Alloy steel plate: "performance customization expert" in the industrial field

With modern industry placing increasingly stringent demands on material performance, ordinary steel plates, relying solely on carbon content to adjust their properties, are no longer able to meet the demands of complex operating conditions such as heavy loads, high temperatures, corrosion, and low temperatures. Alloy steel plates, through the precise ratio of "base steel + alloying elements" and by adjusting the content of elements such as chromium, nickel, molybdenum, and vanadium, achieve customized upgrades in strength, toughness, corrosion resistance, and heat resistance. They have become a core material supporting key sectors such as high-end equipment manufacturing, energy and chemical engineering, and aerospace, earning them the title of "performance customization expert" within the industrial system.

I. Definition and Core Characteristics: More Than a Simple "Steel + Alloy" Composite
Alloy steel plates are made from carbon steel by intentionally adding one or more alloying elements (such as chromium, nickel, molybdenum, vanadium, titanium, and niobium) and undergoing specialized smelting, rolling, and heat treatment processes. Its core value lies not in simply stacking alloying elements, but in precisely controlling the steel's microstructure (such as ferrite, austenite, martensite, and bainite) through the synergistic effects between these elements, thereby endowing the steel plate with specialized properties far exceeding those of ordinary carbon steel.

Compared to ordinary carbon steel plates, the core characteristics of alloy steel plates can be summarized as "three highs and one specialized":

High Strength: Alloying elements (such as manganese, vanadium, and niobium) refine the grain size and strengthen the matrix, resulting in yield strengths generally exceeding 420 MPa. Some high-end alloy steel plates (such as quenched and tempered alloy structural steels) even exceed 1000 MPa, enabling them to withstand greater loads.
High Toughness: Alloying elements (such as nickel and titanium) improve the steel's low-temperature toughness, maintaining excellent impact toughness (impact energy ≥ 27 J) even at extreme temperatures of -40°C or even -60°C, preventing brittle fracture.
High Stability: The addition of elements tailored to specific environments (such as chromium and molybdenum for heat resistance, copper and chromium for weather resistance, and nickel and chromium for corrosion resistance) enables the steel plates to operate stably and long-term in harsh operating conditions such as high temperatures, corrosion, and windblown sand.
Specialized: Performance can be customized to meet specific needs. For example, heat-resistant alloy steel plates are suitable for Wear-resistant alloy steel plates can withstand intense friction in high-temperature environments above 500°C, while corrosion-resistant alloy steel plates can resist attack from strong acids and alkalis.

II. Classification System: Precise Classification by Elements, Applications, and Performance
Alloy steel plates are classified across a variety of dimensions. The core principle is to "match composition design to performance requirements." Different classifications correspond to specific application scenarios, avoiding "overperformance" or "underperformance." (I) Classification by Main Alloying Elements: Elements Determine Core Performance
The impact of different alloying elements on steel plate properties varies significantly, which is the core basis for classification:

Chromium Alloy Steel Plate
Core Element: Chromium (content 0.5%-18%), often used in combination with molybdenum and vanadium.
Performance Characteristics: Chromium significantly improves corrosion resistance (forming a dense chromium oxide film) and heat resistance (inhibiting high-temperature oxidation), while also enhancing strength and hardness.
Typical Grades: 15CrMo (chromium-molybdenum steel, containing 1%-1.5% chromium and 0.25%-0.35% molybdenum), Cr12MoV (chromium-molybdenum-vanadium steel, containing 11%-12.5% ​​chromium).
Applications: 15CrMo is used for thermal power boiler pipes and high-temperature pressure vessels; Cr12MoV is used for large wear-resistant molds (such as stamping dies). Nickel Alloy Steel Plate
Core Element: Nickel (1%-36%), often added in combination with chromium and molybdenum.
Performance: Nickel stabilizes the austenite structure, significantly improving low-temperature toughness (when the nickel content is ≥3.5%, it maintains good toughness at -196°C), while also improving corrosion resistance and weldability.
Typical Grades: 304 Stainless Steel (8%-11% nickel, 18%-20% chromium), Ni9 Steel (8%-10% nickel).
Applications: 304 is used in chemical equipment and food machinery; Ni9 is used in bridges and cryogenic storage tanks in extremely cold regions. Molybdenum Alloy Steel Plate
Core Element: Molybdenum (0.15%-1.5%), often used as a secondary strengthening element in combination with chromium and manganese.
Performance: Molybdenum refines grain size and improves hardenability (enabling hardening even in the core of thick steel plates), while also enhancing high-temperature strength and hydrogen corrosion resistance.
Typical grades: 42CrMo (chromium-molybdenum steel, containing 0.15%-0.25% molybdenum), 20MoG (molybdenum steel, containing 0.45%-0.65% molybdenum).
Applications: 42CrMo is used for high-pressure bolts and heavy machinery shafts; 20MoG is used for high-pressure boiler pipes. Vanadium Alloy Steel Plate
Core Element: Vanadium (content 0.02%-0.2%), often used in combination with manganese and niobium.
Performance: Vanadium refines grain size by forming carbides (VC), significantly improving strength and toughness without compromising weldability.
Typical grades: Q690D (low-alloy high-strength steel, containing 0.02%-0.06% vanadium), 40CrV (chrome-vanadium steel, containing 0.10%-0.20% vanadium).
Applications: Q690D is used in wind turbine tower flanges and construction robot arms; 40CrV is used in high-precision machine tool spindles. Multi-element Composite Alloy Steel Plate
Features: The simultaneous addition of two or more alloying elements achieves "superior performance," such as chromium-nickel-molybdenum steel and chromium-manganese-vanadium-titanium steel.
Typical grades: 316L stainless steel (containing 16%-18% chromium, 10%-14% nickel, and 2%-3% molybdenum), 12Cr1MoVG (chromium-molybdenum-vanadium steel, containing 1%-1.5% chromium, 0.25%-0.35% molybdenum, and 0.15%-0.30% vanadium).
Applications: 316L is used in marine engineering and equipment used in highly acidic environments; 12Cr1MoVG is used in superheaters for supercritical thermal power boilers. (II) Classification by Application: Focusing on Industry-Specific Needs
Alloy Structural Steel Plate: Used in the manufacture of mechanical structural components subject to heavy loads and impact (such as shafts, gears, and bolts), requiring a balance of strength and toughness. Typical grades include 40Cr and 42CrMo, and are widely used in mining machinery and automotive transmissions.
Alloy Tool Steel Plate: Used in the manufacture of cutting tools, molds, and measuring tools, requiring high hardness and high wear resistance. Typical grades include Cr12 and Cr12MoV, suitable for stamping dies and cold working dies.
Heat-Resistant Alloy Steel Plate: Used in high-temperature environments (above 300°C), requiring high-temperature oxidation resistance and stable high-temperature strength. Typical grades include 15CrMo and 12Cr1MoVG, suitable for boilers and cracking furnaces.
Corrosion-Resistant Alloy Steel Plate: Used in corrosive environments (acids, alkalis, seawater, and industrial waste gases), requiring resistance to uniform and pitting corrosion. Typical grades include 304, 316L, and Q355NH (weathering steel), suitable for chemical equipment and offshore platforms.
Low-Temperature Alloy Steel Plate: Used in - Low-temperature environments below 40°C require excellent low-temperature toughness and no risk of brittle fracture. Typical grades include Ni9 and Q345E, suitable for LNG storage tanks and polar vessels.
(III) Classification by Heat Treatment Status: Process Determines Final Performance
The properties of alloy steel plates need to be "activated" through heat treatment. Different heat treatment processes correspond to different performance emphases:

Normalized: Heated to austenitizing temperature followed by air cooling, resulting in uniform microstructure and excellent toughness, suitable for structural parts operating in medium and low temperature conditions (e.g., 15CrMo normalized for low-temperature pressure vessels);
Quenched and Tempered (Quenched + High-Tempering): Quenched followed by high-temperature tempering to form tempered bainite, combining high strength and toughness, suitable for heavy-duty parts (e.g., 42CrMo quenched and tempered for high-pressure bolts);
Annealed: Heated followed by slow cooling to achieve low hardness and good plasticity, making it easier to machine (e.g., annealed Cr12MoV for rough mold machining);
Quenched + Low-Tempering: Quenched followed by low-temperature tempering to form tempered martensite, offering high hardness and excellent wear resistance, suitable for tools and molds (e.g., Cr12 quenched + low-temperature tempered for cold stamping dies).
III. Core Performance: Why Can It Adapt to Complex Operating Conditions? The "customizable performance" of alloy steel plates stems from the synergistic effect of alloying elements and processing. Its core performance indicators precisely match the stringent requirements of industrial applications. This can be specifically addressed from the following four aspects:
(I) The Art of Balancing Strength and Toughness
Ordinary carbon steel plates often face the dilemma of "increasing strength while decreasing toughness." However, alloy steel plates achieve this balance through elemental manipulation:

Strength-Boosting Mechanism: Elements such as manganese, vanadium, and niobium enhance strength through a triple effect: "solid solution strengthening" (incorporating into the iron matrix to enhance lattice strength), "grain refinement" (refining grains and increasing grain boundary area), and "precipitation strengthening" (forming carbides to hinder dislocation motion). For example, Q690D, through the addition of vanadium (0.04%) and niobium (0.02%), achieves a yield strength of 690 MPa, far exceeding the 345 MPa of standard Q345 steel.
Toughness-Enhancing Mechanism: Elements such as nickel and titanium lower the ductile-brittle transition temperature (DBTT) of steel, maintaining toughness even at low temperatures. For example, Ni9 steel (containing 9% nickel) maintains an impact energy of ≥50J at -196°C, making it suitable for liquid nitrogen storage tanks. Chromium and molybdenum improve the microstructure of the heat-affected zone (HAZ) of the weld, preventing a loss of toughness after welding.
This combination of high strength and high toughness enables the alloy steel plate to withstand the combined effects of heavy loads and impacts. For example, when lifting a heavy object, an engineering robot arm will neither bend due to insufficient strength nor break due to impacts (such as the swaying of the load). (II) Environmental Protection

For various corrosive and high-temperature environments, alloy steel plates are designed with a "specialized protection system" through elemental design:

Corrosion Protection: Chromium (≥12%) forms a dense Cr₂O₃ oxide film on the surface, preventing the intrusion of corrosive media (e.g., 304 stainless steel); Molybdenum enhances resistance to pitting corrosion (against chloride ions, such as 316L in seawater); Copper (0.2%-0.5%) improves atmospheric corrosion resistance (e.g., Q355NH weathering steel, used in outdoor bridges);
Heat Protection: Chromium (≥5%) inhibits high-temperature oxidation (e.g., 15CrMo resists oxidation below 550°C); Molybdenum and Tungsten enhance high-temperature strength (e.g., 12Cr1MoVG maintains sufficient strength at 580°C, used in boiler superheaters);
Hydrogen Corrosion Resistance: Molybdenum combines with carbon in steel to form stable carbides, preventing the reaction of hydrogen and carbon to form methane (which causes "hydrogen embrittlement"), such as 20CrMoH used in hydrogenation reactors. (3) "Suitability" for Processing and Welding
In industrial applications, alloy steel plates undergo cutting, drilling, welding, bending, and other processes. Their suitability for processing directly impacts production efficiency:

Machinability: By adjusting the carbon content and alloying elements (such as adding sulfur to improve machinability), alloy steel plates can be made easier to process while maintaining strength. For example, 40Cr steel, with an annealed hardness of ≤229HB, can be turned and milled using high-speed steel tools.
Weldability: Low-alloy, high-strength alloy steel plates (such as Q690D) can be welded using conventional arc welding and submerged arc welding by controlling the carbon equivalent (Ceq ≤ 0.5%). Proper preheating (80-150°C) is sufficient to avoid weld cracks. Stainless steel alloy plates (such as 304) can be welded using specialized stainless steel welding wire (such as ER308L) to achieve high-quality welds.
Formability: Some alloy steel plates (such as Q345E) have an elongation of ≥21%, allowing for cold and hot bending. Bend radius ≥3 times the plate thickness is crack-free, making them suitable for the manufacture of curved structural parts (such as pressure vessel shells). (IV) Uniformity and Stability of Dimensions and Structure
Thick alloy steel plates (thickness > 20mm) are prone to "thickness effect" (significant differences in core and surface properties). Uniformity can be achieved through process control:

Compositional uniformity: Utilizing the "converter + LF refining + RH vacuum degassing" process, alloy element distribution deviation is maintained within ≤5%, preventing local property fluctuations.
Microstructural uniformity: Utilizing the "controlled rolling and controlled cooling (TMCP)" process, rolling temperature and cooling rate are controlled to maintain grain size variation through the thickness of the plate within ≤2 grades (e.g., for a 60mm thick Q690D plate, the core-surface hardness difference is ≤15HB).
Dimensional accuracy: Utilizing wide and heavy plate rolling mills and CNC shearing equipment, thickness deviation is maintained within ≤±0.3mm, and flatness is maintained within ≤2mm/m, meeting the assembly requirements of high-precision equipment (e.g., 40Cr steel plates for machine tool spindles). IV. Production Process: Precision Control from Molten Steel to Custom Steel Plate
The production of alloy steel plate requires precise control throughout the entire process: composition design - smelting - rolling - heat treatment - testing. Each step directly impacts the final performance, and the technical threshold is much higher than that of ordinary steel plate. (I) Composition Design: "Source Customization" of Performance
Based on target performance requirements, the optimal alloying element ratio is determined through thermodynamic calculations and experimental verification:

Example 1: 15CrMo steel plate for high-temperature pressure vessels requires a balance between heat resistance and weldability. The designed composition is: C 0.12%-0.18%, Cr 0.80%-1.10%, and Mo 0.40%-0.55%. This ensures high-temperature strength at 550°C while keeping the carbon equivalent ≤0.45% (facilitating welding).

Example 2: Ni9 steel plate for bridges in extremely cold regions is designed to have a nickel content of 8%-10% to improve low-temperature toughness, while also keeping sulfur ≤0.015% and phosphorus ≤0.020% (to avoid low-temperature embrittlement). (II) Smelting: The "Core Guarantee" of Purity
Raw Material Selection: Use low-sulfur scrap steel (sulfur ≤ 0.01%) and high-quality molten iron (phosphorus ≤ 0.015%) to avoid the introduction of harmful elements.
Smelting Process:
Converter/Electric Furnace Smelting: Initial decarburization and dephosphorization, controlling the carbon content deviation to ≤ 0.02%;
LF Refining: Add alloying elements (such as ferrochrome, nickel plate, and ferromolybdenum) to precisely adjust the composition to the target range, while simultaneously desulfurizing to ≤ 0.005%;
RH Vacuum Degassing: Remove hydrogen (≤ 2 ppm) and nitrogen (≤ 50 ppm) under vacuum to prevent hydrogen-induced cracking and nitride precipitation;
Casting: Use continuous casting (standard specifications) or die casting (high-end specifications, such as nuclear power steel), controlling the casting speed (0.8-1.2 m/min) to avoid internal pores and interlayers. (3) Rolling: Precision Shaping of Microstructure and Dimensions
Heating: The slab is heated to 1150-1250°C (austenitizing temperature) and held for 2-4 hours to ensure full dissolution of alloying elements.
Controlled Rolling: This process is divided into "rough rolling" and "finishing rolling":
Rough rolling: A high reduction (single-pass reduction ≥ 15%) is used to break up coarse grains.
Finishing rolling: Rolling is performed below the recrystallization temperature (e.g., 800-900°C) to refine the grains through strain-induced precipitation. The final rolling temperature deviation is controlled to ≤20°C.
Controlled Cooling: After rolling, laminar cooling is used at a controlled cooling rate (5-20°C/s) to form the target microstructure (e.g., bainite, sorbite) and prevent grain growth.