1. What is the correspondence between core standards and grades?
The Chinese national standard (GB) and the Japanese national standard (JIS) for cold-rolled coils overlap significantly in product applications, but their underlying standard systems and naming logics differ significantly.
**GB Standard:** This involves two main standards. One is GB/T 708, which uses yield strength as the core indicator for grade naming (e.g., Q195, Q235). The other is GB/T 5213, which references the European standard system in its naming (e.g., DC01) and focuses more on stamping performance.
**JIS Standard:** The core standard is JIS G 3141, corresponding to its most basic commercial grade, SPCC (Steel Plate Cold Rolled Commercial).
Correspondence: In general industrial applications, JIS SPCC can be considered equivalent in performance to GB Q195, Q215, or DC01. However, it should be noted that this is only an approximate correspondence, not a completely equivalent replacement.
2.What are the differences between chemical composition and mechanical properties?
While their chemical compositions are very similar, there are key differences in the testing methods for mechanical properties, which must be carefully considered in import trade.
Chemical Composition: Compared to the Japanese standard, the Chinese national standard GB/T 5213 adds requirements for trace elements such as acid-soluble aluminum (Alt) and titanium (Ti).
Mechanical Properties: The biggest difference lies in the measurement method for elongation after fracture. The Japanese standard (JIS G 3141) typically uses an A50 gauge length (proportional gauge length), while the Chinese standard (GB/T 5213) uses an A80mm gauge length (fixed gauge length). Due to the different sample lengths, the values cannot be directly compared; the specific measurement standard must be confirmed.
3.What is the difference between dimensional tolerances and process precision?
This is a very important practical difference, directly related to the product's machinability.
Tolerance Requirements: Taking a 0.5mm thick cold-rolled coil as an example, the thickness tolerance of the Japanese standard (JIS G 3141) is typically ±0.015mm, while the national standard (GB/T 708) is typically ±0.03mm.
Process Impact: This seemingly small 0.015mm difference has a significant impact on high-precision machining. For example, it will affect the perpendicularity of the cut during laser cutting, and may lead to unstable weld penetration in automated welding, increasing rework rates.
4.What are the differences between surface quality and panel shape control?
Both have surface quality grading, but the specific grading standards and expressions differ.
Chinese National Standard Surface Quality Grading: According to GB/T 5213, it is divided into three levels: FB (higher grade surface), FC (high grade surface), and FD (super high grade surface).
Japanese Standard Surface Quality Grading: Its surface quality system is usually indicated by a gloss suffix, for example, "B" represents bright, and "D" represents dull. This difference means that when importing high-surface-requirement appliance panels or automotive exterior panels, one cannot simply look at the grade number; the specific surface grade code must be clearly identified.
5.What are the differences in the extensions and applications of grade designations?
Both standard grade designation systems use suffixes or codes to distinguish different processing states and applications. Understanding these codes is key to correct material selection.
Japanese Standard Grade Suffixes: The Japanese standard system has a rich variety of suffixes, for example:
-S: Represents standard quenching and tempering (annealing), the most common state.
-1/8H, -1/4H, -1/2H, -H: Represent 1/8 hard, 1/4 hard, 1/2 hard, and fully hard (cold-rolled hard coil) states, respectively.
-T: Indicates guaranteed tensile strength (e.g., SPCCT), whose strength is usually higher than standard SPCC.
National Standard Grade Designation Logic: The DC series of GB/T 5213 (e.g., DC01, DC03, DC04) clearly indicates a progressive improvement in stamping performance through numerical increments.