Ultra-high Strength Boron Steel Research Articles

Mechanical and microstructure properties of ultra-high strength boron steel using rapid resistance heating without soaking

Mechanical and microstructure properties of ultra-high strength boron steel using rapid resistance heating without soaking

Microstructure and mechanical properties of a 2000 MPa grade ultrahigh strength boron steel

Press hardening steels (PHS) with very high strength are current key materials for lightweight engineering solutions and corresponding CO2 savings. In the present study, the microstructure and mechanical properties of a 2000 MPa grade ultrahigh strength boron steel for hot stamping were reported. Microstructure evolution experiment revealed that the microstructure of the 2000 MPa grade PHS only consisted of martensite when the cooling rate increased to 20 °C/s. The prior austenite grain size was about 6.8 μm when the sample was austenitized at 900 °C for 300 seconds. The appropriate isothermal quenching process should be austenitized between 900 °C and 950 °C during hot stamping because there is an abnormal grain growth of austenite grains at 950 °C. Following the optimal hot stamping condition, the 2000 MPa grade PHS alloy demonstrated yield strength in excess of 1300 MPa and ultimate tensile strength in excess of 2000 MPa together with a total elongation of about 7%.

Joining aluminum alloy to ultrahigh-strength boron steel through an impact welding approach

Direct welding of ultrahigh-strength boron steels to aluminum alloys is challenging but of great practical importance. The present study reports a new approach in vaporizing foil actuator welding to produce direct spot welds between 6022-T4 Al alloy and Usibor 1500®, a representative boron steel. Mechanical testing results revealed a strong joint with failure occurring at the Al base metal outside the weld nugget. Comparison of joint strengths with other joining techniques revealed performance on par if not better, opening prospects for development of more robust dissimilar welding pairs in the future.

A type of cruciform specimen applied to evaluate forming limits for boron steel under hot stamping conditions

The ultra-high strength boron steel has been intensively used in the hot stamping process to produce complex-shaped structural components in transportation industries. Forming limit diagram (FLD) is a fundamental and useful tool to evaluate the formability of metallic materials under various forming conditions. Since the standardized Nakajima test and Marciniak test are not applicable to perform formability tests for hot stamping applications due to the complex heating and cooling processes required, an in-plane testing method, in which cruciform specimens are deformed under hot stamping conditions in a Gleeble materials simulator combined with a multi-axial tensile rig to convert an input force to an output biaxial force, has been successfully applied to assess the formability of aluminium alloys at elevated temperatures. However, it is challenging to apply this in-plane testing method for boron steel due to higher nonuniformity of temperature distribution in gauge region of the cruciform specimen at a higher temperature. In this paper, a new type of cruciform specimen, together with a new specimen heating strategy, has been proposed to improve the temperature distribution in the gauge region. The dimensions of the newly-designed cruciform specimen have been optimised by a thermo-electrical finite element model embedded with a UAMP subroutine in ABAQUS to improve the uniformity of temperature distribution in the gauge region. In order to validate the new design of cruciform specimen, biaxial tensile tests were conducted under hot stamping conditions by using the in-plane testing method.

Effect of initial blank temperature in hot press forming towards 22MnB5 springback failure

The springback failure of ultra-high strength boron steel (22MnB5) in hot press forming (HPF) process was characterized under bending and membrane conditions. Hot press forming for U–shaped parts with ultra-high strength boron steel were experimented and simulated to study the effect of initial blank temperatures on springback failure in the automotive industry. The results specify the various preheated temperature of 22MnB5 blank effect toward springback occurrences with reference to hot press forming dies design. ANSYS Workbench was used to verify finite element (FE) simulations of the processes in order to consolidate the knowledge of springback. The validated numerical simulation model were used in analyzing the stress and strain distributions along the formed part in the FE models, it was found that the springback angle was related in averaging value throughout quenching, regardless of the forming conditions. Springback failure mainly caused dimension deviation in hot press form parts due to the impact of thermal restoring moments and quenching rate of hot press forming process.

Fracture Profile and Crack Propagation of Ultra-High Strength Hot-Stamped Boron Steel During Mechanical Trimming Process

Mechanical trimming process for ultra-high strength boron steel after hot stamping was carried out in this study. Shear and tensile tests were designed to analyze the influences of stress state on the fracture mode; trimmed fracture surface and profile were observed and compared to other commonly used steels such as DP980 and Q235 etc.; the crack propagation during trimming process was studied through step-by-step tests. The observation and analysis reveal that the fracture mode of hot-stamped boron steel is highly related to the stress state, it belongs to cleavage fracture on low stress triaxiality but dimple fracture on high stress triaxiality. Such phenomenon is reflected in the trimming process, during which the stress state changes from shear-dominated state to tensile-dominated state. In addition, the burnish zone of trimmed boron steel is much smaller than other high strength steels, and the profile of cutting surface shows an ‘S’-like shape which is destructive to the trimming tool. Moreover, during the trimming process, most martensite laths near the cutting edge are stretched and rotated markedly to the direction of the shear band, and the main crack expands along those grain boundaries, which may penetrate through a few martensite laths and form small crack branches.

Analysis of the Design and Mechanical Performance about Hot Stamping Power Fitting Products

Hot stamping ultra-high strength steel not only plays an important role in automotive field, but also has potential application in transmission line fittings industry. According to the current development status of Chinese electrical power fittings and the demand for new technique, combined with hot stamping technology, a new solution for reduction of weight in transmission line fittings has been proposed. This paper focuses on a new lightweight connection fitting with multilayer stack structure of ultra-high strength boron steel, and the key issue in design and manufacturing process of the deep U-shaped hot stamping stacked steel sheet. Numerical simulation method combining the experimental data is also used to predict mechanical performance of the new connection fitting, which provides guidance for the manufacturing process.

New generation ultrahigh strength boron steel for automotive hot stamping technologies

The automotive industry is under increasing pressure to: (1) reduce the weight of vehicles and (2) improve crash performance. ‘Hot stamped’ mild carbon–manganese–boron steel 22MnB5 has become common place in the body structure of the European vehicle over the past decade. The ultrahigh strength martensitic microstructure resulting from hot stamping (proof strength ∼1200 MPa and ultimate tensile strength ∼1500 MPa) enables down gauging while not compromising crash performance. However, with demands for yet higher strength in the final component so to enable further down gauging, novel grades must be developed. In this paper, development of the novel grade 38MnB5 was reported. Following hot stamping, 38MnB5 demonstrated proof strength in excess of 1400 MPa and ultimate tensile strength in excess of 2000 MPa. Owing to the immense strength, the novel grade 38MnB5 was considered to offer significant down gauging and weight reduction opportunities to the automotive industry.

Experimental Study on Conduction Heating of Ultra-High Strength Boron Steel

Conduction heating overcomes the shortage of the furnace, due to rapid heating and high energy efficiency. The heating experiment of ultra-high strength boron steel (LG1500HS) of a rectangular blank was implemented on the conduction heating system. The variations of the average temperature rising rate, uniform temperature region and energy efficiency were investigated under the non-heating-retaining condition. The results show that the average temperature rising rate, uniform temperature region and energy efficiency are 44.5°C/s, 225 mm and 71.6%, respectively.

Flow Behavior of Ultra-High Strength Boron Steel at Elevated Temperature

Ultra-high strength boron steel is widely used in a new hot stamping technology which is hot formed and die quenched simultaneously in order to obtain stamping parts with 1500MPa tensile strength or higher. Tensile experiments were carried out with ultra-high strength boron steel in a range of temperature 500°C~860°Cand strain rate 0.01/s~1/s with the thermal simulation testing machine Gleeble 3800, and the stress-strain curves were obtained. The influences of the deformation temperature and strain rate on the stress-strain curves were analyzed. The results show that hot behavior at elevated temperature of ultra-high strength boron steel consists of strain hardening and dynamic recovery mechanism, which can be accurately described by the mathematic model. Keywords: Ultra-high strength boron steel, hot stamping, hot flow behavior

Investigation on Dynamic Recovery Behavior of Boron Steel 22MnB5 under Austenite State at Elevated Temperatures

Hot forming process of ultrahigh strength boron steel 22MnB5 is widely applied in vehicle industry. It is one of the most effective approaches for vehicle light weighting. Dynamic recovery is the major softening mechanism of the boron steel under austenite state at elevated temperatures. Deformation mechanism of the boron steel can be revealed by investigation on the behavior of dynamic recovery, which could also improve the accuracy of forming simulations for hot stamping. Uniaxial tensile experiments of the boron steel are carried out on the thermo-mechanical simulator Gleeble3800 at elevated temperatures. The true stress-strain curves and the relations between the work hardening rate and flow stress are obtained in different deformation conditions. The work hardening rate decreases linearly with increasing the flow stress. A flow stress model is derived based on Kocks model, and the derivative of the dislocation density with respect to the true strain, which is expressed by the peak stress and initial yield stress, is also deduced considering the dynamic recovery effect. A dynamic recovery efficiency factor h is defined as the ratio of the dynamic recovery effect and the dislocation accumulation effect. The effects of the deformation conditions on the h are analyzed.

Hot Forming Analysis of Car Door Bar for Ultra High Strength Steel

Ultra high strength steel plays an important role of light weighting in automotive industry. The hot forming simulation of car door bar is processed with 22MnB5 ultra high strength boron steel. FEM is built with the 12 nodes shell elements and MAT 106 is selected in LS-DYNA. The hot forming processes include two heat transfers. One is the process from the oven to the tools after the blank is heated. The other is the process after the blank contacts the tools. The hot forming simulation results are obtained by LS-DYNA. The results show that the thickness distribution, the forming limit and the maximum effective plastic strain and other performances attain to standards. It is proved that the hot forming simulation method is correct.

Forming Response of Ultra High Strength Steel Sheet to Stamping Speed during Hot Forming

Stamping speed is an important parameter in sheet metal forming especially in hot forming. In this study, hot forming of a U-shaped part made of ultra high strength boron steel (22MnB5) sheet is simulated with solid elements. The mechanical properties of 22MnB5 steel sheet and the key process parameters are introduced in detail. Emphasis is laid on the forming response of the boron steel sheet to stamping speeds of 3.25m/s, 0.325m/s and 0.0325m/s. The mechanism of stamping speed acting on hot formability and temperature field of the stamped part is analyzed. It is demonstrated that stamping speed affects both formability and the heat transferred from blank to tools and to environment during hot forming. And the coupling effect of material properties, the heat produced during plastic deformation and heat boundary condition decides the formability and temperature field. An appropriate stamping speed is more important for hot forming than that for common cold forming.

Influence of Low Tempering Temperature on Fracture Toughness of Ultra High Strength Boron Steel for Hot Forming

The influence of low tempering temperature (TT) on mechanical properties and fracture toughness of ultra high strength boron steel 22MnB5 for hot forming was investigated by tensile and Kahn tearing experiments. Compared with the conventional quenchable boron steel, The results show that the toughness of tested steel are both preferable when tempered at 200 , 40min. When TT between 100 ～200 , The tear strength (TS) and unit propagation energy (UPE) of quenched steel heightened along with the temperature raised, but in 200~350 temperature range, the toughness appears negatively growth. Appropriate TT could improve the mechanical properties and fracture toughness obviously, and the temper brittleness can be avoided.

High-temperature friction and wear characteristics of hardened ultra-high-strength boron steel

The usage of ultra-high-strength boron steel (UHSS) in automotive industry has increased rapidly in the recent past. Forming of UHSS components is performed at elevated temperatures, which also offers the possibility of hardening through quenching directly after forming. However, the influence of hardening on friction and wear during relative sliding between the tool and the workpiece is unclear. Therefore, the friction and wear characteristics at elevated temperatures of hardened and unhardened UHSS and tool steel pairs are investigated in this study. The results show that both friction and wear at all the investigated temperatures are affected by hardening of the UHSS. For uncoated UHSS, the hardening resulted in lower friction and the tool wear increased at low temperatures, but was not affected at elevated temperatures. This was attributed to the higher hardness after hardening combined with the presence of an oxide scale on the UHSS after heating and quenching. For Al—Si-coated UHSS, the hardening reduced friction and tool steel wear at elevated temperatures, and also reduced the wear of the Al—Si-coated high-strength steel at low temperature mainly owing to the formation of an intermetallic layer on the Al—Si-coated UHSS surface after exposure to elevated temperatures.

Experiments and Numerical Simulation of Hot Bending and Die Quenching for Ultra-High Strength Boron Steel

Hot bending and die quenching for U-shaped parts with ultra-high strength boron steel were experimented and simulated to study its quenching effect and accuracy. The results indicate that through the hot contact bending and die quenching, bending parts with higher strength and forming precision than that of cold stamping can be achieved. And the hot contact bending improves the quenching effect at the bottom of the bending part. The springback of hot contact bending decreases gradually when the blank holder force (BHF) increases, which is mainly negative for the impact of the thermal shrinkage moments. The numerical simulation results are consistent with the experiments’. Therefore, it verifies the reliability of finite element model and lays the foundation for the numerical simulation of the hot stamping process applied in complicated shape parts’ production.