Understanding Q345B: Real-world insights on q345b yield strength
I’ve spent a good chunk of my career on shop floors and in mill labs, and—honestly—few phrases get tossed around more casually than “yield strength.” With Q345B, the number sounds simple, but the context matters: thickness, heat treatment, and testing standards all play a part. Let’s unpack what buyers, fabricators, and QA teams should know, without the fluff.
What Q345B is (and why engineers still ask twice)
Q345B is a Chinese low-alloy, high-strength structural steel defined in GB/T 1591. For thin plate and sheet—say 2.0–3.5 mm—the minimum q345b yield strength is typically 345 MPa. In real production, you’ll often see 350–380 MPa on mill test certificates. Tensile strength usually lands around 470–630 MPa, depending on coil/plate source and rolling schedule. Charpy impact for the “B” grade is verified at room temperature; mills target safe margins, but always check the MTC.
Quick product snapshot (GB/T 1591)
| Product | Low Alloy Steel Q345B GB/T 1591-2008 / Q355B GB/T 1591-2018 |
| Thickness | 2.0–3.5 mm (typical stock) |
| Width | 1251–1500 mm (custom cuts available) |
| Yield strength | ≥345 MPa for t ≤16 mm; real-world ≈350–380 MPa |
| Tensile strength | ≈470–630 MPa (test per ISO 6892-1) |
| Impact energy | Room temp Charpy V-notch, typically ≥27–34 J (GB/T 229) |
| Origin | ROOM 1616, SHOUJING E-WORLD, XINDU DISTRICT, XINGTAI, CHINA |
From melt to MTC: how it’s actually made and tested
Process flow (typical): BOF/EAF → LF/VD refining → continuous casting → hot rolling → controlled cooling/leveling → shear/cut-to-length → UT spot checks (if specified) → packing. Sampling per GB/T 2975. Tensile tests per ISO 6892-1; Charpy per GB/T 229. Dimensional tolerances per relevant GB/T sheets. I know, it sounds dry—but this is where consistency in q345b yield strength is won or lost.
Service life? In mild outdoor environments with basic coating, 15–30 years is normal. In marine or industrial atmospheres, plan for proper paint systems or galvanizing to keep that q345b yield strength advantage intact over time.
Where it shines: applications and feedback
- Ship plate and deck modules (weight reduction without drama).
- Boiler/pressure-supporting structures—ancillary frames, not the pressure boundary.
- Containers and chassis—good weldability, predictable forming.
- Construction machinery—booms, outriggers, lattice members.
Two quick cases. A North China fabricator swapped mild steel for Q345B in container floor beams and shaved 8–10% weight—no weld rework spikes, according to their QA boss. Another client building small harbor cranes reported smoother bending on 3.0 mm sheet (bigger bend radius, fewer micro-cracks), which—frankly—lined up with what we saw in their bend test coupons.
Advantages and current buying trends
Supply has stabilized post-2023, and buyers now push for tighter yield bands and low-sulfur heats. The big win is weldability: moderate carbon equivalent means fewer surprises in HAZ toughness. Also, MOQ can be as low as 1 ton, which is nice for prototyping.
| Vendor | Stock/Size Range | Lead Time | Certs & Testing | Notes |
|---|---|---|---|---|
| Baidy Steel (Hebei) | 2.0–3.5 mm, 1251–1500 mm; cut-to-size | ≈ within 7 days | EN 10204 3.1 MTC; ISO/GB tests | Stable q345b yield strength; FOB/CFR/CIF |
| Local Service Center | Mixed heats; quick blanks | 1–3 days ex-stock | Spot re-test on request | Convenient, slightly higher price |
| Trading House | Wide mill network | Varies by mill slot | Mill MTC only | Check tolerances and heat traceability |
Customization menu
Services include welding, punching, cutting, bending; pickled & oiled finish; temper pass; UT per order; packaging for sea freight. Payment terms: 30% T/T advance + 70% balance; Incoterms FOB/CFR/CIF. Many customers say the cut-to-size program is what saves their schedule, to be honest.
Specs table for quick selection
| Parameter | Spec/Value | Notes |
|---|---|---|
| q345b yield strength | ≥345 MPa (t ≤16 mm) | Per GB/T 1591; actual heats may exceed |
| Tensile strength | ≈470–630 MPa | ISO 6892-1 test method |
| Elongation | ≈20%–26% | Gauge length dependent |
| Impact | ≥27–34 J at RT (B grade) | GB/T 229 Charpy V |
Certification: EN 10204 3.1 MTC; compliance with GB/T 1591-2008 or GB/T 1591-2018 (for Q355 series). Third-party inspection on request (SGS, BV).
Final word
If your design hinges on q345b yield strength, tie the spec to thickness and the test standard in your PO, and keep an eye on heat numbers. Simple, practical, and it saves rework.
Authoritative citations
- ISO 6892-1: Metallic materials — Tensile testing — Part 1
- ISO 148-1: Metallic materials — Charpy impact test (referenced alongside GB/T 229)
- GB/T 1591-2008: High strength low alloy structural steels
- GB/T 1591-2018: High strength structural steels (Q355 series)
- GB/T 2975: Steel and steel products — Location and preparation of samples and test pieces
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