Hot Stamping Conditions Research Articles

Constitutive behaviour under hot stamping conditions

The modelling of the hardening behaviour at high temperatures and a range of strain rates is extensively discussed. The hardening behaviour is characterized using tensile tests done in a Gleeble testing machine. The specimen is heated by an electric current, soaked to get fully austenized, cooled down to its desired testing temperature and then drawn to fracture with a given strain rate without a further drop of the temperature. One of the major challenges of this test is to achieve a uniform temperature distribution over the sample, to ensure homogeneous austenization. A dedicated tensile sample geometry enables a much more uniform temperature distribution than a regular sample geometry. Yielding and hardening behaviour have been characterized with a Kocks–Mecking plot. These characterizations have been used to fit parameters for a physically based hardening model that is applicable in a wide range of strain rates and temperatures. The predicted strains in FEM simulations that use these hardening curves match well with thickness measurements on hot stamped parts.

Effect of hot-stamping process conditions on the changes in material strength

According to the recent trend of increasing demand in weight reduction of car bodies for the improvement of vehicle safety and gas mileage, there exists a forming method that makes use of the hot-stamping process as a means that satisfies all of the conditions above. However, there is insufficient systematic technical information on the method of heating material and setting up the forming process in case of the hot-stamping technique. A material heating device was manufactured in this study and the temperature profile for the forming process was created in order to design the material heating technique and the forming process in the hot-stamping method. Furthermore, in order to study the changes in strength according to the hot-stamping forming variables, high temperature forming was conducted according to forming variables such as forming speed, rapid cooling time, and material rolling direction. In order to verify the subsequent results, the springback was measured to assess the material hardness and to check the forming precision. In addition, a stamping analysis and quenching analysis were conducted on the experiment conditions through a commercial finite element analysis program, and the results of comparing the hardness distribution with the test values showed similar trends. Based on the results of the analysis, having the shortest possible material transport time would allow for the production of products with evenness and high strength.

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.

Damage Investigation of Boron Steel at Hot Stamping Conditions

The uniaxial tension tests of 1500MPa boron steel at strain rates of 0.01-5.0 s-1 and temperatures of 550-850°C were performed. Considering the difference between the deformation necking cross section and the centre measuring section of specimen, a correction method of measuring strain and temperature at necking cross section are developed. For the method, the deformation of necking cross section was obtained which approximate for the trapezoidal change law near the necking zone. The stress strain curves are corrected based on the correction method of the strain and by considering temperature correction. A set of unified constitutive equations has been developed and calibrated. Correlation between the corrected and predicted true stress data from the constitutive equation is presented. The correlation coefficient is 0.969, and the absolute average relative error is 3.73% in the range of allowable experimental conditions.

A New Test Design for Assessing Formability of Materials in Hot Stamping

Hot stamping has been widely used in the forming of lightweight automotive body structural panel components. The assessment of the formability of the materials in the non-isothermal stamping conditions is challenging. A new testing method with a new specimen design is introduced in this paper for the evaluation of the formability and failure of materials in hot stamping processes. A series of tests have been carried out for an AA6082 at different forming rates. The failure mechanisms and failure locations have been analysed. A set of mechanism based stress-state dependent damage equations for AA6082 is introduced and this has been inputted into the commercial FE code, ABAQUS, through the user defined subroutine, VUMAT. An FE model has been created and validated from experimental results. Studies have been carried out to optimise the specimen shape designs, so that the formability and failure of the material can be predicted in hot stamping conditions.

V単独，V-Nb複合添加超高強度ホットスタンピング部材の組織と機械的性質

As a method for producing ultrahigh strength automotive steel components with high gauge accuracy, the hot stamping technology draws attention and the amount of hot stamped automotive components has exponentially increased in the last decade. The concerns about the potential danger of embrittlement and delayed fracture, however, limit the strength of conventional hot stamped components to 1500 MPa. If these concerns are eliminated, automotive steel components with higher strength which is demanded by the automotive industry can be further commercialized.In this study, microstructures and mechanical properties of V and/or Nb bearing ultrahigh strength hot stamped steel components have been investigated. The V addition improves the resistance to delayed fracture by forming VC whose interface with matrix is supposed to act as preferential hydrogen trap site. The Nb addition refines the microstructure, which improves both the toughness and the resistance to delayed fracture. Proper hot stamping conditions for these steels has been discussed to create an ultrafine martensite microstructure and finely dispersed precipitates. The results indicate that 1900 MPa class steel components with good toughness and high resistance to delayed fracture can be produced.

The development of continuum damage mechanics-based theories for predicting forming limit diagrams for hot stamping applications

This paper presents a novel plane-stress continuum damage mechanics (CDM) model for the prediction of the different shapes of forming limit diagrams (FLCs) for aluminium alloys under hot stamping conditions. Firstly, a set of uniaxial viscoplastic damage constitutive equations is determined from tensile experimental data of AA5754 at a temperature range of 350–550℃ and strain rates of 0.1, 1.0 and 10 s−1. The tests were carried out on Gleeble materials simulator (3800). Based on the analysis of features of FLCs for different materials forming at different temperatures, a plane-stress damage equation is proposed to take account the failure of materials at different stress-state sheet metal forming conditions. In this way, a set of multiaxial viscoplastic damage constitutive equations is formulated. The model is calibrated from the FLC data at temperature of 350℃ and strain rate of 1.0 s−1 for AA5754. A good agreement has been achieved between the experimental and numerical data. The effect of the maximum principal stress, effective stress and hydrostatic stress on the materials failure features and on the shape of FLCs is studied individually and in combination. Using the newly developed plane-stress unified viscoplastic damage constitutive equations, the FLC of materials can be predicted at different temperatures and strain rate forming conditions.

Tribological characteristics of high strength steel sheets under hot stamping conditions

Nowadays, the hot stamping of high strength steel sheets is currently utilized to produce automotive components characterized by a high strength-to-weight ratio and an increased resistance to impact. In order to avoid scaling and decarburization during the heating stage, the metal sheets are coated with a specially developed Al–Si coating that have proved a relevant influence also on the tribological behaviour of the metal sheets during the forming stages. The paper presents a fundamental study about the sheet coating characteristics in terms of morphology, surface roughness and tribological behaviour as a function of the process parameters typical of industrial hot stamping processes. The presented results show that the blank heating promotes the iron diffusion into the Al–Si coating giving form to an Al–Fe–Si ternary alloy whose characteristics depend on the thermal cycle parameters; moreover, it is proved that the tribological characteristics of the metal blank surface in terms of friction coefficient depend on the blank temperature and contact pressure.

Development of Estimation Procedure of Contact Heat Transfer Coefficient at the Part–Tool Interface in Hot Stamping Process

Energy efficiency, a high level of passenger safety, and weight reduction of the vehicles still constitute the main functional elements for the design of modern car body structures. Toward this objective, the usage of advanced sheet-metal forming technologies like hot stamping of quenchable boron manganese steel is developing nowadays. Hot stamping is a combination of hot forming and simultaneously quenching and hardening of the blank. In this article, an experimental procedure developed to estimate the thermal conductance at the part–tool interface during a hot stamping procedure is presented. The tools set (punch and die) has been designed to form omega-shaped samples. This work represents the first stage of the procedure development where the standard experiment is a simple compressive load of sample in the bottom of the omega-shape die, considering the real hot-stamping conditions. Tests are carried out under different contact pressure values for two different blank materials (Usibor 1500P and a material B). The results show that the thermal conductivity increase contributes to the reduction of the thermal contact resistance and therefore to a more rapid cooling of the part.

Valuation method for effects of hot stamping process parameters on product properties using hot forming simulator

Hot stamping is one of the hot forming processes for manufacturing products of lightweight construction such as lightweight vehicles. Knowledge on the characteristics of the hot stamping process is significant in designing and optimizing the process conditions, dies and tools; however, until now, the characteristics of this process, such as the relationships between a product property and die temperature and between a product property and cooling rate, and the springback amount, have not been clarified consistently or precisely. A new valuation method used to measure the effects of hot stamping conditions on product properties as a function of process parameters is proposed in this paper. This method utilizes a hot forming simulator, which is a precision press with feedback temperature control and controlled cooling, and permits to know the various characteristics of the hot stamping process. Here, the basic construction of the proposed method is presented, and its application to the characterization of the hot stamping process of high-strength steel sheets, such as an evaluation of the changes in product properties upon varying the forming conditions and process parameters, is shown and discussed.

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.

Comparison of Surface Treatments for H10A Tool Steel in Hot Stamping Process

Trials were carried out under industrial hot stamping conditions to compare the effect of different surface treatments on hot working tool steel. In comparison to conventionally quench and tempered AISI grade H10A tool steel, a PVD CrN coating and plasma nitride treatment were applied to the H10A tool steel. Following tool failure, the microhardness and the microstructure of the surface of the steel were analysed to determine metallurgical changes of the tool steel by the stamping process. The results in the present study show that the plasma nitrided tool increased die life twofold. This increase in wear life is attributed to the increase in surface hardness which decreased gradually with depth, and the presence of residual compressive stresses within the surface. In comparison, the CrN coating was unable to extend tool life. The failure of the CrN coating was attributed to delamination and microcracking brought about by a combination of residual stresses within the coating. This was due to differences in the coefficient of thermal expansion and fracture of the coating under impact loading.