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.     

Development of microalloyed steel for pipeline applications

Abstract

An experimental HSLA steel was produced by the electric arc furnace, vacuum degassing, ladle treatment and continuous casting route. The experimental steel was then rolled in a laboratory using a hot rolling schedule to simulate an industrial controlled hot rolling procedure for the production of plates as closely as possible to investigate the effect of a thermomechanical processing schedule plus the use of water quench, accelerated cooling followed by forced nitrogen gas or air as cooling media, on the mechanical properties of plates. The results showed that the controlled thermomechanical hot rolling schedule of slabs followed by the cooling of plates in either forced nitrogen gas or by accelerated cooling exhibited target properties equivalent to a steel grade API X-80.     

Influence of Hot Press Forming Techniques on Properties of Vehicle High Strength Steels

  Based on the combination of materials science and mechanical engineering, hot press forming process of the vehicle high strength steels was analyzed. The hot forming process included: heating alloys rapidly to austenite microstructures, stamping and cooling timely, maintaining pressure and quenching. The results showed that most of austenite microstructure was changed into uniform martensite by the hot press forming while the samples were heated at 900 ℃ and quenched. The optimal tensile strength and yield strength were up to 1530 MPa and 1000 MPa, respectively, and the shape deformation reached about 23%. And springback defect did not happen in the samples.     

热成型钢及热成型技术

对热成型钢的成分设计、热成型工艺过程以及表面氧化的处理进行了论述。热成型工艺可以通过直接热成型或者间接热成型实现;热成型钢都是含硼钢,硼能抑制成型过程中铁素体形核;通过锰、铬和钼等提高淬透性防止珠光体形成;一般热成型钢中含有0.2%左右的碳。热成型钢可通过喷丸的方法消除表面氧化层或者镀层技术保护其表面不受氧化。     

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。     

冷速和变形对700MPa超级钢组织和性能的影响

采用Gleeble 1 5 0 0热模拟实验机及Instron试验机研究并分析了 70 0MPa超级钢的连续冷却转变曲线 (CCT曲线 )及冷却和变形工艺对实验钢性能的影响。结果表明 ,热变形显著加速钢的相变过程 ,随着冷却速度的提高 ,钢的抗拉强度提高 ,屈服强度降低     

30SiMnCrB5热成形钢的微观组织和力学性能

为了提高热成形钢的综合性能,设计了一种C-Si-Mn-Cr-B系热成形钢,采用热膨胀仪测定并研究了30SiMnCrB5热成形钢的连续冷却转变曲线和相变规律.分析了经轧制、退火及热成形模拟后钢板的微观组织形貌和力学性能,结合等密度线极图的方法,判定了热成形模拟后钢板中马氏体变体与母相的取向关系.30SiMnCrB5热成形钢具有较好的淬透性,临界冷速为5℃·s-1,有效抑制了珠光体和贝氏体的形成,完全马氏体组织的硬度可达600 HV以上.热成形模拟后的微观组织由板条马氏体和残余奥氏体构成,残余奥氏体主要以薄膜状分布在马氏体板条间,质量分数为6%~8%,抗拉强度为1800 Mpa左右,总伸长率可达10%以上,强度和塑性的匹配较好.热成形模拟后30SiMnCrB5热成形钢板中马氏体变体与母相的取向关系更接近N-W关系,12种变体没有都出现在原始奥氏体内.      

新标准下热轧带肋钢筋生产工艺的分析

新标准实施后,为了探究热轧带肋钢筋最佳生产工艺,对广东省不同生产工艺的主要钢企生产的钢筋进行了研究。分析表明,采用穿水冷却工艺,试样的强度偏高、韧性偏低,金相组织不合格,部分试样反向弯曲及抗震性能不合格,说明该工艺已严重不符合新标准要求;采用加铬、钼合金化热轧工艺,试样间强度偏差较大,且反向弯曲性能不合格,说明工艺不易控制、产品质量不稳定;采用常规热轧工艺,试样屈服强度偏低,仅高出标准下限1～2 MPa,说明该工艺生产的钢筋强度不够;采用钒氮合金化热轧工艺,试样各项试验指标均良好,当钒质量分数为0.33%～0.36%、氮质量分数为0.022%～0.025%时,产品综合质量最佳。     

Recrystallization controlled rolling and accelerated cooling for high strength and toughness in V-Ti-N steels

Optimum thermomechanically controlled process parameters have been established for the production of Ti-V-N microalloyed high-strength low-alloy (HSLA) steels. On the basis of laboratory simulation and full-scale processing, it has been shown that nitrogen is an essential alloying element addition and full appreciation of its effects leads to the ability to utilize high nitrogen steel in connection with hot rolling in a high-temperature regime to produce HSLA products with very favorable combinations of yield strength and toughness. The effects of reheating temperature, rolling reduction, cooling rate, and finish-cooling temperature (FCT) on the ferrite grain size and mechanical properties have been examined. It has been shown that the potential for precipitation strengthening is dependent on vanadium, nitrogen, and cooling parameters. Accelerated cooling (ACC) prevents precipitation of vanadium nitrides in austenite and enhances both grain refinement and precipitation strengthening. By adjusting nitrogen content and processing parameters, a yield strength of 500 MPa and impact transition temperature (ITT) below -60 ‡C can be obtained in the as-hot-rolled condition in Ti-V-N steels, using high finish-temperature hot rolling and accelerated cooling.     

Improvement of Mechanical Properties of Automotive Components Using Hot Stamping With Integrated Q-P Process

Cold forming of high strength materials is accompanied by an undesirable spring-back effect and therefore the automotive industry prefers to produce components from high strength steels by hot stamping.Hot deformation and cooling in a die are applied to obtain shaped components with martensitic microstructure and high yield strength and ultimate tensile strength.This article presents new applications of this forming technology by incorporating another innovative heat treatment by the Q-P process,which improves both strength and ductility of obtained structures at the same time.Ultimate strengths over 2000 MPa with ductility above 10% can be achieved by this processing.To test microstructure development,thin sheet was hot formed and a corresponding FEM simulation was created.This processing was applied to low alloyed AHS steel with 0.42% of carbon and with an alloying strategy based on Mn,Si and Cr.Martensitic microstructure with retained austenite was obtained by this processing with a strength of around 2000 MPa and ductility of 10%.     

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.     

Modeling of Microstructure Evolution in 22MnB5 Steel during Hot Stamping简

Automobile manufacturers have been inereasingl~r adopting hot-stamped parts for use in newly designed ve- hicles to improve crash worthiness and fuel efficiency. However, the simulation of hot stamping is rather complex and challenging, and further research still needs to be done on hot stamping hardening mechanism. The microstruc- ture evolution and hardening mechanisms during hot stamping of 22MnB5 steel were thoroughly investigated, using information provided in the literatures as well as experimental results. New models were developed to predict the grain growth during heating and the flow stress of a manganese boron steel （22MnB5） with high hardenability by the Gleeble simulation experimental results. The deformed austenite decomposition during stamping and quenching was emphatically quantified based on the transformation thermodynamic and kinetic theories, and the relationship of mi- crostructure to properties was analyzed. The results showed that the optimal process to obtain homogeneous and small lath martensite is heating at 900--950 ℃ for 5 min and then auenching at 50 ℃/s with a Dressing time about 8 s.     

Effect of Cooling Patterns on Microstructure and Mechanical Properties of Hot-Rolled Nb Microalloyed Multiphase Steel Plates

 The effect of the run-out table cooling patterns on the microstructure and mechanical properties of Nb microalloyed steel plates was investigated by hot rolling experiment. The results showed that the mixed microstructure containing ferrite, bainite and significant amounts of retained austenite can be obtained through three kinds of cooling patterns on the run-out table under the same hot rolling condition. Three kinds of cooling patterns possess different austenite transformation kinetics, which leads to variations in microconstituent characteristics. The yield strength increases, the tensile strength decreases and the total elongation tends to increase as the cooling patterns Ⅰ, Ⅱ and Ⅲ were applied respectively. The yield strength, the total elongation and the product of tensile strength and ductility reach the maximum values (547 MPa, 37.2% and 28384 MPa·%, respectively) for the steel plate processed by cooling pattern Ⅲ.     

相变时间对CSP热轧TRIP钢组织和性能的影响

利用OM、SEM、XRD、EBSD和室温拉伸试验机等研究了CSP热轧TRIP钢中间缓冷时间及贝氏体等温时间对组织和力学性能的影响。结果表明,随着中间缓冷时间的延长,试验钢中的铁素体和残余奥氏体体积分数增加,贝氏体体积分数减少;抗拉强度基本不变,屈服强度逐渐降低,断后伸长率和强塑积变化不明显。中间缓冷时间为6 s时,可满足CSP产线的要求。对贝氏体相变时间的研究表明,当等温时间为15 min时,试验钢中的残余奥氏体主要分布于铁素体/铁素体界面、铁素体/贝氏体界面以及贝氏体中,体积分数约为7.1%,表现出良好的TRIP效应。其抗拉强度、屈服强度、断后伸长率和强塑积分别达到744.0 MPa、522.5 MPa、29.3%和21.8 GPa·%,力学性能最优。当等温时间延长至50 min时,试验钢中的贝氏体含量增加,残余奥氏体体积分数减少至2.7%,强塑积明显下降。     

利用高炉烘炉消除冷却壁铸造内应力的新工艺研究与工业应用

为消除高炉冷却壁铸造内应力对壁体工作的破坏性影响，提出了利用高炉烘炉过程来实现既烘干水分保护耐材，同时又消除内应力的新思路。采用与马钢1号高炉第5代炉役铸钢冷却壁相同的实验冷却壁，使用8种处理工艺，力学性能及X射线应力测试、金相微观组织分析等方法，在实验室进行了系列热态模拟实验。取得的优化工艺条件在1号高炉烘炉中进行了工业应用。结果表明，新烘炉工艺可使ZG冷却壁内应力由出厂铸态的212N／mm^2消除到5．8N／mm^2。有效消除率达97％以上，壁体的力学性能也得到明显提高。     

500 MPa级V-N 微合金化热冲压桥壳用钢的开发

采用OM、SEM和TEM对500 MPa级V-N微合金化热冲压桥壳用钢的组织与性能进行了研究。结果表明,其屈服强度、抗拉强度分别达到了373和544 MPa,断后伸长率达到25.5%,低温冲击性能优异,在-60 ℃时的冲击功达到了145 J。其显微组织主要为铁素体和少量珠光体的混合组织,其中,铁素体基体上存在大量球形析出物,该析出物在规格上分为尺寸为30～50 nm且能谱分析显示主要为VCN的大颗粒第二相和尺寸在20 nm以下且能谱分析显示主要为VC的小颗粒第二相。热冲压后,500 MPa级V-N微合金化热冲压桥壳用钢的屈服强度和抗拉强度分别达到305和450 MPa,屈服强度和抗拉强度的下降率分别控制在18.2%和17.3%。     

Effect of cooling rate on the as-quenched microstructure and mechanical properties of HSLA-100 steel plates

The effect of cooling rate on the as-quenched microstructure and mechanical properties of a 14-mm-thick HSLA-100 steel using various cooling media such as brine, water, oil, air, and furnace has been studied. While quenching in brine, water, and oil resulted in lath martensite structures, the granular bainite and martensite-austenite (M-A) constituents were found in air- or furnace-cooled specimens. The average lath spacing increased slightly on decreasing the cooling rate (300 nm in brine-quenched specimen to 400 nm in oil-quenched specimen). The precipitates of Cu and Nb(C, N) were observed in all the quenching conditions except in the brine-quenched specimen. The as-quenched strength and toughness of the brine-, water-, and oil-quenched specimens were higher (yield strength: 894 to 997 MPa, ultimate tensile strength: 1119 to 1153 MPa, and Charpy V-notch energies: 65 to 70 J at −85 °C) than those of air- and furnace-cooled specimens (yield strength: 640 to 670 MPa, ultimate tensile strength: 944 to 1001 MPa, and Charpy V-notch energies: 10 to 20 J at −85 °C). For industrial production of HSLA-100 steel plates, oil or water quenching is recommended in lower thickness plates (<25 mm). For production of thicker plates, however, water quenching is more suitable.     

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。     

首钢热轧DP580双相钢的工业试生产

采用合金成本低廉的C-Mn-Cr化学成分设计,通过层流冷却段的水冷—空冷—水冷的三段式冷却模式和低温卷取,成功在1 580mm机组上试生产580MPa级热轧双相钢。对试生产钢卷进行了性能均匀性分析,结果表明:除去轧钢头部弱冷区域,整卷性能均匀,马氏体比例可稳定在10%左右。进而得出当前采用的恒速轧制方法有利于热轧双相钢力学性能均匀性控制的结论。试生产中在终轧后实施了2种不同的中间温度,轧制结果显示2种工艺方案下屈服强度和屈强比差距较大。