The yield strength and tensile strength of SGH340+Z galvanized steel sheets are primarily influenced by three core factors: the composition of the base material and rolling process, galvanizing process parameters, and heat treatment conditions. Details are as follows:

1. Influence of the Base Material (Hot-Rolled Substrate) Itself (Core Factor)

The strength of SGH340+Z is fundamentally determined by its hot-rolled substrate, representing the most critical influence:

Chemical Composition:

Carbon (C): Higher content increases substrate strength but reduces ductility. The SGH340 standard specifies C ≤ 0.25%. Minor fluctuations in carbon content during production directly impact tensile/yield strength.

Manganese (Mn): As a strengthening element, Mn enhances substrate strength through solid solution strengthening. The standard specifies Mn ≤ 1.70%, and deviations in its content cause strength variations.

Impurity Elements (P, S): While P slightly increases strength in small amounts, excessive levels increase brittleness; S readily forms sulfide inclusions, compromising overall strength uniformity. Thus, standards strictly limit P ≤ 0.020% and S ≤ 0.005%.

Hot Rolling Process:

Rolling Temperature (Final Roll / Coil Temperature):

- Excessively low final roll temperature refines grains, enhancing strength.

- Excessively high coil temperature may cause grain coarsening, reducing strength.

Reduction Ratio: A higher reduction ratio during rolling results in more complete grain deformation in the substrate, intensifying the work hardening effect. This correspondingly increases yield strength and tensile strength.

2. Impact of Galvanizing Process

While the galvanizing process does not alter the core strength of the substrate, certain parameters may slightly affect the strength of the surface and near-surface regions:

Hot-dip galvanizing temperature: Substrates are heated to approximately 450°C during galvanizing. Excessive holding time may cause slight grain growth in the surface layer, resulting in a marginal decrease in surface yield strength (generally affecting overall strength by less than 5%-10%).

Cooling Rate: Excessively rapid cooling after galvanizing may form fine alloy phases (e.g., Fe-Zn phase) at the substrate-zinc interface. While this enhances interfacial bonding strength, it does not significantly alter the tensile/yield strength of the substrate itself.

3. Impact of Subsequent Processing

Subsequent processing or heat treatment of galvanized sheets may also affect final strength:

Flat-rolling: Post-galvanizing flattening (light cold rolling) induces minor work hardening, slightly increasing yield strength (typically by 10-30 MPa) but reducing elongation.

Straightening Process: Excessive stress applied during straightening may cause localized plastic deformation in the substrate, affecting strength uniformity (e.g., slightly higher local yield strength compared to the overall material).