镀锌基板氧化铁皮在还原过程中的组织演变

氢气还原氧化铁皮无酸洗热镀锌技术具有清洁、环保、低成本的优点。为了促进该技术的推广应用,采取5％压下率冷轧作为预处理工艺,并利用真空管式炉和扫描电镜,研究了不同还原温度对经预处理和未预处理镀锌基板SPHC带钢氧化铁皮还原产物、还原效率和还原机制的影响规律。结果表明:SPHC带钢在30%H₂浓度下,不同温度下还原机制有较大差异,最终产生3种不同的还原产物,500~600 ℃时还原产物为多孔铁,经过预处理的试样多孔铁更致密;700 ℃时还原产物由多孔铁与致密铁共同组成,经过预处理的试样氧化铁皮残留量小于未预处理的试样;800 ℃时还原产物为致密铁,未预处理的试样氧化铁皮中间层依旧存在氧化物残留,经预处理的试样几乎看不见氧化物残留;900 ℃时还原产物为致密铁,预处理和未预处理试样还原效果几乎相同。     

氢还原除鳞后的冷轧带钢退火工艺

采用氢还原法去除带钢表面氧化铁皮绿色环保,但氢还原后的带钢表面出现脱碳现象,且内部组织粗化,冷轧及退火后可能遗传给成品带钢。本文对氢还原后的热轧带钢进行了冷轧和退火试验,研究了氢还原除鳞后的冷轧带钢在退火过程中的组织性能演变规律,分析了退火温度和退火时间对带钢组织性能的影响。结果表明:经过氢还原除鳞的冷轧带钢退火后内部组织发生了回复、再结晶和晶粒长大现象,退火温度越高,退火时间越长,晶粒长大越严重。与酸洗除鳞的冷轧带钢不同,经过氢还原除鳞的冷轧带钢再结晶退火时间相应缩短,在相同条件下,退火后基体晶粒尺寸相对较大,表面脱碳层内的晶粒长大更为明显,其抗拉强度降低10~20 MPa,伸长率提高3%~5%。     

氢还原除鳞后的冷轧带钢退火工艺北大核心CSCD

采用氢还原法去除带钢表面氧化铁皮绿色环保,但氢还原后的带钢表面出现脱碳现象,且内部组织粗化,冷轧及退火后可能遗传给成品带钢。本文对氢还原后的热轧带钢进行了冷轧和退火试验,研究了氢还原除鳞后的冷轧带钢在退火过程中的组织性能演变规律,分析了退火温度和退火时间对带钢组织性能的影响。结果表明：经过氢还原除鳞的冷轧带钢退火后内部组织发生了回复、再结晶和晶粒长大现象,退火温度越高,退火时间越长,晶粒长大越严重。与酸洗除鳞的冷轧带钢不同,经过氢还原除鳞的冷轧带钢再结晶退火时间相应缩短,在相同条件下,退火后基体晶粒尺寸相对较大,表面脱碳层内的晶粒长大更为明显,其抗拉强度降低10～20 MPa,伸长率提高3%～5%。     

热轧SPHC带钢酸洗行为及其高效酸洗工艺研究

冷轧带钢产品的表面质量主要取决于热轧原料的酸洗质量。针对常规热轧(HR)工艺、CSP工艺及ESP工艺生产的热轧SPHC带钢,对其表面氧化铁皮结构及其酸洗历程进行了对比分析研究;在上述基础上,指出缩短孕育期,使带钢快速进入氧化铁皮大面积剥离阶段是提高酸洗效率的关键,提出了热轧SPHC带钢预升温酸洗工艺,并进行了带钢升温、未升温酸洗试验以验证酸洗效果。结果表明:HR带钢、CSP带钢、ESP带钢表面氧化铁皮均由外层的Fe₃O₄和内层为的FeO组成,前两者氧化铁皮厚度约为6~8 μm,ESP带钢表面氧化铁皮两层之间有较为明显的间隙,总平均厚度约为18 μm。3种热轧带钢的酸洗曲线呈现相同的变化趋势,酸洗效率随着酸液温度及紊流度的提高而提高,且在低温和低雷诺系数下增幅明显。HR带钢与ESP带钢的酸洗曲线接近,相对于前两者,CSP带钢的酸洗效率更高、更易酸洗。热轧SPHC带钢氧化铁皮去除符合S型曲线,经历孕育期,加速期和平稳期的时长的占比分别为40%、40%及20%。板带预升温酸洗工艺实施简单,可使表层难酸洗氧化铁皮快速剥离,缩短酸洗时间约50%,显著提高了酸洗效率。     

热轧板氧化铁皮的动力学研究

利用扫描电镜对一种热轧板的表面氧化铁皮形貌进行观察,并在实验室中通过改变加热温度和加热时间对试样的氧化铁皮形成过程进行了模拟。结果表明,热轧板上下表面的氧化铁皮形态具有明显的差异,热轧板上下表面均有氧化铁皮压入的现象,且上表面压实氧化铁皮周围可见氧化铁皮碎化。热轧板氧化增重与保温时间成线性关系,初始氧化层可以一定程度的阻止氧的扩散。模拟得到试样氧化速率方程为W=4.52 e-Ea/RTt。     

煤气中硫化氢含量对610L红色氧化铁皮的影响

针对汽车大梁钢表面存在的红色氧化铁皮缺陷,在生产汽车大梁钢610L时,根据在线表面检测仪采集的带钢表面红色氧化铁皮图像,按照红色氧化铁皮分布数量来量化其程度大小。结果表明,红色氧化铁皮的严重程度与加热时间直接相关,查询对应时间段的煤气中H2S含量达到300 mg/m3以上,可确认H2S是导致红色氧化铁皮的主要原因,为此减少了煤气中H2S含量,红色氧化铁皮得到有效遏制。     

热轧酸洗板氧化铁皮缺陷原因分析及控制措施

针对热轧表检仪不能有效识别的片状、条状、山水画状、边部粗糙酸洗氧化铁皮缺陷,介绍了其形貌特征,对热轧工艺中的影响因素进行了分析和排查,得到了缺陷的形成原因。回炉板坯重复入炉加热,氧化铁皮的生成量将会增加,容易造成酸洗后片状氧化铁皮缺陷。除鳞水压力、喷射角度、喷射面重叠量及除鳞道次对二次氧化铁皮破除能力不足时,容易产生酸洗后条状、山水画状氧化铁皮缺陷。同时,粗轧工作辊轧制公里数较长、中间坯温度过高也会对山水画状氧化铁皮缺陷有一定的影响。热轧带钢终轧或卷取温度较高,薄规格带钢板形较差时,会造成酸洗后带钢边部粗糙氧化铁皮缺陷。为此,对板坯加热工艺、粗轧及除鳞工艺、精轧及层冷工艺进行了优化,大大降低了酸洗板氧化铁皮缺陷的发生率,提高了产品表面质量。     

小压下率冷轧过程中低碳钢氧化铁皮断裂行为研究

针对免酸洗新工艺中采用小压下率冷轧或平整工艺，试验研究了不同压下率冷轧前后SPHC带钢操作侧（OS侧），中间部位，驱动侧（DS侧）3个位置表面和截面的氧化铁皮形貌和厚度，以及氧化铁皮在冷轧过程中的断裂行为。结果表明，试验钢热轧后冷却过程中的热应力使其表面氧化铁皮产生裂纹，且卷取后的冷却速率会对表面氧化铁皮结构产生较大影响；采用10%冷轧压下率时，带钢表面出现较多氧化铁皮块状脱落和粉化脱落现象，表面质量破环严重；采用5%冷轧压下率时，带钢表面仅出现少量氧化铁皮块状脱落，大部分氧化铁皮达到只开裂不剥落状态；氧化铁皮在常温下塑性较差，在冷轧后氧化铁皮厚度未发生随钢基体明显变薄的协同变形。     

小压下率冷轧过程中低碳钢氧化铁皮断裂行为研究

针对免酸洗新工艺中采用小压下率冷轧或平整工艺，试验研究了不同压下率冷轧前后SPHC带钢操作侧（OS侧），中间部位，驱动侧（DS侧）3个位置表面和截面的氧化铁皮形貌和厚度，以及氧化铁皮在冷轧过程中的断裂行为。结果表明，试验钢热轧后冷却过程中的热应力使其表面氧化铁皮产生裂纹，且卷取后的冷却速率会对表面氧化铁皮结构产生较大影响；采用10%冷轧压下率时，带钢表面出现较多氧化铁皮块状脱落和粉化脱落现象，表面质量破环严重；采用5%冷轧压下率时，带钢表面仅出现少量氧化铁皮块状脱落，大部分氧化铁皮达到只开裂不剥落状态；氧化铁皮在常温下塑性较差，在冷轧后氧化铁皮厚度未发生随钢基体明显变薄的协同变形。     

退火工艺对冷轧低碳钢组织和性能的影响

对低碳带钢进行多道次常规冷轧(原始厚度为2.5 mm,轧后厚度为0.4 mm,总压下量为84%),研究退火工艺对冷轧板组织和性能影响。结果表明：轧制完成后,晶粒明显拉长,出现了较高密度的位错。随退火温度升高,位错密度显著下降,晶粒得到细化,550 ℃时,形变组织完全消失,再结晶过程结束,位错密度为1.34×1014 m-2,晶粒尺寸1.24 μm。退火温度高于550 ℃时,晶粒尺寸不断长大,试样的表面活化能高达278 kJ/mol。550 ℃最佳退火温度下,保温时间达到60 min时,再结晶基本完成,显微硬度下降明显,伸长率增加。     

热轧带钢表面氧化铁皮的EBSD制样及表征方法

为了得到适用于热轧带钢表面氧化铁皮EBSD样品的制备方法，以22MnB5热轧板表面生成的氧化铁皮为研究对象，对比分析了冷镶、热镶和夹具夹持3种样品固定方法，以及振动抛光和手工制样两种制样方法下的热轧带钢表面EBSD扫描效果。结果表明：热镶法是最适宜热轧带钢表面氧化铁皮打磨的固定方法，振动抛光和手工制样两种制样方法均可以表征热轧带钢表面氧化铁皮的形貌、相组成和晶粒尺寸等显微结构特征，且提出的手工制样方法能够在保证制样效果的同时缩短制样时间和降低成本。     

Effects of oxide layers on surface defects during hot rolling processes

An oxide layer, which developed on the surface of a commercial hot rolling mill, was examined by forcibly stopping the roller between mill stands during activity. Liquid quartz was sprayed on the strip to prevent further oxide layer growth during cooling after stopping the hot-rolling mills. Then the thickness and shape of the oxide layer was examined in a cross-sectional view using an optical microscope. The thickness of the oxide layer increased through the 1^st and 2^nd passes of hot rolling, and then decreased through successive rolling, because the thickening rate by growth is larger than the thinning rate by deformation in high temperature. The temperature distributions of the oxide layer as well as the strip were predicted using the thermo-mechanical finite element method. As thermal conductivity of the oxide layer is low, the temperature deviation of the oxide layer increases and average temperature decreases as the thickness of the oxide layer increases, suggesting the increased formation of surface defects. With these results, a new cooling device was installed between the hot rolling mills to decrease the surface temperature and the thickness of the oxide layer, resulting in improved surface quality of the strip.     

热轧钢板免酸洗直接冷轧还原退火

以MRT-4CA热轧板为例,通过金相分析及XRD等分析方法,考察了不同冷轧压下量时,氧化皮随基体的变形情况,及试样在750℃、10%H2还原气氛下保温180 s后氧化皮的还原情况。结果表明:MRT-4CA热轧板直接冷轧后,氧化皮可随热轧基底一同变形,冷轧产生大量裂纹提升了氧化皮的还原效率;XRD测试表明还原后试样表面的主要成分为Fe与FeO;压下量为55%时质量损失率最高(为0.107%),压下量进一步增加,质量损失率反而有所下降。     

Mechanism of Iron Oxide Scale Reduction in 5%H<Subscript>2</Subscript>–N<Subscript>2</Subscript> Gas at 650–900 °C

The reduction behaviour of the oxide scale on hot-rolled, low-carbon steel strip in 5%H₂–N₂ gas at 650–900 °C was studied. In general, the reduction rate of the oxide scale at the centre location was more rapid than that at the near-edge location. In both cases, the reduction rates at 650 °C were extremely low and the rates increased with increased temperature, reaching their maxima at 850 °C. Arrhenius plot of the rate constant derived from the early parabolic stage revealed that the reduction mechanism at 650–750 °C differed from that at 750–850 °C, with the former being oxygen diffusion in α-Fe and the latter most likely iron diffusion in wustite. In all cases, a thin iron layer formed on the scale surface within a very short time and then the thickness of this layer remained essentially unchanged, while the scale layer was gradually reduced via outward migration of the inner wustite–steel interface, as a result of inward iron diffusion through the wustite layer to that interface. More rapid oxygen diffusion through the thin surface iron layer than the oxygen supply rate through interface reaction was believed to result in a lower oxygen potential at the outer iron–wustite interface, thus providing a driving force for iron to diffuse through the wustite layer. The inner wustite–iron interface became undulating initially; then with the rapid advance of some protruding sections, some parts of the wustite layer were reduced through first, and finally the remaining wustite islands were reduced to complete the reduction process. Porosities were generated when wustite islands were reduced due to localized volume shrinkage. Higher oxygen concentrations in the scales of the near-edge samples were believed to be responsible for their slower reduction rates than those of the centre location samples.     

A Study of the Scale Structure of Hot-Rolled Steel Strip by Simulated Coiling and Cooling

The morphological development of oxide scale on hot-rolled steel strip under various simulated coiling and cooling conditions was investigated. Oxide-scale structures developed were classified into several categories, and the conditions under which each category formed were identified and mapped. It was found that oxide scale formed under normal coiling and cooling conditions had a structure that was difficult to pickle. Increased cooling rate after coiling, or coiling at a temperature below 350°C, improved this structure. The conditions under which a magnetite layer at the wüstite–steel interface was formed during continuous cooling were also identified and the mechanisms of its formation discussed.     

Oxide-Scale Structures Formed on Commercial Hot-Rolled Steel Strip and Their Formation Mechanisms

The oxide-scale structure developed on commercial hot-rolled steel strip at the mid-coil position was examined. The initial oxide scale after rolling and cooling on the run-out table had a three-layer (hematite, magnetite, and wustite) structure; the thickness was found to be a function of the finishing temperature. From this initial structure, various final scale structures developed after coiling, depending on the coiling temperature, oxygen availability, and cooling rate. For relatively low coiling temperatures (e.g., at 520°C), the final scale structure comprised an inner magnetite/iron mixture layer, an outer magnetite layer, and, at regions away from the center, a very thin outermost hematite layer. For higher coiling temperatures (e.g., in the range of 610 to 720°C), a two-layer hematite/magnetite structure was observed at the edge regions, whereas at the center regions, these two layers were absent and the entire scale layer comprised a mixture of the wustite-transformation products, i.e., a mixture of proeutectoid magnetite, magnetite+iron eutectoid, and a certain amount of retained wustite. At regions between the edges and the center, the oxide structures were similar to those developed at low coiling temperatures (<570°C), i.e., an inner layer comprising a mixture of the wustite-transformation products, an intermediate magnetite layer and at regions near the edges, an outermost hematite layer. In addition, two distinct structures were observed on strips with a coiling temperature of 720°C. One structure comprised a very thick hematite layer (3–5 m) formed near the edges (within 10–20 mm from the edges), while the other structure comprised a substantial amount of retained wustite formed at the center regions. The formation mechanisms of various oxide scale structures are discussed.     

高强低合金钢H420LA表面条斑缺陷成因分析

针对高强低合金钢H420LA退火带钢表面存在的暗色条斑缺陷,采用SEM、EDX等手段对带钢表面正常区及条斑区进行了微观结构对比分析,并追溯了热轧态及模拟酸洗后带钢表面不同区域的宏观及微观形貌差别。结果表明：H420LA热轧带钢表面红锈缺陷可遗传演变为退火成品的表面条斑。通过对红锈缺陷的微观结构及其成因分析,提出了提高出炉温度、增加粗轧除鳞道次的工艺措施,有效地减少了H420LA热轧卷表面的红锈缺陷,冷轧成品表面质量因此得到改善。     

Effect of surface oxidation layer on tensile strength of Cu-Ni alloy in friction stir welding

Friction stir welding (FSW) of thick Cu-Ni plate was successfully completed. The fracture position after tensile testing was located at the weld nugget zone (WNZ), where surface oxidation occurred. The oxidation morphologies on the surface of the base metal were analyzed by SEM, EPMA and XRD, with the oxide layer being obtained by simple and useful way to analyze the oxide products, namely, collecting oxide powders after immersing of the oxidized specimen into HNO3 solution. The results highlighted that an oxide layer of 30 μm thickness consists of a mixture of two phases, Cu2O and NiO, on the surface of the base metal. After FSW, the thickness of the oxide layer on the surface was decreased to approximately 5 μm, and broken oxide particles, which is NiO, penetrated into the WNZ by the rotating tool. NiO was preferentially formed at the surface after FSW because it has a lower Gibbs free energy value at 950 °C, which is the peak temperature measured during FSW. Oxide layer of Cu-Ni plate was clearly only removed by mechanical method grinding with 1200-grit SiC paper. The removal of oxide layer results in improved mechanical strength.