回火温度对高碳钢断裂韧性的影响

研究了回火温度对高碳钢断裂韧性及其他力学性能的影响。利用紧凑拉伸试样测量其平面应变断裂韧度,扫描电镜(SEM)观察回火后的组织演变规律及断裂韧度试样断口形貌。结果表明:随着回火温度的升高,马氏体逐渐分解消失,从过饱和α固溶体中析出的碳化物数量逐渐增多并发生聚集长大,强度和硬度下降,塑性、断裂韧性和冲击韧性上升。位错强化和固溶强化作用减弱是试验钢强度降低、韧性升高的主要原因。     

Recrystallization Mechanism in Carbon Steels

Abstract

A systematic study of the recrystallization mechanisms in carbon steels has been undertaken. The microstructures (martensite, pearlite, tempered martensite containing Fe₃C—particles of various sizes) of the starting-point specimens were prepared in a preliminary heat-treatment experiment. For each of the specimens, the start, progress and end of recrystallization were followed up by means of optical and electron microscopes as well as hardness measurements. Various recrystallization mechanisms were observed, and each was seen to have correlation with a particular starting-point microstructure.     

Recrystallization and formation of austenite in deformed lath martensitic structure of low carbon steels

The effect of prior deformation on the processes of tempering and austenitizing of lath martensite was studied by using low carbon steels. The recrystallization of as-quenched lath martensite was not observed on tempering while the deformed lath martensite easily recrystallized. The behavior of austenite formation in deformed specimens was different from that in as-quenched specimens because of the recrystallization of deformed lath martensite. The austenitizing behavior (and thus the austenite grain size) in deformed specimens was controlled by the competition of austenite formation with the recrystallization of lath martensite. In the case of as-quenched (non-deformed) lath martensite, the austenite particles were formed preferentially at prior austenite grain boundaries and then formed within the austenite grains mainly along the packet, block, and lath boundaries. On the other hand, in the case of lightly deformed (30 to 50 pct) lath martensite, the recrystallization of the matrix rapidly progressed prior to the formation of austenite, and the austenite particles were formed mainly at the boundaries of fairly fine recrystallized ferrite grains. When the lath martensite was heavily deformed (75 to 84 pct), the austenite formation proceeded almost simultaneously with the recrystallization of lath martensite. In such a situation, very fine austenite grain structure was obtained most effectively.     

钢中稀土微合金化作用与应用前景

低氧硫钢中室温下稀土固溶量达10-5～10-4,MnS夹杂完全变质后,稀土固溶量显著增加,酸溶铝,Nb,Ti均有利于提高钢中稀土固溶量.固溶稀土偏聚在晶界,减少硫、磷在晶界的偏析,渗碳体中固溶稀土多于铁素体.稀土和碳相互作用,减少珠光体数量、珠光体片间距和渗碳体厚度,细化珠光体组织,提高硬度.稀土对钢的强度影响具有两面性,稀土能提高锰碳钢的屈强比和有利于改善低合金高强度钢的强韧性.稀土在低合金高强度钢中有广泛的应用前景,发展具有中国资源优势的稀土微合金钢有重要的意义.     

The Effect of the Initial Microstructure on Recrystallization and Austenite Formation in a DP600 Steel

The effects of initial microstructure and thermal cycle on recrystallization, austenite formation, and their interaction were studied for intercritical annealing of a low-carbon steel that is suitable for industrial production of DP600 grade. The initial microstructures included 50 pct cold-rolled ferrite–pearlite, ferrite–bainite–pearlite and martensite. The latter two materials recrystallized at similar rates, while slower recrystallization was observed for ferrite–pearlite. If heating to an intercritical temperature was sufficiently slow, then recrystallization was completed before austenite formation, otherwise austenite formed in a partially recrystallized microstructure. The same trends as for recrystallization were found for the effect of initial microstructure on kinetics of austenite formation. The recrystallization–austenite formation interaction accelerated austenization in all the three starting microstructures by providing additional nucleation sites and enhancing growth rates, and drastically altered morphology and distribution of austenite. In particular, for ferrite–bainite–pearlite and martensite, the recrystallization–austenite formation interaction resulted in substantial microstructural refinement. Recrystallization and austenite formation from a fully recrystallized state were successfully modeled using the Johnson–Mehl–Avrami–Kolmogorov approach.     

回火温度对淬火后30MnB5热成形钢组织与性能影响

将30MnB5热成形钢进行淬火和回火处理,利用扫描电镜、透射电镜、能谱仪和拉伸性能检测等方法研究了不同回火温度后的显微组织和力学性能变化.经200℃保温2 min回火后热成形钢的综合力学性能最佳,抗拉强度为1774 MPa,总伸长率为8%,强塑积达14 GPa·%以上,该性能满足热成形后作为汽车结构件的使用要求;并且随着回火温度的升高,力学性能呈非单调性变化.200℃低温回火后,主要为板条马氏体和ε碳化物,位错密度略有降低,析出的ε碳化物粒子呈针状分布在马氏体板条内,长度方向大小为100 nm左右,并与位错发生钉扎作用.随着回火温度的升高,板条马氏体发生回复和再结晶,板条边界逐渐模糊,并向等轴状铁素体转变,位错密度显著降低,ε碳化物逐渐向低能态的近球形渗碳体转变并粗化至200 nm左右,对位错的钉扎作用也随之减弱.      

Effect of Initial Microstructure on Warm Deformation Behavior of 45 Steel

 In order to investigate the effect of initial microstructure on warm deformation behavior, some specimens of 45 steel were annealed and some quenched. Then the specimens were isothermally compressed on a Gleeble 3500 machine. The deformation temperature range was 550 to 700 ℃ and the strain rate range was 0.001 to 0.1 s-1. An optical microscope (OM) and a transmission electron microscope (TEM) were used to study the microstructures. The results show that the microstructure of annealed specimens is ferrite and pearlite and that of quenched specimens is martensite. The flow stress of quenched specimens is higher than that of annealed ones at 550 ℃ when strain rates are greater than 0.001 s-1. However, at 600 to 700 ℃ and strain rate of 0.001 s-1, the whole flow curves of quenched specimens are below that of annealed ones. Under the rest conditions, the flow stress of quenched specimens is higher at the beginning of compression and then the opposite is true after the strain is greater than a critical value. The microstructure examination proves that the tempering and dynamic recrystallization easily occur in the specimens with martensite during warm compression, which results in the above phenomena.     

冷作硬化非调质钢的连续冷却相变规律

钢的连续冷却相变曲线(CCT)是组织调控的基本依据,为了优化紧固件用冷作硬化非调钢热轧态的组织和力学性能,采用DIL805A相变仪测定了试验钢在0.1～50℃/s不同冷却速率下的热膨胀曲线,结合金相硬度法确定相变类型,并绘制了试验钢的CCT曲线.结果 表明,试验钢马氏体转变点(Ms)为280℃,在不同冷速范围内均有铁素...     

高Co—Ni二次硬化马氏体钢中合金元素对相变的影响

利用“固体与分子经验电子理论”（ＥＥＴ）分析了高Ｃｏ－Ｎｉ二次硬化马氏体钢中几种主要合金元素在不同状态下的价电子结构，认为Ｃｏ、Ｎｉ在奥氏体中与Ｃ相互作用较强，它们能降低Ｃ的扩散能力，推迟马氏体相变。在马氏体回火过程中，Ｃｏ 和Ｎｉ溶入渗碳体形成的合金渗碳体，在较高的温度下分解，使固溶于基体中的Ｍｏ 与位错充分结合。在渗碳体分解后，Ｍｏ 与Ｃ形成富Ｃ、Ｍｏ的溶质簇，或以Ｍ２Ｃ的形式析出，在基体内弥散分布，形成二次硬化。     

稀土元素对重轨钢动态再结晶和珠光体片间距的影响

应用Gleeble-1500D热模拟试验机对重轨钢BNbRE和U71Mn进行单道次压缩试验,测定850～1 150℃下的真应力-真应变曲线,研究不同终轧温度和稀土元素对重轨钢动态再结晶的影响,并用扫描电镜观察变形后空冷的显微组织。结果表明,稀土元素能显著地抑制形变奥氏体的动态再结晶并细化珠光体片层间距。因此,稀土元素可抑制重轨钢动态再结晶,使其在较高终轧温度下仍能得到细小均匀的显微组织。     

回火时间对Fe-0.39C-3.69Mn中锰钢的组织和力学性能的影响

摘要：初始组织为片层珠光体的Fe-0.39C-3.69Mn（wt.%）钢板被快速加热到730℃保温90s后淬火至室温,获得包含13.2%残余奥氏体的板条马氏体组织。借助SEM、TEM、XRD和单向拉伸力学测试等试验手段,研究了在200℃回火时不同保温时间对微观组织和力学性能的影响,特别是对残余奥氏体体积分数和稳定性的影响。结果表明,随回火时间的增加,过渡碳化物逐渐增多并粗化,残余奥氏体含量先减少后不变;硬度、强度和均匀伸长率均逐渐降低,但断后伸长率由于马氏体回复而有所增加。片状残余奥氏体由于富Mn而具有较高稳定性,在200℃长时间回火后基本不发生分解。回火15min后获得最佳的综合力学性能,其屈服强度为1544MPa,抗拉强度为2031MPa,断后伸长率为10.1%。     

热处理对14MnVTiRE钢4 mm薄板组织和力学性能的影响

14MnVTiRE钢[/%:0.10～0.15C、0.30～0.60Si、1.20～1.60Mn、0.03～0.09V、0.07～0.16Ti、0.10～0.15RE(加入量)]4 mm薄板的生产流程为180 t顶底复吹转炉-180 mm×1 260 mm连铸板坯-连轧-卷取工艺。在连铸时由结晶器喂φ2.5 mm RE丝。试验研究了950℃正火,950℃正火+690℃1～5 h回火对薄板组织和力学性能的影响。结果表明,正火后钢的组织为铁素体+细珠光体,在正火+690℃回火1～3 h钢中的珠光体片层碳化物发生球化,分布在晶界和晶粒内部,随回火时间进一步增加,碳化物呈点状和三角形,全部分布在晶界,材料的强度降低很大。14MnVTiRE钢薄板的最佳热处理工艺为950℃正火+690℃1～2 h回火,其屈服强度为510～610MPa,抗拉强度630～680 MPa,伸长率22%～25%。     

The tempering of Fe-C lath martensite

The changes in matrix structure that occur during tempering of an Fe-0.2C martensite at 400° to 700°C have been investigated. Light and electron metallographic observations show that when tempered, the fine martensitic lath structure coarsens while retaining the elongated packet-lath morphology. The as-quenched hardness 504 Khn and total grain boundary area per unit volume 50,800 cm⁻¹ decrease abruptly at the higher tempering temperatures and in seconds reach relatively stable values that decrease slowly with time. The decrease in low angle boundaries accounts for most of the initial grain boundary area change, while the large angle boundary component of total boundary area decreases gradually with tempering time. Recovery processes are responsible for the initial changes in matrix structure, and carbide boundary pinning suppresses recrystallization until grain growth dominates in the later stages of tempering.     

变形温度对CSP热轧65Mn带钢相变行为的影响

利用ThermecMastor-Z型热模拟试验机模拟CSP工艺条件,辅以金相显微镜(OM)、扫描电镜(SEM)和维氏硬度计等,研究65Mn钢的连续冷却转变规律及变形温度对其等温相变的影响。绘制了65Mn钢的动态CCT曲线。结果表明,当轧后冷速小于2℃/s时,试验钢可获得铁素体和珠光体组织。随着冷速的增大,试验钢中将出现贝氏体和马氏体组织,硬度增大。当冷速大于40℃/s时,试验钢中的组织全为马氏体,硬度达到678.05HV。此外,在研究不同变形温度对65Mn钢等温相变的影响时发现,第2道次变形温度为920℃时,珠光体组织多呈片层状,硬度为271.86HV;随着变形温度的降低,试验钢中铁素体含量增加,珠光体球化趋势明显,粒状珠光体含量增多。当变形温度下降至860℃时,试验钢的硬度降低至252.21HV,有利于其后续深加工。     

Microstructure of stainless steel after heat treatment: Data from atomic-force microscopy

The micro- and nanostructure of 40Kh13 stainless steel is studied by optical, scanning electron, and atomic-force microscopy. The images of the steel’s structure and phase composition in three different states (after annealing, quenching, and high-temperature tempering) are compared. The optical images of the ferrite–pearlite structure with considerable content of (Cr, Fe)₂₃C₆ globular carbides obtained after annealing are compared with the results of scanning electron and atomic-force microscopy. It is found that the qualitative conclusions regarding the microstructure of the steel obtained by atomic-force and scanning electron microscopy not only agree with the results of optical microscopy but also provide greater detail. Data from the scanning electron microscope indicate that large carbides are located at the boundaries of ferrite grains. Some quantity of carbides may be found within the small ferrite grains. The size of the inclusions may be determined. The structure formed after quenching consists of coarse acicular martensite. Images from the atomic-force microscope show the acicular structure with greater clarity; three-dimensional images may be constructed. The undissolved carbides are also globular. The size of the martensite plates may be determined. The structure of the steel after high-temperature tempering (tempering sorbite) is formed as a result of the decomposition of martensite to ferrite–carbide mixture, with the deposition of regular rounded carbides. As confirmed by spectral analysis, the individual and row carbides (Cr, Fe)₂₃C₆ that appear contain chromium, which rapidly forms carbides. This structure is stronger than martensite. Data from uniaxial tensile tests are presented for all the states; the hardness HB is determined.     

Phenomena of carbon atom redistribution in martensite

Phenomena related to the redistribution of carbon atoms prior to carbide precipitation (tempering) are considered in this paper. 1) Order-disorder processes are described, and the influence of several factors is considered. It is shown that the lattice conjugation and coherence between different variants of martensite plates as well as between martensite plates and austenite leads to a disappearance of the tetragonal cubic phase transition. 2) A possible mechanism for the formation of “abnormal” martensite having a comparatively low axial ratio is considered to be (011)_M transformation twinning. 3) Diffuse maxima observed in electron diffraction patterns near reciprocal lattice points are discussed. The diffuse scattering is related to the short range order of carbon atoms and a spinodal decomposition of the Fe-C solid solution because of the C-C elastic strain interaction. 4) The influence of irradiation on the carbon atom distribution in martensite is considered, and it is shown that the observed decrease in martensite tetragonality after cooling below 0°C and its restoration upon subsequent heating may be explained by a phase transition involving the condensation of carbon atoms on irradiation-induced defects. 5) A short review on experimental results related to the redistribution of carbon atoms because of the presence of dislocations is given.     

Excess Solute Carbon and Retained Tetragonality in Tempered Fe-0.6C-1Mn Martensite and the Effect of Silicon Addition

The tetragonality and carbon distribution in tempered Fe-0.6C-1Mn martensite were investigated by X-ray diffraction and atom probe tomography to elucidate strain relaxation in the tetragonal lattice during tempering and its relationship with the solubility of excess carbon in martensite. Even though tetragonality (c/a) decreased with an increase in the tempering temperature, it persisted at low levels up to 400 °C. Si addition suppressed the decrease in tetragonality at 400 °C by inhibiting recovery in the dislocated matrix. Such persistence implies that dislocation migration is crucial for the complete release of tetragonal lattice strain at such a temperature, in addition to the decrease in the amount of solute carbon in martensite. A low level of tetragonality was observed for martensite containing carbon in the solid solution below the critical value of ~ 0.2 mass pct, at which a bcc structure was predicted. The amount of solute carbon after tempering was linearly correlated with tetragonality in the solute carbon content range of 0.07 to 0.6 mass pct, and the correlation coefficient was similar to those for as-quenched auto-tempered martensite and bainitic ferrite; these results indicate that the amount of excess carbon is simply determined by the amount of tetragonal lattice distortions remaining after carbide precipitation and recovery.

     

基于淬火回火的热镀锌退火工艺对双相钢组织性能的影响

提出了基于淬火回火的双相钢热镀锌退火工艺,并进行试验研究了该工艺对双相钢显微组织和力学性能的影响。结果表明,随着热镀锌或/及合金化时间的延长,材料的强度下降、塑性升高,相对于热镀锌退火,热镀锌合金化退火的温度更高、时间更长,因此双相钢的强度下降更明显;微合金元素铌延迟回火时马氏体中固溶碳的析出并延迟马氏体的回复再结晶过程,可以提高双相钢的回火稳定性,从而改善热镀锌双相钢的力学性能。     

Microstructure Evolution and Precipitation Behavior of 0Cr16Ni5Mo Martensitic Stainless Steel during Tempering Process

The microstructure,morphology of precipitates and retained austenite and the volume fraction of retained austenite in 0Cr16Ni5 Mo stainless steel during the tempering process were analyzed using optical microscope(OM),transmission electron microscope(TEM),X-ray diffraction(XRD)and scanning transmission electron microscope(STEM).The results show that the microstructure of the tempered steel is mainly composed of tempered martensite,retained austenite,and delta ferrite.In the case of samples tempered from 500 to 700 ℃,the precipitates are mainly M_(23)C_6,which precipitate along the lath martensite boundaries.The precipitate content increases with the tempering temperature.During the tempering process,the content of retained austenite initially increases and then decreases,the maximum content of retained austenite being 29 vol.% upon tempering at 600 ℃.TEM analysis of the tested steel reveals two morphology types of retained austenite.One is thin film-like retained austenite that exists along the martensite lath boundary.The other is blocky austenite located on packet at the boundary and the original austenite grain boundary.To further understand the stability of reversed austenite,the Ni content in reversed austenite was measured using STEM.Results show a significant difference in nickel concentrations between reversed austenite and martensite.     

常化后冷却工艺对1600MPa级超高强钢组织性能的影响

为改善高强度钢的塑性和韧性,对中碳低合金马氏体高强度钢分别采用常化后空冷＋回火和常化后控冷＋回火工艺,研究常化后冷却工艺对钢中残余奥氏体及力学性能的影响.采用扫描电镜获得钢的组织形态,利用X射线衍射和电子背散射衍射技术分析钢中残余奥氏体的体积分数、形貌和分布.发现两种工艺下均得到板条马氏体＋残余奥氏体组织,残余奥氏体均匀分布在板条之间,随工艺参数不同,其体积分数在3%~10%变化.常化后加速冷却能显著细化马氏体板条,提高钢的屈服强度和抗拉强度100 MPa以上,冲击功下降4 J.残余奥氏体的体积分数随常化控冷终冷温度的升高呈现先升高后降低的变化,常化后的控制冷却也可以作为进一步改善马氏体类型钢组织和性能的方法