What Are Steel Grades?

According to the amount of carbon in the steel, the structure of the carbon, whether it contains other alloys, and the amount of other alloys, steel can be divided into many types, and each different steel type has different uses.

Steel types are generally divided into the following categories, and their uses are briefly introduced:

Steel material introduction

As a dominant structural material, steel has always been one of the important indicators of a country's comprehensive strength. After 50 years of struggle, China's steel output has exceeded 100 million tons, ranking first in the world and becoming a large steel country, but it is still not a steel country. At present, the world's per capita steel occupancy is 150 kg per year, compared with 500 kg in industrial developed countries, but less than 100 kg in China. In order for our country to reach the world average level, steel output still needs to be doubled. If it is to reach the level of industrially developed countries, it needs to be quadrupled. This is unbearable for China's capital investment, resource supply, transportation, and environmental protection. For this reason, under the leadership of the Ministry of Science and Technology, in 1998, China launched the "973 Project" of "Major Basic Research on New-Generation Steel Materials" in the National Major Basic Research Development Plan. The ultimate goal of this research is to double the strength and life of the three types of steel such as carbon steel, low alloy steel and alloy structural steel, which account for more than 60% of China s steel output. 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 within 10 years, and is used for the replacement of infrastructure building materials such as roads, bridges, and high-rise buildings. Mainly research and develop 800 MPa easily weldable ferrite-pearlite steel, 1 500 MPa ultra-high-strength steel resistant to delayed damage and fatigue failure, ferritic heat-resistant steel for supercritical pressure power generation equipment and steel for coastal areas Wait. South Korea also started a 10-year 21st century structural steel project in 1997, mainly researching and developing 800 MPa structural steel, 600 MPa weathering steel, and 1,500 MPa bolt steel.

Steel characteristics

The new-generation steel materials are characterized by ultra-fine grains, ultra-cleanliness, high uniformity, and a more reasonable new-generation steel grade. Its strength and life are doubled compared to the original similar steel. Ultrafine grain means the grain size reaches 0.1 ~ 10um; Ultra-cleanliness means that the total content of impurities such as S + p + O + N + H in steel is reduced to 150 ppm; High uniformity refers to the composition, structure and properties Highly uniform and emphasizes the dominance of even organization. Through the comprehensive application of the above technologies, the strength and toughness of steel can be greatly improved. The new generation of iron and steel materials is in the research and development stage. By 2000, research on 400 Mpa and 800 Mpa ultrafine grain steels in China had made substantial progress. Among them, 400 Mpa grade ultrafine grain steel is realized by refining grains and purifying on the basis of Q235 steel; and 800 Mpa grade ultrafine grain steel is refined on the basis of X65 pipeline steel Granulation and purification. It can be expected that the successful research and development of this type of steel materials will revolutionize the steel materials in China.

Steel weldability

Due to the extremely fine grains of the new generation of steel materials, the serious problems faced during welding are the strengthening and toughening of welds and the growth of grains in heat affected zones. In China's next-generation steel material projects, the above-mentioned weldability problems are mainly addressed for 400 Mpa and 800 Mpa grade ultra-fine grain steels. It should be comprehensively solved from many aspects such as welding materials, welding methods and welding processes.

Strengthening and toughening of weld metal

Weld metal is mainly used to control the structure of the weld to achieve toughening. For 400 Mpa grade fine-grained steel, as long as the weld structure is adjusted to obtain acicular ferrite, the ideal strength and toughness can be obtained. For 800 Mpa grade fine-grain steel, it is difficult to match the weld metal to the base metal. Because as the strength level increases, the carbon equivalent increases, and the cold cracking tendency of the weld increases. In order to strengthen and toughen welds and avoid cold cracks, it is necessary to develop welding materials that match the properties of the base metal, but there is no mature experience in this regard. The welding material to be developed in South Korea to match the 800 Mpa grade is a non-preheated ultra-low carbon bainite welding material.

Grain Growth Tendency in HAZ

For ultrafine grain steels, regardless of the 400Mpa or 800 Mpa grades, due to the extremely small grains, there will be a serious tendency for grain growth during welding. Grain growth not only causes embrittlement of the HAZ, but also causes softening of the HAZ. To solve this problem, welding methods such as laser welding, ultra-narrow gap GMA welding, and pulse MAG welding should be used. ^[2]