New steel grades in China and their weldability development

With the advancement of welding technology, the welding structure is increasingly developing in the direction of high parameters and large-scale, which puts higher and higher requirements on the performance of steel for welded structure, not only has good comprehensive mechanical properties, but also has good Process performance (such as weldability). For steels used under special conditions, it is also required to have special special properties such as corrosion resistance, high temperature resistance and low temperature resistance. Therefore, the original carbon structural steel is far from satisfactory, which further promotes the rapid development and wide application of alloy structural steel. The quality of weldability is one of the main indicators for evaluating the performance of steel. Weldability, welding materials and the corresponding welding process are the three basic elements of welded alloy structural steel, and the three are inseparable. Therefore, if the weldability of each alloy structural steel and the supporting welding materials and welding process problems are not solved in time and effectively, it will directly hinder the promotion and application of this type of steel.

In order to continuously improve the weldability of alloy structural steel, foreign countries began to develop and produce micro-alloy controlled-rolled steel with good weldability from the late 1960s, and began to study the next generation of ultra-fine grain steel. The emergence of new steel grades has brought about major changes in the weldability of steel.

1 microalloy controlled rolling steel

The alloy structural steel produced in the past focuses on the properties of the steel itself, with emphasis on oxidative purification, processing and phase change heat treatment. However, less consideration is given to the weldability, which brings a lot of inconvenience to the welding technology and the welding production. In recent decades, foreign countries have paid special attention to fundamentally solving the problem of weldability of steel from metallurgy. In the past two decades, metallurgical workers in China have also paid attention to this. They have produced a number of pipeline steels with good toughness and excellent weldability through metallurgical measures using low-carbon microalloying and controlled rolling and controlled cooling. Bridge steel, steel for pressure vessels, etc., which have made new contributions to the development of alloy structural steel for welding. The production of new steel grades has the following characteristics:

1.1 Clean technology

The cleanliness of the steel will significantly improve the impact toughness of the steel, the crack resistance of the welded joint and the resistance to HIC, and the weldability will be significantly improved, and the corresponding welds must be cleaned accordingly.

1.2 fine crystallization technology

Fine grain strengthening can further reduce the carbon content of low-alloy high-strength steel and reduce the solid solution alloy elements, so that the impact toughness will be further improved. The impact toughness of pipeline steel and bridge steel produced by WISCO using the above technology has reached a high level, and the measured Charpy impact energy of the steel grade is above 100J at -40 °C.

1.3 Microalloy controlled rolling steel requirements for welding materials

1.3.1 Cleaning of weld metal

(1) Cleaning of raw and auxiliary materials for welding.

(2) Cleanliness in welding metallurgical reactions.

1.3.2 Microalloying of welding materials

The toughening of microalloyed controlled rolling steel requires high strength and toughness matching of the weld, which requires the weld metal not only to be cleaned, but also to be finely crystallized. However, the fine crystallization of the weld seam can not be achieved by the controlled rolling and controlled cooling process like the steel plate, and it can only refine the grain by alloying. Studies have shown that the presence of a large amount of acicular ferrite in the weld can significantly improve the toughness of the weld metal. This is because the ferrite needle is very small, the average size is about 1 μm, and the ferrite needle is freely oriented at a large angle. The grain boundary has a strong resistance to crack growth. Therefore, it is the key to improve the toughness of the weld metal by making the weld have a sufficient amount of acicular ferrite. The addition of various trace elements in the weld can inhibit the grain growth of high-temperature austenite on the one hand, and promote the formation of acicular ferrite in the weld. The inclusions in the weld are used as the nucleation core of acicular ferrite. It plays an important role in increasing the nucleation rate of acicular ferrite.

Among the many alloying elements of the weld, Ti, B, Re, and Al have a more important role. In addition, by optimizing the process parameters, controlling the cooling rate can also promote the formation of acicular ferrite.

It should be noted that manganese is the main addition element in microalloyed steel, generally added in an amount of 1.1% to 2.0%. The addition of manganese can not only improve the solid solution strengthening effect, but also reduce the transformation temperature of γ→α, thereby achieving Refine the role of ferrite grains. Properly adjusting the manganese content can transform austenite into acicular ferrite, which further enhances the toughness.

Introduction of 2 microalloy controlled rolling steel weldability

Microalloy controlled rolling steel has high toughness and good weldability due to its low carbon content, high cleanliness and fine grain. Since different application fields have different requirements on the performance of steel, the alloying is different and the weldability is also different. The microalloyed controlled rolling steels produced by Baosteel and Wuhan Iron and Steel are mainly pipeline steel, steel for pressure vessels, bridge steel, etc., and have been successfully applied in engineering. Among them, pipeline steel is the earliest example of microalloying and its application. It has been widely used in the United States, Japan and West Germany in recent decades. In recent years, China has also produced its own pipeline steel, which has gradually replaced imports and has been applied in pipeline engineering in China.

3 new generation of steel materials and their weldability

In 1998, China launched the “973 Project” of “Major Basic Research on Steel Materials” in the National Major Basic Research Development Plan. The ultimate goal of the study is to double the strength and life of “three types of” steels such as carbon steel, low alloy steel and alloy structural steel, which account for more than 60% of China's steel production. If a new generation of steel is used to replace 50% of the traditional steel in the three types of steel, the annual consumption of 1,500 tons of steel can be reduced, and the direct economic benefit is 45 billion yuan. The indirect economic benefits are more significant, such as reducing the construction of steel mills. Infrastructure investment in mines, reducing resource losses and pollution to the ecological environment.

At the same time, Japan launched the STX-21 "Super Steel Materials" scientific research project in 1997, investing 100 billion yen. The goal is to develop super steel that is twice as strong as existing steel materials in 10 years and is used for the upgrading of infrastructure such as roads, bridges and high-rise buildings. It mainly researches and develops 80MPa grade weldable ferritic-pearlite steel, 1500Mpa grade ultra high strength steel resistant to delayed damage and fatigue damage, ferritic heat resistant steel for supercritical pressure power generation equipment and steel for coastal areas.

South Korea also started a 21-century structural steel project in 1977, mainly researching and developing 800Mpa grade structural steel, 600Mpa grade weathering steel and 1500Mpa grade bolt steel.

3.1 Characteristics of a new generation of steel materials

A new generation of steel materials is characterized by a new generation of steel grades with ultra-fine grain, ultra-cleanness, high uniformity and more reasonable cost performance. Its strength and life are doubled compared with the original steel.

3.2 Weldability of a new generation of steel materials

Due to the extremely fine grain of the new generation of steel materials, the serious problems encountered during welding are the toughening of the weld and the grain growth of the heat affected zone. In China's new generation of steel materials project, the above-mentioned weldability problems are mainly solved for ultra-fine grain steel of 400Mpa grade and 800Mpa grade. It should be comprehensively solved from welding materials, welding methods and welding processes.

(1) Strengthening and toughening of weld metal.

(2) The tendency of grain growth in the heat affected zone.

4 Conclusion

With the improvement of metallurgical technology in China, the performance of new steel grades is continuously improved, which requires the development of high-quality welding materials to match and strengthen the toughness of the weld. As the weldability of the new steel grades continues to improve, the weld crack tends to decrease and the welding process is simplified. For the welding technology of this type of steel, it is mainly to develop in the direction of high efficiency and automation. For the major development of a new generation of ultra-fine grain steel, the strength and life are doubled without increasing the alloying elements. This is not only a major change in steel materials, but also a development of welding technology and welding materials. New opportunities and challenges.