Introduce
Steel is ubiquitous in modern society, serving as a foundational material for buildings, industrial tools, consumer accessories and transportation equipment. While its applications are widely recognized, the core mechanical properties that determine its performance are less understood. This document systematically elaborates on the key mechanical properties of steel, providing a technical basis for rational material selection and application.
Yield Point (σs)
When steel or its specimens are subjected to tensile loads, plastic deformation continues to occur without an increase in stress once the stress exceeds the elastic limit. This phenomenon is defined as yielding. The minimum stress value that triggers yielding is the yield point. The calculation formula is as follows:
σ₈=P₈/F₀ (MPa)
In the formula, P⁸ refers to the external force at the yield point, and F₀ refers to the cross-sectional area of the specimen.
Yield Strength (σ₀.₂)
The yield point of some metal materials is not clearly distinguishable, making direct measurement difficult. To quantify the yield characteristics of such materials, the stress that causes permanent plastic deformation equivalent to 0.2% of the original length is specified as the conditional yield strength, referred to as yield strength (σ₀.₂).
Tensile Strength (σb)
Tensile strength represents the maximum stress that a material can withstand from the start of tension until fracture. It is a core index reflecting the material’s fracture resistance. Similar mechanical performance indicators include compressive strength and flexural strength. The calculation formula is as follows:
σb=Pb/F₀ (MPa)
In the formula, Pb refers to the maximum tensile force before material fracture, and F₀ refers to the cross-sectional area of the specimen.
Elongation (δs)
Elongation is the percentage of the plastic elongation length of a material after fracture relative to the original length of the specimen. It is an important parameter for evaluating the plastic deformation ability of steel.
Yield-Strength Ratio (σs/σb)
The yield-strength ratio is the ratio of the yield point (or yield strength) of steel to its tensile strength. A higher ratio indicates higher reliability of structural components. Typical yield-strength ratio ranges are as follows:
Carbon steel: 0.6–0.65
Low-alloy structural steel: 0.65–0.75
Alloy structural steel: 0.84–0.86g-term corrosion protection.
Hardness
Hardness is the ability of a material to resist indentation by a hard object. It is one of the key performance indicators of metal materials. Generally, higher hardness corresponds to better wear resistance. Commonly used hardness testing methods include Brinell, Rockwell and Vickers hardness tests.
- Brinell Hardness (HB)
A hardened steel ball with a specified diameter (typically 10mm) is pressed into the material surface under a fixed load (typically 3000kg) and held for a set duration. After unloading, the Brinell hardness value is calculated as the ratio of the load to the indentation area. - Rockwell Hardness (HR)
Rockwell hardness testing is used as an alternative when HB exceeds 450 or the specimen size is too small for Brinell testing. The test uses a diamond cone indenter with a 120° vertex angle or a steel ball indenter with a diameter of 1.59mm or 3.18mm. Hardness values are derived from indentation depth under specified loads. Three scales are available for different material hardness levels:- HRA: Uses a 60kg load and diamond cone indenter, suitable for ultra-hard materials such as cemented carbide.
- HRB: Uses a 100kg load and 1.58mm hardened steel ball indenter, suitable for low-hardness materials such as annealed steel and cast iron.
- HRC: Uses a 150kg load and diamond cone indenter, suitable for high-hardness materials such as quenched steel.
- Vickers Hardness (HV)
- A diamond square pyramid indenter with a 136° vertex angle is pressed into the material surface under a load of up to 120kg. The Vickers hardness value is calculated as the ratio of the load to the surface area of the indentation.
Conclusion
In summary, a thorough understanding of core mechanical properties—including yield strength, tensile strength, elongation, hardness and yield-strength ratio—is the fundamental basis for the correct selection and application of steel. These indicators collectively determine whether a steel grade is suitable for a specific project, and directly affect the safety, durability and reliability of the final product.
Selecting the appropriate steel grade is critical for projects requiring high strength, toughness or wear resistance. This document aims to provide technical guidance for informed decision-making in material selection.
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