Are there any specific heat treatment processes for Q355GNH? - Knowledge

1. Heat Treatment for Cold-Rolled Q355GNH Plates (e.g., for Boiler Heat Exchangers)

For thin-gauge cold-rolled Q355GNH (common in precision applications like heat exchanger components), the core heat treatment is continuous annealing (continuous retreating) , integrated with the cold rolling process to ensure surface quality and dimensional stability. The specific process is as follows (per Summary 1):

Process Sequence: Hot-rolled coil → Acid pickling → Cold rolling → Continuous annealing → Temper rolling

Key Annealing Parameters (Vertical Continuous Annealing Furnace):

Stage	Temperature Range	Purpose
Heating Stage	800–830°C	Fully austenitize the steel to eliminate internal stress from cold rolling.
Soaking Stage	800–830°C	Maintain temperature to ensure uniform chemical composition and microstructure.
Slow Cooling	650–680°C	Reduce cooling rate to avoid brittle phases (e.g., martensite) and improve toughness.
First Cooling	370–410°C	Controlled cooling to promote the formation of fine, stable precipitates (e.g., Cu-rich phases for weathering resistance).
Overaging Stage	390–410°C	Adjust the size and distribution of precipitates to balance strength and ductility.
Second Cooling	190–210°C	Final cooling to room temperature, ensuring dimensional stability.

Post-Annealing Temper Rolling: A light rolling process with an elongation rate of 0.2–0.6% to correct plate shape (e.g., flatness) and further refine the surface.

2. Heat Treatment for Medium-Thick Q355GNH Plates (e.g., for Bridges, Construction)

Medium-thick Q355GNH plates (thickness 25–40mm, exceeding the standard 20mm limit per Summary 2) prioritize controlled rolling + controlled cooling (TMCP, Thermo-Mechanical Control Process) over traditional heat treatment. This "process instead of heat treatment" approach simplifies production while enhancing strength and weathering resistance:

Core Process: Cast slab heating → Two-stage controlled rolling → Water cooling → Air coolingSlab Heating (Pretreatment):

Heating coefficient: 8–11 min/cm (ensures uniform heating of thick slabs).

Outgoing temperature: 1050–1150°C (avoids over-burning while ensuring full dissolution of alloying elements like Cr, Cu).

Two-Stage Controlled Rolling (Key to Microstructure):

First Stage: Finish rolling at ≥950°C with a compression ratio ≥60% (breaks coarse austenite grains).

Second Stage: Start rolling at ≤920°C, finish rolling at 810–850°C with a compression ratio ≥60% (refines grains to improve toughness and weathering resistance).

Controlled Cooling:

Water cooling immediately after rolling, with a red return temperature of 670–710°C (controls the cooling rate to form a fine ferrite-pearlite structure, avoiding brittle phases).

Subsequent air cooling to room temperature (eliminates the need for stacking slow cooling, shortening production cycles).

3. Surface Heat Treatment (for Enhancing Wear Resistance and Fatigue Strength)

For Q355GNH components requiring improved surface performance (e.g., mechanical parts, landscape structures), high-frequency induction surface heat treatment is commonly used (Summary 3). This process modifies only the surface layer (without affecting the core's weathering resistance):

Process Principle: High-frequency induction current heats the steel surface to 800–1000°C (austenitization), followed by rapid cooling (quenching) to form fine martensite on the surface.

Key Advantages:

Surface hardness increases by 2–3 HRC compared to conventional furnace heat treatment.

Wear resistance and fatigue strength are significantly improved (due to fine martensite grains, high carbide dispersion, and surface compressive stress).

Reduces notch sensitivity of components (critical for load-bearing parts like automobile half-shafts).

4. Compliance with Standards and Core Objectives

All heat treatment processes for Q355GNH align with GB/T 4171-2008 (Weathering Structural Steels). The core objectives are:

Preserve Weathering Resistance: Ensure the formation of stable, dense oxide films (rich in Cr, Cu, Ni) on the surface-avoiding heat treatment parameters that destroy these alloying element distributions.

Balance Mechanical Properties: Achieve a yield strength ≥355MPa, tensile strength ≥490MPa, and elongation ≥22% (per Summary 4), while maintaining good low-temperature impact toughness (KV2 ≥34J at -20°C).

Adapt to Product Forms: Cold-rolled plates focus on annealing for ductility and flatness; medium-thick plates use TMCP for efficiency and strength; surface treatment targets localized wear resistance.

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