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.

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.

Key role of niobium stabilization in ultra-pure ferritic stainless steels for construction and home-appliance applications

The beneficial effects of niobium addition on properties such as high-temperature strength, toughness, and formability of ferritic stainless steels have been addressed. Based on the Thermo-Calc analysis, precipitation of niobium carbonitride and solubility of niobium have been predicted and characterized via scanning electron microscope （SEM） and transmission electron microscope （TEM） observations. It is shown that addition of niobium has a beneficial effect on improving the high-temperature strength, toughness, formability, and corrosion resistance of ferritic stainless steel. Soluted niobium is very effective in improving the high-temperature strength, which is beneficial to reducing the sticking propensity during hot rolling. Although niobium increases the recrystallization temperature, niobium-added ferritic stainless steels show a high mean r value, or a high plastic strain ratio, as long as the annealing temperature is high enough. Furthermore, because niobium helps to inhibit the formation of chromium carbides, ferritic stainless steel can keep an effective chromium content in the matrix, leading to improved corrosion resistance. Applications of these ferritic stainless steels for construction and home appliances have also been presented.

Effects of tungsten on the oxidation resistance and high-temperature strength of 18CrNbTi ferritic stainless steel

18CrNbTi ferritic stainless steel is a low-cost material mainly used for the fabrication of manifolds, which usually work at temperatures below 950℃. With the development of engine technology, exhaust manifolds tend to work above 1 000 ℃ and this may be even higher in the future. For developing a new kind of steel to satisfy these requirements,the effects of tungsten （W）addition on the high-temperature strength and oxidation resistance of 18CrNbTi ferritic stainless steel are discussed in this study. The test results show that W enhances high-temperature strength at 1 000 ℃ and significantly improves oxidation resistance. However, W addition tends to degrade oxide layer adhesion,causing spalling during alternate hot and cold conditions.

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.

Development of heat-resistant ferritic stainless steels for exhaust lines at Nisshin Steel

Exhaust emission regulations of the automotive are enforced in each country to prevent air pollution and global warming,and the restriction standard tends to become severer.Various techniques such as the combustion improvement of gasoline,upgrades of the catalyst,and the thermal capacity decreases in the exhaust lines are adopted to suit the regulations,and these lead to an increase of the maximum temperature of the exhaust gas. Recently,ferritic stainless steels are mainly used to parts of exhaust lines,as their thermal expansion coefficient is small,and the cyclic oxidation resistance and the thermal fatigue property are better than austenitic stainless steels. This paper presents newly developed heat-resistant stainless steels from Nisshin Steel for exhaust lines usage,and describes the currents of the steel development that could be envisaged in the future.With regard to improving the high-temperature strength of ferritic stainless steels,the addition of Nb,Mo and Cu is effective in solution hardening and precipitation hardening at 700℃,while the addition of Nb,Mo and W is effective in mainly solution hardening at 900℃.The addition of Cr,Si and Mn suppress the breakaway oxidation in air at 950℃up to 200 h of ferritic stainless steels containing 14%Cr.Especially,the addition of 0.8%or higher Mn would effectively improve the adherence of oxide scale.It is confirmed that ferritic stainless steels,NSSHR-1(14Cr-lMn-0.9Si-Nb) and NSSHR-2(10Cr-0.9Si-Nb-Ti ),is having a superior heat resistance,formability and cost performance compared to conventional Type441 and Type439 respectively.

Intergranular corrosion behavior of high nitrogen austenitic stainless steel

The intergranular corrosion （IGC） behavior of high nitrogen austenitic stainless steel （HNSS） sensitization treated at 650-950℃ was investigated by the double loop electrochemical potentiodynamic reactivation （DL-EPR） method. The effects of the electrolytes, scan rate, sensitizing temperature on the susceptibility to IGC of HNSS were examined. The results show that the addi-tion of NaCl is an effective way to improve the formation of the cracking of a passive film in chromium-depleted zones during the reactivation scan. Decreasing the scan rate exhibits an obvious effect on the breakdown of the passive film. A solution with 2 mol/L H2SO4＋1 mol/L NaCl＋0.01 mol/L KSCN is suitable to check the susceptibility to IGC of HNSS at a sensitizing temperature of 650-950℃ at a suitable scan rate of 1.667 mV/s. Chromium depletion of HNSS is attributed to the precipitation of Cr2N which results in the susceptibility to IGC. The synergistic effect of Mo and N is suggested to play an important role in stabilizing the passive film to prevent the attack of IGC.

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.

Characteristics of SEN clogging and adhesive behavior of oxide inclusion during continuous casting of Ti-stabilized ultra-pure ferritic stainless steels

Submerged entry nozzle(SEN)clogging during continuous casting of Ti-stabilized ultra-pure ferritic stainless(Ti-UPFS)steels was systematically investigated via cross-sectional analysis and acid dissolution treatment.The SEN deposit profile was characterized as occurring in three major layers:(1)an eroded refractory layer;(2)an initial adhesive layer comprised an Al_(2)O_(3)-ZrO_(2) composite sub-layer and a dense Al_(2)O_(3)-based deposit sub-layer;and(3)a porous multiphase deposit layer mainly consisting of MgO·Al_(2)O_(3),CaO-Al_(2)O_(3),and CaO-TiOx.The MgO·Al_(2)O_(3)-rich inclusions did not adhere directly to the eroded refractory but were entrapped during the deposit growth.Results of inclusion characterization in the tundish revealed that the MgO·Al2O3-rich particles present in the tundish served as the primary source of clogging deposits.Furthermore,a novel cavity-induced adhesion model by circular approximation was established to explain the effects of complex inclusion characteristics and refractory material type on adhesion force.A high number of small MgO·Al_(2)O_(3) inclusions were expected to accelerate the buildup of clogging deposits.Improving the modification of MgO·Al_(2)O_(3)-rich inclusions in the size range of 2-4μm by Ca treatment was crucial to minimizing the risk of SEN clogging during the continuous casting of Ti-UPFS steels.

Comparison of feasibility,microstructure and performance of hybrid laser arc,activated flux tungsten inert gas and friction stir welding for thick plate of innovative ultra-pure ferritic stainless steel

An innovative grade of ferritic stainless steel,ultra-pure 18Cr–2Mo thick plate,was designed and produced for special industrial application.In order to maintain its mechanical properties after joining,three advanced joining methods,hybrid laser arc welding,activated flux tungsten inert gas welding and friction stir welding,were selected and conducted to connect the thick plates.The feasibility of three joining methods,the microstructure and mechanical properties were compared,and the results have demonstrated that the sound joint was successfully produced using the selected parameters through friction stir welding.The obtained hardness and impact toughness of the weld zone were satisfying.In terms of activated flux tungsten inert gas welding,the crack will be created due to microstructural brittleness.And as for hybrid laser arc welding,the weld zone is narrow,and the addition of wire during welding for the top weld metal area leads to higher formation ratio of low-angle grain boundaries,which is beneficial to performance of the joint.However,there is still a weak area in the fusion line of the welded joint.The result has illustrated that the welding of innovative ultra-pure ferritic stainless steel thick plate by friction stir welding is feasible.

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 %.

Distribution of TiN inclusions in Ti-stabilized ultra-pure ferrite stainless steel slab

In order to clarify and control the silver defect on surface of cold-rolled sheet of the Ti-stabilized ultra-pure ferrite stainless steel, the distribution of TiN inclusions on the cross section of hot-rolled plate was studied using automated scanning electron microscopy/energy-dispersive X-ray spectroscopy inclusion analysis (ASPEX 1020 system). It was found that the number density decreases sharply from the surface to the center of the hot-rolled plate, whereas the average size increases. Then, the distribution of TiN inclusions on the cross section of continuously cast slab was investigated. Similarly, numerous small-sized TiN inclusions were generated at the subsurface of the slab. The average size rapidly increased and the number density dramatically decreased from the subsurface to 1/4 thickness, while from 1/4 thickness to 1/2 thickness, the increase in average size and the decrease in number density were slight. Thermodynamics results showed that TiN inclusion was formed below the liquidus temperature, which indicated that TiN inclusions could not be formed during secondary refining. Considering the microsegregation of solute elements and the equilibrium of TiN formation during solidification, TiN precipitated in the mushy zone when the solid fraction was close to 0.2. The growth of TiN was analyzed based on the diffusion-controlled growth model. With the increase in cooling rate, the time for TiN growth decreased and the size of TiN inclusions was diminished, which revealed the size distribution of TiN inclusions in the cast slab qualitatively.

Effect of Mo on High-Temperature Fatigue Behavior of 15CrNbTi Ferritic Stainless Steel

In order to understand the effect of Mo element on the high-temperature fatigue behavior of 15CrNbTi ferritic stainless steel, the stress-controlled fatigue tests have been performed for both 15CrNbTi and 15Cr0.5MoNbTi ferritic stainless steels at 800 ℃ in laboratory air. The fatigue test results indicate that the fatigue resistance of 15Cr0.5MoNbTi steel is manifestly higher than that of 15CrNbTi steel at the maximum stress below 57 MPa; the 15Cr0.5MoNbTi steel possesses a fatigue limit of 35 MPa, which is higher than that of 15CrNbTi steel. The TEM observations reveal that the Mo element can suppress the formation of coarsened Fe3Nb3C precipitates and result in the fatigue resistance enhancement. The dislocation networks formed during the cyclic load favor to improve the fatigue resistance of 15Cr0.5MoNbTi steel at 800 ℃.

Metallurgical characteristics of armour steel welded joints used for combat vehicle construction

Austenitic stainless steel(ASS) and High nickel steel(HNS) welding consumables are being used for welding Q&T steels, as they have higher solubility for hydrogen in austenitic phase, to avoid hydrogen induced cracking(HIC) but they are very expensive. In recent years, the developments of low hydrogen ferritic steel(LHF) consumables that contain no hygroscopic compounds are utilized for welding Q&T steels. Heat affected zone(HAZ) softening is another critical issue during welding of armour grade Q&T steels and it depends on the welding process employed and the weld thermal cycle. In this investigation an attempt has been made to study the influence of welding consumables and welding processes on metallurgical characteristics of armour grade Q&T steel joints by various metallurgical characterization procedures. Shielded metal arc welding(SMAW) and flux cored arc welding(FCAW) processes were used for making welds using ASS, LHF and HNS welding consumables. The joints fabricated by using LHF consumables offered lower degree of HAZ softening and there is no evidence of HIC in the joints fabricated using LHF consumables.

具有高强匹配焊缝金属的LDX2101钢接头显微组织表征

综述了涉及高强匹配焊缝金属的LDX2101钢接头显微组织的特征。结果表明,节约型双相不锈钢LDX2101焊接材料的选用,可以采用“准成分匹配”选用原则,即焊缝中所加元素的种类与母材相同,但各元素的含量可能有较大的变化,以确保熔敷金属中具有适量的相平衡,获得满意的综合性能。相同的热循环条件下,接头的组织主要取决于化学成分,即Creq/Nieq比值;在一定范围内,随Creq/Nieq比值降低,提高了铁素体固溶温度,在较高温度下发生δ→γ转变,获得较多的奥氏体。随热输入增加,在所有焊缝金属中,γ奥氏体的含量增大,而且γ奥氏体的含量高于δ-铁素体的含量;在HAZ,随热输入增大,γ奥氏体的含量增大。在650℃时效试件中,焊接接头的组织变化主要是Cr2N析出;650℃时效15 min时,主要是铁素体晶内析出Cr2N;650℃时效90 min时,在接头3个区的晶界上均析出Cr2N。铁素体含量的变化:焊缝为37%,HAZ为40%,母材为45%。

高速激光熔覆马氏体不锈钢涂层与电镀层性能对比与研究

为对比研究不同液压缸服役工况下高速激光熔覆涂层替代电镀硬铬层的可行性,分别制备了马氏体不锈钢熔覆层XG-1、XG-2和电镀硬铬涂层,开展了三种涂层组织、硬度、腐蚀性能及模拟不同工况(划伤磨损、干砂摩擦磨损、滑动摩擦磨损)下磨损性能测试,探讨了涂层失效行为及其应用工况。结果表明:XG-1、XG-2涂层微观组织致密且均匀,平均显微硬度为720.5 HV、653 HV;电镀硬铬涂层分布孔隙和裂纹等缺陷,自腐蚀电流密度为10.45μA/cm^(2),耐腐蚀性能最差;三种磨损形式下电镀硬铬涂层均发生了开裂及剥落,高速激光熔覆涂层表现出更优异的耐磨性能,适用于活塞杆易拉伤、外界富集硬质颗粒污染物、大侧向载荷等液压缸服役工况。

S44535铁素体不锈钢高温氧化行为

为了探究稀土La对S44535铁素体不锈钢高温氧化行为的影响,采用加速氧化对添加0.26%La的S44535钢在800、900、1000℃下的静态空气中进行高温氧化研究。采用恒温氧化法对比S44535铁素体不锈钢在三种温度下的氧化动力学曲线,用扫描电镜(SEM)、能谱分析仪(EDS)和X射线衍射(XRD)对钢的氧化层进行表征分析,探究了稀土La对铁素体不锈钢高温抗氧化性能的影响。结果表明,氧化产物主要为Cr2O3和MnCr2O4,随着温度的升高,表面氧化膜增厚且更加致密,其外层富含Mn,内层富含Cr,说明稀土改变了氧化膜的生长方式,且呈现出较为优异的高温抗氧化性能。

合金元素对中铬铁素体不锈钢组织性能的影响

采用金相、电化学试验、点腐蚀以及盐雾试验等手段,对比分析了添加合金元素Cr、Cu和Mo的常规碳氮中铬铁素体不锈钢与超低碳氮中铬超纯铁素体不锈钢组织性能。试验结果表明:中铬铁素体不锈钢中高C、N含量会导致基体析出大量铬碳化物,造成碳化物周围基体贫铬,影响耐点蚀性能;添加0.2%合金元素Cu提高了不锈钢的电极电位,在3.5%NaCl中性溶液中拥有高的自腐蚀电位和低的自腐蚀电流密度;添加合金元素0.2%Cu和Mo的常规碳氮中铬铁素体不锈钢500h中性盐雾腐蚀试验可以达到超低碳氮中铬铁素体不锈钢同样水平。

高温水蒸气环境中TP439不锈钢的应力腐蚀开裂敏感性

目的 研究TP439不锈钢在高温水蒸气环境中的应力腐蚀开裂行为,并探讨水蒸气和温度对其应力腐蚀开裂敏感性的影响规律。方法 采用慢应变速率拉伸试验方法研究了TP439不锈钢在400~600℃水蒸气环境中的应力腐蚀开裂行为,利用SEM和EDS分析试样断口区域的形貌及元素分布。结果 同一应变速率(2×10^(-5) s^(-1))下,随着温度在400~600℃范围内升高,TP439不锈钢在空气和水蒸气环境中的屈服强度、抗拉强度和断裂能均逐渐降低,延伸率逐渐增大。400℃和500℃时,试样在水蒸气环境中的抗拉强度较空气环境中有所降低,而延伸率较空气环境中增大。600℃时试样在水蒸气环境中的力学性能较空气环境中无明显差别。试样在400、500、600℃水蒸气环境中的应力腐蚀开裂敏感性指数分别为0.7%、1.2%和-2.8%,应力腐蚀开裂敏感性较低。试样在400~600℃水蒸气环境中的断口均呈现韧性断裂特征,断口形貌整体由韧窝和微孔组成,颈缩现象显著,断口附近未发现二次裂纹。温度在400~600℃范围内升高时,断口的韧窝特征更加明显,颈缩程度逐渐增大,600℃时断口侧面的氧化膜表面Cr含量明显降低,主要由Fe的氧化物形成。结论 水蒸气对TP439不锈钢的应力腐蚀开裂行为起促进作用。基于应力腐蚀开裂敏感性指数和断口的分析,在应变速率为2×10^(-5) s^(-1)的400~600℃水蒸气环境中,TP439不锈钢的应力腐蚀开裂敏感性较低。