures.     

Effects of Mo on high-temperature strength of fire-resistant steel

A series of Fe–Mo–C steels with Mo addition from 0.1 to 0.8 wt.% have been prepared to study the effects of Mo on high-temperature strength of fire-resistant steel. The high-temperature hardness tests were carried out to investigate the strengthening mechanisms of Mo in fire-resistant steel. The results show that the hardness of Fe–Mo–C steels increases with the increase of Mo content at a given temperature, and the strengthening effect of Mo becomes remarkable when the temperature is on the rise. Theoretical analysis indicates that the solid-solution strengthening of Mo is the dominant high-temperature strengthening mechanism in fire-resistant steel, but this strengthening effect becomes relatively weak when Mo content is more than or equal to 0.5 wt.%. Moreover, the bainite strengthening plays an important role in improving the high-temperature strength of fire-resistant steel. Furthermore, the analysis indicates that the ferrite grain size has less effect on high-temperature strength of fire-resistant steel. The present results also provide fundamentals to design low-cost fire-resistant steels with excellent high-temperature properties and the most reasonable range of Mo addition is 0.2–0.3 wt.%.     

Copper bearing microalloyed ultrahigh strength steel on a pilot scale: Microstructure and properties

In the present study, copper bearing low carbon microalloyed ultrahigh strength steel has been produced on a pilot scale. Transformation of the aforesaid steel during continuous cooling has been evaluated. The steel sample has been thermomechanically processed followed by either air cooling or water quenching. Variation in microstructure and mechanical properties at different finish rolling temperatures has been studied. A mixture of granular bainite, bainitic ferrite and precipitation of nano-sized (Ti, Nb)C particles is the characteristic microstructural feature of air cooled steel. On the other hand, predominantly lath martensitic structure along with the similar type of microalloying precipitates of air cooled steels and Cu precipitates are obtained in case of water quenched steel. The best combination of strength (1364-1403 MPa) and ductility (11-14%) has been achieved for the selected range of finish rolling temperature of water quenched steel.     

High strength low carbon hot rolled δ-ferritic steel: microstructure and mechanical properties

Abstract

The present study concerns the development of high strength low carbon hot rolled bainitic and martensitic δ transformation induced plasticity steels. Equilibrium and para-equilibrium phase evolution have been examined by carrying out thermodynamic calculation using MT-DATA software. Microanalysis demonstrates that both manganese and aluminium partition between liquid and solid phases. Isothermal treatment and tempering at 350°C for bainitic and martensitic microstructures respectively have yielded the best combination of strength and ductility. All the steels have exhibited the continuous yielding behaviour and favourable ratio of yield and tensile strength, which are desirable for formability. The annealed steel has yielded a high level of tensile strength with the static toughness value in between the conventional transformation induced plasticity assisted and dual phase steels.     

片层状贝氏体铁素体长大动力学研究

为研究合金元素含量对钢中贝氏体铁素体长大动力学的影响,采用Zener-Hillert和Bosze-Trivedi动力学模型,通过选取热力学和动力学参数,计算了合金成分不同的钢的片层状贝氏体铁素体长大速度.研究表明:Fe-0.59C、Fe-0.81C和Fe-0.478C-4.87Ni合金在贝氏体相变时,贝氏体铁素体长大速度可以用无分配局部平衡条件下的扩散模型很好地描述,Fe-0.69C-1.8Ni-0.8Mo合金贝氏体铁素体长大速度略慢于理论值,Fe-C-8.7Ni合金贝氏体铁素体的长大速度比理论值约慢2个数量级;合金元素含量高的钢的贝氏体铁素体长大速度无法用扩散控制模型很好地描述;结合对贝氏体相变机制的讨论,提出贝氏体相变机制可能与相变温度和钢的成分相关.     

Stress induced transformation to bainite in Fe–Cr–Mo–C pressure vessel steel

Abstract

The kinetics of the bainitic transformation in a polycrystalline Fe–Cr–Mo–C alloy designed for applications in energy generation systems has been studied, with particular attention to the influence of mild tensile stresses on transformation behaviour. The steel was found to exhibit the incomplete reaction phenomenon, in which transformation to bainite stops well before the residual austenite acquires its paraequilibrium carbon concentration. It was found that even in the absence of an applied stress, the growth of bainitic ferrite caused anisotropic changes in specimen dimensions, consistent with the existence of crystallographic texture in its austenitic condition and, significantly, with the nature of the invariant-plane strain shape change that accompanies the growth of bainitic ferrite. Thus, transformation induced plasticity could be detected in fine grained polycrystalline samples, even in the absence of applied stress. The application of an external stress was found to alter radically the transformation behaviour, with clear evidence that the stress tends to favour the development of certain crystallographic variants of bainite, even though the stress may be well below the single phase yield strength. It is concluded that the transformation is influenced significantly by stresses as low as 45 MN m^?2, even though the effect may not be obvious in metallographic studies. The results are analysed and discussed in terms of the mechanism of the bainite transformation.

MST/1394     

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.     

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.     

Precipitation of MC phase and precipitation strengthening in hot rolled Nb–Mo and Nb–Ti steels

Hot rolled Nb–Mo steel of yield strength 600 MPa and Nb–Ti steel of yield strength 525 MPa with polygonal and acicular ferrite microstructure have been developed. Using physicochemical phase analysis, XRD, TEM and EDS, the distribution, morphology, composition, crystal structure and particle size of precipitates were observed and identified in these steels. The results revealed that the steels containing both Nb and Mo exhibited fine and uniformly distributed MC-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb and Ti. The physicochemical phase analysis showed MC-type carbides contain both Nb and Mo, and the ratio of Mo/Nb was 0.41. Meanwhile, the mass% of the fine particles (<10 nm in size) of Nb–Mo steel was 58.4%, and higher than that of Nb–Ti steel with 30.0%. Therefore, the results of strengthening mechanisms analysis showed the higher strength of Nb–Mo steel than that of Nb–Ti steel is attributed to its relatively more prominent precipitation strengthening effect. The yield strength increments from precipitation hardening of Nb–Mo steel attained 182.7 MPa and higher than that of Nb–Ti steel.     

3Cr2Mo塑料模具钢连续冷却相变行为

为了调节塑料模具钢3Cr2Mo的组织,以实现在线预硬化,使用Gleeble1500热模拟试验机、光学显微镜以及透射电子显微镜等研究3Cr2Mo钢变形及未变形奥氏体的连续冷却相变行为及相变组织.实验结果表明,3Cr2Mo钢奥氏体稳定性较高,在所研究的实验条件下,连续冷却过程中没有出现先共析铁素体和珠光体,而是发生贝氏体和马氏体相变.热变形使奥氏体发生了机械稳定化,贝氏体相变推迟到较低温度下才完成.随着冷却速度的降低,贝氏体的形态由常规板条状变成粒状,最终可获得粒状贝氏体组织.     

微合金元素Nb对低碳铸钢强度和冲击韧性的影响

采用中频炉冶炼制备不同Nb含量的微合金低碳铸钢,用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、液压万能强度试验机、半自动冲击试验机等手段研究了Nb微合金化对低碳铸钢显微组织、强度和冲击韧性的影响.结果表明,添加合适的微合金元素Nb可以使低碳铸钢的晶粒尺寸减小20.8％～34.6％,同时促进细小NbC析出相的形成,能有效提高低碳铸钢的强度和冲击韧性,晶粒细化和析出强化为其主要的强韧化机制.其中,含Nb量为0.044％的微合金铸钢屈服强度为350 MPa,抗拉强度为520 MPa,室温冲击功为119.7 J.与普通低碳铸钢相比,其塑性基本保持不变,但屈服强度、抗拉强度和室温冲击功分别提高了20.7％、7.2％和25.6％.     

基于S35140钢的超低碳成分设计和力学性能研究

S35140钢是一种基于25Ni-20Cr的奥氏体耐热钢,为了获得高强度,通常会提高碳含量,但碳含量较高不利于高温时效稳定性和长期耐腐蚀性能.本文在S35140钢的基础上,大幅度降低碳含量,并通过调控N和Nb等微合金元素含量,以及加入Ti元素,促使析出新的强化相,弥补减少碳含量所导致的强度降低.同时引入一定量的Al元素,增强S35140钢的高温强度和抗氧化性能.研究表明,加入微量Nb,N,Ti元素的热轧态实验钢析出了氮化物和细小的Laves相,这些析出相具有较强的强化作用,使其拉伸性能不亚于具有较高碳含量的S35140钢;加入4.7％Al(质量分数)元素后,实验钢的基体中出现了铁素体和奥氏体双相组织,同时析出大量B2-NiAl相,使其室温和高温拉伸强度以及室温韧性均高于其它成分的实验钢.     

贝氏体钢的研究现状和发展展望

综述了国内外贝氏体钢的研究开发和应用现状,分类介绍了Mo-B系、Mn-B系、新型准贝氏体钢、低温贝氏体钢、超细组织空冷贝氏体钢等.贝氏体钢已经在生产中得到广泛推广和应用,是21世纪性能优良的新一代钢种.