Hot Stamping Steel Research Articles

High temperature deformation behaviors of hot dip 55 wt% Al-Zn coated steel

The high temperature tensile test of hot dip 55 wt% Al-Zn coated steel was performed to study the fracture behaviors of hot dip 55 wt% Al-Zn coating as a protective coating for hot-stamping steel. During austenitization, the 55 wt% Al-Zn coating had transformed to an alloy layer composed an Fe2Al5Znx (with few FeAl and Zn) outer layer and an FeAl inner layer. When the austenitization heat treatment was fixed at 5 min, the sample heated at a higher heating rate (20 K/s) was subjected to a longer soaking time at 900 °C. This resulted in a thicker FeAl inner layer at the steel/Fe2Al5Znx (FeAl, Zn) interface. During the initial stages of deformation, cracks formed in Fe2Al5Znx (FeAl, Zn) layer. Subsequently, cracks developed in the FeAl inner layer. With continued deformation, cracks widened and debonding occurred at the FeAl/Fe2Al5Znx (FeAl, Zn) interface for the samples heated at lower heating rates (5 K/s or 10/s). Finally, crack widening accelerated the exposure and subsequent oxidation of the steel substrate. The absence of debonding can be beneficial for reducing the susceptibility to powdering during hot stamping.

Evolution of Phase Transition and Mechanical Properties of Ultra-High Strength Hot-Stamped Steel During Quenching Process

Hot stamping process is widely used in the manufacture of the high strength automotive steel, mainly including the stamping and quenching process of the hot-formed steel. In the hot stamping process, the steel is heated above the critical austenitizing temperature, and then it is rapidly stamped in the mold and the quenching phase transition occurs at the same time. The quenching operation in the hot stamping process has a significant influence on the phase transition and mechanical properties of the hot-stamping steel. A proper quenching technique is quite important to control the microstructure and properties of an ultra-high strength hot-stamping steel. In this paper, considering the factors of the austenitizing temperature, the austenitizing time and the cooling rate, a coupled model on the thermal homogenization and phase transition from austenite to martensite in quenching process was established for production of ultra-high strength hot-stamping steel. The temperature variation, the austenite decomposition and martensite formation during quenching process was simulated. At the same time, the microstructure and the properties of the ultra-high strength hot-stamping steel after quenching at different austenitizing temperature were experimental studied. The results show that under the conditions of low cooling rate, the final quenching microstructure of the ultra-high strength hot-stamping steel includes martensite, residual austenite, bainite and ferrite. With the increase of the cooling rate, bainite and ferrite gradually disappear. While austenitizing at 930 °C, the tensile strength, yield strength, elongation and strength-ductility product of the hot-stamping steel are 1770.1 MPa, 1128.2 MPa, 6.72% and 11.9 GPa%, respectively.

Effects of Ti Alloying on Resistance to Hydrogen Embrittlement in (Nb+Mo)-Alloyed Ultra-High-Strength Hot-Stamping Steels

Effects of Ti Alloying on Resistance to Hydrogen Embrittlement in (Nb+Mo)-Alloyed Ultra-High-Strength Hot-Stamping Steels

Transient Softening at the Fusion Boundary of Resistance Spot Welds: A Phase Field Simulation and Experimental Investigations for Al–Si-coated 22MnB5

This work gives an insight into the transient softening at the fusion boundary of resistance spot welds on hot stamped steel. Metallographic investigations and hardness mapping were combined with finite phase–field modeling of phase evolution at the fusion boundary. Saturation of weld nugget growth in the welding process was observed. For industrially relevant, long welding times, the fusion boundary of a spot weld is therefore isothermally soaked between the peritectic and solidus temperatures. This leads to a carbon segregation to the liquid phase due to higher carbon solubility and possibly to δ-Fe formation at the fusion boundary. Both results in a local carbon depletion at the fusion boundary. This finding is in good agreement with carbon content measurements at the fusion boundary and the results of hardness measurements.

The Effects of Process Temperatures on the Microstructure and Tensile Properties of Warm-Stamped Nb-Bearing Medium-Mn Steel

The effects of process temperatures for warm stamping, such as annealing (Tann), austenitizing (Taus), and stamping temperatures (Ts), on the tensile properties of Nb-bearing medium-Mn (5.9 wt pct) steel were investigated. The cold-rolled tensile specimens were first annealed at 650 °C to 750 °C, austenitized at 650 °C to 900 °C, held at Ts (500 °C to 700 °C), and then air-cooled to room temperature. Of the three process temperatures, only Taus significantly influenced the tensile properties of medium-Mn steel; in the Taus range of 650 °C to 900 °C, the amount of variation in yield strength (YS), ultimate tensile strength (UTS), and total elongation (TE) were 555 MPa, 570 MPa, and 20 pct, respectively. Particularly, when the specimens were austenitized at 730 °C to 790 °C, high YS (1060 to 1100 MPa), UTS (1760 to 1795 MPa), and TE (10.0 to 11.7 pct) were obtained due to fine-grained martensite embedded with nano-sized Nb carbides. The Nb-bearing medium-Mn specimen was successfully warm-stamped into a T-part of a B-pillar prototype at 600 °C without any cracks after austenitized at 750 °C for 5 minutes, and then air-cooled to room temperature. The warm-stamped Nb-bearing medium-Mn steel revealed the higher TE (~ 9.3 pct) than the hot-stamped 30MnB5 steel (5.3 pct) at the similar level of UTS (1855 to 1900 MPa).

Impact and damage behaviour of FRP-metal hybrid laminates made by the reinforcement of glass fibers on 22MnB5 metal surface

Hot stamping of 22MnB5 steel is know to yield very high strength of up to 1500 MPa depending on the final microstructure. In this work, different microstructures of 22MnB5 steel were produced by quenching with different cooling rates and then utilized to develop Fiber Reinforced Plastic (FRP)-metal hybrid laminates. By developing FRP-metal hybrid laminates using these hot stamped steels, the superior mechanical of it can be transferred to FRP-metal hybrid laminates. Two different types of FRP-metal hybrid laminates were developed in the study, one by reinforcing thermoplastic based glass/PA-6 FRP on steel surface using press-forming technique and other by reinforcing thermoset based glass/epoxy FRP using vacuum assisted resin transfer moulding process. By developing the FRP-metal hybrid laminates, optimal exploitation of the lightweight and high strength potential can be achieved. Both thermoplastic and thermoset based FRP-metal hybrid laminates developed by using heat treatment steel has better flexural properties compared to steel without heat treatment. In overall the PA-6 has better adhesion towards steel surface compared to epoxy polymer. The developed unsymmetrical hybrid laminates will consist of one side steel and other side with FRP and the influence of the damage behaviour with respect to the impacted side were investigated. Different failure behaviours were identified with respect to the impacted side. 22 J impact on the metal side of thermoplastic based FRP-metal hybrid laminates, developed by the hot stamped steel, requires 127.88% higher load compared to FRP laminates and 33.32% higher load compared to commercially available 22MnB5 steel.

Effect of Nb–Ti multi-microalloying on the hydrogen trapping efficiency and hydrogen embrittlement susceptibility of hot-stamped boron steel

Multi-microalloying technique has been widely used to improve strength and ductility in steels, however, the effect of multi-microalloying on the hydrogen diffusion and hydrogen embrittlement (HE) behavior of steels is still not completely understood. This study evaluates the effect of Nb–Ti multi-microalloying on the hydrogen trapping efficiency and HE of hot-stamped steel by a combination of hydrogen permeation test, slow strain rate tensile (SSRT) test and quantitative analysis. The microstructural examination and hydrogen permeation tests showed that with increasing Nb + Ti content, more nano-sized (Nb, Ti) C precipitates were formed, and the martensite grain boundaries areas increased, thus increasing the irreversible and reversible hydrogen trap sites and decreasing the hydrogen diffusion coefficient in the hot-stamped steel. A quantitative analysis of the hydrogen traps demonstrated that the number of hydrogen traps induced by the (Nb, Ti) C precipitates was larger than that of the grain boundaries and dislocations in the Nb–Ti bearing steels. In addition, the SSRT tests showed that the HE susceptibility of the test steel decreased with increasing Nb + Ti content. This was because the additional hydrogen traps generated by the Nb–Ti addition hindered localized hydrogen accumulation at prior austenite grain boundaries, the cracking resistance was improved by a lower Σ3 boundary fraction, and the pinning role of the (Nb, Ti) C precipitates on the movable dislocation could inhibit H-dislocation interaction.

Effect of Hot Stamping Heat Treatment Temperature on Resistance Spot Weldability of Al-10% Si Coated 30MnB5 Steel

Hot stamping is the most efficient manufacturing process to produce high strength automotive parts with reduced springback. Recently, 30MnB5 steel has been developed for hot stamping steel which can provide above 1700 MPa. However, resistance spot weldability of this steel is insufficient due to the need for a large amount of carbon addition. In this study, the effect of the hot stamping heat treatment temperature on resistance spot weldability of Al-10% Si coated 30MnB5 was evaluated to improve the resistance spot weldability of 30MnB5. In terms of nugget diameter, weldable current range, tensile shear strength and cross tension strength, the specimen heat treated at 850 oC achieved better performance than the specimen heat treated at 950 oC. There was a coating microstructure difference between the specimens heat treated at 850 oC and 950 oC whereas both specimens had similar microstructure and mechanical properties of their substrates. The Fe-rich layer of the specimen heat treated at 950 oC was twice as thick as the specimen heat treated at 850 oC because the diffusion rate was much higher at 950 oC. The thick Fe-rich layer deteriorated the resistance spot weldability due to high electrical resistivity during resistance spot welding. Key words: hot stamping, Al-10%Si coating, 30MnB5, resistance spot welding, dynamic resistance

A newly-designed hot stamping plus non-isothermal Q&P process to improve mechanical properties of commercial QP980 steel

A brand-new process with the combination of hot stamping and non-isothermal quenching and partitioning (Q&P) on a controlled servo press has been applied on a commercial QP980 steel to generate a microstructure of martensite and retained austenite. The results indicate that non-isothermal partitioning can be successfully carried out in this newly proposed novel hot stamping process. Compared to the conventional hot stamped steel, the total elongation can be increased from 12.5% to 14.6%, the uniform elongation can be increased by 19% (from 5.28% to 6.27%) and the product of strength and elongation can be enhanced by 9% (from 19.3 GPa% to 21.0 GPa%) for the steels treated by the new hot stamping plus non-isothermal Q&P process. These improved mechanical properties can be attributed to the increased fraction of retained austenite, as a result of the non-isothermal partitioning.

Effects of Prior Austenite Grain Size on Hydrogen Delayed Fracture of Hot-Stamped Boron Martensitic Steel

Understanding the hydrogen delayed fracture requires elucidating the interaction of relevant cracks based on the microstructural features. The microstructural features of a hydrogen delayed fracture were studied for martensitic steel under different soaking times and prior austenite grain sizes based on the furnace temperature. A considerable correlation is investigated between the coarseness of the austenite grain and degree of susceptibility on a hydrogen delayed fracture. The deterioration of hydrogen delayed fracture is attributed to reversible hydrogen in the microstructure sites with a low trapping energy weakening the grain boundary and rapidly propagating the crack initiation.

Hot stamping steel grades with increased tensile strength and ductility - MBW-K 1900, tribond 1200 and tribond 1400

The use of hot formed 22MnB5 steel with an aluminium-silicon (AS) coating is well established for high strength automotive parts to combine weight reduction and passenger safety. Recent developments aim to increase the material strength for further weight saving and to improve the ductility. Especially an improved ductility makes hot formed parts suitable for additional applications, such as parts with higher energy absorption or even axially loaded parts. thyssenkrupp Steel Europe AG has developed new hot stamping grades. With an increased strength of about 1900 MPa, the MBW-K 1900 allows the application of even lighter parts compared to 22MnB5. Due to the increased strength level, delayed fracture has to be evaluated. Furthermore, the recommended parameters for processing MBW-K 1900 are regarded.A high crash performance of a material implies a high strength, while the material still shows no cracking during folding or heavy bending of the automotive part. This is the basis for the development of tribond 1200 and tribond 1400. As a new concept for hot stamping, tribond 1200/1400 are composite materials, which bond two outer layers of a ductile 500 MPa steel grade with a high strength 1500 MPa steel core layer. Both grades are already well known as monolithic steels MBW 500 and MBW 1500 for hot stamping. An aluminium-silicon coating allows tribond 1200/1400 to be processed in the same way as AS coated 22MnB5. Nevertheless tribond 1200/1400 show a reduced susceptibility to hydrogen embrittlement in the 4-point bending test compared to 22MnB5. By their improved crash behaviour, tribond 1200/1400 enable the manufacturing of more kinds of hot formed parts such as longitudinal beams. An advanced simulation method takes the composite structure of tribond 1200/1400 into account to evaluate the crash behaviour.

Oxidation resistant silane coating for hot-dip galvanized hot stamping steel

A silane coating was developed for hot-dip galvanized (HDG) steel via a sol-gel roll coating method and its oxidation resistance during austenitization was detailed. The HDG steel with silane coating retains metallic appearance after 900 °C austenitization for 5 min. Moreover, the silane coating preserves more Zn in the alloy layer, conferring better corrosion resistance. Liquid metal embrittlement is absent when a thinner Zn coating (∼ 7 μm) is applied, yet imparts cathodic protection to the austenitized silane-coated HDG steel with a cosmetic coating. The silane coating markedly inhibits oxidation and confers enhanced corrosion resistance for hot-stamped HDG steel parts.

Influence of inclusions on hydrogen-induced delayed cracking in hot stamping steels

The hydrogen-induced delayed cracking (HIDC) behaviors of two types of 1500 MPa grade hot stamping steels (HSSs) have been investigated by the method of slow strain rate tensile test and hydrogen permeation, where one is manufactured by compact strip production (CSP) process which is a revolution to the traditional HSS and the other by the traditional cold rolling process. The results show that the performance of HSS produced by CSP is superior to that of the traditional HSS, due to lower hydrogen embrittlement index, lower hydrogen diffusion coefficient and lower hydrogen content. It has been found that HIDC behavior is closely associated with inclusions. The inclusions of HSS produced by CSP are mainly spherical Al–Ca–O and CaS, while the inclusions in the traditional HSS are TiN + Al2O3 + MnS with sharp edges and corners. Based on these results, the influence of composition, shape and distribution of inclusions in HSS on HIDC and the mechanism of HIDC from the perspective of inclusions were analyzed and discussed.

Effect of quenching temperature on the microstructure and mechanical properties of 30MnBNbV hot stamping steel

A low carbon steel microalloyed with Nb-V subjected to hot stamping process, then quenching to different temperatures (30 °C, 230 °C, 300 °C) followed by air cooling to room temperature was investigated. The method in this article was significantly simplified compared with the conventional quenching & tempering (QT) process. The results shown that the best comprehensive mechanical properties could be obtained at 230 °C. The yield strength (YS) and ultimate tensile strength (UTS) were 1128.8 MPa and 1768.8 MPa respectively, and the total elongation was 8.8% without much compromise. The excellent properties were attributed to a multi-morphology microstructure which consisted of large amount of martensite (block fresh martensite and lath tempered martensite) and trace carbon-enriched retained austenite. In addition, the recovery of block fresh martensite (FM) and decomposition of unstable austenite were discussed during air cooling to room temperature process.

A study of annealing time to surface characteristics and hydrogen embrittlement on AlSi coated 22MnB5 during hot stamping process

Hydrogen embrittlement is influenced by material sensitivity, residual stress and hydrogen uptake. During hot stamping process, there are various factors that affect hydrogen embrittlement, such as annealing time during the heating process. As the annealing time increases, the color of the metallic coating gradually turns from blue to red and microstructure becomes larger. When the surface properties of automotive steel are changed, it is necessary to check whether there is any problem in electro-coating process and corrosion resistance. In this study, we investigated the effect of the metallic layers by diffusion of Fe and defect sensitivity resulting of the electrocoating process such as pinhole, corrosion resistance and hydrogen embrittlement sensitivity of steel, with annealing time in the hot stamping heating furnace. Coating workability shows similar results regardless of annealing time, but the sensitivity of hydrogen embrittlement is greatly influenced by annealing time in the hot-stamping furnace. And we have found a way to manage optimal annealing time to reduce of hydrogen embrittlement sensitivity on hot stamping steel by chroma- meter.

Effect of Hydrogen Content and Strain Rate on Hydrogen-induced Delay Cracking for Hot-stamped Steel

Hot-stamped steel has been widely used in automobile bumper and other safety components due to its high strength. Therefore, this paper investigates the effect of hydrogen content and strain rate on hydrogen-induced delay cracking (HIDC) behavior. The results showed that the plasticity of the steel significantly decreased with an increase in hydrogen content or a decrease in the strain rate. Fractography was analyzed after tensile tests. It was found that all of the pre-charged specimens cracked at large-sized inclusions when stretched at a strain rate of 1 × 10−3 s−1, which indicates that, in this case, the defect itself in the material had great influence on the extend properties. No inclusions were found at the main fracture origin area for hydrogen steady-state specimens, when stretched at a strain rate of 1 × 10−6 s−1, which demonstrated that a slower strain rate causes greater influence by hydrogen. However, for the non-pre-charged samples, the fractures surface showed that cracking originated from the defect near the sample surface, which was independent of strain rates.

Study on Microstructure and Properties of a New Warm-Stamped Niobium-Alloyed Steel

The warm stamping technology is a promising technology to meet the needs of car weight reduction and energy conservation. In order to compare with the mechanical properties of the traditional hot-stamped boron-alloyed steel 22MnB5, a new warm-stamped niobium-alloyed steel 22Mn3SiNb was designed and tested. The optimal heating parameters for warm forming process were explored through mechanical tests, and the process of their microstructure evolution was investigated by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD), etc. The experimental results indicate that the optimal heating parameters for the niobium-alloyed steel 22Mn3SiNb are a heating temperature of 800 °C and a soaking time of 5 min. Compared to the hot-stamped boron-alloyed steel 22MnB5 under their respective optimal heating parameters, the properties and microstructure characteristics of 22Mn3SiNb are greatly improved, and nearly no decarburized layer is found on the surface of the niobium-alloyed steel 22Mn3SiNb. In addition, the addition of Nb produces the effects of grain refinement and precipitation strengthening due to the introduction of plenty of nano-precipitated particles and dislocations. In the end, it can be predicted that the new warm-stamped niobium-alloyed steel will replace the conventional hot-stamped boron-alloyed steel.

Enhancement of Fatigue Endurance by Al-Si Coating in Hot-Stamping Boron Steel Sheet

Most structural components undertake cyclic loads in engineering and failures always cause catastrophic economic losses and casualties. In the present work, the phase evolution of Al-Si coating of high-strength boron steel during hot stamping was investigated. Two types of 1500 MPa grade boron steel sheets, one with Al-Si coating and the other without, were studied to reveal the effect on the high-cycle fatigue behavior. The as-received continuously hot-dip Al-Si coating was composed of α(Al), eutectic Al-Si and τ5. After hot stamping at 1193 K, three phases formed in this coating: β2, Fe(Al,Si)2 and α(Fe). The experimental results showed that the endurance limit of the coated steel sheet was 370 MPa under 107 fully reversed tension-compression loading cycles as opposed to 305 MPa in the uncoated sheet. Both the coated and the uncoated specimens showed surface-induced transgranular fatigue fractures. In the uncoated sheet, the fatigue cracks were generated from the decarburization surface, but the Al-Si coating effectively prevented the occurrence of near-surface decarburization during high-temperature hot stamping, and the only cracks in the coated steel sheet were initiated at wire-cutting surfaces.

Baking Effect on Desorption of Diffusible Hydrogen and Hydrogen Embrittlement on Hot-Stamped Boron Martensitic Steel

Recently, hot stamping technology has been increasingly used in automotive structural parts with ultrahigh strength to meet the standards of both high fuel efficiency and crashworthiness. However, one issue of concern regarding these martensitic steels, which are fabricated using a hot stamping procedure, is that the steel is highly vulnerable to hydrogen delayed cracking caused by the diffusible hydrogen flow through the surface reaction of the coating in a furnace atmosphere. One way to make progress in understanding hydrogen delayed fractures is to elucidate an interaction for desorption with diffusible hydrogen behavior. The role of diffusible hydrogen on delayed fractures was studied for different baking times and temperatures in a range of automotive processes for hot-stamped martensitic steel with aluminum- and silicon-coated surfaces. It was clear that the release of diffusible hydrogen is effective at higher temperatures and longer times, making the steel less susceptible to hydrogen delayed fractures. Using thermal desorption spectroscopy, the phenomenon of the hydrogen delayed fracture was attributed to reversible hydrogen in microstructure sites with low trapping energy.

Tailoring by Direct Contact Heating During Hot Forming/Die Quenching

The well-known drawbacks of furnace-based technologies used for hot-stamping ultrahigh strength steel (UHSS) parts motivate development of alternative approaches for austenitizing blanks, among them direct contact heating. While the objective of most hot-stamping operations is to produce fully martensitic parts, there is growing interest in developing tailoring techniques for making parts having inhomogeneous mechanical properties for improved crash performance. This work shows how direct contact heating can produce parts having highly controlled microstructures by virtue of the different thermal effusivities of steel and ceramic regions on the heating die, even though the contact surface remains nearly isothermal. The process is designed with the aid of a finite element model that incorporates an austenitization submodel for 22MnB5, a common ultrahigh strength steel. Microhardness measurements and metallographic analysis on heated and quenched blanks are consistent with modeled austenite phase fractions, and highlight the potential of this technique for producing strong and lightweight automotive parts.