Ultra-Pure Ferritic Stainless Steels-Grade,Refining Operation,and Application

The grades of ultra-pure ferritic stainless steels, especially the grades used in automobile exhaust system, were reviewed. The dependence of properties on alloying elements, the refining facilities, and the mechanism of the reactions in steel melts were described in detail. Vacuum, strong stirring, and powder injection proved to be effective technologies in the melting of ultra-pure ferritic stainless steels. The application of the ferritic grades was also briefly introduced.

Development and prospect of secondary refining technology for ultra pure ferritic stainless steel

Ultra pure ferritic stainless steel has more advantages in the performance than the ordinary ferritic stainless steel because of ultra low carbon and nitrogen content,such as corrosion resistance,toughness and weldability,etc.Such steel has therefore been applied in many fields,leading to the very rapid development over the past 40 years.This study focuses on the secondary refining process which is the most important step of the whole steelmaking process for the ultra pure ferritic stainless.Firstly,some difficulties of the secondary refining process are described,including the high purification in terms of both carbon and nitrogen contents, high efficient and stable control.Secondly,the development and progress of the secondary refining technology for ultra pure ferritic stainless is introduced in terms of the refining equipments,metallurgical process and assistant technologies.Finally,the prospect was made for the development of secondary refining process for ultra pure ferritic stainless in the future.

Effect of cold-rolled annealing processes on microstructure and properties of ultra-pure ferritic stainless steel

In this study, the influence of annealing processes for cold-rolled sheets on the microstructure and mechanical performance of ultra-pure 430 ferritic stainless steel was investigated. Thermo-Calc calculation, organization observation, SEM detection,and tensile tests were used to discern the optimal annealing process. It is found that the microstructure is made up by the fine and uniform recrystallized ferritic grains after annealing. The optimum annealing process for ultra-pure 430 stainless steel is 950 ℃ for 90 s. After annealing, the stainless steel can obtain the optimum microstructure,recrystallization texture, and mechanical properties.

Low molybdenum ultra-pure ferritic stainless steel for solar water heaters

In order to meet the demands of service life and the synthetical performance/price ratio of stainless steel in the solar water heater industry, the low molybdenum ultra-pure ferritic stainless steel （FSS） B445J1M was developed at Baosteel. In this study, comparative studies were carried out on the mechanical properties, the formability and the corrosion resistance of B445J1M ,304 and 444 ,and the advantages and application fields of B445J1M were summarized.

Development of ferritic stainless steel B445R with superior corrosion resistance for architectural applications

The dull-finish ferritic stainless steel （FSS） sheet B445R for architectural roofing has been developed by Baosteel. This steel product exhibits excellent corrosion resistance superior to that of SUS 316L with a lower cost. It can be easily formed into roofing panels by ordinary processes. Moreover,the thermal strain of it is less than SUS 316L because of its lower thermal expansion coefficient, and its reflectivity is lower due to the dull-finish treatment. All of these features make it capable of being used as architectural roofing materials in coastal regions.

Inclusions for Ultra-pure Ferritic Stainless Steels Containing 21% Chromium

As stabilizing elements added into ultra-pure ferritic stainless steels, niobium and titanium react with car- bon and nitrogen to form carbonitrides and have great effects on the ratio of equiaxed zone and the grain size of solidi- fication structure of ingots, which remarkably affect the quality of cold-rolled sheets. Combined with thermodynamic calculation, style and precipitation progress of inclusions in ultra-pure ferritic stainless steels were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy and energy dispersive spectros- copy. The results indicate that the inclusions are mainly Ti-Al-N- O system inclusions in ultra-pure ferritic stainless steels. Al2Oa starts to precipitate firstly and then TiOx and TiN precipitates sequently. The inclusions are mainly single TiN particles and complex inclusions with Al2O3-Ti2O3 as cores and covered with TiN under the condition of 0.31% titanium addition and mainly Al2O3 under the condition of 0.01% titanium addition. A few （Nb,Ti）N parti- cles precipitate because of no enough titanium to react with nitrogen when titanium addition is 0.01 %. In addition, fine Nb（C, N） particles with size of less than 500 nm precipitate at relatively low temperature.

Mechanisms of Solidification Structure Improvement of Ultra Pure 17 wt% Cr Ferritic Stainless Steel by Ti,Nb Addition

The grain structures and the precipitates in the solidification microstructure of the ultra pure 17 wt% Cr ferritic stainless steels with different Ti and/or Nb micro-alloying were investigated both experimentally and theoretically. It was found by the grain structure observation that the addition of Ti or Nb to the steel reduced the grain size （D） and elongation factor （E）, and improved the equiaxed grain proportion （P） and globularity factor （￡）. Among the four steels studied, the minimum grain size and maximum equiaxed grain proportion were obtained by jointly adding both Ti and Nb to the steel. The SEM observation indicated that several kinds of precipitations, such as TiN, MC （rich in Nb）, Laves phase （Fe2Nb） and so on, formed in the corresponding steels. In addition, the results calculated using the Thermo-Calc software illustrated that TiN precipitates in the liquid at proper Ti and N contents. Meanwhile, the solidification interval （△T） was enlarged by the addition of Ti or Nb, and the effectiveness of Nb was stronger than Ti. Based on the experimental and calculation results, the mechanisms of grain refinement and increment in equiaxed grain proportion were discussed.

Inclusions and solidification structures of high pure ferritic stainless steels dual stabilized by niobium and titanium

As the raw materials in the post process of rolling and heat treatment, ingots have great effects on the properties of the final products. Inclusions and solidification structures are the most important aspects of the quality of ingots. Niobium and titanium are usually used to react with carbon and nitrogen to improve the properties of ferritic stainless steels. In this research, combined with thermodynamic calculation, effects of niobium and titanium on the inclusions and solidification structures in three kinds of high pure ferritic stainless steels with different titanium additions were investigated by optical microscope（OM）, scanning electron microscope（SEM）, transmission electron microscope（TEM）, and energy disperse spectrometer（EDS）. Results show that Al2O3 and a few（Nb,Ti）N particles form when titanium addition is 0.01 %.Furthermore, inclusions are mainly Ti N and Al2O3–Ti Ox–Ti N duplex inclusions when titanium addition is more than0.10 %. Those two types of inclusions are in well distribution, and can afford nuclei to the solidification process.Therefore, the ratio of equiaxed zone increases with the increase of titanium addition. The ratio increases from42.1 % to 64.0 % with the titanium addition increasing from 0.01 % to 0.10 %, and it increases to 85.7 % when the titanium addition reaches 0.34 %.

不锈钢冶炼及凝固过程氮的控制

总结了氮在不锈钢中有害和有利正反两方面的作用。通过热力学计算和实测数据分析了温度和氮分压对不锈钢熔体中氮溶解度的影响,理论分析了不锈钢熔体吸氮和脱氮的动力学,指出了真空和高压分别是生产超低氮和高氮钢的主要方法。结合以往的研究成果和生产实践提出了生产超低氮铁素体不锈钢和高氮不锈钢的具体工艺技术措施。

铁素体不锈钢高温氧化层生长及热轧粘辊研究

通过高温氧化实验研究了超纯21%Cr铁素体不锈钢高温氧化层的生长情况,并采用开发的模拟铁素体不锈钢热轧粘辊实验方法研究了表面氧化对超纯21%Cr铁素体不锈钢热轧粘辊的影响,探讨了铁素体不锈钢的热轧粘辊机理.实验发现:超纯21%Cr铁素体不锈钢的炉生氧化铁皮较厚,而热轧过程中二次氧化铁皮生长较缓慢;锤头在循环轧制过程中出现的表面裂纹为热轧粘辊提供了形核地点;在高温变形过程中,表面氧化对铁素体不锈钢带钢表面具有保护作用,可以减轻热轧粘辊.

热输入对445J2超纯铁素体不锈钢焊接接头显微组织和力学性能的影响

研究了不同热输入445J2超纯铁素体不锈钢TIG焊焊接工艺。设计三组热输入分别为低热输入(1.67J/mm),中热输入(1.75J/mm)和高热输入(1.95J/mm),系统地分析了接头组织和力学性能。结果表明:接头焊缝均由中心等轴晶和外围柱状晶组成,HAZ均为粗化的等轴晶;低热输入接头的静载拉伸强度最高;拉伸试样断裂位置均位于接头柱状晶区;随着热输入的增大,拉伸断裂机理由韧窝断裂向准解理转变;对弥散分布的第二相SEM和EDS分析,其成分为Ti或Nb的氮化物,有利于焊缝中心柱状晶向等轴晶的转变。

冷轧变形率对超纯铁素体不锈钢439板材退火组织与性能的影响

将超纯铁素体不锈钢439板材在不同冷轧变形率下冷轧变形,然后将轧制态冷板在950~1050℃下进行退火,研究冷轧变形率对该材料退火组织及性能的影响。结果表明：随着变形率的增加,晶粒尺寸变小;50%变形率条件下冷板最高伸长率为36%,80%变形率条件下最高伸长率可达40%;基于金相试验数据,利用回归分析的方法计算了不同变形率条件下的晶粒长大激活能,得出50%和80%冷轧变形率条件下再结晶晶粒长大激活能分别为60.19 kJ/mol和65.68 kJ/mol;采用晶粒尺寸与晶界迁移激活能间的定量关系来描述晶粒长大过程,最终建立了该合金在试验温度范围内再结晶晶粒长大的动力学模型。

85 t VOD精炼超纯铁素体不锈钢的脱氮工艺及效果

分析了85 t VOD精炼时相关工艺参数对超纯铁素体不锈钢00Cr18Mo2和00Cr17Mo终点氮含量的影响。结果表明,随初始碳含量增加,初始氮含量降低,钢水温度提高,适当增加脱氮时间,VOD钢水终点氮含量降低;当控制钢水初始碳含量0.4%～0.9%、处理温度≥1 590℃、真空度≤70 Pa、脱氮时间15～20 min、吹氩搅拌强度8～15 L/(min·t)、初始氮含量≤0.017 0%,VOD终点钢水氮含量为0.006 4%～0.009 5%。

建筑用高耐蚀性铁素体不锈钢B445R的研制

宝钢研制出了建筑屋顶装饰用铁素体不锈钢B445R毛面产品。该产品的耐蚀性比奥氏体不锈钢SUS316L更优越,成本也比SUS316L低,采用常规的成形方法就可以将其加工成屋顶面板。此外,由于该产品的热膨胀系数比SUS316L小,故其热变形也比SUS316L小,同时由于进行了毛化处理,其表面具有较低的光反射能力。所有这些优点使其有能力成为沿海地区建筑物的屋顶装饰材料。

超纯铁素体不锈钢B439M 200 mm×1260 mm连铸板坯中TiN夹杂物的分布

采用自动扫描电镜Aspex研究了超纯铁素体不锈钢B439M(/%:0.01C,0.40Si,0.25Mn,0.020P,0.001S,17.50Cr,0.15Ni,0.20Ti,0.0080N,0.0040O)200 mm×1 260 mm连铸板坯中TiN夹杂物的分布,结果表明,由于连铸坯凝固过程中由外向内冷却速率减小,在连铸坯宽度方向边部,生成了大量小尺寸(平均1.26μm)TiN夹杂物;而在连铸坯宽度方向的1/4处和中部,由内弧表面到厚度1/4处再到厚度方向中部,TiN形核速率降低,TiN夹杂物的数量减少而尺寸增大。动力学分析结果得出,冷却速率减小会使得TiN夹杂物的生成尺寸增大,与TiN夹杂物在连铸坯中的分布规律相吻合。

退火温度对443超纯铁素体不锈钢冷轧板性能的影响

采用电子背散射衍射(EBSD)技术和拉伸试验等研究了退火温度对443超纯铁素体不锈钢冷轧板微观组织、织构和力学性能的影响。结果表明:随着退火温度从880℃升高到980℃,晶粒尺寸增大,平均晶粒尺寸从32.5μm增大到64.3μm。随着退火温度升高,{111}织构增强,{111}<112>织构强度由6.6增加到10.3。不同退火温度下试验钢的屈服强度、抗拉强度和硬度变化不显著。经过930℃退火,伸长率和综合力学性能最佳,组织均匀。

TiN夹杂物对超纯铁素体不锈钢点蚀的影响

通过电化学试验和浸泡试验,利用扫描电镜(SEM)、透射电镜(TEM)和原子力显微镜(AFM)表征了超纯铁素体不锈钢的点蚀坑尺寸、TiN夹杂物的微观结构以及TiN与基体的电势差和高度差,分析了TiN对试样点蚀发生和扩展的作用机理。结果表明:试样的腐蚀电位和点蚀电位分别为94 mV和353 mV,点蚀坑的平均宽度和深度分别为203.6μm和114.74μm, TiN夹杂物以单个或团簇的形式存在;TiN比基体的电势高65 mV,点蚀优先在TiN与基体界面处发生,基体成为阳极发生微电偶腐蚀;由"孔内活化-孔外钝化"腐蚀电池引起的自催化过程加速了点蚀的扩展。

TTS443高铬铁素体不锈钢的开发

针对304(Crl8-Ni8型)奥氏体不锈钢和430(Cr17型)铁素体不锈钢的特性,通过试验和分析Cr、Cu、Nb、Ti等合金元素对铁基合金材料性能的影响,开发出一种高铬铁素体不锈钢-TTS443(/%:O.010C、21Cr、0.40Cu、0.25Nb、0.20Ti、O.012N)。该钢种的耐蚀性能与304奥氏体不锈钢相当,具有良好的成形性与焊接性能,TTS443铁素体不锈钢是304奥氏体不锈钢理想代替材料。

时效温度对SUS444超纯铁素体不锈钢冲击韧性的影响

采用光学显微镜、扫描电镜和电子探针研究了时效温度对SUS444超纯铁素体不锈钢的显微组织及冲击韧性的影响。结果表明:SUS444不锈钢经850℃时效处理10 min后,室温冲击吸收能量从4.4 J提高至55 J,韧脆转变温度从20℃以上降低至-5℃。电子探针分析表明TiN相时效处理后转变为TiN、NbC和贫Nb相的三层复合结构,提高了其与基体界面的结合强度,导致实验钢冲击韧性的大幅提高。

000Cr26Mo1超纯高铬铁素体不锈钢烧碱加热器研制总结

阐述了０００Ｃｒ２６Ｍｏ１ 超纯高铬铁素体不锈钢（ＥＢ２６ － １） 化学成分、机械性能和焊接工艺，介绍该材质加热罐制造及应用情况。