Q235 Steel vs 45 Steel vs 40Cr Steel

In engineering practice, steel selection directly influences performance, 제조, 신뢰할 수 있음, and cost of components.

Three commonly referenced steels in Chinese and international standards — Q235, 45 강철, 그리고 40Cr — cover a broad spectrum of design requirements, from basic structural support to high‑strength mechanical parts.

Although each is based on iron‑carbon metallurgy, their alloying strategies, microstructural behavior, 기계적 성능, and optimal applications differ substantially.

This article provides a multi‑perspective, 권위 있는, and in‑depth comparison to guide material selection and engineering decision‑making.

1. Metallurgical Identity and Classification

Q235 Steel

Q235 is a low-carbon structural steel widely used in general engineering and construction applications.

It is the most common Chinese 탄소강 등급, equivalent to ASTM A36 그리고 EN S235JR. Q235 offers a balance of strength, 연성, 및 용접성, making it suitable for bridges, 건물, ship structures, 파이프라인, and machinery frames.

특징

• 화학 성분: Carbon ≤ 0.20–0.25%, Mn 0.30–0.70%, trace S and P.
• 기계적 성질: Yield strength ≈ 235 MPa, tensile strength ≈ 375–500 MPa.
• Weldable and formable: 쉽게 절단 가능, 용접된, and cold-formed.
• 비용 효율적: Economical option for general structural applications.
• 응용: Construction beams, 구조 프레임, 조선, 압력 용기.

45 강철 (also known as C45 or 1.1191)

45 steel is a medium-carbon steel widely used in China and internationally for mechanical parts requiring higher strength and hardness than low-carbon steels.

It corresponds roughly to 아이시 1045. It is suitable for shafts, 기어, and fasteners that are mechanically loaded and can be heat-treated.

특징

• 화학 성분: Carbon ≈ 0.42–0.50%, Mn 0.50–0.80%, S/P <0.05%.
• 기계적 성질 (단련된): Tensile strength ≈ 570–700 MPa, yield strength ≈ 330–500 MPa.
• Heat-treatable: Can be quenched and tempered to achieve higher hardness and wear resistance.
• Good machinability and moderate toughness: Balances strength and processability.
• 응용: 샤프트, 기어, 볼트, 차축, 커넥팅로드, and mechanical parts under moderate loads.

40Cr Steel (라고도 1.7035)

40Cr is a medium-carbon, 크롬-alloyed steel widely used in applications requiring 더 높은 강도, 경도, 및 내마모성 than ordinary medium-carbon steels.

Chromium improves hardenability, 내식성, 그리고 피로강도. It is roughly equivalent to 아이시 5140.

특징

• 화학 성분: Carbon ≈ 0.37–0.44%, Chromium ≈ 0.80–1.10%, Mn 0.50–0.80%, S/P <0.035%.
• 기계적 성질 (정규화): Tensile strength ≈ 745–930 MPa, yield strength ≈ 435–600 MPa.
• Excellent hardenability: Can be quenched and tempered to achieve high hardness (최대 HRC 50) 내마비 부품의 경우.
• Good fatigue resistance and toughness: Suitable for critical mechanical components.
• 응용: 샤프트, 기어, 크랭크샤프트, 무거운 차축, 스핀들, and other high-strength mechanical parts.

2. 화학 성분 비교: Q235 Steel vs 45 Steel vs 40Cr Steel

The chemical composition of steel directly determines its phase transformation behavior and mechanical properties.

The following table presents the standard composition ranges (per Chinese national standards) and the functional mechanisms of key elements for the three steels:

Key differences:

Q235 has low carbon and no intentional alloying elements, focusing on processability; 45 steel has higher carbon and stricter impurity control, enabling heat treatment;

40Cr adds chromium to optimize hardenability and mechanical properties, bridging the gap between carbon steel and high-alloy steel.

3. 미세 구조적 특성: From As-Delivered to Heat-Treated States

Microstructure is the link between chemical composition and mechanical properties.

The three steels exhibit distinct microstructures in different states, directly affecting their performance:

As-Delivered State (Hot Rolled)

• Q235 Steel: Consists of ferrite (α-Fe) + 펄라이트 (lamellar mixture of ferrite and cementite). Ferrite is the main phase (70–80%), ensuring good ductility and weldability.
  Pearlite content (20–30%) provides moderate strength. The structure is coarse-grained due to low alloy content and simple hot rolling process.
• 45 강철: 페라이트 + 펄라이트, with higher pearlite content (40–50%) than Q235 due to higher carbon content.
  The structure is finer and more uniform (고품질 강철), with fewer inclusions, leading to better strength and toughness balance.
• 40Cr Steel: 페라이트 + 펄라이트 + trace chromium-rich carbides. Chromium refines the grain size, making the pearlite lamellae thinner than 45 강철.
  The presence of chromium carbides (Cr₃C) lays the foundation for subsequent heat treatment strengthening.

Heat-Treated State (담금질 + 템퍼링, 큐&티)

• Q235 Steel: Poor hardenability; 담금질 (수냉) only forms martensite in the surface layer, with the core remaining ferrite-pearlite.
  Heat treatment is rarely used, as it cannot significantly improve overall performance and may cause deformation/cracking.
• 45 강철: 담금질 후 (840–860℃ water/oil cooling), the structure transforms into lath martensite (hard but brittle).
  Tempering at 200–300℃ (low tempering) produces tempered martensite, improving toughness while maintaining high hardness.
  Tempering at 500–600℃ (medium tempering) forms sorbite, achieving a balance of strength (σᵤ≥600 MPa) 그리고 연성 (δ≥15%).
• 40Cr Steel: Excellent hardenability; oil cooling (instead of water cooling) can achieve full martensite transformation even for workpieces with diameter ≤50 mm.
  After medium tempering (520–560℃), the structure becomes tempered sorbite (fine-grained sorbite + 분산 된 탄화물), with higher strength and toughness than 45 강철. Chromium stabilizes the martensite structure, reducing temper brittleness.

4. Mechanical Properties Comparison — Q235 Steel vs 45 Steel vs 40Cr Steel

5. Heat Treatment Characteristics: Hardenability and Process Adaptability

Heat treatment responsiveness (경화성, temper stability) determines the scope of application of steel. The three steels differ significantly in this regard:

Hardenability

• Q235 Steel: Very poor hardenability. The critical cooling rate is high; only thin workpieces (≤5 mm) can form a small amount of martensite after water cooling, while thick workpieces remain ferrite-pearlite.
  Heat treatment is not economically viable, so it is used in the as-delivered state.
• 45 강철: Moderate hardenability. Workpieces with diameter ≤20 mm can achieve full martensite by water cooling; for thicker workpieces (20-40mm), oil cooling leads to incomplete hardening (core is sorbite).
  It is suitable for medium-sized, medium-load parts requiring heat treatment.
• 40Cr Steel: Excellent hardenability. Chromium reduces the critical cooling rate, enabling full martensite transformation in workpieces with diameter ≤50 mm by oil cooling (avoiding water cooling-induced deformation/cracking).
  For workpieces up to 80 mm, water-oil quenching can achieve uniform hardening, making it suitable for large, heavy-load parts.

Common Heat Treatment Processes and Effects

• 가열 냉각: Q235 annealing (600–650℃) relieves rolling stress; 45/40Cr annealing refines grains and reduces hardness for machining. 40Cr annealing also dissolves chromium carbides, preparing for quenching.
• 정규화: Q235 normalizing (880–920℃) improves structure uniformity; 45/40Cr normalizing enhances strength and toughness, used as a pre-treatment for complex parts.
• 담금질 + 템퍼링: The core process for 45/40Cr. 45 steel uses water quenching + medium tempering; 40Cr uses oil quenching + medium tempering, achieving better comprehensive performance and lower deformation.
• 표면 경화: 45/40Cr can undergo induction hardening or carburizing (45 강철) to improve surface hardness (HRC 50–60) 내마비 부품의 경우.
  40Cr’s chromium content enhances surface hardening effect and wear resistance.

6. Processing Performance: 주조, 단조, 용접, 및 가공

Processing performance directly affects manufacturing efficiency and cost, and is a key factor for material selection in mass production:

Casting Performance

• Q235 Steel: Poor castability. Low carbon and alloy content lead to poor molten fluidity and high shrinkage rate, prone to shrinkage cavities and porosity. Rarely used for casting; mainly for rolling and forming.
• 45 강철: Moderate castability. Higher carbon content improves fluidity compared to Q235, but still prone to hot cracking. Used for small to medium-sized cast parts with low precision requirements.
• 40Cr Steel: Better castability than 45 강철. Chromium refines the cast structure, reducing shrinkage and hot cracking tendency.
  Suitable for precision cast parts requiring heat treatment, but casting cost is higher than rolling.

Forging Performance

• Q235 Steel: Excellent forging performance. Forging temperature range (1150–850℃) is wide, with good plasticity and low deformation resistance. Suitable for hot forging of simple shapes (예를 들어, 볼트, 괄호).
• 45 강철: Good forging performance. Forging temperature (1100–800℃); requires uniform heating to avoid cracking. Forged parts have refined grains, improving heat treatment effect.
• 40Cr Steel: Moderate forging performance. Chromium increases deformation resistance, requiring higher forging force and stricter temperature control (1100–820℃).
  Post-forging annealing is necessary to eliminate internal stress and prepare for heat treatment.

용접성능

• Q235 Steel: Excellent welding performance. Low carbon content avoids martensite formation in the heat-affected zone (위험요소), with no preheating or post-weld heat treatment (PWHT) required for thin workpieces. Compatible with all welding methods (SMAW, GMAW, GTAW).
• 45 강철: Poor welding performance. High carbon content leads to hard martensite in the HAZ, prone to cold cracking.
  예열 (150–200℃) and PWHT (tempering at 600–650℃) are mandatory. Welding is only used for repair, not for load-bearing welds.
• 40Cr Steel: Worse welding performance than 45 강철. Chromium increases HAZ hardenability, making cold cracking and temper brittleness more likely.
  Strict preheating (200–300℃), low heat input welding, and PWHT are required. Welding is generally avoided; mechanical joining (bolting, 리벳 팅) 선호된다.

가공 성능

• Q235 Steel: Excellent machining performance. Low hardness and good plasticity make cutting easy, with low tool wear.
  Suitable for high-speed machining and automated production lines (예를 들어, machining of brackets, 접시).
• 45 강철: Good machining performance in the as-delivered state (HBW 190–230). 열처리 후 (hardness ＞ HRC 30), machining difficulty increases, requiring hard alloy tools. It is a typical “machinable heat-treated steel”.
• 40Cr Steel: Moderate machining performance in the as-delivered state. Chromium increases cutting resistance, so tool wear is higher than 45 강철.
  After Q&티 (HBW 280–320), machining requires higher cutting speed and feed rate control, with machining cost 15–20% higher than 45 강철.

7. 부식 저항

All three steels are carbon/alloy structural steels without intentional corrosion-resistant alloying elements (Cr content in 40Cr is too low for passive film formation), so their corrosion resistance is generally poor, with slight differences:

• Q235 Steel: 내식성이 좋지 않음. High impurity content (에스, 피) and low alloy content accelerate atmospheric and freshwater corrosion, with a corrosion rate of 0.1–0.3 mm/year in industrial atmospheres. Must be protected (그림, 아연 도금) for outdoor service.
• 45 강철: Slightly better corrosion resistance than Q235. Lower impurity content and finer structure reduce corrosion initiation sites.
  Corrosion rate is 0.08–0.25 mm/year in industrial atmospheres, still requiring protection for long-term service.
• 40Cr Steel: Best corrosion resistance among the three. Chromium forms a thin oxide film on the surface, inhibiting corrosion.
  Corrosion rate is 0.05–0.20 mm/year in industrial atmospheres, and it has better resistance to mild acids/bases than Q235 and 45 강철.
  하지만, it still suffers from pitting corrosion in high-chloride media, requiring anti-corrosion treatment (chromizing, 그림).

8. Application Scenarios Q235 Steel vs 45 Steel vs 40Cr Steel

The application of the three steels is strictly based on their performance and cost, covering different industrial fields:

Q235 Steel

Low-cost, general-purpose structural steel. 응용 프로그램은 다음과 같습니다:

• Building and construction: Steel frames, 광선, 기둥, steel plates, and reinforcement bars for ordinary buildings, 교량, and workshops.
• Mechanical manufacturing: Non-load-bearing parts (괄호, 기지, 커버), 볼트, 견과류, and washers for low-load equipment.
• Pipeline and container: Low-pressure water pipelines, 저장 탱크, and brackets for non-corrosive media.

45 강철

Medium-strength, heat-treatable carbon steel. 응용 프로그램은 다음과 같습니다:

• Mechanical parts: 기어 샤프트, 커넥팅로드, 크랭크샤프트, 볼트, and nuts for medium-load equipment (예를 들어, small motors, 슬리퍼, and agricultural machinery).
• Tool components: 블레이드, 펀치, and dies for low-speed, low-wear tools (after surface hardening).
• 자동차 산업: Non-critical parts (예를 들어, 브레이크 페달, 스티어링 너클) for low-end vehicles.

40Cr Steel

고강도, alloy structural steel. 응용 프로그램은 다음과 같습니다:

• Mechanical transmission parts: High-load gear shafts, 드라이브 샤프트, 기어, and bearings for heavy machinery (예를 들어, engineering machinery, 공작기계).
• 자동차 항공우주: Critical parts (예를 들어, engine crankshafts, 캠축, transmission gears) for high-end vehicles and light aircraft.
• Petrochemical industry: High-pressure pipeline flanges, 밸브, and pump shafts for medium-corrosion, high-load environments.

9. Cost and Cost-Effectiveness Comparison

Cost is a key factor in large-scale production. The relative cost (taking Q235 as the baseline) and cost-effectiveness of the three steels are as follows:

10. 결론

The comparative analysis of Q235 steel, 45 강철, and 40Cr steel highlights how 탄소 함량, 합금, 그리고 열처리 influence mechanical performance, 제조, 응용 프로그램 적합성.

• Q235 steel 는 low-carbon structural steel with excellent ductility, 용접성, 및 성형성.
  Its cost-effectiveness makes it ideal for general structural and fabrication applications, but it has limited strength and requires corrosion protection.
• 45 강철 는 medium-carbon, heat-treatable steel offering higher strength and hardness than Q235.
  언제 담금질과 템퍼링, it achieves significantly improved tensile strength and wear resistance, 에 적합하게 만드는 것 mechanical parts such as shafts, 기어, and axles.
• 40Cr steel 는 medium-carbon chromium-alloy steel 설계 high-strength and fatigue-resistant applications.
  그것은 deep hardenability and wear resistance allow it to perform under heavy cyclic loads, 볼 수 있듯이 크랭크샤프트, 커넥팅로드, and high-load machinery components.

Bottom line: Material selection should balance 힘, 인성, 가공성, 용접성, 그리고 비용 against service requirements.
Q235 suits structural and low-load applications, 45 steel covers moderate-load mechanical parts, and 40Cr steel excels in high-strength, high-fatigue, and wear-critical components.

 

자주 묻는 질문

What is the main difference between Q235, 45, and 40Cr steels?

• Q235 is low-carbon structural steel; 45 steel is medium-carbon and heat-treatable; 40Cr is a medium-carbon chromium-alloy steel with high strength and hardenability.

Can Q235 steel be heat-treated to improve strength?

• 아니요, Q235’s low carbon content limits heat-treatment hardening. Strength improvements rely on cold working or design optimization.

Which steel is best for shafts and gears?

• 45 steel is suitable for moderate-load shafts and gears; 40Cr is preferred for high-strength, high-fatigue, and wear-resistant mechanical components.

Is 40Cr steel corrosion-resistant?

• Not inherently. 보호 코팅, 도금, or design considerations are needed for corrosive environments.

How does heat treatment affect 45 and 40Cr steels?

• Quenching and tempering significantly improve tensile strength, 경도, 및 피로 저항, making them suitable for mechanically demanding components.