High-strength steel is of great significance to the development of metallurgical industry in the past 10 years. At present, it can meet the requirements of strength, plasticity, toughness, formability and weldability, making the annual output of low-cost steel account for about 10% of the world's structural steel.
Review the development of high strength structural steel. In the early 20th century, structural engineers used a single type of steel, which was called "low carbon steel", meaning "low carbon, soft, easy to process". Low carbon steel did not intentionally use alloying elements other than carbon. Strengthened, Mn in steel is used for deoxidation, and also contains stable sulfides. It is generally considered that the chemical composition range of low carbon steel is 0.1% to 0.25% C, 0.4% to 0.7% Mn, 0. 1% to 0.5% Si, the balance being S, P and other elements. The yield strength of mild steel is about 250 MPa.
Prior to 1940, the main requirement for structural steel was to increase tensile strength. In order to obtain higher strength, the C content is increased to 0.3%, and the Mn content is about 1.5%. This steel has a wide application range and cannot meet the requirements of modern high-strength structural steel. It has the following disadvantages: (a) steel with a thickness of 30 mm, the yield strength is too low, can only reach 360 MPa; (b) the yield strength decreases greatly when the thickness is increased; (c) the high C, Mn content makes the steel The welding performance is deteriorated; (d) the fracture toughness is lower than that of the low strength steel.
Competition with other materials, especially reinforced concrete, has contributed to the development of structural steel. In order to ensure market share, many simple production and refining technologies have been developed, which make the weldability and notch impact toughness of steel more and more high. The investigation and research on the metal fracture behavior of bridges and ships, especially the famous "freewheel" during the period from 1942 to 1949, laid the foundation for metal fracture mechanics.
The properties of the steel are mainly improved by the following processes: limiting the C content; improving the cleanliness, including reducing the S and P contents; Al deoxidation, microalloying, normalizing rolling and later controlled rolling. The above process refines the structure of the steel and improves the strength and toughness of the steel.
After 1960, the revised standards introduced some new steel numbers: the French DIN17102 standard, the British BS4360 standard, and the French NFA35-504 standard. These national standards became the basis of the later European unified standard, fine grain steel EN10113. The research work has established the principle of determining the resistance of steel to brittle fracture. These work used the usual notched impact test results and KAc values ​​for the fracture mechanism.
Under the following conditions, the material needs to have higher toughness: engineering structure under fatigue load; low service temperature; high yield strength; thick section steel. The new European architectural design specification includes the temperature at which the minimum impact energy is 27 J, and the corresponding steel number is selected accordingly.
The development of the marine industry in the Arctic has greatly contributed to the development of structural steel, which requires the assembly of engineering structures under severe low temperature conditions. Due to the limited carrying capacity of marine structures and the development of deep-sea oil and oil and gas fields, weight reduction has become a top priority for marine structures, and high-strength steel has become the focus. Special standards have been developed for steels used in these harsh environments, such as EN10225 or API2MT2.
Table 1 summarizes the characteristics of modern structural steel grades.
1. Production Process
Long strips can be smelted by oxygen converter, or can be smelted by electric arc furnace, and more continuously produced by continuous casting process. Continuously cast into billets, billets and slabs, as semi-finished products, have recently been cast into I-beam shapes. According to the 1964 test, near-final casting of I-beam was started in 1968. The technology was later adopted by companies such as Kawasaki, Japan Steel and Japan Steel, Japan's Newcoma Yam-ato, Chaparrol, Northwest Steel, and Europe's ProfilARBED.
Different types of semi-finished products for hot rolled products of the European PrefilARBED Group. The I-beam has a maximum width of 1118 mm or a maximum thickness of 125 mm. Hot-rolled I-beams account for a large proportion of structural steel. Therefore, the following discussion focuses on I-beams, but the main principles apply equally to other steels of equivalent thickness.
The molten steel produced by ProfilARBED in large I-beams is smelted by electric arc furnace and continuously cast into I-beam. After continuous casting, before the initial rolling, the I-beam is reheated in the stepping furnace, rolled by two reversible universal mills and rolled by the universal mill. The mill has different hole shapes and different sections of the rolled product.
1.1 Traditional rolling process of large I-beam
The semi-finished product is heated to about 1250 ° C and rolled through 15 to 20 passes. For the ingot, it needs to be heated to 1300 °C, and it may need to be rolled by 40 passes. The pass reduction rate of the I-beam is 4%-20%, the finish rolling temperature is higher than 1000 °C, and the temperature on the I-beam. Uneven distribution: the root and waist joints have the highest temperature, the waist has the lowest intermediate temperature, and the temperature difference is related to the size of the I-beam. The maximum temperature difference can reach 100 °C. It is rolled according to the process and graded according to the ASTM standard, and the grain size of the I-beam having a thickness of 40 mm is 7 grades.
In order to refine the microstructure of the steel, Ti-Nb microalloying can be used to make the austenite grains relatively small (up to 50 μm instead of 200 to 300 μm) during reheating, and the recrystallized structure is also relatively small. Laboratory simulation results show that the required microstructure can be obtained with a reduction of 15% per pass, and the mechanical properties reach 50Ksi (tensile strength ≥ 50Ksi, equivalent to S355). Table 2 shows the chemical composition of the 50Ksi I-beam.
This ingredient design has been successfully used in industrial production. The high temperature during hot rolling means that Nb in the steel remains in a solid solution state, and even at the finish rolling temperature, no carbide of Nb precipitates. When Nb exists in a solid solution state in steel, the ferrite grains are refined by delaying the phase transformation, and a certain amount of bainite is obtained, thereby increasing the strength of the steel. The typical structure of the steel is about 80% ferrite, the remainder being bainite and pearlite. Under the same rolling process conditions, the grain size of C-Mn steel is 7 grades according to ASTM standards, while the grain size of Nb-containing steel is 9 grades. In the phase change process or after the phase change, if NbC precipitates are formed in the ferrite, the strength of the steel can be further improved. The grain can only be refined in a limited manner by the conventional rolling process. For steels with a strength higher than 50 Ksi or a thickness greater than 20 mm, a controlled rolling process must be employed to meet the toughness requirements.
1.2 normalizing heat treatment
The normalizing is above the Ac3 phase transition point (usually Ac3 + 50 ° C). The hot rolled state of S355 steel is ferrite-pearlite. The purpose of normalizing is to refine the structure, make the structure uniform and improve the toughness of the steel.
The degree of organizational refinement is related to the original organization. For steels that cannot be controlled for rolling, especially for thick-section steels, a fine graining effect can be achieved by normalizing. For thin-section steels, normalizing may not achieve the purpose of refinement. In this case, the rolling process can be considered as a controlled rolling process, which is commonly referred to as "normalizing rolling", and the structure of the hot-rolled state and Performance is similar to tissue and performance after normalizing.
Nb is generally used to increase the tensile strength of normalized steel, and Nb can prevent austenite grain growth and enlarge the γ phase region. This effect is particularly pronounced in Si-containing steels. The normalizing temperature is between 900 ° C and 1050 ° C, and the Nb content is 0.02% to 0.04%, which is sufficient to achieve a grain size of 10 grades. In contrast, when the Si-containing steel does not contain Nb, the grain size is 7 when the normalizing temperature is 1000 °C.
Nb carbonitrides, like Al nitrides, can prevent austenite grain growth at 1050 ° C. This effect is particularly important, even in furnaces where the furnace temperature is not uniform, fine ferrite can be obtained. Body-pearlite structure.
The thickness of the hot-rolled I-beam is 7 to 9 grades. After normalizing, the grain size of 910 ° C × 30 ′ reached 11 grades. vT27>40 °C. After normalizing, vT27 <-45 °C, after normalizing, the strength decreased slightly.
Based on the above test results, the alloy design principle of S355 steel was formulated.
In order to meet the welding performance requirements, it is necessary to have a lower carbon equivalent. Compared to controlled rolled steel, S460 steel has a higher carbon equivalent, which limits its production. It must also be pointed out that especially for thin section steels, heat treatment tends to cause deformation. After deformation, it must be straightened with a straightener.
1.3 controlled rolling process: controlled rolling
When controlled rolling is performed in the low temperature region of austenite, the Nb-containing steel can satisfy the requirements of strength and toughness. During the rolling process, the austenite is first deformed at 1050 ° C or higher to refine the austenite grains. If a total reduction ratio of 70% is given, fine austenite grains can be obtained by static recrystallization after each pass rolling. Then, the final rolling is carried out until the temperature is below 900 °C. The recrystallization in the Nb-containing steel is very slow, and the austenite grains become a cake shape, thereby effectively refining the crystal grains.
The high free nitrogen content of the Si-containing steel smelted by the electric arc furnace is very significant. In the above steel, the total nitrogen content is 100 ppm or more. In the air-cooled steel containing Si, the relationship between the free nitrogen content and the total nitrogen content is N free = 0.43 N total. The toughness of Si-containing steels is related to the free nitrogen content. When the free nitrogen content is 32 to 33 ppm or less, vT40J is about -10 °C. Once the free nitrogen content exceeds 35 ppm, vT40J quickly reaches >+30 °C.
There are two ways to improve the toughness: one is to reduce the finish rolling temperature from 960 °C to 870 °C, and the ferrite grain size is increased from 7 to 9. This process significantly improves the toughness of the steel; System to form nitrides and reduce free nitrogen in steel. The combination of the two, the steel's toughness is best.
Al is commonly used to reduce the free nitrogen content in steel, and elements such as Ti, Nb, and V can also be used. V and Nb have advantages over Al and Ti, and they do not cause continuous casting problems such as nozzle clogging or defects during continuous casting. The determination of the free nitrogen content can be used to determine the effect of the nitride forming element on the fixed nitrogen. From this, the calculation formula of Al equivalent is determined as Aleq = Al + 2Ti + Nb + V (%). The alloying element is selected according to the mechanical properties of the I-beam, and the relationship between vT40 and yield strength is obtained. Al, Ti, Al+Ti, Ti+V are used for microalloying. When the yield strength is about 320 MPa, the vT40 is between -60 and -70 °C. The strength is similar to that of the C-Mn steel used for comparison. There is no precipitation in the C-Mn steel. hardening. When the yield strength of Nb microalloyed steel is 375 MPa, vT40 = -55 °C, Nb produces a significant precipitation hardening effect and refines the structure. When Ti+Nb is added in combination, the precipitation hardening effect is alleviated due to the interaction of TiN and Nb.
Similarly, the addition of Ti to the V-containing steel also reduces the hardening effect of V because Ti fixes nitrogen and reduces the hardening effect of the nitride of V.
Although controlled rolling can meet the requirements of strength and toughness, there are some disadvantages. Reducing the finish rolling temperature increases the load on the mill. Many mills are not designed with this added load in mind. Compared to C-Mn steel, the presence of Nb prevents recrystallization, which increases the load on the mill. Since there is a temperature-to-war process in the controlled rolling process, the rolling time is increased and the production efficiency is lowered.
The chemical composition, carbon equivalent and Nb content required to achieve the strength corresponding to a certain thickness of S355 I-beam. Different chemical compositions can be used to achieve the desired strength.
Compared with the alloy design principle, the C content is increased by 0.06% and the Nb (V) content is lowered. These two components have the same strength, but have a great influence on the toughness. The C content is low, and the toughness in the longitudinal direction is improved when Nb is microalloyed, but as the thickness is increased, it is more difficult to satisfy the toughness requirement.
The EN10225 standard for marine welded structural steels proposes more stringent toughness requirements, which involve toughness requirements in the transverse or thickness direction. For lower carbon content steels, lateral impact energy requirements are increased, and toughness in thickness direction In terms of, it can be achieved by lowering the S content.
The controlled rolling process is also used to produce S460 steel. Of course, the controlled rolling process cannot produce the largest thickness range.
For thicker steels, the rolling temperature is increased and the cooling rate after rolling is reduced, resulting in coarsening of the structure. In order to meet the strength requirements, the alloy content must be increased. Due to the requirements of welding performance and the limitation of carbon equivalent, it is not possible to produce S460 steel with a thickness of 50 mm or more.
1.4 controlled rolling process: accelerated cooling
To overcome the limitations of controlled rolling, ProfilARBED, in conjunction with the Metallurgical Research Center and the British Steel Company, developed a process for accelerated cooling after rolling.
In the quenching + self-tempering process, after the last rolling, the entire surface of the I-beam is sprayed with water to chill. Before the core is quenched, the water spray is stopped, and the outer layer of the I-beam is self-tempered by the heat transferred from the core to the surface layer. Figure 5 is a schematic illustration of the heat treatment process. From the final roll directly into the cooling frame, the temperature is about 850 ° C, the surface of the entire workpiece after cooling, the temperature of self-tempering starts ≥ 600 ° C. In general, the prerequisite for the quenching + tempering process is that the temperature of the entire section of the I-beam is uniform, so that during the rolling process, the highest temperature part of the I-beam, that is, the joint between the leg and the waist, is required. Selective cooling. Figure 6 is a schematic illustration of the process. This technique eliminates temperature differences across the I-beam section.
The Fukuyama Plant of Nippon Steel Pipe Co., Ltd. has developed a process similar to Online Accelerated Cooling (OLAC). OLAC has been in use in steel sheet production since 1980. For thick section steels, this technique encounters technical difficulties in the application of such materials due to the complex cross-sectional shape. Since the thermal deformation is difficult to overcome and the deformation-free cooling is employed, the product quality is difficult to control because the size and steel number of the product are very dispersed. The Japanese steel company developed an accelerated cooling device for large I-beams.
The hot-rolled microstructure obtained by different rolling processes is as described above. The grain size obtained by conventional rolling is 7 grades, the grain size during controlled rolling is about 9 grades, and the grain size can be accelerated by accelerated cooling. Up to level 11. Such a fine structure also has good toughness at a very low temperature. According to the special standard of EN10113, the transition temperature of 41J at a thickness of 125 mm is below -50 °C.
When the structure is refining by chilling, the action of the Nb refining structure is not exhibited, and in the hot-rolled steel, the addition of Nb does not improve the toughness. However, in high-strength steel, the weldability is improved by adding Nb to lower the carbon equivalent, which is especially important for thick steel.
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