Effect of prior hot rolling on the microstructures and mechanical properties of duplex stainless steel containing tempered martensite and ferrite

A duplex structure of δ-ferrite and lath martensite with interlath retained austenite film is developed in this study by modifying the alloy addition. The presence of δ-ferrite can further strengthen the grain refinement of austenite during hot rolling. As a result, the amount of retained austenite is enhanced. Tempered martensite embrittlement occurs due to the decomposition of retained austenite, and grain refinement can in fact ameliorate the tempered martensite embrittlement by delaying the onset of the embrittlement to a higher temperature. The combined effect of uniform and small grains as well as a large amount of retained austenite provides a further increase in the mechanical properties. After identical tempering treatments, all mechanical properties measured in the as-rolled condition were found to be higher than those of direct quenching without rolling. After hot rolling, the increase in the hardness and tensile strength was not accompanied by a drop in the ductility and toughness.     

Improvement of the cavitation erosion resistance of UNS S31803 stainless steel by duplex treatment

A duplex surface treatment consisting of High Temperature Gas Nitriding (HTGN) followed by Low Temperature Plasma Nitriding (LTPN) was carried out in an UNS S31803 duplex stainless steel. The HTGN treatment was intended to produce a relatively thick and hard fully austenitic layer giving mechanical support to the thinner and much harder expanded austenite layer. HTGN was performed at 1200 °C for 3 h, in a 0.1 MPa N₂ atmosphere while LTPN, was carried out in a 75% N₂ + 25% H₂ atmosphere, at 400 °C for 12 h, under a 250 Pa pressure, and 450 V. An expanded austenite γ_N layer, 2.3 μm thick, 1500 HV0.025 hard, was formed on top of a 100 μm thick, 330 HV 0.1 hard, fully austenitic layer, containing 0.9 wt% N. For comparison purposes LTPN was carried out with UNS S30403 stainless steel specimens obtaining a 4.0 μm thick, 1500 HV 0.025 hard, expanded austenite layer formed on top of a fully austenitic matrix having 190 HV 0.1. The nitrided specimens were tested in a 20 kHz vibratory cavitation-erosion testing equipment. Comparison between the duplex treated UNS S31803 steel and the low temperature plasma nitrided UNS S30403 steel, resulted in incubation times almost 9 times greater. The maximum cavitation wear rate of the LTPN UNS S30403 was 5.5 g/m²h, 180 times greater than the one measured for the duplex treated UNS S31803 steel. The greater cavitation wear resistance of the duplex treated UNS S31803 steel, compared to the LTPN treated UNS S30403 steel was explained by the greater mechanical support the fully austenitic, 330 HV 0.1 hard, 100 μm layer gives to the expanded austenite layer formed on top of the specimen after LTPN. A strong crystallographic textured surface, inherited from the fully austenitic layer formed during HTGN, with the expanded austenite layer showing {101} crystallographic planes//surface contributed also to improve the cavitation resistance o f the duplex treated steel.     

固溶处理对马氏体/铁素体双相不锈钢组织和性能影响

利用金相显微镜、扫描电镜等试验手段,研究了固溶温度及固溶时间对马氏体/铁素体双相不锈钢微观组织和力学性能的影响。试验结果显示,随着固溶温度的升高,第二相逐渐溶解于基体中,马氏体含量逐渐增加,在力学性能方面表现为强度逐渐增大,塑韧性降低。通过相分析可知,基体中第二相为M₂₃C₆,且固溶温度越高,第二相（M₂₃C₆）在基体中的残留越少,当固溶温度高于1100 ℃时,第二相完全溶解到基体中。     

双相区轧制对铁素体/马氏体双相钢组织性能的影响

采用双相区(α+γ)轧制及双相区短时保温处理相结合的方式,制备了一种高强高韧性低碳低合金铁素体/马氏体双相钢,并采用SEM、室温拉伸试验和维氏硬度检测等手段研究了不同轧制工艺对铁素体/马氏体双相钢组织和性能的影响。结果表明:相对于普通的连续轧制工艺,等温轧制和道次之间短时保温处理相结合的工艺对铁素体/马氏体双相钢的相比例、形貌和尺寸有重要影响。等温轧制及短时保温处理的双相钢的组织明显细化,马氏体相比例增加,组织均匀性显著改善,屈服强度提升了34%,达到1229 MPa,屈强比高达0.78,断口为韧性断口特征,呈细小韧窝状,具有良好的综合力学性能。     

B436L铁素体不锈钢热轧带钢辊迹缺陷改善

针对B436L铁素体不锈钢带钢表面易产生辊迹缺陷问题,分析了B436L带钢生产工艺,得出带钢表面辊迹缺陷是热轧过程中带钢与轧辊发生粘结而产生的。为此,对轧辊材质、板坯加热温度、轧制规程、轧制计划等进行了优化,使因粘结导致的辊迹缺陷的发生率由38.4%降低到1.0%以内。     

Improvement on the pitting corrosion resistance of 304 stainless steel via duplex passivation treatment

A remarkable improvement in the pitting corrosion resistance of 304 stainless steel was attempted using a novel duplex passivation treatment method. First, chemical passivation in nitric acid followed electrochemical passivation via potential polarization of step cycling in sodium nitrate electrolyte. Compared with traditional chemical passivation, breakdown potential was increased from 0.31 V_SCE to positive than 0.9 V_SCE at 70°C in a solution bearing 0.6 M [Cl^?] concentration. The critical pitting temperature was enhanced from 21.5°C to above 70°C in a solution with 6 M [Cl^?] concentration. Impedance analysis and X‐ray photoelectron spectroscopy results show that a more compact passive film with a higher ratio of chromium oxide on iron oxide was achieved by electrochemical passivation compared with chemical passivation. Morphology observation suggested that the potential polarization of step cycling slightly increased the dissolution of inclusions after being subjected to chemical passivation. The probable reason for the improvement on pitting resistance is discussed in detail based on inclusion dissolution and the protectiveness in passive film.     

2205双相不锈钢冷轧过程中磁性的研究

针对2205双相不锈钢冷轧过程中磁性发生显著变化的问题,通过室温冷轧(70%压下率)的方法对冷轧前后材料的磁性用光学显微镜(OM)、铁素体测量仪、X-射线衍射和EBSD做了研究。结果表明:由于双相不锈钢常温下冷轧过程中α相不发生分解,且材料中Cr超过20%,γ相在冷轧过程中不容易发生马氏体转变,硬度测量也没有显示发生马氏体转变。铁素体仪测量冷轧后试样和原始铸态相比示数下降15%,主要来源于轧制过程中铁素体磁性的变化。冷轧过程中晶粒发生细化、铁素体在冷变形过程中位错发生缠结生成大量的亚晶界、变形导致的内应力场以及织构的影响,使铁素体的磁性发生显著的下降。     

双相不锈钢2205热变形行为的实验

利用扫描电镜的EBSD技术,通过对单道次压缩热模拟实验后淬火试样组织的深入系统分析,研究了2205双相不锈钢热变形过程中的软化行为。研究结果表明,2205双相不锈钢的主要软化机制为铁素体的连续动态再结晶和奥氏体与铁素体之间的相转变,变形速率为主要影响参数,并通过影响应变在两相之间的分配控制组织在变形过程中的软化进程。变形速率很小时,铁素体的动态再结晶和铁素体向奥氏体的相转变是互为竞争的两个软化机制;随着变形速率的增加,主导软化机制转变为铁素体的动态再结晶和奥氏体相向铁素体相的转变。     

奥氏体不锈钢热轧稳定性的分析

针对奥氏体不锈钢的轧制难点,结合宝钢1 780 mm热连轧生产线近几年来奥氏体不锈钢的生产实践经验,分析了加热、粗轧、热卷箱、精轧及卷取等生产环节的控制要点。     

双相不锈钢车体电阻点焊接头组织和性能

采用电阻点焊对2205不锈钢进行焊接,通过金相显微镜、扫描电镜和万能试验机研究了焊接接头组织和性能。结果表明,双相不锈钢电阻点焊当焊接电流过大时,接头易产生飞溅和裂纹缺陷。焊接电流较小时,柱状晶从熔合线附近形成并延伸至结合面。随着焊接电流的增加,柱状晶组织变小,接头韧性提高。随着焊接电流的增加,双相不锈钢点焊接头剪切力先增加后减小,焊接电流9.5 k A时达到最大值15.4 k N。接头断口由解理断裂向韧性断裂转变。焊接电流超过11.5 k A时,断口出现裂纹。     

铁素体/马氏体双相钢的组织及性能

采用金相显微镜、SEM等试验方法,研究了中碳铁素体/马氏体双相钢的组织及性能。结果表明:在785~800℃淬火,起始组织为铁素体加珠光体的A型组织钢和起始组织为马氏体的B型组织钢随两相区淬火温度的升高强度升高;原始组织不同两相区淬火后钢的组织及性能不同,经785℃×30 min淬火的B型组织钢强度明显高于A型组织钢,经800℃×30 min淬火的B型组织钢伸长率和断面收缩率高于A型组织钢;785℃保温10 min淬火的B型组织钢相比于A型组织钢奥氏体化过程加速,钢的强度及塑性均好于A型组织钢;两相区淬火具有双相组织的钢具有连续屈服和快速应变硬化现象及低的屈强比,785℃×30 min两相区处理的钢与调质处理的钢相比塑性低但强度明显提高,785℃×10 min两相区处理的B型组织钢强度略低于调质钢,但塑性略有增加。     

深冷容器用奥氏体不锈钢冷成形封头铁素体量的控制

结合测量技术,对相应的标准进行了背景依据论述与细节对比,指出了目前的铁素体含量专用测量标准以磁性法为主,非专用标准以面积比例法为主。分析了铁素体含量测量的影响因素与目前存在的问题,实际应用需求为非破坏性检验、微区三维体积比例测量及形态表征、适用热影响区及焊道横截面等。对铁素体含量测量技术与标准进行了展望,提出了通过系统开展双盲试验、加强基础研究等手段进一步提升铁素体含量测量结果准确度、可靠性。

     

Microstructure and pitting corrosion resistance of annealed duplex stainless steel

The transition from metastable to stable pitting was studied in 0.5 M NaCl water solution for two cast duplex stainless steels under different microstructural conditions achieved by annealing in the range from 900 °C to 1200 °C. The ensuing microstructural changes in heat treated steels were defined and correlated with established pitting potentials (E_p) and sites of corrosion damage initiation. The variations in E_p have been discussed in terms of secondary phases precipitation. The critical condition for pit stability was quantified and used to select an appropriate microstructural state, resulting in the higher potential at which stable pit growth is first observed.     

焊缝金属铁素体含量对氢裂敏感性的影响

采用试验室制造的不同成分的焊丝,用Ar+H₂混合气体保护的TIG焊方法,通过纵向焊道拉伸拘束裂纹(LB-TRC)试验和焊缝金属恒应变速率拉伸(WM-SERT)试验,定量研究了双相不锈钢焊缝金属中铁素体含量对氢致裂纹敏感性的影响。结果表明,当双相不锈钢焊缝金属中δ铁素体含量小于50%时。焊缝金属对氢致裂纹不敏感;当δ铁素体含量大于50%时,随铁素体含量的增加,氢裂敏感性显著增加。     

回火温度对铁素体/马氏体双相不锈钢组织与性能的影响

利用扫描电镜、透射电镜等试验手段,研究了回火温度对铁素体/马氏体双相不锈钢组织与性能的影响规律。结果表明,试验钢的组织为马氏体+铁素体,组织存在大量位错,经回火处理后,马氏体板条间以及双相界面处均发现了M23C6相析出,其颗粒尺寸随着回火温度的升高而逐渐增大。回火温度对力学性能的影响主要表现为,随着回火温度的升高,抗拉强度与规定塑性延伸强度均有所降低,断后伸长率与断面收缩率基本保持一致,而冲击性能却显著提高。此外,经回火处理后试验钢的磁性能得到显著改善。     

Corrosion resistance of low‐nickel duplex stainless steel rebars

The use of stainless steel bars in reinforced concrete structures may be an effective method to prevent corrosion in aggressive environments where high amounts of chlorides may penetrate in the concrete cover. For an estimation of the service life of structures where stainless steel bars are used, the chloride threshold for these rebars should be defined, and the influence of chemical composition and metallurgical factors that may affect the corrosion resistance (strengthening, welding, etc.) should be assessed. To reduce the cost of stainless steel reinforcement, duplex stainless steels with low nickel content have been recently proposed as an alternative to traditional austenitic steels, even though, few results are available regarding their corrosion performance in chloride contaminated concrete. This paper deals with the corrosion resistance of low‐nickel duplex stainless steel rebars (1.4362 and 1.4162) as a function of the chloride content. Comparison is made with traditional austenitic steels. An attempt to define a chloride threshold for the different stainless steels is made by comparing the results of several test procedures both in concrete and in solution.     

00Cr12NiNbTi不锈钢热轧中析出行为

对00Cr12N iNbTi铁素体不锈钢进行加热、粗轧、精轧及卷取过程的模拟实验,应用扫描电镜、透射电镜、化学相分析及热模拟等方法对试样中析出物进行了定性定量分析。结果表明:加热到1140℃,保温0、45和90 m in后,粒子主要为TiN,Ti的固溶率为66.3%,N的固溶率为1.3%;从粗轧到精轧,微米级的小颗粒减少,大颗粒增多,颗粒平均尺寸由1.8μm增大到3.2μm,有聚集长大现象;在热轧过程中微米级粒子形貌为方形或球形,基本保持稳定;纳米级的(Nb,Ti)C颗粒在精轧阶段开始析出,卷取保温及缓冷至室温过程中大量析出,多分布于晶界及晶内处,在较高温度下卷取会析出较多的纳米级(Nb,Ti)C颗粒。     

Enhancement of abrasion and corrosion resistance of duplex stainless steel by laser shock peening

The results for laser shock peening of duplex stainless steel (22% Chromium-5% Nickel) using a pulsed Nd:YAG laser (wavelength = 532 nm, pulse width = 8 ns) for the application to high-capacity pumps for reverse-osmosis type seawater desalination plants are reported. By properly selecting the process parameters such as laser intensity of 10 GW/cm², laser pulse density of 75 pulse/mm², and 100 μm thick aluminum foil as a protective coating layer, wear volume and corrosion rate of duplex stainless steel could be reduced by 39% and 74.2%, respectively. The number and size of corrosion pits produced on wear track during copper accelerated acetic acid salt spray test decreased approximately by half as a result of laser shock peening. It is shown that laser shock peening is a practical option to improve abrasion and corrosion properties of a seawater desalination pump parts.     

碳钢/不锈钢复合厚板热轧头部翘曲有限元模拟

针对复合厚板轧制过程头部翘曲问题,采用ABAQUS软件进行有限元建模分析,对网格无关性进行了验证,得出了合理的单元尺寸;研究了板坯来料厚度、轧制压下率、板坯上下表面温差及轧辊速比对复合厚板头部翘曲的影响规律。