Hot Stamping Steel Research Articles

Study on the Effect of Heat Treatment on Microstructures and High Temperature Mechanical Properties of Welding Spots of Hot Stamped Ultra-High Strength Steel Patchwork Blanks

Insufficient strength of welding spots is a common problem in the hot stamping process of ultra-high strength steel patchwork blanks (UHSSP). In this paper, the welding spots of 22MnB5 boron steel with thicknesses of 1.2 and 1.5 mm were austenitized and then air-cooled to 650–850 °C for high temperature tensile shear tests and high temperature cross-tension tests, respectively. To study the mechanical properties of the welding spots at room temperature after heat treatment, the austenitized welding spots were quenched in cold water to room temperature, and microhardness tests and microstructure observations were performed. The results indicated that compared to the original welding spots, the heat-affected softening zone disappeared after heat treatment, and the hardness values of the fusion zone, heat-affected zone and base material were basically the same, at about 500 HV. After heat treatment, the welding spots were mainly martensite. With the increase in deformation temperature, the peak loads of the tensile shear and the cross tension of the welding spots decreased. At 750 °C, the peak loads of the welding spots decreased less, energy absorption was larger, and the welding spots had the comprehensive mechanical properties of strength and ductility.

A mean-field homogenization approach to predict fracture in as-quenched microstructures of Ductibor® 500-AS steel: characterization and modelling

Ductibor® 500-AS is a hot-stamping steel that has been designed to introduce local ductile regions within hot-stamped tailor-welded blanks (TWBs) used for energy-absorbing sections of automotive structures. This study investigates the flow and fracture behavior of a range of the microstructures of this alloy, corresponding to mostly ferritic (~96% ferrite plus 4% martensite), intermediate ferritic + martensitic (~57% ferrite plus 43% martensite), and mostly martensitic (~10% ferrite plus 90% martensite), obtained by applying various quench rates after austenitization. A series of mechanical tests in various stress states, ranging from simple shear to biaxial tension, was performed to characterize fracture for the developed microstructures. A mean-field homogenization (MFH) technique was then used to predict the flow and fracture response as a function of the microstructure. An MFH scheme designated as the interpolative Samadian-Butcher-Worswick 1 (INSBW1) within the first-order secant-based linearization method was considered to predict the macroscopic hardening behavior of the ferritic-martensitic microstructures. In the MFH modelling, the flow response of the ferritic and martensitic phases was modelled based on a dislocation-based hardening model, taking into account the chemical composition, carbon partitioning between phases, and dislocation generation and annihilation. Damage accumulation and the onset of fracture were predicted using a phenomenological damage indicator defined within each constituent phase given their calculated fracture loci and instantaneous stress and strain states. The experimental results showed that the mostly-martensitic microstructure had the lowest ductility and highest strength, and ductility and strength increased and decreased, respectively, as the martensite volume fraction in the microstructure decreased. The predicted flow curves and fracture strains as well as the fracture-limit curves for all of the microstructures agreed well with the measured data and interpolated modified Mohr-Coulomb (MMC) fracture loci.

Effects of Al-Si Coating on Static and Dynamic Strength of Spot-Welded Hot-Stamping Steel Joints

Al-Si is the most popular coating used to prevent oxidation on the surfaces of hot-stamped steel sheets during the heating process. However, like other coatings, it affects the strength of the spot welds in joining the hot-stamped steel parts. In this study, the effects of Al-Si coating on the tensile strength of the resistance spot-welded joints in hot-stamped steel are discussed. Two types of 1.8 mm hot-stamped steel, in uncoated and Al-Si coated forms, were resistance spot-welded, and the tensile shear behavior of the welded joints was studied in both static and dynamic tests. To do this, a special fixture for impact tensile shear tests was designed and fabricated. In the case of the Al-Si coated steel, the presence of the molten Al-Si over the fusion zone, especially its aggregation in the edge of the weld nugget, caused a decrease in the maximum tensile load capacity and a failure of energy absorption in static and dynamic tests, respectively. Additionally, it increased the probability of changing its failure mode from pull out to interfacial fracture in the dynamic test. This study shows that the tensile strength behavior of the welded joints for the Al-Si coated hot-stamped steel is lower than the uncoated steel during static, and especially dynamic, force.

The effect of laser welding on microstructure and mechanical properties in heavy-gage press hardening steel alloys

The hot stamping steel grade 22MnB5 steel producing a target tensile strength of 1500 MPa is standardly used for press hardened components in passenger cars. The thickness of such components is usually limited to 2 mm or less. More recently, the application of press hardened components has been also considered for components of heavy commercial vehicles requiring much thicker sheet gages. The heavier gage poses increased challenges towards the press hardening process as well as to assembly technology. This study, being part of a larger development program, evaluates laser welding of heavy gage press hardened steel. The steel used is hot rolled standard 22MnB5 as well as a variant modified by niobium and molybdenum microalloying for optimizing hardenability and toughness. The press hardened steel samples were subjected to double-pass laser welding. The welded samples of the modified steel variant were found to have a more favorable microstructure in the heat affected zone resulting in improved mechanical properties and more ductile fracture behavior. The improvements relate to the inhibition of austenite grain coarsening in the heat affected zone. The mechanisms of grain growth limitation during laser welding of the modified steel will be outlined.

MBW 1200 – Hot Stamping Steel with Increased Ductility

Hot stamping steels are trending towards increased ductility without sacrificing too much stiffness. Thus a new aluminum silicon coated grade, MBW 1200 + AS has been developed, with typical yield strength after hot stamping and paint baking of YS ≈ 1000 MPa, tensile strength TS ≈ 1200 MPa and A80 > 5.0 %. The highly increased ductility compared to 22MnB5 expresses through the particularly increased bending angle of >75° and the logarithmic true thickness strain of ≈0.90. Hence MBW 1200 shows the desired significantly higher ductility compared to 22MnB5 in lateral crash testing without crack appearance. The process stability has been approved by different tests, e.g. increased furnace dwell time, tool temperature and transfer time. Verifying MBW 1200 in patchwork blank applications with total thicknesses of 3.0 and 3.5 mm showed only a minor decrease of YS and TS between 30 and 50 MPa and leaves the Vickers hardness at ≈400 HV10 with fully martensitic microstructure. In partial press hardening tests, using tools heated of up to 550 °C, bending angles reach the test’s maximum and YS falls below 500 MPa with a hardness of ≈210 HV1. Finally in a comparison between experiment and numerical hot stamping simulation it can be shown, that the determined material modelling parameters can well be used in the feasibility analysis of new automotive components.

Hydrogen embrittlement characteristics of hot-stamped 22MnB5 steel

Hydrogen embrittlement (HE) characteristics of 22MnB5 steel (U-bent specimen) manufactured using hot-stamping process at various temperatures were experimentally and numerically investigated. Steel resistance to HE was examined through delayed failure tests under static and cyclic loading during hydrogen charging. First, the low cyclic loading caused severe HE, in which a clear difference in the extent of HE was obtained depending on the hot-stamped sample, which directly affected the microstructural characteristics and stress–strain distribution. The hot-stamped samples with large martensite phase showed low resistance to HE compared with those with small martensite phase because of the high concentration of hydrogen trapped in the phase boundaries. Moreover, the dual phase (ferrite and martensite) of the hot-stamped samples reduced their resistance to HE, which is caused by the hydrogen trapped in the laminar-shaped pearlite phase. The resistance to HE was improved by low-temperature heating at 200 °C for 1 h because of the generation of ε-carbides as trap sites as they render the hydrogen non-diffusible. Furthermore, the internal strain in the U-bent sample could accelerate HE because of the high concentration of hydrogen. These results were verified by experimental and numerical analyses. Thus, the hydrogen trapping mechanism was proposed as a valid mechanism for HE in 22MnB5.

Investigation of the Microstructural Evolution during Hot Stamping of a Carburized Complex Phase Steel by Laser-Ultrasonics.

Prior carburization of semi-finished steel sheets is a new process variant in hot stamping to manufacture parts with tailored properties. Compared to conventional hot stamping processes, a complex phase typed steel alloy is used instead of 22MnB5. Yet recent investigations focused on final mechanical properties rather than microstructural mechanisms cause an increase in strength. Thus, the influence of additional carburization on the microstructural evolution during hot stamping of a complex phase steel CP-W®800 is investigated within this work. The phase transformation behavior, as well as the grain growth during austenitization, is evaluated by in-situ measurements employing a laser-ultrasound sensor. The results are correlated with additional hardness measurements in as-quenched condition and supplementary micrographs. The experiments reveal that the carburization process significantly improves the hardenability of the CP-W®800. However, even at quenching rates of 70 K/s no fully martensitic microstructure was achievable. Still, the resulting hardness of the carburized samples might exceed the fully martensitic hardness of 22MnB5 derived from literature. Furthermore, the carburization process has no adverse effect on the fine grain stability of the complex phase steel. This makes it more robust in terms of grain size than the conventional hot stamping steel 22MnB5.

Effect of hierarchical martensitic microstructure on fatigue crack growth behavior of ultra-high strength hot stamping steel

Martensite is a typical hierarchical microstructure, but which substructure plays a major role on fatigue crack growth behavior is still not clear. Therefore, this paper studied the microstructure and mechanical properties of 22MnB5 hot stamping steels, the influence of hierarchical martensitic microstructure on fatigue crack growth behavior was analyzed. Results show that higher hot stamping temperature results in larger size of prior austenite grain and substructure, the tensile strength decreased with increasing temperature, while the fatigue life and critical crack length were higher. It was concluded that block is the effective substructure to control the fatigue crack growth behavior of the steel. The fatigue fracture of stable-growing region was typical fatigue striations along with secondary cracks, crack closure was observed in rapid-growing region.

The Effect of 3D Representative Volume Element Grain Morphology Resolution on the Prediction of Fracture and Damage Accumulation for Multiphase Hot‐Stamped Steels

This work investigates the effect of 3D representative volume element (RVE) grain morphology resolution on the prediction of fracture and damage evolution for tailor hot stamped steels. Scanning electron micrographs were used to build RVE meshes from coarse‐8 × 8 (<1 elements μm−3) to high‐50 × 50 (53–81 elements μm−3) element densities. For a uniaxial stress‐state, the tailored material condition (TMC1) (ferrite‐pearlite‐martensite) RVE nucleated voids due to separation between small martensite grains and the RVE fracture strain was 0.41 and 0.53 for the 50 × 50 and 25 × 25 RVE, respectively. The TMC3 (ferrite‐bainite‐martensite) RVE predicted high strain partitioning between the irregularly shaped martensite grains and the higher resolution RVE predicts a 39% lower fracture strain. The large grained martensite‐bainite TMC5 RVE predicted relatively similar void accumulation and strain partitioning behavior for the 50 × 50 and 25 × 25 RVE models, while the coarser 13 × 13 and 8 × 8 models predicted higher fracture strains due to low grain morphology resolution. Compared to an experimentally derived fracture locus, plane strain and equibiaxial deformation of the RVEs was shown to predict the equivalent fracture strains reasonably well, but like in the uniaxial case, the coarser RVEs overpredict the fracture strain.

Phase Composition of Al-Si Coating from the Initial State to the Hot-Stamped Condition.

The chemical and phase composition of the coating and the coating/substrate interface of an Al-Si-coated 22MnB5 hot stamped steel was investigated by means of SEM-EDS, XRD, micro-XRD and electron diffraction. Moreover, the surface profile was analyzed by XPS and roughness measurements. The XPS measurements showed that the thickness of the Si and Al oxide layers increased from 14 to 76 nm after die-quenching, and that the surface roughness increased as well as a result of volume changes caused by phase transformations. In addition to the FeAl(Si) and Fe2Al5 phases and the interdiffusion layer forming complex structures in the coating, electron diffraction confirmed the presence of an Fe2Al5 phase, and also revealed very thin layers of Fe3(Al,Si)C, Fe2(Al,Si)5 and Al-bearing rod-shaped particles in the immediate vicinity of the steel interface. Moreover, the scattered nonuniform layer of the Fe2Al8Si phase was identified in the outermost layer of the coating. Despite numerous studies devoted to researching the phase composition of the Al-Si coating applied to hot stamped steel, electron diffraction revealed very thin layers and particles on the coating/substrate interface and outermost layer, which have not been analyzed in detail.

Influence of Thermal Modification on Al-Si Coating of Hot-Stamped 22MnB5 Steel: Microstructure, Phase Transformation, and Mechanical Properties

The hot-stamped steel with ultrahigh strength is a promising material for the fabrication of automotive components. However, the coating on the sheet surface leads to a softening problem in the welded joint. Instead of the costly coating removal process, heat treatment is an economical and effective method for the diffusion process, which can decrease the Al concentration in the coating. In this study, a preheating treatment was carried out on Al-Si-coated 22MnB5 hot-stamped steels for the homogeneity of Al, followed by laser welding and hot stamping. The effects of the preheating on the microstructure and mechanical properties of the laser-welded joints were investigated. With the preheating treatment, the Al-Si coating transformed into an Fe-Al intermetallic compound and the difference in Al content between the coating and substrate was reduced. The Al content in the weld of the specimen with the preheating treatment was reduced, compared with that without the preheating treatment. The amount of δ-ferrite in the weld after laser welding was reduced largely. The distribution of long-bland-like segregation was changed to a fine and uniform distribution. With the preheating treatment, the tensile strength of the welded joint was significantly improved and comparable to that of the decoated joint. In conclusion, the preheating treatment before the welding is an effective method to suppress the formation of δ-ferrite and improve the mechanical properties of the welded joint.

Effects of solid solution and grain-boundary segregation of Mo on hydrogen embrittlement in 32MnB5 hot-stamping steels

Hydrogen embrittlement (HE) has become an important issue in ultra-strong automotive steel applications. The addition of Mo to commercial 32MnB5 hot-stamping steel is preferred to enhance the strength levels with little ductility loss. However, the effects of solute Mo on HE have been rarely studied for developing 32MnB5 steel, and most studies on the alloying of Mo for interfacial cohesion have been conducted theoretically by calculating the cohesive energies in the Fe lattice. In this study, 0.15 wt.% Mo was added to the 32MnB5 steel and the resistance to HE was evaluated experimentally via electrochemical H-charging. The H-charged reference steel shows a large ductility loss (50–79%) after H-charging, while the addition of Mo significantly reduces the loss (17–26%) with sufficient post-elongation, indicating a higher resistance to HE. This is because the solute Mo decreases the H diffusivity, resulted from the high H affinity and repulsive strain field owing to the large atomic size of Mo. The direct observation of crack propagation reveals that the H-induced crack path changes from the prior austenite grain boundaries (PAGBs) to the grain interiors of H-enhanced slip planes. This is attributed to the reduced H- and strain-localization on the PAGBs by the solute Mo and the enhanced grain-boundary cohesion by Mo segregation. This work thus demonstrates the beneficial effects of the addition of Mo on the tensile properties and the intrinsic resistance to HE for the development of ultra-high-strength steels.

Numerical and Experimental Study of AlSi Coating Effect on Nugget Size Growth in Resistance Spot Welding of Hot-Stamped Boron Steels

In recent years, increasing automotive safety by improving crashworthiness has been a focal point in the automotive industry, employing high-strength steel such as press hardenable steel (PHS). In addition to the improved strength of individual parts in the body of the vehicle, the strength of the resistance-spot-welded joints of these parts is highly important to obtain a safe structure. In general, dimensions of weld nuggets are regarded as one of the criteria for the quality of spot-welded joints. In the presented research, a three-dimensional axisymmetric finite element model is developed to predict the nugget formation in resistance spot welding (RSW) of two types of PHS: the uncoated and AlSi-coated 1.8 mm boron steel after hot stamping. A fully coupled electro-thermo-mechanical analysis was conducted using the commercial software package Abaqus. The FE predicted weld nugget development is compared with experimental results. The computed weld nugget sizes show good agreement with experimental values.

A novel method for the spring-back analysis of a hot stamping steel

Spring-back of sheet metal affects the dimensional accuracy of the components after stamping and has to be eliminated or corrected. Advanced high strength steels (AHSS) are prone to spring-back while forming at ambient temperatures. Hot stamping process is an effective technique to reduce the spring-back upon forming. The forming temperatures and the phase transformations associated with quenching are known to influence the spring-back. However, the detailed experimental analysis on spring-back behaviour is limited due to the difficulty in physically simulating the thermal-mechanical conditions prevailing in a typical hot forming process. In the present work, a novel V-bending technique is presented for effectively measuring the spring-back using a thermal-mechanical simulator, Gleeble™. V-bending is carried out by applying a compressive force along the longitudinal plane of AHSS steel strips at forming temperatures. Upon V-bending, samples were cooled at different cooling rates while maintaining the forming loads to simulate die quenching. Results indicate that the spring-back decreases and becomes close to zero (at a cooling rate of 20 K s−1) with an increase in cooling rate for a hot stamping steel. Further, an increase in cooling rate results in spring-forward owing to the formation of an increased volume fraction of martensite.

Effect of different lapping orders on mechanical performance and nugget forming process for three-sheet dissimilar resistance spot welding joints between DC06 and unequal-thickness hot-stamped B1500HS steel sheets

In this study, the mechanical properties of two welded joints under different lapping orders (B1500HS-1.4 mm/B1500HS-1.6 mm/DC06, denoted as type I; B1500HS-1.4 mm/DC06/B1500HS-1.6 mm, denoted type II) were compared. The nugget formation mechanism was analysed by a coupled electrical-thermal finite element model (FEM). It is found that different lapping orders significantly affect the mechanical properties of three-sheet RSW joints. All RSW joints tend to fail in the pull-out or tearing failure mode. The peak load of the two interfaces of type II RSW joint is more balanced, and the failure load of which is much higher than that of the type I RSW joint; The warpage was observed at type I welded joint. Considered the load-carrying capacity, type II was excellent. The simulation results indicate that the diameter of the weld nugget at the upper interface of the type I RSW joint was larger than that on the other interfaces, which agree well with the experimental results. The nugget formation mechanism of dissimilar high strength steel three-sheet RSW joint was obtained that forming the nugget firstly from two interfaces and final formed nuggets were asymmetric.

Interaction between Dislocations, Precipitates and Hydrogen Atoms in a 2000 MPa Grade Hot-Stamped Steel

Interaction between Dislocations, Precipitates and Hydrogen Atoms in a 2000 MPa Grade Hot-Stamped Steel

Effect of Cooling Path on Microstructures and Hardness of Hot-Stamped Steel

The final mechanical properties of hot-stamped steel are determined by the microstructures which are greatly influenced by the cooling process after hot stamping. This research studied the effect of the cooling path on the microstructures and hardness of 22MnB5 hot-stamped steel. The cooling path was divided into continuous and discontinuous (primary and secondary) processes. After cooling, the Vickers hardness along the thickness of the specimens was measured. The results indicate that, for a continuous cooling process, there was a critical cooling rate of 25 °C/s to obtain fully martensitic microstructure. For the discontinuous cooling process, the slower was the cooling rate, the higher was the degree of auto-tempering that occurred, and the greater was the amount of carbides that formed, regardless of the primary or secondary cooling rate. When the cooling rate was lower than the critical value, a higher primary cooling rate suppressed the auto-tempering of lath martensite and increased the quenched hardness. By contrast, the hardness was not sensitive to the cooling rate when it exceeded the critical value.

Irreversible hydrogen embrittlement study of B1500HS high strength boron steel

The reversible/irreversible recovery of mechanical properties, and the microstructure characteristics of a typical hot-stamped steel B1500HS have been studied under different conditions of hydrogen permeation. Initially, all tested specimens were permeated by hydrogen atoms through an electrochemical hydrogen charging scheme. Then, the comparisons between different currents and charging time were performed. The influence of different storage time was compared as well. Additionally, the effect of the plastic strain introduced by pre-stretching was also investigated. The experimental results showed that the negative impact of hydrogen embrittlement was altered from reversible to irreversible as the magnitude of the charging current increased. The hydrogen blistering and the hydrogen charging-induced cracks were both observed and inspected in the tested samples regarding the irreversible situation. Moreover, the adverse influence of hydrogen embrittlement was enhanced by plastic pre-straining or extending the charging period. At the micro-level, hydrogen charging-induced cracks generally were generated at defect locations, such as the prior austenite grain boundaries and lath martensite interfaces. Particularly, crack direction occurred perpendicular to the orientation of lath martensite, and transgranular fracture occurred at the prior austenite grains.

Self‐Piercing Riveting of Hot Stamped Steel and Aluminum Alloy Sheets Base on Local Softening Zone

Self‐piercing riveting (SPR) has been used to joint steel to aluminum alloy sheets in multimaterial car body. It is a challenge to joint high strength steel to aluminum alloy by SPR process, especially for materials with high strength and large thickness. Herein, the SPR process for 1500 MPa hot stamped steel and aluminum alloy sheets is studied by experiment and simulation method. The laser heat treatment process is applied to the hot stamped steel sheet before joining. The relationship between heat treatment process and hardness distribution is studied and the softening zone with hardness less than 300 HV is obtained. Furthermore, the local softening zone is joined to an aluminum alloy sheet by SPR process. In the process, the rivets with high hardness are used to pierce the steel sheet. The failure mode and maximum tensile shear force of the joint are studied. At the same time, the joints without laser heat treatment are compared. The results show that the interlock and the maximum shear force for laser heat treatment increase by 91% and 26%, respectively.

Influence of Austenitizing Parameters on the Microstructures and Mechanical Properties of 22MnB5 Hot Stamping Steel Produced by TSCR Process

Thin gauge hot strip steel can be produced by thin slab continuous casting and rolling (TSCR) process without the aid of cold rolling. However, few attempts have been made to produce hot stamping steel by using TSCR technology. As given steel, the hot stamping steel produced by the TSCR process was used to study the influence of austenitizing parameters on the final microstructures and mechanical properties. A Gleeble3500 thermal simulator was employed to heat the samples to obtain different austenite microstructures. After quenching, the Vickers hardness examination along the thickness of the sample and tensile testing were conducted. The microstructure was observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that on the condition of relatively lower temperature and shorter holding time, the sample exhibited better mechanical properties at the cost of microstructure uniformity along the thickness, although higher austenitizing temperature enhanced the carbide dissolution which would increase the effective carbon content in the martensite matrix. Lower heating temperature led to small prior austenite grain size and small blocks which contributed greatly to the mechanical properties.