Q370R Steel Plate: The Normalized Vessel Grade of GB/T 713
Q370R is a low alloy, normalized steel plate defined in GB/T 713, the Chinese standard for pressure vessel steels. The grade was introduced as the replacement for the older 15MnNbR material, and its main appeal to fabricators is the combination of a guaranteed minimum yield strength of 370 MPa with a mandatory low temperature impact test. In the designation Q370R, the letter Q denotes a steel specified by yield strength, 370 is the nominal minimum yield strength in megapascals, and the letter R identifies a grade intended for pressure vessel and boiler service.
Because Q370R plate is delivered in the normalized condition, it has a uniform ferrite pearlite microstructure with fine grains stabilized by niobium. That gives predictable toughness through the full thickness, which matters for the shells, heads and compression components of pressure equipment working at ambient and moderately elevated temperatures.
Chemical Composition of Q370R Vessel Steel
Niobium is the microalloying element that separates Q370R from a plain carbon vessel steel. Niobium refines the austenite grain size during normalizing, so the yield strength rises without the loss of toughness and weldability that a simple carbon increase would cause. Carbon is deliberately kept low, and both phosphorus and sulphur are tightly restricted because of their effect on toughness and on resistance to hydrogen induced cracking.
| Element | Requirement |
|---|---|
| C, carbon | 0.18 % max |
| Si, silicon | 0.55 % max |
| Mn, manganese | 1.20 - 1.60 % |
| Nb, niobium | 0.015 - 0.050 % |
| P, phosphorus | 0.025 % max |
| S, sulphur | 0.015 % max |
Aluminium is added as a grain refining element and its minimum content is controlled by the product standard. When a project also calls for resistance to wet hydrogen sulphide service, the plate can be ordered in a modified variant with further restrictions on sulphur and on the carbon equivalent, and such plate is supplied with its own test certificate.
Mechanical Properties Across the Thickness Range
Yield strength falls gently as the plate becomes thicker, which is normal for normalized vessel steels because the cooling rate after normalizing slows down in heavy sections. The table below collects the values tabulated for the grade.
| Thickness t, mm | Yield strength, min, MPa | Tensile strength, MPa | Elongation, min, % |
|---|---|---|---|
| 10 < t ≤ 16 | 370 | 530 - 630 | 20 |
| 16 < t ≤ 36 | 360 | per GB/T 713 | per GB/T 713 |
| 36 < t ≤ 60 | 340 | 520 - 620 | per GB/T 713 |
Tensile strength and elongation are specified band by band in GB/T 713, so a plate is normally ordered against the complete standard rather than against a single thickness band. The tensile to yield ratio stays comfortably below the level at which ductility becomes a concern, which is one reason the grade is accepted for welded pressure equipment.
Delivery Condition, Impact Testing and Thickness Range
Q370R is supplied in the normalized condition. Normalizing is not optional for this grade: it is the heat treatment that produces the fine grained structure on which the toughness figures depend. Under GB/T 713 the plate must pass a Charpy V notch impact test at minus 20 degrees Celsius, so the grade is a low temperature vessel steel in the same sense as the other impact tested grades of the same standard. Where a design temperature is lower again, a plate with impact testing at minus 40 degrees Celsius should be selected instead.
Production thickness for Q370R runs from about 6 mm up to 180 mm. Plates at the heavy end of that range are usually normalized in a walking beam furnace and then controlled in cooling to avoid an uneven structure through the section. Lighter gauges are commonly supplied in plate widths that suit the rolling of shells with a minimum of longitudinal welds.
Where Q370R Plate Is Used
Q370R belongs to the group of vessel steels used for compression components and for containment in the middle and normal temperature range. Typical end uses include:
Petroleum and petrochemical process equipment, including columns and reactor shells.
Chemical plant vessels, reactors and separators.
Gas separation, gas storage and gas transportation containers.
Towers, heat exchangers, storage tanks and road or rail tankers.
Boiler shells, drums and other welded pressure parts in the moderate temperature range.
General heavy welded fabrications where a 370 MPa yield level and guaranteed impact toughness are both needed.
Because the grade carries a guaranteed impact energy at a sub zero test temperature, it is often chosen where a plain carbon vessel steel would need extra thickness or additional testing to satisfy the design code.
Fabrication Notes for Welders and Buyers
Q370R is readily welded by the common arc processes, including shielded metal arc, submerged arc and gas shielded methods, provided that consumables of matching strength are selected and that the heat input is controlled. A carbon equivalent in the region typical of this grade means preheat is normally modest for thin plate and increases with thickness and with the degree of restraint.
The niobium microalloying has little effect on cold forming quality, and plate can be cold rolled into shells within the normal bend radii for a steel of this strength level. Flame cutting and plasma cutting are both acceptable; for thicker plate a light preheat before cutting avoids a hardened cut edge. When plate is to be used in sour service, hardness limits on the weld and heat affected zone should be written into the purchase order.
From a purchasing point of view, the important items to state are the grade, the delivery condition, the thickness with its tolerance class, the impact test temperature, the required inspection level and the certificate type. Stating the thickness band is significant because the yield requirement changes with thickness, as shown above.
Frequently Asked Questions
Q: What does the designation Q370R mean?
Q indicates a steel classed by yield strength, 370 is the nominal minimum yield strength in MPa, and R shows that the grade belongs to the pressure vessel and boiler family.
Q: Which older grade does Q370R replace?
Q370R replaced 15MnNbR. Both are niobium bearing vessel steels, but Q370R is defined and certified under GB/T 713 with its own composition and property requirements.
Q: What is the delivery condition of Q370R plate?
The plate is supplied normalized. Normalizing produces the fine grained microstructure that delivers the impact toughness required by the standard.
Q: What impact test temperature applies to Q370R?
GB/T 713 requires a Charpy impact test at minus 20 degrees Celsius. If the design or test temperature is lower, a grade tested at minus 40 degrees Celsius is the correct choice.
Q: What thickness range is available?
Q370R vessel plate is produced from about 6 mm up to 180 mm. Yield strength requirements step down as thickness increases, so the thickness band must be stated on the order.
Q: Can Q370R be welded without difficulty?
Yes. It welds with conventional processes and matching strength consumables. Preheat depends on thickness and restraint, and heat input should be controlled to protect the impact properties of the welded joint.
