1 Overview

In recent years, the rapid development of China's mold manufacturing industry has driven the consumption of mold steel, and China's mold steel production has ranked among the top in the world. However, the development of mold steel in China is currently very unbalanced, with an oversupply of mid to low-end products, while nearly 50% of high-end mold steel relies on imports, especially for large, precise, complex, and long-life mold steel with a greater gap. There are issues with the level of production technology, but the lack of guidance and standardization of relevant technical standards is also a key factor restricting the development of high-end mold steel in China.

From 2012 to 2016, relying on the standardization research capabilities of China's steel industry, organized by the Special Steel Branch Technical Committee of the National Steel Standardization Technical Committee, six national standards for high-end mold steel were developed. Among the six standards, three projects for hot work mold steel and cold work mold steel have been approved for release, and three projects for plastic mold steel have passed the expert review of the committee and been reported to the National Standardization Management Committee, which is in the approval and release stage. The formulation of these six standards reflects the current level of technology and industrial development in the field of mold steel in China, improves the standard system of mold steel in China, has strong operability, and has a certain degree of foresight. The standard technical indicators are advanced and reasonable, which can better regulate the current production of mold steel in China, and the overall level of standards reaches the international advanced level.

2 Main technical points

2.1 Scope of application, brand name, and chemical composition of the standard

The chemical composition specified in the six high-end mold steel standards formulated this time has all removed the allowable deviation of the chemical composition of finished steel products, that is, all specified in the standards are finished product components. At present, only ASTM A681 has abolished the allowable deviation in the chemical composition of finished steel products. Other national/organizational standards, such as BS4659, NF A35-590, DIN17350, ISO4957, JIS G4404, etc., have all specified the allowable deviation in the chemical composition of finished steel products. Cancelling the allowable deviation in the chemical composition of finished steel and further narrowing the range of composition is beneficial for improving the internal quality of the product.

2.2 Dimensions and appearance

The dimensions and appearance requirements of hot-rolled, forged round steel, square steel, flat steel, and machined steel involved in the standards for plastic mold steel and cold work mold steel shall comply with the relevant provisions of GB/T1299-2014 "Tool and Mold Steel". Steel plates, cold drawn steel bars, and silver bright steel bars shall comply with GB/T709 "Dimensions, Appearance, Weight, and Allowable Deviations of Hot Rolled Steel Plates and Strips" respectively GB/T 905 "Dimensions, Shape, Weight, and Allowable Deviations of Cold Drawn Round, Square, and Hexagonal Steel" and GB/T 3207 "Silver Bright Steel".

The forged round steel and square steel involved in hot working die steel do not comply with the provisions of GB/T1299-2014. The main consideration is that the size and appearance of forged round steel and square steel in GB/T1299-2014 comply with the provisions of GB/T908-2008 "Forged Steel Bars - Dimensions, Shape, Weight, and Permissible Deviations", while the allowable deviation of forged round steel and square steel in GB/T908-2008 consists of positive and negative deviations. In actual ordering, the vast majority of users hope that the allowable size deviation of the metallurgical plant is only positive. Therefore, in order to fully reflect the user's needs, this standard only allows positive deviations in the dimensions of forged round and square steel. The thickness tolerance of forged flat steel is consistent with the provisions of GB/T1299-2014, and the width tolerance is strictly controlled.

2.3 Smelting method and delivery status

While specifying commonly used smelting methods, the standard also stipulates that other smelting methods that can meet the requirements of the standard can be used.

2.4 Delivery hardness

All six standards have clear regulations on the delivery hardness of steel, and the specified hardness values are consistent with the provisions of the general standard GB/T1299-2014.

2.5 Macrostructure

The regulations for the microstructure of plastic mold steel and cold work mold steel shall comply with the strictest (group 1) regulations in GB/T1299-2014. The microstructure of hot work die steel has been strictly regulated based on Group 1 of GB/T1299-2014.

2.6 Non metallic inclusions

The qualification level of non-metallic inclusions is a key indicator of product quality. Among the six standards developed this time, the qualification level of non-metallic inclusions refers to the highest level provisions in GB/T1299-2014. Except for the consistency between the highest level provisions in "Plastic Mold Steel Part 1: Non alloy Steel" and GB/T1299-2014, the other five standards have been tightened based on the highest level, In particular, the "Hot Work Die Steel Part 1: Steel for Die Casting Dies" standard is consistent with the North American Die Casting Association NADCA # 207-2016 standard, reflecting the high level of product quality.

2.7 Decarburization layer

The plastic mold steel Part 3: Corrosion resistant steel is consistent with the provisions of GB/T1299-2014. The decarburization layer provisions in the standards for hot work mold steel and cold work mold steel comply with the provisions of the highest level (group 1) in GB/T1299-2014.

2.8 Microstructure

Hot work mold steels - Part 1: Steel for die-casting molds "require the inspection of the strip structure and annealing microstructure of the steel, and the inspection standards and qualification level shall be determined through consultation between the supply and demand parties. Compared with GB/T1299-2014, the requirements for the unevenness of eutectic carbides in Cold Work Die Steels - Part 1: High Toughness and High Wear Resistance Steels have been stricter for the unevenness levels of carbides in various specification ranges, and the unevenness requirements for flat steel have been clarified.

2.9 Ultrasonic testing

The Ultrasonic testing provisions of the standards for plastic mold steel and cold work mold steel refer to the provisions of GB/T1299-2014, and the qualification level basically adopts the provisions of the highest level (group 1). The standard Ultrasonic testing qualification level of hot work die steel is stricter than the highest level (group 1) specified in GB/T1299-2014

2.10 Surface quality

The surface quality of the six standards refers to the provisions of GB/T1299-2014, which have made clear requirements for different products.