Usage Of High Strength Steels Research Articles

Effect of post-weld heat treatment on mechanical and microstructural properties of high strength steel weld metal

The usage of high strength steel (HSS) is steadily increasing, primarily driven by the pursuit of weight reduction, leading to a subsequent decrease in greenhouse gas emissions. This paper investigates the impact of various post-weld heat treatment (PWHT) temperatures of 525 °C, 550 °C and 575 °C with a holding time of 2 h on both the microstructure and mechanical properties of weld metal produced using a HSS metal cored wire. The investigation reveals that PWHT does not significantly alter strength but has a more pronounced influence on toughness. The as-welded condition exhibited the highest toughness. Among the samples subjected to the PWHT, the one treated at 575 °C showed the highest impact energy, reaching 69 J at −60 °C. This outcome is attributed to the increased presence of acicular ferrite in the microstructure, surpassing that of samples subjected to PWHT at different temperatures.

EXPERIMENTAL INVESTIGATION OF SPRINGBACK OF LOCALLY HEATED ADVANCED-HIGH STRENGTH STEELS

Sheet metal bending is one of the important metals forming processes at ambient temperature. The usage of high-strength steel is one of the stronger materials for the construction of components in the automotive industries. However, for complex shapes, cold forming is not always sufficient, and heating the workpiece plays a major role in manufacturing these shapes. Bendability may increase with increasing forming temperature and currently, hot forming of advanced high strength steels (AHSS) is becoming more attractive. While hot forming of AHSS is beneficial for high formability, subsequent quenching is required to maintain final strength. This procedure extends the production time. In this study, temperature gradients, bending loads, and springback after V-bending were investigated. The experimental study was carried out on a 2 mm thick Docol 1500M steel at various temperatures by using a speed-controlled servo press machine. The bending regions of the specimens were locally heated to 200, 300, 400, 500, and 600oC by using a high-frequency induction heating device. The results show that; punch loads were significantly lowered with increasing the local heating temperature during bending. There were no cracks observed on the specimens. The amount of spring back is decreasing with the bending temperature and around 500oC almost no springback was measured. Negative spring back was observed for the bending temperatures higher than 500oC

Bending Parameters in Heat Assisted Air Bending of High Strength Steels

The usage of high strength steels is a viable option for improving existing sheet metal constructions manufactured by air bending. This improvement can be achieved through increased payload, lighter structures or enlarged safety margins. Unfortunately, as these materials have limited cold formability, usually high strength steels cannot be formed with the same tooling as used for conventional steels. Thus, in the case of air bending, the forming punch for conventional steels should be substituted by a punch with a bigger radius. This substitution implies changes in the geometry of the final part and leads to redesigning of the complete structure due to the bigger bending radius of formed parts. Additional problems related with the usage of high strength steels are bigger bending forces during the forming process and much higher springback. These issues can be attenuated by heat assisted air bending. In this work, 6 mm plates of two different high strength steels - Strenx 700 MC and Strenx 1300 - are bent with near infrared lamps as a heating method. The experimental campaign has been performed to understand the evolution of following bending parameters: minimal bending radius, bending force, springback and bend allowance. The experimental campaign shows that heat assisted air bending with near infrared lamps is an attractive opportunity for overcoming problems related to bending of high strength steels.

Experimental investigation of large radius air bending

Usage of high-strength steels with limited bendability in conventional air bending is not obvious due to the deformation limits of these materials. An attractive solution for deformation reduction is the utilization of large radius punches. The bending process for large radius tooling is dissimilar to conventional air bending, due to the presence of the multi-breakage phenomenon. This phenomenon has been observed by many researchers, but understanding of the influencing parameters is limited. Multi-breakage poses a serious obstacle, since it changes the loading scheme from the typical three-point to four-point bending. In this contribution, an extensive experimental exploration of large radius bending leads to conclusions on the influence of tool dimensions and material characteristics. Air bending experiments have been performed for five materials, four thicknesses, four punch radii, and four die openings. Based on the observed force measurements, the process can be defined as large radius bending when the ratio punch radius/die opening is larger than 1:4. Trends for the influencing parameters of the measured data can be used for further investigation of large radius air bending.

Properties of welded joints made in high strength steel using laser technology

The usage of high strength steels (HSS) provides numerous benefits in the form of the cross- section area reduction causing weight decrease, improvement in exploitation parameters and in result positive financial effect. The most advantageous results are observed in the parts loaded by tensile stresses. The factors most influencing the fatigue strength of the metals, especially the steels of high value of yield point, are stress concentration in the originated geometrical notches, local changes of the stress state or the microstructure of paternal material. Improvement in the fatigue behaviour of HSS steels can be achieved by changing the commonly used technique for high-energy joining technology, namely laser beam welding. Relatively narrow welds and reduced heat-affected zone (HAZ) have minimized the residual stresses and distortions. The paper presents the results of the studies focused on mechanical properties of welded joints of high strength fine-grained steel S960QL. There was investigated the impact of heat input on the microstructure, microhardness- and residual stresses in the weld and HAZ. The crucial part is devoted to the fatigue research and fractographic investigation of the fatigue ruptures of laser-made welds. There were also found in the origins the cause of crack initiation. Keywords: mechanics, fatigue life, high strength steel, laser welding

Contact Modelling of Large Radius Air Bending with Geometrically Exact Contact Algorithm

Usage of high-strength steels in conventional air bending is restricted due to limited bendability of these metals. Large-radius punches provide a typical approach for decreasing deformations during the bending process. However, as deflection progresses the loading scheme changes gradually. Therefore, modelling of the contact interaction is essential for an accurate description of the loading scheme. In the current contribution, the authors implemented a plane frictional contact element based on the penalty method. The geometrically exact contact algorithm is used for the penetration determination. The implementation is done using the OOFEM - open source finite element solver. In order to verify the simulation results, experiments have been conducted on a bending press brake for 4 mm Weldox 1300 with a punch radius of 30 mm and a die opening of 80 mm. The maximum error for the springback calculation is 0.87° for the bending angle of 144°. The contact interaction is a crucial part of large radius bending simulation and the implementation leads to a reliable solution for the springback angle.

Quantification of micro-cracks on the bending surface of roll formed products using the GTN model

With the rising interest in lightweight construction, the usage of high strength steel has increased remarkably during the last years. Unfortunately, there is manufacturing problem: high strength steel has a higher occurrence rate of cracks than mild steel. The Gurson-Tvergaard-Needleman (GTN) model is used to precisely predict the occurrence of cracks in roll forming. The flow stress and the GTN material parameters are identified using a load-displacement curve obtained from tensile test. In detail, the material parameters that have to be determined were selected and based on this 3-level orthogonal array is made. The coefficients of the objective function are derived using the response surface method with the orthogonal array based on the simulation results. The material parameters are obtained by minimizing the objective function and determined by comparing the simulation and test results. The reliability of the material parameters determined by the tensile test increased through cross-validation using the bending test. The roll forming simulation with the determined parameters is fulfilled, and the experiments are performed to determine the occurrence of micro-cracks according to bending angles. Micro-cracks are quantified by comparing the void volume fraction (f) from the GTN model, and the micro-cracks on the surface are measured using a scanning-electron-microscope.

Design and welding method for high-strength steel structure using laser-arc hybrid welding

The study intended for rational usage of high-strength steel in structure by applying laser-arc hybrid welding has been conducted. Key design requirements and improvement target have been clarified considering their key performances as structural members. The evaluation methods for risk of brittle fracture and cold cracking have been established for quality control. Safety of welded joint has been confirmed and their fatigue strength has been improved by optimizing welding process.

The Effect of Cooling Rate on Mechanical Properties of 22MnB5 Steel Sheet during Hot Press Forming

There has been a growing usage of high strength steels (HSS), particularly in automobile applications mainly as structural parts in view of their light weight and high strength properties. These materials are also being considered for dynamic applications. However, the multi-phase microstructure, which is at the base of the strengthening mechanisms in most of these steels, leads to unacceptably high stresses during forming and significant springback phenomena, thus making traditional sheet metal forming technologies unsuitable. To avoid the disadvantages, a new process method was introduced – Hot Press Forming. Hot press forming (HPF) process is a forming method which can provide various advantages such as excellent mechanical properties and formability, good weldability and little springback. Here, the experiment parameters which include locations of the cooling holes and the flow rate of the cooling water play an important role in the HPF process. In this paper, the Al-Si coated boron steel sheet was researched by heating it up to 930oC for 5 min and formed by a hydraulic press. In this study, microstructural evolutions and the associated mechanical properties were investigated in terms of the flow rates of the cooling water.

동재하시험결과 분석을 통한 송도지역 590MPa급 고강도 강관말뚝의 항타관입성 및 지지력 특성 분석

강관말뚝의 경쟁력 향상을 위해 개발되어 현장적용되고 있는 590MPa급 고강도 강관말뚝의 항타관입성과 지지력 특성을 분석하기 위하여 인천 송도지역에 총 18본의 다양한 지름/두께 및 강도별로 강관말뚝을 시공하고 이에 대한 재하시험을 수행하였다. 항타관입성은 일반강도와 고강도간의 타격횟수, 관입깊이, 건전도 등을 확인하여 비교하였으며, 파일 지지력은 동재하시험을 수행하고 대표 말뚝에 대해서는 정재하시험을 수행하여 동재하시험의 적절성을 평가하였다. 또한 동재하시험결과를 GRLWeap 해석결과와 비교하고 입력변수에 대한 역해석을 수행하여 향후 고강도 강관말뚝을 송도지역에 시공할 때 보다 적절한 항타장비 선정 및 지지력 예측이 가능하도록 표준관입시험의 N값을 이용한 지반반력계수에 대한 보정식을 제안하였다. Domestic usage of high strength steel for pile has been limited to steel with yield strength (YS) of 490 MPa. However, design and construction cases abroad show beneficial usage of steel pipe with YS ranging in 500~700 MPa. In this study, YS 590 MPa steel pipe has been tested for driven pile foundation in Songdo area. Pile dynamic analysis (PDA) was carried out for 18 piles of which 16 piles have been reviewed for comparison of the PDA test results with those of GRLWeap analysis using SPT N value. Back analysis of PDA analysis was also carried out to narrow the deviation of standard SPT N value used in GRLWeap analysis. A regression equation is suggested for the shaft and toe resistance according to SPT N values for future GRLWeap analysis that can be used in the designing stage at Songdo area.

Analysis of microstructure and mechanical properties of different high strength carbon steels after hot stamping

Usage of high strength steels may reduce the weight of automobiles and improve the crash safety and low down the gas emissions. Besides cold forming, hot stamping has gained much interest for the production of car body components. Boron alloyed steels have been the point of focus for the materials choice in hot stamping. In this paper, four high strength non-boron alloyed steels were hot stamped using water and nitrogen cooling media. Microstructural analyses, lateral and surface hardness profiling as well as tensile tests of hot stamped samples were performed. These steels provided yield strength (Y.S.) values of 600–1100MPa and ultimate tensile strength (U.T.S.) values of 900–1400MPa. Increasing cooling rates, i.e. by using nitrogen cooled punch (NCP) during hot stamping resulted in mostly martensitic microstructure and maximum strength, while hot stamping using water cooled punch (WCP) resulted in maximum formability index due to presence of some ferrite phase.

Friction and wear behaviour of high strength boron steel at elevated temperatures of up to 800 °C

There has been a growing usage of high strength steels, particularly in automobile applications mainly as structural parts in view of their light weight and high strength properties. These materials are also being considered for dynamic applications. However, the understanding of their tribological behaviour vis-a-vis their hot forming and also as tribological materials is highly inadequate. The present work thus aims at creating new knowledge about the tribological characteristics of high strength steels and bridging this existing gap. High temperature tribological studies on different tool steels (with and without surface treatment) sliding against high strength boron steel (with and without coating) and studies on self-mated hardened high strength boron steel under dry reciprocating sliding conditions have been conducted. High temperature tribological studies keeping in view the hot metal forming aspects were conducted by using an SRV machine whereas a two-disc machine was employed for investigating their fundamental friction and wear behaviour. The results from the high temperature studies indicate that the friction is dependent on temperature since a reduced friction level was observed with increasing temperature. The wear of the tool steels increased with increasing temperature and nitriding of the tool steels provided better protection against severe wear. The results from the study on self-mated hardened high strength boron steel showed that sliding speed has a marginal effect on friction whereas the effect of contact pressure is more pronounced (decreasing friction with increased contact pressure). The specific wear rate decreased with increased sliding speed.