Microstructures and Properties of 550 MPa Grade High Strength Thin-walled H-beam Steel

The microstructures and mechanical properties of 550 MPa grade lightweight high strength thin-walled H-beam steel were experimentally studied. The experimental results show that the microstructure of the air-cooled H-beam steel sample is consisted of ferrite, pearlite and a small amount of granular bainites as well as fine and dispersive V（C,N） precipitates. The microstructure of the water-cooled steel sample is consisted of ferrite and bainite as well as a small amount of fine pearlites. The microstructure of the water-cooled sample is finer than that of the air-cooled sample with the average intercept size of the surface grains reaching to 3.5 gna. The finish rolling temperature of the thin-walled high strength H-beam steel is in the range of 750 ~C-850 ~C. The lower the finish rolling temperature and the faster the cooling rate, the finer the ferrite grains, the volume fraction of bainite is increased through water cooling process. Grain refinement strengthening and precipitation strengthening are used as major strengthening means to develop 550 MPa grade lightweight high strength thin- walled H-beam steel. Vanadium partially soluted in the matrix and contributes to the solution strengthening. The 550 MPa grade high-strength thin-walled H-beam steel could be developed by direct air cooling after hot rolling to fully meet the requirements of the target properties.

Kinetics and formation mechanisms of intragranular ferrite in V?N microalloyed 600 MPa high strength rebar steel

To systematically investigate the kinetics and formation mechanisms of intragranular ferrite（IGF）, isothermal heat treatment in the temperature range of 450℃to 600℃ with holding for 30 s to 300 s, analysis of the corresponding microstructures, and observation of the precipitated particles were conducted in V-N microalloyed 600 MPa high strength rebar steel. The potency of V（C,N） for IGF nucleation was also analyzed statistically. The results show that the dominant microstructure transforms from bainite（B） and acicular ferrite（AF） to grain boundary ferrite（GBF）, intragranular polygonal ferrite（IPF）, and pearlite（P） as the isothermal temperature increases from 450℃ to 600℃. When the holding time at 600℃ is extended from 30 s to 60 s, 120 s, and 300 s, the GBF content ranges from 6.0vol% to 6.5vol% and the IPF content increases from 0.5vol% to 2.8vol%, 13.1vol%, and 13.5vol%, respectively, because the ferrite transformation preferentially occurs at the grain boundaries and then occurs at the austenite grains. Notably, V（C,N） particles are the most effective nucleation site for the formation of IPF, accounting for 51% of the said formation.

Effect of Ti content on the yield strength of Nb-Ti microalloyed steels

The hot rolling experiment investigates into the relationship between the microstructures and the mechanical properties of Nb-Ti microalloyed steels with various Ti contents. The results indicate that the effect of the bainite fraction of Nb-Ti microalloyed steels on the yield strength of the steels is not related to Ti content, while the slope of the Hall- Petch relationship decreases with the increase of Ti content. Accordingly, the Misra model for the yield strength of Nb-Ti microalloyed steels is modified, and the factors which cause the change in the slope of Hall-Petch relationship are discussed.

Nb-Ti微合金化超低碳低合金高强度钢中第二相的析出行为

研究了Nb-Ti微合金化超低碳低合金高强钢在控轧控冷及回火过程中含Nb和Ti相的析出行为及其与硬度的关系。结果表明,Nb-Ti微合金化超低碳低合金高强钢在控轧控冷驰豫过程及随后的回火过程中均有第二相析出,在变形过程中Nb(CxNy)相析出颗粒更为细小,在回火过程中Nb(CxNy)依附于TiN形核长大,600℃回火2 h后其硬度高于其它温度回火后的硬度。

Nb-Ti高强IF钢板最优再结晶退火温度确定

以工业生产的Nb-Ti高强IF钢为试验材料,在实验室研究了退火温度对试验钢的显微组织、力学性能和织构的影响。结果表明:在试验条件下,退火温度750℃时,试验钢为未完全再结晶组织,退火温度升高至780～870℃时,试验钢均为完全再结晶组织;随着退火温度的升高,试验钢的ReL、Rm逐渐降低,A50、Ae逐渐增加。退火温度810～840℃时,试验钢的屈服强度、抗拉强度、延伸率、塑性应变比及应变硬化指数分别在300 MPa、410 MPa、36.5%、1.7、0.22左右;随着退火温度的升高,α和γ取向线织构减弱。且织构有向{111}织构靠拢的趋势。提出试验钢工业生产中的最佳再结晶退火温度为810～840℃。

600MPa级Nb-Ti微合金高强度钢的开发

开发的Nb-Ti微合金高强钢(/%:0.04C、0.34Si、1.40Mn、0.010P、0.004S、0.098Nb、0.020Ti、0.045Al、0.002 5N)由真空感应炉冶炼、50 kg钢锭40 mm锻造板坯经试验室单架轧机于1 200℃7道次轧制成10mm板,末道次压缩比≥15%,终轧温度880℃,喷水冷却至600℃,置于热处理炉600℃30 min,炉冷至室温,分别模拟层流冷却和卷取工艺。该钢经Gleeble 3500热模拟机试验得出,高温低塑性区为650～800℃和≥1 300℃。力学性能试验结果为下屈服强度R_(el)625～640 MPa,抗拉强度R_m705～710 MPa,伸长率18.0%～19.5%。所开发的钢具有碳当量低,焊接性能好,成本低等特点。

960MPa级Nb-Ti微合金化高强钢中第二相粒子回溶行为研究

研究了960MPa级Nb—Ti微合金化高强钢中第二相粒子在不同温度和保温时间下的回溶行为。研究结果表明，原始样品中存在尺寸与形状明显不同的3种类型的第二相粒子：尺寸大于1μm的方形TiN粒子；尺寸为0．2～1．0μm的方形和椭球形TiS或Ti（C，S）粒子；尺寸在0．5μm以下的球形、方形和椭球形Nb—Ti复合析出物。随着加热温度的降低，小尺寸析出相数量不断增加。当加热温度为1150℃时，随着保温时间的延长，小尺寸球形和椭球形的析出相逐渐溶解，大尺寸方形析出相数量逐渐增加，且棱角变得模糊。综合考虑Nb—Ti微合金化高强钢中第二相粒子能够充分溶解及原始奥氏体晶粒尺寸，960MPa级Nb—Ti微合金化高强钢的加热温度选择1250℃、保温时间选择80min较合适。

回火工艺对Nb-Ti微合金化高强度大梁钢性能的影响

利用YB-MK马弗炉研究了300℃～750℃回火工艺下10 mm的Nb-Ti微合金化高强度大梁钢力学性能和韧性变化规律。发现了Nb-Ti微合金化高强度大梁钢650℃回火时屈服和抗拉强度强度达到峰值和400℃左右回火时强度和韧性综合性能较好的规律。同时还根据性能变化情况给出火焰矫梁工艺烘烤温度不大于700℃的建议。

Determination of Optimal Recrystallization Annealing Temperature of Nb-Ti Microalloyed High Strength IF Steel

The Nb-Ti microalloyed high strength IF steel sheet was used to study the effect of annealing temperature on the microstructures,mechanical properties and textures.The experimental results show that experimental steel is incomplete recrystallization at 750℃ annealing,but complete recrystallization from 780℃ to 870℃ under experimental conditions.When the annealing temperature was increased,the yield strength and tensile strength would gradually reduce,the plastic strain ratio and yield point elongation would gradually increase.The yield strength,tensile strength,elongation,the plastic strain ratio and the strain hardening exponent were approximate 300MPa,410MPa,36.5%,1.7 and 0.22 respectively under annealing temperature 810℃ to 840℃.When the annealing temperature was increased,the α-textures and γ-textures were gradually weakened,and the α-textures have a trend to {111} texture.Therefore,the suggestion of the optimal recrystallization annealing temperature is about 810℃ to 840℃ in industrial production.

Nb-Ti微合金化高强度汽车用钢研发及应用

柳钢在2 800 mm中厚板生产线试制汽车车厢用钢过程中,采用Nb-Ti复合强化成分设计,通过制定合理的冶炼、连铸、轧制工艺,成功开发出600 MPa级Nb-Ti微合金化高强度、薄规格超宽汽车车厢用钢板。其产品尺寸、板型、组织性能等完全满足用户使用需求,同时具有较低的脆性转变温度,并在商用改装车车厢上得到批量应用。

Feasibility of 0.02% Nb-Based Microalloyed Steel for the Application of One-Step Quenching and Partitioning Heat Treatment

To attain an enhanced combination of mechanical properties for low alloyed steel, the current study has been made to fulfill that growing need in the industry. Its results are introduced within this paper. One step Quenching and Partitioning (Q&P) heat treatment has been applied on Niobium-based microalloyed steel alloy with 0.2 %C, in the form of 2 mm thickness sheets. The target of this study is to investigate the viability of applying that significantly recommended, results-wise, heat treatment on the highly well-suited alloy steel samples, to achieve the main target of enhanced properties. A single temperature of 275°C was used as quenching and Partitioning temperature. Four Partitioning periods (30, 200, 500, and 1000 Seconds) were used for soaking at the same temperature. The results were analyzed in the light of microstructural investigation and mechanical testing. All applied cycles did not enhance the strength but moderately improved the ductility and toughness, mainly caused by the slightly high soaking temperature used. Niobium impact of grain refining was apparent through all cycles. The cycle of 500 Seconds Partitioning time obtained optimum values at that particular temperature. The 1000 Seconds Cycle obtained the worst combination of properties. A set of recommendations are set. More research is required at this point, where a lower Partitioning temperature is advised. In the light of the applied combination of parameters, the Partitioning period at such temperature is advised to be between 500 and 1000 Seconds. A high probability that periods closer to 500 than 1000 Seconds will produce better results. More research is needed between those two values of Partitioning time to precisely determine the optimum time at that temperature on that specific alloy.

微量稀土元素对冷成型超高强度汽车钢组织性能的影响

随着新能源汽车产销量的快速崛起,超高强度汽车钢的需求量越来越大。而超高强度汽车钢冷成型过程中的回弹和变形开裂问题成为制约超高强度汽车钢应用的主要因素。因此,开发一种强度高且成型性能优良的汽车钢具有实际应用价值,市场潜力巨大。许多研究发现,在汽车钢中加入稀土元素可以提高其综合性能。文章设计了两组成分方案,采用相同的逆相变退火(ART)工艺参数,通过拉伸试验、扫描电镜以及XRD等手段研究了稀土元素对冷成型超高强度汽车钢的组织性能影响机制。结果表明,在ART退火工艺下,试验钢微观组织均为马氏体、残余奥氏体和微量铁素体,但两种成分方案中添加稀土试验钢的残余奥氏体含量略高于不含稀土试验钢,为后续成型过程中的TRIP效应提供了组织基础。在800℃淬火保温5 min,随后645℃退火保温15 min的最佳热处理工艺参数下,方案一中含稀土试验钢的残余奥氏体含量比不含稀土的提高了8.1%,断后伸长率提高了21.8%,强塑积提高了17.2%,达到了28.47 GPa·%;方案二中含稀土试验钢残余奥氏体含量比不含稀土的提高4%,断后伸长率提高了2.8%,强塑积提高了5.1%,达到了34.8 GPa·%,说明添加稀土元素可以不同程度地改善超高强度汽车钢的综合性能。

氮含量与终轧温度对钛微合金化高强钢CGLC700低温冲击韧性的影响

针对钛微合金化高强钢CGLC700低温冲击韧性差的问题,通过热力学计算与高温原位观察,采用电子背散射衍射、透射电镜、扫描电镜和光学显微镜对含Ti高强钢的夹杂物、第二相粒子、断口形貌和低温冲击韧性等进行了研究。结果表明,含Ti高强钢低温冲击韧性差的原因与钢中大尺寸脆性夹杂物和Ti(C,N)、TiN析出相有关。将钢中w[N]从0.0049%降低至≤0.0035%时,可以有效降低钢中脆性夹杂物的数量和尺寸,从而提高钢材冲击韧性;终轧温度从885~895℃降低至875~885℃,可以促使纳米级TiC第二相粒子析出和大角度晶界的生成,并降低有效晶粒尺寸,从而明显改善钢材的低温冲击韧性;同时降低氮含量至≤0.0035%与终轧温度在875~885℃时,钛微合金化高强钢中平均晶粒尺寸从3.1μm降至2.7μm,小尺寸有效晶粒占比高,大尺寸夹杂物及数密度降低,大角度晶界中占比增长了16.6%,钢材低温冲击功可以从14.75 J提高到37.35 J。

Mo微合金化对82B钢高温力学性能及显微组织的影响

研究了添加Mo(质量分数)为0.02%、0.04%、0.07%、0.11%的82B钢对不同温度下高温拉伸后的组织和性能的影响及Mo对82B高碳钢的强韧化机理。结果表明:(1)200~300℃高温拉伸时,4组试验钢力学性能较好,300℃达到峰值,Mo为0.11%试验组最优,抗拉强度由1139 MPa增至1192 MPa,屈服强度为736 MPa,略有降低,伸长率与断面收缩率均有显著提高,分别为22%和72.9%。相较于Mo为0.02%试验钢,虽屈服强度下降约5%,但抗拉强度升高5%,伸长率、断面收缩率大幅增加,且塑性随Mo含量同步提高,随着温度继续升高,由于材料开始软化,各项力学性能均呈下降趋势。(2)Mo为0.02%和0.04%时,显微组织除渗碳体随拉伸温度的提高逐渐增多外,索氏体与珠光体未出现明显变化,与室温相同,当Mo增至0.07%和0.11%时,索氏体、珠光体团逐渐细化且分布均匀。同时,4组试验钢在300℃高温拉伸时出现蓝脆现象,通过断口观察发现,有Al、Si等元素的夹杂物存在,且失效方式为韧性断裂,断口主要可分为剪切唇区和纤维区,剪切唇区的面积随着温度的升高逐渐缩小。

In situ observation and modeling of austenite grain growth in a Nb-Ti-bearing high carbon steel

In situ measurements of austenite grain growth were made at various temperatures in the range of 1273-1473 K with subsequent isothermal holding time of 3600 s for the Nb-Ti-bearing and Nb-Ti-free high carb on steel by using a confocal laser scanning microscope.The solid solute behavior of Nb-Ti carbides during austenitizing process was analyzed.The experimental results indicate that the austenite grains of both steels grow up gradually with increasing the heating temperature and holding time;the size and growth rate of austenite grain of Nb-Ti-bearing high carbon steel are much lower than those of Nb-Ti-free high carbon steel.A large amount of(Nb,Ti)(C,N)nanoparticles are observed in Nb-Tibearing steel,which retain the strong pinning effect on austenite grain boundary.The kinetics model of austenite grain growth of Nb-Ti-bearing steel during isothermal heat treatment is obtained and the predicted values calculated by using the model meet the experimental values very well.

耐延迟断裂性能优良的高强度螺栓钢

参照 42 Cr Mo钢 ,通过增加 Mo量 ,添加微合金元素 V、Nb、降低 Mn量和杂质元素 P、S含量 ,研制开发出一种高强度螺栓钢 ADF1 ,在 1 30 0 MPa级的强度水平下具有良好的耐延迟断裂性能 ,并对此进行了分析。

新颖的贝氏体/铁素体双相低碳微合金钢

利用特殊微合金设计及终轧控冷工艺得到超细贝氏体/铁素体双相低碳微合金钢.该钢的组织由原奥氏体晶界上及晶粒内部的约5μm的准多边形铁索体及超细化的贝氏体板条束组成.铁素体的体积分数约20％.该双相低碳微合金钢的强度比同成分的全贝氏体钢略低,但其延伸率却大幅度提高.采取适当的回火处理,该双相钢屈服强度可达到700MPa,而延伸率大于25％,是一种具有高强度、高塑性的新型低碳微合金双相钢.

Nb、Ti对高强度耐候钢组织和性能的影响

为模拟CSP(紧凑式带材生产 )工艺 ,由实验室 10kg真空感应炉冶炼后轧成 6mm钢板 ,试验研究了Nb、Ti对成分 (% )为 0 0 6 0～ 0 0 76C ,0 2 5～ 0 31Cu ,0 4 5～ 0 5 6Cr,0 2 9～ 0 30Ni的 4 0 0MPa和 4 6 0MPa级高强度耐候钢组织和性能的影响。结果表明 ,含 0 0 3%Ti钢σs 和σb 分别为 4 5 0MPa和 5 4 5MPa ,含 0 0 3%Nb、0 0 2 %Ti钢σs 和σb 分别为 5 5 0MPa和 6 15MPa ,不含Nb、Ti钢σs 和σb 仅为 375MPa和 4 80MPa。 4 0 0MPa级耐候钢的组织为铁素体 +少量珠光体 ,含Ti钢的主要析出物为CuS2 (2 0～ 2 5nm)和TiN(80nm) ,含Nb、Ti4 6 0MPa级钢的组织主要为粒状贝氏体 ,析出物CuS2 和 (NbTi)CN(30～ 6 0nm) ,从而提高了钢的强度。

EAF-CSP流程钛微合金化高强钢板的组织和性能研究

珠钢采用Ti微合金化技术在EAF CSP流程上成功地开发出屈服强度为450～700 MPa的高强度热轧钢板.系统地研究了试验钢的组织和性能,并分析了组织与性能的关系.结果表明,随钛含量增加或成品厚度减薄钢板的屈服强度显著提高,最高达到695 MPa;钛的质量分数低于0.024%时对屈服强度影响不大;当钛的质量分数低于0.045%时,钢板屈服强度的提高主要来自于晶粒细化,而当钛的质量分数大于0.045%后,钢板强度的进一步提高来自于沉淀强化.

晶内铁素体型高强韧性微合金非调质钢的进展

晶内铁素体型微合金非调质钢成分一般为(%):0.2～0.4C,0.2～0.8Si,1.0～1.7Mn,0.05～0.15V,采用再结晶锻造或轧制、控制相变区间冷却速率以促进形成细小弥散的晶内铁素体组织是提高铁素体珠光体型微合金非调质钢强韧性的重要手段。选择合适的脱氧工艺,获得一定组成的细小、弥散的氧化物,使之成为MnS、VN和V4 C3等的析出核心,利于晶内铁素体析出的氧化物冶金是近来提高该钢强韧性的重要进展。介绍了国内外晶内铁素体型微合金非调质钢的化学成分和生产工艺,并分析了晶内铁素体形成机理。