Kovar Alloy strip

Quality Control of Production Process of Kovar Alloy 4J29 (marketing department of Shanghai HY Industry Co., Ltd)

1. Kovar Alloy Overview

Kovar alloy 4j29 has a single-phase austenite structure between -70°C and welding temperature, the martensitic transformation temperature ms is lower than -70°C, the alloy does not contain volatile impurity elements, and is in a vacuum state after sealing It does not affect the vacuum degree, has good stamping and welding properties, and has the characteristics of forming a dense oxide film and a firm bond with the substrate after oxidation treatment, so it is widely used in the electronics industry and the electric vacuum industry. Hard glass sealing expansion Kovar Alloy 4j29 has the largest output in the production of expansion alloys.

In recent years, in addition to improving the performance requirements of Kovar Alloy 4j29, users have higher and higher requirements for its surface quality. In order to meet market requirements and according to its own equipment conditions, our company’s precision alloy cold-rolled strip factory has explored a set of Kovar Alloy 4j29 Constant expansion alloy rolling process system, product quality meets the needs of the market. The production line of the precision alloy cold-rolled strip plant is mainly composed of the following equipment: semi-continuous heating furnace, four-roll reversible hot rolling mill, 8.2m2 trolley furnace, grinding machine, ammonia decomposition continuous bright annealing unit, four-roll reversible cold rolling machine, twelve-high reversing cold rolling mill, hob milling machine, split shearing machine. In this paper, the factors affecting the quality and performance of hot-rolled and cold-rolled expansion alloy 4j29 steel strips are analyzed according to the technological process.

2. Kovar Alloy Hot rolling

The hot rolling blanking process consists of a semi-continuous heating furnace and a four-roll reversible hot rolling strip machine.
Strip heating process system: 4j29 furnace entry temperature <800°C, heating temperature 1180~1200°C, preheating time 30~40min, holding time<10min, starting rolling temperature>1160°C, final rolling temperature>900°C, after final rolling air cooled.
The rolling mill adopts a four-roller reversible hot-rolled billet machine with a maximum rolling speed of 180m/min. The specification of the work roll: φ180~195×450mm, the specification of the back-up roll: φ415~435×450mm allows the maximum rolling force of 1568kn, and the specification of the billet used for rolling: 30~40×70~320×800~1200mm, rolled product specifications: 3.0~10×70~320×2000~6000mm.

Rolling process: 40→35→29→23→18→14→10→8→6→4.5mm, total deformation 88.75%, 9 passes.
The quality of hot-rolled coils largely affects the final quality of cold-rolled products. The slab after forging and grinding is heated by a semi-continuous heating furnace, and the strip heating process system is strictly implemented. The heat preservation area must be turned over frequently to prevent overheating or uneven heating temperature of the slab. During the heating process, the atmosphere in the furnace should maintain a positive pressure to reduce oxidation, and the air volume and oil volume should be adjusted frequently. Strictly spray heavy oil directly on the billet to avoid local overheating or overburning. The heated strip is hot rolled by a four-roll reversing hot rolling mill. In order to ensure the surface quality of the hot-rolled strip billet of the four-high reversing hot rolling mill, the operating requirements are as follows:

The quality of the work rolls should be changed every shift. During the rolling process, when the surface of the strip surface is damaged due to the damage of the work rolls, the work rolls should be replaced in time. The surface quality of the back-up rolls should be checked shift by shift, and the surface defects of the back-up rolls should be ground in time. The width of the front and rear guides is generally 10~15mm larger than the final width of the blank. During the rolling process, if the billet is cambered, if it is caused by the size and temperature of the billet, it should be turned over and rolled in the next pass; if the left and right pressing is inconsistent, it should be adjusted in time to ensure that the hot-rolled billet is straight.

Surface quality problems that are prone to occur in the four-roll reversible hot rolling mill:

  • Air skin is due to the cracks on the surface of the slab after the forging of the forged steel have not been ground clean, and the grinding of the forged steel must completely remove the cracks on the surface of the slab.

  • The edge cracking is caused by overheating and overburning of the edge of the slab, and the heating temperature of the slab in the semi-continuous heavy oil heating furnace is too high, and the slab heating process system must be strictly implemented during production.

  • The oxide skin is pressed in because the heating time is too long. During the heating process, the heat preservation time should be strictly controlled, and it should not be too long or too short.

  • Roll bag, sticky roll, and slag are caused by damage to the surface of the work roll, and the work roll should be replaced in time after discovery.

  • Scratches, the guide width is not suitable, adjust the guide width. 90% of the surface defects of 4j29 in the cold-rolled strip mill come from the semi-continuous oil heating furnace and the four-roll reversible hot rolling blanking machine. This process plays a decisive role in the surface quality of 4j29.

3 Kovar Alloy heat treatment, pickling

The hot-rolled steel strip is heat-treated and annealed in an 8.2m2 trolley furnace, and the maximum annealing temperature is 1000°C. Heat treatment process system: Kovar Alloy 4j29 heating temperature 900~920℃, heat preservation 2~3h, air cooling.

After heat treatment, the steel strip is subjected to alkali immersion, water quenching, triple pickling, high-pressure water washing, and neutralization to remove heat-processed scale. Before alkali immersion and pickling, the steel strip should be unrolled one by one, and there should be no mutual sticking between the coil layers. When the steel strip is dipped in alkali, the steel strip must be fully immersed in the lye to ensure that the lye and the oxide scale fully react, so that the oxide scale is fully loose and easy to be washed off in the tri-acid solution. Strictly prevent the steel strip from “sticking to the face” during pickling. After pickling, the strip steel should be free of oxide skin and oxide film, and the surface should have the natural color of metal. Pinhole defects where overacids appear are not to be tolerated. After the steel strip is pickled, it should be rinsed with high-pressure water immediately, and then put into a clean water tank for washing.

4. Kovar Alloy Grinding

Defects caused by hot rolling, heat treatment, and pickling should be removed in this process, and welded into coils at the same time, and supplied to cold rolling mills to roll semi-finished and finished strips. Process flow: opening→straightening→water milling→dry polishing→cutting head→welding→rinsing→drying and coiling.
The pickled steel strip is decoiled, water milled, dry polished, plasma welded, and coiled to form a preliminary rolled steel coil. The width of the grinding wheel is generally greater than or equal to the width of the steel belt 3~5mm. The number of times of water grinding on each side of the steel strip should be ≥ 4 times, until the surface roughness of the steel strip reaches ra=2~3μm, and there are no scratches, cracks, peeling and other defects on the surface. The principle of water milling is multi-pass small pressure and light grinding. Forced pressure grinding is easy to press the grinding wheel surface into the surface of the steel strip and affect the surface quality of the steel strip. After water grinding, some steel strips still have local defects, which should be removed by hand grinding wheels. Such local defects refer to pits, heavy skin, and severe scratches. New scratch-like defect. As an expansion alloy 4j29 strip steel product with high surface quality requirements, it should be dry polished by a polishing machine after water grinding. Dry polishing must be completely polished twice, and the surface of Kovar Alloy 4j29 after polishing is like a mirror surface.

Before welding, press the steel strip to be welded firmly, align the weld seam, and strive to make each steel strip relatively straight after welding, so as to prevent side bending due to improper alignment of the weld seam, and remove oil stains on the weld seam. During welding, there must be no gaps, missing welds, or false welds at the welds to avoid cold-rolled strip breakage in subsequent processes. For the steel strip that has been ground, whether it can produce a finished strip that meets the standard depends on whether the grinding can completely remove all the defects of the previous process.

5, Kovar Alloy cold rolling

The main factors affecting the surface quality and performance of cold-rolled steel strip are: rolling surface, rolling speed, rolling pass, reduction rate, emulsion, roll material and surface accuracy, etc.
The rolling mill adopts a four-high reversing cold rolling mill with a maximum rolling speed of 90m/min. The specification of the work roll: φ175~190×500mm, the specification of the backup roll: φ430~450×500mm allows the maximum rolling force of 2940kn, and the measured value is 2254kn. The specification of the billet used for rolling: 2.5~6.0mm×100~320mm, the specification of the rolled product: 0.35~3.0mm×100~300mm.
Twelve-high reversible cold rolling mill, work roll specification: φ36~43×350mm, intermediate transmission support roll specification: φ45.5~50×301mm, outer support roll specification: φ120×80mm, maximum rolling speed 90m/min, rolling The specification of the billet used: <0.6mm×60~270mm, the specification of the rolled product: 0.05~0.3mm×60~270mm.

Matters needing attention during cold rolling:

  • During rolling, the lower surface of the original rolling needs to be changed to the upper surface of this rolling, so as to remove the surface defects generated during the previous rolling.

  • The higher the rolling speed, the greater the surface roughness of the rolled steel strip.

  • The more rolling passes, the smaller the surface roughness of the steel strip and the smoother the surface; in order to improve the surface quality and dimensional tolerance accuracy, it is allowed to increase the passes appropriately.

  • The greater the reduction rate, the smaller the surface roughness of the cold-rolled steel strip. When the reduction rate of the expansion Kovar Alloy 4j29 is ≥ 72%, a cold-rolled texture will be produced. When stamping and forming, the crystals will be oriented along a certain direction, and the ear will be produced along a certain direction , If such stamped parts are sealed with ceramics, the metal body itself will crack. If the amount of cold deformation is too small (for example, 30%), the material is in a critical deformation state, and coarse recrystallized grains are formed after annealing. Therefore, it is best to control the deformation of the finished product within 65%. Since the delivery tolerances of expanded alloys are all negative tolerances, the dimensional control must be controlled at the center line of the negative tolerance zone during rolling.

  • The greater the viscosity of the emulsion, the greater the surface roughness of the steel strip. In cold rolling, the emulsion is required to have high process cold-wetting performance, chemical stability, and easy removal from the surface of the steel strip; according to the monthly production and The degree of pollution should be replaced in time, and it should not be improvised.

  • The smaller and smoother the surface roughness of the work roll, the smaller the surface roughness of the steel strip. The roll material of cold-rolled expansion Kovar Alloy 4j29 adopts rolling bearing 9cr2. In order to obtain a strip with high-quality surface, the roll surface is required to have the highest smoothness.

  • Semi-finished products and finished products must be lined with paper. If rerolling and blanking, pad paper when rerolling.

Common surface defects in strip rolling:

  • Press-in: Press-in is caused by contamination on the surface of the strip blank, dirt and wool on the feeding clips, reels, guide rails, and pinch rollers.

  • Scratches and strangles: Stretches are caused by tight coil scratches caused by mutual stagger between coils at a given tension when the tension is too small when rewinding and rewinding, and the coils are loose. When making and unloading, the splint needs to be clean and clamped, and the steel strip needs to be padded with paper. Scratches are caused by the guide roller not moving when unloading.

  • Roll marks and indentations: The soft spots caused by the uneven quenching of the roll itself react on the surface of the rolled strip to form periodic bulges to form roll marks, and the strip is seriously deviated during rolling to cause the roll to continue rolling Periodic indentations reflected on the surface of the strip. If the roller printing is found, the only way to change the roller is to regrind immediately.

  • Peeling and slag loss: Excessive gas absorption and diffuse distribution during the smelting process, caused by bubble rupture during the cold rolling process. The slag loss is more in the cold expansion alloy 4j29, which is caused by local overheating and overburning during the strip heating process.

  • Uneven pressure on both sides of the rolling mill or uneven thickness of the billet during rolling produces unilateral waves, and when slight, camber occurs.

  • Double-sided wave: The double-sided wave is mainly caused by the uneven lateral deformation of the strip, and the reduction on both sides is greater than that in the middle. The main reason is that the roll has no convexity or the convexity value is small. Adjusting the convexity mechanism or increasing the convexity of the rollers can eliminate the bilateral waves.

  • Ladle warp: ladle warp is mainly caused by the uneven transverse deformation of the strip, and the depression in the middle is greater than that of the edge. Severe ladle warpage will form waves in the middle of the strip. The main reason is that the roll crown is too large and the thickness of the middle part of the billet Larger, large temperature rise in the middle of the roll, etc. Adjusting the crown mechanism or reducing the crown of the roll can eliminate the double ladle.

  • Diagonal corrugation: Diagonal corrugation is caused by excessive roll convexity or small front tension, unstable roll system, and vibration of the strip. Increasing the reduction and increasing the tension can eliminate or improve this defect.

  • Vibration pattern: Vibration pattern is due to the fact that σb≤570mpa is soft in the Kovar Alloy 4j29 soft state, and the roll will vibrate when the reduction is large, causing horizontal stripes to appear on the surface of the strip. The way to eliminate it is to reduce the compression and rear tension, and increase the front tension. When the roll is ground, the feed rate should avoid sudden cutting and large feed rate, so as to prevent the roll from overheating or burning, and the roll mark formed during rolling will affect the surface quality of the steel strip. The generation of tool marks is generally caused by the poor precision of the bed itself or the defect of the traverse mechanism. Uneven hardness of the grinding wheel can also cause this defect. Vibration lines are generally not easy to find, and can only be seen when the roll is rotating or from the surface of the rolled strip. The main causes of vibration marks are: the dynamic rigidity of the grinding machine itself is not enough, the action is not accurate, the vibration of the roll itself or the unstable support, and external vibration.

6. Kovar Alloy Bright annealing

Bright annealing uses a protective atmosphere to prevent or reduce the oxidation of the alloy during high-temperature annealing. It is used for intermediate annealing of strips with high surface requirements or annealing of finished strips delivered in a soft state. It is an extremely important link in the production of expanded Kovar Alloy 4j29 strip steel. . Intermediate annealing eliminates work hardening produced in cold rolling, improves metal plasticity, reduces deformation resistance, and facilitates subsequent processing. The purpose of finished product annealing is to control the final performance of the product, ensure compliance with national technical standards, and meet user requirements. The precision alloy cold-rolled strip plant uses a 350mm protective atmosphere bright annealing unit for softening annealing of semi-finished products and annealing of finished products.

Semi-finished products are cleaned after annealing, and finished products are stripped after annealing. Annealing Notes:

  • Heat treatment process system: the heating temperature of the expansion alloy 4j29 semi-finished product is 1050 °C, and the heating temperature of the finished product is 950 °C. The maximum line speed of the mechanical equipment of the production line is 15m/min.

  • Protective atmosphere control: bright annealing working atmosphere ammonia decomposition atmosphere 75% hydrogen and 25% nitrogen mixed gas, the maximum consumption of ammonia decomposition gas is 45m3/h, the dew point of ammonia decomposition gas is ≤ -60°C, and the residual oxygen of ammonia decomposition gas is ≤ 10ppm , Ammonia decomposition gas residual ammonia ≤ 5ppm.

  • Tension and steel strip exit temperature: the tension in the heating zone of the bright annealing furnace is 78~2695n, and the steel strip exit temperature is ≤90°C.

  • Cleaning and degreasing: The steel strip goes through a degreasing agent cleaning machine, a rinsing machine, and an air dryer to remove oil stains on the surface of the steel strip and then enters the heating room for heat treatment.

  • The semi-finished product must be cleaned after softening and annealing to improve the surface quality of the strip steel. The cleaning brush uses a wire brush and must be front and back.

  • When the finished product is divided and cut, the surface of the steel belt should be strictly prevented from being scratched when it is moving. The knife must be accurate and the burrs should be small. Precisely adjust the gap and overlap of the blades to ensure that the burr, side bending and plate shape of the slitting meet the standard requirements.

7. Conclusion

After years of production practice, it is concluded that the surface quality of the expanded Kovar Alloy 4j29 has always been the focus of quality control in the production process due to the soft material of the steel strip. For the cold steel strip with a thickness of 1.0mm~1.5mm, it is easy to have surface slag drop, foreign matter intrusion, and the phenomenon that the difference between the same plate is out of tolerance. For steel strips with a thickness of 0.2mm~1.0mm, color spots formed after foreign matter is easily pressed in. From the perspective of the entire production process, each process will produce various defects. Hot rolling is the main process that produces 90% of surface defects, so hot rolling is the key process of quality control in the production process. Grinding, finishing and cleaning are the key processes to improve the surface of the steel strip.

Shanghai HY Industry Co., Ltd has international first-class material production equipment such as high temperature alloy isothermal forging production line, CONSARC vacuum melting furnace, VAR vacuum arc remelting furnace etc., with strong professional ability and highly responsible management team. We are a member of China Application of nuclear energy materials, Member of the International Council of the Aeronautical Sciences. Years of experience in Heat Resistant Steels & Superalloys. When you have superalloy material inquiry, please contact us without hesitation. Shanghai HY Industry Co., Ltd is Corrosion & fatigue nickel alloy professional manufacturer,We will provide you with sincere and thoughtful service.

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