Thick steel plates: the “load-bearing backbone” of industrial infrastructure

In modern industrial systems and large-scale infrastructure projects, thick steel plate, with its exceptional load-bearing capacity, excellent mechanical properties, and stable structural characteristics, has become a core material supporting heavy equipment, super-high-rise buildings, bridges, tunnels, and other critical areas. From the frames of 10,000-ton hydraulic presses to the steel box girders of kilometer-span bridges, and even the containment vessels of nuclear power reactors, thick steel plate, with its "heavy body," bears the weight of industrial production and infrastructure, and is a key indicator of a country's steel industry's technological prowess.

1. Definition and Classification: Precise Definitions from Thickness to Application
Thick steel plate is more than simply "thick steel plate." Instead, it has clear industry standards and a classification system. Its core characteristics are "thickness, high strength, and stable performance," enabling it to withstand the demanding requirements of heavy loads, low temperatures, and corrosion in complex operating conditions.
(I) Definition: "Heavy Material" Determined by Thickness

According to the Chinese national standard "GB/T 15574-2016 Steel Product Classification," thick steel plate generally refers to steel plate with a thickness of 4mm or greater. Further subdivided, it can be divided into "medium-thick plate" (thickness 4-20mm) and "extra-thick plate" (thickness >20mm, with some extra-thick plates exceeding 600mm). Compared to thin steel plate (thickness <4mm), thick steel plate has a more complex production process (requiring controlled internal microstructure uniformity) and more pronounced mechanical properties (requiring high strength and toughness). It is primarily used in applications requiring heavy loads or structural stability.

(II) Core Classification: Classification by Material, Application, and Performance

The classification of thick steel plate is directly related to its application. Different types of thick steel plate have significant differences in composition design and heat treatment processes to meet the specific needs of different industries. 1. Classification by Material (Chemical Composition)
Carbon structural thick steel plate: Primarily composed of iron and carbon, with a carbon content of 0.06%-0.22%, with small additions of elements such as silicon and manganese. It offers moderate strength (yield strength 235-345 MPa), good ductility, excellent weldability, and low cost.
Applications: Steel columns and beams in ordinary industrial plants, bases and brackets in machinery manufacturing, and heavy equipment casings with low strength requirements.
Typical grades: Q235B and Q345B (low-alloy, high-strength steel containing small amounts of manganese and vanadium. It boasts higher strength than ordinary carbon steel and is classified as "low-alloy," but is often used alongside carbon steel).
Alloy structural thick steel plate: Alloying elements such as chromium, nickel, molybdenum, and vanadium are added to carbon steel to enhance strength, toughness, wear resistance, or heat resistance. Yield strength can reach 420-960 MPa. Some plates undergo tempering (quenching followed by high-temperature tempering) to significantly enhance their overall mechanical properties. Applications: Gears and shafts for heavy machinery (such as main shafts in mining machinery), high-pressure vessel casings (such as reactors in petrochemicals), and structural components for engineering machinery requiring high strength (such as track shoes for excavators).
Typical grades: 40Cr (chrome steel, used for shafts), 42CrMo (chrome-molybdenum steel, used for high-pressure bolts), and Q690D (low-alloy high-strength steel, used for wind turbine tower flanges).
Special performance thick steel plates: Designed for specific operating conditions, they offer specialized properties such as weather resistance, corrosion resistance, heat resistance, and low-temperature toughness. These plates often feature more complex compositional designs (e.g., the addition of copper, nickel, and titanium). Applications:
Weather-resistant thick steel plate (such as Q355NH): Used for outdoor bridges and towers, it resists atmospheric corrosion and reduces paint maintenance.
Heat-resistant thick steel plate (such as 15CrMoR): Used for thermal power boilers and high-temperature pipelines, capable of long-term stable operation at temperatures of 400-550°C.
Low-temperature toughness thick steel plate (such as Q345E): Used for bridges and containers in cold regions, it maintains excellent impact toughness (impact energy ≥ 27J) at temperatures as low as -40°C.
Stainless steel thick plate (such as 304, 316L): Used for chemical equipment and nuclear power pipelines, it resists acid and alkali corrosion and can be over 50mm thick.
2. Classification by Application
Structural thick steel plate: Accounting for over 60% of all thick steel plate use, it is used in load-bearing structures in buildings, bridges, and machinery, requiring a balance of strength and toughness, and excellent weldability. Examples include steel columns in super-high-rise buildings (using Q355GJD high-rise steel) and steel box girders in sea-crossing bridges (using Q690qE bridge steel). Thick steel plates for containers: Used in the manufacture of pressure vessels (such as storage tanks and reactors), they require high strength, high toughness, and no internal defects (such as pores and interlayers). They must pass UT/MT testing. Typical grades include Q345R (general purpose container steel) and 15CrMoR (high-temperature container steel).
Thick steel plates for machinery: Used in core components of heavy machinery (such as frames and hammer heads), they require high hardness, wear resistance, or impact resistance, and often require heat treatment (such as quenching and tempering). Examples include NM450 wear-resistant steel plates (used in crusher liners).
Thick steel plates for nuclear power: Used in the containment vessels and steam generators of nuclear power reactors, they require extreme purity (low sulfur and phosphorus), low-temperature toughness, and radiation resistance. These are high-end thick steel plates. Typical grades include SA508Gr.3Cl.2 (reactor pressure vessel steel). II. Core Performance: The Key to Supporting Heavy Loads and Severe Operating Conditions
The core value of thick steel plate lies in its exceptional load-bearing capacity and stability in complex operating conditions. These properties, determined by the composition, smelting process, and heat treatment, are key differentiators from ordinary thin steel plate.
(I) High Strength: The "Core Capability" for Heavy Load Bearing
The strength indicators (yield strength and tensile strength) of thick steel plate are far higher than those of ordinary thin steel plate, and remain stable as thickness increases (avoiding the strength drop caused by the "thickness effect").

Ordinary carbon thick steel plate (Q235B): Yield strength ≥ 235 MPa, tensile strength 375-500 MPa, capable of withstanding the vertical loads of factory steel columns.
Low-alloy high-strength thick steel plate (Q690D): Yield strength ≥ 690 MPa, tensile strength 770-940 MPa. Used in flanges for wind turbine towers, they can withstand the tremendous torque during wind turbine operation.
Alloy structural thick steel plate (42CrMo): After quenching and tempering, yield strength ≥ 1080 MPa, tensile strength ≥ 1230 MPa. Used in bolts for high-voltage equipment, they can withstand pressures of hundreds of megapascals.
This high-strength property enables thick steel plate to carry greater loads with a thinner thickness. For example, the use of Q690qE thick steel plate in the steel box girders of kilometer-span bridges can reduce weight by over 20% compared to ordinary steel, reducing the overall load on the bridge. (2) High Toughness: A "Safety Guarantee" Against Impact and Low Temperatures
Toughness is the "safety line" for thick steel plates, especially under low-temperature and impact conditions (such as bridges and heavy machinery collisions in cold regions). High toughness prevents "brittle fracture" (sudden, unpredictable breakage).

Low-temperature toughness: Using "impact energy (Ak)" as an indicator, for example, Q345E thick steel plates with an impact energy of ≥27J at -40°C can be used in cold regions such as Northeast China and Siberia without becoming brittle in winter due to low temperatures.

Impact toughness: For example, NM500 wear-resistant thick steel plates used in construction machinery are not only highly hard (Brinell hardness ≥500HB) but also have an impact energy of ≥30J at room temperature, allowing them to withstand stone impacts without breaking.

Welding toughness: Thick steel plates are often joined by welding (such as steel box girders in bridges). The toughness of welded joints is crucial. High-end thick steel plates (such as Q690qE) have an impact energy of ≥47J in the heat-affected zone (HAZ) at -40°C, ensuring the safety of the overall structure after welding. (III) Low Defects and High Uniformity: Avoiding "Hidden Risks"
Thick steel plates are prone to internal defects such as pores, interlayers, and segregation (compositional inhomogeneity). These defects can significantly reduce load-bearing capacity and even lead to structural failure (such as pressure vessel explosion). Therefore, high-quality thick steel plates must meet the following "low defect and high uniformity" requirements:

High Purity: Extremely low sulfur and phosphorus content. For example, sulfur content of thick steel plates for nuclear power plants is ≤0.005% and phosphorus content is ≤0.015%, to avoid brittleness caused by sulfides and phosphides.

Internal Defect Free: Must pass 100% ultrasonic flaw detection (UT) and meet the GB/T 2970-2016 Level I standard (no recordable defects) to ensure a dense internal structure.

Uniform Microstructure: Minimal variation in grain size and hardness across the thickness. For example, for Q355GJD high-rise building steel, the thickness hardness difference is ≤15HB. This avoids stress concentration caused by localized strength deficiency. (IV) Excellent Machinability and Weldability: Suitable for Engineering Assembly
Thick steel plates often require cutting, drilling, welding, and bending before they can be formed into final structural components. Therefore, machinability and weldability are crucial:

Weldability: Ordinary carbon thick steel plates (Q235B) can be welded without preheating; high-strength thick steel plates (Q690D) require moderate preheating (e.g., 80-150°C), but reliable welding can still be achieved with specialized welding wire (e.g., ER55-G).

Machinability: Alloy structural thick steel plates (e.g., 40Cr) have a high hardness and require carbide tools for machining. However, by adjusting cutting parameters (e.g., reducing cutting speed), they can meet precision machining requirements.

Formability: Some thick steel plates (e.g., Q345R) can be cold- or hot-bent, such as into curved pressure vessel shells. Surface cracks are not observed when the bend radius is ≥3 times the plate thickness. III. Production Process: "Precision Forging" from Molten Steel to Thick Plate
The production of thick steel plate is one of the most technically challenging areas in the steel industry, requiring multiple processes including "smelting → casting → rolling → heat treatment → flaw detection." Each step requires precise control to ensure performance meets standards.
(I) Smelting: Laying the "Foundation of Purity"
Converter/Electric Furnace Smelting: Using molten iron (converter) or scrap steel (electric furnace) as raw materials, slag-forming agents such as lime and dolomite are added to remove harmful elements such as sulfur and phosphorus. For high-end thick steel plates (such as those used in nuclear power plants), a "converter + LF furnace (refining furnace) + RH vacuum degassing" process is used to reduce the hydrogen content in the molten steel to below 2 ppm (to avoid hydrogen-induced cracking) and the sulfur content to below 0.005%. Composition Control: Alloying elements are precisely added based on the intended use of thick steel plates. For example, when producing Q690D, manganese (1.0%-1.6%) and vanadium (0.02%-0.06%) are added to enhance strength. When producing weathering steel, copper (0.2%-0.5%) and chromium (0.3%-0.8%) are added to improve corrosion resistance.
(II) Casting: Avoiding "Internal Defects"
Continuous Casting/Mold Casting: Ordinary thick steel plates utilize a continuous casting process, where molten steel is cast into thick slabs (200-300mm thick). High-end thick steel plates (such as those used in nuclear power plants) utilize a mold casting process, where molten steel is cast into ingots (weighing up to hundreds of tons) and then forged into slabs. This reduces internal segregation and porosity during the continuous casting process.
Slow Cooling: After casting, the slabs must be cooled slowly (for a minimum of 24 hours) to avoid internal stress or cracks caused by excessively rapid cooling. (III) Rolling: Controlling Thickness and Microstructure
Heating: The slab is heated to 1100-1250°C (austenitizing temperature) to ensure good steel plasticity and facilitate rolling.
Hot Rolling: Multiple passes are performed using a wide and heavy plate mill (roller length ≥ 3.5m). The "Controlled Rolling and Controlled Cooling (TMCP)" process (controlling rolling temperature, reduction, and cooling rate) refines grain size and improves strength and toughness. For example, in the production of Q690qE bridge steel, laminar cooling is used after rolling, with a cooling rate controlled at 10-20°C/s to form a fine ferrite + pearlite structure.
Cutting to Length: After rolling, the thick steel plate is cut into fixed lengths (e.g., 12m x 3m) using CNC shears or flame-cut into special shapes (e.g., curved plates for bridge box girders) according to customer requirements. (IV) Heat Treatment: Optimizing "Performance Indicators"
Normalizing: Heating thick steel plates to above Ac3 (approximately 900-950°C), holding, and then air cooling. This refines the grain size, homogenizes the microstructure, and improves toughness. Suitable for container steel (e.g., Q345R);
Quenching and Tempering (Quenching + High-Tempering): Heating thick steel plates to quenching temperature (850-900°C), water quenching, and then tempering at 500-650°C creates a tempered bainite structure, which combines high strength and toughness. Suitable for alloy structural steel (e.g., 42CrMo);
Stress Relief Annealing: After welding, thick steel plate structural components (e.g., steel box girders) require annealing (heating to 600-650°C, holding, and then slow cooling) to eliminate welding stress and prevent subsequent deformation or cracking. (V) Quality Inspection: Ensuring "Zero Defects"
Mechanical Property Testing: Samples are taken to test yield strength, tensile strength, elongation, and impact energy to ensure compliance with standards.
Non-destructive Testing: 100% ultrasonic testing (UT) is used to detect internal defects, and magnetic particle testing (MT) is used to detect surface cracks.
Chemical Composition Analysis: Spectroscopic analysis is used to determine the content of each element in the steel to ensure that the composition meets standards.
Dimensional Accuracy Testing: A laser thickness gauge is used to measure thickness deviation (thick steel plate thickness deviation ≤ ±0.3mm), and a flatness tester is used to measure plate shape (flatness ≤ 2mm/m).

IV. Application Areas: Core Support for Industry and Infrastructure
Thick steel plates are used primarily in areas requiring heavy loads and resistance to harsh environments. They are an indispensable "framework material" for large-scale projects and heavy equipment, directly impacting the safety and stability of the project. (I) Architecture and Infrastructure: The "Skeleton" of Supertall and Long-Span Buildings
Supertall Buildings: The steel columns, beams, and core walls of skyscrapers are constructed from thick high-grade steel plates (such as Q355GJD and Q460GJE), with thicknesses ranging from 50-100mm. These plates bear the building's weight and wind loads. For example, the core wall of the 632-meter Shanghai Tower utilizes thick Q690GJD steel plates with a yield strength of 690 MPa, significantly reducing the number of steel components and improving structural stability.
Long-Span Bridges: The steel box girders and towers of sea-crossing bridges and high-speed rail bridges utilize thick bridge steel plates (such as Q345qD and Q690qE), with thicknesses ranging from 20-60mm. These plates must exhibit excellent low-temperature toughness and weldability. For example, the steel box girders of the Hong Kong-Zhuhai-Macao Bridge utilize 420,000 tons of Q690qE thick steel plate, which can withstand corrosion from marine environments and typhoon impacts.

Tunnels and Pipeline Galleries: The roofs of urban underground pipeline galleries and tunnel shield segments utilize thick, weather-resistant steel plates (such as Q355NH) with a thickness of 10-30mm, eliminating the need for frequent painting and reducing maintenance costs.

(II) Energy and Chemical Industry: High-Pressure and High-Temperature "Containers"

Thermal/Nuclear Power: The steam drums of thermal power boilers and the pressure vessels of nuclear power reactors utilize heat-resistant, high-strength thick steel plates (such as 15CrMoR and SA508Gr.3Cl.2) with a thickness of 50-200mm. These plates must operate stably and long-term in high-temperature and high-pressure environments. For example, the pressure vessels of million-kilowatt nuclear power reactors use 200mm thick SA508Gr.3Cl.2 steel plates, which must pass rigorous flaw detection and performance testing.
Petrochemical: Large storage tanks, reactors, and oil pipelines use thick container steel plates (such as Q345R and Q420R) with a thickness of 10-80mm. These plates must have good weldability and hydrogen corrosion resistance. For example, 10

The 10,000 cubic meter crude oil storage tank's cladding is constructed of thick Q345R steel plates, welded in multiple layers. The thickness of each layer gradually increases from 12mm to 30mm to withstand the pressure of the crude oil's liquid column.
(III) Heavy Machinery and Equipment: Heavy-Duty and Wear-Resistant "Core Components"

Construction Machinery: Excavator track shoes and loader buckets utilize thick, wear-resistant steel plates (such as NM450 and NM500) with a thickness of 20-50mm and a Brinell hardness of ≥450HB, capable of withstanding the friction and impact of rocks. Crane booms utilize high-strength thick steel plates (such as Q690D) with a thickness of 16-30mm, offering both strength and toughness, enabling the lifting of hundreds of tons.

Heavy Machine Tools: 10,000-ton hydraulic presses and rolling mill frames utilize thick alloy structural steel plates (such as 45# and 40Cr) with a thickness of 100-300mm. These plates require extremely high rigidity and wear resistance to ensure accurate operation.

Shipbuilding and Offshore Engineering: Large ship hulls and offshore platform legs utilize thick marine-grade steel plates (such as AH36 and DH36) with a thickness of 100-300mm. 12-50mm, requiring excellent low-temperature toughness (impact energy ≥ 34J at -20°C) and seawater corrosion resistance to withstand the erosion of the marine environment.
(IV) Other Special Applications: "Applications" of Ultimate Performance
Defense and Military Industry: Tank armor plates and missile launchers utilize high-strength alloy steel plates (such as 30CrNiMo8), 50-100mm thick, offering both ballistic resistance and toughness.
Aerospace: Aircraft engine combustion chambers and rocket fuel tanks utilize heat-resistant alloy steel plates (such as GH4169), 5-20mm thick, capable of maintaining strength at temperatures exceeding 600°C.
Mold Manufacturing: Large stamping mold templates utilize alloy tool steel plates (such as Cr12MoV), 20-80mm thick. After quenching and tempering, they can achieve a hardness of HRC 58-62, providing excellent wear resistance and dimensional stability.