基于纳米铁碳析出物的钢综合强化机理

简要叙述了关于钢强化机理的研究现状,用化学相分析+X射线小角散射、RTO方法及高分辨透射电镜对薄板坯连铸连轧钛微合金化高强耐候钢中纳米粒子的属性进行了综合分析。发现钛微合金化高强耐候钢中尺寸<36 nm的粒子,除纳米TiC以外,还存在大量的纳米Fe3C,其体积分数为同尺寸TiC体积分数的4.4倍,析出强化作用比纳米TiC粒子大,不可忽略;提出了钢的综合强化机理,指出对不同种类、不同尺寸的纳米析出粒子,应分别根据位错切割和位错绕过机理计算出析出强化贡献,然后与固溶强化和细晶强化贡献加和,求得钢的屈服强度;讨论了细晶强化与位错强化不能加和的原因以及相变对钢强度影响的表现形式,对钛微合金化高强耐候钢屈服强度的理论计算与生产结果相符,实验钢σs=630～676 MPa,实际σs=630～680 MPa。简述了钢综合强化机理的应用情况,指出了有待进一步研究的科学问题。     

钛微合金化低碳钢的研究进展

工业水平的发展对低合金低碳钢的性能提出了更高的要求.较高的强度、良好的韧性和抗疲劳能力以及优异的耐蚀性等是低合金高强钢开发的主要方向.微合金化处理通过在钢中加入微量的合金元素可以明显地改善材料的性能.钛微合金化成本较低,能够明显细化奥氏体晶粒,提高材料的强度,具有广泛的应用价值.微合金化元素钛与钢中的碳元素、氮元素反应生成的TiN、TiC以及Ti(C,N)第二相粒子所产生的沉淀强化、细晶强化等作用能够明显改善材料的性能.TiN粒子析出温度较高,细小的TiN粒子可以抑制高温下晶粒的长大;而粗大的TiN粒子对材料的性能不利.TiC粒子可以在铁素体基体中随机沉淀析出,并与基体保持一定的位相关系,还可以钉扎位错、细化晶粒.Ti(C,N)粒子由TiC与TiN互溶形成,可以钉扎位错,产生析出强化.第二相粒子的尺寸受热处理工艺等影响,因此,需要严格调控材料的热处理工艺,避免粗大第二相粒子的形成.钛的微合金化作用还受到钢中其他合金元素的影响,钛与钼、锰、硼等元素可以产生协同作用,相互促进,有利于材料的强韧性匹配.将钛元素与铌元素、钒元素中的一种或两种同时引入合金钢中进行复合微合金化处理,钛铌复合可以在提高材料强度的同时避免塑性的大量损失,钛钒复合可以降低强度提高时对材料韧性的损害并有效提高材料的淬透性,铌钒钛复合可以结合三种元素的优点更好地改善材料的性能.但是,复合微合金化对合金元素含量具有较高的要求,含量控制不当会严重影响材料的性能.文中主要介绍了近年来国内外关于低碳钢的钛微合金化的研究现状,并针对微合金化的强韧化机理研究进展进行了分析和评述,以期为制备性能优良、适合实际生产的微合金化钢提供参考.     

Super304H奥氏体耐热钢微观组织研究

为了深入认识新型奥氏体耐热钢Super304H(0.1C-18Cr-9Ni-3Cu-Nb,N)的微观组织,利用光学显微镜、扫描电子显微镜、电子探针和X射线衍射等手段,研究了Super304H钢合金元素的分布和析出相的组成及分布形态.试验结果表明,Super304H钢在供货状态下的显微组织由γ-基体和析出相组成.与传统的18-8不锈钢相比,这种奥氏体钢晶粒均匀细小,晶粒尺寸约46μm.析出相主要由Nb(C,N)和富铜相组成,Nb(C,N)有呈方向性分布的条块状和呈弥散分布的细小颗粒状两种形态,条块状的Nb(C,N)是软化过程中残留下来的,而弥散分布的是固溶处理及冷却过程中析出形成的.其中弥散分布的Nb(C,N)与富铜相对细化晶粒和改善钢的高温强度起重要作用,而多种复合强化机制使得Super304H钢具有优异的高温性能.     

N含量对Cr-Mo-V系超低碳贝氏体钢组织性能和析出行为的影响

通过光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和能谱仪(EDS)等实验方法,研究了三种不同N含量的超低碳贝氏体钢的显微组织和析出相的成分、尺寸、形貌以及分布等特征。结果表明:低氮含量的钢组织为粒状贝氏体,高氮含量的钢组织为粒状贝氏体+少量的针状铁素体。当实验钢中V/N比为3.4时,通过细晶强化和沉淀强化综合作用,可以使材料的屈服强度和抗拉强度分别增加231MPa和95MPa。与氮含量低的钢相比,高氮含量的钢具有更细小的贝氏体铁素体板条亚结构,且析出相尺寸减小,体积分数增加。基体中存在两种尺寸的纳米级析出相:一种尺寸在10～15nm之间,为V(C,N)析出相,弥散分布在贝氏体板条内部;另一种是含有Cr和V尺寸在10nm以下,具有面心立方结构的(V,Cr)(C,N)复合析出相。     

N含量对Cr-Mo-V系超低碳贝氏体钢组织性能和析出行为的影响EI北大核心CSCD

通过光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和能谱仪(EDS)等实验方法,研究了三种不同N含量的超低碳贝氏体钢的显微组织和析出相的成分、尺寸、形貌以及分布等特征。结果表明:低氮含量的钢组织为粒状贝氏体,高氮含量的钢组织为粒状贝氏体+少量的针状铁素体。当实验钢中V/N比为3.4时,通过细晶强化和沉淀强化综合作用,可以使材料的屈服强度和抗拉强度分别增加231MPa和95MPa。与氮含量低的钢相比,高氮含量的钢具有更细小的贝氏体铁素体板条亚结构,且析出相尺寸减小,体积分数增加。基体中存在两种尺寸的纳米级析出相:一种尺寸在10～15nm之间,为V(C,N)析出相,弥散分布在贝氏体板条内部;另一种是含有Cr和V尺寸在10nm以下,具有面心立方结构的(V,Cr)(C,N)复合析出相。     

含Nb细晶高强IF钢热变形行为的析出热力学

以新型含Nb细晶高强IF钢为研究对象,运用规则溶液亚点阵模型计算了实验钢从均热(1523K)冷却至室温(293K)的各个工艺过程中碳氮化物析出相的平衡体积分数、析出相的组成成分、各组元的平衡质量分数。计算结果表明,从均热到冷却至室温(1523K~293K)的过程中,随着温度的降低,固溶于基体中的Nb、C、N元素的质量分数逐渐减小,析出相平衡体积分数先增大后减小,NbN在析出相中比例逐渐降低,NbC在析出相中比例逐渐增加。至室温(293K)时,Nb和N元素几乎全部析出,碳氮化物不再继续析出。     

V-N微合金化抗震钢筋铁素体中V(C,N)析出行为分析

采用回火硬度法研究了两种不同V,N含量的抗震钢筋回火等温过程组织及显微硬度变化规律,根据J-M-A(Johnson-Mehl-Avrami)理论定量计算了V(C,N)在铁素体中的析出热动力学,并与实验所测PTT(析出-温度-时间)曲线进行对比。透射电镜下观察了V(C,N)沉淀析出规律。结果表明,由于大量弥散的第二相颗粒的作用,V-N微合金化钢筋(0.04V-0.0135N)的回火组织比V微合金化钢筋(0.076V-0.0055N)更加均匀细小。回火硬度法测得两种材料的PTT曲线与计算所得吻合,V微合金化钢筋呈"C"型且在670℃左右的鼻温区析出动力学加快,V-N微合金化钢筋是一单调曲线,这主要考虑N含量的影响。本次实验同时观察到纤维状碳化物,V(C,N)在过饱和铁素体中相间析出以及位错线析出,并就其形成机制进行讨论。     

超快冷条件下Mn-Nb-B系低碳贝氏体高强钢组织与性能研究

采用Mn-Nb-B减量化成分设计的低碳贝氏体高强钢为研究对象,通过热模拟实验研究实验钢热变形行为和相变行为。结合中厚板生产线特点制定控制轧制与超快速冷却相结合生产工艺路线,充分利用超快速冷却条件下的细晶强化、析出强化等综合强化机制,实现综合力学性能优良的低成本高强工程机械用钢的试制和生产。产品屈服强度和抗拉强度分别达到678MPa和756MPa,伸长率A50为33%,-20℃低温冲击达到261J。产品显微组织由粒状贝氏体、针状铁素体和板条贝氏体组成,基体组织内弥散分布着细小的点状、粒状M/A岛和均匀细小的(Nb,Ti)(C,N)析出粒子以及大量位错组织。     

Nb、Ti微合金钢中碳氮化物固溶与再析出的研究

利用光学显微镜(OM)、透射电子显微镜(TEM)研究了再加热温度、奥氏体区变形温度和组织转变温度的变化对Nb、Ti微合金钢组织性能及其碳氮化物固溶与再析出行为的影响.结果表明:钢中加入铌,主要利用铌的碳氮化物在奥氏体形变过程中的再析出,抑制形变奥氏体的再结晶,在随后的组织演变过程中细化了组织;而钢中加入较高含量的钛,主要利用钛的碳化物在铁素体中的析出,产生明显的沉淀强化作用.这主要是铌、钛的碳氮化物固溶后,在奥氏体和铁素体中再析出的不同所造成的.钢中复合加入Nb-Ti后既起到细化晶粒的作用,又起到析出强化的作用.细晶强化既提高钢的强度又提高钢的韧性,但沉淀强化在大幅提高钢的强度的同时恶化了钢的韧性.     

复合微合金化对Al-Mg合金组织与性能的影响

研究了Sc和Ti复合微合金化对Al-Mg合金显微组织与拉伸性能的影响.结果表明:Sc和Ti复合微合金化可以显著提高Al-Mg合金的强度,并可细化铸态合金的晶粒组织.微量Sc和Ti的加入可使合金中形成大量细小弥散的球形Al3(Ti,Sc)粒子,这些Al3(Ti,Sc)粒子对位错和亚晶界具有强烈地钉扎作用,因而能强烈抑制合金的再结晶.Sc和Ti复合微合金化的Al-Mg合金的强化作用主要来源于Al3(Ti,Sc)粒子的析出强化和亚结构强化以及细晶强化.     

正火型V-N-Ti和Nb-V-Ti海工钢焊接粗晶热影响组织和韧性研究

用焊接热模拟方法研究了V-N-Ti和Nb-V-Ti微合金化正火型海工钢模拟粗晶热影响区(CGHAZ)组织和韧性的变化规律。结果表明,组织的不同使V-N-Ti设计正火型海工钢的模拟CGHAZ韧性比Nb-V-Ti钢的好。对于V-N-Ti钢,较高的N含量提高了富Ti(Ti, V)(C, N)粒子析出温度和铁素体形核能力,使模拟CGHAZ原始奥氏体晶粒和(取向差角为15°)晶粒细化,并生成能阻止或使解理裂纹的偏转细小多边形铁素体,因此具有良好的低温韧性。而Nb-V-Ti钢模拟CGHAZ原奥氏体晶界上的链状M-A、粗大的原始奥氏体晶粒和有效晶粒尺寸,是模拟CGHAZ韧性差的原因。     

C和V对奥氏体不锈钢焊缝的强化

研究了元素C、V对高强度奥氏体不锈钢焊缝金属的强化作用。结果显示,提高C含量,具有明显的固溶强化效果,但C含量的增加导致大量碳化物在晶界上析出;提高V含量使细小弥散的碳化钒在晶内析出,产生明显的沉淀强化效果。在强化效果基本相同时,后者的韧性高于前者。     

The effect of titanium and reheating temperature on the microstructure and strength of plain-carbon,vanadium- and niobium-microalloyed steels

A series of plain-carbon, vanadium- and niobium-microalloyed steels with or without titanium addition were used to evaluate the effect of a small amount of titanium addition on the properties of steels. Titanium inhibits austenite grain coarsening during reheating and grain refinement was observed when the reheating temperature was below the austenite grain coarsening temperature. The lower the reheating temperature, the less was the observed precipitation strengthening effect of V(C, N). The addition of titanium to microalloyed steels reduces the precipitation strengthening effect of V(C, N) but has no visible effect on that of Nb(C, N). The mechanism of reducing the strengthening effect of V(C, N) is possibly caused by the depletion of available nitrogen content for V(C, N) formation.     

Microstructures and Mechanical Properties of Nb-Ti Bearing Hot-rolled TRIP Steels

Nb-Tihot-rolled TRIP-assisted steel with high plasticity and appropriate volume percentage of retained austenite based on fine ferrite grain have been developed in the experiment. The test results showed that niobium tend to exist in solution state in matrix with less precipitation, and niobium-titanium could be precipitated in form of （Nb, Ti）C or （Nb, Ti） （C, N）, which play an important role in increasing yield strength （from 495 MPa to 610 MPa）. Besides, the retained austenite had a positive effect on improving the plasticity by transformation into martensite during tensile deformation.     

The effect of deformation-induced-ferrite-transformation on nanometre-sized carbides in Ti–Mo ferritic steel

In this study, the effect of deformation-induced-ferrite-transformation (DIFT) rolling on precipitation in Ti–Mo ferrite matrix micro-alloyed steel was investigated by comparing to rolling in austenite non-recrystallisation region. The precipitate volume fraction and precipitation-starting time-temperature (PTT) curves under two kinds of process were calculated by thermodynamic and kinetic calculation, and the effects of non-recrystallisation rolling and DIFT rolling on the contribution of strengthening mechanisms were quantitatively analysed. The results showed that comparing with rolling in the austenite non-recrystallisation region, carbides in the steel by DIFT rolling were finer and more uniform. Moreover, DIFT rolling could enhance fine grain strengthening and precipitation strengthening simultaneously, and the increments of fine grain and precipitation strengthening were 34 and 63.7?MPa, respectively.     

Transformation and precipitation behaviour of Ti–Mo bearing high strength medium-carbon steel

Abstract

The authors describe here the transformation and precipitation behaviour of Ti–Mo bearing high-strength medium-carbon steel during continuous cooling, using a combination of thermo-simulation and microscopy approach. The study demonstrates that Ti and Mo carbides precipitate during austenite-to-ferrite, austenite-to-bainite and even during austenite-to-martensite transformations, contributing to precipitation strengthening. Four different types of precipitates in the size range of 3–200 nm were observed during the transformation. They are spherical (Ti,Mo)C and TiC, cuboidal (Ti,Mo)(C,N) and long thin strips of Fe_xC. The size of the precipitates was large and the density was less during austenite transformation. However, the size decreased and density increased during the austenite-to-bainitic ferrite transformation. During the austenite-to-martensite transformation, a high density of fine and spherical-shaped precipitates comprising of Ti and Mo in the range of 3–10 nm were observed.     

Effect of nitrogen and vanadium on austenite grain growth kinetics of a low alloy steel

Austenite grain growth kinetics in a steel containing 0.4% C, 1.8% Cr with different nitrogen contents (in the range 0.0038–0.0412%) and a micralloying addition of 0.078% V were investigated. The investigations were carried out in an austenitising temperature range of 840–1200 °C for 30 min. The results of investigations showed that N promotes the grain growth of austenite. The microalloying addition of vanadium protects the austenite grain growth because of carbonitride V(C,N) precipitation and the grain boundary pinning effect of undissolved particles of V(C,N). Using a thermodynamic model, the carbonitride V(C,N) content, undissolved at the austenitising temperature was calculated. At temperatures when a coarsening and dissolution of carbonitride occurs, the austenite grains start to growth. The effect of nitrogen on the type of chord length distribution of austenite grains was analysed.     

Effects of precipitates on strength and toughness of vanadium structural steels

Abstract

The effects of precipitates formed by the addition of vanadium, nitrogen, and aluminium on the strength and toughness in 0·15%C steels were studied by cooling suitable steels at varying rates from temperatures giving a constant austenite grain size. Yield strength increments resulting from precipitation were derived from the experimental results. Similar increments were produced by vanadium carbide (VC) and vanadium carbonitride (V(C,N)) and in each case a reduction in cooling rate resulted in reduced strengthening because of increased particle coarsening. In the presence of aluminium rather less strengthening was obtained from V(C,N) precipitates. No strengthening resulted from the presence of aluminium nitride (AIN) particles. When the carbon content of the steel is greatly in excess of the stoichiometric quantity, the carbonitride formed is relatively low in nitrogen and the yield strength increments caused by low nitrogen V(C,N) and VC are similar. Impact transition temperature increments resulting from precipitation were also derived from the experimental data. These showed that V(C,N) precipitates are less detrimental to transition temperatures than are VC precipitates giving comparable yield strength increments. Shelf energies were also reduced to a greater extent by VC than by V(C,N). The presence of AIN particles had little effect on shelf energy, but had a detrimental effect on transition temperature. The high nitrogen V(C,N) precipitates are less detrimental to both impact transition temperatures and upper shelf energies of the steels than are low nitrogen V(C,N) or VC precipitates. Removal of soluble nitrogen in the form of nitrides has been confirmed as a means of improving the toughness of steels. Aluminium has been found to be more effective than vanadium in the removal of soluble nitrogen.

MST/935     

微钛低碳钢板的微观组织观察与分析

利用金相显微镜和电子显微镜等方法对微Ｔｉ低碳钢板的微观组织和第二相粒子的沉淀行为进行了研究。结果表明，基体组织由极细晶铁素体和亚晶粒组成，铁素体晶粒平均尺寸为４．８７μｍ。细小的第二相粒子在铁素体中沉淀析出，粒子的平均尺寸为１２．４ｎｍ，沉淀的粒子体积百分数为０．０３６％。在亚晶界上观察到大粒子的沉淀，位错被细小呈球形的粒子所钉扎．相分析表明，这些粒子为ＴｉＮ，Ｔｉ（Ｃ、Ｎ）或ＴｉＣ．经计算，沉淀强化与细晶强化值分别为４０ＭＰａ和２４０ＭＰａ。