基于过冷奥氏体动态相变的热轧TRIP钢组织控制 Ⅱ.动态相变后冷却速率

通过热模拟压缩实验,研究了铁素体相变前的奥氏体晶粒尺寸对基于动态相变的热轧C-Mn-Al-Si系TRIP钢组织及力学性能的影响。结果表明,减小原始奥氏体晶粒尺寸,可促进动态相变时的铁素体相变动力学,有利于铁素体、贝氏体及残余奥氏体等相分布更为均匀,获得的贝氏体束及贝氏铁素体尺寸较小,残余奥氏体的体积分数及C含量均较高,细小的颗粒状残余奥氏体数量较多且弥散分布,因此可获得具有较高强度和优良塑性的热轧TRIP钢。     

热轧双相钢的发展

1 前言热轧双相钢是由一定成分的低碳或低合金钢经控轧和控冷而得到具有铁素体和马氏体双相组织的钢。这种钢有75％～85％的铁索体及15％～25％的弥散于铁素体基体的岛状马氏体。同时,保留少量的残余奥氏体。     

基于动态相变热轧C-Mn-Si系TRIP钢的组织及性能

通过热模拟压缩实验,研究了基于动态相变的热轧C-Mn-Si系TRIP钢的组织特征及性能特点.结果表明,通过基于动态相变的热轧工艺,可以获得具有铁素体晶粒尺寸细小、贝氏体团径较小、贝氏铁素体板条较短且位向混乱、颗粒状残余奥氏体较多且分布弥散等特点的热轧TRIP钢.与750℃等温相变的热轧工艺相比,基于动态相变的热轧工艺制备的TRIP钢具有较高的强度和良好的塑性.     

基于动态相变的热轧C-Mn-Al-Si系TRIP钢组织演变

通过热模拟压缩实验,研究了基于过冷奥氏体动态相变的C-Mn-Al-Si系热轧TRIP钢的组织演变规律.结果表明,在过冷奥氏体动态相变过程中,铁索体相变速率较快,生成铁索体的体积分数与应变量有着一定的对应关系,铁素体晶粒细小且未相变的奥氏体更加分散.在贝氏体等温处理时,过冷奥氏体动态相变后较大尺寸的奥氏体能够较快地发生贝氏体相变,但生成的贝氏体束尺寸较小,位向较为混乱;而位于相邻铁素体晶粒间,尺寸在0.5-1.5μm之间的细小奥氏体岛稳定性较高,不易于发生贝氏体相变.通过过冷奥氏体动态相变和随后的等温贝氏体处理,可以获得晶粒细小的铁索体、体积分数较高的残余奥氏体、贝氏体和残余奥氏体分布更加弥散的热轧TRIP钢.     

冷却速度对C-Si-Mn系热轧双相钢组织性能的影响

通过实际的热轧实验,研究了轧后冷却速度对C-Si-Mn系热轧双相钢组织形貌和力学性能的影响。研究发现,轧后冷却速度对热轧双相钢的显微组织和力学性能有很大的影响。热轧双相钢具有三种典型的组织形貌,而不同的组织形貌通过不同的强化机制赋予热轧双相钢不同的力学性能。通过不同冷却速度下的双相钢热轧实验,可以使热轧双相钢获得良好的力学性能。     

20Mn2钢晶粒超细化及性能

借鉴超塑预处理细化晶粒的思路，在20Mn2钢中加入与其液态(1650℃）密度相同、且不溶于钢的ZrC粒子，使其成为钢在热轧时的形变核心和奥氏体及形变诱导铁素体的再结晶核心。并综合引入形变强化、相变强化、第二相弥散强化效应及细晶强化效应，使两种状态试验钢晶粒尺寸均细化到1—2μm。与20Mn2钢相比，淬火态试验钢σ0.2和σb分别提高187．0％和131．8％。同时，延伸率也有所提高；油淬态试验钢的σ0.2和σb分别提高为39．9％和34．2％，与20Mn2钢的塑性指标相比，油淬及低温回火态延伸率分别提高了90％和111％。     

Nb含量对Si-Mn系热轧双相钢组织与力学性能的影响

研究了Nb含量对热轧Si-Mn系双相钢组织与力学性能的影响。结果表明,不同的Nb含量可改变双相钢中马氏体的显微结构,从而出现不同的力学性能。当Nb含量控制在0.073%左右时,双相钢显微组织为不规则的多边形铁素体、少量的残余奥氏体以及岛状和板条状马氏体,其中马氏体的体积分数为18.3%,且内部位错密度较高,强度级别可以达到780~800 MPa。     

Cr对低碳Si-Mn系TRIP钢组织与力学性能的影响

主要研究了Cr对低碳Si-Mn系TRIP钢组织与力学性能的影响。首先利用Formastor-F型膨胀仪测定了含Cr和不含Cr两种低碳钢的连续冷却转变(CCT)曲线,分析指出了Cr对连续退火工艺的潜在影响;然后采用Gleeble-3800热/力模拟试验机对两种钢的薄板试样进行了连续退火模拟实验,并通过拉伸试验测定了力学性能;最后采用金相、扫描电镜、X-射线衍射分析等技术考察分析了两种钢的显微组织。结果表明:含Cr的TRIP钢的组织比较细小,铁素体晶粒近似等轴分布;两种TRIP钢的残余奥氏体含量相近,但含Cr钢的残余奥氏体中的含碳量较高。分析认为这是由于含Cr钢在热轧阶段较易生成细小的组织,而在热处理阶段则抑制贝氏体的生成,最终获得稳定的残余奥氏体。     

高氮奥氏体不锈钢组织调控及加工硬化机制

研究了高氮奥氏体不锈钢在不同热轧条件下的组织与性能的演变规律,探讨控轧参数对试验钢组织及加工硬化机制的影响。结果表明:随着终轧温度的降低,奥氏体晶粒由偏等轴变化到压扁拉长状,试验钢中铁素体的含量和球状碳化物析出量增多,抗拉强度和屈强比升高,伸长率降低,断面收缩率变化不大;空冷比水冷得到试验钢的抗拉强度大,而冷却方式对伸长率、断面收缩率和屈强比影响不大。在拉伸变形过程中应变硬化机制是相互转换和竞争的,在变形刚开始时(强加工硬化阶段),位错强化起主要作用,当变形进行到一定程度时(多阶段硬化阶段),位错强化和孪晶强化机制共同起作用。     

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 在未控轧控冷的轧制条件下,Q345E钢材终轧温度较高,铌推迟形变奥氏体再结晶的作用不明显。虽然铌/钒复合微合金化钢晶粒有一定程度的细化,由于微合金元素的沉淀强化及热轧态组织中贝氏体的出现,导致含铌微合金化钢低温韧性不能满足使用要求。通过对Q345E钢化学成分进行控制,结合控轧控冷技术,并采取合理的热处理工艺,使Q345E钢在保证高强度的基础上,－40 ℃低温冲击韧性得到明显提高。     

相同成分DP钢和TRIP钢部分力学性能的比较

对同一种钢板进行不同热处理分别制成具有相同铁素体含量的双相钢(DP钢)和相变诱发塑性变形钢(TRIP钢),并对其部分力学性能进行对比。比较发现,铁素体基体上不同的第二相使得材料力学性能产生巨大差异:马氏体使DP钢具有很高的抗拉强度,残余奥氏体则赋予TRIP钢优良的伸长率;DP钢拥有更加优良的加工硬化能力,TRIP钢则具有较为理想的烘烤硬化能力。试验还表明,考察DP钢和TRIP钢的烘烤硬化能力时,除柯氏气团外,内应力的消除也应该考虑其中。     

Microstructural Evolution and Deformation Behavior of a Hot-Rolled and Heat Treated Fe-8Mn-4Al-0.2C Steel

The microstructural evolution following tensile deformation of a hot-rolled and heat treated Fe-8Mn-4Al-0.2C steel was studied. Quenching in the range of 750-800 °C followed by tempering at 200 °C led to a ferrite-austenite mixed microstructure that was characterized by excellent combination of tensile strength of 800-1000 MPa and elongation of 30-40%, and a three-stage work hardening behavior. During the tensile deformation, the retained austenite transformed into martensite and delayed the onset of necking, thus leading to a higher ductility via the transformation-induced plasticity (TRIP) effect. The improvement of elongation is attributed to diffusion of carbon from δ-ferrite to austenite during tempering, which improves the stability of austenite, thus contributing to enhanced tensile ductility.     

高锰TRIP/TWIP效应共生钢高速变形过程中的组织演变及变形行为

对18Mn-3Al-3Si和21Mn-3Al-3Si高锰TRIP/TWIP效应共生钢动态变形过程中的变形行为,应变硬化速率、真应力和应变硬化指数随真应变的变化,以及应变硬化和基体软化间的相互作用等进行了研究,采用OM,SEM,TEM和XRD等方法对变形前后的组织进行了分析.结果表明,高应变速率下,TRIP/TWIP效应共生钢应变诱发相变途径为γ→ε→α;高速变形对滑移的抑制、奥氏体向马氏体的相变和形变孪晶对奥氏体晶粒的细化是应变硬化的主要因素;造成基体软化的原因是绝热温升效应、ε→γ的逆相变和孪晶的动态再结晶.     

Extended strain hardening by a sequential operation of twinning induced plasticity and transformation induced plasticity in a low Ni duplex stainless steel

Extended strain hardening was realized by a sequential operation of twinning induced plasticity (TWIP) followed by transformation induced plasticity (TRIP) in a Fe-20Cr-3Mn-2Cu-1Ni-1Si-0.2N duplex stainless steel (DSS). As a result, the present DSS exhibited an excellent combination of strength — ductility of 900 MPa which was 75% superior to that of conventional DSSs. The deformed microstructures of the present DSS revealed that strain induced martensite (SIM) causing TRIP primarily nucleated at intersections of mechanical twins without formation of ε martensite which is an intermediate phase during SIM transformation. In addition, the sequential operation of TWIP-TRIP enables strain hardening to be extended to higher strains compared to the operation of TWIP alone.     

Transformation hardening of steel sheet for automotive applications

Among high-strength steels, transformation hardening steels such as dual-phase (DP) steel and transformation-induced plasticity (TRIP) steel offer a superior relationship between tensile strength (TS) and elongation (El) on a commercial scale. As demand has grown for lighter-weight automobiles, so also has the demand for higher TS, lower yield ratio, and higher hole expansion ratio grown. Recently DP steel has been developed with precipitation hardening and grain refining by TiC. A new TRIP steel composed of 5Mn-2Si and control-rolled with niobium addition suggests the formation of retained austenite (γ _R ) as much as 30% and TS × El = 3,000 kgf/mm²·%.     

THE CHARACTERISTICS OF MICROSTRUCTURES AND PROPERTIES FOR ADVANCED ALUMINUM／SILICON CONTAINED HOT-ROLLED TRIP STEELS

The microstructure characteristics with super fine ferrite grain size less than 5mm,appropriate retained austenite fraction around 5.0% and or removable abundant dislocations have been obtained by controlled rolling and cooling, which leads to well balance comprehensive properties with high tensile strength of 510 and 615MPa, high elongation of 40% and 27%, low ratio of yield strength to tensile strength 0.83 and 0.80, as well as low ductilebrittle transition temperature less than -80 and -70℃ for advanced aluminum hot-rolled TRIP steel and silicon hot-rolled TRIP steel respectively.     

Effect of V–Ti Addition on Microstructure Evolution and Mechanical Properties of Hot-Rolled Transformation-Induced Plasticity Steel

In order to reveal the effect of V–Ti addition on the microstructure evolution and the mechanical properties of hot-rolled transformation-induced plasticity(TRIP) steel, two steels with 0.072 V–0.051 Ti steel(Bear-V–Ti steel) and 0.001 V–0.001 Ti steel(Free-V–Ti steel) were designed, respectively, and the comparison analyses were carried out by performing thermodynamic calculation and an experiment. With the thermodynamic calculation, the critical annealing temperature of a large fraction of retained austenite(～51%) obtained via solute enrichment was determined, and an optimized quenching at 650 °C and tempering at 200 °C adopted on the as-hot-rolled steel. The results show that the V–Ti TRIP steel displays more optimum mechanical stability during the tensile deformation, since the fraction and the mechanical stability of retained austenite are improved and the microstructure is also ultrarefined by V–Ti alloy precipitation. The yield strength of Bear-V–Ti steel increases from 650 to 800 MPa, and the ductility reaches 37%, showing that the comprehensive mechanical properties are greatly improved.     

非比例加载路径下TRIP钢的应变硬化行为

通过大试样拉伸与小试样拉伸相结合的方法,分析冷轧TRIP780钢的瞬时应变硬化速率随应变水平的变化规律,详细讨论不同应变路径下TRIP钢应力应变曲线体现出的硬化特性,并对微观组织的演变进行扫描电镜观察。结果表明,TRIP钢不仅具有较高的初始硬化能力,而且具有较长应变范围内的后续硬化能力;非比例加载路径下TRIP钢应力应变曲线表现出了软化效应、瞬时硬化能力和相交效应;加载路径的改变,可引起位错的运动方向和稳定性发生改变,从而影响硬化行为。     

Development of Ti–V–Mo Complex Microalloyed Hot-Rolled 900-MPa-Grade High-Strength Steel

A new Ti–V–Mo complex microalloyed hot-rolled high-strength steel sheet was developed by controlling a thermo-mechanical controlled processing(TMCP) schedule, in particular with variants in coiling temperature. The effects of coiling temperature(CT) on various hardening mechanisms and mechanical properties of Ti–V–Mo complex microalloyed high-strength low-alloy steels were investigated. The results revealed that the steels are mainly strengthened by a combined effect of ferrite grain refinement hardening and precipitation hardening. The variation in simulated coiling temperature causes a significant difference in strength, which is mainly attributed to different precipitation hardening increment contributions. When the CT is 600 C, the experimental steel has the best mechanical properties: ultimate tensile strength(UTS) 1000 MPa, yield strength(YS) 955 MPa and elongation(EL) 17%. Moreover, about 82 wt% of the total precipitates are nano-sized carbide particles with diameter of 1–10 nm, which is randomly dispersed in the ferrite matrix.The nano-sized carbide particles led to a strong precipitation hardening increment up to 310 MPa.     

Effect of Cooling Rates in Coiling Process on Microstructures and Mechanical Properties in Al-Bearing Hot-Rolled TRIP Steel

In this study, the effect of cooling rates on microstructures and mechanical properties in a Al-bearing hot-rolled transformation-induced plasticity steel was investigated. The experiments were carried out using hot simulation machine and hot rolling mill, where the samples were cooled at different cooling rates. The results showed that with the increase in cooling rates, film-like retained austenite gradually disappeared and only blocky retained austenite was retained at higher cooling rates. The volume fraction of retained austenite was 9–11% at cooling rates of 0.05–1°C/s and 4–6% at cooling rates of 5–10 °C/s. In addition, martensite/austenite island was observed because of the heterogeneous carbon distribution. The samples cooled at 0.05 °C/s and 0.5 °C/s exhibited excellent mechanical properties, with tensile strengths of 712 MPa and 726 MPa, total elongations of 42% and 36% and strength and ductility balances of 29.91 GPa% and 26.15 GPa%, respectively. During plastic deformation, the instantaneous work hardening exponent of the sample cooled at 0.05 °C/s increased continuously until it reached the maximum value, while the instantaneous work hardening exponent of the sample cooled at 0.5 °C/s remained stable.