High Strength Steel Parts Research Articles

Control of Microstructure and Properties of Welded Joints of Heavy Structures of Low Alloy High Strength Steels

In order to prevent the cold cracking of the welded joints and the overheating of the heat affected zone of the welded joints of the heavy and heavy parts of low alloy high strength steels, We have carefully studied the welding materials and methods for heavy parts of alloy high strength steel. We should pay attention to the preparation before welding, strictly control welding energy parameters, temperature parameters and operation parameters, adjust welding line energy and preheating temperature, control welding heat cycle, and adopt appropriate post-weld heat treatment and post-weld inspection process measures. By improving and optimizing the welding technology of heavy and heavy parts of low alloy high strength steel, we can effectively avoid the cold crack and overheating of heat affected zone in the welding of heavy and heavy parts of low alloy high strength steel, and greatly improve the mechanical properties of welded joints.

Influencing Intermetallic Layers of Hot Stamped Steel for Adhesively Bonded Plastic Metal Hybrids

Hot stamping of high strength steel parts is an established way to realize today ́s structural lightweight components in car bodies through sheet thickness reduction. The steel typically used for hot stamping is a boron-manganese 22MnB5 steel reaching up to 1,900 MPa in strength. New boron-manganese alloys achieving higher strength are expected to be developed, generating challenges for the manufacturing process by means of formability. Thus, a further reduction of steel thickness resulting in lighter components is not expected. Hybrid lightweight design approaches aim at weight reduction by reducing steel thickness and applying fiber-reinforced plastics (FRP) to regain structural stiffness and strength. The use of residual heat remaining from the hot stamping process allows to activate adhesives to bond FRP to hot stamped steel. The performances of adhesive bonds depend strongly on surface characteristics. To avoid scaling and decarburization during the heating process several coatings for hot stamped steels are used forming intermetallic layers through heat treatment. The most common coating in today’s automotive application is an Al-Si coating. Thus, the overall performance of the adhesive bonded hot stamped metal polymer hybrid is not only depending on adhesives performance but also strongly on the hot stamped steel’s coating performance. In this paper, the characteristics of hot stamped steel 22MnB5 Al-Si coating are investigated with regard to adhesion performance. Therefore, hot stamped specimens are manufactured under realistic industrial conditions investigating the influence of furnace temperature and dwell time on the overall coating and intermetallic layers of Al-Si coating. The specimens are investigated with respect to Al-Si coating thickness, lap shear strength of hybrid specimen and tensile strength of hot stamped steel demonstrating the dependency of the overall hybrid specimen performance from the coating performance.

Development of Hot Stamping Technology for High Strength Steel Parts with Tailored Properties

In the context of lightweight vehicles, tailored properties high-strength steel parts play a vital role in improving safety under car collisions. Hot stamping technology provides good formability for manufacturing high-strength steel parts with tailored properties, and the hot stamping parts have high strength, small springback and good industrial promotion prospect. This paper summarizes the current state-of-the-art processes for the implementation of hot stamped parts with tailored properties, through intrinsic strategy and tailored welded / rolled blanks.

Development of an Electrical Resistance Sensor from High Strength Steel for Automotive Applications.

This work focuses on a demonstration of the monitoring of corrosion processes taking place in high strength steel in automotive applications. This is performed by means of a corrosion sensor, which operates as an electrical resistance sensor. It was developed from the same type of material that is used for the high-strength steel parts produced in the automotive industry. Using the sensor, real time corrosion processes can be measured. It is attached to a location inside the vehicle’s engine and is equipped with a data logger, which enables wireless transfer of the measured data. In this study the development, operation, and evaluation of the monitoring process are presented. Corrosion estimation is verified by means of electrochemical methods. A metallographic investigation was included in order to verify the similarity between the microstructural properties of the sensor and those of the as-received high-strength steel sheet.

Multi-scale simulation for hot stamping quenching & partitioning process of high-strength steel

The quenching and partitioning process has proven to be an efficient way to improve the ductility of high-strength steel parts. In this study, two-step quenching and partitioning combined with the hot stamping process was investigated. A multi-scale coupling simulation was performed, in which the micro-scale carbon diffusion, interface migration and austenite domain were coupled into the macro-scale thermomechanical simulation by Abaqus user-defined subroutines. The effect of stress on martensite start temperature (Ms) was taken into account for calculating the retained austenite volume fraction after the first die quenching. The non-uniformity results caused by non-synchronous cooling and the effect of stress were evaluated with the coupled simulation. The carbon diffusion and interface migration were greatly affected by the loading history during the partitioning process. A careful comparison between quasi-static partitioning and dynamic partitioning results indicates that the heating history during the partitioning process cannot be neglected. Finally, a U–shape part was produced by hot stamping quenching and partitioning process to verify the multi-scale simulation. Initial martensite and fresh martensite were distinguished using scanning electron microscopy. The retained austenite volume fractions were quantitatively measured using X-ray diffraction and compared with predictions.

Additive manufacturing of high-strength weathering steel parts fabricated by gas tungsten arc: Microstructure and mechanical properties

This study focuses on microstructure and mechanical properties as a function of location in additively manufactured high-strength weathering steel components using gas tungsten arc as the heat source. Variations of microstructure and mechanical properties in various locations are presented and analysed. The as-deposited high-strength weathering steel is composed of columnar grain morphology with proeutectoid ferrite, acicular ferrite, side plate ferrite and a small amount of pearlite microstructure in the top region, equiaxed grains with ferrite and pearlite in the middle region, and columnar grains in the near-substrate region with the microstructure similar to that in the top region. There exist obvious layer bands in the middle region, and the forming mechanism of the bands is addressed. Microhardness measurement and tensile strength testing indicate obvious changes in different regions, depending on location and direction of testing specimens. The microhardness in the middle region is inferior to that in both near-substrate and top regions. The ultimate tensile strengths in the travel and deposition height directions are approximately 553 and 506 MPa, respectively. Different locations exhibit heterogeneous tensile strength and elongation due to various microstructures and boundaries. The results indicate the feasibility to fabricate high-strength weathering steel components with good tensile properties using gas tungsten arc–based additive manufacturing.

Hot isostatic diffusion bonding tungsten alloy and high-strength steel Part I: Design and preparation of Ni-Si-B interlayer by magnetron sputtering

Ni-Si-B interlayer film was deposited on 90WNiFe alloy substrate by magnetron sputtering. The thickness of Ni-Si-B film was measured and the macroscopic morphology was observed. The adhesive strength between film and substrate was determined, the microstructure and element distribution of the film, as well as the interface between film and substrate were researched. The research result indicates that, the film and substrate shows tight metallurgical combination, and the film exhibits the good integrality without any breakage. With increase of the sputtering time, the deposited film thickens gradually, the thickness of film d and sputtering time t meets the relationship of d = 0.33998 t+0.02583 t2. However, with the increasing sputtering time, adhesive strength between film and substrate decreases gradually. For the films with sputtering times of 2 h and 6 h, there is no cracking under the scratch load of 100 N, while for those with sputtering times of 10 h, 16 h and 24 h, the affordable maximum scratch load decreases to 51.3 N, 34.6 N and 13.8 N, respectively. The element distribution of the film is uniform and the chemical composition is consistent with that of Ni-Si-B target well, which results in the similar melting points of film and target.

EBSD Study of 55 Wt.% Al-Zn Coating and Corrosion Behavior Effected by Austenitization Heat Treatment

Reduction of CO2 emission and enhancement of fuel efficiency are an important environmental issue and the design trend of vehicle structures. Hot stamping process has been employed to for advanced high-strength steel (AHSS) parts for the automobile components. Several coating systems, such Al-Si, Zn, and Zn-Al, on the AHSS for the hot stamping process have been reported. The microstructural evolution during austenitization differs in the type of the coating, which is mainly characterized using the SEM and TEM, as well as the EDS. Electron back-scattering diffraction (EBSD) is a kind of the crystal identification technologies. Due to the difference in the crystal orientation of phases, the phase mapping is power for detailing large-scale microstructural evolution. In this study, the EBSD was employed to identify the phase evolution of a 55 wt.% Al-Zn coated steel during different heat treatments. The result showed that the Fe2Al5 was the major phase formed at 800℃ for 5 min and no Fe-Zn IMCs was found in the coating. As the temperature of the heat treatment was increased to 900℃, the FeAl phase formed on the top of the coating and gradually became the major constituent of the coating. Moreover, the Zn did not react with the FeAl phase and gathered on the top of the coating to form the Zn-oxide. The result of pole figure indicates that the texture orientation of the FeAl phase had changed; however, the orientation change of the Fe2Al5 phase was not obvious and showed preferred orientation. Finally, the chemical stripping results show that the potential of the coating was active than the steel substrate and the potential gap between the coating and steel substrate was approximately 500 mV, which is closed to the result of Zn-coated steel without heat treatment.Key words: 55 wt.%Al-Zn coating, EBSD, austenitization heat treatment, Fe-Al IMCs, chemical stripping Figure 1

Selective laser melting of high strength and toughness stainless steel parts: The roles of laser hatch style and part placement strategy

Laser hatch style and part placement strategy are acknowledged to have a considerable effect on processing quality and mechanical performance of selective laser melting (SLM) products. In an effort to obtain 316L stainless steel products with favorable properties, hatch style integration and part placement optimization were adopted in this study. The tensile property of SLM parts at different placement angle ψ was identified and the corresponding deformation behavior and failure mechanism were investigated. Interestingly, the part prepared by double stagger melt (DSM) style presented a strong <001> texture along the transverse direction (TD) rather than the growth orientation of the columnar grains (building direction, BD). This crystallographic variation imposed an immense effect on yield strength of SLM-processed parts by modifying the orientation relationship between texture and growth direction of grains. As ψ was set to 60°, the DSM-fabricated parts exhibited a lower Schmid factor, hence achieving a higher average yield strength σs of 536.6 MPa. The static fracture toughness UT was closely associated with the normal component force Fn imposed on preferential loading plane. By means of hatch style integration and part placement optimization, the dangerous “opening or tensile” loading mode was effectively avoided and reasonable compromise between the strength and ductility was achieved elaborately, thus obtaining an ultrahigh UT (3.478 × 108 J/m3) of SLM-processed parts. This study scientifically showed the great potentials of the SLM process to prepare 316L stainless steel products demanding challenging high-strength and high-toughness performance.

Study on forming process and spring back control of high strength sheet based on Dynaform

The drawing process of a high strength steel part without blank holder force was numerically simulated based on Dynaform. In present investigation, the drawing velocity and velocity profile motion of punch was studied by simulating the drawing operation of high strength steel part. The results show that restricting drawing velocity and controlling velocity profile motion of punch could all reduce the spring back. The measure of restricting drawing velocity could reduce non-pressure forming spring back about 31% and Trapezoidal motion mode of punch is the most beneficial to reduce spring back.

Finite element based analysis of two-stage forming for advanced high strength steel part

Advanced high strength (AHS) steel sheets have been increasingly used for producing various automotive structural parts. The components of new generation vehicles mostly exhibit very complex shapes, for which multiple step forming procedures are required in order to achieve the desired geometry. By sheet metal forming of such parts with large deformation, fracture and springback phenomenon are significantly crucial, especially, in case of the AHS steels. In this work, a two-stage forming process of an automotive part made of AHS steel grade 780 was investigated. Initially, the forming limit of steel was experimentally determined according to the Nakajima test. Subsequently, the specific two-step forming process and its corresponding FE simulations were conducted. In the simulations, the forming limit curve (FLC) was used as failure criterion and calculated formabilities of deformed parts were evaluated. Additionally, the Yoshida-Uemori kinematic hardening model and that coupled with the anisotropic Yld89 yield criterion were applied. Then, the springback occurrences of parts formed after each forming step were examined, in which the measured and computed results were compared. It was thus found that the obtained FLC could precisely describe the material formability, while the Yoshida-Uemori model more acceptably predicted the springback angles of AHS parts in the two-stage forming process.

Friction and wear mechanisms during hot stamping of AlSi coated press hardening steel

With increasing demand for light-weight constructions and improved safety standards, hot stamping becomes a desirable process to produce high strength steel parts. However, for an effective process design, a thorough understanding of tribological interactions between the tool surface and the sheet metal is of major importance. The goal of this paper is to characterize tribological interactions between the tool and the sheet metal coating during hot stamping and furthermore to investigate the different phenomena which affect the local friction coefficient and tool wear.For this purpose, the friction and wear mechanisms occurring during hot friction draw tests between the uncoated tool steel and the AlSi-coated press hardening steel (PHS) at 700°C are investigated. Most importantly, the mutual interaction between the friction and tool wear mechanisms is probed. The results show complex friction and wear mechanisms with several phenomena taking place simultaneously and/or in quick succession within the strip-tool contact system. Cumulative wear effects are also found to occur from one draw test to the next as a result of different phenomena. Furthermore our results show that abraded tool material could embed in the relatively soft sheet coating which subsequently causes ploughing marks on the tool. These interactions manifest a change in the friction mechanism as material transfer takes place from sheets to the tool. Our results provide a clear insight on the sheet metal–tool interactions during the hot stamping process. These results can be used for a more realistic modelling of the hot stamping process and its optimization.

Research on Q&amp;P hot stamping process integrated with fractional cooling strategy

Abstract Hot stamping technology has been widely used to produce high strength steel parts, but the elongation of the production is limited which leads to low ductility. In this paper, the quenching & partitioning treatment (Q&P) has been introduced into the existed hot stamping process to develop the Q&P hot stamping technology. Moreover, a fractional cooling strategy was also introduced, which improves the formability and helps to reduce the cooling requirement of the tool as well. By the presented process, the specimen was firstly quenched quickly to a suitable forming temperature by water spray method, then it was stamped and quenched inside the tool, and subsequently partitioned and secondly quenched to room temperature. The presented process can improve the elongation effectively and maintain the strength in a high level, which was proved by a series of Gleeble tests and real forming tests. OM and SEM observations were also provided to study the mechanisms of the process.

Improvement of Shape Accuracy in Press-Formed Parts of High-Strength Steel by Springback-Root-Cause Analysis

Springback is one of the most serious problems in high-strength steel-sheet forming to produce automotive body parts. A springback-root-cause analysis method was developed to identify the areas of stresses at the bottom dead point, which are the most influential in springback. The analysis method of using springback driving stresses, that is, the difference between stresses before and after springback, is more accurate to eliminate the effect of the residual stresses on springback. This analysis method was applied to both a simple model and the forming of an actual part to verify this analysis method. The areas of stresses that have a major impact on springback were identified. A countermeasure against the actual part springback based on this result was devised and it was clarified that the countermeasure is effective on the springback.

Coupled Eulerian–Lagrangian simulation of granular medium sheet forming process and experimental ınvestigation at elevated temperature

The granular medium forming process is a newly proposed flexible die forming technique. It is mainly applied to fabricate thin-walled lightweight components such as titanium alloys and high-strength steel parts at relatively high temperatures (above 500 °C) with good forming quality. In order to investigate the flow behavior of the granular medium in the forming process, tri-axial compression test with different confinement pressures is performed. Test shows that the selected granular medium exhibits pressure-dependent properties with low-friction angle. Then, three classical geotechnical models, i.e., the Mohr–Coulomb, Drucker–Prager, and Duncan–Chang models are established for the numerical simulation process with the parameters obtained from the tri-axial compression test. To avoid the undesired mesh distortion during FEM simulation of forming process, Coupled Eulerian–Lagrangian (CEL) analysis method is developed. This enables the simulation of granular material involving large deformation. Finally, the granular medium forming experiment is implemented at 500 °C using commercially pure TA1 titanium sheet, and comparison study with the FEM simulation is carried out. Result indicates that the Mohr–Coulomb model exhibits the best performance regarding the calculation accuracy of shape dimension and thickness distribution.

The role of alkalizing agent on the manganese phosphating of a high strength steel part 2: The combined effect of NaOH and the amino group (NH4OH, mono-ethanolamine and NH4NO3) on the degradation stage of the phosphating mechanism

Abstract This work shows the effect that presents NaOH in combination with several additives of the amino group on the formation of manganese phosphate coatings on steel. These combinations of additives does not modify the structure of the coating (hureaulite) but can affect the stages of corrosion, degradation and crystalline reorganization of the cyclic phosphating mechanism. These amino additives behave as cathodic corrosion inhibitors and are auxiliary agents in the acid leaching of manganese from the prismatic crystals of hureaulite which accelerates the crystalline reorganization stage and thus modifies the frequency of the cycles of growth and reduction of the prismatic crystals. Some of the additives act as chelating agents, retaining the Mn2 + and affecting the kinetics of precipitation and dissolution of manganese rich hureaulite crystals, but increasing the sites of nucleation and growth of the crystals (crystal density). The formation of the phosphate coating follows the cyclic phosphating mechanism for all the baths. The effects on crystal size (S(t)) of the equilibrium conditions in the bulk of the bath (S0), corrosion (C(t)) and degradation (D(t)) stages in each cycle can be resumed through the equation: S(t) = S0 + C(t) sin[D(t)].

Sensitivity analysis and optimization of hot-stamping process of automotive components using analysis of variance and Taguchi technique

Nowadays, the demand for achieving cars with higher strength, lower fuel consumption, and better safety considerations propels automakers to produce parts with extremely high strength-to-weight ratios. This criterion is done through the hot-stamping process considered as a novel technology employed for producing high-strength steel parts with low springback, particularly appropriate for the vehicle body. In this paper, firstly, numerical simulation of the hot-stamping process of a blank consisted of boron-alloyed steel 22MnB5 (with commercial name of Usibor 1500) is performed. Secondly, effects of different design parameters including blank holder force, die radius, gap between the punch and die, and forming time on the final temperature distributions as well as springback of the part is investigated. Consequently, optimization has been performed using Taguchi L16 orthogonal array to obtain the parameters which minimize above-mentioned parameters as two objective functions. Obtained results are verified based on performing numerical simulation and comparison to a similar work in the literature. Accuracy of the results is also assessed via the technique of plotting normal probability graphs of both objective functions. Finally, via evaluation of contribution percentage associated with analysis of variance considering each design parameter, a discussion is done by proposing the optimum design.

The role of alkalizing agent on the manganese phosphating of a high strength steel part 1: The individual effect of NaOH and NH4OH

This work shows the effects of NaOH and NH4OH on the formation of manganese phosphate coatings on steel. Three phosphating baths are studied using the same composition (Mn, H3PO4, HNO3) and working pH (2.6), these are classified according to the additive used to adjust the pH: 1) sodium hydroxide solution, 2) sodium hydroxide flakes and 3) concentrated NH4OH solution. The X-ray diffraction patterns of the coatings show that these additives do not modify the structure of the hureaulite; however, the effects of each additive are observed through the crystal size vs immersion time curves and the polarization curves recorded for each bath. Since the growth and size of the hureaulite crystals in the outer layer of the coating are related to the uniform and localized attack of the steel, these additives, and their corresponding ions, are studied as corrosion inhibitors that affect the formation of the phosphate coatings. The results reveal that the refining of the crystals in the outer layer of the phosphate coating is proportional to the inhibition of the corrosion of the steel. The formation of the phosphate coating follows the cyclic phosphating mechanism for all the baths.

Assessing 3 and 5 Degrees of Freedom Toolpath Strategy Influence on Single Point Incremental Forming

Single Point Incremental Forming (SPIF) is a promising manufacturing technology concerning the production of customized products, low batches or prototyping of ready-to-use parts, given its easy implementation and absence of dedicated tooling. The range of application is wide, covering many materials and virtually unlimited geometries. Indeed, nowadays’ boundaries of the process are more related to the machines limitations than to the process itself. The SPIFA machine [1] developed at the University of Aveiro allies high payload capacity to flexibility driven by a kinematics based on a Stewart Platform. In this work, it will discussed the effects of employing five degrees of freedom toolpaths to produce aluminium and high strength steel parts.

Influence of stress on martensitic transformation and mechanical properties of hot stamped AHSS parts

Non-isothermal tension and compression tests of 22MnB5 boron steel were carried out in this study. How different stress state influences the martensitic transformation of advanced high strength steel (AHSS) parts was analyzed. The analysis reveals that the martensitic transformation starting temperature (Ms) changes with different stress states. Specifically, the Ms temperature rises with increasing tensile stress, however, it rises first and then drops with increasing compressive stress. Moreover, a higher initial forming temperature leads to a higher Ms temperature under the same stress. Simulation of an actual hot-formed AHSS B-pillar together with the microscopic metallography, hardness and martensitic content shows that in higher tensile stress dominated area, the martensitic content and hardness are usually higher than in other areas. Although the stress can promote the Ms temperature, a lower cooling rate may lead to less martensite fraction.