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.     

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.     

Effect of prestrain and baking on the secondary deformation properties of cold-rolled ultra-high-strength steel sheets

Two experimental steels with tensile strength above 980 MPa were prepared to investigate the effect of prestrain and baking on their mechanical and fracture behaviors. The experimental results reveal that,for both experimental steels,with increases in the prestrain level,the bake hardening value increases before reaching a maximum point,and then decreases with further increases in the prestrain level. The results of a "bending-baking-secondary bending"test indicate that the secondary bendability deteriorates at a high level of prestrain. The yield strength of the experimental steels was found to increase and the elongation to decrease after high levels of prestrain and bake hardening. Fracture morphology images indicate that a high prestrain level is associated with shallow dimples and more and larger local cleavage areas.     

Influence of silicon and phosphorous on the mechanical properties of both ferrite and dual-phase steels

A study has been made of the influence of up to 2 pct Si and 0.42 pct P upon the strength and ductility of ferrites over a wide grain size range; the grain size was varied fromd ^−1/2 mm^−1/2 = 4 to 14. Although the ductility decreased with increasing strength for all the alloys, the 2 pct Si alloy had the best combination of strength and ductility. The ferrites containing 2 pct Si and 0.2 pct P had greater uniform elongations than conventional HSLA steels at the same tensile strength; it is thought that the ductility of the ferrites is enhanced by the presence of Si while the ductility of conventional HSLA steels is reduced by the presence of carbide precipitates. With the theory for a composite of two ductile phases and the results for the fine-grained alloyed ferrites, the change in uniform elongation as a function of tensile strength was predicted for dual-phase (martensite plus ferrite) steels. Good agreement was found between the prediction and experimental results for dual-phase steels containing up to 0.2 pct P or 2 pct Si; the 2 pct Si alloy had the best combination of strength and ductility of all dual-phase steels so far reported. This study again emphasizes the importance of the high strength, high ductility ferrite in controlling the properties of dual-phase steels.     

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.     

Hydrogen embrittlement of dual-phase steels

Reversible hydrogen embrittlement (HE) is usually only found in quenched and tempered steels with yield stresses in excess of 1035 MPa (150 ksi). A study of the HE phenomena in two dual-phase steels with tensile strengths of about 690 MPa (100 ksi) has shown that these steels are susceptible to the presence of hydrogen. HE results in a reduction in fracture strength, although no preyield failures are observed, and a change in fracture mode from ductile dimpling to transgranular cleavage. After prestraining and HE, it is found that the greater the prestrain the higher is the fracture stress. It is concluded that the presence of the 15 to 20 pct high carbon (0.6 pct C) high strength martensite in the dual-phase steels is responsible for the HE; tempering studies give results consistent with this idea. Delayed failure tests on notched specimens showed that for the as-received condition, the run-out stress (stress for no failures in 50 to 100 h) to be above the macroscopic flow stress. A condition for HE failure in dual-phase steels appears to be considerable macroscopic deformation.     

High‐Strength Steels in Welded State for Light‐Weight Constructions under High and Variable Stress Peaks

In design codes (Eurocode, British Standard and others) for the dimensioning of welded joints, no distinction is made between low, medium and high strength steels. Because of a lack of general knowledge about the benefits of high‐strength steels and also because of missing information in design codes, in many cases design engineers still use low or medium strength steels (R_p0.2 < 400 MPa) and compensate for high loads under constant or variable amplitude loading or overloads by increasing dimensions. Given this situation, it was deemed necessary to establish criteria for the design of light‐weight welded constructions under high and variable stress peaks using new classes of high strength steels, such as S355N (normalized), S355M (thermomechanically treated), S690Q (water quenched) and S960Q (water quenched), and to perform more reliable evaluations of the fatigue performance of high strength steel structures subjected to complex loading with regard to light‐weight design and economics. For the comparison of the fatigue strengths of the investigated steels the notch factors present were taken into account. Additionally, the real damage sums were determined in order to give recommendations for the fatigue life estimation. Under constant amplitude loading, no significant difference in the bearable local stress amplitudes for the butt welds can be detected for the four investigated steels. Under variable amplitude loading, the butt welded (lower notch factor) high strength steel S960Q has advantages in the case of the normal Gaussian spectrum and in the case of overloads, especially under pulsating loading. For the transverse stiffeners (high notch factor), slight advantages for the high strength steel S960Q exist, only in the case of pulsating overloads. However, the advantages of high strength steels in case of static loading are indisputable. In most of the investigated cases, overloads lead to a benefit in fatigue life.     

Calculability of the Bake Hardening Effect of Hot Rolled Multiphase Steels

In recent years high strength hot rolled multiphase steels with a thickness of less than 2 mm have become available. These multiphase steels show a good Bake Hardening effect, which makes them important for automotive constructions, because an increase of yield strength may lead to a weight optimised body structure and/or a better crash resistance. In this investigation five different hot rolled multiphase steels were examined. The steels were exposed to different prestrains, temperatures and holding times. Design of Experiments and statistical methods were used to reduce the quantity of experiments. After evaluation of the results it was possible to develop a formula to predict the yield strength for a given prestrain, temperature and time for each steel. Furthermore, a method was found to decide easily if a steel is qualified for further investigation of the Bake Hardening effect, or more exactly, of its response to different annealing temperatures and times.     

Shear Fracture of Advanced High Strength Steels

Failure experiments were carried out through a stretch-bending test system for advanced high strength steels, i.e. dual-phase （DP） steels and martensitic steels （MS）. The die radius in this system was designed from 1 to 15 mm to investigate the failure mode under different geometries. Two failure modes were observed during the ex- periments. As a result, critical relative radii （the ratio of inner bending radius R to sheet thickness t） for DP590 and DP780 steels were obtained. The stretch-bending tests of DP980 display some trends unlike DP590 and DP780 steels, and curve of DP980 in different thicknesses does not coincide well. High blank holder force exhibits more possibility of shear fracture tendency than low blank holder force. The unique character of high strength martensitic steel （1500MS） is that no shear fracture is found especially over small bending radius （R =2 mm） under the same experi- mental conditions. Microstructure analysis indicates that there are obviously elongated grains on shear fracture sur- face. It shows smaller diameter and shallower depth of the dimples than the necking failure.     

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.     

日本研制高强度无取向电工钢的新技术

为了适应新能源汽车——电动车（EV）和混合动力电动汽车（HEV）电机对高性能电工钢板的需求,日本住友金属工业有限公司应用位错强化方法,已经成功研制出一种具有高强度、低铁损的无取向电工钢,主要介绍新产品690～780MPa、0．35mm厚的高强度无取向电工钢的磁性能和力学性能都优于980MPa双相高强度钢,并且成本非常低,因此得到新能源电动车（EV）和混合动力电动汽车（HEV）用高效永磁（IPM）电机铁芯的广泛应用。     

高等级管线钢的发展现状

回顾总结了近年来国际上高等级管线钢的发展.现代管线钢向高强度、大厚度、抗应变和抗HIC方向发展.为降低长距离天然气管线的建设成本,开发了X100和X120超高强度管线管,并进行了X100和X120管线试验段建设,取得了显著的进展.开发了以双相显微组织为特征的满足"基于应变设计"的抗大应变高强度管线钢,强度等级从X65至X100,可应用于冻土带、地震区和水土流失区域的管线建设.海底管线用钢和抗HIC管线管的强度等级已从以往的X65提高到X70,X70管线管的最大壁厚可达34.1 mm,并已批量在工程中应用.新型的HTP高强度管线钢采用超低碳高铌含铬的成分设计,具有十分优良的性能,用铬替代钼可显著地降低成本,生产的X80管线管已应用于美国第一条X80管线.     

Inclusion behaviour in aluminium-killed steel during continuous casting

The inclusion behaviour during continuous casting were investigated in ultra low carbon (ULC) and high strength low alloy (HSLA) steels. From tundish to cast slabs, the total oxygen (T.O) content decreased from 19 to 12?ppm, 12 to 11?ppm in ULC steel and HSLA steel, respectively. The number density of inclusions in ULC decreased by about 40%, while it kept almost constant in HSLA steel during casting. A simple calculation on the T.O removal agreed well with the measured value, which showed that T.O removal was much larger through mould flux absorption than nozzle attachment. The T.O removal in ULC steel is substantially higher than that of HSLA steel due to the fact that alumina particles in ULC steel tend to agglomerate easier than calcium aluminates in HSLA steel, which facilitates inclusion removal to the mould powder.     

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.     

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。     

The influence of prestrain and ageing on near-threshold fatigue-crack propagation in as-rolled and heat-treated dual-phase steels

The influence of prestrain and ageing on near-threshold fatigue-crack propagation (FCP) in as-rolled and heat-treated dual-phase steels (DPS) was investigated over a wide range of 10^?10 to 10^?7 mm/cycle in laboratory air under a load ratio of R = 0. It was found that the fatigue-crack propagation threshold value increases with increasing grain size and decreasing yield stress, and that a combination of 10% prestraining with 175°C/30 min ageing showed almost no effect on the threshold level of as-rolled dual-phase steel, but decreased that of heat-treated dual-phase steels more than 37%. This different behaviour was suggested to result from the remarkable differences in grain size and volume fraction of martensite (Vm) in these two kinds of dual-phase steels. An estimation of the plastic zone size of fatigue-crack tip supported this suggestion.     

Annealing Response of Cold-Rolled Vanadium Microalloyed Sheet Steels

Vanadium microalloying is widely employed in hot-rolled HSLA products;because of its relatively high solubility in austenite,vanadium plays a special role in thin-slab cast hot-rolled products where reheating is limited prior to direct-rolling.In cold-rolled and annealed sheet steels,vanadium technology has been employed in bake-hardenable drawing-quality steels,and in HSLA products.Recent studies have investigated the continuous and batch annealing response of vanadium-containing cold-rolled HSLA steels,with an emphasis on aluminum and nitrogen variations (because of the special importance of nitrogen in vanadium microallyed steels,and interesting interactions between vanadium,aluminum and nitrogen).The present contribution reviews some key aspects of vanadium-microalloyed coldrolled sheet steels,and highlights the results of selected studies showing the influence of steel composition and processing.     

High Strength Low Alloy (HSLA) Steels in Building Construction

Majority of the buildings,including industrial buildings,are constructed using either structural steel (plates and structural shapes) or deformed bar steel reinforced concrete.Such buildings,however,must be designed to be safe and serviceable during construction and during use and occupancy.These objectives can be easily achieved by the use of steels having superior mechanical properties,ductility,weldability,fire resistance,etc.Over the years,the steel industry has made improvements in steel making technologies resulting in high strength low alloy (HSLA) steels with superior steel properties well suited for building construction.First part of this paper presents the structural design considerations,and the constructional considerations associated with the building structures in general,and steel structures in particular.This second part of the paper looks at the acceptance criteria for HSLA steels for North American building codes and construction.The third part of the paper presents the structural properties of currently available HSLA steels for building construction.The discussion focuses on hot-rolled structural steel shapes as well as deformed steel bars for concrete reinforcement.The paper argues that Niobium microalloying is the key to achieving superior properties in such steels.