Press-hardened Steel Research Articles

Development of Niobium Alloyed Press Hardening Steel with Improved Properties for Crash Performance

Press hardening steel has become a much used material in car body manufacturing due to its excellent safety and lightweight potential. In some recent car models press hardening steel has reached already a weight share of more than 20% in the body structure while it is estimated that it could reach even around 40% in the future. However conventional press hardening steel based on the alloying concept 22MnB5 was designed originally not for automotive application. In spite of the high strength level, press hardening steel has generally low toughness due to the relatively high carbon content and its martensitic microstructure. Particularly important is the ductile-to-brittle transition temperature at low temperature, which might lead to unexpected failure in cold climate regions. Furthermore, hydrogen embrittlement is a major concern in press hardening steel as previous results indicated that only a few ppm of hydrogen could induce delayed cracking. Generally it is important that impact energy should be absorbed by the material to avoid unexpected disintegration of the structure. This material capability is usually characterized by the toughness. All these characteristics have not yet been systematically investigated in press hardening steel. Consequently, no attempts have been made to optimize conventional press hardening steel for obtaining better toughness, lower ductile-to-brittle transition temperature and reduced sensitivity to hydrogen induced cracking.A generally proven approach of improving the resistance against brittle failure in high strength steel is the refinement of microstructure, which can be efficiently achieved by Nb microalloying. This paper will introduce modified alloy designs for press hardening steel and explain fundamentally the metallurgical effects of Nb microalloying on the improvement of crack propagation resistance, bendability and delayed cracking behavior induced by hydrogen penetration in press hardening steel. The results reveal better toughness, improved bending behavior and increased resistance against hydrogen embrittlement.

Hot Stamping Technology Applied on Vehicles on EuroCarBody

EuroCarBody, offer the most important forum for defining and discussing the state of the art in modern series car body engineering. The present paper concerns the materials of press hardened steels and its’ manufacturing technology (Hot stamping) applied on vehicle on EuroCarBody from 2009 to 2013 through presentations and benchmark datas. The using percentage of press hardening steels (PHS) on body in white (BIW) are summarized and analyzied. It can be noted that the number and weight of BIW parts using of press hardening steels are increasing from 2009 to 2013, some kind of vehicle, such as Audi A3, the using percentage of PHS reaches 21.6%. The press hardening steels have started used on commercial vehicle from EuroCarBody 2013. More and more new technologies applied on hot stamping processing, such as Joule heating for hot stamping, Hot-form-process partial tempering, Tailor Rolled Blank (TRB) technolgy, etc. With the developing of the automotive industry, more and more hot forming parts will be applied on vehicles in the future.

Effects of Austenitizing Temperature on Microstructure and Properties of Hot-Formed Steel

Press hardening steel is the best solution for application of extremely high strength steel in automotive structures in order to reduce the weight of body-in-white. Effect of austenitizing temperature on the grain coarsening of a press hardening steel has been investigated by using dilatometer at first. The mechanical properties of press-hardened steel austenitized at temperature between 850 to 950oC by using a pilot hot stamping line have been investigated. The strength, especially the ultimate tensile strength, was improved by the grain refinement with lower austenitization temperature.

Research Status of Advanced Hot Forming Technology

For the past decade, BaoSteel has been working on the development of press hardened steels (PHS) and hotforming (HF) technologies, aiming to provide technical package solutions for customers and made some achievements. In this paper, the latest research development on advanced hotforming processes such as patchwork, tailored property, door ring and low-cost short-cycle was introduced.

Solutions for Hydrogen-Induced Delayed Fracture in Hot Stamping

One of the main targets in automotive industry is to reduce the weight of vehicle as well as increase the safety. To accomplish this goal, press-hardening steel and hot stamping parts have been used in car body. However, the possibility of hydrogen-induced delayed fracture (HDF) of hot stamping parts exits, which will decrease the car’s passive safety. A solution has been presented to reduce the sensitivity of HDF and improve hydrogen-induced delayed fracture resistance (HDFR) by Niobium micro-alloying technology. Traditional press-hardening steel 22MnB5 and new steel 22MnBNb2, 22MnBNb5 and 22MnBNb7 were studied, and it is shown that the appropriate addition of Nb is beneficial to the improvement of the delayed fracture resistance of the hot stamping steel, which indicates that Niobium micro-alloying technology is an effective solution to the HDF in hot stamping steels.

Effects of Initial Material Conditions on the High Temperature Surface Oxidation of Press-Hardening Steels

The 22MnB5 steel of Wuhan Iron with three early treatment (pickling, cold-rolling and cold-rolled annealing), then high-temperature oxidation, surface morphology and element of the oxide layer were observed after oxidation testing. The result shows that the oxide layer thickness of cold-rolled annealed sheet is maximal, followed by cold rolled sheet, the pickling sheet is minimum; The surface roughness is associated with degree of oxidation; Si and Cr content increase sharply in oxide layer of 22MnB5 steel after annealing, annealing and cold rolling have little effect of Mn content in the oxide layer.

Investigation into the tribological behaviors of press hardening steels on the tailored conditions

There has been a growing demand for safety parts with tailored properties in automobile industry. However, the understanding of tribological behavior of press hardening steels (PHS) on the tailored conditions is highly inadequate. The present work aims at creating new knowledge about the tribological characteristics of PHS on the tailored conditions and bridging this existing gap. The paper proposes an improved hot drawing tribo-simulator to simulate the realistic experimental conditions industry. Investigations were carried out on the condition of different initial heating temperatures, tool temperatures, austenitizing temperatures, cooling rates and microstructures. The presented results show that the whole frictional process is divided into three stages for both coated and uncoated steels. The frictional factor changes a lot and the peak value of frictional factor occurs for serious adhesive wear. The frictional factor rises as the tool temperature and austenitizing temperature rise. The surface morphology of tools indicates that the coating adhering to tool gets thicker as the tool temperature increases. With the increase of cooling rate, the frictional factor declines firstly and then rises to some extent. Flat dies with different temperatures are used to form specimens with different microstructures, which also affects the frictional factor and wear.

Development of a Press-Hardened Steel Suitable for Thin Slab Direct Rolling Processing

The thin slab casting and direct rolling process is a hot-rolled strip production method which has maintained commercial quality steel grades as a major material in many industrial applications due to its low processing cost. Few innovative products have however been developed specifically for production by thin slab direct rolling. Press hardening or hot press forming steel grades which are now widely used to produce structural automotive steel parts requiring ultra-high strength and formability may however offer an opportunity for thin slab direct rolling-specific ultra-high strength products. In this work, a newly designed press hardening steel grade developed specifically for thin slab direct rolling processing is presented. The press hardening steel has a high nitrogen content compared with press hardening steel grades produced by conventional steelmaking routes. Boron and titanium which are key alloying additions in conventional press hardening steel such as the 22MnB5 press hardening steel grade are not utilized. Cr is added in the press hardening steel to obtain the required hardenability. The properties of the new thin slab direct rolling-specific 22MnCrN5 press hardening steel grade are reviewed. The evolution of the microstructure and mechanical properties with increasing amounts of Cr additions from 0.6 to 1.4 wt pct and the effect of the cooling rate during die-quenching were studied by means of laboratory simulations. The selection of the optimum chemical composition range for the thin slab direct rolling-specific 22MnCrN5 steel in press hardening heat treatment conditions is discussed.

Microstructural and Nanostructural FE-SEM, TEM, and STEM Investigations of High Strength Boron-Alloyed Steel Used in the Automotive Sector

Abstract More and more steel grades with martensitic microstructure are used for highly stressed structural components in the automobile branch. They are used in the field of light weight construction and for crash safety. Principally, press-hardened boron-alloyed steels with low carbon content are used here. In this study, the fine microstructure is investigated by means of FE-SEM and TEM microscopy. Possibilities but also limits of the FE-SEM investigation methodology are clearly pointed out and demonstrated presenting concrete examples. In addition to that, further microstructural investigations are performed using a modern scanning transmission electron microscope (STEM). The device allowed for switching from the TEM mode to the SEM mode and thus enabled an investigation of one and the same microstructural section in SEM and TEM conditions; first experiences are presented.

Microstructure of liquid metal embrittlement cracks on Zn-coated 22MnB5 press-hardened steel

Zn-coated press-hardened steel, used for ultrahigh-strength structural parts of passenger cars, is sensitive to liquid-metal-induced embrittlement cracking during die quenching. The microstructure of the intergranular liquid-metal-induced cracks was analyzed in detail. A new model for crack formation is proposed. Intergranular cracking is due to the grain boundary penetration of a liquid Zn alloy phase along a crack tip propagating on prior austenite grain boundaries weakened by the Zn diffusion-mitigated phase transformation to ferrite.

Microstructural Evolution of the 55 Wt Pct Al-Zn Coating During Press Hardening

Press hardening is increasingly being used to produce ultra-high strength steel parts for passenger cars. Al-Si, Zn, and Zn-alloy coatings have been used to provide corrosion protection to press hardening steel grades. The use of coatings has drawbacks such as coating delamination or liquid metal-induced embrittlement. In the present work, the microstructural evolution of Al-Zn coating during press hardening was studied. The 55 wt pct Al-Zn coating can in principle provide both Al barrier protection and Zn cathodic protection to press hardened steel. During the heat treatment associated with the press hardening, the 55 wt pct Al-Zn alloy coating is converted to an intermetallic surface layer of Fe2Al5 and a FeAl intermetallic diffusion layer. The Zn is separated from both intermetallic compounds and accumulates at grain boundaries and at the surface. This Zn separation process is beneficial in terms of providing cathodic protection to Al-Zn coated press hardening steel.

Application of Quenching and Partitioning (Q&P) Processing to Press Hardening Steel

Press hardening steel (PHS) has been increasingly used for the manufacture of structural automotive parts in recent years. One of the most critical characteristics of PHS is a low residual ductility related to a martensitic microstructure. The present work proposes the application of quenching and partitioning (Q&P) processing to improve the ductility of PHS. Q&P processing was applied to a Si- and Cr-added Q&P-compatible PHS, leading to a press hardened microstructure consisting of a tempered martensite matrix containing carbide-free bainite and retained austenite. The simultaneous addition of Si and Cr was used to increase the retained austenite fraction in the Q&P-compatible PHS. The Q&P processing of the PHS resulted in a high volume fraction of C-enriched retained austenite, and excellent mechanical properties. After a quench at 543 K (270 °C) and a partition treatment at 673 K (400 °C), the PHS microstructure contained a high volume fraction of retained austenite and a total elongation (TE) of 17 pct was achieved. The yield strength (YS) and the tensile strength were 1098 and 1320 MPa, respectively. The considerable improvement of the ductility of the Q&P-compatible PHS should lead to an improved in-service ductility beneficial to the passive safety of vehicle passengers.

Innovative and Highly Productive Joining Technologies for Multi-Material Lightweight Car Body Structures

Driven by increasing costs for energy and raw material and especially by the European CO2-emission laws, automotive industry faces the challenge to develop more lightweight and at the same time still rigid and crash-stable car bodies, that are affordable for large-scale production. The implementation of weight-reduced constructions depends not only on the availability of lightweight materials and related forming technologies, but also on cost-efficient and reliable joining technologies suitable for multi-material design. This article discusses the challenges and requirements for these technologies, based on the example of joining aluminium with press-hardened boron steels, what is considered as a very important material combination for affordable future lightweight mobility. Besides a presentation of recent developments for extending the process limits of conventional mechanical joining methods, new promising technologies such as resistance element welding are introduced. In addition, the performance, advantages, and disadvantages of the presented technologies are compared and discussed.

Determination of hydrogen input in welded joints of press-hardened 22MnB5 steel

Over the last decades, press hardening or hot forming has become all pervasive for automotive body-in-white design. Press-hardened boron-microalloyed steels are widely used in modern body structure for high safety standard and lightweight achievement. Considering the welding of press-hardened components, some special considerations have to be taken into account compared to conventional deep-drawing steel grades. The welding process results in a significant hardness drop in the heat-affected zone (HAZ) which may lead to a change in failure mode or premature failure. Furthermore, the mechanical properties of the welded joints are influenced by the weld metal hydrogen content. Hydrogen pickup of press-hardened steel may result from the heat treatment or the welding process. Due to the martensitic microstructure and the high strength level, a critical hydrogen content may cause hydrogen embrittlement or hydrogen-assisted cold cracking (HACC). This paper contributes to the determination of the diffusible hydrogen content in resistance spot-welded and gas metal arc-welded joints of press-hardened boron-microalloyed steel. It also promotes the understanding of the effect of diffusible hydrogen on the mechanical properties of those joints. Hydrogen was deliberately introduced during welding by wetting of the sheet surface with hydrogenated fluids. The hydrogen content in the weld metal was quantified by thermal desorption mass spectrometry (TDMS) technique. Finally, the influence on the mechanical properties of the welded joints was determined based on a simple component test. A GMA-welded component was set under a constant static load and evaluated for delayed hydrogen-assisted cracking.

Effect of Al Content on the Microstructure of Hot-Depped Zinc Coating on Hot Stamping Steel

Different galvanized coatings on hot stamping steel was obtained by using hot dip galvanizing simulation experiment device dipping in zinc baths with four different Al contents. The effect of Al content on the microstructure of galvanized was investigated using SEM/EDS and glow discharge optical emission spectrometer(GDOES).The results show that press-hardened steel has good reactive wetting in different Al content zinc bath. The Al content(mass fraction) in zinc bath has a significant influence on coating/steel interface composition, morphology and light property. Enrichment of Mn, Si elements occurred in the steel surface during bright annealing process, and the Mn-Si oxides react(redox reaction) with Al in zinc bath. For 0.11% dissolved Al bath, continuous ζ phases formed at the coating/steel interface. For 0.17% dissolved Al bath, the coating/steel interface consist of ζ phases and Fe- Al phases. A continuous Fe2 Al5 inhibition layer achieved in baths with Al content 0.25% and 0.43%, and the morphology of interface layer became coarser when Al content in zinc bath was 0.25%. In addition, with the increase of Al content in zinc bath, the light property of zinc coatings gradually became worse.

Two-beam Laser Brazing of Thin Sheet Steel for Automotive Industry Using Cu-base Filler Material

This work shows the potential of two-beam laser brazing for joining both Zn-coated steel and 22MnB5. Brazing of Zn-coated steel sheets using Cu-Si filler wire is already state of the art in car manufacturing. New press-hardened steels like 22MnB5 are more and more used in automotive industry, offering high potential to save costs and improve structural properties (reduced weight / higher stiffness). However, for joining of these ultra-high strength steels investigations are mandatory. In this paper, a novel approach using a two-beam laser brazing process and Cu-base filler material is presented. The use of Cu-base filler material leads to a reduced heat input, compared to currently applied welding processes, which may result in benefits concerning distortion, post processing and tensile strength of the joint. Reliable processing at desired high speeds is attained by means of laser-preheating. High feed rates prevent significant diffusion of copper into the base material.

Surface Oxide Formation during Rapid Heating of Zn-coated Press Hardening Steel

During the conventional die-quenching processing of a galvanized PHS steel, a thick ZnO layer is formed at the surface. When the heating rate is increased, the oxide at the surface is a thin Al2O3 layer. This remarkable change in surface oxide during rapid heating is due to the partial melting of the coating instead of the solidification of the coating and the formation of Fe–Zn intermetallics. In the present study, the characterization of the surface oxide formation at different heating rates is presented.

Investigations on the Manufacturability of Thin Press Hardened Steel Components

In the recent years, the automotive industry is focusing on the reduction of the vehicles weight, so as to minimize the CO2 emissions, while improving the crashworthiness levels. In order to achieve a further body-in-white weight reduction and exploit the potential for lightweight construction of the hot-dip aluminized press hardening manganese-boron steel, further research on the design and development of a process chain for the production of thin hot stamped components is carried out. For this purpose the impact of different blank thickness-dependent process parameters on the component properties is determined through both simulation and experimental analysis. The temperature profile of the heating process is determined for different blank thicknesses and the development of the diffusion layer between the AlSi-coating and the base material is examined. With respect to the transfer process of the thin austenitized blanks from the roller hearth furnace into the forming tool, the time slot is determined via the analysis of the corresponding time-temperature curves. In addition, different simulation models are developed, aiming at the validation and optimization of the transfer process. In order to further investigate the manufacturability of thin press hardened components, the simulation of a hot stamping process is carried out. The developed models are verified by an optical 3D forming analysis of the hot stamped components. As a conclusion of the current investigations, thin hot stamped components can be manufactured only under the precondition that the process is optimally designed and the process chain properly adjusted.

Effect of strip entry temperature on the formation of interfacial layer during hot-dip galvanizing of press-hardened steel

Strip entry temperature is one of the important factors that affect alloying reactions at the steel/coating interface. This study focuses on the influence of strip entry temperature on phase formation at the press-hardened steel (PH) substrate/coating interface in zinc baths with 0.12wt.% and 0.2wt.% Al content respectively. The microstructure of the interfacial layer was characterized by Scanning Electron Microscope (SEM) and Glow Discharge Optical Emission Spectrometer (GDOES). It was determined that the microstructure components of steel/coating interface are significantly influenced by the strip entry temperature within 440–480°C. In the zinc bath with 0.12wt.% Al, the interfacial layer consists of mainly ζ phase (FeZn13), and the size of the ζ phase increases with the strip temperature. However, the amount of ζ phases and the Al content of the interfacial layer decreases with the strip temperature. Meanwhile, in the zinc bath with 0.2wt.% Al, a continuous inhibition layer Fe2Al5 was formed on the steel surface. The Al enrichment at the interfacial layer increased with increasing strip temperature, accompanied with which is the coarser inhibition layer morphology.

Analysis of Laser and Resistance Spot Weldments on Press-Hardened Steel

For the manufacture of safe, lightweight vehicles, the demand for ultra-high-strength steel in the automotive industry is increasing. Although transformation-induced plasticity (TRIP) and dual-phase (DP) steels have a strength of under 1 GPa, boron-alloyed steel produced using the hot press forming process has a strength of more than 1500 MPa. Laser and resistance spot welding processes are used to join press-hardened steel, but the characteristics of the resulting weldments are not yet fully understood. In this study, the thermal cycles for both welding processes were investigated using finite element (FE) analysis. Resistance spot welding was analyzed using a combination of thermal, electric, and mechanical models, whereas the thermal behavior of laser welding was predicted using only a thermal model. The calculated bead shapes were compared with experimentally measured ones to validate the simulation models. The mechanical and metallurgical characteristics of the weldments were explained using the thermal history of each welding process.