Fracture morphologies of a hot stamped steel and comparisons with several sheet metals

Hot stamping has been widely used in car industry to produce safety components.Most existing researches focused on the stamping and quenching process,but less on the mechanical properties of stamped parts.The fracture behaviors of hot stamped boron steel B1500 HS have been studied,and other four commonly used sheet metals with different strengths,including Q235,TRIP780,QP980 and MS1300,were also introduced for comparison.Both uniaxial tests and mechanical trimming tests were performed,and the fracture surfaces under different stress states were observed and discussed.The SEM observations showed that the fracture models are closely related to the stress states,i.e.,the tensile surfaces have ductile rupture characters while the trimming surfaces have brittle rupture characters.Compared with other steels,the quenched boron steel has smaller dimple size accompanied by shear planes in the tensile surface,and has smaller burnish zone in the trimming surface,and its cutting surface with‘S'like shape is also very different with others.Furthermore,two fitted empirical models were derived to describe the quantitative correlations between the average dimple diameter and the steel strength and between the percentage of burnish zone and the steel strength.     

Metallurgy of continuously annealed high strength TRIP steel sheet

The effects of heat‐treatment conditions on mechanical properties are comprehensively investigated to optimise the industrial process of the 590 MPa grade TRIP steel sheet with the metallurgical understanding. The substantial effect of the thermal conditions are first clarified by laboratory investigation, which includes the effects of annealing conditions, cooling conditions from intercritical temperature to austempering temperature and austempering conditions. The results indicate that the optimum annealing temperature is between 800 and 850 °C and the mechanical properties are hardly influenced by the annealing time between 30 and 120 s at an annealing temperature of 825 °C. It is also suggested that the optimum quenching rate is 45 °C/s to obtain the stable properties of the products and the optimum austempering conditions are 425 °C with over 300 s in case of a constant temperature austempering. Based on the laboratory investigation, mill trial is performed using the NKK No.4‐CAL in Fukuyama works. The heat treatment conditions are intentionally varied to examine minutely the stability of the production. The mechanical properties are sensitive to the austempering start temperature, when the austempering temperature is gradually decreased during austempering in the industrial conditions for the stable operation without meanders. Excellent mechanical properties can be obtained by controlling the austempering start temperature between 445 and 460 °C. On the contrary, the properties deteriorate in case of the austempering start temperature over 470 °C although the amount of retained austenite is the same or slightly larger than the material which exhibits excellent properties. This is because the retained austenite is less stable in the high‐temperature austempered material caused by less bainite transformation.     

温成形中锰钢的组织性能评价

 热成形零件已在汽车安全件上广泛应用,为了进一步提升零件碰撞安全性、提高表面质量、降低成本,基于中锰钢提出了一种降低加热温度的热成形技术,通过将完全奥氏体化的中锰钢在模具中淬火成马氏体组织获得超高强度力学性能,与22MnB5钢热成形相比,在获得1 500 MPa抗拉强度时,中锰钢温成形的加热温度可降低150 ℃以上,断后伸长率提高30%以上,同时提高零件的表面质量。综述并评价了中锰钢经温成形后的微观组织与力学性能以及冷弯性能、成形性能、电阻点焊等工艺性能,并与22MnB5钢热成形进行了系统地比较,体现出温成形中锰钢节能环保、提高碰撞安全性的技术优势。     

Analysis of Microstructure and Mechanical Properties of Different Hot Stamped B‐bearing Steels

Hot stamping is a technique to produce ultra high strength automobile components. The common material used in hot stamping process is coated and/or uncoated 22MnB5 boron alloyed steel. Ferritic‐pearlitic microstructure in as‐delivered sheets is transformed to fully lath martensitic after hot stamping. In the present research, hot stamping under water or nitrogen cooling media was investigated using different boron alloyed steel grades. Microstructural analyses, linear and surface hardness profiling as well as tensile tests of hot stamped samples were performed. Various microstructures of fully bainitic and/or fully martensitic were produced. The resulting microstructures provided yield strengths of 650–1370 MPa and tensile strengths of 850–2000 MPa. There is an optimum carbon equivalent content for which the highest formability index value, UTS × A₂₅, is achieved. Using a nitrogen cooled punch resulted in higher yield strength without significant changes in ultimate tensile strength. It is concluded that a wide range of B‐bearing steels having an extended carbon equivalent range with an acceptable formability index value can be used by increasing the cooling rate in the die assembly.     

Diamond-hard alloy macrocomposite material: Development and application

A new diamond-hard alloy macrocomposite material consisting of diamond grits (0.5–0.8 mm or more in size) and a WC-Co matrix has been developed. The material is characterized by high mechanical properties of the matrix (the same as for WC-Co monolithic hard alloys) while diamonds remain completely intact (no graphitization or dissolution in cobalt melt). This process does not require superhigh pressures. Hard-alloy samples sintered beforehand in conditions that ensure the maximum mechanical properties (1450–1500°C, 30–60 min holding) are used as initial materials. Hollows (cells) or ditches of specific sizes (depth, width) are made in these samples with mechanical or electrophysical methods, and then diamond crystals (grits) commensurate with the hollows, cells, or ditches are placed in them. Vacuum infiltration (brazing) at moderate temperatures (900–1150°C) with adhesion-active alloys (or metallized diamonds) is the final stage in the formation of the composite. Therefore, strong adhesion-mechanical fixation of diamond grits in the hard-alloy matrix is ensured. The new material is efficiently used in diamond bits.     

Metallurgy,Microstructure and Properties of a New Nitride Strengthened Nickel‐Chromium Superalloy

The nitrogen content of Ni‐base superalloys for high temperature service is generally kept below about 0.05 wt.‐% to avoid detrimental precipitation of nitrides. These nitrides are said to have a harmful influence on mechanical properties and workability of these alloys. However, some recent studies and research conducted with nitrogen strengthening of Ni‐Cr‐alloys have resulted in an alloy with excellent physical and mechanical properties. The applied PESR (Pressurized Electro‐Slag Remelting)‐Technology provided up to 1.0 wt‐% nitrogen in a NiCr7030‐alloy.The homogeneously distributed nitrides prevent the alloy from excessive grain growth thus providing stable mechanical properties, i.e. impact toughness even after long term exposure. The new alloy easily exceeds R_m= 850 MPa at room temperature and 600 MPa at 600 °C as relevant design values. This paper introduces this new alloy with its very special metallurgy, microstructure, and its physical and mechanical properties.     

BZ37汽车车轮用冷冲压钢板的开发

BZ37热轧钢板主要用于冷冲压加工汽车车轮。该钢要求抗拉强度波动范围窄,塑性指标高,冷冲压加工性能良好,因此要严格控制影响钢板强度、塑性、冲压性能的元素含量,生产过程中要求纯净钢质．组织和性能均匀,保证钢板的冲成率。     

Strength and ductile-phase toughening in the two-phase Nb/Nb5Si3 alloys

The effect of heat treatment on the mechanical properties of Nb-Nb₅-Si₃ two-phase alloys having compositions Nb-10 and 16 pct Si (compositions quoted in atomic percent) has been investigated. This includes an evaluation of the strength, ductility, and toughness of as-cast and hot-extruded product forms. The two phases are thermochemically stable up to ∼1670 °C, exhibit little coarsening up to 1500 °C, and are amenable to microstructural variations, which include changes in morphology and size. The measured mechanical properties and fractographic analysis indicate that in the extruded condition, the terminal Nb phase can provide significant toughening of the intermetallic Nb₅Si₃ matrix by plastic-stretching, interface-debonding, and crack-bridging mechanisms. It has been further shown that in these alloys, a high level of strength is retained up to 1400 °C.     

Prediction and Control of Wrinkle and Fracture for Stamping Regular Polygonal Box

Based on the deformation characteristic of regular polygonal box stamped parts and the superfluous triangle material wrinkle model, the criterion of regular polygonal box stamped parts without wrinkle was deduced and used to predict and control the wrinkle limit. According to the fracture model, the criterion of regular polygonal box stamped parts without fracture was deduced and used to predict and control the fracture limit. Combining the criterion for stamping without wrinkle with that without fracture, the stamping criterion of regular polygonal box stamped parts was obtained to predict and control the stamping limit. Taken the stainless steel 0Cr18Ni9 (SUS304) sheet and the square box stamped part as examples, the limit diagram was given to predict and control the wrinkle, fracture and stamping limits. It is suitable for the deep drawing without flange, the deep drawing and stretching combined forming with flange and the rigid punch stretching of plane blank. The limit deep-drawing coefficient and the minimum deep-drawing coefficient can be determined, and the appropriate BHF (blank holder force) and the deep-drawing force can be chosen. These provide a reference for the technology planning, the die and mold design and the equipment determination, and a new criterion evaluating sheet stamping formability, which predicts and controls the stamping process, can be applied to the deep drawing under constant or variable BHF conditions.     

中锰钢温成形技术探讨

使用Gleeble-3800热模拟试验机研究了中锰钢(温成形钢)和22MnB5钢(热成形钢)的微观组织、力学性能和高温拉伸性能.结果表明:与22MnB5钢相比,在获得1 500 MPa级的抗拉强度时,中锰钢的加热温度可从950℃降低到800℃,钢的组织明显细化并且没有发生表面脱碳,断后伸长率从7.5％提高到10％.高温拉伸试验结果表明:中锰钢比22MnB5钢具有更高的延伸率和硬化指数,可以减小成形过程中局部减薄过高导致的样件破裂.     

热成形工艺参数对AC1500HS钢板性能的影响

以鞍钢热成形用钢ACl500HS为研究对象．采用热模拟试验机测定了ACl500HS的CCT曲线并模拟了热成形工艺,确定了ACl500HS钢热成形工艺参数：对钢板进行热成形汽车零件试制,结果表明,该钢热成形性良好,零件尺寸精度达到要求。抗拉强度达到1500MPa以上,满足汽车零件装车要求。     

The Influence of Precooling on Liquid Metal Embrittlement and Microstructure Properties of Galvanized 22MnB5 Tubes

To address the issue of liquid metal embrittlement (LME) susceptibility in galvanized 22MnB5 steel during the hot stamping process, the material's performance is affected. This study proposes a method combining precooling and tube hydroforging to produce galvanized tubular hot stamping parts. The primary workflow comprises four distinct stages: the heating phase, thermal retention phase, precooling phase, and the tube hydroforging phase. Notably, the precooling stage employs two distinct approaches: air precooling and boiling water precooling. The composition and morphology of the zinc coating and the microstructure and mechanical properties of the 22MnB5 steel are investigated using two precooling methods at different hydroforging temperatures. The results show that when the initial hydroforging temperature is below 800 °C, the precooling combined with the tube hydroforging process eliminates LME. With increasing precooling time, the oxidation reaction forms ZnO and Fe₂O₃ on the coating. By comparing the composition, morphology, and mechanical properties of the coatings at hydroforging temperatures of 800 °C and 500 °C, it is concluded that the boiling water precooling aligns more effectively with the requirements of industrial production, with the optimal forming temperature being within the range of 750–800 °C.     

The effect of thermal exposure on the mechanical properties of the directionally solidified superalloy TRW-NASA VIA

The nickel-base superalloy TRW-NASA VIA was studied in the directionally solidified (DS) condition utilizing metallographic and residue analysis techniques in conjunction with mechanical property tests to determine the effect of thermal exposure on the microstructure and mechanical properties. Exposure conditions of 1000 h at temperatures from 1500 to 1900°F (816 to 1038°C) were investigated. Four minor phases (two varieties of MC, MgC and M₃B₂) plus gamma-prime were identified in the gamma matrix of the DS material. Significant variations were observed to occur in the mechanical properties with thermal exposure. Microstructural evaluation indicated that as in the equiaxed condition these variations were due principally to gamma-prime agglomeration or ripening. Comparison of the findings from the DS and the equiaxed process conditions indicated several factors which contributed to the property enhancement observed in the DS condition. These included the virtual elimination of the transverse grain boundaries by the DS process which improved the 1400 (760°C) and 1800°F (982°C) properties, the heterogeneous distribution of the blocky and the spherical-like gamma-prime which primarily improved the room temperature and the 1400°F (760°C) strength properties, the generally larger gamma-prime size as well as the intragranular columnar form which improved the ductility properties and the longitudinal grain boundaries containing the stabilized Hf enriched MC and the MgC carbides which in conjunction with the intergranular gamma-prime formation also improved the ductility properties.     

一种Si-Mn系超高强度钢板的热冲压成形试验研究

 采用弯曲件热冲压成形试验研究了板料加热温度、保温时间及移送时间等工艺参数对一种Si-Mn系超高强度钢板热成形零件的力学性能及微观组织的影响规律。结果表明,通过控制热成形工艺参数,在所设计的模具上可实现Si-Mn系超高强度钢板热成形零件的有效淬火,在合适的工艺参数下,可获得细小均匀的马氏体组织,从而获得抗拉强度1700MPa以上,伸长率10%以上的性能,达到原始板料抗拉强度的3倍左右,并明显高于传统的Mn-B系超高强度钢板。     

Investigation of Microstructure and Mechanical Properties of A335 P11 Main Steam Pipe in Stesen Janaelektrik Jambatan Connaught Power Plant,Malaysia

This study aimed at examining the mechanical property, hardness and microstructure of the main steam pipe in the waste heat boiler electric power plant to prevent any possible failure and determine its lifetime, reliability and efficiency. A used alloy steel main steam pipe with ASTM standard grade (ASTM A335 P11) and standard chemical composition (1.25Cr–0.5Mo) was taken from the waste heat boiler electric power plant, Tenaga Nasional Berhad, Connaught Bridge Power Station, Klang Selangor, Malaysia. Specimen from each critical point P1 (incoming from WHB1B) at 485 °C and 39 bar, P2 (incoming from WHB1A) at 485 °C and 39 bar, P3 (joint of WHB1A & WHB1B) at 110 °C and 65 bar and P4 (outgoing to ST1C) at 212 °C and 4 bar were cut for sample preparations. Microstrural spectra of all the samples revealed that all the points were being transformed from pearlite to spheroidite and critical point P3 (at 110 °C and 65 bar) contained cavities in its grain boundaries. Also, the results showed that the P3 (at 110 °C and 65 bar) had the highest ductility (34.17), Poisson’s ratio (0.2683), lowest yield strength (286.83 MPa) and lowest ultimate tensile strength (423.26 MPa) against other locations. Other critical points; P1 (at 485 °C and 39 bar), P2 (at 485 °C and 39 bar) and P4 (at 212 °C and 4 bar) showed the average of tensile properties which is slightly low compared to new pipe material due to stress and high temperature exposed in main steam pipe for a long period time with little increase in ductility. Hardness test showed that the critical point P3 (at 110 °C and 65 bar) possessed the lowest hardness strength value (140.7 HV) due to complex state of stress which assisted in modification of microstructure prematurely in comparison with other locations.     

The influence of microstructure on fracture of drawn tungsten wire

The microstructures and longitudinal fracture resistances of 0.635 mm diam lamp-doped and undoped tungsten wire were examined in the as-drawn condition and after anealing at temperatures between 600 and 1500°C. A variety of experimental techniques were employed, including Auger Electron Spectroscopy, Scanning Electron Microscopy, Transmission Electron Microscopy and a newly developed mechanical testing technique. The longitudinal fracture mode was intergranular for all wires and a second phase was observed on the grain boundaries of all doped wires. High concentrations of the dopant element potassium were present on the fracture surfaces of doped wires and experimental evidence was obtained which suggests they may be due to postfracture surface diffusion. Doped wires demonstrated increasing amounts of structure coarsening up to 1500°C whereas large equiaxed grains were formed in undoped wires annealed at 1300 and 1500°C. The longitudinal fracture resistance of undoped wire was unaltered by annealing at 1050°C and below, but decreased dramatically after annealing at 1300 and 1500°C. In contrast the fracture resistance of doped wire decreased after annealing at 1050 and 1300°C, but increased after annealing at 1500°C. Fracture resistance is discussed in terms of microstructure and fracture surface chemistry. A. W. FUNKENBUSCH, formerly Research Metallurgist with General Electric Refractory Metals Product Department     

Effect of Composition and Distribution of Phases after Aging on Stamp Ability for Aluminum Alloy D16 (AA2014) Sheets

The relevance of this research is connected to the increasing requirements for accuracy in stamped parts produced from aged aluminum alloys, and can also be applied for making layered composites. Indexed requirements can be provided by controlling the structure of sheet blanks, particularly by the phase composition and distribution behavior. Results of experimental research into the effect of aging modes on composition, dispersion behavior, and stamping number of sheet samples of the D16 aluminum alloy (AA2014) are presented. Heat treatment includes quenching from a temperature of 500°С into water of room temperature and further aging: natural aging for 7 days and artificial aging at temperatures of 50, 100, 150, and 200°С with a duration at each temperature of 15, 30, 60, 120, and 240 min. A method of estimating the quantity of the characteristics of dispersion phases is proposed for the microstructural picture. Stamp ability is evaluated using the stamping number, which is the ration between yield stress and tensile strength. It is found that increasing aging temperature and durance leads to the growth of the stamping number, which shows a low ability for sheet-stamping operations of alloy. Aging at 50°С did not lead to the sedimentation of dispersion phases for either optical metallography or scan electron microscopy. The inhomogeneity of phase dispersion inside the grain grows at the initial stages of aging, when durance is less than 1 h and temperature is 100, 150 and 200°С. Further increasing durance to 4 h leads to inhomogeneity decreasing. There is no correlation between the uniformity of phase dispersion and the stamping number. The chemical composition of phases plays the main role in stamping number, outside of phase-dispersion uniformity. The phase-composition changes depend on the mode of heat treatment: at an annealed and naturally aged state, the θ and S phase is sediment. After aging at a temperature lower 150°С after a short durance of less than 1 h, the θ, S and T phases are revealed; the θ phase appears after aging at temperatures higher than 150°C and long durance reaching 4 h.     

Structure and mechanical properties of Fe-Cr-Mo-C alloys with and without boron

A study of the structure and mechanical properties of Fe-Cr-Mo-C martensitic steels with and without boron addition has been carried out. Nonconventional heat treatments have subsequently been designed to improve the mechanical properties of these steels. Boron has been known to be a very potent element in increasing the hardenability of steel, but its effect on structure and mechanical properties of quenched and tempered martensitic steels has not been clear. The present results show that the as-quenched structures of both steels consist mainly of dislocated martensite. In the boron-free steel, there are more lath boundary retained austenite films. The boron-treated steel shows higher strengths at all tempering temperatures but with lower Charpy V-notch impact energies. Both steels show tempered martensite embrittlement when tempered at 350 °C for 1 h. The properties above 500 °C tempering are significantly different in the two steels. While the boron-free steel shows a continuous increase in toughness when tempered above 500 °C, the boron-treated steel suffers a second drop in toughness at 600 °C tempering. Transmission electron microscopy studies show that in the 600 °C tempered boron-treated steel large, more or less continuous cementite films are present at the lath boundaries, which are probably responsible for the embrittlement. The differences in mechanical properties at tempering temperatures above 500 °C are rationalized in terms of the effect of boron-vacancy interactions on the recovery and recrystallization behavior of these steels. Although boron seems to impair room temperature impact toughness at low strength levels, it does not affect this property at high strength levels. By simple nonconventinal heat treatments of the present alloys, martensitic steels may be produced with quite good strength-toughness properties which are much superior to those of existing commercial ultra-high strength steels. It is also shown that very good combinations of strength and toughness can be obtained with as-quenched martensitic steels.     

Determination of Thermal and Mechanical Material Properties of Ultra‐High Strength Steels for Hot Stamping

Direct and indirect hot stamping presently constitutes one of the most innovative forming technologies in the automotive industry through the combination of forming and hardening in one process step or line. Thus, structural components with strength up to 1600 MPa can be accomplished with the quench hardenable ultra‐high strength steel 22MnB5. With respect to the numerical investigation of the feasibility of different parts the knowledge of various thermal and mechanical material characteristics determined under process relevant conditions are required. Within the scope of this paper different experimental methods will be introduced for the determination of material properties according to the typical time‐temperature characteristics of the hot stamping process, as well as the modelling of it as input data for the FE analysis.     

Development of a Lean Duplex Stainless Steel

The classic series of duplex stainless steels shows very high corrosion resistance and can be used for very demanding applications. A new lean duplex steel, LDX 2101® (EN 1.4162, UNS S32101), has been developed with corrosion resistance on a par with standard austenitic grades. Application areas include: structural components, chemical industry, tanks and containers. The steel was designed to have equal amounts of ferrite and austenite in annealed condition and with an austenite that is stable against strain‐induced martensite. Thanks to its high nitrogen content, the steel has a fast austenite reformation when subjected to thermal cycling, e.g. welding. Unlike conventional duplex grades, the formation of intermetallic phase is very sluggish, although precipitation of nitrides and carbides has a certain impact on material properties after exposure in the temperature range 600 to 800°C. The precipitation behaviour after different isothermal treatments is described and its influence on different product properties is shown. A good agreement was found between impact toughness and corrosion resistance for a wide range of thermal treatments.