Boron Steel 22MnB5 Research Articles

Delaying hydrogen permeation by Mg and Sr added Al-Si coating during hot-press-forming of boron steels

This study explores the hydrogen permeation resistance of Al-Si-Mg and Al-Si-Mg-Sr coated 22MnB5 boron steel in hot-press-forming. The Al-Si-Mg coating exhibits structural changes and Mg segregation, replacing the oxide layer. Sr addition is found segregated at the surface, further transforming the oxide layer. These modifications improve hydrogen permeation resistance, reducing both reversible and irreversible hydrogen permeation during hot-press-forming. Mg or Sr addition presents promising avenues for enhancing hydrogen permeation resistance in steel, potentially extending the service life of ultra-high-strength steel.

FEM model optimization study of mechanical properties and material selection in stamping process

Abstract This paper establishes a finite element model of stamping and forming and proposes solution algorithms for static and dynamic forces. The finite element simulation of the hot stamping process is carried out to analyze the thermophysical parameters of BR1500HS ultra-high-strength steel sheet material and H13 steel mold material. Set the temperatures of austenitic material in the transfer process and molding process, obtain the rheological stress data of BR1500HS ultra-high-strength steel plate during plastic deformation at high temperature, and determine the basic mechanical properties of the material at high temperature. A geometric finite element model of thermal-force-phase coupling of the hot stamping and forming process is established to simulate the temperature field change of the steel plate during the hot stamping and forming process by combining the process conditions and process parameters. At the same time, the ultra-high-strength boron steel 22MnB5 is selected to simulate the hot forming and tempering process of U-shaped parts made of high-strength steel sheet material, and the feasibility of the finite element model is verified. Different hot stamping process parameters are set to analyze the forming quality of both BR1500HS ultra-high strength steel and ultra-high-strength boron steel 22MnB5. When the stamping speed is increased from 50mm/s to 100mm/s, the maximum equivalent force of BR1500HS ultra-high strength steel decreases, and thus, when the stamping speed is 100mm/s and the holding time is 5s, the part forming performance is better, which meets the requirements of the gradient performance hot stamping process.

Effects of soaking temperature on the microstructure and mechanical properties of a medium Mn steel for hot/warm forming

The increasing demands for better crashworthiness of vehicles require structural components with excellent energy absorption performance and anti-intrusion resistance. Here, we explored the potential of using a medium Mn steel with a 10 wt.% Mn content for hot/warm forming with the aim to produce automotive components with improved strength–ductility combination. The soaking temperature was varied from 670 to 870 °C for tuning the microstructure and the resultant mechanical properties. It is found that a duplex microstructure consisting of ferrite with a high dislocation density and retained austenite with a volume fraction of 53.5% is formed at 720 °C, leading to an exceptional strength–ductility combination (yield strength: 1016 MPa; ultimate tensile strength: 1407 MPa; total elongation: 32.7%). The energy absorbed before fracture is 39.8 GPa·%, which is four times more than that of the widely-used 22MnB5 boron steel. On basis of the experimental results, the effects of soaking temperature on austenite reverse transformation, ferrite recrystallization and martensitic transformation are discussed in depth to provide guidance for optimizing the strength–ductility combination of medium Mn steels processed by hot/warm forming.

Experimental and Numerical Study on Mechanical Behavior of Steel/GFRP/CFRP Hybrid Structure under Bending Loading with Adhesive Bond Strength Assessment.

Adhesive bonding between steel and carbon-fiber-reinforced polymer (CFRP) composite leads to hybrid structures that combine the high strength and ductility of steel with the excellent specific strength and stiffness of CFRP composite. There is, however, a concern regarding possible galvanic corrosion when steel and carbon fibers are bonded together. One way to overcome this problem is placing glass fiber-reinforced polymer (GFRP) composite between the steel and CFRP composite, creating a more complex steel/GFRP/CFRP hybrid structure. Therefore, experimental and numerical studies on the mechanical behavior of the adhesive bonds between the steel sheet and the GFRP/CFRP hybrid composite were carried out. Among the different failure patterns, mode II was chosen for analysis because metal-polymer composite structures are usually subjected to bending, and debonding may occur due to in-plane shear stress. The tested steel/GFRP/CFRP hybrid structure was made of a hot-formed 22MnB5 boron steel sheet, intermediate single-ply bidirectional GFRP composite, and three-ply unidirectional CFRP composite. Additional mechanical tests were also carried out to determine various engineering constants of the components to simulate the debonding process. A finite element model of the steel/GFRP/CFRP hybrid structure with a typical cohesive interface was established and verified against the experimental data. The results showed that due to the use of various materials, the dominant failure modes in the hybrid structure under bending loading were a brittle fracture of the CFRP composite and debonding between the steel and the GFRP composite. However, the load-bearing capacity of the hybrid structure was five times greater than that of a non-reinforced steel sheet. In addition, its mass was only 28% greater than the non-reinforced steel sheet. The obtained results provided valuable conclusions and useful data to continue further research on the mechanical behavior of steel/GFRP/CFRP hybrid structures.

Investigation of variability in apparent values of materials properties in thermo-mechanical uniaxial tensile tests on sheet metals

Thermo-mechanical uniaxial tensile testing is commonly carried out to characterise the mechanical properties of materials under conditions which mimic advanced industrial forming processes, such as hot stamping of steels and aluminium alloys, and to generate microstructures for metallographic investigation. However, in this type of testing, heat loss to the specimen grips can lead to nonuniform temperature distributions along the gauge length, resulting in challenges in determining absolute values of materials properties at the nominal temperature of interest. The present study investigates the effect of these nonuniform temperature distributions on the variability in the thermo-mechanical properties as measured in the tests, and in the microstructures of the tested specimens. For this purpose, uniaxial tensile tests on the boron steel 22MnB5 and aluminium alloy AA6082 were performed under hot stamping conditions using a Gleeble 3800 thermal-mechanical physical simulation system, in which the specimens were heated using resistance heating and the strain fields were measured using digital image correlation (DIC). The nonuniformity of the temperature distributions along the gauge length was quantified. Both the strains and the strain rates along the gauge length were then computed and the effects of factors such as pre-forming gauge length, post-forming gauge length and specimen design on the spatial distribution of strains and strain rates were investigated. The effects of these factors on the values of thermo-mechanical properties determined from the tests, such as the ductility and the ultimate tensile strength (UTS), were also analysed and quantified. This study reveals the variability in the apparent values of materials properties as determined by thermo-mechanical testing resulting from nonuniform temperature distributions, and provides experimental data for the development of new standards for thermo-mechanical tests in future.

Comparison of two CDM-based models for necking and fracture limits of metal sheets under hot stamping conditions

A unified viscoplastic constitutive model based on continuum damage mechanics (CDM) has been recently developed for the prediction of forming limit curves (FLCs) and fracture forming limit curves (FFLCs). In this CDM model, two damage variables which are strain path dependent (so called strain-based model) have been introduced to model the necking and fracture limits in sheet metal forming. In this study, the two damage variables have been modified by replacing the strain components with stress components to model stress path dependence (so called stress-based model). Then the two sets of CDM models have been analysed and compared in respect to their computational accuracy and efficiency. For this purpose, all the material constants in the models have been calibrated using the recently published data of 22MnB5 boron steel sheet under hot stamping conditions. Subsequently, the two models together with the calibrated material constants have been implemented into the commercial software ABAQUS using a user subroutine VUMAT, and applied to biaxial tests for the computation of both the necking and the fracture limit strains at hot stamping temperatures. Computational results show that there is little difference between the two CDM models with respect to the computed limit strain values, but the strain-based CDM model is more time-efficient.

Wear Characteristics of 22MnB5 Boron Steel under Friction Micro Pin-On-Disks Test

22MnB5 Boron Steel can be considered as emerged material for high strength and low weight application. This material potentially used in abrasive condition such as cutting tool or brake pad where high friction resistance applies. In this study, the wear characteristics of 22MnB5 was investigated under the frictional tests via micro pin-on-disk. 22MnB5 Boron Steel was prepared the form of round shape within the size of 2.6 mm thickness and 12 mm diameter by using laser cutting. 4 different samples were tested namely blank (sample A), self-hardening heat treatment (sample B), 60 HRC hot stamped (sample C) and 70 HRC hot stamped (sample D). The results show that Coefficient of Friction (COF) increased as the hardness of 22MnB5 decreased. Low COF of 0.2114 recorded for sample D with 70 HRC hardness. The COF increased to 0.24, 0.29 and 0.3 when sample C (60 HRC), sample B (52 HRC) and sample A (45.5 HRC) applied respectively. For pin-on disc test, worn area decreased as the hardness increased. 22MnB5 that prepared with the highest hardness of 70 HRC presented smallest wear area of 700 µm x 2400 µm. It is followed by 800 µm x 2400 µm, 1000 µm x 2400 µm, 1600 µm x 2800 µm, when sample C, B and A were scratched. Observation on the worn surface revealed delamination of 22MnB5 surface in the form of fragmented flaking debris.

Development of Resistance Sintering Furnace for Heat Treatment of 22MnB5

This paper presents the development of resistance heating furnace based on the direct electrical conduction method that can perform with lower energy. Based on the concept of resistance welding, resistance heating furnace was developed for heat treatment of 22MnB5 Boron steel with the aim to change the microstructure from pearlite to austenite phases before quenched into martensite phase. The main component for this machine consisted of transformer, copper electrodes, copper cable coil and wood frame structure. The design of the machine resembled resistance welding with upper and lower electrodes were assembled together mounted by the wood frame. The heating energy collected from the electrical current that flow through cooper cable that rolled through toroidal transformer. The result shows that the machine capable to heat the 22MnB5 Boron steelwith maximum temperature of 721.2°C. However, the machine demonstrates overheat after 7 minutes due to the insulation burn of cooper cable. Further improvement of cooling system and copper insulation need to be addressed to prolong the heating operation.

A CDM-based unified viscoplastic constitutive model of FLCs and FFLCs for boron steel under hot stamping conditions

Forming limit curves (FLCs), which are constructed using the limit strains at localised necking, are the most widely used tools for the evaluation of the formability of sheet metals. Fracture forming limit curves (FFLCs) are more recently developed, complementary tools for formability evaluation which are instead constructed using the limit strains at fracture. Since the formability depends strongly on forming conditions such as strain state, temperature and strain rate, models for predicting FLCs and FFLCs are essential for the optimisation and further application of hot forming processes in which these forming conditions vary significantly with both position and time. However, no model has so far been developed to predict FFLCs either alone or in conjunction with FLCs for sheet metals such as boron steel under hot stamping conditions. In this study, a set of unified viscoplastic constitutive equations for the prediction of both FLCs and FFLCs based on continuum damage mechanics (CDM) has been formulated from a set of recently developed constitutive equations for dislocation-based hardening, in combination with two novel coupled variables characterising the accumulated damage leading to localised necking and fracture. The novel variables take into account the effects of strain state, temperature and strain rate on the formability of sheet metals. The material constants in the CDM-based constitutive equations have been calibrated using experimental data comprising true stress-true strain curves and limit strains of a 22MnB5 boron steel obtained at a range of temperatures and strain rates. Investigation of the effect of varying selected parameters in the coupled damage variables on the resulting computed FLCs and FFLCs has demonstrated the flexibility of the model in enabling curves of different shapes and numerical values to be constructed. This indicates the potential of the CDM-based constitutive model for application to other materials for warm or hot stamping processes.

Stress Response Behavior, Microstructure Evolution and Constitutive Modeling of 22MnB5 Boron Steel under Isothermal Tensile Load

22MnB5 boron steel has become one of the main choices for lightweight vehicles due to its extremely high mechanical properties. To explore the intrinsic relationship between the thermoforming process and thermo-mechanical behavior for constitutive modeling and thermoforming of vehicle structure, thermal tensile tests in wide ranges of deformation temperature (500 °C to 950 °C) and strain rate (0.01 s−1 to 10 s−1) were performed using a Gleeble-1500D thermal simulator with hot-rolled 22MnB5 boron steel. With increasing applied strain and strain rate, the flow stress increases gradually and then tends to saturation after reaching peak stress, except for that at 0.01 s−1 and 500 °C. With increasing deformation temperature, the microstructure transforms from a mixture of bainite, ferrite and pearlite to lath-shaped martensite accompanied with some residual austenite. At 950 °C, the average size of martensite decreases with increasing applied strain rate. After thermoforming with austenitizing temperature of 950 °C, lath-shaped martensite accompanied with some residual austenite is obtained in a thermoformed U-shaped structural part, resulting in a dramatical increase in tensile strength. In contrast, the tensile strength of sidewall is slightly higher than that of bottom. Based on the Arrhenius-type constitutive model, a modified constitutive model is constructed with a relative error of less than 5%, which can well describe the flow stress behavior of the studied 22MnB5 boron steel.

Experimental studies of necking and fracture limits of boron steel sheet under hot stamping conditions

Boron steel is the most widely used material in hot stamping applications for forming automotive body panels with complex shapes and ultra-high strength. Due to the high austenitic transformation temperatures and the complex thermal cycle required for hot stamping, however, it is difficult to evaluate the formability of the material using standard punch test methods developed for room-temperature testing. In this study, a high-temperature modification of a recently developed biaxial test method has been used to determine, in a single test procedure and for the first time, forming limit curves (FLCs) and fracture forming limit curves (FFLCs) for 22MnB5 boron steel sheet with a thickness of 1.5 mm under thermal conditions that are representative of industrial hot stamping processes. A direct resistance heating strategy has been developed, and a recently proposed cruciform specimen design has been modified for high-temperature use. For tests with target temperatures in the range of 750 to 925 °C, the resulting test specimens had the highest temperature at the specimen centre and a temperature difference of less than 45 °C in the gauge area and fracture occurred close to the centre of this area under all test conditions investigated. Limit strains at the onset of necking and at fracture for the material have been determined by applying digital image correlation (DIC) to obtain full-field strain measurements, providing an experimental foundation for constructing both FLCs and FFLCs for industrial applications.

The effects of die temperature and cooling condition on friction behavior of bare 22MnB5 boron steel in hot stamping

ABSTRACT Hot stamping is an effective method to improve both the elongation and plasticity of ultra-high strength steels, thus a higher specific strength could be achieved, making it possible for weight reduction in motor vehicles. However, due to the die temperature rise and the cooling liquid within hot stamping applications, the friction in its forming stage is always a non-isothermal mode. Therefore, a high-temperature sliding-on-sheet-strip (SOSS) tribo-tester to simulate sliding friction in hot stamping of 22MnB5 hot stamping steel sheet was designed and custom-made. Then, it was employed to exploit High-temperature friction behavior and mechanism of bare 22MnB5 hot-stamping boron steel sheet under different die temperature and extra cooling conditions. Results show that the friction coefficient obtained under the pure die cooling conditions is lower than that obtained under extra cooling conditions, which is attributed to the formation of thicker oxide layers on the surface of specimen during the cooling process. In addition, the friction coefficient is obviously decreased when the die temperature increased to 200 °C, which is ascribed to the co-action of the destruction of the oxide layers and the formation of bainite.

Wear Characteristics of 22MnB5 Boron Steel under Friction Micro Pin-on-Disks Test

22MnB5 Boron Steel can be considered as emerged material for high strength and low weight application. This material potentially used in abrasive condition such as cutting tool or brake pad where high friction resistance applies. In this study, the wear characteristics of 22MnB5 was investigated under the frictional tests via micro pin-on-disk. 22MnB5 Boron Steel was prepared the form of round shape within the size of 2.6 mm thickness and 12 mm diameter by using laser cutting. 4 different samples were tested namely blank (sample A), self-hardening heat treatment (sample B), 60 HRC hot stamped (sample C) and 70 HRC hot stamped (sample D). The results show that Coefficient of Friction (COF) increased as the hardness of 22MnB5 decreased. Low COF of 0.2114 recorded for sample D with 70 HRC hardness. The COF increased to 0.24, 0.29 and 0.3 when sample C (60 HRC), sample B (52 HRC) and sample A (45.5 HRC) applied respectively. For pin-on disc test, worn area decreased as the hardness increased. 22MnB5 that prepared with the highest hardness of 70 HRC presented smallest wear area of 700 µm x 2400 µm. It is followed by 800 µm x 2400 µm, 1000 µm x 2400 µm, 1600 µm x 2800 µm, when sample C, B and A were scratched. Observation on the worn surface revealed delamination of 22MnB5 surface in the form of fragmented flaking debris.

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.

Effect of heat treatment conditions on the fatigue resistance of press hardened 22MnB5 steel evaluated through rapid testing technique

Fatigue strength is considered as a crucial parameter for automotive applications subjected to cyclic loads during their long service life, as chassis parts. The high yield stress of press hardened steels poses them as good candidates for lightweight solutions with improved fatigue resistance. However, their high strength leads to an increase in notch sensitivity which can ruin the whole part’s integrity. This behaviour was observed in previous works on press hardened steels, where their high fatigue strength was significantly affected by the surface conditions and by heat treatment conditions. Nevertheless, press hardening steels are still good candidates to manufacture complex geometry parts reaching high performance.Aiming at increasing the existing knowledge on the fatigue behaviour of press hardened steels, this paper analyses the fatigue performance of boron steel (22MnB5) under different time austenitizing times. Fatigue resistance is evaluated using a novel rapid fatigue testing technique based on the stiffness evolution. The method permits a fast and reliable determination of the fatigue limit. Based on results obtained with this rapid testing method, the most suitable heat treatment to mitigate fatigue notch sensitivity and then achieving the best fatigue performance for chassis applications is discussed.

Effect of Mold Heating on High-Temperature Friction and Wear Characteristics of Uncoated 22MnB5 Boron Steel

Effect of Mold Heating on High-Temperature Friction and Wear Characteristics of Uncoated 22MnB5 Boron Steel

Determining Forming Limit Diagrams Using Sub-Sized Specimen Geometry and Comparing FLD Evaluation Methods

Sheet metal forming boundaries are established using the forming limit diagram (FLD). The Nakajima and Marciniak tests, which are based on stretching a material using a punch, are the most commonly used methods for determining the FLD or fracture forming limit diagram (FFLD). The results are usually evaluated by calculating local strain, strain rates, specimen thickness reduction or fracture strain. When the amount of experimental material is insufficient, miniaturization of the testing specimens may be a solution. However, the interchangeability of the results for standard and miniaturized specimens has not been proven yet. In this study, the Nakajima tests were performed using standard and sub-sized specimens made of manganese–boron steel 22MnB5, commonly used in the automotive industry. Afterwards, four FLD/FFLD evaluation methods were applied and compared. The miniaturized specimens yielded higher strain values, which was explained by the varied ratio of material thickness/punch diameter and different bending conditions. The highest compliance of the results was recorded for the standard and miniaturized FFLD.

Study on the constitutive equation of hot forming of boron steel sheet

The high temperature constitutive equation of boron steel sheet can describe the relationship of the thermodynamic parameters on the hot forming process of boron steel sheet. It can reflect the relationship between flow stress and strain, the temperature and strain rate, and is indispensable in the forming simulation of hot stamping forming process. In this paper, around the ultra-high-strength boron steel sheet 22MnB5, conduct thermodynamic tensile test on it. And the high-temperature constitutive equation of the high-strength steel sheet under the high-temperature austenite state is established according to the deformation theory. Finally, it is confirmed that the improved Norton-Hoff constitutive equation fitted with the experimental results is in good agreement with the flow stress relationship of boron steel 22MnB5.

Determination of Interfacial Heat Transfer Coefficient of 22mnb5 Boron Steel in Hot Forming Based on Inverse Heat Transfer Method

The heat transfer coefficient at the blank-die interface is difficult to determine the values from experiments due to the influence of various factors, such as forming load, coating material, coating thickness, roughness of surfaces and gap formation caused by the deformation of blank and die, etc. In the present paper, the interfacial heat transfer coefficient (IHTC) between the blank and die is identified by using the method of inverse analysis based on measured temperatures. The results show that the method of inverse analysis is a feasible and effective tool for determination of the blank-die IHTC. In addition, it is found that the identified IHTC varies with temperature and load during hot stamping. The characteristics of the IHTC variation have also been discussed.

Corrosion behavior of Al-Si-Mg coated hot-press-forming steel

This study introduces enhanced corrosion performance of the Al-Si-Mg coated 22MnB5 boron steel. Structural changes of the coating after hot-dip-aluminizing and hot-press-forming processes with the presence of Mg included compacter surface with Mg segregation and thicker crack-free layer. Electrochemical corrosion tests indicated higher polarization resistance from the Al-Si-Mg sample compared to the reference Al-Si sample, especially in an early period of immersion. Such enhanced anti-corrosion performance can be attributed to the aforementioned structural changes and pore-filling effect of corrosion products. This study depicts that the Al-Si-Mg coating can be considered as another option for hot-press-forming steel by its enhanced barrier protection vs. Al-Si coating.