Hot Rolled Steel: Properties and Key Applications

Hot rolled steel is a widely used category of steel that is produced through the hot rolling process, where steel is heated above its recrystallization temperature and then deformed into desired shapes. This process results in a steel that is malleable and can be formed into various shapes and sizes. Hot rolled steel is typically classified as low-carbon mild steel, which contains a low percentage of carbon (usually less than 0.25%), making it ductile and easy to work with. The primary alloying elements in hot rolled steel include iron (Fe), carbon (C), and small amounts of manganese (Mn), phosphorus (P), and sulfur (S).

Comprehensive Overview

The characteristics of hot rolled steel are defined by its mechanical properties, which include high ductility, good weldability, and moderate strength. The hot rolling process also imparts a rough surface finish, which can be beneficial for certain applications but may require further processing for aesthetic purposes.

Advantages of Hot Rolled Steel:
- Cost-Effective: Generally less expensive than cold rolled steel due to lower processing costs.
- Versatile: Suitable for a wide range of applications, including structural components, automotive parts, and machinery.
- Good Formability: Can be easily shaped and formed into complex geometries.

Limitations of Hot Rolled Steel:
- Surface Finish: The rough surface may not be suitable for all applications without additional finishing.
- Dimensional Tolerances: Hot rolled steel typically has looser tolerances compared to cold rolled steel, which may affect precision applications.
- Lower Strength: Compared to cold rolled steel, hot rolled steel generally has lower tensile strength.

Historically, hot rolled steel has played a significant role in the development of modern infrastructure and manufacturing, being a foundational material in construction and heavy machinery.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	G10100	USA	Closest equivalent to A36
AISI/SAE	A36	USA	Common structural steel grade
ASTM	A992	USA	Used for wide-flange beams
EN	S235JR	Europe	Similar properties, often used in construction
DIN	S235	Germany	Comparable to A36, with minor compositional differences
JIS	SS400	Japan	Equivalent to A36, widely used in Japan
GB	Q235	China	Similar to A36, commonly used in Chinese construction

The differences between these grades, while often minor, can affect performance in specific applications. For example, A992 is designed for better performance in structural applications due to its higher yield strength compared to A36.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.05 - 0.25
Mn (Manganese)	0.30 - 0.60
P (Phosphorus)	≤ 0.04
S (Sulfur)	≤ 0.05
Fe (Iron)	Balance

The primary role of carbon in hot rolled steel is to enhance strength and hardness. Manganese contributes to improved toughness and wear resistance, while phosphorus and sulfur are considered impurities that can adversely affect ductility and weldability.

Mechanical Properties

Property	Condition/Temper	Test Temperature	Typical Value/Range (Metric)	Typical Value/Range (Imperial)	Reference Standard for Test Method
Tensile Strength	Hot Rolled	Room Temp	400 - 550 MPa	58 - 80 ksi	ASTM E8
Yield Strength (0.2% offset)	Hot Rolled	Room Temp	250 - 350 MPa	36 - 51 ksi	ASTM E8
Elongation	Hot Rolled	Room Temp	20 - 25%	20 - 25%	ASTM E8
Hardness (Brinell)	Hot Rolled	Room Temp	120 - 180 HB	120 - 180 HB	ASTM E10
Impact Strength	Hot Rolled	-20 °C	27 - 34 J	20 - 25 ft-lbf	ASTM E23

The combination of these mechanical properties makes hot rolled steel suitable for applications requiring good ductility and moderate strength, such as structural beams, frames, and automotive components.

Physical Properties

Property	Condition/Temperature	Value (Metric)	Value (Imperial)
Density	Room Temp	7850 kg/m³	490 lb/ft³
Melting Point	-	1425 - 1540 °C	2600 - 2800 °F
Thermal Conductivity	Room Temp	50 W/m·K	29 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	Room Temp	0.49 kJ/kg·K	0.12 BTU/lb·°F
Electrical Resistivity	Room Temp	0.0000017 Ω·m	0.0000017 Ω·ft

The density of hot rolled steel contributes to its overall strength and durability, while its thermal conductivity is important for applications involving heat transfer. The specific heat capacity indicates how much energy is required to raise the temperature of the material, which is crucial in processes involving thermal cycling.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Atmospheric	-	-	Fair	Susceptible to rust
Chlorides	Low	Ambient	Poor	Risk of pitting corrosion
Acids	Dilute	Ambient	Poor	Not recommended
Alkalis	Dilute	Ambient	Fair	Moderate resistance

Hot rolled steel exhibits limited corrosion resistance, making it susceptible to rust in moist environments. It is particularly vulnerable to pitting corrosion in the presence of chlorides. Compared to stainless steels, such as AISI 304, which offer excellent corrosion resistance, hot rolled steel is less suitable for applications in corrosive environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	400 °C	752 °F	Suitable for moderate temperature applications
Max Intermittent Service Temp	500 °C	932 °F	Short-term exposure without degradation
Scaling Temperature	600 °C	1112 °F	Begins to oxidize significantly

At elevated temperatures, hot rolled steel can experience oxidation, which may lead to scaling and reduced mechanical properties. It is important to consider these factors when designing components that will operate at high temperatures.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
MIG	ER70S-6	Argon/CO2	Good for general applications
TIG	ER70S-2	Argon	Provides high-quality welds
Stick	E7018	-	Suitable for thicker sections

Hot rolled steel is generally considered to have good weldability, making it suitable for various welding processes. Preheating may be required for thicker sections to avoid cracking. Post-weld heat treatment can improve the properties of the weld zone.

Machinability

Machining Parameter	Hot Rolled Steel	AISI 1212	Notes/Tips
Relative Machinability Index	60	100	Hot rolled steel is less machinable than AISI 1212
Typical Cutting Speed	30 m/min	50 m/min	Adjust speeds based on tooling

Hot rolled steel has moderate machinability, and while it can be machined, it may require more robust tooling and slower cutting speeds compared to more machinable grades like AISI 1212.

Formability

Hot rolled steel exhibits excellent formability, making it suitable for various forming processes, including bending, stamping, and drawing. It can be cold worked to achieve desired shapes, but care must be taken to avoid excessive work hardening.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	600 - 700 °C / 1112 - 1292 °F	1 - 2 hours	Air or water	Softening, improving ductility
Normalizing	800 - 900 °C / 1472 - 1652 °F	1 - 2 hours	Air	Refining grain structure
Quenching	800 - 900 °C / 1472 - 1652 °F	30 minutes	Oil or water	Hardening, increasing strength

Heat treatment processes such as annealing and normalizing can significantly alter the microstructure of hot rolled steel, enhancing its mechanical properties. Annealing softens the steel, while normalizing refines the grain structure for improved strength and toughness.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Construction	Structural beams	High strength, good weldability	Essential for load-bearing structures
Automotive	Chassis components	Ductility, formability	Allows for complex shapes and strength
Manufacturing	Machinery frames	Cost-effectiveness, moderate strength	Economical for large-scale production

Other applications include:
- Pipes and Tubes: Used in plumbing and structural applications.
- Automotive Parts: Such as brackets and supports.
- Heavy Equipment: Components in construction machinery.

Hot rolled steel is chosen for these applications due to its balance of strength, ductility, and cost-effectiveness, making it a versatile choice in various industries.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	Hot Rolled Steel	A36 Steel	S235 Steel	Brief Pro/Con or Trade-off Note
Key Mechanical Property	Moderate Strength	Moderate Strength	Moderate Strength	Similar properties across grades
Key Corrosion Aspect	Fair	Fair	Fair	All grades have similar corrosion resistance
Weldability	Good	Good	Good	Suitable for various welding processes
Machinability	Moderate	Moderate	Moderate	All grades require robust tooling
Formability	Excellent	Excellent	Excellent	All grades are suitable for forming
Approx. Relative Cost	Low	Low	Low	Cost-effective options available
Typical Availability	High	High	High	Widely available in various forms

When selecting hot rolled steel, considerations include cost-effectiveness, availability, and the specific mechanical properties required for the application. While hot rolled steel is generally less expensive than cold rolled alternatives, its surface finish and dimensional tolerances may necessitate further processing for certain applications. Additionally, its magnetic properties make it suitable for applications where magnetic interference is a concern.

In summary, hot rolled steel is a versatile and widely used material in various industries, offering a balance of strength, ductility, and cost-effectiveness. Understanding its properties and limitations is crucial for selecting the right grade for specific applications.