Ultra-high Strength Steel Parts Research Articles

Development of a digital material shadow for the press hardening route of medium manganese steel

Press-hardened ultra-high strength steel parts are widely used in the automotive sector for their lightweight and safety advantages. Medium-Manganese Steels (MMnS) are being explored as an alternative to boron-manganese steels due to their high strength and ductility after quenching, achieved at lower annealing temperatures thus reducing energy usage and carbon emissions. However, industrial adoption of MMnS is hindered by challenging processing requirements, e.g. in cold-rolling and press hardening.To expedite and improve the process development, data-driven decisions based on process parameters hold promise. Establishing a link between process data and the final produced part necessitates the development of a framework for a Digital Material Shadow (DMS).This paper investigates the development of a DMS framework for the cold rolling and press hardening process chain. In conjunction with conventional data acquisition methods employed for cold rolling, novel data acquisition techniques are introduced specifically tailored for press hardening, ensuring the comprehensive availability of relevant data. Moreover, a data pipeline is implemented to enable automatic processing, visualization, and analysis of process data. To facilitate seamless data linkage across processes in the DMS, an ID-system is introduced. Finally, the developed framework’s validity is demonstrated by creating a DMS for press-hardened MMnS parts, showcasing its potential for practical applications.

Investigation of 300M ultra-high-strength steel deposited by wire-based gas metal arc additive manufacturing

300 M ultra-high-strength steel (UHSS) is widely used to produce landing gear components for aircraft. The conventional manufacturing route for these components involves extensive machining and significant material wastage. Here, the application of wire-based gas metal arc additive manufacturing to produce 300 M UHSS parts was investigated. In particular, the influence of torch shielding atmosphere on the process stability and material performance of 300 M UHSS was investigated. The shielding gases used for comparison are pure Ar, Ar with 2.5% CO2, Ar with 8% CO2, Ar with 20% CO2, and Ar with 2% CO2 and 38% He. It was found that the arc length decreased, the transfer mode changed from spray to droplet mode, and spattering became more severe as the CO2 proportion increased. Additionally, replacing Ar with He led to a broader arc core, and a slightly shorter arc length and maintained a spray transfer, which decreased spatter. The wall surface roughness followed the trend in spatter, becoming worse with the increasing CO2 proportion, and better with He addition. Adding CO2 and He in pure Ar significantly increased the bead and wall width. The microstructure and mechanical properties exhibited a strong location dependence in the as-built state, with fresh martensite and higher strength in the top region, and tempered martensite and better ductility in the reheated bulk. Generally, torch shielding gas composition appeared to have no significant effect on the microstructure evolution. This study provides a reference for the subsequent application of gas metal arc additive manufacturing to aircraft landing gear mass production to achieve a high deposition rate and process stability simultaneously.

Fracture Analysis of Ultrahigh-Strength Steel Based on Split Hopkinson Pressure Bar Test

Impact loading is an important cause of fracture failure of ultrahigh-strength steel parts during service. Revealing the fracture mechanism of ultrahigh-strength steel under impact loading has important reference significance for the material preparation, part design, and manufacturing of such steel. Based on the split Hopkinson pressure bar (SHPB) test, the mechanical response characteristics of 45CrNiMoVA steel under impact loading were analyzed, and the true stress–true strain curves under a high strain rate (103 s−1) were obtained. It was found that under the simultaneous action of forward and tangential loading forces, a severe plastic deformation layer with a thickness of 20–30 μm was generated in the near impact-loading end face, which is the main cause for crack initiation and propagation. Under the condition of a high strain rate, the plastic flow stress of 45CrNiMoVA steel was characterized by the equilibrium of strain hardening and strain softening, and its impact fracture toughness decreased by 43.6%, resulting in increased quasi-cleavage fracture. Hence, severe surface plastic deformation during 45CrNiMoVA steel machining should be avoided, as it may lead to early failure.

Corner Strengthening by Local Thickening and Ausforming Using Planar Compression in Hot Stamping of Ultra-High Strength Steel Parts

Hot-stamped products are widely used for the body-in-white of an automobile as they are lightweight and improve crashworthiness. A hot-stamping process using planar compression was developed to strengthen corners of ultra-high strength parts by local thickening and hardening. In this process, the corners are thickened by compressing the blank in the planar direction with the upper and lower dies while blocking the movement of both edges with stoppers in the latter stage of forming. Thickening of the corners largely heightens the strength of the formed parts. Not only the thickness but also the hardness of the corner was increased by large plastic deformation and die quenching. For a hot hat-shaped part, a 30% increase in thickness and a 530 HV20 hardness around the corners were attained. The bending rigidity and strength of the formed parts thickened by 30% in the corners increased by 25% and 20%, respectively. In addition, the improvements of the part shape accuracy and the sidewall quenchability were obtained.

Tailored tempering without die heating in hot stamping of ultra-high strength steel parts

Three tailored tempering processes without die heating were developed to manufacture tailored parts having high strength and high ductility zones. In a 1st forming stage, only the high strength zone was formed with the partial dies in order to decrease the cooling rate in the high ductility zone by no contact with the dies, and then both zones were formed with the sufficiently wide dies in a 2nd forming stage. Two cooling stages were introduced to prevent the martensite transformation of the high ductility zone. The 2nd cooling stage was omitted by partial forced cooling in the 1st stage having a higher cooling rate. The 2nd forming stage was further omitted by keeping high temperature in the high strength zone and by slowly cooling the high ductility zone in the 1st cooling stage. A tailored part having hardnesses of about 470 and 230 HV1 in the high strength and high ductility zones, respectively was produced, and the cycle time was 15 s, 4 shots per minute.

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.

Effects of Hot Trimming Conditions on Sheared-edge Quality and Mechanical Properties in Hot Stamping of Ultra-high Strength Steel Parts

The effects of hot trimming conditions on the sheared-edge quality and mechanical properties in hot stamping of ultra-high strength steel parts were examined. In hot stamping, the temperature of the parts rapidly drops during stamping, and the subsequent trimming operation is greatly influenced by the temperature. The sheared-edge quality was high above the temperature at which the martensitic transformation starts for a comparatively small clearance. The trimming temperature had a great influence on the occurrence of delayed fracture, whereas the clearance had no influence. In hot trimming, delayed fracture was prevented by decreasing the tensile residual stress and hardness near the sheared edge. The static and fatigue strengths of the parts are reduced by delayed cracks and hydrogen embrittlement, whereas the fatigue limit is unaffected. In hot trimming of thin sheets, delayed fracture tends to occur as a result of a large temperature drop.

Improvement of formability using partial cooling during transfer in hot stamping of ultra-high strength steel parts

The formability in hot stamping was improved by partial cooling of a heated sheet during the transfer from a furnace to dies. Using the transfer device having grips on the heated sheet, the temperature of the portions undergoing large deformation during forming is decreased to increase the flow stress. The cooling pressure and the contacting width in transfer device were changed to control the temperature distribution in the sheet. In W-bending, a temperature difference of 250 °C and a flow stress difference of 300 MPa were obtained, and the depth of the W-bent sheet was increased by 26.1%.

2-stage progressive-die hot stamping of ultra-high strength steel parts using resistance heating

Abstract A 2-stage progressive-die hot stamping process using partial resistance heating was developed to produce a small-sized ultra-high strength steel parts without additional heat treatments. In the 1st stage, a uncoiled long plate was successively heated by partial resistance heating, and then is simultaneously blanked and punched. The blank is inserted in the blanked plate to be conveyed. In the 2nd stage, the blank is bent and die-quenched. The resistance heating and die quenching operations are synchronised to double the productivity. An automobile seat belt buckle having 1500 MPa in tensile strength was successfully produced without springback.

Hot stamping of ultra-high strength steel parts

The demand for new processes to produce high strength parts, under appropriate cost and productivity, has grown with weight reduction and crash safety improvements in automobile design. The hot stamping processes of quenchable steel sheets potentially offer not only small forming load and high formability, but also high strength and no springback by die quenching. This paper aims to provide an overview of the state-of-the-art in such hot stamping processes, including quenchability, formability, heating and cooling approaches and lubrication. The paper also includes a description of the mechanism of formability and quenching, tailoring, analysis of hot stamping processes and applicability.

Reduction in length of transient region in successive forging of tailored blank for hot stamping of ultra-high strength steel parts

Abstract Tailored blanks having a thickness distribution without welds were produced by successive forging, and the length of the transient region in which the thickness gradually decreases was reduced by an infinitesimal feed. The effect of the infinitesimal feed on the reduction of the length of the transient region was evaluated. A tailored blank having a desired thickness was successive-forged, and was hot-stamped into a roof bow. The produced roof bow had sufficient hardness and high paintability.

Inclusion of hot stamping operations in progressive-die plate forging of tailored high strength gear part

Hot stamping operations of a quenchable steel sheet were included in progressive-die plate forging to produce a tailored high strength gear part without additional heat treatments. As process design for the inclusion, the former stages in the progressive-die operations for punching and trimming were cold stamping, and subsequently, resistance heating, blanking, forging and die quenching were performed. The plate was partially resistance-heated with a pair of electrodes to produce a tailored gear part having high strength teeth and a ductile remainder. The tooth portion was uniformly heated around 1050 °C in the transversal current direction, and a gear part having teeth having 520 HV2 in hardness was produced. The quality of the teeth was improved by hot semi-blanking with local compression and cold scrap removing. It is found that the combined process of plate forging and hot stamping is useful to produce ultra-high strength steel parts.

Deep Drawability and Bendability in Hot Stamping of Ultra-High Strength Steel Parts

The deep drawability and bendability in hot stamping of ultra-high strength steel parts were examined. Although the cold drawability is greatly influenced by the blank shape, the limiting drawing depths for the square and circular blanks were equal for hot stamping because of small flow stress. In hot hat-shaped bending using draw-and form-type tools, the effect of the blankholder force generated with the draw-type tools on the springback was small, and the seizure for the form-type tools was smaller than that of the draw-type tools. Since both edges in contact with the electrodes are not heated for resistance heating, cracks were caused at the edges for resistance heating in the transversal directions in hot stamping of an S-rail with form-type tools, and thus it is required to control deformation of the non-heating zones.

Hot semi-punching and cold scrap removing processes for hot stamping of ultra-high strength steel parts

Hot semi-punching and cold scrap removing processes for hot stamping of ultra-high strength steel parts were developed to eliminate laser cutting having low productivity after hot stamping. A quenchable steel sheet is semi-punched without separation of punching scraps during hot stamping, and subsequently, the scraps are removed from the hot-stamped part at room temperature. For hot semi-punching, additional channels for taking the punching scraps out of dies are not required. Minimum remainder without separation of punching scraps and no clearance between the die and punch were optimal for the hot semi-punching process. The hot punching and cold removing loads of the quenched sheet were considerably smaller than the cold punching load, the quality of the hole edge was high and the delayed fracture around the sheared edge was prevented. Hot semi-punching and cold scrap removal were applied to a hot hat-shaped bending process.

Improvements in productivity and formability by water and die quenching in hot stamping of ultra-high strength steel parts

To improve the productivity in hot stamping of ultra-high strength steel parts, the parts are quenched not only with dies but also in water during holding at the bottom dead centre. Since the cooling speed for water quenching is higher than that for die quenching, the hold time for hardening is reduced. In hot stamping, water is kept in a lower die to increase the cooling rate during die quenching. In addition, local thinning around the punch corner was prevented by water and die quenching, and thus the drawablity increases.

1-Shot hot stamping of ultra-high strength steel parts consisting of resistance heating, forming, shearing and die quenching

A 1-shot hot stamping process consisting of resistance heating, forming, shearing and die quenching was developed to produce small- and medium-size ultra-high strength steel parts. A rectangular sheet was resistance-heated to obtain a uniform distribution of temperature, and just after the end of heating, a sequence of forming, shearing and die quenching was performed by one shot to prevent the drop in temperature. An ultra-high strength steel spur gear having a hardness of 540 HV2 was produced by 1-shot hot stamping composed of heating, blanking and die quenching. The rollover was improved by partial compression of the blanked gear. An ultra-high strength stainless steel part having a hardness of 580 HV2 was produced by 1-shot hot stamping consisting of the heating, bending, shearing and die quenching, and no springback and quenching distortion of the produced part were observed by holding at the bottom dead centre of the press. An operation for thickening the edge of the punched hole was included in 1-shot hot stamping to improve the strength of a product.

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.

Experimental heat transfer coefficient measurements during hot forming die quenching of boron steel at high temperatures

Hot forming die quenching is used by the automotive industry to produce ultra high strength steel parts with a high strength-to-weight ratio. Crash-resistant structural parts with distributed properties can be obtained by controlling the local cooling rate during the quenching step using heated dies. This procedure requires detailed spatial knowledge of the heat transfer coefficient at the blank/die interface. Hot stamping experiments were conducted on Usibor® 1500P boron steel blanks to investigate how pressure and blank and die temperatures influenced the heat transfer coefficient, which was inferred using inverse heat conduction analysis. The heat transfer coefficient was found to vary throughout the experiment with deformation of the surface roughness peaks and evolution of the blank and die temperatures. Whereas the heat transfer coefficient at the beginning of the stamping process increases with the initial die temperature, it converges to a value that depends only on applied pressure. Moreover, the experimental heat transfer coefficient for zero pressure was found to match the air gap conductance predicted by semi-empirical models, but the pressure-dependent component was lower than the model-predicted solid contact conductance.

Hot Semi-punching of Quenchable Steel Sheet

Hot semi-punching of a quenchable steel sheet was carried out to eliminate laser cutting conventionally used for hot stamping of ultra-high strength steel parts. A quenchable steel sheet is semi-punched without separation of punching scraps during hot stamping, and subsequently, the scraps are removed from the hot-stamped part at room temperature. Additional channels for taking punching scraps out of dies are not required. Minimum remainder without detachment of punching scraps and no clearance between the die and punch were optimal for the hot semi-punching process. The hot punching and cold removing loads of the quenched sheet were considerably smaller than the cold punching load, the quality of the hole edge was high and the delayed fracture around the sheared edge was prevented.

超高強度鋼部材のホットスタンピングにおけるせん断加工

超高強度鋼部材のホットスタンピングにおけるせん断加工