Press-hardened Steel Research Articles

The Effect of Tempering on the Microstructure and Mechanical Properties of a Novel 0.4C Press-Hardening Steel

In this paper, the effects of different tempering temperatures on a recently developed ultrahigh-strength steel with 0.4 wt.% carbon content were studied. The steel is designed to be used in press-hardening for different wear applications, which require high surface hardness (650 HV/58 HRC). Hot-rolled steel sheet from a hot strip mill was austenitized, water quenched and subjected to 2-h tempering at different temperatures ranging from 150 °C to 400 °C. Mechanical properties, microstructure, dislocation densities, and fracture surfaces of the steels were characterized. Tensile strength greater than 2200 MPa and hardness above 650 HV/58 HRC were measured for the as-quenched variant. Tempering decreased the tensile strength and hardness, but yield strength increased with low-temperature tempering (150 °C and 200 °C). Charpy-V impact toughness improved with low-temperature tempering, but tempered martensite embrittlement at 300 °C and 400 °C decreased the impact toughness at −40 °C. Dislocation densities as estimated using X-ray diffraction showed a linear decrease with increasing tempering temperature. Retained austenite was present in the water quenched and low-temperature tempered samples, but no retained austenite was found in samples subjected to tempering at 300 °C or higher. The substantial changes in the microstructure of the steels caused by the tempering are discussed.

Investigation of Mechanical Tests for Hydrogen Embrittlement in Automotive PHS Steels

The problem of hydrogen embrittlement in ultra-high-strength steels is well known. In this study, slow strain rate, four-point bending, and permeation tests were performed with the aim of characterizing innovative materials with an ultimate tensile strength higher than 1000 MPa. Hydrogen uptake, in the case of automotive components, can take place in many phases of the manufacturing process: during hot stamping, due to the presence of moisture in the furnace atmosphere, high-temperature dissociation giving rise to atomic hydrogen, or also during electrochemical treatments such as cataphoresis. Moreover, possible corrosive phenomena could be a source of hydrogen during an automobile’s life. This series of tests was performed here in order to characterize two press-hardened steels (PHS)—USIBOR 1500® and USIBOR 2000®—to establish a correlation between ultimate mechanical properties and critical hydrogen concentration.

Effect of galvanneal-coating evolution during press-hardening on laser welding of 22MnB5 steel

ABSTRACTUnderstanding phase evolution and growth kinetics of Zn-coating during press-hardening is essential in predicting welding behaviour of press-hardened 22MnB5 steel. The results showed that during press-hardening, the initial δ and Γ-Γ1 phases in the coating transform into α-Fe(Zn), Γ, and ZnO phases. As heat-treating continued at a constant temperature, the α-Fe(Zn) thickness grew and the overall Zn-content decreased. This resulted in the full-penetration weld window shifting towards higher heat-inputs. However, when heat-treating at higher temperatures, severe oxidation and Fe-Zn inter-diffusion led to the reduction of Zn-content in the evolved coating. It was found that higher heat-treatment times resulted in wider weld beads, thereby increasing joint strength.

Effects of Al-Si coating and Zn coating on the hydrogen uptake and embrittlement of ultra-high strength press-hardened steel

Press-hardened steel (PHS), used for automotive safety-related structure parts, is sensitive to hydrogen embrittlement due to its martensitic microstructure. Hydrogen is introduced in PHS during the hot press forming (HPF) process, by an atmospheric corrosion process. In this study, the hydrogen embrittlement behavior of uncoated, aluminized, and galvanized PHSs was investigated. The Al-10%Si coating promoted the absorption of diffusible hydrogen at elevated temperature during the HPF while the reacted coating layer prevented the absorbed hydrogen from out-diffusing through the reacted coating surface layer at room temperature. Therefore, the aluminized PHS showed a greater sensitivity to both the hydrogen uptake and the resultant embrittlement, as compared to the uncoated and galvanized PHSs. Use of galvanized PHS for HPF application reduces the risk of hydrogen embrittlement, since the Zn coating effectively prevents the hydrogen uptake. The greater embrittlement resistance of the galvanized PHS is possibly due to the inhibition of the hydrogen generation reaction by the surface ZnO oxide layer and the low rate of hydrogen transport through the liquid Zn phase.

High Temperature Oxidation Resistance of Cr Coatings on 22MnB5 Steel Electroplated in Trivalent Cr Baths

22MnB5 steel is one kind of advanced high-strength steels commonly used for vehicles to enhance passengers’ safety and reduce carbon dioxide emission[1]. However, 22MnB5 steel is vulnerable to high-temperature oxidation and a protective coating is essential to inhibit oxidation during hot stamping process. For example, hot-dip Al-10 wt % Si coating has been extensively applied on press hardening steel although it does not provide sacrificial protection on the steel parts[2]. In contrast, hot-dip Zn coating, which confers sacrificial protection, tends to suffer oxidation and liquid metal embrittlement during hot stamping[3, 4]. This study investigated the structure and properties of Cr coatings on 22MnB5 steel electroplated in a trivalent Cr (Cr(III)) bath. The DC electroplating was conducted in a chloride-based Cr(III) electrolyte with and without the addition of sulfur-containing organic compounds. The morphology, microstructure, and composition of the Cr coating were characterized using scanning electron microscope (SEM), X-ray diffraction (XRD), and electron dispersive spectroscopy (EDS). The thickness of the deposit before and after high-temperature heat treatment (900℃ for 5 min in air) was measured using cross-sectional SEM. Experimental results show that the sulfur-containing organic compound in the electrolyte reduces the carbon content and increases the sulfur content of the Cr coating, which, in turn, reduces the residual stress of the coating. As a result, a crack-free Cr coating can be obtained in the electrolyte with the organic additive. In contrast, many cracks develop on the Cr coating plated in the electrolyte without the organic additive. After the heat treatment at 900℃ for 5 min, extensive oxidation was observed on the crack-free Cr coating, but not on the Cr coating with cracks. It is likely that the presence of sulfur in Cr coating deteriorates the oxidation resistance. Crack openings of the Cr coating free of sulfur are filled with compact oxides after 900℃heat treatment for 5 min. The steel substrate with a Cr overlay remains intact, indicating the Cr coating prevents the steel substrate from oxidation at 900℃.Finally, a 2-mm-thick Cr coating can be electroplated in the Cr(III) bath without the organic additive at a current density of 10 A/dm2and a plating time of 1 min and effectively protects the 22MnB5 steel from high-temperature oxidation. Reference [1] H. Karbasian, A.E. Tekkaya, Journal of Materials Processing Technology, 210 (2010) 2103-2118. [2] D.W. Fan, H.S. Kim, J.-K. Oh, K.-G. Chin, ISIJ international, 50 (2010) 561-568. [3] C.W. Lee, D.W. Fan, I.R. Sohn, S.-J. Lee, B.C. De Cooman, Metallurgical and Materials Transactions A, 43 (2012) 5122-5127. [4] M. Razmpoosh, A. Macwan, E. Biro, D. Chen, Y. Peng, F. Goodwin, Y. Zhou, Materials & Design, (2018).

Evaluation of Mechanical Properties and Microstructures of Molybdenum and Niobium Microalloyed Thermomechanically Rolled High-Strength Press Hardening Steel

This article studied the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory control rolled steels based on grade 22MnB5. The constructed continuous cooling transformation diagrams revealed that an addition of Mo significantly increased the hardenability. Especially in the case of austenite compressed below its recrystallization temperature, an Mo addition delayed ferrite and bainite formation, and it increased martensite hardness. Laboratory hot-rolling experiments with a finish rolling temperature of 850°C produced a flattened pancaked austenite. After hot rolling and direct quenching, samples were rapidly re-austenitized at 900°C followed by water quenching to simulate an actual press hardening process. Especially in the case of Nb-Mo steel, a strongly pancaked austenitic structure was achieved after hot rolling, which led to a fine, uniform prior austenite grain structure after re-austenitization and quenching. The Nb-Mo steel had a tensile strength > 1500 MPa and ~ 11% total elongation combined with good impact toughness, which can be considered excellent for this type of press hardening steel.

High-temperature deformation behavior and constitutive relationship of press-hardening steel 38MnB5

With the rapid development of global economy, problems in energy production and environmental protection are becoming severe, and the automotive industry is under increasing pressure to reduce the weight of vehicles and improve crash performance. Due to the demand for reduced vehicle weight as well as improved safety and crashworthiness, hot-stamped components from ultra-high strength steels have been utilized for automobile manufacturing. Currently, the most widely used hot-stamped steel plate is 22MnB5. Its tensile strength is 1500 MPa and yield strength is 1200 MPa. In contrast, as the demands for steel strength have increased, the demand for high strength grades of steel has been quickly put on the production agenda. In recent years, a novel hot-stamped steel, 38MnB5 has been developed, with a tensile strength exceeding 2000 MPa. The high temperature deformation behavior of 38MnB5 steel was investigated by the Gleeble-3500 thermal-mechanical simulator. The isothermal uniaxial tensile tests of the steel were performed within deformation temperature range of 650-950℃ under strain rates of 0. 01, 0. 1, 1, and 10 s-1, and the typical true stress-strain curves of 38MnB5 at relative conditions were analyzed. The experimental results show that the flow stress rises with decreasing deformation temperature under the same strain rate, and with an increasing strain rate. When the strain rate gradually increased, dynamic recovery and dynamical recrystallization exhibited an apparent effect on the hot deformation process, while the inconspicuous impact receded with rising temperature. In consideration of the multiple influences on deformation temperature, strain rate and strain, a phenomenological, constitutive relationship was developed to depict the hot deformation process of 38MnB5. In the established equation, the material constants dependent on the deformation temperature, strain rate, and strain were obtained using regression analysis of the experimental data for flow stress, strain, strain rate, etc. The comparison between the calculated data and the experimental data show that the calculated data derived from the constitutive models are found to be in satisfactory agreement with the experimental results.

Characterization of Static Performance and Failure of Resistance Spot Welds of High-Strength and Press-Hardened Steels

The static performances and failure characteristics of resistance spot welds (RSW) of steel sheets were studied in this work. Both similar and dissimilar joints of press-hardened steel grade 22MnB5 and as-rolled high-strength steel grade 1000 were examined. From metallographic analyses and micro-hardness measurements, it was shown that the microstructures of base metal (BM), heat affected zone (HAZ) and fusion zone of steel joints were noticeably different. In particular, the HAZ close to BM or subcritical HAZ (SCHAZ) of 22MnB5 steel exhibited greatly decreased hardness due to occurring tempered martensite, in which carbide precipitation was clearly observed. Furthermore, tensile shear test and cross-tension test of the welded steel sheets were carried out. The determined maximum loads, energy absorbability and corresponding failure mechanisms were analyzed and compared. The RSW samples of similar steel grade 1000 provided the highest load-carrying capacity and absorbed energy in both tests. Due to the large hardness difference and soft zone in weldment of dissimilar steel specimen, failure loads and static absorbed energies were deteriorated up to 17 and 68%, respectively. The observed fracture occurrences of all investigated joints were pullout failure mode, in which BM and HAZ were the crack initiation sites of grades 1000 steel and 22MnB5 steel, respectively. The SCHAZ of welded boron steel considerably caused the failure onset in tension shear test, but seemed to have negligible effect on cross-tension test.

Influence of austenitizing parameters on microstructure and mechanical properties of Al-Si coated press hardened steel

The influence of the austenitizing parameters on the microstructure and mechanical behavior of Al-Si coated press hardened 22MnB5 steel was evaluated in this study. Increasing the austenitizing temperature and hold time resulted in microstructural homogenization, an increase in the prior austenite grain size (PAGS), and increased thickness of the substrate-coating interdiffusion layer. The mechanical properties were evaluated using smooth-sided tensile testing, double edge-notch tensile testing, and free bend testing. Refinement of the PAGS was, in general, beneficial for the mechanical performance of the press hardened steel (PHS); i.e. strength, notch displacement, maximum bending load, and bend angle at maximum load all generally increased with decreasing PAGS. The stress and strain state experienced during each type of test influenced the sensitivity of mechanical and fracture behavior of the PHS to austenitizing conditions. Furthermore, the thickening of the substrate-coating interdiffusion layer, composed of brittle intermetallic phases, at higher austenitizing temperature negatively influenced bendability of the PHS. Ductile fracture was commonly observed in the steel matrix composed of a lath martensitic structure. However, quasi-cleavage fracture was observed for the martensitic matrix with the largest PAGS, which implies a loss of toughness, possibly because of the increased substructure size.

Suppression of δ-ferrite formation on Al-Si coated press-hardened steel during laser welding

Mechanical properties of Al-Si coated press-hardened steel laser welded joint can not meet requirement due to the presence of δ-ferrite. This study investigated the suppression mechanism of δ-ferrite using a nickel interlayer during fiber laser welding. The fraction of primary solidification δ-ferrite was reduced to 57% with a 5.85% Ni in welding pool, and completely transformed into austenite during peritectic reaction (L + δ → γ) and solid phase transformation (δ → γ). Full martensite microstructure was obtained in welded seam and fusion line without Al-segregation. Tensile strength of welded joint had a ∼17.1% improvement compared to as-received welded, similar to that of removing Al-Si coating.

Bake-partitioning in a press-hardening steel

Bake-partitioning, a process integrating paint-baking of car-body assembling to induce carbon partitioning into austenite, has been proposed for enhancing the ductility of press-hardening steels. Carbon is estimated to diffuse a maximum distance of 2.7 nm in austenite at 170 °C for 20 min. Austenite films with a thickness of approximately 5 nm are therefore desired for sufficient bake-partitioning. This was realised by the ultra-fine prior austenite grain due to 7 wt% of manganese addition combined with vanadium carbide precipitation. The novel alloy exhibits 1800 MPa ultimate tensile strength combined with 16% elongation, which has never been achieved in press-hardening steels.

Effect of TiN-Particles on Fracture of Press-Hardened Steel Sheets and Components

It is known that large TiN particles deteriorate the toughness of high-strength steels. The toughness of press-hardened steel (PHS) sheets with full-martensite microstructure for safety critical components of a car body has, however, seldom been studied. Consequently, the effect of TiN particles on either the toughness of PHS sheets or the performance of hot-stamped components have not been investigated. In this research, the size and distribution of TiN particles are quantitatively analyzed for two 22MnB5 PHS sheets containing nitrogen of 29 ppm and 44 ppm in mass. Higher populations of coarse TiN particles have a negligible effect on standard tensile properties, but significantly reduce impact toughness by Charpy V-notch impact tests. Hot-formed components with smaller fractions of TiN particles show greater fracture resistance by a three-point bending test.

The use of Scanning Vibrating Electrode Technique to Evaluate the Effect of Hot Stamping on the Corrosion Resistance of 22MnB5 press-hardened Steel Electroplated with Zn-Ni

Carmakers must achieve the worldwide targets for lightweight materials, safety and reducing the fuel consumption. The use of press-hardened steel (PHS), in vehicle structures has been contributing with these requirements. This type of steel is widely used for the hot-stamping which consists in heating the steel blank to total austenitization temperature and then transferring it from the furnace into the die tool where the steel is formed and quenched at the same time. PHS is usually protect with metallic coatings in order to avoid both steel oxidation and decarburization. Hot-dip Al-Si coating is currently the main used in this application. However, alternative coatings, like zinc-based, are under investigation. This work aims at evaluating the corrosion resistance of the 22MnB5 grade PHS, electroplated with Zn-Ni, before and after hot stamping, using the scanning vibrating electrode technique (SVET). Results from SVET showed that samples prior to hot stamping, the corrosion mechanism was uniform over the exposed surface, and was mainly related to selective dissolution of zinc from the coating. On the other hand, hot stamped samples showed localized corrosion mechanism with decrease in current with time of immersion due to the formation of corrosion products on the exposed surface.

Effect of Laser Beam Welding on the Cyclic Material Behavior of the Press-hardened Martensitic Chromium Steel X46Cr13

For the application of high-strength materials in welded joints, a point of principle is how the strength of the sheet metal is affected by cyclic loading and by welding. For the investigation of the cyclic material behavior of the press-hardened martensitic chromium steel X46Cr13, strain-controlled fatigue tests were performed and evaluated. The aim of compensating the limitations in the weldability of this press-hardened material is achieved by a reduced heat input of the laser beam welding and a defined heat treatment. The effect of laser beam welding on the fatigue properties is shown by the cyclic behavior of butt joints. The cyclic material behavior is the basis of strain-based fatigue assessment approaches. Both cyclic stress-strain curves and strain-life curves are used for the fatigue life estimation. No clear difference between the press-hardened base material and butt joints has been found in the cyclic stress-strain curves. Transient effects are found by comparison of hysteresis loops of the initial loading, at the cyclically stabilized state and at crack initiation. Cyclic hardening is concluded from the initial loading and the cyclically stabilized state. By comparison of strain-life curves, a difference in the number of cycles to crack initiation between the base material and butt joints is found. Cycles to crack initiation of butt joints tested under strain control result in over 50 % of the base material’s fatigue strength at 1·106 cycles to failure.

Process development for self-pierce riveting with solid formable rivet of boron steel in multi-material design

Abstract Mechanical joining techniques are important joining processes for the implementation of lightweight structures in automotive engineering and other branches of industry. However, existing processes such as self-pierce riveting with semi-tubular rivet (SPR-ST) or solid rivet (SPR-S) reach their limits in certain applications, e.g. joining press-hardened boron steel sheets in multi-material constructions of automotive bodies. A new method to join these materials reliably and with high quality is the self-pierce riveting with solid formable rivet (SPR-SF). This paper shows different problems during the mechanical joining of press-hardened steel 22MnB5 by conventional SPR-S and illustrates the numerical as well as the experimental development of the new rivet concept: the solid formable rivet. On the basis of validated simulation models for conventional SPR-S numerical sensitivity analyses are carried out to investigate the correlations between the rivet properties (geometry parameters, material) and important quality characteristics of the joint like interlock, punching force and upsetting force for the new SPR-SF joining process. With the knowledge of these studies, specific rivet geometries are determined and prototypes manufactured for joining the material combination EN AW-6016 T4 (t = 1.5 mm) into 22MnB5 (t = 1.2 mm) in good quality. So, the application potential of the new SPR-SF is shown.

Effect of Original Microstructure on Microstructure and Mechanical Properties of High Strength Steel WHF1500H during Hot Forming

The press hardening steel sheets WHF1500H with different original structures produced by compact strip production (CSP) line in Wuhan Iron and Steel Ltd. were austenitized at 950 °C for 5 min, and then hot stamped and quenched by using the flat die. The microstructure and mechanical properties were investigated by optical microscope, transmission electron microscope and universal testing machine. The results show that the microstructure after stamping and quenching is fully composed of lath martensite. The mechanical properties of the steel samples with different original structures are different after hot stamping and quenching, and this difference is smaller than that before hot stamping and quenching. When the original structure consists of ferrite and pearlite, the austenite grains after austenitization are fine and uniform. In addition, the martensite structure obtained after hot stamping and quenching is also uniform and fine, leading to higher mechanical properties. With the increase of the strength of the original steel, when the original structure is martensite, the austenite grains after austenitization are coarse, and the martensite structures obtained after quenching is also coarse, and thereby decreasing the strength.

Microstructure based modeling of deformation and failure of spot-welded advanced high strength steels sheets

Microstructures and hardness of different weld zones of spot-welded high strength steel grade 1000 and press-hardened 22MnB5 steel were characterized. Tensile shear and cross tension test of welded specimens were conducted, in which load responses and fracture developments were identified. Then, samples with microstructures of each weld zone in all examined joints were generated by specific heat treatment processes. 2D representative volume elements (RVEs) for these microstructures were modelled. Flow stress curves of the individual phases were described by micromechanics based models under consideration of local chemical compositions and phase constituents. It was clearly shown that predicted overall stress-strain responses of all weld regions were well in accordance with the tensile test results of heat-treated samples. Furthermore, fracture criteria were determined for each weld zone by using samples having varying geometries for a wide range of stress triaxialities along with the digital image correlation technique. Finally, FE simulations of the tensile shear and cross tension tests of all investigated joints were carried out, in which the obtained flow stress curves and fracture criteria were defined for the individual weld regions. The experimental force-displacement curves up to fracture and observed failure occurrences could be correctly described by the simulation results.

Phase Composition and Microstructure of Zn Coated Press Hardening Steels for Direct Hot Stamping

Decarburization and oxidation occur during production of press hardening steels (PHS) used in automotive. Therefore, coatings are applied on the steel surface to prevent this. The most common Zn coatings have not been processable by available direct hot stamping technology ordinarily used to produce the PHS and have been replaced by Al-Si coatings. However, the Al-Si coatings have a negative impact on joining technologies and further processing. Recently, the modification of direct hot stamping, which enables process the PHS with Zn coatings, was introduced. We investigated the microstructure, phase composition and distribution of particular phases (SEM-EDS, XRD), as well as micro-hardness of the Zn coated PHS prepared by the modified direct hot stamping. The PHS revealed the coating with 25 ± 3 μm thickness and continuous interface consisting of α-Fe(Zn) solid solution, Γ and Γ1 intermetallic phases and zinc oxides enriched with manganese. The thorough description of the material is important in development of further processing steps such as joining technologies and corrosion resistance.

Mechanical properties of repair welded joints for automobile body structures

The purpose of this study is to evaluate the mechanical properties of repair spot welding and plug welding for automobile body structures. The lifecycle of a modern car includes different phases. From research and development to production and assembly and finally to the use of the car by the customers. These are the most examined phases, whereas the after-sales sector is a nonetheless important part of the lifecycle. After sales includes the repair of damaged automobile structures after road accidents. Increasing demands for safety and comfort lead to the request for feasible repair methods which create equal mechanical characteristics of the body structures such as the original set up. The joining technologies for modern press-hardened high-strength steels are essential in this context. Testing of lap shear and cross-tensile specimens is used to show mechanical properties of two repair welding technologies. The results show a major loss of strength in the tested joints when using plug welding instead of state of the art repair spot welding. Various analytical methods are moreover used to get a detailed understanding of the influence of both repair welding technologies on the material structure.

Influence of vanadium on the hydrogen embrittlement of aluminized ultra-high strength press hardening steel

Improved safety standards and reduced automotive body-in-white weight have led to a strong interest in martensitic press hardening steel (PHS). As the sensitivity to hydrogen embrittlement of martensite increases at higher strength level, the very small uptake of diffusible hydrogen by aluminized PHS during the austenitization stage of the hot press forming process is of concern. The hydrogen uptake was found to reduce the plasticity of the PHS considerably. The PHS with a higher strength was more susceptible to hydrogen-induced brittle fracture. The present work reports that vanadium additions, which serve to trap the hydrogen, considerably reduce the negative impact of the hydrogen uptake in aluminized 1800 MPa and 2000 MPa PHSs. The contribution also proposes a mechanism for the uptake of hydrogen during the processing of aluminized PHS.