What exactly is the Q345R alloy steel—the primary material of the oxygen chamber?

I. Introduction to Q345R

1. Q345R is a major category of steel plates—pressure vessel plates. The grade designation method is as follows: Q345R, 16MnR, 16MnG.

2. Q345R: Q—initial letter of the Chinese pinyin for “qu” (yield strength). 345—value of yield strength. R—initial letter of the Chinese pinyin for “rong” (allowable stress).

3. According to the new classification stipulated in “GB 713-2008 Steel Plates for Boilers and Pressure Vessels,” effective September 1, 2008, the steels 16Mng, 16MnR, and 19Mng have been combined into Q345R. Q345R is a general low-alloy steel commonly used for boilers and pressure vessels. Its mechanical properties are similar to those of Q345 (16Mn)—specifically, the yield strength of 16mm-thick plates exceeds 345 MPa—while its tensile strength ranges between (510–640) MPa, its elongation rate is greater than 21%, and its zero-degree V-notch impact energy exceeds 34 J. The process reference standard for Q345R is GB 713-2014.

4. Delivery condition: Hot-rolled controlled rolling or normalizing; classified as low-alloy steel.

5. Manufacturing method: The steel is refined in an oxygen converter or an electric furnace; the continuous casting billet shall have a compression ratio of no less than 3.

II. Q345R steel is a pressure vessel-specific plate with a yield strength of 345 MPa. It boasts excellent comprehensive mechanical properties and good processability. Its phosphorus and sulfur contents are slightly lower than those of the low-alloy high-strength steel Q345 (16Mn). In addition to higher requirements for tensile strength and elongation compared to Q345 (16Mn) steel, it also mandates guaranteed impact toughness. Currently, this steel is the most widely used and highest-volume pressure vessel-specific steel in China.

III. Precautions for the Use of Q345R

1. It is necessary to consider the operating conditions of the equipment (such as design pressure, design temperature, and characteristics of the medium), the weldability of the materials, their cold and hot working performance, heat treatment requirements, and the structure of the vessel.

2. On the premise of satisfying the first condition, consider economic feasibility:

① When the required steel plate thickness is less than 8 mm, carbon steel plates should be preferred over low-alloy high-strength steel plates whenever possible (except for multilayer vessels).

② In applications where stiffness or structural design is the primary consideration, general carbon steel should be selected whenever possible. In applications where strength design is the primary consideration, steel plates such as Q235B, 20R (20g), and Q345R (16MnR) should be selected in sequence, based on operational constraints including pressure, temperature, and medium.

③ When the required stainless steel thickness exceeds 12 mm, structural forms such as clad materials, composites, and surfacing welding should be preferred whenever possible.

④ Stainless steel should, as far as possible, not be used as a heat-resistant material for applications where the design temperature is less than or equal to 500 degrees Celsius.

⑤ Pearlite heat-resistant steels should, whenever possible, not be used as heat-resistant steels for applications where the design temperature is less than or equal to 350 degrees Celsius. When it is necessary to use pearlite heat-resistant steel for heat-resistant or hydrogen-resistant applications, efforts should be made to minimize and consolidate the types and specifications of steel materials.

3. For Q345R steel plates with a thickness greater than 60 mm, the upper limit of carbon content can be increased to 0.22%.

4. Niobium, vanadium, and titanium may be added to Q345R steel plates; their contents shall be indicated in the certificate of quality. The combined content of these three elements shall not exceed 0.050%, 0.10%, or 0.12%, respectively.

IV. Chemical Composition of Q345R

C ≤ 0.20

Si ≤ 0.55

Mn 1.20~1.60

P ≤ 0.025

S ≤ 0.010

Alt ≥0.020

Note: For Q345R steel plates with a thickness greater than 60 mm, the maximum carbon content may be increased to 0.22%.

V. Mechanical and Process Performance of Q345R

Steel plate thickness/mm 3~16 >16~36 >36~60 >60~100 >100~150 >150~200

Tensile strength Rm/(N/mm²) 510~640 500~630 490~620 490~620 480~610 470~600

Yield strength ReL/(N/mm²) ≥345 ≥325 ≥315 ≥305 ≥285 ≥265

Elongation at break A/% ≥21 ≥21 ≥21 ≥20 ≥20 ≥20

Temperature/℃ 0 0 0 0 0 0

V-notch impact absorption energy Akv/J ≥41 ≥41 ≥41 ≥41 ≥41 ≥41

180-degree bending test (b = 2a): d = 2a, d = 3a, d = 3a, d = 3a, d = 3a, d = 3a

As per the buyer’s requirements and upon agreement between the supplier and the buyer, the Q345R steel plate may undergo an impact test at -20℃ instead of the 0℃ impact test specified in the table. The impact values shall comply with the specifications given in the table.

VI. High-Temperature Mechanical Properties of Q345R

Thickness/mm Test temperature/℃

200 250 300 350 400 450

Yield strength σs (MPa) shall not be less than

>20~36 255 235 215 200 190 180

>36~60 240 220 200 185 175 165

>60~100 225 205 185 175 165 155

>100~150 220 200 180 170 160 150

>150~200 215 195 175 165 155 145

7. Applications of Q345R

Q345R steel plates are widely used in industries such as petroleum, chemical engineering, power plants, and boilers, and are employed in the manufacture of high-pressure hyperbaric chambers, reactors, heat exchangers, separators, spherical tanks, oil and gas storage tanks, liquefied gas tanks, pressure vessels for nuclear reactors, boiler steam drums, liquefied petroleum gas cylinders, high-pressure water pipes for hydropower stations, and turbine volutes, among other equipment and components.