Abstract : The characteristics of ultra-fine grain steel are introduced. For these characteristics, the weldability of ultra-fine grain steel is discussed, including HAZ performance, weld performance, HAZ and crack tendency of welds. 1 ultra-fine grain steel 2 Weldability of ultra-fine grain steel Next page Inorganic Fabric Fire Shutter Door Inorganic Fabric Fire Shutter Door,Inorganic Fabric Curtain Roller Up Shutter Door,Fireproof Rolling Shutter Door,Inorganic Fire Door Dongguan HengTaichang Doors Co, Ltd. , https://www.dghtcdoors.com
Key words : ultra-fine grain steel, weldability, HAZ, grain growth
1.1 Classification
In conventional steel, the grain size below 100 μm is called fine grain steel, which is a traditional fine grain steel. With the continuous advancement of metallurgical technology and production processes, the size of fine crystals has been shrinking, even reaching micrometers and submicrometers. The ultrafine grain steel mentioned herein does not include conventional fine grain steel.
According to the development process of ultra-fine grain steel and its size, it can be divided into the following categories.
(1) TMCP steel
Immediately after the controlled rolling, the steel produced by the cooling is accelerated, which is called TMCP (Thermo-Mechanical Control Process) steel. In the laboratory using the TMCP process, the grain size can reach several microns, but in actual industrial production, the resulting steel has a grain size of less than 50 μm and a minimum of 10 μm. This steel meets the needs of the oil and gas industry, which provides excellent weldability for high strength, high toughness and low carbon equivalent.
(2) A new generation of steel materials
Comprehensive experience in the continuous improvement of low-alloy high-strength steels, making full use of the advantages of alloying and technological advancement in production technology, developing a new generation of steel material products and conducting basic theoretical research. The main features of the new generation of steel materials that are currently in the development stage: under the conditions of full consideration of economy, the steel has the characteristics of high cleanliness, ultra-fine grain and high uniformity, and the strength is doubled compared with the commonly used steel. Life expectancy has doubled. High cleanliness means that the total content of S, P, O, N and H elements is less than 80×10-6, which not only improves the original properties of the steel, but also imparts new properties to the steel; ultra-fine structure, grain With a size between 0.1 and 10 μm, refining the grain is the only way to increase the strength without reducing the toughness or even the toughness; high uniformity means that the composition, structure and properties are very uniform and the fluctuation range is small. In the chemical composition-process-organization-performance relationship of steel, the dominant position of the organization is emphasized, that is, its ultra-fine microstructure exhibits excellent comprehensive performance.
1.2 Chemical composition and metallurgical characteristics
Fine-grained steel has low carbon and low carbon equivalent and low impurity content, which not only benefits its weldability, but also helps to improve other properties of steel, such as the toughness of HAZ and base metal in joints and hydrogen induced cracking (HIC). Sulfide stress corrosion cracking (SSCC) resistance, etc. Fine-grained steel also contains a small amount of microalloying elements such as Nb, V, and Ti. Its main purpose is to form carbon and nitrogen compounds, thereby effectively preventing grain growth. Due to the low S, P, N element content of fine-grained steel and the control of the added microalloying elements, the presence of nitride-forming elements will reduce free nitrogen, reduce the aging effect, and contribute to the improvement of toughness.
The metallurgical characteristics of fine-grained steel with high cleanliness and high uniformity are mainly aimed at how to improve its cleanliness, ie reduce the content of elements such as S, P, N, O and H. Its metallurgical and production process technology has been very large. Progress: The "three-off" (desiliconization, desulfurization and dephosphorization) process and the less slag smelting process of the hot metal established by the idea of ​​"segmented refining"; established to meet the requirements of H2S corrosion resistance of petroleum pipeline steel Hot metal bag Mg-Ca desulfurization pretreatment process, vacuum powder desulfurization process; refining outside the furnace; production process of defect-free continuous casting billet.
1.3 Process methods and strengthening characteristics
In order to obtain ultra-fine grain steel, various processes have been developed: repeated heat treatment under the same rapid heating conditions, metal powder mechanical grinding, controlled rolling, controlled cooling, TMCP, composite TMCP method, and the like. The use of production technology is the main means to obtain ultra-fine grains, and it is the determinant of excellent toughness and comprehensive properties of ultra-fine grain steel. Therefore, the difference between ultra-fine grain steel and traditional steel is that its chemical composition cannot be used for prediction. The strength of the steel.
Compared with conventional steel of the same strength, ultra-fine grain steel has the main feature of its chemical composition, which is low in carbon content, which is beneficial to improve its weldability. Therefore, the strengthening means does not increase the carbon content and the alloying element content, but Grain refinement, phase transformation strengthening, precipitation strengthening and the like are combined to achieve the purpose of improving the toughening. Grain refinement (including deformation refinement and phase change refinement) is the only method that can simultaneously increase the strength and toughness of steel, and thus becomes the best strengthening mechanism for ultra-fine grain steel. Precipitation strengthening using second phase particles is another strengthening mechanism used in ultrafine grain steel. Particles formed in austenite at high temperature are effective for controlling grain growth, but do not cause strengthening, strengthening particles. It is formed in austenite or ferrite at low temperatures, and dislocation and substructure strengthening is also an effective strengthening method.
The strengthening and toughening mechanism of ultra-fine grain steel is different from that of traditional steel. Therefore, the welding problem must be fully considered. There are two main problems: 1 due to its ultra-fine grain, the grain grows under the action of welding heat. The driving force is large, which will inevitably lead to severe coarsening of the HAZ grain, which will affect the performance of the entire joint and the performance of the base metal; 2 to obtain the welded joint with the same performance as the base metal, the welding material, the welding method and the welding process are reasonable. select.
Ultrafine grain steel and its weldability