09G2S and Q345B, Q355B: low-alloy plate for loaded structures

09G2S is the working grade for trusses, beams, tanks and frames where St3 falls short on strength and alloy steel is unnecessary. The designation reads by GOST: 09 is carbon around 0.09 %, G2 is manganese around 2 %, S is silicon up to 1 %. Chinese mills do not roll this grade; they roll Q355B to GB/T 1591-2018, and at this point in the specification the question appears: is it the same thing?

Q345B and Q355B: one grade, two names

Before the 2018 edition of GB/T 1591 the grade was called Q345B: yield strength not below 345 N/mm² for thickness up to 16 mm, tensile strength 470–630, elongation not below 20 %. In 2018 the standard was aligned with ISO 630 and EN 10025: the minimum rose to 355 N/mm², elongation to 22 %, and the grade became Q355B. The steel at the mill is the same; the name and the lower limit changed. Certificates of earlier batches and traders’ price lists still carry the old name; the contract takes the current one, Q355B to GB/T 1591-2018.

Four grades in numbers

For 09G2S the yield strength drops with thickness in GOST 19281 steps: 345 N/mm² up to 10 mm, 325 for 10–20 mm, 305 for 20–32 mm. Q355B and S355JR have their own steps: 355 up to 16 mm, 345 for 16–40 mm, 335 for 40–63 mm. For a 20 mm plate this means the GB grade is formally 20 N/mm² stronger than the GOST grade, for an 8 mm plate 10. The designer calculates to the standard written in the project; procurement cannot change it.

Frost: where the standards part ways

09G2S is valued not for strength but for impact toughness in the cold: the categories of GOST 19281 set the test at minus 40 °C and minus 70 °C. The letter B in Q355B and JR in S355JR mean a test at plus 20 °C, enough for a heated shop or a frame in Tashkent, not enough for an open pipe rack on the Ustyurt plateau or a tank in the mountains. The closest GB grades by temperature: Q355D is tested at minus 20 °C, Q355E at minus 40 °C. In EN these are S355J2 (minus 20 °C) and S355K2. The specification takes the grade with the right letter, not “an equivalent of 09G2S”.

What to write into the contract

• The grade to GB/T 1591-2018 with the temperature letter: Q355B for structures in above-zero conditions, Q355D or Q355E for cold; next to it the project grade 09G2S and its category to GOST 19281.
• The thickness and the yield strength for that thickness as a number, so that acceptance checks the certificate against the contract rather than against memory.
• The impact test record in the mill certificate with the temperature stated, not just the result but the conditions.
• The old name Q345B only as a note, if the mill still writes it in its own papers; the contract grade is Q355B.

Grade equivalents

Equivalents are the closest match, not identical: chemistry and tolerances differ between standards. The exact grade and standard are written into the contract.