Effect of lower bainite/martensite/retained austenite triplex microstructure on the mechanical properties of a low-carbon steel with quenching and partitioning process

We present a study concerning Fe–0.176C–1.31Si–1.58Mn–0.26Al–0.3Cr (wt%) steel subjected to a quenching and partitioning (Q&P) process. The results of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and tensile tests demonstrate that the microstructures primarily consist of lath martensite, retained austenite, lower bainite (LB), and a small amount of tempered martensite; moreover, few twin austenite grains were observed. In the microstructure, three types of retained austenite with different sizes and morphologies were observed: blocky retained austenite (~300 nm in width), film-like retained austenite (80–120 nm in width), and ultra- fine film-like retained austenite (30–40 nm in width). Because of the effect of the retained austenite/martensite/LB triplex microstructure, the specimens prepared using different quenching temperatures exhibit high ultimate tensile strength and yield strength. Furthermore, the strength effect of LB can partially counteract the decreasing strength effect of martensite. The formation of LB substantially reduces the amount of retained austenite. Analyses of the retained austenite and the amount of blocky retained austenite indicated that the carbon content is critical to the total elongation of Q&P steel.     

Structural refinement of 00Cr13Ni5Mo2 supermartensitic stainless steel during single-stage intercritical tempering

The 00Cr13Ni5Mo2 supermartensitic stainless steel was first tempered at 570–730℃ for 2 h to observe the microstructure and hardness changes. The tempering temperature was set to 600, 650, and 700℃, which is below, equal to, and above the austenite transformation start temperature, respectively, for each holding period to investigate the effects of tempering time on the structure and properties of the steel. The microstructure of the specimens was examined by optical microscopy and transmission electronic microscopy, and the phase composition was detected by X-ray diffraction. As expected, lath refinement was observed in the steel tempered at 700℃, and the refinement degree significantly depended on the tempering time. Contrary to normal steel softening by tempering, the hardness performance of the steel was significantly enhanced primarily because of the refinement of martensite laths after single-stage intercritical tempering. It is believed that the reverse transformation of martensite (α′) to austenite (γ) is responsible for the refinement.     

低碳Si-Mn系DQ&P钢组织演变模拟研究

以低碳Si-Mn钢为研究对象,采用直接淬火-配分工艺,研究了不同变形温度和配分时间对组织演变行为的影响.结果表明,QP钢典型组织由马氏体、铁素体和少量残余奥氏体组成.其中马氏体呈现板条马氏体、孪晶马氏体等特征.随着配分时间的延长,显微组织呈现回火转变趋势.当变形温度为950℃和880℃时,残余奥氏体体积分数先增加后减少,均在配分150 s时达到最大值,分别为9.1%,10.1%.当变形温度在820℃时,由于先共析铁素体的存在易于获得较多的残余奥氏体,体积分数高达11.9%,并且残余奥氏体中平均碳含量相对较低.     

淬火介质对耐磨铸钢组织和性能的影响

分别研究了以水和机油作为淬火介质的42CrMo钢和双相耐磨钢淬火+回火后的显微组织和性能.确定了双相耐磨钢和42CrMo钢的最佳热处理工艺及性能.结果表明:双相耐磨钢经水淬和油淬回火后的显微组织均为板条马氏体+针状贝氏体+碳化物;42CrMo钢经水淬和油淬回火后的显微组织分别为回火马氏体+残余奥氏体和回火马氏体+铁素体+贝氏体.经过480h的低应力磨料磨损实验发现,水淬+280℃回火处理的42CrMo钢耐磨性能最好,其相对耐磨性可达高锰钢的1.558倍.     

一种Mn-Al系TRIP钢的临界区奥氏体稳定化研究

利用实验室MMS-200热模拟试验机对Fe-0.2C-7Mn-3Al钢的临界区奥氏体稳定化行为进行研究.通过SEM,EPMA,TEM和XRD等手段观察并分析了实验钢的微观组织演变以及C和Mn元素的配分过程.实验结果表明,不同的临界区退火温度下,实验钢中均存在25%~30%左右的粗大压扁状δ铁素体.随着退火温度的升高,微观组织中残余奥氏体的含量先增加后减小,体积分数为10.2%~32.5%,残余奥氏体与临界区铁素体呈板条状相间分布,板条宽度约200~300 nm.最佳的临界区退火温度为750℃.C,Mn,Al元素的协同作用促进了临界区奥氏体的稳定化,使得实验钢能够在较短的时间内完成有效的配分.     

Variation in retained austenite content and mechanical properties of 0.2C–7Mn steel after intercritical annealing

The effects of annealing time and temperature on the retained austenite content and mechanical properties of 0.2C–7Mn steel were studied. The retained austenite content of 0.2C–7Mn steel was compared with that of 0.2C–5Mn steel. It is found that 0.2C–7Mn steel exhibits a similar variation trend of retained austenite content as 0.2C–5Mn steel. However, in detail, these trends are different. 0.2C–7Mn steel contains approximately 7.5vol% retained austenite after austenitization and quenching. The stability of the reversed austenite in 0.2C–7Mn steel is lower than that in 0.2C–5Mn steel; in contrast, the equilibrium reversed austenite fraction of 0.2C–7Mn steel is substantially greater than that of 0.2C–5Mn steel. Therefore, the retained austenite content in 0.2C–7Mn steel reaches 53.1vol%. The tensile results show that long annealing time and high annealing temperature may not favor the enhancement of mechanical properties of 0.2C–7Mn steel. The effect of retained austenite on the tensile strength of the steel depends on the content of retained austenite; in contrast, the 0.2% yield strength linearly decreases with increasing retained austenite content.     

预处理组织对低碳钢IQ&P工艺下组织及性能影响

采用γ单相区和γ+α双相区轧制并淬火工艺以及双相区再加热-淬火-碳配分( IQ&P)工艺,研究预处理组织对低碳钢室温状态多相组织特征及力学性能的影响规律. 实验用低碳钢经两种工艺轧制并淬火处理,获得马氏体和马氏体+铁素体的预处理组织,再经双相区IQ&P工艺处理后均获得多相组织. 马氏体预处理钢的室温组织由板条状亚温铁素体、块状回火马氏体以及一定比例的针状未回火马氏体和8. 2%的针状残余奥氏体组成;马氏体+铁素体预处理钢由板条状亚温铁素体、块状和针状未回火马氏体以及14. 3%的短针状或块状残余奥氏体组成. 在相同的双相区IQ&P工艺参数下,预处理组织为马氏体的钢抗拉强度为770 MPa,伸长率为28%,其强塑积为21560 MPa·%;而预处理组织为马氏体+铁素体的钢抗拉强度为834 MPa,伸长率增大到36. 2%,强塑积达到30190 MPa·%,获得强度与塑性的优良结合.     

两种贝氏体塑料模具钢的相变与残余奥氏体

将两种塑料模具钢FT600 和FT600mod 奥氏体化后, 以连续冷却或等温方式, 获得贝氏体组织. 随后分别在350, 580, 700 °C下进行回火, 研究了Si, Mn元素微调后贝氏体组织及残余奥氏体转变情况. 对两种贝氏体钢在不同热处理工艺下的宏观硬度、残余奥氏体含量、微观组织进行了表征及分析. 研究结果表明, 在FT600 钢的基础上降Si 增Mn 得到了FT600mod 钢, 其贝氏体转变后的残余奥氏体含量大幅降低; FT600 钢中的残余奥氏体在不同温度下回火, 其转变机制不同; FT600mod钢中的残余奥氏体含量较少, 组织稳定, 更适用于非调质工艺处理.     

高强高韧低合金锰钢的研制

采用多元合金化思路设计了一种新型低合金高强度高韧性锰钢.研究了该钢的静态CCT曲线、显微组织、断口形貌以及热处理工艺对钢的力学性能的影响.结果表明:该钢中过冷奥氏体的稳定性高,具有高淬透性及高回火稳定性,经890～930℃淬火及200～230℃回火后获得回火板条马氏体组织,使该钢具有高的强韧性(抗拉强度Rm≥1500MPa,冲击韧性Akv≥85J)匹配;钢中适当提高锰含量,符合我国资源情况,具有较高的性价比.     

Influence of original microstructure on the transformation behavior and mechanical properties of ultra-high-strength TRIP-aided steel

The transformation behavior and tensile properties of an ultra-high-strength transformation-induced plasticity (TRIP) steel (0.2C-2.0Si-1.8Mn) were investigated by different heat treatments for automobile applications. The results show that F-TRIP steel, a traditional TRIP steel containing as-cold-rolled ferrite and pearlite as the original microstructure, consists of equiaxed grains of intercritical ferrite surrounded by discrete particles of M/RA and B. In contrast, M-TRIP steel, a modified TRIP-aided steel with martensite as the original microstructure, containing full martensite as the original microstructure is comprised of lath-shaped grains of ferrite separated by lath-shaped martensite/retained austenite and bainite. Most of the austenite in F-TRIP steel is granular, while the austenite in M-TRIP steel is lath-shaped. The volume fraction of the retained austenite as well as its carbon content is lower in F-TRIP steel than in M-TRIP steel, and austenite grains in M-TRIP steel are much finer than those in F-TRIP steel. Therefore, M-TRIP steel was concluded to have a higher austenite stability, resulting in a lower transformation rate and consequently contributing to a higher elongation compared to F-TRIP steel. Work hardening behavior is also discussed for both types of steel.     

回火对非调质SG钢组织和性能的影响

采用低频力学谱仪、扫描电子显微镜和透射电子显微镜并结合JMatPro软件分析了贝氏体型非调质SG钢在350 °C回火后的组织及强韧化机制,分析其内耗与温度的关系曲线中Snoek峰、SKK峰及其微观形貌,探讨了贝氏体中铁素体的碳原子分布及析出行为、粒状贝氏体中岛状马氏体/奥氏体的分解规律.结果表明：未回火SG钢的Snoek峰强度极低,且贝氏体中铁素体的固溶碳原子含量较低;经350 °C回火2 h后,SG钢的屈服强度最高.这是由于析出强化和位错强化共同作用的结果,其中分解出现的弥散碳化物Cr7C3相起到了主要作用.     

9Ni钢中逆转变奥氏体的稳定性

为了研究逆转变奥氏体在应力和低温条件下的稳定性,通过透射电子显微镜对淬火+回火(QT)和淬火+两相区淬火+回火(QLT)处理9Ni钢中逆转变奥氏体进行形貌观察.结果发现QT处理钢中逆转变奥氏体以块状形态存在;QLT处理钢中逆转变奥氏体以块状和薄膜状存在,薄膜状逆转变奥氏体分布在板条边界上.通过X射线衍射对液氮浸泡前后逆转变奥氏体含量和其碳含量的变化进行测量,发现QLT工艺处理后在液氮中稳定存在的逆转变奥氏体的含量要高于QT工艺.通过三点弯曲试验、单轴拉伸和压缩试验研究其机械稳定性,发现拉应力和压应力均能促使逆转变奥氏体转变,大部分残留奥氏体分布在晶内.     

马氏体钢力学性能和耐磨性的关系

对自行研制的新型马氏体耐磨钢20Si2Ni3进行了Q-P-T(淬火-分配-回火)及传统淬火-回火(Q-T)热处理,用MLD-10型动载磨料磨损实验机比较在1.5J冲击能量、石英砂磨料条件下,20Si2Ni3钢和高锰钢ZGMn13的冲击磨料磨损性能,并用X射线测量磨损实验前后20Si2Ni3钢残余奥氏体量的变化,用扫描电镜(SEM)分析磨损机理。结果表明:经两种热处理工艺后的20Si2Ni3钢,其冲击磨料磨损性能均优于高锰钢。相比传统Q-T热处理工艺,经Q-P-T热处理后的20Si2Ni3钢,在保持较高硬度的同时,冲击韧性得到了较大提高;不同热处理工艺后,在硬度均约HRC45.5时,20Si2Ni3钢的冲击磨料磨损性能随着冲击韧性的增加而提高,磨损机理以显微犁削为主。     

Element distribution and diffusion behavior in Q&P steel during partitioning

Carbon, manganese, and silicon distribution in quenching and partitioning （Q＆P） steel during partitioning process was investigated to reveal the diffusion behavior. The microstructure and chemical composition were analyzed by means of scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and three-dimensional atom probe. It is shown that the studied Q＆P steel consisted of martensite laths and thin, film-like retained austenite showing extraordinary phase transformation stability. Carbon atoms mostly diffused to the retained austenite from martensite at a higher partitioning temperature. In the experimental steel partitioned at 400℃ for 10-60 s, carbides or cementite formed through carbon segregation along martensite boundaries or within the martensite matrix. As a result of carbon atom diffusion from martensite to austenite, the carbon content in martensite could be ignored. When the partitioning process completed, the constrained carbon equilibrium （CCE） could be simplified. Results calculated by the simplified CCE model were similar to those of CCE, and the difference between the two optimum quenching temperatures, where the maximum volume fraction of the retained austenite can be obtained by the Q＆P process, was little.     

高强度马氏体钢显微组织对腐蚀行为影响的研究进展

The high strength martensite steels are widely used in aerospace, ocean engineering, etc., due to their high strength, good ductility and acceptable corrosion resistance. This paper provides a review for the influence of microstructure on corrosion behavior of high strength martensite steels. Pitting is the most common corrosion type of high strength stainless steels, which always occurs at weak area of passive film such as inclusions, carbide/intermetallic interfaces. Meanwhile, the chromium carbide precipitations in the martensitic lath/prior austenite boundaries always result in intergranular corrosion. The precipitation, dislocation and grain/lath boundary are also used as crack nucleation and hydrogen traps, leading to hydrogen embrittlement and stress corrosion cracking for high strength martensite steels. Yet, the retained/reversed austenite has beneficial effects on the corrosion resistance and could reduce the sensitivity of stress corrosion cracking for high strength martensite steels. Finally, the corrosion mechanisms of additive manufacturing high strength steels and the ideas for designing new high strength martensite steel are explored.     

低温压力容器钢回火过程组织转变规律

以07MnNiMoVDR钢为研究对象,通过热膨胀试验、TEM检测相结合的方法,研究其回火过程中各回火温度区间中组织转变和碳化物析出的变化规律.结果表明:300℃回火时,残余奥氏体分解,片状θ-碳化物位于仍具有较高位错密度的α相板条之间;650℃回火时,α相除回复外,部分α相还发生了再结晶,析出物分布于已回复的板条之间以及再结晶晶界上.     

Microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels

The microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile test. The results show that Si can promote the transformation of austenite (γ) to ferrite (α), enlarge the (α+γ) region, and increase the aging stability of martensite by inhibiting carbide precipitation. Adding Cr leads to the formation of retained austenite and martensite/austenite (M/A) constituents, as well as the decomposition of martensite during the overaging stage. Both of the steels show higher initial strain-hardening rates and two-stage strain-hardening characteristics. The C-Mn-Si-Nb steel shows the higher strain-hardening rate than the C-Mn-Cr-Nb steel in the first stage; however, there is no significant difference in the second stage. Although the tensile strength and elongation of the two steels both exceed 1000 MPa and 15%, respectively, the comprehensive mechanical properties of the C-Mn-Si-Nb steel are superior.     

Hot ductility behavior of V-N and V-Nb microalloyed steels

The hot ductility of V-N and V-Nb microalloyed steels was investigated on a Gleeble-1500 thermomechanical simulator, and the results were compared with those of V and Nb microalloyed steels. A ductility trough is found in both the steels in the temperature range of 700 to 1050℃. Compared to the V steel, the V-N steel has a wider and deeper ductility trough with the increase of N content, due to the increased precipitation of V(C, N) in the steel. Above 930℃, when 0.047wt% V is added to the 0.028wt% Nb-containing steel, the ductility becomes worse, owing to the rise of the onset dynamic recrystallization temperature. However, the ductility gets better at 800 to 930℃ because of the coarsening of precipitates in austenite. With the improvement in ductility, the fracture mechanism is changed from intergranular to high ductile fracture in the temperature range of 800 to 1050℃.     

Effect of microstructure on corrosion behavior of high strength martensite steel——A literature review

The high strength martensite steels are widely used in aerospace, ocean engineering, etc., due to their high strength, good ductility and acceptable corrosion resistance. This paper provides a review for the influence of microstructure on corrosion behavior of high strength martensite steels. Pitting is the most common corrosion type of high strength stainless steels, which always occurs at weak area of passive film such as inclusions, carbide/intermetallic interfaces. Meanwhile, the chromium carbide precipitations in the martensitic lath/prior austenite boundaries always result in intergranular corrosion. The precipitation, dislocation and grain/lath boundary are also used as crack nucleation and hydrogen traps, leading to hydrogen embrittlement and stress corrosion cracking for high strength martensite steels. Yet, the retained/reversed austenite has beneficial effects on the corrosion resistance and could reduce the sensitivity of stress corrosion cracking for high strength martensite steels. Finally, the corrosion mechanisms of additive manufacturing high strength steels and the ideas for designing new high strength martensite steel are explored.     

汽车用高强度Q&P钢的组织与力学性能

研究了0.21C--1.43Si--1.35Mn钢在两相区及完全奥氏体区采用QP(Quenching and Partitioning)工艺加热后的微观组织与力学性能.结果表明:两相区加热可获得马氏体、残余奥氏体和铁素体组织,钢的抗拉强度为1 013 MPa,延伸率为25%,强塑积为25 655 MPa.%;完全奥氏体区加热可获得马氏体和残余奥氏体组织,钢的抗拉强度为1 257 MPa,延伸率为17%,强塑积为21 454 MPa.%;QP钢中的马氏体主要为板条状,伴有大量位错,并且发现有少量孪晶马氏体,分析认为由配分过程后的淬火过程转变而来;通过QP工艺可得到体积分数高达10.67%的残余奥氏体,分布在板条马氏体间,呈薄膜状.