1. Overview
CK67 and CK75 are high carbon spring steel grades commonly used in hardened and tempered steel strips.
These steels offer an outstanding combination of hardness, toughness, and spring properties, making them the preferred materials for rolling shutter springs, flat springs, saw blades, and hand tools.
Jiaxiao MTC manufactures CK67 / CK75 strips under strict thermal and dimensional control, ensuring excellent flexibility, surface finish, and consistent mechanical behavior during coiling and bending.
2. Chemical Composition and Equivalent Grades
| Grade | C (%) | Mn (%) | Si (%) | P max | S max |
|---|---|---|---|---|---|
| CK67 | 0.64–0.68 | 0.60–1.10 | 0.17–0.37 | ≤0.035 | ≤0.035 |
| CK75 | 0.72–0.80 | 0.50–0.80 | 0.17–0.37 | ≤0.035 | ≤0.035 |
Equivalent international grades:
- AISI 1065 / 1075
- EN C67S / C75S
- BS CS70 / CS75
These steels belong to the high-carbon class (0.65–0.80% C), providing superior hardness after quenching and excellent elasticity after tempering.
3. Heat Treatment and Production Process
CK67 / CK75 strips are produced through a precisely controlled hardening and tempering process at Jiaxiao MTC:
- Cold Rolling:
Reduces hot rolled coils to the desired thickness while ensuring excellent flatness. - Hardening:
Heated above the critical temperature (approx. 800–850°C) and quenched rapidly to achieve high hardness. - Tempering:
Reheated to 300–450°C to balance hardness with toughness and flexibility. - Edge and Surface Treatment:
- Edge types: slit, rounded, or square
- Surface finishes: natural, blue, yellow, white, or polished
All treatments are performed in controlled atmosphere furnaces to prevent oxidation or decarburization.
4. Mechanical Properties
| Condition | Hardness | Tensile Strength | Surface Finish | Edge Type |
|---|---|---|---|---|
| Hardened & Tempered | 30–55 HRC | 1500–1800 N/mm² | Blue / Yellow / Polished | Rounded / Square |
Thickness Range: 0.1–5.0 mm
Width Range: 2–500 mm
The resulting strips show high fatigue life, uniform spring tension, and excellent repeatability during coiling, ensuring reliable operation for mechanical springs and shutter doors.
5. Rolling Shutter Spring Application
One of Jiaxiao MTC’s most recognized applications of CK67 / CK75 is in rolling shutter door springs.
In this field, flexibility and consistent performance are key factors — the steel must withstand repeated winding and unwinding cycles without cracking or losing elasticity.
Each strip undergoes:
- Edge deburring to prevent failure under tension,
- Hardness and width testing for dimensional stability, and
- Flexibility testing to ensure it meets required bending cycles.
Typical shutter spring specifications:
| Spring Size (mm) | Load (kg) | Material Example |
|---|---|---|
| 60 × 1.20 | 29 kg | CK75 |
| 55 × 1.20 | 26 kg | CK67 |
| 50 × 1.40 | 28 kg | CK75 |
| 45 × 1.00 | 15 kg | CK67 |
These values demonstrate how strip thickness and width determine spring torque and load capacity — a critical factor in shutter and door balance mechanisms.
6. Advantages of CK67 / CK75 Steel Strips
- High tensile strength and elasticity after controlled tempering
- Excellent fatigue resistance for repeated stress cycles
- Smooth surface finish with optional polished or blue-tempered coating
- Uniform flatness and edge accuracy for automatic coiling machines
- Stable mechanical properties due to high-quality quenching and tempering lines
7. Other Industrial Applications
Beyond shutter springs, CK67 and CK75 are widely used for:
- Flat and spiral springs in machinery and tools
- Saw blades and scraper knives requiring sharpness and resilience
- Putty knives, trowels, and flexible blades in construction tools
- Shoe shank and toe cap materials for durability and flexibility
These applications rely on the steel’s combination of hardness (HRC 30–55) and controlled toughness, making it ideal for bending, forming, and punching operations.
8. Dimensional Tolerances and Inspection
Jiaxiao MTC strictly follows standard tolerance controls:
- Thickness tolerance: ±0.015–0.070 mm
- Width tolerance: ±0.20–0.80 mm
- Straightness tolerance: Class R1–R4
- Flatness deviation: ≤0.3% of width
Each batch undergoes:
- Chemical composition testing (spectrometer)
- Hardness verification (Vickers, Rockwell)
- Surface inspection under magnification
- Elasticity and flexibility testing on coiling simulators
9. Packaging and Delivery
Every coil is:
- Lightly oiled and wrapped in plastic film and anti-rust paper,
- Packed in seaworthy wooden crates or steel pallets,
- Labelled with grade, hardness, size, and batch information for traceability.
This ensures safe international delivery and protection against corrosion or deformation.
10. About Jiaxiao MTC
Guangzhou Jiaxiao Material Technology Co., Ltd.
is a professional manufacturer of high carbon steel strips, alloy steel strips, and stainless steel strips in both hardened & tempered and cold-rolled annealed forms.
With advanced facilities for acid washing, cold rolling, annealing, slitting, quenching, tempering, and polishing, Jiaxiao MTC ensures consistent quality across its 15,000 tons annual capacity.
Its products are trusted worldwide in industries such as spring manufacturing, cutting tools, automotive components, and construction hardware.
