PTJ-2 Wet Skin Pass Rolling Liquid is a water-dilutable rolling fluid formulated for recirculating wet skin pass mill systems in flat-rolled steel and stainless strip processing. The application fields documented below are limited to established downstream production routes where wet skin pass is used to suppress yield point elongation, transfer work roll roughness, flush zinc or chromium oxide dust from the strip surface, and cool the roll bite under controlled friction. Each scenario states the governing product standards for the strip, the working dilution range, the mill configuration and process window, and the terminal product class. Bath concentration is controlled by conductivity or refractometric dosing, with tramp oil removal and filtration matched to the contaminant load of the specific line. Formulation components are selected to exclude cadmium, lead, mercury, hexavalent chromium, and chlorinated paraffins; REACH pre-registration is maintained for all substances supplied into the European Economic Area.
What Limits Surface Carbon Retention and Paint Adhesion After Wet Skin Pass on Zinc-Coated Exposed Panels?
Zinc-coated strip manufactured to EN 10346 and ASTM A653/A653M for automotive exposed body panels is skin-passed after hot-dip coating at elongation levels between 0.5% and 1.2%. PTJ-2 is dosed into demineralized water at 1.5–2.5 vol%, with makeup water conductivity below 5 µS/cm and working bath conductivity maintained between 800 µS/cm and 1,500 µS/cm. The conductivity window is selected to minimize zinc dissolution and post-skin-pass white rust while maintaining the required lubricating film strength at roll-bite temperatures. Bath pH is held at 8.2–9.0; fluid temperature is controlled at 35–50 °C through plate heat exchangers. Volume flow to the spray headers is set so that strip surface coverage exceeds 0.8 L/min per 100 mm of strip width. Flat-jet nozzles operate at 3–6 bar on the entry and exit sides of the roll bite on a 4-high or 6-high skin pass mill. Roll force is set between 2.0 kN/mm and 4.0 kN/mm strip width, and line speed is typically 80–180 m/min.
Surface carbon residue after drying is measured by combustion/infrared or extraction methods aligned with VDA 230-203-type surface cleanliness protocols; exposed-panel limits are commonly below 3 mg/m² total carbon. Paint adhesion is checked after phosphate treatment and electrocoating by cross-cut according to DIN EN ISO 2409, with acceptance at ≤ 1. The wet skin pass fluid is filtered through 10–25 µm bag filters and cartridge polishing filters; tramp oil is removed by belt skimmer or coalescer to keep free oil below 0.2 vol%. Nozzle clogging on line is observed when zinc dust exceeds approximately 200 mg/L; magnetic separators are therefore installed in return lines. Terminal products include exterior door outer panels, hood skins, roof panels, fenders, and decklids with zinc coating masses from 50 g/m² to 70 g/m² per side.
On double-reduced tin mill product lines processing strip widths from 600 mm to 1,050 mm, PTJ-2 is run at 2.0–3.5 vol% in recirculating water, with the bath maintained at 40–55 °C, pH 8.5–9.5, and conductivity 1,200–2,200 µS/cm. The higher concentration compared with automotive galvanized strip is selected because double-reduced tinplate is processed at entering thicknesses of 0.12–0.25 mm and exit thicknesses down to 0.10 mm, where boundary friction dominates and oil retention must remain low. Substrate compliance is governed by ASTM A623/A623M and EN 10202; residual surface oil after skin pass must permit subsequent electrocleaning and tin plating and must be compatible with food-contact lacquering under 21 CFR 175.300. Amine-containing rust inhibitors should not be blended into the bath because they can generate tin oxide staining on thin tinplate edges.
The downstream process is a single-stand 2-high or 4-high skin pass mill positioned after continuous annealing or after double reduction. Rolling force is typically 2.5–5.0 kN/mm, and elongation is set at 0.4–1.0% for double-reduced stock to avoid edge thinning and pinching. PTJ-2 is supplied to the roll bite through high-volume low-pressure nozzles at 4–8 bar, with return-line filtration at 5–10 µm to prevent work roll build-up and strip surface slivers. Heat exchanger capacity is sized to hold recirculation temperature swing within 8 °C during sustained line speed operation. Terminal products are beverage can bodies, drawn and wall-ironed aerosol cans, food can ends, crown corks, and rectangular general-purpose containers.
Stainless Steel Skin Pass Rolling Practice with High-Detergency Wet Rolling Fluid
Stainless steel strip produced to ASTM A480/A480M and EN 10088-2 is skin-passed after bright annealing to remove yield point elongation, adjust surface roughness, and improve flatness without leaving organic deposits that would require post-cleaning before passivation. PTJ-2 is used at 3.0–5.0 vol% in demineralized water, with bath pH between 8.0 and 9.0 and temperature between 30 °C and 45 °C. The higher concentration range is required because austenitic and ferritic stainless steel strip generates higher roll-bite shear stress under polished work rolls, and the fluid must maintain a coherent film at reductions of 0.3–0.8% without introducing silicate or amine residues that reduce corrosion resistance after subsequent pickling or passivation. Compatibility with high-alloy surfaces is verified by intergranular corrosion tests according to ASTM A262 Practice C or equivalent when specified for the end product.
On a bright annealing and skin pass line, strip enters a 2-high or 4-high mill with roll force between 3.0 kN/mm and 6.0 kN/mm, at line speeds up to 400 m/min. The fluid is sprayed through air-atomizing nozzles at 2–4 bar onto top and bottom work rolls; it removes fine chromium oxide dust and prevents roll coating defects on bright-annealed surface. Filtration is staged at 25 µm primary and 5 µm polishing to avoid nozzle clogging and maintain surface quality. Mixing with cationic polyelectrolytes or chlorinated paraffin-containing coolants should be avoided; incompatibility can destabilize the dilution and form deposits in the return flume. Terminal products include architectural sheet, elevator cabin panels, kitchen equipment, fuel cell bipolar plate substrate, and automotive bright trim strip.
When High-Strength Low-Alloy Strip Enters the Skin Pass Mill After Overaging
When high-strength low-alloy or advanced high-strength steel strip enters the skin pass mill after continuous annealing overaging, the process must simultaneously suppress Lüders bands, transfer prescribed surface topography, and limit residual stress that enlarges springback. PTJ-2 is dosed at 2.0–3.0 vol% into softened water, with pH 8.2–9.0 and bath temperature 40–55 °C. The fluid is selected for this route because wet rolling allows lower friction than dry skin pass at the 1.0–1.5% elongation typically specified for HSLA and AHSS grades. Governing product standards include ASTM A1008/A1008M, EN 10268, and VDA 239-100 for cold-formable high-strength sheet. Friction response can be checked by drawbead simulation or ball-on-disc tests, but published data for this specific product-grade combination is limited; mill validation is required for each substrate and coating system.
The wet skin pass process on a 4-high or 6-high mill involves roll forces between 3.5 kN/mm and 8.0 kN/mm, strip tensions from 20 N/mm² to 80 N/mm², and line speeds from 60 m/min to 220 m/min. Heat generated by high contact pressure is removed by recirculating fluid through plate heat exchangers; the return-to-delivery temperature rise is controlled within 8 °C to prevent viscosity collapse at the roll bite. Work roll roughness is specified to produce a final strip surface of Ra 0.6–1.2 µm, with peak count measured according to ISO 4287 and controlled by roll change schedule.
The critical processing conflict is the elongation window: below approximately 0.8%, Lüders bands may persist after forming; above approximately 1.5%, excessive work hardening increases stamping springback and edge cracking. PTJ-2 stabilizes the roll-bite friction so that a consistent elongation can be maintained without dry-bite chatter at high roll force. Finished product categories are structural door beams, bumper reinforcement carriers, seat rails, cross members, suspension arms, and longitudinal side members.
| Strip class | Working dilution | Bath pH | Bath temperature | Primary product standards |
|---|---|---|---|---|
| Automotive zinc-coated exposed | 1.5–2.5 vol% | 8.2–9.0 | 35–50 °C | EN 10346, ASTM A653/A653M |
| Double-reduced tin mill products | 2.0–3.5 vol% | 8.5–9.5 | 40–55 °C | ASTM A623/A623M, EN 10202 |
| Stainless steel bright-annealed strip | 3.0–5.0 vol% | 8.0–9.0 | 30–45 °C | ASTM A480/A480M, EN 10088-2 |
| HSLA / AHSS cold-rolled high-strength | 2.0–3.0 vol% | 8.2–9.0 | 40–55 °C | ASTM A1008/A1008M, EN 10268 |
| Appliance-grade low-carbon cold-rolled | 1.0–2.0 vol% | 8.0–9.0 | 25–40 °C | ASTM A1008/A1008M, EN 10130, ISO 3574 |
| Semi-processed non-oriented electrical steel | 0.8–1.5 vol% | 6.8–7.8 | 20–35 °C | ASTM A683/A683M, EN 10106 |
Because appliance-grade cold-rolled strip is often subjected to coil coating, dry lubricant removal, or direct polyester powder coating after skin pass, the wet rolling fluid must leave no measurable amine or silicone residue that would interfere with pretreatment adhesion. PTJ-2 is applied at 1.0–2.0 vol% in demineralized water, bath pH 8.0–9.0, temperature 25–40 °C, and conductivity below 1,000 µS/cm. Substrate specifications include ASTM A1008/A1008M, EN 10130, and ISO 3574 for low-carbon cold-rolled sheet. Residual organic carbon on dried strip is controlled below 2 mg/m² by infrared or solvent extraction, because residual film above this range can reduce phosphate crystal density and subsequent paint adhesion.
The production process is a single-stand 4-high skin pass mill placed at the exit of continuous annealing or ahead of hot-dip metallic coating. Elongation is set between 0.5% and 1.0%, roll force from 2.0 kN/mm to 4.0 kN/mm, and line speed from 150 m/min to 350 m/min. Recirculating fluid is filtered at 10 µm and a demister is applied to the mill housing to control aerosol. Roll surface roughness is maintained at Ra 0.8–1.5 µm, with Rp peak height controlled to avoid paint short-wave waviness. Terminal products include refrigerator side panels, washing machine cabinets, oven shells, water heater jackets, and office furniture panels.
Electrical Steel Surface Conditioning with Controlled Residual Stress and Insulation Compatibility
During skin pass of semi-processed non-oriented electrical steel strip after continuous annealing, PTJ-2 is applied for flatness correction and residual stress reduction without degrading the insulation/stick-through coating or leaving ionic contamination that would reduce interlamination resistance. The working dilution is 0.8–1.5 vol% in demineralized water, pH 6.8–7.8, temperature 20–35 °C, and conductivity below 500 µS/cm. Applicable substrate standards are ASTM A683/A683M and EN 10106 for semi-processed non-oriented electrical steel. Magnetic property verification is conducted per IEC 60404-2 using Epstein frame tests at 1.0 T and 50 Hz; published data for this specific PTJ-2 application is limited, and process validation on the target grade is required before series use.
The skin pass mill for electrical steel typically uses a 2-high or 4-high stand with polished work rolls to avoid creating surface peaks that would bridge insulation coatings. Elongation is kept below 0.3%, roll force below 1.5 kN/mm, and line speed between 50 m/min and 150 m/min. The fluid is applied in a once-through or low-residence-time spray system rather than a large recirculating sump, to prevent accumulation of iron fines that could alter stack resistance and to maintain low ionic contamination on the strip surface. Terminal products are motor laminations, small transformer cores, appliance motor stators, and segmented traction motor laminations.
