In the fields of mechanical manufacturing, construction engineering, the automotive industry, and hardware processing, carbon steel bar, due to its economical cost, flexible performance adjustment, and wide processing adaptability, has become one of the most basic and widely used metal profiles. From machine tool spindles and automotive crankshafts to construction rebar and hardware fasteners, carbon steel bar, with its core characteristics of "easy processing, controllable strength, and abundant resources," supports the full range of manufacturing needs, from precision components to heavy structures, making it an indispensable "universal basic profile" in the industrial system.

I. Basic Understanding of Carbon Steel Bar: Definition, Core Components, and Production Process
1. Definition and Morphological Characteristics
Carbon steel bar, also known as "carbon structural steel bar," is a solid bar profile composed of iron (Fe) as the matrix, carbon (C) as the primary alloying element, and small amounts of impurities such as silicon (Si), manganese (Mn), sulfur (S), and phosphorus (P). Its cross-sectional shapes vary, with common ones including round (round steel bar), square (square steel bar), hexagonal (hexagonal steel bar), and flat (flat steel bar). Diameters and side lengths typically range from 5mm to 500mm, and lengths can be customized (6m-12m fixed or custom lengths) to suit different processing scenarios.
2. Correlation between Core Composition and Performance
The core performance of carbon steel bar is determined by its carbon content. Minor differences in impurity and alloying elements also affect its workability and stability. The specific correlation is as follows:

Carbon (C): A key element that determines strength and hardness—higher carbon content increases strength and hardness, but lowers ductility and toughness. Low-carbon steel bar (C ≤ 0.25%) is easy to weld and cold work; medium-carbon steel bar (0.25%-0.60%) requires quenching and tempering to strengthen; high-carbon steel bar (C > 0.60%) requires quenching and tempering to improve wear resistance. Silicon (Si, 0.17%-0.37%): A beneficial element that improves steel's strength and oxidation resistance without significantly reducing its ductility. It is a common strengthening element for carbon steel bars.
Manganese (Mn, 0.30%-0.80%): A beneficial element that improves steel's toughness and hardenability, offsetting the harmful effects of sulfur (preventing hot brittleness). The manganese content in medium- and high-carbon steel bars can be appropriately increased (e.g., 16Mn steel) to enhance strength.
Sulfur (S≤0.050%) and Phosphorus (P≤0.045%): Harmful impurities. Sulfur can cause steel to become "hot brittle" (prone to cracking during high-temperature processing), while phosphorus can cause steel to become "cold brittle" (prone to cracking at low temperatures). Therefore, the standard strictly limits the content of both.
3. Core Production Process
The production of carbon steel bars is centered around continuous rolling, a mature and scalable process that includes three key steps:

Raw Material Preparation: Molten iron from blast furnaces is used as the raw material. Steelmaking is carried out in converters or electric furnaces, where impurities are removed before casting into "billets" (round or square, with a cross-section of 150mm-300mm). The billets undergo inspection to ensure that their composition meets specified standards.
Rolling: The billets are heated to 1100°C-1250°C (austenitizing temperature) and progressively rolled through continuous rolling mills (roughing, intermediate, and finishing) into bars of the desired cross-section and size. The hot rolling process directly produces hot-rolled carbon steel bars (with a scaled surface). For higher precision, hot rolling can be followed by cold rolling or cold drawing to produce cold-rolled carbon steel bars (with a smooth surface and high dimensional accuracy). Finishing: Hot-rolled bars undergo cooling, straightening (to ensure straightness), cutting to length (cutting as needed), and surface cleaning (to remove oxide scale). Cold-rolled/cold-drawn bars also undergo annealing (to eliminate internal stresses) and polishing (to improve surface quality). Finally, they undergo inspection, packaging, and shipment.

II. Core Classification of Carbon Steel Bars: By Carbon Content, Process, and Cross-Section
Carbon steel bar classification is based on three key dimensions: carbon content, production process, and cross-sectional shape. Different categories differ significantly in performance, cost, and application, directly determining their downstream application scenarios.

1. Classification by Carbon Content: Low Carbon Steel Bar, Medium Carbon Steel Bar, and High Carbon Steel Bar

This is the most core classification of carbon steel bars. Carbon content directly determines their mechanical properties and processing characteristics, and is the primary factor in selection. (1) Low carbon steel bar (C≤0.25%)
Typical grades: Q235 (most commonly used, C≈0.14%-0.22%), 10# (C≈0.07%-0.14%), 20# (C≈0.17%-0.24%);
Performance characteristics: good plasticity (elongation ≥25%), excellent weldability (weldable without preheating), low strength (tensile strength 235-375MPa), easy cold processing (can be bent and stamped);
Core advantages: lowest cost, most convenient processing, suitable for scenes requiring welding or forming;
Typical applications: construction steel bars (Q235 threaded steel bars), general mechanical parts (such as flanges, brackets), welded structural parts (such as steel structure columns), seamless pipe blanks for pipelines (20# steel bars). (2) Medium carbon steel bar (0.25%＜C≤0.60%)
Typical grades: 35# (C≈0.32%-0.40%), 45# (C≈0.42%-0.50%, most commonly used), 50# (C≈0.47%-0.55%);
Performance characteristics: Balanced strength and toughness (tensile strength 450-650MPa), can be improved through "quenching and tempering treatment (quenching + high temperature tempering)" to improve hardness and wear resistance, medium weldability (needs preheating to prevent cracking), general cold workability (needs annealing before forming);
Core advantages: adjustable performance after heat treatment, suitable for medium stress scenarios;
Typical applications: mechanical transmission parts (such as gears, shafts - 45# quenched and tempered steel bar), automobile crankshafts, bolts and nuts (35# steel bar), mold blanks (50# steel bar). (3) High carbon steel bar (C>0.60%)
Typical grades: 60# (C≈0.57%-0.65%), 65# (C≈0.62%-0.70%), T8 (tool steel, C≈0.75%-0.84%), T10 (C≈0.95%-1.04%);
Performance characteristics: high hardness (HRC 50-60 after quenching), strong wear resistance, high strength (tensile strength ≥700MPa), poor plasticity (elongation ≤15%), poor weldability (almost not used for welding);
Core advantages: outstanding wear resistance, suitable for scenes with friction or impact;
Typical applications: hardware tools (such as knives, drill bits - T8/T10 steel bars), springs (65# steel bars), bearing balls, agricultural machinery parts (such as plowshares), steel wire billets for wire ropes (60# steel bars). 2. Classification by production process: hot-rolled carbon steel bar, cold-rolled/cold-drawn carbon steel bar
Process differences determine the surface quality, dimensional accuracy and application scenarios of carbon steel bars. The two cover areas with different precision requirements.
(1) Hot-rolled carbon steel bar
Process characteristics: high-temperature rolling, surface oxide scale (black-brown), low dimensional accuracy (diameter tolerance ±0.5mm), general straightness (bending per meter ≤3mm);
Performance characteristics: coarse grains, good plasticity, and can be processed without annealing;
Cost and efficiency: short production cycle, low cost (30%-50% lower than cold rolling), large output (accounting for more than 80% of the total carbon steel bar output);
Typical applications: construction steel bars, large mechanical structural parts, seamless pipe billets, low-precision parts (such as brackets, counterweights). (2) Cold-rolled/cold-drawn carbon steel bars
Process characteristics: Using hot-rolled bars as base material, cold-rolled (room temperature rolling) or cold-drawn (drawing forming), smooth surface (silver-gray, Ra 0.8-3.2μm), high dimensional accuracy (diameter tolerance ±0.05mm), good straightness (bending per meter ≤0.5mm);
Performance characteristics: grain refinement, slightly higher strength than hot-rolled bars, internal stress exists after cold working, and annealing is required to restore plasticity;
Cost and efficiency: long production process, high cost, suitable for high-precision requirements;
Typical applications: precision machinery parts (such as motor shafts, instrument shafts), bolts and nuts (high-precision thread processing), medical device parts (such as surgical instrument connecting rods), automotive precision parts (such as gearbox gear shafts). 3. Classification by Cross-sectional Shape: Adapting to Different Processing Requirements
Cross-sectional shape directly determines the processing method and final form of carbon steel rods. Common types and applications are as follows:

Round carbon steel rod (round steel rod): The most widely used, suitable for machining shafts, bolts, bearings, and other components. Complex parts can be made through turning and milling.

Square carbon steel rod (square steel rod): Used to manufacture square structural parts (such as steel structure supports and mold templates), suitable for welding or cutting.

Hexagonal carbon steel rod (hexagonal steel rod): Can be used as raw material for hexagonal bolts and nuts without additional processing, reducing turning steps and improving efficiency.

Flat carbon steel rod (flat steel rod): Used to manufacture springs, tool edges, and embedded components in construction, suitable for bending or stamping.

Threaded carbon steel rod (rebar): Features spiral ribs on the surface to enhance bond strength with concrete. It is specifically used in reinforced concrete structures (such as beams and columns).

III. Core Advantages and Limitations of Carbon Steel Rod
The value of carbon steel rod stems from its "fundamental attributes"—low cost, abundant resources, and flexible processing. However, it is also limited by its performance boundaries, and its scope of application requires objective evaluation. 1. Core Advantages: The Key to Becoming an "Industrial Basic Profile"
Outstanding Cost-Effectiveness: Compared to stainless steel bars (3-8 times the price) and copper bars (5-10 times the price), carbon steel bar raw materials (carbon steel) are readily available, the production process is mature, and the unit price is only 1/5-1/10 of that of specialty metals, making it suitable for large-scale, low-cost demand scenarios (such as construction rebar and general machinery parts).

Adequate Resources and Production Capacity: Global carbon steel production accounts for over 90% of total steel production, and carbon steel bar production capacity is substantial (a single plant can produce thousands of tons per day). With short lead times (1-3 days for standard specifications), it can quickly respond to the needs of large-scale projects (such as infrastructure projects).

Strong Performance Adjustability: By adjusting the carbon content (low carbon, medium carbon, high carbon) and the heat treatment process (tempering, quenching), it is possible to achieve "high plasticity, medium strength, and high hardness." Comprehensive performance coverage, suitable for applications ranging from welded structural parts to wear-resistant tools.

Wide Processability: It can be formed using nearly any metalworking process, including turning, milling, drilling, welding, bending, stamping, and heat treatment. It requires no specialized equipment and can be processed by small and medium-sized fabricators, lowering the barrier to entry for downstream manufacturing.

High Recyclability: The recycling rate for carbon steel bars exceeds 95%, allowing scrap steel bars to be remelted into new bars without loss of performance. This aligns with the "dual carbon" trend and offers low long-term operating costs. 2. Major Limitations: Application Shortcomings to Avoid
Poor Corrosion Resistance: Carbon steel bars are susceptible to electrochemical corrosion (rust) in humid, acidic, alkaline, or salty environments. Without anti-corrosion treatment (such as painting or galvanizing), their service life is only 1-3 years (e.g., untreated outdoor steel bars), making them unsuitable for use in corrosive environments.
Limited High-Temperature Stability: When exposed to temperatures above 500°C for extended periods, the strength of carbon steel bars decreases significantly (e.g., the tensile strength of 45# steel drops below 200 MPa at 600°C), making them unsuitable for use in ultra-high-temperature applications (e.g., boiler furnaces and aircraft engine components).
Inadequate Low-Temperature Toughness: High-carbon steel bars are prone to brittle cracking (cold brittleness) in low-temperature environments (below -20°C). Even medium-carbon steel bars require the addition of elements such as nickel and chromium (e.g., Q345ND low-temperature steel) for use in extremely cold regions.
Poor Suitability for High-Strength Applications: Tensile strength is generally below 800 MPa, making them unsuitable for use in alloy structural steels (e.g., 40Cr) is used in high-pressure, high-load scenarios (such as large motor shafts and aircraft landing gear).
IV. Typical Applications of Carbon Steel Rods
Carbon steel rods are used throughout the core sectors of the national economy. Different types of carbon steel rods precisely match the basic manufacturing needs of various industries, forming a clear "performance-to-application" match.
1. Construction and Infrastructure: Core Raw Materials for Load-Bearing Structures
Reinforced Concrete Structures: Q235 and HRB400 (medium-carbon low-alloy) threaded steel rods serve as the main reinforcement for building beams, columns, and floor slabs. The surface threads enhance the bond with concrete, supporting the building's weight.
Steel Structure Engineering: Q235 round and square steel rods are used for columns and beams in steel structure workshops. 20# flat steel rods are used for connecting plates and supports in steel structures, forming stable frames through welding.
Infrastructure Engineering: Q235 carbon steel rods are used in embedded bridge components and road guardrail posts. 45# steel rods are used in the drive shafts of large tower cranes, supporting the heavy loads of infrastructure projects. 2. Mechanical Manufacturing: The Main Force in Transmission and Structural Components
Shaft Parts: 45# quenched and tempered round steel bar is the core raw material for motor shafts, machine tool spindles, and reducer shafts. After turning and grinding, it can achieve an accuracy of IT6, meeting transmission requirements.
Gears and Cams: 35# and 45# steel bars are quenched and tempered and surface hardened (such as high-frequency hardening) to produce transmission components such as gears and cams, balancing strength and wear resistance.
General Parts: Q235 steel bar is used to make low-precision parts such as flanges, brackets, and bearing seats. 20# steel bar is used as seamless steel tube billets to be processed into oil pipes for mechanical hydraulic systems. High-carbon steel bar (65#) is used to make springs and clutch plates. 3. Automotive and Transportation: Power and Structural Support
Powertrain: 45# and 50# steel bars are used in automotive crankshafts and connecting rods. After quenching and tempering, they withstand the reciprocating impact of the engine. 60# steel bars are used in automotive shock absorber springs, providing cushioning.
Chassis and Body: Q235 square and flat steel bars are used in automotive chassis frames and body anti-collision beams, creating rigid structures through welding. Cold-drawn low-carbon steel bars are used in automotive bolts and nuts, ensuring secure connections.
Rail Transit: Q345 (medium-carbon, low-alloy) steel bars are used in train wheel blanks and rail connector plates, enduring long-term heavy loads and friction. 4. Hardware and Tools: A Wear-Resistant and Practical Choice
Hand Tools: T8 and T10 high-carbon steel bars are made into kitchen knives, scissors, and wrenches. After quenching, they achieve high hardness and strong wear resistance, resulting in a sharp and durable edge.
Hardware Accessories: Q235 and 20# steel bars are made into hinges, hinges, and door handles, formed by bending and stamping. Cold-drawn hexagonal steel bars are directly made into hexagonal bolts and nuts, eliminating the need for additional hexagonal surface processing.
Daily Hardware: Low-carbon steel bars are made into nails, wire, and wire mesh. High-carbon steel bars (65#) are made into spring mattress springs and tension springs for rolling shutters. 5. Energy and Chemical Industry: Infrastructure Security
Energy Equipment: 20# and 35# steel bars are used to make supports for power plant boilers and tube sheets for heat exchangers, enduring medium temperatures and low pressures. Q345 steel bars are used to make supports for oil and gas production platforms, supporting the weight of the equipment.
Chemical Equipment: Low-carbon steel bars (20#) are used to make supports for chemical storage tanks and flanges for pipelines. Anti-corrosion treatment (such as epoxy paint) protects against low-level corrosion. Medium-carbon steel bars are used to make impellers for chemical pumps, suitable for medium-pressure delivery.
V. Carbon Steel Bar Selection and Maintenance: Accurately Matching Requirements to Extend Lifespan
The selection of carbon steel bars directly determines product performance and cost, while maintenance impacts safety and lifespan. A plan should be developed based on the application scenario. 1. Core Selection Principles: Determine Suitable Carbon Steel Rods in Four Steps
Step 1: Determine Carbon Content Based on Load and Operating Conditions
Low Loads and Welding Requirements (e.g., Brackets, Steel Structures): Select Low-Carbon Steel Rods (Q235, 20#);
Medium Loads and Heat Treatment Requirements (e.g., Shafts, Gears): Select Carbon Steel Rods (45#, 35#);
High Wear Resistance and Low Plasticity Requirements (e.g., Tools, Springs): Select High-Carbon Steel Rods (T8, 65#);
Corrosive Environments: Avoid Direct Use of Carbon Steel Rods. If Use is Necessary, Select Low-Carbon Steel Rods and Provide Strict Corrosion Protection (e.g., Galvanizing and Painting).
Step 2: Determine the Carbon Steel Rods Based on Accuracy and Specifications

Determine the process based on surface requirements.
Low precision and low cost (e.g., construction rebar and brackets): Choose hot-rolled carbon steel bar.
High precision and high surface quality (e.g., precision shafts and bolts): Choose cold-rolled/cold-drawn carbon steel bar.
For subsequent turning or milling: Choose hot-rolled bar (machining allowance allows for scale removal).
For direct use without machining (e.g., hexagonal bolts): Choose cold-drawn bar (higher dimensional accuracy).
Step 3: Determine the cross-sectional shape based on the processing method.
Shafts and round parts: Choose round steel bar.
Square structural parts and formwork: Choose square steel bar.
Hexagonal bolts and nuts: Choose hexagonal steel bar (reduces machining time).
Springs and flat parts: Choose flat steel bar. Step 4: Confirm Quality According to Standards
Common domestic standards: GB/T 702 (Dimensions, Shape, Weight, and Tolerances of Hot-rolled Carbon Steel Bars), GB/T 699 (High-quality Carbon Structural Steel), and GB/T 13793 (Cold-drawn Round, Square, and Hexagonal Steel).
Quality Verification: Suppliers are required to provide a composition report (carbon content and sulfur and phosphorus content meet standards), a mechanical property report (tensile strength, elongation), and a visual inspection (to ensure there are no cracks, bends, or excessive oxide scale). 2. Daily Maintenance Key Points: Corrosion Prevention and Performance Maintenance
Anti-corrosion Treatment (Core):
Outdoor or humid environments: Apply anti-corrosion paint (such as epoxy zinc-rich paint or alkyd paint) with a thickness of ≥80μm, and repaint every 2-3 years.
Moderately corrosive environments: Hot-dip galvanizing (zinc layer thickness ≥85μm) for a service life of 10-15 years.
Severely corrosive environments: Lined with plastic or coated with an anti-corrosion coating (such as polytetrafluoroethylene), or use stainless steel bars.
Storage Protection: Store in a dry, ventilated indoor warehouse, avoiding open-air storage to prevent rainwater and rust. Hot-rolled bars can be stored bare, but must be elevated to protect against moisture. Cold-rolled/cold-drawn bars should be wrapped in plastic film to prevent surface scratches.
In-Process Protection: When welding low-carbon steel bars, use matching welding rods (such as J422) to avoid cracking due to the use of high-carbon steel rods. When heat treating medium-carbon steel bars, strictly control the heating temperature and cooling rate (e.g., 45# quenching and tempering requires a 30-minute coolant). (840°C quenching, 600°C tempering) to prevent deformation or cracking.

Daily Inspection: Regularly inspect outdoor carbon steel rods for corrosion, and promptly repaint if paint peeling is detected. Carbon steel rods used in mechanical parts (such as shafts and gears) should be regularly inspected for wear and cracks, and replaced promptly if any abnormalities are found to prevent breakage and failure.

VI. Future Development Trends of Carbon Steel Rods: Adapting to Industrial Upgrades and Pushing Performance Limits

With the advancement of the "dual carbon" goals and manufacturing upgrades, the carbon steel rod industry is moving towards high performance, low carbonization, refinement, and customization, further consolidating its position as a "basic profile." 1. High Performance: Pushing the Boundaries of Strength and Weather Resistance
Microalloyed Carbon Steel Rods: By adding microalloying elements such as vanadium (V), niobium (Nb), and titanium (Ti), high-strength carbon steel rods (such as Q690 grade) are developed. The tensile strength is increased to over 690 MPa, allowing them to replace some alloy structural steels for applications such as heavy machinery and bridges, reducing material usage (weight reduction of over 20%).
Weather-Resistant Carbon Steel Rods: By adding elements such as copper (Cu), chromium (Cr), and nickel (Ni), "weather-resistant steel rods" (such as Q355NH) are developed. These rods form a dense oxide film in outdoor environments, increasing corrosion resistance 3-5 times that of ordinary carbon steel. They can be used without painting, making them suitable for outdoor applications such as buildings and bridges. 2. Low-Carbon Manufacturing: Responding to the Dual Carbon Goals
Short-Process Steelmaking: Promoting the "electric furnace + scrap steel" steelmaking process to replace the traditional "blast furnace + converter" process, reducing carbon emissions during carbon steel bar production (the short-process reduces carbon emissions by over 60% compared to the long process);
Waste Heat Recovery and Energy Saving: Introducing a waste heat recovery system in the rolling process to use waste heat from the heating furnace and rolling mill for power generation or billet heating, reducing production energy consumption (energy consumption is reduced by 15%-20%);
Upgrading Recycling: Establishing a closed-loop system from "scrap carbon steel bars to recycled carbon steel bars." Through intelligent sorting (AI identification of carbon content and grade), the quality and stability of recycled steel bars are improved, achieving resource recycling. 3. Refinement and Customization: Meeting High-End Demands
High-Precision Rolling: Upgrading rolling mill equipment (such as the AGC automatic thickness control system on continuous rolling mills) allows hot-rolled carbon steel bars to have a dimensional tolerance of ±0.1mm, approaching the precision of cold rolling, reducing downstream processing allowances (reducing scrap by over 10%).
Special Section Customization: Targeting the needs of new energy vehicles and precision machinery, we develop carbon steel bars with special cross-sections (such as elliptical and trapezoidal bars) that can be used directly without secondary processing, improving manufacturing efficiency.
Customized Performance: Based on customer needs, we provide carbon steel bars with defined carbon content and heat treatment conditions (such as pre-tempered 45# steel bars and low-temperature toughness Q235 steel bars), eliminating heat treatment steps and shortening production cycles.
Conclusion
As a basic metal profile, carbon steel bars lack the corrosion resistance of stainless steel or the electrical conductivity of copper. However, their core advantages of low cost, abundant resources, flexible processing, and adjustable performance make them the "invisible cornerstone" supporting industrial manufacturing and infrastructure. From the rebar of towering skyscrapers to the shafts of precision machinery, from automotive crankshafts to everyday hardware tools, carbon steel rods have always played an irreplaceable role in even the most ordinary of roles. In the future, with the advancement of high-performance and low-carbon technologies, carbon steel rods will continue to evolve towards cost reduction, quality improvement, and carbon reduction, providing more solid material support for global industrialization and infrastructure upgrades.