Application Of High Strength Steel Research Articles

Comparison of the hot-stamped boron-alloyed steel and the warm-stamped medium-Mn steel on microstructure and mechanical properties

The application of high strength steels (HSS) for automotive structural parts is an effective way to realize lightweight and enhance safety. Therefore, improvements in mechanical properties of HSS are needed. In the present study, the warm stamping process of the third generation automotive medium-Mn steel was discussed, the characteristics of martensitic transformation were investigated, as well as the microstructure and mechanical properties were analyzed, compared to the popular hot-stamped 22MnB5 steel in the automotive industry. The results are indicated as follows. Firstly, the quenching rate of the medium-Mn steel can be selected in a wide range based on its CCT curves, which is beneficial to the control of forming process. Secondly, the influence of stamping temperature and pressure on the Ms temperature of the medium-Mn steel is not obvious and can be neglected, which is favorable to the even distribution of martensitic microstructure and mechanical properties. Thirdly, the phenomenon of decarbonization is hardly found on the surface of the warm-stamped medium-Mn steel, and the ultra-fine-grained microstructure is found inside the medium-Mn steel after warm stamping. Besides, the warm-stamped medium-Mn steel holds the better comprehensive properties, such as a lower yield ratio, higher total elongation and higher tear toughness than the hot-stamped 22MnB5 steel. Furthermore, an actual warm-stamped B-pillar of medium-Mn steel is stamped and ultra-fine-grained martensitic microstructure is obtained. The mechanical properties are evenly distributed. As a result, this paper proves that the warm-stamped medium-Mn steel part can meet the requirements of lightweight and crash safety, and is promising for the industrial production of automotive structural parts.

Hot Hydroforming of 22MnB5 Tube by Resistance Heating

To promote the application of high strength steels in automobile bodies, the practicability of hot hydroforming of tube by resistance heating is illustrated. From the results of experiments conducted to measure temperature distributions of the tube during the forming process, a method to improve temperature uniformity has been proposed and achieved. Validity was evaluated by examining the effects of hot gas forming on the microstructure and hardness. Results indicate an obvious temperature difference along the axial direction for two cross-sectional shapes: the temperature in the middle zone of the tube is higher than that at its ends. Both thermal convection and cross-sectional shape have only a limited effect on the temperature distribution. The main reason for non-uniform temperature distribution is the heat transition between the electrodes and the tube ends. The temperature difference decreased as the heating rate increased. In contrast, the temperature distribution was even along the circumferential direction for both cross-sectional shapes. Adjusting the contact resistance is a useful method of reducing the temperature difference. In this study, the temperature difference was successfully decreased to 20°C, while reaching a maximum temperature of 750°C, which is adequate for both forming and quenching. A rectangular component was formed to validate the practicality and efficiency of tube hot hydroforming by resistance heating. The hardness and microstructure met the requirements of 22MnB5, which demonstrates both the forming efficiency and quantity advantages of hot gas hydroforming by resistance heating.

Behavior of Q690 high-strength steel columns: Part 1: Experimental investigation

With the increasing application of high-strength steel over the past years, the urgent problem is to determine buckling curves in current codes for design of axial compression columns fabricated by high-strength steel. The current design curves cannot cover the high-strength steel columns due to that the available curves were based on research of mild carbon steels. To study the overall buckling behavior of high-strength steel columns, an experimental investigation on the ultimate bearing capacity of weld box and H columns with a nominal yield stress of 690MPa subjected to axial compression was carried out. Twelve columns with slenderness ratios ranging from 30 to 70 were comprised in the experiment. The geometrical dimensions and initial geometrical imperfection involving initial deflection and loading eccentricity were measured. The buckling deformation and bearing capacity of the columns were studied on the basis of the test results. By comparing the experimental results with the buckling curves stipulated in GB50017-2003, Eurocode 3, and ANSI/AISC 360-10, it is found that the current design curves underestimate the ultimate strength of the axially loaded box and H columns welded by 690MPa high-strength steel plates. The curves “a” in the corresponding codes such as GB50017-2003 and Eurocode 3 are found to be suitable for the welded 690MPa high-strength steel box- and H-columns buckling about minor axis. The test results are intended to assess the accuracy of the numerical method presented in Part 2 of the paper.

Effect of a conically shaped hollow electrode on advanced high strength steel in three-sheet resistance spot welding

Application of high strength steel is one of the most promising solutions to the problem of vehicle weight reduction. However, weldability problems occur when three sheet metals of the high strength steel are welded. This is because under such situation, the suitable welding range in the lobe diagram is too narrow and the strength between the contact zones is lower than the required value, and also, expulsions between the sheets occur. In this work, a conically shaped hollow electrode was applied at the lower electrode to improve resistance spot weldability of three-sheet welding including high strength steel. Shear strength test and analysis of welding signal and weld cross section were conducted. It was found that the proposed lower electrode has some advantages in welding the three-sheets welding compared with conventional electrodes or hollow electrodes. Consequently, a welding process using the proposed electrode was studied and verified with numerical analysis.

Mechanical properties and design recommendations of very high strength steel S960 in fire

Due to the lack of experimental evidence in literature and specific provisions in current leading design standards for (very) high strength steel structures, the fire-resistance design of steel structures with high strength steel (HSS) and very high strength steel (VHSS) is either too conservative or safety-compromised in practice, which seriously retards the application of HSS and VHSS in civil engineering as well as leads to safety risk. This paper presents a detailed experimental study on VHSS S960, under both steady state fire condition and transient state fire condition. The elastic modulus, yield strength and ultimate strength of S960 in fire are obtained and compared with the recommendations of leading design standards for steel structures (i.e. EC3, ASCE, AISC, AS 4100 and BS 5950), as well as with HSS S460 and S690 reported in previous study. The comparison with design standards reveals the necessity of including specified recommendations on the deterioration of mechanical properties in fire for very high strength structural steels in current leading design standards. Further, some unique predictive equations are proposed for the elevated-temperature mechanical properties of VHSS S960. The research results could be employed as fundamental data to numerically or theoretically analyze the fire performance of steel structures with members made of S960 and to carry out accurate fire-resistance design.

Stress corrosion cracking behavior of high strength casing steel in working fluids

Aiming at the stress corrosion cracking behavior of a high strength steel, the air working conditions, the annulus protection fluid and the organic salts mud were simulated under the temperature of 60 °C, 100 °C, 150 °C by slow strain rate tests (SSRT). Tensile strength and elongation and stress corrosion cracking susceptibility index and fracture morphology were analyzed by stress–strain curves and SEM. To determine the applicability of high strength steel under different working conditions, the extent of damage were analyzed quantitatively. The results showed that less degree of stress corrosion occurs when temperature below 100 °C or even lower, and severe increase of stress corrosion occurs when temperature is 150 °C. The high strength steel stress corrosion cracking tendency increase as the temperature grows, and so do the stress corrosion cracking susceptibility index F(σ) and F(δ). Besides, the tendency of increase of F(σ) is more significant than that of the F(δ). In the working conditions of the organic salt mud, the changes of mechanical properties of the high strength casing steel are more significant. The corrosion pits on the sample surface contributed to such changes as a mainly factor. The crack nucleation can occur under the action of stress concentration at the bottom of the corrosion pits, and results in crack initiation extension inside the sample along the direction perpendicular to the tensile stress. In addition, the annulus protection fluid protected the strength and toughness of the casing excellently.

On effect of pre-bending process on low cycle fatigue behaviour of high strength steel using lock-in thermography

The application of High Strength Steel (HSS) on different structural components is becoming more attractive. They have a fine grain structure, low carbon content for improved weldability, and controlled internal purity. These steel grades are frequently used for structural applications. There are a lot of steel components of the considered applications that are subjected to bending and fatigue loading conditions, respectively. It is well known that under critical loading conditions, cyclic stress, which exceeds the material yield stress, can occur at some critical locations such as the inner side of bent components. Combining these two aspects, i.e. the bending process followed by fatigue loading, causes multiple micro-crack initiation inside the inner surface of the bending area, which is followed by propagation of the cracks up to final rupture of material.The main objective of the present study is to investigate the effect of pre-bending process of HSS subjected to low cycle fatigue loading conditions, since so far only very limited amount of research has been focused in this direction. For this purpose, in the first step, a new test set-up was designed to take into account the effect of pre-bending process when the fatigue load has been applied. Lock-in thermography technique was used to monitor the incremental temperature variation during fatigue cycling at the bending root. Using the temperature evolution, the crack initiation and propagation lifetimes were separated from total lifetime. Fractography and Scanning Electron Microscopy (SEM) analyses were performed to study the fracture surface of specimens after bending and fatigue testing. Furthermore, numerical technique approach was used to model the bending and spring back processes along with the fatigue loading in order to understand the effect of bending process on fatigue behaviour of tested material. The developed finite element model provides more information about the multiaxial strain and stress states at and near the bending root after bending process and applied axial fatigue load.

応力反転後の非線形弾塑性挙動の考慮によるスプリングバック予測精度向上

Abstract Prediction of springback during press forming has become more important due to the recent trend of application of high strength steels in automotive white body parts. The accuracy of springback prediction depends on the estimation of recovery strain after die release. As it is well known that unloading behavior after pre-strain shows a nonlinear stress-strain relationship, a material model which considers nonlinear elastoplastic behavior is necessary for accurate springback prediction. In this study, the nonlinear behaviors of high strength steel sheets under uniaxial and biaxial stress states were measured. A new material model that considers nonlinear behavior was proposed and implemented in finite element method (FEM) software. Press-forming tests of curved hat-shaped parts and the corresponding springback analyses were performed, and the springback angles were evaluated. The springback predictions made using the proposed model showed good agreement with the experimental results.

Analysis of the applicability of high strength steel of YS=1300 MPa in the marine industry

Analysis of the applicability of high strength steel of YS=1300 MPa in the marine industry

Влияние плазменной резки на микроструктуру и твердость высокопрочной стали

The application of high-strength steels during the creation of products satisfying all the modern requirements of the market is one of the most promising areas of modern engineering. The use of these steels reduces metal consumption of produced constructions due to reducing the thickness of the metal while maintaining the same structural strength, increases the service life of products thanks to increasing durability, as well as improves the reliability of the produced product. The processing of high-strength steel has a number of features associated with increased sensitivity of such steels to a thermal effect. The aim of this study is to research the influence of plasma-arc cutting on the microstructure and hardness in the heat-affected zone of high-strength steel having hardness of 500 HBW. To do that, the research of microstructure in the heat-affected zone of plasma-arc cutting of high-strength steel with 500 HBW hardness was performed. The results of research showed the presence of parts with different structures and properties in the heat-affected zone of plasma-arc cutting: decarburized layer, hardening layer, incomplete quenching layer, tempering layer, and the base metal. While moving off the boundary of the cut the cooling rate and temperature of metal are reduced. It is the cause of lowering steel hardness. The microstructure of the base metal has a needle form, and consists of strips of different orientation and size. The structure of different parts of the heat-affected zone are: hardening layer – martensite, incomplete quenching layer – martensite + ferrite, tempering layer – tempered martensite. The total length of the heat-affected zone and the length of its individual sections were also determined.

Formability Investigations of High-Strength Dual-Phase Steels

Car manufacturing is always regarded as the key industry behind sheet metal forming, and thus, the requirements of and developments in car manufacturing play a decisive role in the development of sheet metal forming. The automotive industry is faced with contradictory demands and requirements: better performance with lower consumption and less harmful emissions, more safety and comfort; these are extremely difficult to supply simultaneously with conventional materials and conventional manufacturing processes. The fulfillment of these often contradictory requirements is one of the main driving forces in the automotive industry and thus in the material and process developments in sheet metal forming, as well. In recent years, significant developments can be observed in the application of high-strength steels. In this respect, the application of various dual-phase steels is one of the best examples. However, the application of these high-strength steels often leads to formability and manufacturing problems. One formability problem is the springback occurring after sheet metal forming. In the current research, we have dealt mainly with advanced high-strength steels, primarily with dual-phase steels. When applying them, the springback phenomenon is one of the most critical issues. To reduce the tremendous amount of experimental work needed, we also applied numerical simulation using isotropic–kinematic hardening rules. The isotropic–kinematic hardening behavior of a given material in the applied AutoForm numerical package may be characterized with three independent material parameters γ, χ and K (a detailed explanation of their meaning will be given in the main part of this paper). However, we found that the material data included in simulation packages for these new high-strength steels are not fully adequate. For the determination of more reliable material parameters and to achieve better simulation results, a new testing device was developed. Numerical simulations were performed using the material parameters determined by the new device to show the sensitivity of springback behavior to these material parameters.

Concrete transformation of buildings in China and implications for the steel cycle

Urbanization and real estate development are two mighty impetuses for the growth of China. An enhanced dynamic modeling has been devised to explore stocks and flows of buildings in the country and to quantify the related steel cycle. The uncertainties of the variables and results are investigated by the means of Monte Carlo method and sampling analysis. The building stocks are expected to increase to some 85–130 billion m2 in the mid-century, about 40–100% up from the current level. Throughout China but in urban areas in particular, concrete structures are replacing the buildings made of wood, clay brick, and primitive materials. By 2050 every two out of three buildings in China will be reinforced concrete- or steel-framed, leading to substantial demand for ferrous metals.Scenarios analysis shows that a slowing down in the building stock expansion will likely occur in China in no more than ten years. This may open up a transition with profound industrial and resource implications. Increasing businesses for the construction industry may emerge from maintenance, retrofitting, and end-of-life management of existing buildings. The steel industry shall reform its capacity to conform to the growingly available secondary resources and the declining requirement for construction steel. Efficient and appropriate recycling of steel content from waste concrete will play an important role in material conservation. A collaboration of improvements in process material efficiency with lifetime extension and application of high-strength steel may save nearly 40% of primary iron ores for building use in the coming four decades.

Moment redistribution of continuous composite I-girder with high strength steel

【The continuous composite I-girder should have a sufficient rotation capacity (or ductility) to redistribute the negative bending moment into an adjacent positive bending moment region. However, it is generally known that the ductility of the high strength steel is smaller than that of conventional steel, and application of high strength steel can cause ductility problems in a negative moment region of the I-girder. In this study, moment redistribution of the continuous composite I-girder with high strength steel was studied, where high strength steel with yield stress of 690 MPa was considered (the ultimate stress of the steel was 800 MPa). The available and required rotation capacity of the continuous composite I-girder with high strength steel was firstly derived based on the stress-strain curve of high strength steel and plastic analysis, respectively. A large scale test and a series of non-linear finite element analysis for the continuous composite I-girder with high strength steel were then conducted to examine the effectiveness of proposed models and to investigate the effect of high strength steel on the inelastic behavior of the negative bending moment region of the continuous composite I-girder with high strength steel. Finally, it can be found that the proposed equations provided good estimation of the requited and available rotation capacity of the continuous composite I-girder with high strength steel.】

Estudo do comparativo do comportamento mecânico em uniões soldadas dos aços de alta resistência de 700 e 1.000MPa

The application of high strength steels is strongly diff used in the engineering industry, especially in applications where weight reduction is important, such as the industry of agricultural machines and trailers industry. This investigation aims at conducting a comparative analysis on the mechanical behavior (micro hardness profile and tensile test) of welded joint by GMAW – Gas Metal Arc Welding – using different values of current power and voltage. The welding process was made using robotic system to maintain homogeneity along the joint, the right distance and position during the total weld joint length. The mechanical behaviors were performed with the optical microscope, micro hardness profiles analysis and tensile test. The results showed that the mechanical properties between the base metal and the weld in a structural steel welded joint have a primary importance in the final test results. Keywords: welding, micro hardness profile, tensile test, high strength low alloy steel.

Review on High Strength Steel Bolted End-Plate Connections

Past three decades have seen the rapid development of high strength steel (HSS) in its application in structural engineering. However, so far the mechanical performance of a HSS beam-to-column connection has not been systematically studied, especially for bolted end-plate connections, the commonly employed beam-to-column connections in steel structures, which could restrict the application of HSS. Therefore, this paper aims to represent the basic methods, current achievements, recent applications, and the existing problems that lie in the way. In doing so, this paper is composed of three parts, experimental results, numerical analysis as well as component method. At the end, this paper indicates that future investigation should be based upon experimental analysis and proper finite element modeling, to verify a numerical model and to refine design standards.

Application potential of high performance steels for weight reduction and efficiency increase in commercial vehicles

The fast-growing economy and the gradually established highway system have boosted the road transportation for both passenger and cargo over the last decade in China. From 2000 to 2010 Chinese GDP increased by around 10.15% annually and the sales of medium and heavy trucks by around 18.87% (sales increased from 0.2 million in 2000 to 1.3 million in 2010) according to the National Bureau of Statistics of People’s Republic of China. Today commercial vehicles consume almost the same amount of fuel as passenger cars in China although the number of commercial vehicles is only about one fourth of passenger cars. It is estimated that around 50% of imported fuel to China each year will be consumed by vehicle transportation. This situation will worsen fuel shortage problems in the long run and at the same time it is partially responsible for the ever-worsening air pollution in China. Due to the widespread overloading in China, lightweight development in commercial vehicles has fallen far behind that of passenger cars with the consequences that Chinese commercial vehicles consume in average about 20% more fuel, especially the heavy trucks, compared to European models. Under these circumstances it is essential to reduce the vehicle fuel consumption and increase the transport efficiency. The key solution thereby is to implement lightweight design in commercial vehicles as it has been successfully practiced over the last decade in the passenger cars. This paper summarizes highlights given in presentations during the “International seminar on the application of high strength steels in light weight commercial vehicles” with the focus on the development and application of Nb alloyed high performance steels made for lightweight commercial vehicles.

Guest editorial of “Application of high strength steels in lightweight commercial vehicles”

New and increasingly sharp laws for the reduction of emissions from commercial vehicles entered into force in the European Union since 1993 with the aim of substantially reducing pollutants and emissions from trucks and buses. Thereby the maximum levels for the emission of particulate matter (basically soot particles) as well as oxides of nitrogen (NOx) have been subsequently lowered. To comply with these demands, commercial vehicle producers had to introduce new emission reducing technologies of which exhaust-gas recirculation (EGR), selective catalytic reduction (SCR) and diesel particle filters (DPF/ CRT) are vital. However, the implementation of such technologies in a commercial vehicle causes a weight increase that can be estimated to reach 300 kg. Practically this means that for instance the capacity of a bus would have to be reduced by four passenger seats unless the extra vehicle weight is reduced in other areas of the vehicle. Unlike in passenger cars, maximum CO2 emissions are not yet being specified by European regulations for commercial vehicles (see Fig. 1). CO2 emissions (of course all other emissions) are directly proportional to the fuel consumption of the vehicle, which in turn can be reduced by higher efficiency in the powertrain, lower vehicle weight and improved aerodynamics. However, what actually counts beyond the absolute emissions and efficiency of a commercial vehicle is its relative performance per ton of transported cargo. A lighter vehicle simply allows transporting more cargo respecting a given total admitted vehicle weight on the road. Accordingly, the emission per ton of transported cargo becomes less. Improved design, better-performing materials and innovative manufacturing technology each by itself enable light weighting of vehicle structures and efficiency gains in the powertrain. However, the optimum benefit can only be obtained by a holistic approach involving all three approaches in a coordinated way. China Automotive Lightweight Association (CALA) was installed in 2008 to work out strategies of weight reduction in the automotive sector. In November 2014 CITIC Metals and Companhia Brasileira de Metalurgia e Mineracao (CBMM) organized a seminar in Beijing focusing on the ‘‘Application of high strength steels in lightweight commercial vehicles’’. Around 90 experts from organization delegated by commercial vehicle producers, CALA, China Iron and Steel Research Institute (CISRI), steelmakers and supply companies participated to identify the opportunities and needs for producing lighter and more fuel-efficient trucks in China. Two facts emphasize the urgency of these activities:

Formability of High Strength Sheet Metals with Special Regard to the Effect of the Influential Factors on the Forming Limit Diagrams

Car manufacturing is one of the main target fields of sheet metal forming: thus sheet metal forming is exposed to the same challenges as the automotive industry. The continuously increasing demand on lower consumption and lower CO2 emission means the highest challenges on materials developments besides design and construction. As a general requirement, the weight reduction and light weight construction principles should be mentioned together with the increased safety prescriptions which require the application of high strength steels. However, the application of high strength steels often leads to formability problems. Forming Limit Diagrams (FLD) are the most appropriate tools to characterize the formability of sheet metals. Theoretical and experimental investigations of forming limit diagrams are in the forefront of todays’ research activities.

Research Progress for High Strength Structural Steel and Key Issues of Application in Seismic Zone

A review of recent research on mechanical performance of high strength structural steel (HSSS) in the level of both materials and members is presented, including the static material performance, toughness and mechanical property under cyclic loadings of HSSS, buckling and hysteretic behavior of high strength steel columns (HSSC), the ductility and resistance of bolted connection with high strength steel slices etc. Previous research work shows that the application of high strength steel (HSS) is limited in seismic zone .Finally, two design methods of using HSS in seismic zone are proposed based on current seismic design philosophy.

특수구조 대상으로 고강도 강재(HSA800)의 현장 적용성 연구

The purpose of this study is to increase applicability of high strength steel, HSA800 to the structure. Selected study of structure is to consider high strength steel, and following parts, 1) Tensile member with no consider of buckling, 2) Truss existing both tension and compression members with small slenderness ratio. This studied structure is included tension column hang on to the upper bridge truss. The structure element quantity with apply HSA800 instead of SM570 is reduced about 38.9% of tension column and 29.7% of bridge truss. In addition, the number of element`s division is reduced about two sections due to reduction of self weight that the crane is able to lift up. This improves to reduce erection sequence and construction period which can save about a month. All connections are reviewed as welding and bolt. Also, the cost of welding is reduced about 41.3% due to apply HSA800. In conclusion, applying HSA800 to the hanging structure aggressively can secure economic and constructability.