What are the dimensional accuracy defects in cold drawn flat steel?

The dimensional accuracy defects of cold drawn flat steel primarily concentrate in four core dimensions: cross-sectional size, shape regularity, length, and straightness, which directly affect assembly compatibility, load uniformity, and performance. Specific categories and details are as follows:

Sectional Dimension Deviation (Core Size Out of Tolerance)

Thickness/width dimensions are oversized (either larger or smaller).

• Performance: Actual thickness (in the direction of flat steel thickness) or width (in the direction of flat steel width) exceeds the tolerance range specified by the product standard or order requirements (e.g., standard requires thickness 8±0.2mm, actual is 8.3mm or 7.7mm).
• Common Scenarios:
• Thickness Excess: Molding cavity design excessively thick; mold wear not promptly repaired (diameter enlarged).
• Thinner Thickness: Excessive cold drawing deformation, initial material thickness was too thin and工艺was not adjusted.
• Excessively wide/narrow: Design error in the mold width direction, uneven force distribution along the width during stretching.
• Impact: Inability to accommodate assembly requirements (such as being too wide to fit into the slot or too thin for insufficient load-bearing capacity) directly results in product scrapping or rework.

2. Uneven cross-sectional dimensions (local variations in thickness/wideness)

• Performance: Inconsistent thickness/width within the same cross-section (e.g., flat steel with one side thickness of 8.2mm and the other 7.8mm; width-wise, wider in the middle and narrower at the sides).
• Reason: Insufficient precision in mold hole shaping (such as tilted holes, asymmetry), misalignment of guiding devices during stretching (unbalanced force on one side), and inherent unevenness in the cross-section of the raw material (thickness/width fluctuations of hot-rolled billets).
• Impact: Local stress concentration under load, prone to deformation or fracture; poor fit during assembly, affecting connection stability.

Sectional Shape Distortion (Irregular Shape)

Planarity Exceeds Standard (Warping, Bending)

• Performance: The flat surface (contact surface) of the flat steel is uneven, showing upward/downward bowing, localized undulations, exceeding the tolerance for straightness/planearity (e.g., warpage per meter length > 0.5mm).
• Causes: Inconsistent deformation on the upper and lower surfaces during cold drawing, asymmetric fillet at the mold entry, lack of straightening or incorrect straightening parameters after cold drawing, and stress release from internal material residual stress leading to deformation.
• Impact: Inability to fit tightly against the contact surface during installation, with excessive gaps, affecting structural stability (e.g., uneven bolt loads during mechanical connections).

2. Sectional shape distortion (diamond, trapezoidal, elliptical)

• Performance: Ideal cross-section is rectangular, but actual shape is rhombus (with non-right angles at the corners), trapezoidal (top and bottom bases are not of equal width), or the edge arc is too large (exceeding the radius tolerance).
• Reason: Unreasonable mold hole design (e.g., rectangular hole angle deviation), insufficient lateral constraint force during stretching, and uneven pressure on the straightening roller.
• Impact: Inaccurate fit with mating parts during assembly (e.g., rectangular slots not matching diamond-shaped cross-sections, leading to loosening); force direction deviates from design expectations, reducing load-bearing efficiency.

Edge Defects (Burrs, Rolled Edges)

• Performance: Sharp burrs (exceeding tolerance) at the section edge or localized塌边 (edge indentation).
• Reason: Mold cutting edge wear, excessive or insufficient mold clearance, and uneven metal flow at the edges during stretching.
• Effects: Burrs can easily scratch assembly personnel or mating parts; curled edges reduce effective load-bearing width, affecting strength.

Section 3: Length Dimension Deviation

Length variance (either too long or too short)

• Performance: Finished product length exceeding order requirements (e.g., required 6 meters, actual 6.2 meters) or insufficient length due to breakage during stretching (e.g., only 5.8 meters).
• Cause: Inaccurate stroke control on the stretching machine, excessive stretching speed leading to "over-stretching," miscalculation in raw material length, and internal cracks in the material (which cause breakage during stretching).
• Impact: Excess length leads to material waste and requires additional cutting; insufficient length fails to meet usage requirements, resulting in direct scrapping.

2. Inconsistent Lengths (Variations in Standard Length)

• Performance: Excessive length variation within the same batch (e.g., some at 6.0 meters, some at 6.1 meters, some at 5.9 meters), exceeding the batch dimension tolerance requirements.
• Reason: Unstable stretching machine speed, insufficient positioning accuracy of cutting devices (such as sawing), and failure to adjust processes in a timely manner for fluctuations in raw material length.
• Impact: Size confusion during bulk assembly requires individual sorting, reducing production efficiency.

Straightness / Torsion Deviation Exceeded

1. Lateral bending (insufficient straightness)

• Performance: Flats are bent along the length, forming an "S" or "C" shape, with the straightness deviation per meter exceeding the standard (e.g., bending amount per meter > 1mm).
• Causes: Unbalanced tension at both ends during stretching, misalignment of guiding devices, wear or uneven pressure on straightening rolls, or improper stacking during storage (compression-induced bending).
• Impact: Installation not feasible (e.g., components requiring straight assembly cannot fit properly), necessitating additional correction and increasing processing costs.

2. Twisted and deformed

• Performance: The flat steel twists along its length (rotating the cross-section around the longitudinal axis), resulting in non-parallel ends of the cross-section.
• Causes: Asymmetric clamping at both ends of the billet during stretching, offset between the mold hole shape and the centerline of the billet, and uneven distribution of internal stress in the material (such as segregation).
• Impact: Misalignment during assembly can lead to additional torque under load, which is prone to fatigue failure.

Core Impact of Dimensional Precision Defects

• Assembly Level: Inability to accurately match with mating parts, resulting in loosening, jamming, or inability to install.
• Performance-wise: uneven stress distribution, reduced load-bearing capacity, prone to deformation and fracture.
• Cost-wise: The need for rework correction (such as cutting, straightening) or direct scrapping increases production costs and timeline.

Key Preventive Measures

1. Molds: Precision design of mold holes, regular inspection of wear, and timely polishing/replacement.
2. Process: Maintain consistent cold drawing deformation and stretching rates, and eliminate stress through intermediate annealing as needed.
3. Equipment: Regular calibration of stretching machines, straighteners, and cutting devices to ensure guide and positioning accuracy.
4. Raw Materials: Rigorously inspect the uniformity of the cross-sectional dimensions of hot-rolled billets to avoid excessive initial deviations.