How to Resolve Internal Structure Defects in Cold Drawn Square Steel?

Internal defects in cold drawn square steel, such as coarse and uneven grain structure, excessive residual stress, abnormal fibrous structure, inclusions, and segregation, are primarily caused by unreasonable cold drawing deformation, improper heat treatment processes, and substandard raw material quality. These defects are also hidden, directly affecting mechanical properties (strength, toughness, and processing stability). Below are targeted solutions for various internal structure defects, balancing process operability and stability.

Large and Uneven Grain Structure / (Leading to Decreased Strength and Tensile Properties, Prone to Brittle Fracture)

Core Solution Logic: Refining grain structure and achieving uniform organization through "Reasonable Deformation + Precise Annealing"

1. Enhance Cold Drawing Deformation Control

• Single-pass deformation amount: Low carbon steel is controlled at 10%-15%, medium to high carbon steel (such as 45#, 65Mn) at 8%-12%, to avoid excessively small deformation amounts (<8%) that prevent grain refinement or excessively large deformation amounts (>20%) that result in uneven grain distortion.
• Multiple Stretching: Total deformation must be greater than 15% (to ensure sufficient grain fracturing) but not exceed 30% (to avoid excessive deformation leading to grain orientation stretching), completed in 2-3 stretches with equal distribution of deformation per stretch (e.g., first stretch 12%, second stretch 10%).

1. Precise control of the intermediate annealing process

• Annealing Temperature: 650-700°C (for low-carbon steel, use the upper limit; for medium to high-carbon steel, use the lower limit), avoiding temperatures that are too high (>750°C) which can lead to abnormal grain growth, or too low (<600°C) which prevents grain recrystallization.
• Insulation Duration: Adjusted by the length of square steel edges (for edges ≤20mm, insulation for 1 hour; for edges 20-50mm, insulation for 1.5-2 hours), ensuring thorough recrystallization of the internal structure and uniform grain distribution.
• Cooling Method: Slowly cool in the furnace to below 300°C, then naturally cool to room temperature to prevent rapid cooling from causing variations in grain size.

1. Raw Material Grain Pretreatment

• Selected hot-rolled billets that have been normalized (normalizing temperature 850-900℃) to ensure fine and uniform initial grain size (grain size ≤ 30μm), and reject billets that are overheated or have coarse grains.
• Ultrasonic inspection of raw materials is conducted to eliminate ingredients with internal疏松ness and shrinkage holes, which cause grain inhomogeneity.

Excessive residual stress (leading to subsequent processing deformation and cracking during use)

Core Solution Logic: Release stress and balance stress distribution through "Process Optimization + Stress Relieving Treatment"

1. Optimized cold drawing process parameters, reducing stress accumulation

• Stretching speed: Maintain between 2-3m/min to avoid uneven metal flow and delayed stress release due to excessive speed (>5m/min).
• Deformation Allocation: Avoid large deformation in a single pass (＞15%). During multi-pass stretching, pause for 5-10 minutes after each pass to allow partial stress release.
• Lubrication Optimization: Selecting extreme pressure lubricants (such as those containing MoS₂) reduces the friction between molds and metals, thereby minimizing frictional stress.

1. Cold drawn followed by stress-relief treatment

• Low-Temperature Stress Relieving Annealing (Standard Requirement): Temperature 200-250°C, hold for 2-3 hours, air cool, can reduce residual stress by 60%-80% (suitable for most cold-drawn square steel).
• Medium-temperature stress relief (high precision required): For square steel used in machining with high dimensional stability requirements, a holding temperature of 350-400°C for 1.5 hours is applied to reduce residual stress to ≤100MPa, preventing subsequent cutting deformation.
• Natural Aging (Auxiliary Method): After cold drawing, allow the material to rest at room temperature for 7-15 days to gradually release stress, suitable for products with less stringent production cycle requirements and can be used in conjunction with low-temperature annealing.

1. Reduce Sectional Stress Concentration

• Mold Entry Radius Design: R=3-5mm to avoid sharp angles causing local stress concentration.
• Square steel corner deburring: After cold drawing, lightly deburr the corners (R=0.5-1mm) to reduce uneven stress caused by cross-sectional突变.

Section 3: Abnormal Fiber Organization (Increased anisotropy, poor lateral toughness)

Core Solution Logic: By reconstructing the fiber structure through "multiple-stretching + intermediate annealing," reduce anisotropy.

1. Multi-pass Stretching + Intermediate Annealing Combination Process

• Cold drawing is performed in 2-3 passes, with each pass controlling the deformation rate between 8%-12% to prevent excessive fiber orientation due to large deformation in a single pass.
• Following each stretch, an intermediate annealing (650-680°C, holding for 1 hour) is conducted to re-distribute the fiber structure uniformly through recrystallization after stretching, breaking the single orientation.
• Total deformation is controlled between 20%-25%, balancing strength enhancement and anisotropy to prevent excessive total deformation (＞30%) from preventing the reconfiguration of the fiber tissue.

1. Adjust tension and elongation force status

• Ensure that the centerline of the billet aligns with the centerline of the mold cavity to prevent uneven fiber distribution due to uneven force on one side.
• For rectangular steel with large side lengths (>50mm), the "pre-stretching + straightening + final stretching" process is employed. The straightening process can partially disrupt the fiber orientation, reducing anisotropy.

1. Final Product Heat Treatment (optional)

• For products requiring high lateral toughness (such as mechanical structural components), after cold drawing, perform normalizing treatment (850-900°C, hold for 1 hour, air-cooled) to completely reconstruct the structure, converting the fibrous structure into uniform equiaxed grains, significantly reducing anisotropy.
• Avoid using cold drawn materials (not annealed) directly, otherwise the fiber structure may remain for a long time, resulting in transverse impact toughness that could be only 50%-70% of the longitudinal toughness.

Four: Inclusions and segregation (originating from raw materials, reducing plasticity and fatigue strength)

Core Solution Logic: Source Control + Process Optimization, Reduce the Impact of Inclusions, Alleviate Segregation

1. Rigorous Quality Control of Raw Materials (Core)

• High-quality steel billets are selected, with uniform chemical composition (elements such as carbon and manganese with segregation ≤0.05%) and non-metallic inclusions (oxides, sulfides) up to level 2 as per GB/T 10561.
• Non-destructive testing of raw materials (ultrasonic testing, magnetic particle testing) to remove severely flawed materials with internal inclusions, shrinkage holes, and layered defects.
• Avoid using steel with excessive sulfur and phosphorus content (S, P content ≤ 0.04%), as sulfur and phosphorus tend to form brittle inclusions, exacerbating inhomogeneity in the structure.

1. Process Optimization Alleviates Segregation Impact

• Preheating the billet before cold drawing (to 400-500°C, holding for 1 hour) can partially alleviate the inhomogeneity in structure caused by element segregation.
• The "small deformation + multiple annealing" process allows for a slight mitigation of segregation during the annealing process, reducing the impact of inclusions on mechanical properties.
• After cold drawing, the material undergoes a tempering process (quenching + high-temperature tempering), suitable for medium to high carbon steel squares. This process ensures uniformity in the material's structure, reducing the promotion of inclusion-induced crack propagation. Note: The tempering process will offset the strength enhancement achieved through cold drawing, and a balance must be struck.

1. Remedial Measures for Mixed Defects

• If local inclusions exceed the standard, they can be removed through mechanical processing of the affected area (such as turning or grinding), followed by stress-relieving annealing.
• Severely mixed finished products are immediately scrapped to prevent them from flowing downstream and causing fractures or failures.

Internal Cracks (Fatal Defect, Originating from Stress Concentration + Material Issues)

Core Solution Logic: Source Prevention + Process Control, to prevent crack formation and expansion

1. Rigorous raw material quality control

• High purity billets are selected to avoid using raw materials with shrinkage holes, looseness, or皮下bubbles, as these defects are prone to develop into internal cracks during cold drawing.
• Maintain the hydrogen content of the steel billet at ≤2ppm, as excessive hydrogen can easily lead to "hydrogen-induced cracks" after cold drawing.

1. Process Parameter Optimization

• Cold Drawing Deformation: The deformation amount for medium-high carbon steel should not exceed 10% in a single pass, and for low carbon steel, it should not exceed 15% to avoid excessive deformation that could lead to stress exceeding the material's yield strength, causing internal cracks.
• Stretching Speed: Medium to high carbon steel ≤ 2m/min, low carbon steel ≤ 3m/min; avoid excessive speed to prevent localized stress concentration.
• Ensure thorough lubrication: Make sure the lubricant fully covers the billet surface to reduce friction stress and prevent localized overheating, brittleness, and cracking.

1. Crack Detection and Treatment

• Finished products are inspected using non-destructive testing (ultrasonic testing, penetrant testing), with a focus on square steel bars with side lengths greater than 30mm. Any internal cracks found are immediately scrapped.
• For products suspected of having cracks, conduct macroscopic sampling and perform metallographic analysis to trace the raw material and process parameters, preventing batch issues.

Six: Core Control System for Internal Organizational Deficiencies (Ensuring Stable Effectiveness)

1. Process Standardization

• Develop comprehensive SOPs to clearly define the deformation allocation for different materials (low carbon steel, medium-high carbon steel), varying side lengths of square steel, annealing temperature/time, and cooling methods to minimize operational discrepancies.
• Documenting process parameters (deformation amount, annealing temperature, holding time) for each batch to create traceability records for future optimization.

1. Process Monitoring and Inspection

• Each batch selects 1-2 finished products for metallographic analysis to test grain size (required ≤ 40μm), fiber tissue distribution, and inclusion grade, ensuring the tissue meets standards.
• Randomly inspect finished products using the residual stress tester (3 samples per batch), ensuring residual stress is ≤150MPa (standard requirement) or ≤100MPa (high precision requirement).

1. Equipment and Molding Assurance

• Regularly calibrate the tensile sensor and stroke control device of the stretching machine to ensure precise deformation measurement.
• The mold hole design is rational, ensuring even stress distribution during stretching, thus avoiding localized stress concentration and organizational defects.

Summary

The core solution to internal defects in cold drawn square steel is "source control (raw materials) + process optimization (deformation amount + annealing) + closed-loop detection": Avoiding inherent defects such as inclusions and segregation through high-quality raw materials; resolving defects such as grain size, stress, and fiber structure through reasonable multi-pass stretching and precise annealing; ensuring timely detection of defects through metallographic analysis and non-destructive testing. Among them, "intermediate annealing" is a key process for solving most structural defects, requiring strict control of temperature and holding time to avoid over-annealing, which can lead to decreased strength, or under-annealing, which fails to address the issue.