1. Differences in chemical composition: a game between precise control and loose range
In the field of low-carbon steel, taking Q235 (GB) and SS400 (JIS) as examples, the control of carbon content shows obvious differences. The upper limit of Q235 carbon content is 0.22%, while SS400 is strictly controlled within 0.17%. In terms of manganese, Q235 requires a range of 0.30%-0.70%, while SS400 allows a higher upper limit of 1.40%. This difference is more significant in the control of phosphorus and sulfur impurities. The upper limit of phosphorus and sulfur content of 45 steel (GB) is 0.045%, while S45C (JIS) is controlled below 0.030%.
The difference in the field of medium-carbon steel is also prominent. The carbon content of 45 steel (GB) is 0.42%-0.50%, and S45C (JIS) is accurate to 0.42%-0.48%. In terms of manganese element control, 45 steel allows 0.50%-0.80%, while S45C requires 0.60%-0.90%. This difference in composition directly affects the material properties. For example, S45C is better than 45 steel in hardenability and is suitable for more precise mechanical parts.
In the field of stainless steel, the difference in nickel content between 304 (GB) and SUS304 (JIS) is worth noting. The GB standard requires 8.00%-11.00%, while the JIS standard limits it to 8.00%-10.50%. This subtle difference makes SUS304 slightly better in deep drawing performance and suitable for food processing equipment with complex shapes.
2. Mechanical properties: the art of balancing strength and plasticity
In the yield strength dimension, SS400 (JIS) requires ≥245 MPa, surpassing Q235 (GB)'s ≥235 MPa. This difference is particularly critical in the field of building structures, and the Japanese standard pays more attention to the stability of materials under dynamic loads. In terms of tensile strength, SS400's 400-510 MPa range shows more stable strength performance compared to Q235's 370-500 MPa.
The elongation index shows the opposite trend, Q235 requires ≥26% (thickness ≤16mm), while SS400 only requires ≥20%. This difference stems from the fact that the GB standard pays more attention to the forming performance of the material, especially when welding steel structures, higher elongation can effectively reduce the risk of cold cracks. In the field of medium carbon steel, the tensile strength of 45 steel is 600-755 MPa, while that of S45C is 570-700 MPa, but S45C shows better dimensional stability in gear processing through stricter surface decarburization control.
3. Test method: comprehensive control from micro to macro
In terms of metallographic inspection, the GB standard requires the inspection of low-magnification structures, inclusions, decarburization and other items, while the JIS standard allows the buyer and seller to agree. For example, GB/T 699-2015 requires that carbon structural steel must be tested for low-magnification structural defects, while JIS G3101 only conducts relevant tests when requested by the buyer. This difference results in the metallographic qualification rate of Chinese exported steel being generally higher than that of Japanese standards.
In terms of dimensional tolerance control, the JIS standard has more stringent requirements on the out-of-roundness of round steel. Taking a 50mm diameter round steel as an example, JIS allows an out-of-roundness of ≤1.3mm, while the GB standard relaxes it to ±2.5mm. This difference is particularly critical in the field of precision machining, directly affecting the assembly accuracy of parts such as bearings.
4. Identification and certification: the key to market access
The JIS brand system adopts a three-segment coding of "material-shape-strength", such as "SS" in SS400 represents structural steel, and "400" represents the lower limit of tensile strength. The GB standard adopts the naming method of "yield strength + quality grade", such as "345" in Q345B represents yield strength. This difference often leads to brand correspondence errors in international trade. It is recommended to use a chemical composition comparison table for conversion.
In terms of certification system, JIS certification requires factories to pass quality management system review, and the certificate is valid for 3 years, during which all products automatically meet the standard. The GB standard mainly adopts a batch inspection mode. This difference has increased the certification cost of Chinese steel in the Japanese market by about 30%. It is recommended that export companies start the JIS certification process 6 months in advance and establish a complete production record traceability system.
5. Application field: market mapping of standard differences
In the field of building structures, Q345B (GB) is widely used in super high-rise buildings in China due to its high strength and low-temperature toughness, while the Japanese market prefers SN430B (JIS), which has a yield strength of ≥325 MPa and has special requirements for copper elements. In the field of machinery manufacturing, S45C (JIS) is used in 65% of Japanese machine tool spindles, while similar Chinese products mostly use 45 steel, and the performance gap is compensated by adjusting the heat treatment process.
In the automotive industry, Japan uses FCD400 (JIS) ductile iron to manufacture crankshafts, while China uses QT400-18 (GB). Although the tensile strength of the two is similar, the elongation requirement of FCD400 is ≥18%, which leads to the need to add a normalizing process for crankshaft blanks exported from China, increasing the cost by about 15%.
6. Material selection strategy: Optimization scheme based on standard differences
For steel structure enterprises exporting to Japan, it is recommended to adopt a dual-standard production mode: the chemical composition is controlled according to JIS, and the mechanical properties meet the requirements of GB and JIS at the same time. For example, the carbon content of Q345B is reduced from 0.20% to 0.18%, and the manganese content is adjusted from 1.70% to 1.50%, which not only meets the requirements of SN430B, but also maintains cost advantages.
In the field of mechanical parts processing, it is recommended to use S45C instead of 45 steel. By controlling the silicon content (JIS requires 0.15%-0.35%) and phosphorus and sulfur content (≤0.030%), the surface hardness of the parts can be increased by 10%-15%, while reducing the amount of grinding. For enterprises that need JIS certification, it is recommended to establish a dedicated production line and configure impact testing equipment such as impact testing machines. The annual certification maintenance cost can be controlled within 500,000 yuan.
This article systematically analyzes the differences between Chinese and Japanese steel standards to provide enterprises with accurate technical references. It is recommended to establish a standard conversion database, include more than 500 brand correspondences, and combine AI algorithms to achieve automatic matching, reducing the material selection error rate from the industry average of 12% to below 3%.