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冷轧不 锈钢退火酸洗新工艺及其优点 Cold rolled stainless steel annealing pickling new process and its advantages

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【摘要】:
根据欧洲标准EN10088/2,当前供 应的冷轧不锈钢带通常分为两种表面:1)2D/2B表面:冷轧、退火、酸洗并平整后的表面;2)2R表面:冷轧、经光亮退火(BA)并平整后的表面。   2D/2B表面,通常用 卧式高产量的连续退火酸洗线生产,产量可达150t/h,钢带的 退火是在氧化气氛中进行(燃烧气体混合物)。退火中 所产生的氧化层必须被去除,然后恢复钝化状态。该工序 通常由电解除鳞和化学酸洗
  根据欧洲标准EN10088/2,当前供 应的冷轧不锈钢带通常分为两种表面:
 
  1)2D/2B表面:冷轧、退火、酸洗并平整后的表面;
 
  2)2R表面:冷轧、经光亮退火(BA)并平整后的表面。
 
  2D/2B 表面,通常用 卧式高产量的连续退火酸洗线生产,产量可达150t/h,钢带的 退火是在氧化气氛中进行(燃烧气体混合物)。退火中 所产生的氧化层必须被去除,然后恢复钝化状态。该工序 通常由电解除鳞和化学酸洗工艺来完成,化学酸 洗一般采用使用混合酸液(硝酸和氢氟酸)。由此产 生的大量的污染物,如氮氧化物、废水中 的硝酸盐以及污泥,需要特 殊的处理工艺来处理。
 
  2R表面,在立式 光亮退火机组上生产。带钢在 氢氮混合物中进行退火,控制好露点,从而防 止了带钢表面的氧化,而无需酸洗工艺。然而,立式炉限制了生产率,目前最大只能达到20t/h-25t/h左右。由于其 镜面特性和良好的外观,2R表面产品比2D/B更受欢迎。
 
  通过结 合两种工艺的主要优点,开发出 一项创新的工艺技术,并应用 于不锈钢冷轧带钢的退火酸洗处理。
 
  1 工艺和技术
 
  市场全 球化正在驱使钢铁制造商寻求新的工艺和技术来减少生产成本、提高竞争力、改善产品质量,同时还 要减少生产过程对于环境的影响。为了追求这个目标,开发出 了一种能够得到接近于2R的增强 表面质量的新不锈钢带生产技术、同时可 以维持与传统退火酸洗线相同的生产能力和生产成本的工艺,其进一 步的优势还表现在能减少化学酸洗工艺对环境的影响。
 
  与传统 退火酸洗线相比,新工艺 能急剧减少带钢在退火段产生的氧化。因此,化学酸 洗处理也就可以取消或者减少使用,从而获 得好的表面质量,节约酸 洗处理的费用并减少了需要中和处理的废液量。控制氧化层的形成,是通过 特殊的带钢热处理周期和精确控制每个工艺步骤(加热和冷却段)的退火 气氛中的氧化量,特别是在高温工艺段。该工艺 技术的关键步骤包括以下几段:
 
  1)控制性 氧化气氛下的快速加热段,在这里,氧化晶 核和一层薄的氧化膜形成;
 
  2)退火段,完成冶金学的转化(得到要求的力学性能、晶粒尺寸、碳的固熔等等),该退火在非氧化气氛(氮气)下进行,以限制氧化层的生长;
 
  3)在非氧 化气氛下的冷却段,冷却速 率能避免碳化物析出;
 
  4)有效的电解除鳞段;
 
  5)轻微的化学酸洗段,可以减 少对环境影响和产生钝化的表面。要获得最好的表面,该步骤可以免去。
 
  2 研究和开发 
 
  从已经 拥有的不锈钢连续退火和酸洗的生产工艺和厂家的技术诀窍开始,首先在 实验室中的处理设施上进行初步研发。后来,新工艺 的实验验证在特别设计和建造的连续退火实验机组上进行。并进行 了退火样品板的表面特性和酸洗的测试。
 
  退火
 
  研究AISI 304不锈钢 在传统退火和新工艺下产生的氧化现象,结果表明,在新工 艺下退火的初始加热段(850℃~950℃),氧化的 减少是通过采用较传统工艺下高的空燃比获得的,从而在 控制性气氛下形成了一层保护性的薄的氧化层。在更高温度下的退火(通常这 个阶段大量的氧化形成),通过采 用非氧化性气氛防止了氧化物的增长,同样的,在冷却段,采用非 氧化性气氛也防止了进一步的氧化物产生。
 
  (2)  酸洗
 
  传统的 冷轧退火酸洗线的酸洗段通常分为两部分。一个电 解段溶解氧化层,一个化学段(混合硝 酸和氢氟酸池或者生态盐池)通过溶 解内层的反应层来去除残余氧化铁皮和恢复钝化表面。
 
  因为新 工艺减少了氧化层的产生,薄氧化 层能够通过在传统电解段的简单电解除鳞处理很容易被去除。电解处理后,带钢表面无氧化,呈现接近与2R的表面质量。
 
  最后,轻微的化学处理(低温,低氢氟酸含量,短处理时间)保证了 即使在退火酸洗线非标准工作环境下(如因为 生产产量和来料的改变而引起的炉子的过渡期,生产线降速等)也能实现完全酸洗。因为极 大地减少了氧化现象从而降低了钢带表面反应性,最终的 轻微化学酸洗跟传统工艺相比,极大减 少了材料的损耗和酸洗的时间。
 
  (3)  试验工厂  
 
  所建造 的退火试验线能够以最高机组速度10m/min连续处理0.4mm~2.0mm厚和300mm宽的带钢,机组是模块化设计的,能够复 制不同的退火周期和炉子气氛。使用的 耐火材料能够承受加热区域最高1400℃的温度,并且配 备了工艺控制仪表。当带钢 到达炉子的第二个区域时,温度达到最高,此时由 电阻加热或者辐射管进行间接加热。大量的 实验在这条机组上进行,致力于 寻求最好的工艺条件、按比例增加的标准,并定义 了相应的工业生产用工艺技术。
 
  3 应用领域
 
  使用该技术,所有的不锈钢种(奥氏体、铁素体和双相不锈钢)可以获 得接近于光亮退火(BA)和标准2B~2D的表面质量。退火工艺中,只有需 要传送到钢带上总能量的约20%的热量 由非直接加热系统(如电阻 加热或辐射管加热)提供。
 
  4 成本评估
 
  成本分 析是基于当地能源和材料价格进行的,考虑了下列生产费用:
 
  a)退火段能源(燃料)和工艺气体消耗;
 
  b)电解段 和轻化学酸洗段的电力和化学品消耗;
 
  c)废物及废气的处理;
 
  d)工厂运 行的直接人力成本;
 
  e)由于退 火和酸洗导致的金属损失(成材率);
 
  基于上述生产费用,新工艺 与传统退火酸洗工艺相比,生产AISI 304成本可以降低35%~40%左右。
 
  5 主要优点
 
  总而言之,新工艺 技术的主要优点体现为:
 
  1)具有和 传统退火酸洗线相同的生产能力;
 
  2)高的灵活性,无论是生产2D产品的 表面还是接近于2R的表面;
 
  3)减少甚 至取消化学酸洗,从而节 约了设备投资和管理成本;
 
  4)减轻对于环境的影响,从而降 低了排放和废物中和处理的费用;
 
  5)更好的产品表面质量;
 
  6)该技术 无论是对于新建工厂或者现有工厂改造都适合。对于现有工厂改造,只需改造炉子段,而无需 对酸洗段做任何改变,仅由于 减少了带钢的氧化,从而提 高了产量而获得盈利。
 
According to European standard en 10088/2, cold-rolled stainless steel bands currently supplied are usually divided into two surfaces:
 
 
1) 2D/2B surface: the surface after cold rolling, annealing, pickling and leveling;
 
 
2) 2R surface: the surface after cold rolling, bright annealing (BA) and leveling.
 
 
2D/2B surfaces, usually produced with horizontal high-yield continuous annealing acid washing lines, yield up to 150t/h, and steel bands are annealed in an oxidation atmosphere (combustion gas mixture). The oxide layer produced by annealing must be removed and then returned to passivation. The process is usually completed by electrodesorption scale and chemical pickling process, which usually USES mixed acid solution (nitric acid and hydrofluoric acid). The resulting large amount of pollutants, such as nitrogen oxides, nitrate in wastewater, and sludge, require special treatment processes.
 
 
2R surface, in the vertical bright annealing unit production. The strip is annealed in a mixture of hydrogen and nitrogen to control the dew point so as to prevent the oxidation of the strip surface without the need of pickling process. However, the vertical furnace limits productivity, which is currently only around 20t/h-25t/h. 2R surface products are more popular than 2D/B due to their mirror-like properties and good appearance.
 
 
By combining the main advantages of the two processes, an innovative technology was developed and applied to annealing and pickling of stainless steel cold-rolled strip steel.
 
 
1 process and technology
 
 
Market globalization is driving the steel manufacturers to seek new technology and techniques to reduce production costs, enhance competitiveness, improve the product quality, but also reduce production process for environmental impact. In pursuit of this objective, developed a way to get close to 2 r enhance the surface quality of the new stainless steel belt production technology, at the same time to maintain the same as the traditional annealing pickling line of production capacity and the cost of production process, the further add to the advantage of pickling process can reduce the chemical's impact on the environment.
 
 
Compared with the traditional annealing pickling line, the new process can greatly reduce the oxidation of strip steel in annealing section. Therefore, chemical pickling treatment can be canceled or reduced, so as to obtain good surface quality, save the cost of pickling treatment and reduce the amount of waste liquid that needs to be neutralized. Control the formation of oxide layer, is through the special strip heat treatment cycle and precise control of each process step (heating and cooling section) annealing oxidation in the atmosphere, especially in the high temperature process. The key steps of the technology include the following:
 
 
1) the rapid heating section in the controlled oxidation atmosphere, where the oxide crystal nucleus and a thin oxide film are formed;
 
 
2) annealing section, complete metallurgical transformation (get the required mechanical properties, grain size, carbon solid fusion, etc.), the annealing in the oxidizing atmosphere under (nitrogen), in order to limit the growth of the oxide layer;
 
 
3) in the cooling section of the non-oxidizing atmosphere, the cooling rate can avoid carbide precipitation;
 
 
4) effective electric descaling section;
 
 
5) the slight chemical pickling section can reduce the environmental impact and produce passivated surface. To get the best surface, this step can be avoided.
 
 
2. Research and development
 
 
Starting from the continuous annealing and pickling process of stainless steel and the manufacturer's technical know-how, the preliminary research and development of the treatment facilities in the laboratory is carried out. Later, the experimental verification of the new process was carried out on a specially designed and constructed continuous annealing experimental unit. The surface characteristics and pickling of annealed sample plate were tested.
 
 
annealing
 
 
Study of AISI 304 stainless steel in traditional annealing and new process of oxidation phenomenon, the results show that under the new technology of annealing the initial heat (850 ℃ ~ 950 ℃), oxidation reduction is by using the traditional process under high air-fuel ratio, thereby under controlled atmosphere to form a protective layer of thin oxide layer. At higher temperature annealing (usually this stage a lot of oxide formation), by adopting the oxidizing atmosphere to prevent the growth of the oxide, likewise, in the cooling section, using non oxidizing atmosphere also prevents further oxide to produce.
 
 
(2) the pickling
 
 
The pickling section of the traditional cold-rolled annealing pickling line is usually divided into two parts. An electrolytic section dissolves the oxide layer, and a chemical section (mixed nitric acid and hydrofluoric acid pool or ecological salt pool) dissolves the inner reaction layer to remove the residual oxide sheet and restore the passivated surface.
 
 
Because the new process reduces the production of the oxide layer, the thin oxide layer can be easily removed by simply removing the scale from the conventional electrolyte section. After electrolytic treatment, the surface of strip steel is not oxidized, presenting a surface quality close to 2R.
 
 
Finally, mild chemical treatment (low temperature, low content of hydrofluoric acid, short processing time) to ensure the even in annealing pickling line, non-standard work environment (such as for production and material caused by the change of the transition period of the furnace, production line speed down, etc.) can achieve full pickling. As the oxidation phenomenon is greatly reduced and the surface reactivity of the steel strip is decreased, the final slight chemical pickling greatly reduces the material loss and pickling time compared with the traditional process.
 
 
(3) test factory
 
 
Built by the annealing test facilities to continuous processing at their highest rate for unit 10 m/min 0.4 mm ~ 2.0 mm thick and 300 mm wide strip, units are modular design, different annealing cycle can be replicated and furnace atmosphere. Use of refractory material to withstand heating area up to 1400 ℃ temperature, and is equipped with process control instrument. When the strip reaches the second area of the furnace, the temperature reaches the highest, which is then heated by a resistance or a radiation tube. A large number of experiments were carried out on this unit, aiming to find the best technological conditions, increase the standard by proportion, and define the corresponding industrial production technology.
 
 
3 application fields
 
 
Using this technique, all stainless steel species (austenite, ferrite, and duplex stainless steel) can obtain surface quality close to bright annealing (BA) and standard 2B ~ 2D. In annealing, only about 20% of the total energy that needs to be transferred to the steel strip is supplied by a non-direct heating system, such as resistance heating or radiant tube heating.
 
 
4 cost assessment
 
 
The cost analysis is based on local energy and material prices, taking into account the following production costs:
 
 
A) energy (fuel) and process gas consumption in the annealing section;
 
 
B) consumption of electricity and chemicals in electrolytic and light chemical pickling sections;
 
 
C) waste and waste gas treatment;
 
 
D) direct labor costs for factory operation;
 
 
E) metal loss due to annealing and pickling (yield);
 
 
Based on the above production cost, compared with the traditional annealing pickling process, the cost of AISI 304 production can be reduced by 35% ~ 40%.
 
 
5 main advantages
 
 
In a word, the main advantages of the new technology are as follows:
 
 
1) having the same production capacity as the traditional annealing acid washing line;
 
 
2) high flexibility, no matter the surface of 2D product or the surface close to 2R;
 
 
3) reduce or even cancel chemical pickling, thus saving equipment investment and management costs;
 
 
4) reduce the impact on the environment, thereby reducing the cost of emissions and waste neutralization;
 
 
5) better surface quality of products;
 
 
6) this technology is suitable for both new factories and existing ones. For the existing plant transformation, only the furnace section needs to be modified without any change to the pickling section. Only by reducing the oxidation of strip steel, the production can be improved and the profit can be made.

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