Carbon steel bars: the "backbone of mechanics" in industry

As one of the most basic profiles in the steel material system, carbon steel bars, with their excellent mechanical properties and processing adaptability, play a key structural support and transmission role in the fields of machinery manufacturing, construction engineering, and the automotive industry. From precision bolts with a diameter of a few millimeters to heavy shaft parts with a diameter of hundreds of millimeters, carbon steel bars have built a solid skeleton of modern industry through a variety of specifications and performance combinations.
1. Definition and classification system of carbon steel bars
Definition: Carbon steel bars refer to bar-shaped steel with iron as the matrix and a carbon content between 0.05% and 1.5%. Its mechanical properties are mainly adjusted by the carbon content, and a small amount of silicon, manganese and other elements can be added to improve the process performance. According to GB/T 699-2015 standard, the sulfur and phosphorus content of carbon steel bars is usually controlled at ≤0.04%, and high-quality steel can be as low as ≤0.03%.
Classification by cross-sectional shape and production process
Cross-sectional shape:
Round bars (diameter 6-500mm): used for shaft parts, accounting for more than 70% of carbon steel bar output;
Square bars (side length 10-200mm): used for frame structures and agricultural machinery parts;
Hexagonal bars (distance from edge 8-100mm): directly process bolts and nuts to reduce cutting volume;
Special-shaped bars (such as I-shaped and T-shaped): special for special structural parts.
Production process:
Hot-rolled bars: heated to 1050-1200℃ for rolling, thick surface oxide scale, accuracy ±0.5mm, used for general structural parts;
Cold-rolled bars: rolled at room temperature, smooth surface (Ra≤3.2μm), accuracy ±0.05mm, used for precision parts;
Cold-drawn bars: formed by die drawing, dimensional accuracy can reach h6 level, used for hydraulic piston rods, etc.;
Forged bars: formed by free forging or die forging, dense structure, used for important load-bearing parts.
2. Production process and key technological breakthroughs
Full process analysis
Steelmaking:
Converter steelmaking: Ordinary carbon steels such as Q235 use top and bottom double-blowing converters, with a smelting cycle of 30-40 minutes and a carbon content control accuracy of ±0.02%;
Electric furnace steelmaking: High-quality carbon steels such as 45# use electric arc furnace + LF refining, and the sulfur content can be reduced to ≤0.015%, which is suitable for important parts.
Continuous casting technology:
Small billet continuous casting (less than 150mm×150mm): used to produce bars with a diameter of ≤100mm, with a drawing speed of 4-6m/min;
Large round billet continuous casting (diameter 300-800mm): using electromagnetic stirring technology to reduce center segregation, used for heavy shafts.
Rolling and finishing:
High-speed wire rolling: φ6-20mm round bars are produced by high-speed wire rolling mills, with a speed of up to 120m/s and uniform surface quality;
Precision cold rolling: Rolled by multi-roll mills (such as 12-roll Sendzimir mills), the dimensional tolerance can be controlled at ±0.02mm;
Heat treatment process:
Annealing: eliminate stress and improve cutting performance (such as 45# steel hardness ≤200HBW after annealing);
Quenching and tempering: quenching + high temperature tempering to obtain comprehensive mechanical properties (such as σb≥980MPa after quenching and tempering of 40Cr);
Surface quenching: induction heating quenching, the surface hardness reaches HRC50-55, used for gear tooth surface.
Cutting-edge technology cases
Headless rolling technology: Tangshan Iron and Steel Company of Hesteel Group uses headless rolling technology to produce carbon steel bars, eliminating the temperature difference between the head and tail of traditional rolling, improving dimensional accuracy by 30% and yield rate by 2.5%;
Controlled rolling and controlled cooling (TMCP): Baowu Group uses low-temperature rolling + ultra-fast cooling technology for 60Si2Mn spring steel bars, increasing the yield strength from 800MPa to 1000MPa, while maintaining toughness AKV≥30J;
3D printing bars: 304L stainless steel bars (diameter 1.75mm) developed by Anshan Iron and Steel are purified by electroslag remelting, with an oxygen content of ≤50ppm, suitable for metal wire 3D printing.
3. Mechanical properties and engineering applications
Mechanical manufacturing field
Transmission system:
40Cr quenched and tempered rods are used to manufacture automobile transmission shafts (diameter 70-120mm), bearing torque ≥5000N・m;
35CrMo forged rods are used for wind power equipment main shafts (diameter 300-500mm), fatigue strength ≥450MPa, and can withstand alternating loads for more than 20 years.
Basic parts:
12mm diameter 35# cold heading rods are used to manufacture 8.8 grade high-strength bolts, and the efficiency of cold heading is 10 times higher than that of cutting processing;
Hexagonal rods (20mm opposite sides) are directly processed into machine tool handles, and the material utilization rate is increased from 40% of cutting processing to 85%.
Building and bridge engineering
Structural support:
HRB400E hot-rolled ribbed steel bars (φ16-40mm) are used for high-rise building concrete structures, with a yield strength of ≥400MPa and seismic performance that meets the requirements of 8-degree fortification;
Q355B round bars (φ200mm) are used as bridge cable anchors, with a single bar bearing a tensile force of ≥5000kN.
Special projects:
Q390GJD round bars (φ150mm) used in the Hong Kong-Zhuhai-Macao Bridge Island Tunnel Project have passed the -40℃ low-temperature impact test (AKV≥34J) and are resistant to marine environmental corrosion;
20MnNiMo forged bars (φ400mm) for nuclear power plant containment are 100% ultrasonically inspected to ensure that there are no internal defects.
Automobile and rail transit
Automobile parts:
20MnB cold-drawn rods (φ12mm) are used to manufacture engine connecting rod bolts. After carburizing and quenching, the surface hardness is HRC58-62, and the fatigue life is ≥10^7 times;
45# cold-rolled rods (φ8mm) are used for automobile seat slides. The surface is hard chrome plated (thickness 10-15μm), and the wear resistance is ≥50,000 times.
Rail transit:
60Si2Mn spring steel rods (φ60mm) are used for subway wheels. The surface hardness is HRC48-52 after induction heating and quenching, and the service mileage is ≥1 million kilometers;
Q235B round rods (φ10mm) are used for high-speed rail contact networks. The thickness of the zinc coating is ≥85μm, and the service life in acid rain areas is more than 15 years.
Tool and mold manufacturing
Cutting tools:
T12A high carbon steel rod (φ25mm) is spheroidized annealed + quenched to make woodworking chisels, with a blade hardness of HRC60-62, and can cut pine wood up to 5,000 times without curling;
9SiCr alloy tool steel rod (φ30mm) is used to make stamping dies. After quenching + tempering, the wear resistance is 3 times higher than that of 45# steel.
Wear-resistant parts:
ZGMn13 high manganese steel rod (φ100mm) is used to make crusher jaws. The surface is hardened under impact load (hardness increased from HB200 to HB500), and the service life is extended by 5 times than ordinary carbon steel;
440C stainless steel rod (φ15mm) is used to make surgical blades. After deep cryogenic treatment (-196℃), the hardness is HRC58-60, and the blade retention is ≥100 operations.
IV. Market structure and technology trends
Global industry status
Capacity distribution: In 2024, the global carbon steel bar output will be about 180 million tons, of which China will account for 58% (Hebei, Jiangsu and Shandong are the main production areas), Europe (Germany and Italy) will account for 15%, and North America (the United States and Canada) will account for 12%;
Price dynamics: The average price of 45# hot-rolled round bar (φ45mm) in 2024 will be about US$680/ton, which will be 9% higher than that in 2023 due to the price of iron ore (the average price of 62% grade Australian ore is US$125/ton) and the price of scrap steel (US$320/ton);
Import and export structure: China exports about 8 million tons of carbon steel bars annually, mainly to Southeast Asia (accounting for 35%), and imports about 1.2 million tons, mainly high-end cold-drawn bars (such as S45C-CSP cold-drawn bars imported from Japan, which are 40% more expensive than domestic ones).
Technology development direction
Green manufacturing technology:
Electric arc furnace short process steelmaking: using scrap steel + photovoltaic power to produce carbon steel bars, CO₂ emissions per ton of steel are reduced from 2.1 tons to 0.8 tons (such as the EAF+CSP process of Nucor in the United States);
Low-temperature rolling technology: 950℃ low-temperature rolling is used for 20# steel, which reduces energy consumption by 12%, while refining the grains (grain size 10), and increasing strength by 10%.
High-performance innovation:
Ultra-fine grain carbon steel bars: 20# steel bars prepared by ARB (accumulative rolling) process, with grain size refined to 1μm, yield strength increased from 240MPa to 500MPa, used for lightweight automotive parts;
Functionally gradient material bars: 45# steel bars with WC-Co coating on the surface by laser cladding, surface hardness HRC65-70, core toughness maintained, used for mine drilling drill rods.
Intelligent production:
Intelligent rolling system for bars: The AI ​​rolling system deployed by Baowu Group predicts the rolling force through machine learning, controls the diameter tolerance of φ20mm round bars to ±0.03mm, and increases the qualified rate from 92% to 99.5%;
Unmanned finishing line: Shagang Group's carbon steel bar automatic flaw detection-packing line uses robots + visual inspection, which is 8 times more efficient than manual work, and the defect recognition rate reaches 99.2%.
Surface treatment upgrade:
Chromium-free Dacromet coating: Replaces traditional galvanizing, used for carbon steel bars for automobile chassis, salt spray test (1000 hours) without rust, in compliance with EU REACH regulations;
Nano-ceramic coating: Deposit Al₂O₃-TiO₂ nanolayer on the surface of 42CrMo steel bars, reduce the friction coefficient from 0.6 to 0.15, and is used for aircraft engine bearing push rods.
5. Selection Guide and Maintenance Points
Selection Decision Factors
Load Type:
Select 45# quenched and tempered rod (σb≥600MPa) for tensile load;
Select 40CrNiMoA rod (fatigue strength≥480MPa) for alternating bending load;
Select 20CrMnTi carburized rod (core hardness HRC30-35, surface HRC58-62) for impact load.
Environmental conditions:
Select Q235B galvanized rod (zinc layer thickness ≥55μm) for humid environment;
Select 35CrMo rod (strength retention rate ≥70% at 450℃) for high temperature environment;
Select 316L stainless steel rod (chloride ion corrosion resistance) for corrosive environment.
Processing technology:
Cold heading forming: ML35 cold heading steel bar (hardness ≤170HBW after spheroidizing annealing);
Cutting: Y15 free-cutting steel bar (add 0.2% sulfur, cutting speed increased by 30%);
Forging: 35# steel bar (initial forging temperature 1150℃, final forging temperature ≥850℃).
Storage and maintenance technology
Rust prevention:
Short-term storage (≤3 months): apply water-based rust inhibitor (concentration 5%), cost about 0.5 yuan/square meter;
Long-term storage (>1 year): wrap with VCI vapor phase rust-proof paper, with drying bag (humidity ≤30% RH), the rust prevention period can reach 5 years;
Ocean transportation: the whole bundle of carbon steel bars is wrapped with heat shrink film, and built-in silica gel desiccant (dosage 500g/ton) to prevent salt spray corrosion.
Defect repair:
Surface scratches (depth < 0.5mm): Use abrasive belt grinding (grit size 240#) + polishing;
Minor rust: Clean with 10% citric acid solution (temperature 50-60℃, time 10-15 minutes), and then re-apply anti-rust oil;
Dimensional deviation (≤0.1mm): Cold-drawn rods can be precisely straightened by a straightening machine (pressure 30-50 tons), and the straightness can be corrected to ≤0.1mm/m.