Effect of Boron on Delayed Fracture Resistance of Medium-Carbon High Strength Spring Steel

The delayed fracture behavior of medium-carbon high strength spring steel containing different amounts of boron (0.000 5%, 0.001 6%) was studied using sustained load delayed fracture test. The results show that delayed fracture resistance of boron containing steels is higher than that of conventional steel 60Si2MnA at the same strength level and it increases with the increase of boron content from 0.000 5% to 0.001 6%. The delayed fracture mode is mainly intergranular in the boron containing steels tempered at 350 ℃, which indicates that the addition of boron does not change the fracture character. However, the increase of boron content enlarges the size of the crack initiation area. Further study of phase analysis indicates that most boron is in solid solution, and only a very small quantity of boron is in the M3(C, B) phase.     

Precipitation and fine structure in medium-carbon vanadium and vanadium/niobium microalloyed steels

Hot-rolled and continuously cooled, medium-carbon microalloyed steels containing 0.2 or 0.4 pct C with vanadium (0.15 pct) or vanadium (0.15 pct) plus niobium (0.04 pct) additions were investigated with light and transmission electron microscopy. Energy dispersive spectroscopy in a scanning transmission electron microscope was conducted on precipitates of the 0.4 pct C steel with vanadium and niobium additions. The vanadium steels contained fine interphase precipitates within ferrite, pearlite nodules devoid of interphase precipitates, and fine ferritic transformation twins. The vanadium plus niobium steels contained large Nb-rich precipitates, precipitates which formed in cellular arrays on deformed austenite substructure and contained about equal amounts of niobium and vanadium, and V-rich interphase precipitates. Transformation twins in the ferrite and interphase precipitates in the pearlitic ferrite were not observed in either of the steels containing both microalloying elements. Consistent with the effect of higher C concentrations on driving the microalloying precipitation reactions, substructure precipitation was much more frequently observed in the 0.4C-V-Nb steel than in the 0.2C-V-Nb steel, both in the ferritic and pearlitic regions of the microstructure. Also, superposition of interphase and substructure precipitation was more frequently observed in the high-C-V-Nb steel than in the similar low-C steel.     

几种高速列车用弹簧钢的奥氏体晶粒长大倾向

 弹簧钢的原奥氏体晶粒大小对其力学性能和疲劳性能有重要影响,采用光学显微镜研究了51CrV4、52CrMoV4、60Si2CrVA、60Si2MnA 4种高速列车用弹簧钢的原奥氏体晶粒在加热后的长大倾向,结合透射电镜的观察分析了4种弹簧钢具有不同奥氏体晶粒粗化温度的原因。试验结果表明,化学成分对其奥氏体晶粒长大倾向具有重要影响,弹簧钢中加入Cr、V、Mo能有效阻止原奥氏体晶粒的长大,奥氏体晶粒的粗化温度与微合金碳氮化合物的固溶温度有关。 在800~1100℃温度范围内加热,51CrV4中的奥氏体晶粒长大趋势最小,52CrMoV4和60Si2CrVA次之,60Si2MnA最大。     

Nb微合金化汽车用TWIP钢的研究进展

汽车是日常生活中主要交通工具之一,其用钢质量的优劣直接关系到汽车本身及乘坐人员的安全,因此研发高性能汽车用钢至关重要。微合金化是有效改善汽车用钢性能的手段之一,微合金元素铌可细化晶粒,提高材料的强韧性及氢致延迟断裂性能,备受研究者的青睐。总结了微合金元素铌对汽车用TWIP钢组织的影响,综述了铌对汽车TWIP钢力学性能、耐磨性能及抗氢致延迟断裂性能等的作用及相应机制,并提出了现阶段铌微合金化汽车用TWIP钢研究过程中存在的问题,为后续低成本、高效地发挥铌元素在高强度汽车钢中的应用提供参考依据。     

Production of ultra-high strength wire rod steels by vanadium microalloying

Ultra-high strength high-carbon wire rod steels have been produced using vanadium-microalloying technique instead of the conventional expensive and environment polluting lead patenting treatment. The strength increment attained in the hot rolled steels due to vanadium additions is maintained in the cold drawn wire. By using this technique, high tensile strength levels of 1550-1600 N/mm² were attained either by cold drawing of 0.17% V microalloyed high-carbon steel to 45-47% reduction or by cold drawing of 0.20% V microalloyed high-carbon steel to 25-30% reduction. An equation has been developed to predict the tensile strength from the chemical composition, cooling rate and reduction of area due to cold drawing. A combination of vanadium microalloying and accelerated cooling resulted in additional strength increment due to refining of microstructure and increasing the precipitation strengthening component. Inspite of the decrease in the amount of vanadium precipitates due to the increase in cooling rate, it is suggested that an increase in precipitation strengthening due to refining of these precipitates by accelerated cooling more than offsets the loss of precipitation strengthening due to decreasing the precipitates fraction.     

V-N微合金化Q460GJC钢板的研发

 为降低传统的Nb-V复合微合金化Q460建筑用钢板中的合金含量及生产成本,利用第3代TMCP技术和V-N微合金化技术,研制出单独V-N微合金化(wV=0.06%~0.08%)、不含铌的合金设计,并通过合理地控轧控冷工艺,在中厚板轧机上成功地生产出厚度40和50mm的V-N微合金化Q460GJC钢板。产品具有良好的综合性能,屈服强度大于470MPa,0℃冲击功超过150J,屈强比仅为80%。分析表明,细晶及析出强化对强度的贡献比例达到66%。     

Niobium in Microalloyed Steels Unchallenged for Its Main Applications

During the past five decades niobium has emerged as the most important microalloying element in high strength steels.The first theories to explain its role were presented in the seminal publications of Woodhead,Morrison and Gray.Today niobium microalloyed steels are unchallenged for their main applications.Some examples include linepipe for gas transportation,automotive steels used in body-in-white and structural parts,shipbuilding,steel towers and steel structures in civil construction that exhibit better fire resistance and seismic load performance.Modern technology for steelmaking helped emphasize the superior overall properties that can be attained with the use of niobium.Gas linepipe is the best example of cleaner steel with carbon levels below 0.05% manufactured economically.Modern niobium microalloyed steels present the optimal balance of strength,toughness,weldability and formability.This paper describes the evolution of the technology associated with niobium in microalloyed steels,technology that secured its status as the efficient solution to solving today’s challenges:safety,energy efficiency and environmental concerns.The paper also presents evidence that enhances the case that there is a secure,stable,long-term supply of the ferroniobium used to manufacture these steels.     

Study of the mechanical characteristics of low-carbon plate steels

The Azovstal’ Metallurgical Combine has conducted tests of the production of plates of low-carbon (0.06–0.08% C) low-alloy steels 06GBD and 06G2B. The plates, ranging up to 45 mm in thickness and having yield points ≥390 and ≥440 N/mm², respectively, are intended for use in structures employed for critical applications. Both steels are microalloyed with niobium, vanadium, molybdenum, and additions of copper. It is shown that in the quenched-and-tempered state the two steels fully conform to the specifications. The plates are characterized by excellent cold resistance, impact toughness, and weldability (C_e = 0.34–0.37 for steel 06GBD and C_e = 0.38–0.41 for steel 06G2B) and satisfactory values of the ratio σ _y/σ _u (≤0.90 for plates with a thickness ≤20 mm and ≤0.85 for plates with a thickness >20 mm). The construction of serial curves describing impact work and the percentage of the ductile component in the fracture of Charpy specimens prepared from plates of steel 06GBD showed that the plates have a high resistance to brittle fracture (impact work was at least 145 J at ?70°C, and the percentage of ductile fracture was 100% for 14-mm-thick plates and 80% for 32-mm-thick plates).     

The effect of silicon and nickel additions on the sulfide spacing and fracture toughness of a 0.4 carbon low alloy steel

This paper discusses the effects of silicon and nickel additions on the mechanical properties of a 0.4 carbon low alloy steel. The four experimental steels used in the study were obtained by making additions of 1.5 wt pct nickel and 2 wt pct silicon, both separately and in combination, to a 0.4C/1.5Ni/1.0Cr/0.5Mn base steel. The base + Ni + Si steel, resulting from combined nickel and silicon additions, has a yield strength of 1682 MN/m² and impact and fracture toughnesses of 65 J and 115 MN/m^3/2, respectively. The other three steels have comparable strength levels but more typical impact and fracture toughnesses of about 30 J and 75 MN/m^3/2, respectively. The microstructures of the four steels are almost identical. The only significant observed difference among the four steels is that the sulfides in the base + Ni + Si steel are almost three times as large as those in the other three steels. As the four steels have similar sulfide volume fractions, there is a similar difference in sulfide spacings. The improved toughness of the base + Ni + Si steel is attributed to the increased sulfide spacing due to the influence of combined nickel and silicon additions on the average sulfide size.     

铌对60Si2Mn弹簧钢表面脱碳敏感性的影响

 采用金相法测量了不同铌含量的弹簧钢60Si2Mn的脱碳层深度,系统研究了铌及其含量对60Si2Mn弹簧钢表面脱碳敏感性的影响。研究结果表明：添加铌元素可以有效降低60Si2Mn的脱碳敏感性,但脱碳层深度并未随铌含量的微量增加而降低。利用Thermo-Calc DICTRA软件对不同温度下碳在奥氏体中的扩散系数进行了计算,考虑了铌及其他合金元素对碳的扩散影响。通过菲克第二定律获得了试验钢中脱碳层深度随温度的变化规律,并与实测值相吻合。为研究其他微合金元素对脱碳层的影响提供了一种更为有效的手段。     

Niobium in Engineering Steels for Automotive Applications

In the last decade, the European niobium consumption in automotive strip and sheet has more than tripled. The development of high strength and advanced high strength steel grades for strip and plate products focused upon constantly increasing strength levels combined with excellent fatigue properties as well as formability and weldability. Until now, the high potential of microalloyed steel has not been used to the same extent in forgings and springs. This paper reviews automotive components as transmissions, suspensions and springs highlighting new material solutions for special steels having optimized processing and in‐use properties. Special consideration is given to the metallurgical background as well as to examples for the processing and application of niobium microalloyed steel in forgings and springs from low carbon and higher carbon containing steels. Niobium metallurgy permits the reduction of carbon to meet more stringent end‐user requirements demanding high strength combined with improved toughness, fatigue strength and weldability at a competitive manufacturing cost.     

Delayed fracture of tensile strength 980 MPa grade steels for automotive applications

Three different types of tensile strength( TS) 980 MPa grade advanced high-strength steels used in automotive applications,namely,980 MS( martensite steel),980DP( dual phase) and 980QP( quenching and partitioning) steels were examined. The delayed fracture resistance of the steels was evaluated using a U-bend test,slowstrain rate test( SSRT) and a constant load tensile test. The results indicated that all the steels could pass the300h HCl solution immersion test and none of the U-bend specimens was fractured in the test. However,the steels exhibited different susceptibilities to delayed fracture under SSRT and the constant load tensile tests. 980 DP exhibited the highest resistance to delayed fracture among all the samples,while 980 MS was found to be the most susceptible to delayed fracture.     

Study on hydrogen induced delayed fracture of 1500 and 2000MPa hot stamping steels

The application of hot-stamping steel (HS) in the automobile is an inevitable trend, but the hydrogen embrittlement sensitivity of HS steel still needs to be studied and improved. The hydrogen diffusion behavior and hydrogen embrittlement sensitivity of 1500 and 2000MPa hot stamping steels were studied by means of hydrogen penetration, slow strain rate tensile (SSRT), and fracture analysis. The results show that the apparent diffusion coefficient Dap (1.71×10-7cm2/s) of 1500HS is significantly less than the Dap (3.45×10-7cm2/s) of 2000HS; delayed fracture resistance of 1500HS is superior to 2000HS. From the fracture analysis, under the same hydrogen charging conditions, the fracture morphology of 1500HS changed from typical dimple ductile fracture to quasi cleavage brittle fracture, while 2000HS changed from dimple morphology to intergranular brittle fracture with the increase of hydrogen charging current density. While the deformation degree of 2000HS was very small, the local hydrogen content and stress value had reached the critical deal. The hydrogen reduced the bonding force between grains, resulting in the nucleation and propagation of microcracks. Therefore, with the improvement of the strength of HS steel, Ti and V micro alloyed elements should be properly added to form nano precipitates, as irreversible hydrogen traps to capture hydrogen atoms, hinder their diffusion and segregation, and effectively refine the structure and pinning dislocations, to improve the resistance to hydrogen induced delayed fracture of HS steel.     

Studies of Evaluation of Hydrogen Embrittlement Property of High-Strength Steels with Consideration of the Effect of Atmospheric Corrosion

Hydrogen embrittlement of high-strength steels was investigated by using slow strain rate test (SSRT) of circumferentially notched round bar specimens after hydrogen precharging. On top of that, cyclic corrosion tests (CCT) and outdoor exposure tests were conducted prior to SSRT to take into account the effect of hydrogen uptake under atmospheric corrosion for the evaluation of the susceptibility of high-strength steels. Our studies of hydrogen embrittle properties of high-strength steels with 1100 to 1500 MPa of tensile strength and a prototype ultrahigh-strength steel with 1760 MPa containing hydrogen traps using those methods are reviewed in this article. A power law relationship between notch tensile strength of hydrogen-precharged specimens and diffusible hydrogen content has been found. It has also been found that the local stress and the local hydrogen concentration are controlling factors of fracture. The results obtained by using SSRT after CCT and outdoor exposure test were in good agreement with the hydrogen embrittlement fracture property obtained by means of long-term exposure tests of bolts made of the high-strength steels.     

Static recrystallisation kinetics of austenite in Ti,Nb and V microalloyed steels during hot deformation

Abstract

Static recrystallisation kinetics during hot deformation of two microalloyed steels (C–Ti–V and C–Ti–Nb) has been quantified. Double hit compression test using a Gleeble 1500 thermomechanical simulator was conducted to determine the recrystallisation kinetics. The kinetics of static recrystallisation was found to be more sluggish in the case of microalloyed steels with niobium additions as compared with other grades of microalloyed steels. The rate of recrystallisation increases with increasing temperature, strain and strain rate. The results are compared with other grades of microalloyed steels, mainly with Ti, Ni and Cu additions, from literature. For niobium containing microalloyed steels, a higher temperature is required for recrystallisation as compared with other microalloyed steels.     

汽车紧固件用钢的发展动向

简要介绍了汽车紧固件用钢的性能要求,冶炼及轧制的特点;对相关的洁净螺栓钢、微合金非调质钢、免退火钢、硼钢、耐延迟断裂高强钢的现状和发展动向进行阐述;指明了实施汽车紧固件用钢国产化的努力方向。     

φ20～42iniil60Si2MnA弹簧钢的脱碳控制

介绍了石钢三轧厂加热炉在加热60Si2MnA弹簧钢时存在的缺陷。为了减小中φ20-42mm60Si2MnA弹簧钢的脱碳层深度,进行了加热工艺参数调整,根据用风量调整炉内氧化性气氛,调节烧嘴轴向风和切向风的比例等试验,使φ20～42mm60Si2MnA弹簧钢脱碳检验的一次合格率,稳步提高,达到96．O％,提高了产品质量。     

Effect of Microalloying Additions on the Microstructure and Mechanical Properties of Low Carbon Steel

Low carbon steels are characterized by good weldability,formability and fracture toughness properties.However,the low strength levels of these steel grades limit their wide applications.On the other hand,increasing the strength by increasing the carbon content and alloying elements deteriorates the other properties.In this study,the microalloying technique was used to examine the possibility of attaining low carbon steels with good combination of strength,ductility and impact properties.A low carbon steel microalloyed with single addition of vanadium and another one microalloyed with combined addition of vanadium and titanium were used in this investigation and their properties were compared with non-microalloyed low carbon steel having the same base composition.Furthermore,other two nonmicroalloyed and V-microalloyed steels with higher carbon,silicon and manganese contents were also investigated to reveal the effect of base composition.Tensile,hardness,room and zero temperature Charpy V-notch impact tests were conducted to evaluate the variations in the mechanical properties of low carbon hot forged steel containing vanadium and combinations of vanadium and titanium.In addition,the microstructures of the different investigated steels were observed using both optical microscope and scanning electron microscope.Furthermore,the hardness of the ferrite phase was also determined using micro-hardness technique.The results showed improvement of the mechanical properties of the investigated steels by both single V-and combined V + Ti-microadditions.Tensile,hardness and impact tests results indicated that good combinations of strength,ductility and impact properties can be achieved by V-microalloying addition.Steel with combination of V and Ti microaddition has much higher hardness,yield strength,ultimate tensile strength and impact energy at both room and zero temperatures compared with non-microalloyed and single Vmicroalloyed steels.Higher C,Si and Mn contents result in increasing the strength accompanied with decrea     

矿山钻杆用60Si2MnA弹簧钢的研制开发

采用转炉+LF(VD)精炼工艺生产60Si2MnA弹簧钢,研究了转炉冶炼、LF(VD)精炼、连铸及轧制关键工艺技术和过程控制要点,对钢的性能指标及使用情况进行了分析。结果表明:产品实物质量良好,完全满足标准要求;该钢应用于加工矿山钻杆产品,综合性能及疲劳寿命优异。     

Parameters affecting sag resistance in spring steels

Recent trends toward reducing the weight of automobile suspension springs have led to the development of a number of microalloyed steels and a variety of processing treatments which have claimed to improve the sag resistance of springs while increasing their operating stresses. More often than not, however, the subtle effects of varying levels of hardness and prior austenite grain size, as well as small but significant differences in critical elements such as Si, are over-looked in comparing these new steels with the conventional grades. Hysteresis loops measured in tension (related to the Bauschinger effect) were used to determine the relaxation behavior of a number of microalloyed and standard grade spring steels. The effects of hardness level, austenitizing temperature, prior austenite grain size, and warm prestressing on the Bauschinger hysteresis loops were also established. Silicon was found to be the most important factor influencing the size of the hysteresis loops; the greater the Si content of the steel (up to 2.2 pct), the larger the loops at a given strength level and the greater the expected relaxation resistance of the spring. The standard AISI 9261 steel containing 2.2 pct Si showed the same Bauschinger loops as the microalloyed grades containing Nb and V at the same hardness of 50 HRC.