Stamping Of High Strength Steel Research Articles

Experimental and Numerical Investigations of Wear Resistance Characteristics of XCr13 during Advanced High-Strength Steel Stamping

Owing to the high strength and hardness of advanced high-strength steels (AHSS), the die materials for AHSS sheet stamping must have superior mechanical properties. A novel carburization technology on high-chromium, low-carbon, low-cost stainless steel XCr13 was used to achieve a hardness of approximately 60 HRC, with a carburized layer of approximately 0.5 mm; these specifications meet the requirements of AHSS sheet stamping. To evaluate the wear characteristics of the treated XCr13, regular pin-on-disk wear tests were performed under dry sliding conditions at room temperature. JIS SKD11, a popular cold work die material, was also tested and compared with XCr13. A numerical simulation model for AHSS sheet flanging with a W-shaped specimen was set up in ABAQUS, and the user subroutine UMESHMOTION was used to evaluate the wear resistance of the two die materials, including the wear surface profile, wear depth, and wear severity index value. For JIS SKD11, the agreement between the simulation and experiment results was good, which verifies the reliability and accuracy of the proposed wear simulation model and method. It was found that the wear resistance of carburized XCr13 was at the same level as that of JIS SKD11 because of the carburized zone. Further, carburized XCr13 can work well as a die material for AHSS stamping.

Effect of Material Property on the Springback in Stamping of High Strength Steel

Effect of Material Property on the Springback in Stamping of High Strength Steel

Study on the Cold Stamping of Oil Sump Tank for the Thickness and Springback

In recent years, the high strength steel has been used widely to prove the advantage of lightweight used in automotive industry. The drawbacks occurred in the stamping procedure are thinning, springback and so on. And many process parameters affect the forming quality significantly, such as friction coefficient and blank holder force (BHF). In this paper, the effect of process parameters on the forming defects is investigated. The design of test was utilized and the finite element model was established and applied. The aim of this study is to get a further understanding on the stamping of high strength steel.

Application of a Model for Quenching and Partitioning in Hot Stamping of High-Strength Steel

Application of quenching and partitioning process in hot stamping has proven to be an effective method to improve the plasticity of advanced high-strength steels (AHSSs). In this study, the hot stamping and partitioning process of advanced high-strength steel 30CrMnSi2Nb is investigated with a hot stamping mold. Given the specific partitioning time and temperature, the influence of quenching temperature on the volume fraction of microstructure evolution and mechanical properties of the above steel are studied in detail. In addition, a model for quenching and partitioning process is applied to predict the carbon diffusion and interface migration during partitioning, which determines the retained austenite volume fraction and final properties of the part. The predicted trends of the retained austenite volume fraction agree with the experimental results. In both cases, the volume fraction of retained austenite increases first and then decreases with the increasing quenching temperature. The optimal quenching temperature is approximately 290 °C for 30CrMnSi2Nb with the partition conditions of 425 °C and 20 seconds. It is suggested that the model can be used to help determine the process parameters to obtain retained austenite as much as possible.

A Material Perspective on Consequence of Deformation Heating During Stamping of DP Steels

Recent studies showed that, during stamping of high strength steels at industrially relevant production rates, local temperature in the blank may rise up to 200°C – 300°C due to deformation heating. Moreover, die temperature may also rise up to 100°C – 150°C for progressive stamping dies. Based on the common assumption that the blank softens as the temperature increases, thermal softening creates a margin in Forming Limit Diagram (FLD) and therefore the FLD determined at room temperature can safely be used for those cases. In this article, the validity of this assumption on DP590 steel is questioned by high temperature tensile tests (RT - 300°C) at various strain rates (10-3 s-1 – 1 s-1). The results indicated a decrease both in uniform and total elongation in 200°C – 300°C range together with several other symptoms of Dynamic Strain Aging (DSA) at all strain rates. Concurrent with the DSA, the simulated FLD confirms the lower formability at high temperature and strain rates. Thus, it is concluded FLD determined at RT may not be valid for the investigated steels.

A thermal forming limit prediction method considering material damage for 22MnB5 sheet

Hot stamping of high-strength steel is an innovative technology to achieve automobile lightweight and guarantee security simultaneously. However, the formability of high-strength steel is still limited at elevated temperatures by the evolution of void damage inside materials. Thus, the establishment of an efficient forming limit prediction method is urgently demanded. In the present work, the Gurson-Tvergaard-Needleman (GTN) model is extended by containing Hosford anisotropic yield criterion which can characterize void damage in normal anisotropic materials. The flow behavior of matrix material 22MnB5 at different forming temperatures is simulated by the modified Norton-Hoff hardening law. And the damage-related parameters are calibrated through FEM inverse analysis. Furthermore, the prediction method of forming limit combined with void damage is realized within the framework of the Marciniak and Kuczynski (M-K) model. Meanwhile, the high-temperature Nakazima test is conducted to obtain experimental thermal forming limit diagram (TFLD). The comparison of results by theoretical prediction and experimental test shows good consistency. Based on the proposed method, the effects of temperature, void damage, initial thickness imperfection, and anisotropy on the forming limit are analyzed. The formability improved with the increasing of deformation temperature but worse for other factors.

Die Designs of Cold Roll Forming Process for Car Bumper Using Advanced High Strength Steel

Abstract Cold roll forming (CRF) process can be adopted for making channels and tubes with complex cross section efficiently and integrate the welding and the heat treatment facilities to shorten production period. Advanced high strength steel is a hard-to-form material using the conventional forming method. Hot stamping process is a good way to reduce the forming load of the advanced high strength steel stamping. The heating and the cooling process is complex and difficult to control for the hot stamping method. Cold roll forming process is an alternative method to shape the advanced high strength steel using a series of rolls under the condition of room temperature. This paper proposed the CRF process and die designs for a car bumper structure with a cross section of two un-symmetric closed chambers using the advanced high strength steel with tensile strength of 1400 MPa. Designs of the roll flower, the forming rolls were evaluated using FEM method. The forming rolls were designed with relief and trap features to control the material locally and prevent the defects of folding and wrinkling. The simulation results showed the folding defect at the edge occurred during the closing step of the separated chambers due to the one side roll contact. Only the top roll touched the forming area of the upper blank without the support of the bottom roll due to the constraint of the chamber geometry. A fin pass design was adopted to make a proper preform of the top edge of the chambers. The simulation results showed the proposed design was able to prevent the folding defect and a sound product was obtained.

Cracking Prediction in Hot Stamping of High-Strength Steel by a Temperature-Dependent Forming Limit Surface Approach

Hot stamping of high-strength steel (HSS) can significantly improve ultimate tensile strength (UTS) of hot-stamped part and thus meet the increasing demands for weight reduction and safety standards in vehicles. However, the prediction of forming defect such as cracking in hot stamping using traditional forming limit curve (FLC) is still challenging. In this paper, to predict HSS BR1500HS cracking in hot stamping, a temperature-dependent forming limit surface (FLS) is developed by simulations combined with experiments of biaxial tension of the plate with a groove at different temperatures. Different from the FLC, the newly developed FLS in which temperature is included suits the hot stamping of HSS. Considering the interplay among phase transformation, stress and strain, a finite element (FE)-coupled thermo-mechanical model of the hot stamping is developed and implemented under ABAQUS/Explicit platform where the developed FLS is built-in to predict strain distributions and HSS BR1500HS cracking in the hot stamping. Finally, the developed FLS is used to evaluate hot formability of HSS BR1500HS by using a hot stamping experiment for forming a box-shaped part. Results confirm that the developed FLS can accurately predict HSS BR1500HS cracking occurrence in the hot stamping.

Tribological Behavior of Laser Textured Hot Stamping Dies

Hot stamping of high strength steels has been continuously developed in the automotive industry to improve mechanical properties and surface quality of stamped components. One of the main challenges faced by researchers and technicians is to improve stamping dies lifetime by reducing the wear caused by high pressures and temperatures present during the process. This paper analyzes the laser texturing of hot stamping dies and discusses how different surfaces textures influence the lubrication and wear mechanisms. To this purpose, experimental tests and numerical simulation were carried out to define the die region to be texturized and to characterize the textured surface topography before and after hot stamping tests with a 3D surface profilometer and scanning electron microscopy. Results showed that laser texturing influences the lubrication at the interface die-hot sheet and improves die lifetime. In this work, the best texture presented dimples with the highest diameter, depth, and spacing, with the surface topography and dimples morphology practically preserved after the hot stamping tests.

The finite element analysis of ductile damage during hot stamping of 22MnB5 steel

Abstract This study deals with the finite element investigation of hot stamping of high strength steel. The main objective is to predict when and where the crack can appear in the work-piece during the hot stamping operation. A set of thermo-elastic–plastic constitutive equations coupled with material damage is presented in detail based on continuum damage mechanics. The damage parameters involved in the constitutive equations were determined from uniaxial tensile experimental data of 22MnB5 steel at a temperature range of 650–800 °C and strain rates of 0.01, 0.1, 1.0 s−1. Using the calibrated constitutive equations, a finite element program was developed based on dynamic explicit algorithm and applied to simulate the deformation process and ductile damage of 22MnB5 under hot stamping conditions. The capability of the program as a predictive tool has been evaluated by comparing experimental and calculated results of the hot stamping process of an automotive B-pillar. And the effects of blank holding force and friction on formability, damage evolution and punch force were analyzed.

Hand-Held Blue Light 3D Measurement Technology and its Application in Hot Stamping

Over the last few years hot stamping for its high strength, lightweight characteristics has developed at an alarming rate, At the same time the measurement process a higher demand. Compared with the traditional three-dimensional coordinate measurement, Hand-held Blue structured Light 3D measurement technology for its fast, high precision, good flexibility, etc., has become the best choice of high-strength steel stamping in non-contact 3D measurement and accuracy detection. This paper developed a set of holding blue light 3D measurement system. The system project blue structured pattern to the surface of the object, and captured by two high-speed cameras synchronized. Then using the grabbed pictures reconstruct 3D topography of the object. Blue light projection technology can eliminate the use of influence of different surface reflectance measurement accuracy, So that a single measurement accuracy of 0.02mm, overall measurement accuracy of 0.05mm / m, Single measurement time is less than 0.12 seconds. This measuring system can be widely used in the rapid detection of various types of three-dimensional measurement and precision casting.

Tribological performances of Zn-based coating in direct hot stamping

In hot stamping of High Strength Steels, the severe tribological conditions make the metal sheet coating one of the most critical choices for the technical and economical success of the process. This paper presents the approach and the results in evaluating a Zinc-based coating applied to boron steel sheets in hot stamping operations. Thermal and physical-simulation experiments were carried out to evaluate the chemical interactions between the coating and the metal sheet and the tribological performances during the process. The coating proved to overcome most of the drawbacks of the currently utilized Al–Si coating.

Hot Stamping of High Strength Steel with Tailored Properties by Two Methods

Hot stamped components with full martensite exhibit ultrahigh tensile strength and hardness, whereas the poor ductility restricts their application on certain components, such as B pillar, which needs multi strength in different regions. It is essential to partly achieve softer structure with high ductility, to enhance the crashworthiness and energy absorption of the whole component and thus improving the safety performance. This work described the hot stamping of high strength steel with tailored properties by two methods, namely applying different die temperatures and annealing processes. The hardness distribution, tensile strength and ductility in different regions of the hot stamped components were gained under the conditions of above two methods to realize the tailored properties. Regardless of the decrease of tensile strength, the total elongation of annealed zone can reaches 20.6% while the heating zone can only obtain its elongation of 10.8%. The results show that annealing hardened part to get tailored properties brings about better performance.

Hot Stamping of Door Impact Beam

In recent years, the automotive industry is making an effort to reduce the vehicle weight for reducing the CO2 emissions. The hot stamping of high strength steels offers the possibility to obtain significant reduction of weight. The conventional door impact structure design is composed of one or two tubes and two brackets by stamping. There are many advantages if conventional design was changed to hot stamping design such as cost saving, tools amount and weight reduction. The development of door impact beam by hot stamping is illustrated in this paper. It includes the product design, process evaluation, die design, CAE simulation, and prototype manufacturing. Finally it uses hardness test, thinning rate analysis, and quasi-static three-point bending test to check the hot stamping parts performance, including mechanical properties, plastic deformation capacity and absorbing energy ability. It could be a reference for automotive industry to develop the similar products.

Interlaboratory comparison for heat transfer coefficient identification in hot stamping of high strength steels

The topic of the paper is the identification of the heat transfer coefficient (HTC) in hot stamping of boron steel sheets under conditions very close to the industrial ones. Two approaches followed by one lab in Germany and one lab in Italy are presented for HTC identification, showing the two experimental apparatuses that were set-up to conduct the tests, the procedures developed and applied to identify the HTC. The obtained results are compared in terms of dependence of HTC from the applied contact pressure, the similarities and the differences of the two approaches are outlined and commented.

Die structure analysis of stress distribution during High-Strength Steel stamping process and experimental validation

High-Strength Steel (HSS) is being widely used in automotive industry. However, load condition on die structure for HSS stamping increases sharply. A method for die-structure FEA analysis was proposed in this paper. The interaction force between forming blank and die faces was mapped onto die faces to serve as load boundary conditions for die structure analysis. A hyperbolic-bottomed-cup-drawing die was analysed and validated by experiment under several forming conditions. Results show that the predicted Von Mises stress values at specified positions coincides with the experimental ones as well within 15% error.

Interlaboratory Comparison of Forming Limit Curves for Hot Stamping of High Strength Steels

The paper presents the approaches followed by two labs – LFT at the University of Erlangen-Nuremberg (Germany) and DIMEG at University of Padua (Italy) – in evaluating formability limits of 22MnB5 sheets when processed under hot stamping conditions. Details about the two testing apparatuses and the testing procedures are outlined, and the results in terms of Forming Limit Curves FLC compared and critically commented.

Mastering the stamping of high strength steels in Slovene toolmaking industry

The Slovene toolmaking industry is forced to follow the trends of world automotive industry demanding increased use of steels with improved mechanical properties like high strengths, high elongations etc. The higher strengths of so-called 'high strength low alloyed' steels and 'advanced high strength' steels cause higher tool pressures and loads during forming, higher springback effects of the products after forming and demand special design of the tools. In the paper, the material properties of H220, H420LA and TRIP700 steels were analysed and the bending tests were performed in order to compare the springback effects of all three materials. Based on the obtained data, first guidelines were prepared for the design of the tools. These guidelines were taken into account when the first forming tools for HSLA and AHSS materials were designed in Gorenje Orodjarna d.o.o. tool factory. The FEA approach was used for the tool design of the part made of H420LA steel.

An Approach to Model Sheet Failure After Onset of Localized Necking in Industrial High Strength Steel Stamping and Crash Simulations

This paper describes how, in large-scale industrial simulations the numerical prediction of fracture in sheet metal forming operations as well as in crash events is still a challenging task of high social and economic relevance. Among several approaches presented in literature, the authors and their colleagues developed a model which accounts each for three different mechanisms leading finally to fracture in thin sheet metals: the local instability (necking), ductile normal fracture and ductile shear fracture. This paper develops and validates a new approach to improve the predictive capabilities for fracture triggered by localized necking for a wide variety of steel grades. It is well known that after the onset of a local instability additional strain is still necessary to induce fracture. In a numerical simulation using shell elements this post-instability strain becomes of increasing importance when the ratio of the characteristic shell element edge length to its thickness decreases. Today's shell element lengths in industrial applications can be of the same order as the width of the necking zone. Therefore, the post-instability strain may contribute to a significant percentage of the shell elements total elongation up to fracture and cannot be neglected any longer. The enhanced necking model termed Post-Instability Strain Model for Shells (PIS Model) combines the model for localized necking with the model for ductile shear fracture. Guided by careful Nakajima-type tests of three representative steel grades, the development of the PIS Model also focuses on minimizing the influence of varying shell element edge lengths. The model is implemented in such a way that the elongation of a shell element after onset of necking is highly independent of the element edge length. The improvements achieved with this model are demonstrated by validation examples which include small specimens (tensile specimen with circular hole), technological sheet metal forming experiments (Nakajima tests) and finally the 3-point bending test of an automotive component.

Testing formability in the hot stamping of HSS

This paper presents an innovative experimental procedure, based on Nakazima test, for evaluating the formability limits in the hot stamping of high strength steel (HSS) which is capable of generating formability data suitable for an FE modelling of the process. The approach is based on two complementary tests which are aimed at evaluating the actual phase transformation kinetics for the material under deformation conditions and the combinations of microstructure, temperature and straining path that lead to localized necking or fracture.