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.     

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。     

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。     

搅拌摩擦加工IF钢的组织性能

对3 mm厚的DC04冷轧IF钢板进行搅拌摩擦加工,研究加工区域的微观组织与力学性能.在旋转速度为950 r·min-1,加工速度为60 mm·min-1时,采用加工后强制冷却技术可获得光滑平整且没有缺陷的加工表面.搅拌摩擦加工后组织显著细化,加工中心的平均显微硬度约为HV 135.6,是母材硬度的1.4倍,表面细晶层硬度最高可达到HV 312.8,细晶层和过渡层的抗拉强度分别比母材的抗拉强度提高50.9%和47.6%,加工前后试样的拉伸断口均呈微孔聚合韧性断裂特征.细晶强化对材料抗拉强度的提高起主要作用.      

微合金钢第Ⅲ脆性区的形成机理

为研究微合金钢第Ⅲ脆性区形成机理及其影响因素,控制连铸坯的表面裂纹,采用Gleeble热力模拟机测定了S355微合金钢在不同温度下的抗拉强度及断面收缩率。使用扫描电镜对拉伸断口进行观察分析,同时采用透射电镜对析出物进行观察分析。在此基础上对拉伸试样进行金相实验,对第二相析出进行热力学计算,分析了组织状态及第二相析出规律对脆性区的影响。结果表明,在第Ⅲ脆性区(660~850℃)内,拉伸断口呈冰糖状,韧窝较浅,形貌表现为沿晶脆性断裂。铁素体网膜沿奥氏体晶界优先析出、第二相沿晶界析出是第Ⅲ脆性区形成的主要原因。     

Investigation of Mechanical Property of Oil Tube Steel P110S in High Temperature and High Pressure Environment Containing H2S/CO2

With the exploitation of high sour and deep oil well in china,the corrosion medium containing CO 2,H 2 S,Cl-and organic acid threaten the safety of oil equipment and tube steel severely.In this article,many experiments,including corrosion tests,tensile tests,hydrogen concentration measurement tests and SEM examinations,were performed to investigate the effect of H 2 S partial pressure and loading stress on the mechanical property of oil tube steel P110S in high temperature and high pressure environment containing H 2 S/CO 2.The tensile strength and yield strength during fracture process of P110S steel were obtained experimentally and the fracture morphology was analyzed by SEM.The results indicate that both tensile strength and yield strength decreased after corrosion.The damage of tensile strength and yield strength became serious with an increase in H 2 S partial pressure and loading stress.According to the fracture analysis,the fracture mode exhibited quasi-cleavage and dimple mixed fracture,and the area of quasi-cleavage pattern increased with H 2 S partial pressure and loading stress increasing.According to the results of tensile test and hydrogen concentration measurement test,the tensile property degradation is closely associated with diffusible hydrogen concentration of P110S steel in the H 2 S/CO 2 environment.     

A Numerical and Experimental Investigation into Hot Stamping of Boron Alloyed Heat Treated Steels

Hot stamping of steel sheets using water or nitrogen cooling media was studied on a laboratory scale. Sheets of grade 22MnB5 boron steels in three different thicknesses were investigated and the results of experimental hot stamping tests were considered. Microstructural analysis, linear and surface hardness profiling as well as tensile tests of formed samples were carried out. After hot stamping, mostly fully martensitic microstructures, which yield ultra high strength levels, were produced. It is concluded that die cooling media, i.e., water or nitrogen, have a significant effect on material properties after hot stamping. Using liquid nitrogen as coolant in the punch instead of water increases yield strength by 50 to 65MPa. Moreover, the evolution of the temperature and force during the hot stamping process was simulated by using a coupled thermomechanical FEM program. The results of numerical simulation and experimental results are in good agreement.     

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.     

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。     

Effects of Prestrain and Inner Stress on the Susceptibility of Hot Stamped Boron Steel to Hydrogen Embrittlement

Hot stamped boron steel parts with martensite and bainite combined microstructures are widely used in lightweight manufacturing of automotive bodies. The final products usually experience the manufacturing process, such as trimming or punching, which inevitably introduces inner stress and plastic strain. Hot stamped parts with complex microstructures and manufacturing effects exhibit some special characteristics of hydrogen embrittlement (HE) in service. In this paper, hardened boron steels with a total martensite content and different martensite-bainite combinations, called martensite or bainite dominant microstructures (MDM/BDM), are experimentally obtained by a flexible quenching tool, and the HE susceptibility considering the effects of prestrain and inner (tensile/compressive) stress is investigated based on slow strain rate tensile tests by developing hydrogen charging equipment under constant loads. SEM and EBSD analyses on hydrogen-inducted cracks (HICs) and thermal desorption analysis of hydrogen for different microstructures and strain/stress states are carried out to illustrate the HE mechanism. The HE susceptibility of the as-quenched specimen decreases with increasing volume fraction of bainite, with the fractograph exhibiting intergranular fracture in the MDM and the dimples feature in the BDM. Prestrain weakens the HE susceptibility in the MDM but raises it in the BDM due to the enhancement of hydrogen adsorption, promoting local plasticity in the evolution of the HICs. Inner stresses increase the HE susceptibility in the MDM, which lifts the hydrogen segregation at the grain boundaries to intensify the hydrogen-enhanced decohesion effect. Tensile stress intensifies the sensitivity to HE in the BDM. The promotion of hydrogen adsorption facilitates crack initiation. However, compressive stress has opposite effects on the HE susceptibility due to the improvement of the material ductility by reducing the hydrogen content in the specimen.

     

9CrMoWV耐热钢的高温持久蠕变性能及组织行为研究分析

为了表征9CrMoWV耐热钢在实际应用中的力学行为,对9CrMoWV钢在不同温度和应力条件下进行持久试验,并对持久试样进行OM、SEM、TEM观察,分析不同温度、应力和持久试验时间对9CrMoWV钢显微组织和宏观、微观断口的影响,同时统计Laves相的尺寸变化及其他第二相分布情况。结果表明,随着试验时间和温度的增加,断口韧窝尺寸增大,板条状马氏体回复程度增大,Laves相的尺寸变化明显,分布于晶界的M₂₃C₆与Laves相均发生不同程度聚集粗化,这是高温低应力区试样断裂的原因之一。     

Study on hydrogen induced delayed fracture of 1500 and 2000MPa hot stamping steels

The application of hot-stamping steel (HS) in the automobile is an inevitable trend, but the hydrogen embrittlement sensitivity of HS steel still needs to be studied and improved. The hydrogen diffusion behavior and hydrogen embrittlement sensitivity of 1500 and 2000MPa hot stamping steels were studied by means of hydrogen penetration, slow strain rate tensile (SSRT), and fracture analysis. The results show that the apparent diffusion coefficient Dap (1.71×10-7cm2/s) of 1500HS is significantly less than the Dap (3.45×10-7cm2/s) of 2000HS; delayed fracture resistance of 1500HS is superior to 2000HS. From the fracture analysis, under the same hydrogen charging conditions, the fracture morphology of 1500HS changed from typical dimple ductile fracture to quasi cleavage brittle fracture, while 2000HS changed from dimple morphology to intergranular brittle fracture with the increase of hydrogen charging current density. While the deformation degree of 2000HS was very small, the local hydrogen content and stress value had reached the critical deal. The hydrogen reduced the bonding force between grains, resulting in the nucleation and propagation of microcracks. Therefore, with the improvement of the strength of HS steel, Ti and V micro alloyed elements should be properly added to form nano precipitates, as irreversible hydrogen traps to capture hydrogen atoms, hinder their diffusion and segregation, and effectively refine the structure and pinning dislocations, to improve the resistance to hydrogen induced delayed fracture of HS steel.     

Investigations on Dissimilar Weld of UNS S32205 DSS and ASTM A517 Steel

In the present research, microstructure and mechanical properties of 2205 duplex stainless steel/A517 quench and tempered low alloy steel dissimilar joint were investigated. For this purpose, gas tungsten arc welding was used with ER2209 filler metal. Characterizations were conducted by optical microscopy, scanning electron microscopy equipped with an energy dispersive spectroscopy and X-ray diffraction. Mechanical properties were evaluated in micro-hardness, tensile and impact tests. Microstructure in the weld zone included an austenitic continuous network in the matrix of primary ferrite. No brittle phases were formed in the weld metal and stainless steel heat affected zone (HAZ). The weld metal/A517 interface showed higher hardness than other regions. Tensile tests indicated that the values of the yield and tensile strength were 663 and 796 MPa, respectively. Impact tests indicated that the weld zone had almost the same impact energy as base metals. The minimum impact energy of 12 J was related to A517 HAZ. The results of scanning electron microscopy for fracture surfaces indicated that weld zone, 2205 HAZ and A517 HAZ had ductile, ductile–brittle and brittle fracture mode, respectively.     

Influence of alloying elements and grain refiner on microstructure,mechanical and wear properties of Mg-Al-Zn alloys

In the present investigation, the effects of alloying elements (Sn, Pb) and grain refiner (Ag, Zr) on microstructure, mechanical and wear properties of as-cast Mg-Al-Zn alloys were studied. The alloys were prepared through melting-casting route under a protective atmosphere and cast into a permanent mould. The microstructure of the base alloy consisted of α-Mg, Mg₁₇Al₁₂ continuous eutectic phase at the grain boundary and Mg-Zn phase was distributed within the grains. Addition of Sn and Pb suppressed the formation of continuous Mg₁₇Al₁₂ eutectic phase and formed Pb enriched Mg₂Sn precipitates at the grain boundary as well as inside the grain. The Ag and Zr addition to Mg-Al-Zn-Sn-Pb alloy suppressed the Mg₁₇Al₁₂ phase formation and refined the grains leading to improve mechanical properties. Addition of Sn, Pb and grain refiner (Ag, Zr) significantly enhanced the tensile strength and elongation but reduced hardness. The Ag addition imparted best tensile properties, where ultimate tensile strength (UTS) and elongation are 205?MPa and 8.0%, respectively. The fracture surfaces were examined under SEM which revealed cleavage facets and dimple formation. Therefore, the cleavage fracture and dimple rupture were considered as the dominant fracture mechanisms for developed Mg alloys. The cumulative volume loss of Mg alloys increased with sliding distance and applied load. The coefficient of friction decreased with sliding distance. The microscopic observation, analysis of the wear surface and coefficient of friction revealed that the wear mechanism of developed Mg alloys changes from abrasion oxidation to delamination wear.     

1800 MPa热成形钢与CR340LA低合金高强钢激光焊接性能

采用光纤激光焊接设备对1800 MPa级热成形钢与CR340LA低合金高强钢进行对接激光拼焊,研究了不同激光焊接功率和焊接速度下焊接接头的组织演变规律及热冲压成形性能,并对焊接接头的力学性能和硬度进行了分析。结果表明,3种焊接工艺下激光拼焊原板综合力学性能相差较小,由焊接接头造成的伸长率和抗拉强度的损失均在母材的28.3%和9.1%以内。激光焊接后焊缝区均为粗大、高硬度的马氏体结构;两侧热影响区组织主要为铁素体和马氏体,接头未出现明显的软化区。激光拼焊原板拉伸试样均断裂于CR340LA母材区,距离焊缝12 mm左右,且存在焊缝隆起现象。选取焊接功率和焊接速率分别为4000 W和0.18 m·s?1的焊接试样在高温下进行热冲压成形检测,未出现焊缝开裂,热成形后拼焊板具有良好性能,满足汽车激光拼焊板使用要求,拉伸结果表明,试样断裂位置与未热冲压成形前一致,均位于CR340LA母材区,拉伸过程中,焊缝向高强度母材侧偏移,在弱强度母材侧产生应力集中并缩颈断裂。      

焊接电流对镁/镀锌钢 TIG熔--钎焊接头显微组织与力学性能的影响

采用TIG熔-钎焊焊接方法,以镁合金焊丝为填充材料,对镁合金与镀锌钢进行连接实验,并分析热输入量对接头显微组织和力学性能的影响.热输入量过小会阻碍镁/钢界面反应层的形成而使得焊缝难以焊合,热输入量过大又会促进焊缝内部脆性第二相的长大,降低接头力学性能.接头强度随着焊接电流和焊接速度的增大都呈现先上升后下降的趋势,电流为70 A时强度达到最大,该值接近AZ31B母材的88.7%.此时断裂发生于焊缝熔焊区,断面出现大量韧窝和撕裂棱,呈现出塑性断裂特征.      

超高强热成形钢的应变速率敏感性

利用CMT5105电子万能试验机和HTM 16020电液伺服高速试验机对超高强热成形钢进行拉伸试验,应变速率范围为10-3~103 s-1,模拟热成形零件在不同应变速率下的碰撞情况.结果表明:在低应变速率阶段(10-3~10-1 s-1)实验钢的应变速率敏感性不高,随应变速率的升高,实验钢的强度和延伸率变化不大;在高应变速率阶段(100~103 s-1)实验钢具有高的应变速率敏感性,随应变速率的升高,实验钢的强度和延伸率都呈增大的趋势,并且抗拉强度的应变速率敏感性要大于屈服强度.这主要是由于在高应变速率阶段拉伸时产生的绝热温升现象和应变硬化现象共同作用造成的.实验钢颈缩后的延伸率随应变速率的增大而减小,主要是由于高应变速率下马氏体局部变形不均匀造成的.实验钢吸收冲击功的能力随应变速率的升高而增大,实验钢达到均匀延伸率时吸收冲击功的大小对应变速率更敏感.与低应变速率阶段相比,实验钢在高应变速率阶段的断口韧窝的平均直径更小,韧窝的深度更深,这与高应变速率阶段部分马氏体晶粒的碎化有关.通过扫描电镜和透射电镜观察发现,在高应变速率拉伸时晶粒有明显的拉长趋势,并且在应力集中的地方有一些微空洞的存在,应变速率为103 s-1时部分区域有碎化的现象.      

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。     

Al对310S耐热钢高温拉伸性能的影响

通过压力机和手动轧制机对WS-4非自耗真空电弧炉熔炼制备的Al的质量分数为0%、2%、4%的310S耐热钢在950~1 250℃进行开坯轧制,研究不同Al含量的310S耐热钢热轧板材在800℃下的高温拉伸性能,用SEM观察拉伸断口。结果表明:不含Al和Al的质量分数为2%的合金,它们的高温抗拉强度均为210 MPa左右,但当Al的质量分数增加到4%时,合金的热强性显著提高。3种合金拉伸断面均可以看到大小明显不同的显微韧窝,合金在断裂前经历了比较大的塑性变形,所以均为韧性沿晶断裂。随Al含量的增加,合金断口中的显微韧窝所占的比例随之增加。     

Effect of the Small Reaustenized Grains on the Mechanical Properties of a HSLA Steel

Grain refinement improves not only strength but also toughness.Small reaustenized grains were discovered to form along the grain boundaries and lath boundaries in an as rolled high-strength low-alloy (HSLA) steel when heated to double phase region.The number and size of the small reaustenized grains were revealed to depend mainly on temperature in the range of 700 to 760℃.These small reaustenized grains were deduced to form by consuming precipitates.The mechanical property tests showed that they resulted in slight changes to tensile properties.However,the sample heated at 740℃ produced a peak Charpy V-notch (CVN) impact energy of 248J when tested at-30℃.Fractography showed that the facture surface of the 740℃ heated impact sample exhibited the smallest brittle zone away from the notch root and a large dimple zone lay ahead of the notch tip,which demonstrated the highest toughness.