GB/T 31315 Q235 Cold-Rolled Precision Welded Steel Tubes for Mechanical Structures

GB/T 31315 Q235 cold-rolled precision welded steel tubes are dimensionally controlled carbon steel tubes manufactured for mechanical components requiring tighter outside-diameter tolerances, improved straightness, consistent wall thickness, and cleaner surfaces than conventional structural ERW tubing.

The tube is initially formed and electric-resistance welded from steel strip, followed by cold drawing or cold rolling to refine the outside diameter, inside diameter, wall thickness, roundness, and surface condition. Q235 provides a practical balance of strength, weldability, formability, and manufacturing cost for general mechanical structures.

Specifications

The stated commercial tolerance capability depends on tube diameter, wall-thickness ratio, steel grade, delivery condition, straightness requirement, and ordered quantity. Final acceptance tolerances should be confirmed in the purchase specification.

Main Characteristics

Controlled Dimensional Accuracy

Cold sizing after welding improves dimensional repeatability along the tube length. For suitable size combinations, the outside diameter can typically be controlled to approximately ±0.05 mm, reducing the amount of turning, grinding, or sizing required before assembly.

Improved Roundness

Cold drawing or cold rolling reduces the ovality normally associated with standard welded tubing. Better roundness supports more consistent clamping, press fitting, bearing installation, bending, and automated feeding.

Stable Wall Thickness

The drawing die and internal tooling control the tube section during reduction. This produces more consistent wall thickness than ordinary structural ERW tubing and improves the predictability of bending and forming operations.

Refined Weld Zone

The welded tube is cold worked after the longitudinal seam has been formed. This improves dimensional continuity around the circumference. When heat treatment is specified, residual forming stresses can also be reduced before subsequent machining or fabrication.

Defined Delivery Conditions

The mechanical response can be adjusted through the selected delivery condition:

Reliable Weldability

Q235 is a low-carbon structural steel with a carbon content generally not exceeding 0.22%. It is suitable for conventional welding processes such as MIG/MAG, resistance welding, and robotic welding when the welding procedure is matched to the component thickness and service conditions.

Our Advantages

1. Dimensional Requirements Are Confirmed Before Production

A common procurement problem is specifying only the outside diameter and wall thickness while leaving ovality, straightness, inside diameter, and delivery condition undefined.

Before production, the following parameters can be reviewed together:

• OD, ID, and wall-thickness tolerance
• Maximum ovality
• Straightness per metre
• Cut-length tolerance
• Weld-bead condition
• Surface roughness
• Heat-treatment condition
• Final bending, flaring, or machining process

This reduces the risk of receiving tubes that comply with a general standard but cannot be processed on the customer’s equipment.

2. Weld Integrity Can Be Verified for Critical Components

For mechanically loaded or fatigue-sensitive parts, dimensional compliance alone is insufficient. The longitudinal weld can be inspected using eddy-current testing, ultrasonic testing, flattening tests, flaring tests, or metallographic examination according to the agreed inspection plan.

Inspection records can be linked to the heat number and production batch for traceability.

3. Delivery Condition Is Matched to the Customer’s Forming Process

Cold-finished tubes may crack, spring back, or distort when the delivery condition is unsuitable for the next manufacturing step.

The delivery condition can therefore be selected according to the customer’s operation:

• +C for high strength and limited further forming
• +LC for light bending and general fabrication
• +SR for machining and dimensional stability
• +A for deep forming, expansion, and severe bending
• +N for balanced strength and ductility

Trial bending or forming samples can be prepared before full production when the component geometry is demanding.

Execution Standards

Specific Application: Automotive Seat Frame Cross Tube

Q235 precision welded steel tube can be used as an automotive seat frame cross tube connecting the left and right seat-frame structures.

For this component, the tube may require:

• Stable outside diameter for robotic fixtures
• Controlled straightness for automated assembly
• Consistent wall thickness for bending
• Reliable resistance-welded joints
• Clean surfaces for electrophoretic coating
• Repeatable cut length and end squareness
• Sufficient ductility for local forming and bracket welding

A typical component specification may use a 20–35 mm outside diameter, 1.2–2.5 mm wall thickness, fixed cut lengths, deburred ends, and a normalized or stress-relieved delivery condition. Final dimensions and properties must follow the seat-frame drawing and validation requirements.

Chemical Composition

The following values are representative maximum ladle-analysis limits for the grade family used with GB/T 31315 tubes. The applicable material standard and quality class must be stated in the purchase order.

Mechanical Properties