Short answer for the drawing: Q235B is a Chinese low-carbon structural steel to GB/T 700-2006, not a free-machining or heat-treatable grade. It suits welded frames, brackets, base plates and lightly loaded turned or milled parts where strength is not limiting. Do not specify it where you need through-hardening, high fatigue life, tight as-machined finishes on slender features, or corrosion resistance. On a fatigue-loaded shaft or precision bushing, substituting Q235B for 1045 or 4140 quietly removes the margin your design assumed.
Material identity first: what Q235B actually is
The designation is literal: “Q” is qifu (yield point), “235” the minimum yield in MPa for the thinnest sections, and “B” a quality grade adding a room-temperature impact requirement. Q235B is defined by GB/T 700-2006 “Carbon structural steel”, covering the plain-carbon structural family Q195, Q215, Q235 and Q275. It is hot-rolled, low-carbon, ferrite-plus-pearlite, supplied as-rolled or normalised.
Q235B is often called “AISI 1018” or “equivalent to ASTM A36” online. Both are loose: GB/T 700-2006 and ASTM A36 are separate standards with separate limits, so the grades are near-equivalents in function, not interchangeable on paper.
Grade, standards and condition of supply
Q235 is split into grades A, B, C and D. The chemistry band is largely shared; the grades differ mainly in the impact test:
- Q235A — no impact requirement; static or lightly loaded structures.
- Q235B — Charpy V-notch impact at 20 °C, minimum 27 J. The common default.
- Q235C — impact tested at 0 °C.
- Q235D — impact tested at −20 °C.
Product forms have separate standards (GB/T 3274-2017 for hot-rolled plate and strip; separate GB/T standards for bar and sections). State the product form and governing standard on the purchase order, because thickness range determines guaranteed strength.
Chemical composition and what each element is doing
Composition limits for Q235 grade B under GB/T 700-2006. Grade B caps sulfur at 0.045 % (grade A permits 0.050 %), marginally cleaner for welding.
| Element | Limit (mass %) | Function in Q235B |
|---|---|---|
| C | ≤ 0.20 | Strength and hardenability; kept low to preserve ductility and weldability |
| Si | ≤ 0.35 | Deoxidation and modest solid-solution strengthening |
| Mn | ≤ 1.40 | Solid-solution strengthening, improved toughness and deoxidation |
| P | ≤ 0.045 | Impurity; embrittles as it rises, so capped |
| S | ≤ 0.045 | Impurity; forms MnS inclusions that can aid chip breaking but reduce ductility |
Source: GB/T 700-2006 composition limits for Q235 grade B.
There is no chromium, nickel or molybdenum addition in Q235B. If a mill certificate shows meaningful Cr or Ni, question it — it signals stray material or high scrap-borne residuals.
Mechanical properties and the thickness effect
GB/T 700-2006 minimum properties depend on product thickness: the thinnest sections carry the highest guaranteed yield, thicker plate is permitted to be weaker (slower centre-line cooling, coarser grain).
| Property | Thickness condition | Value | Standard / method |
|---|---|---|---|
| Yield strength ReH (min) | ≤ 16 mm | ≥ 235 MPa | GB/T 700-2006; tensile test to GB/T 228.1 |
| Yield strength ReH (min) | 16–40 mm | ≥ 225 MPa | GB/T 700-2006 |
| Yield strength ReH (min) | 40–60 mm | ≥ 215 MPa | GB/T 700-2006 |
| Tensile strength Rm | Up to 100 mm | 370–500 MPa | GB/T 700-2006; GB/T 228.1 |
| Elongation A (min) | ≤ 40 mm gauge | ≥ 26 % | GB/T 700-2006 |
| Charpy V-notch impact (min) | Grade B, tested at 20 °C | ≥ 27 J | GB/T 700-2006; test to GB/T 229 |
Two cautions: these are specification minimums, not typical values from a specific heat — a certificate may sit anywhere inside the tensile band. And the yield floor applies only at the stated thickness; do not apply 235 MPa to a 50 mm plate.
No hardening route: the property that most often trips up designers
Q235B is not heat-treatable in any practical sense. With carbon capped at 0.20 % and no alloying elements, quenching gives almost no hardening response and risks cracking. You cannot harden-and-temper Q235B to 40 HRC as you would a medium-carbon or alloy grade. Heat treatment here means stress relief or normalising:
- Stress relief: roughly 600–650 °C, hold then slow cool — relaxes residual stress so a part does not walk during final finishing.
- Normalising: around 900–930 °C then air cool — refines a coarse hot-rolled structure and homogenises toughness.
Typical workshop ranges for plain low-carbon structural steel; confirm against your own process specification and furnace capability. GB/T 700-2006 does not guarantee these temperatures.
If the part needs more hardness, wear resistance or strength, change the grade rather than trying to extract it from Q235B.
Corrosion and temperature behaviour
Q235B is an ordinary carbon steel and behaves as one. Indoors, dry or oiled, it is serviceable for years. Outdoors or in humid, salt-bearing or chemical exposure it rusts at a rate set by the environment, not the grade; no alloying element forms a passive layer. Consequences:
- Exposed or visible parts need a defined finish: paint, powder coat, zinc plating, or phosphate plus oil.
- On thin sections rust does not stay on the surface; untreated thin plate can lose useful section over time, so specify a finish before approving the design.
- Impact toughness is proven at 20 °C only for grade B. For service down to 0 °C or −20 °C use Q235C or Q235D. Do not assume room-temperature Charpy data extends to a freezer or winter outdoors.
Machinability: honest expectations
Q235B is easy to cut relative to alloy and stainless steels, but it is not a free-machining grade. Free-machining steels such as 12L14 carry deliberate sulfur or lead to break chips; Q235B has none, so chips are long and ductile on soft, coarse hot-rolled stock. In turning that means stringy swarf wrapping the tool and chuck; in drilling, chips that are hard to evacuate from deep holes.
A second issue is residual stress. Hot-rolled plate and bar arrive stressed from unequal cooling. Remove material asymmetrically and the part can bow or twist — often after the final light pass, not during roughing. It is process-caused distortion, not a material defect, and the most common complaint on thin Q235B plate.
Practical machining notes (conditional starting points, not guaranteed parameters)
Starting ranges for coated carbide on a rigid machine with good workholding. Results depend on workpiece rigidity, machine condition, tool grade and coating, coolant strategy, and whether stock is as-rolled or stress-relieved. No figure is a production commitment, and none transfers to another grade.
| Operation | Starting surface speed | Starting feed | Comment |
|---|---|---|---|
| Turning (roughing) | 150–250 m/min, coated carbide | 0.20–0.35 mm/rev | Positive rake; chipbreaker for long chips |
| Turning (finishing) | 200–300 m/min | 0.08–0.15 mm/rev | Sharper edge; watch chatter on slender parts |
| Milling | 120–200 m/min | 0.05–0.15 mm/tooth | Solid carbide; climb milling for finish |
| Drilling | 80–120 m/min (HSS) | 0.10–0.25 mm/rev | Peck deep holes; coolant for evacuation |
Setup guidance for Q235B: coated P-class carbide, positive geometry and a chipbreaker sized to the feed; flood cooling on turning and milling; rigid workholding and support of thin walls, because low-carbon steel deflects before it chips; and a roughing-plus-finishing sequence with an intermediate stress-relief step when flatness on a large plate matters.
Useful internal references: design for manufacturability guidelines for CNC parts, how to select proper tolerances for CNC parts, and understanding surface finish requirements.
Comparing Q235B with the grades it gets confused with
When substituting grades, compare the same condition and the same metric. Below are specified values from the respective standards, not measured results from one heat.
| Grade | Standard | Yield (min) | Tensile | Practical difference |
|---|---|---|---|---|
| Q235B | GB/T 700-2006 | ≥ 235 MPa (≤16 mm) | 370–500 MPa | Plain structural steel; weldable, cheap, not hardenable |
| Q355 | GB/T 1591 | Higher than Q235 | Higher than Q235 | Low-alloy; choose when 235 MPa yield is insufficient |
| ASTM A36 | ASTM A36 | ≥ 250 MPa | 400–550 MPa | US structural near-equivalent; own standard and limits |
A36 does not share Q235B’s guaranteed yield or tensile band, so “our Q235B meets A36” must be verified against the mill certificate and the applicable standard. A hardness conversion between these grades is equally invalid — none is specified by hardness, so an HRC equivalence implies precision the standards do not provide.
Where Q235B is the right answer — and where it is not
Good fits: welded machine frames and bases, mounting plates and brackets, gussets, fixtures, guards, static or low-cycle spacers, and turned or milled prototypes where cost and weldability matter more than strength or finish.
Poor fits: anything to be through- or case-hardened; fatigue-critical shafts, spindles and axles; precision bushings and wear surfaces; uncoated parts in aggressive corrosion; cold service below 0 °C without moving to Q235C or Q235D. Where these apply, change the grade rather than over-specifying tolerances to compensate.
What to put on a drawing or RFQ for a Q235B part
- Full designation and standard: “Q235B to GB/T 700-2006”, plus product form and its governing standard (e.g. GB/T 3274-2017 for plate).
- Condition of supply: as-rolled, normalised or stress-relieved, and whether stress relief precedes final machining.
- Mill certificate: composition and mechanical properties to GB/T 700-2006, heat number traceable to the parts.
- Drawing: material callout, critical dimensions, tolerances, datum scheme, thread and fit specifications.
- Surface finish: functional Ra for sealing or sliding faces; cosmetic requirements for visible faces.
- Finish/coating: paint, plating or phosphate specification if exposed.
- Quantity: prototype and production quantities.
- Inspection: which dimensions are inspected, and whether a dimensional report or material certificate ships with the order.
Getting a quote
If you have a Q235B or comparable low-carbon structural part and want a manufacturability review and quote, send the drawing with the material specification and grade, the required condition of supply, the quantity, the tolerances and the surface finish requirements. Those details let a shop confirm the grade is suitable before the first chip is cut. Request a custom quote for the fastest response.
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