Fatigue properties of high strength-low alloy steels

High strength-low alloy (HSLA) steels are a relatively new group of alloys similar to hot rolled low carbon steel (HRLC) but having higher strengths as a result of composition and processing variations. Because these steels are of potential use in a variety of structural applications involving cyclic loading a knowledge of their fatigue behavior is important. Fatigue experiments were performed on several 80 ksi yield strength HSLA steels and on conventional HRLC steel for comparison. The HSLA steels were all found to exhibit similar fatigue resistance, and were superior to HRLC steel at longer lives. The effects on fatigue behavior of two types of plastic prestrain were determined. While prestrains caused large increases in monotonic strength properties, such improvements were largely lost in fatigue due to cyclic softening. Tensile prestrains are more detrimental to fatigue resistance than compressive prestrains. Finally, it was found that HSLA steel has a higher fatigue notch sensitivity than HRLC steel, however its notch fatigue resistance is still superior to that of HRLC steel.     

Fatigue properties of high strength-low alloy steels

High strength-low alloy (HSLA) steels are a relatively new group of alloys similar to hot rolled low carbon steel (HRLC) but having higher strengths as a result of composition and processing variations. Because these steels are of potential use in a variety of structural applications involving cyclic loading a knowledge of their fatigue behavior is important. Fatigue experiments were performed on several 80 ksi yield strength HSLA steels and on conventional HRLC steel for comparison. The HSLA steels were all found to exhibit similar fatigue resistance, and were superior to HRLC steel at longer lives. The effects on fatigue behavior of two types of plastic prestrain were determined.While prestrains caused large increases in monotonic strength properties, such improvements were largely lost in fatigue due to cyclic softening. Tensile prestrains are more detrimental to fatigue resistance than compressive prestrains. Finally, it was found that HSLA steel has a higher fatigue notch sensitivity than HRLC steel, however its notch fatigue resistance is still superior to that of HRLC steel.     

Cross Correlations of Modal Responses of Tall Buildings in Wind-Induced Lateral-Torsional Motion

Recent trends towards developing increasingly taller and irregularly-shaped buildings imply that these complex structures are potentially more responsive to wind excitation. Making accurate predictions of wind loads and their effects on such structures is therefore a necessary step in the design synthesis process. This paper presents a framework for dynamic analysis of the wind-induced lateral-torsional response of tall buildings with three-dimensional (3D) mode shapes. The cross correlation reflecting the statistical coupling among modal responses under spatiotemporally varying dynamic wind excitations has been investigated in detail. The effects of intermodal correlations on the lateral-torsional response of tall buildings with 3D mode shapes and closely spaced natural frequencies are elucidated and a more accurate method for quantifying intermodal cross correlations is analytically developed. Utilizing the wind tunnel derived synchronous multipressure measurements, a full-scale 60-story asymmetric building of mixed steel and concrete construction is used to illustrate the proposed framework for the coupled dynamic analysis and highlight the intermodal correlation of modal responses on the accurate prediction of coupled building acceleration.     

高层建筑用钢的发展

近年来，我国各大城市高层建筑发展迅猛，为充分利用有限的土地资源、发展钢结构建筑、急需大量建筑材，为此本文重点好国内外建筑用钢的发展，指出了高强度、高性能、大型化、功能化是建筑用钢发展的主要方向。     

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.     

近年来低合金高强度钢的进展

 从洁净钢生产、薄板坯连铸连轧无头轧制、薄带铸轧以及以快速冷却为核心的TMCP工艺等几个方面介绍了HSLA钢生产工艺技术的最新发展,并系统介绍了汽车用钢、船舶及海洋工程用钢、管线钢、建筑结构钢、核电用钢、压力容器用钢、工程机械用钢及集装箱用钢等行业所用的HSLA钢品种开发方面新进展。认为未来HSLA钢将向高强、高性能和低成本方向发展,对HSLA钢的发展有指导作用。     

Steel variability effects on low cycle fatigue behavior of a single grade of high strength low alloy steel

High strength low alloy (HSLA) steels are a relatively new group of alloys having higher strength as a result of composition and processing variations. Because these HSLA steels are being widely used in applications susceptible to a few cycles of stress or strain in the plastic region, the low cycle fatigue (LCF) properties of these steels are very important to design engineers; and as a result, a knowledge of the extent of the effects of several steel variables (viz, thickness, composition, processing variables, and so forth) and cold-work on the LCF properties of a single steel grade is very desirable from design safety considerations. Specimens obtained from a commercial grade of hot-rolled, pickled and oiled Nb-bearing, fine grained HSLA steel covering three thicknesses, two heats and four coil-positions were characterized as regards to their monotonie and cyclic properties. The present study indicates that the LCF behavior of this commerical grade steel is very insensitive to “steel variables” such as composition, thickness and coil-position. The large increase in monotonie strength obtained by cold-work is not retained under cyclic loading and the LCF behavior of cold-worked samples approached that of undeformed material.     

21st Century Niobium-Bearing Structural Steels

Value-added applications of niobium (Nb) microalloyed steels continue to be developed for commercial implementation to meet increased material demands and improved properties for 21st century structural applications. These applications demand Nb-bearing steels that deliver improved toughness, fracture and fire resistance and weldability. Such applications include medium and jumbo beam, boiler, bridge, container, heavy equipment, long product, pressure vessel, ship, storage tank and windtower applications. Steel producers are challenged to develop microalloyed steel grades that cost effectively meet end user demands for higher strength at thinner cross sections, better low temperature toughness to resist brittle fracture in building, pressure vessel and ship structures, sustain higher loads per unit area in earthquake and hurricane zone product applications, demonstrate improved fire-resistance in buildings, bridges and tunnels and provide overall improved weldability. Niobium is often a key element to achieve these results. This paper will discuss Nb market opportunities and key operational practices required to successfully melt, cast and roll these high strength steel grades. Niobium process metallurgy is important to leverage the ability of niobium to obtain ultra-fine grain, homogeneous structural steel microstructures with superior mechanical properties. The process metallurgy, physical metallurgy and resultant properties are significantly determined by mill capabilities, mill practices, operational understanding and the culture of the steel mill. The optimal combination and implementation aspects that are unique to each mill we call metallurgical operational integration (MOI) . MOI is the bridge that links the product requirements to mill capability and process implementation.     

Research and Practice of Production Technology for High Performance Low Alloy Steels in WISCO

The product system of high strength low alloy steels (HSLA) produced by Wuhan Iron and Steel (Group) Corporation (WISCO) was presented.A series of high performance structural steels have been developed for important construction projects around the world in different areas,such as bridge,pressure vessel,pipe line,shipbuilding,architecture,machinery and railway wagons.In order to promote the development of large heat input welding steel,high grade thick pipeline steel and long span bridge steel,a lot of advanced production technologies have been achieved through intensive investigation,such as the control of high purity,high homogenization and free defects in steel slabs,the rules of high melting point particle formation and distribution in micro and nano scale during steel production,and the TMCP technology for ultra low carbon bainite steel for medium and heavy plates.WISCO will carry on research and application of economic and weather-resistant steels and promote the developing and producing technologies of HSLA steel.     

Dynamic Strain Aging of Various Steels

The dynamic strain aging characteristics of two dual phase steels, a high strength low alloy (HSLA) steel, a 1008 steel and an interstitial free (IF) steel were determined from tensile properties at temperatures in the range 295 to 460 K (22 to 187 °C) and strain rates between 6 × 10^-6 to 10^-2s^-1. All except the IF steel were found to be susceptible to dynamic strain aging, as evidenced by increases in tensile strength. The largest positive change was observed in the 1008 steel while the dual phase and HSLA steels showed much smaller increases. Also, large decreases (up to 75 pct) in uniform elongation were noted for the 1008 steel while the decreases were minimal for the dual phase and HSLA steels. The IF steel did not strain age and showed a slight increase in uniform elongation with increasing temperature. Based upon uniform elongation as an indicator of formability, formability might be improved in dual phase or HSLA steels by reducing the concentration of free interstitials in the ferrites through chemistry control.     

Microstructural model for hot strip rolling of high-strength low-alloy steels

The microstructural evolution during hot-strip rolling has been investigated in four commercial high-strength low-alloy (HSLA) steels and compared to that of a plain, low-carbon steel. The recrystallization rates decrease as the Nb microalloying content increases, leading to an increased potential to accumulate retained strain during the final rolling passes. The final microstructure and properties of the hot band primarily depend on the austenite decomposition and precipitation during run-out table cooling and coiling. A combined transformation-ferrite-grain-size model, which was developed for plain, low-carbon steels, can be applied to HSLA steels with some minor modifications. The effect of rolling under no-recrystallization conditions (controlled rolling) on the transformation kinetics and ferrite grain refinement has been evaluated for the Nb-containing steels. Precipitation of carbides, nitrides, and/or carbonitrides takes place primarily during coiling, and particle coarsening controls the associated strengthening effect. The microstructural model has been verified by comparison to structures produced in industrial coil samples.     

冷却速度对高强度低合金钢组织和性能的影响

采用热模拟试验机、拉伸试验机以及光学显微镜等设备,根据CCT曲线和拉伸实验的结果,研究了冷却速度对700MPa级超级钢组织和性能的影响。在实验室热轧实验的基础上,比较了500MPa和700MPa两个强度级别HSLA钢的组织和性能随冷却速度的变化情况。结果表明,冷却速度对两种钢的影响不同：冷却速度增大时,500MPa级钢的晶粒明显变细,而700MPa级钢的晶粒尺寸变化不大。     

锆处理低碳低合金高强度钢的杂质控制

针对传统工艺技术生产的高强度低合金钢(HSLA)中杂质较高、性能不稳定等缺陷,改进了冶炼及连铸过程中一些工艺,形成了一套自主操作技术.生产的锆处理HSLA钢杂质含量显著降低,加入合金元素锆经改进工艺生产的钢板中夹杂物的大小、形态及分布均得到了明显改善,形成颗粒细小、圆形或近圆形、呈弥散分布的夹杂物.经电子探针和能谱分析表明,夹杂物一般是由Ti2O3、Al2O3、ZrO2及MnS组成的复合夹杂物,Ti2O3、Al2O3、ZrO2位于复合夹杂物的中心部位,MnS位于外层.锆处理和新工艺联合应用有效地改善了钢中夹杂物的特性,生产的HSLA钢板质量大幅度提高,具有良好的力学性能.     

大型冶金工业厂房钢结构安装施工研究

以韶钢大转炉主厂房钢结构安装施工为例,对主厂房钢结构吊装方案的选定、施工前的准备、主要构件安装方法、高强螺栓的保管与紧固以及安全设施等环节进行了阐述,总结了几点施工体会,为同类型大型冶金工业钢结构厂房的安装施工提供借鉴.     

Strain-aging of vanadium, niobium or titanium-strengthened high-strength low-alloy steels

Four commercially available high-strength low-alloy (HSLA) steels were evaluated in this study. It was determined that all four steels were susceptible to strain-aging by interstitial solutes. The increase in strength due to strain-aging was similar to that observed in a low carbon steel studied for comparison. At high levels of prestrain, the percent loss in ductility in the HSLA steels was comparable to that observed in the low-carbon steel in specimens prestrained to the same fraction of the total elongation of the as-received metal. However, when considered on an absolute basis, the residual ductility in the HSLA steels was 25 to 50 pct of that observed in the low-carbon steel. The kinetics of strain-aging were briefly examined. Indications are that the kinetics are slower in the HSLA steels than they are in the low-carbon steel.     

Strain-aging of vanadium,niobium or titanium-strengthened high-strength low-alloy steels

Four commercially available high-strength low-alloy (HSLA) steels were evaluated in this study. It was determined that all four steels were susceptible to strain-aging by interstitial solutes. The increase in strength due to strain-aging was similar to that observed in a low carbon steel studied for comparison. At high levels of prestrain, the percent loss in ductility in the HSLA steels was comparable to that observed in the low-carbon steel in specimens prestrained to the same fraction of the total elongation of the as-received metal. However, when considered on an absolute basis, the residual ductility in the HSLA steels was 25 to 50 pct of that observed in the low-carbon steel. The kinetics of strain-aging were briefly examined. Indications are that the kinetics are slower in the HSLA steels than they are in the low-carbon steel.     

连铸坯热送热装生产工艺的实践

叙述了低合金高强度钢在中国钢铁工业继续发展中的作用，介绍了国外低合金高强度钢在产品、合金化和工艺设计方面的发展。     

Twin roll strip casting of low carbon steels

Abstract

Strip casting of low carbon steels has been investigated using a laboratory twin roll machine with copper rolls. The following grades have been studied: a deep drawing low carbon (LC) steel, a LC steel with titanium addition, and a high strength low alloy (HSLA) grade. The casting behaviour of these steels has been examined, and the observed surface defects classified. Defects are most pronounced for the LC grade and significantly less for the HSLA steel and the LC steel with titanium addition. The as cast structure has been analysed. It can be modified by post-cast treatment, e.g. by normalisation or in line rolling. The mechanical properties of cold rolled and annealed strip materials and their textures are presented. Satisfactory sheet properties can be achieved both as hot band and as cold rolled sheet when adequate treatment steps are applied. Consequences for strip casting applications and future research are discussed.     

A Study of Spot Welding of Advanced High Strength Steels for Automotive Applications

The application of advanced high strength steels in automotive industry has highlighted the need for research into spot weldability of these steels.Using weld lobe diagrams,the spot weldability of DP600 steel was found to be poor with conventional weld schedules.An enhanced weld schedule consisting of two pulses with reduced current on the second pulse gave a substantial increase in the lobe width;the first pulse removed the zinc coating and the second pulse controlled the nugget growth.A data acquisition system was designed to monitor weld expulsion during the weld operation.Of the three possible control strategies proposed,especially with AC welding equipment,the dynamic resistance signal is easily obtained and the least expensive.Expulsion phenomena,microstructural characterization and mechanical properties of spot-welded hot dipped galvanized DP600 steel and interstitial free steel were investigated.Further work on dissimilar welds in DP 600 and HSLA 350 was also conducted with emphasis on tensile and fatigue properties and fracture characteristics.The performance of dissimilar spot welds was different from that of the similar spot welds in each of the HSLA350 and DP600 steels.The DP600 weld properties played a dominating role in the hardness and tensile properties of the dissimilar spot welds.However,the fatigue performance of the dissimilar welds was similar to that of the HSLA welds.Details will be presented at the conference.     

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。