JFE develops high-performance H-shaped steel fonts using TMCP technology

Japan's JFE has developed high-performance H-beams using advanced TMCP processes.

Japan Steel Engineering Holdings Co., Ltd. (hereinafter referred to as JFE) developed high-performance H-beams using advanced thermo-mechanical control technology (TMCP). In order to produce high-strength and high-toughness H-beams, it is important to obtain a refined bainite microstructure through reasonable alloy design, ideal hot rolling conditions and accelerated cooling after hot rolling. JFE recently developed high-performance H-beams, such as 520MPa grade (tensile strength) H-beams for high-rise buildings, and 490MPa grade H-beams with excellent low-temperature toughness, such as excellent toughness, weldability and seismic performance. Alloy design and profile steel accelerated rapid cooling facilities that achieve microstructure refinement under specific hot rolling conditions are key factors in the development of high performance H-beams.

The market needs TMCP for H-beam

TMCP is a general term for the technology of air cooling, controlled cooling and accelerated cooling in the hot rolling process based on the control of heating temperature, rolling temperature and reduction.

On the one hand, in recent years, steel structures have been developed to high-rise, large-scale and larger spans, and therefore H-beams are required to be developed to larger sizes. In order to meet this market demand, JFE developed and produced a large fixed-size H-beam (hereinafter referred to as SHH steel) with a web height of 1000 mm. On the other hand, in order to make the design of the building more efficient and energy-saving, it is necessary to minimize the thickness and weight of the steel structure, which puts higher requirements on the design strength of the steel member.

A strong earthquake can seriously damage the beam end joints of steel structures. Therefore, there is a need for high quality steel having a low yield ratio, high toughness (including welded portions), and excellent weldability.

Thick steel plates are used for box columns, other columns, and steel pipes as structural columns. These steel plates can be produced by TMCP, and high-strength, high-performance steel plates have been developed using advances in hot rolling processes and accelerated cooling technologies. TMCP is also an effective technology for achieving high strength H-beam production. However, since the complex shape and various specifications of the H-beam are formed during the hot rolling process, it is necessary to design a TMCP technique dedicated to H-beam rolling different from the TMCP technology of the steel sheet.

TMCP technology innovation in H-beam

H-beam rolling characteristics and austenite recrystallization behavior. In the H-beam rolling process, in order to ensure the formability during the pass rolling and the universal rolling process, the material is heated to a temperature of 1250 ° C or higher, which is higher than the heating temperature of the sheet rolling. At this high temperature, austenite grains will grow rapidly. Moreover, in the H-beam hot rolling process, the reduction and total compression ratio of each pass are smaller than that of steel sheet rolling. Therefore, in order to ensure ductility and toughness, it is particularly important to sufficiently refine the initial austenite grain size during hot rolling.

The niobium-containing steel exhibits a mixed microstructure composed of austenite fine crystals and coarse crystals because the recrystallization behavior of austenite is inhibited by niobium. If accelerated cooling is performed at this time, bainite coarse crystals are formed, which reduces the ductility and toughness of the material. The inhibition of austenite grain growth by fine precipitates dispersed in steel is an effective way to promote the further refinement of austenite.

TMCP technology for H-beam rolling. In order to promote the refinement of the initial austenite grains and the recrystallization of the austenite phase during hot rolling, it is necessary to design a suitable chemical composition. Although niobium is a useful element in TMCP steel, the amount of niobium added and the rolling schedule must be carefully selected when producing H-beams. In hot rolling, the amount of compression in the high temperature zone is first ensured to ensure sufficient recrystallization of the initial austenite coarse crystals, followed by rapid cooling to produce high quality H-beams of high strength, high ductility and high toughness.

To study the strength and toughness of traditionally controlled rolled and TMCP steels, JFE simulated the H-beam rolling process in the laboratory. The comparison between the microstructure of TMCP steel and conventionally controlled rolled steel shows that the microstructure of conventionally controlled rolled steel is ferrite + pearlite structure, while the microstructure of TMCP steel is fine bainite structure. Although TMCP steel is at the same level as conventionally controlled rolled steel in terms of strength, TMCP steel has better toughness.

Low yield ratio H-shaped steel has good comprehensive performance

Chemical composition and production conditions. The typical chemical composition of the low-ratio ratio SM520 grade SHH steel is shown in Table 1. It has the same carbon content and carbon equivalent as the general 430MPa grade (tensile strength) steel. The production process is: after the temperature is maintained at a temperature above 1250 ° C, hot rolling is carried out at a high temperature (taking into consideration the reduction ratio and rolling temperature), and then rapidly cooled by a profiled steel accelerated cooling device (Super-OLACS). The steel grade produced by JFE is: H900 mm × 400 mm × 19 mm × 40 mm and H 1000 mm × 400 mm × 16 mm × 32 mm fixed-size H-section steel.

　　 Table 1 Typical chemical composition (% by mass) of SHH steel

　　 Element C Si Mn PS Ti and other carbon equivalents

　　 Content 0.17 0.33 1.28 0.020 0.003 — 0.40

Material properties. The microstructure of the H-shaped steel having a flange thickness of 40 mm at a portion of the flange 1/6 width to 1/4 width is a fine bainite structure. Tensile test results and Charpy impact test results show that the H-shaped steel flange 1/6, chamfer and abdomen have achieved high strength, the yield ratio is lower than 80%; Charpy impact absorption is 200 Above the coke, the base material of the newly developed H-beam has good strength and toughness.

The H-shaped steel is protected by CO ₂ gas. The y-slope welding crack test is carried out according to the JIS Z3158 standard under the preheating temperature of 5 ° C and a humidity of 60%. The results show that under the preheating temperature of 5 ° C, there is no Welding cracks show good weldability. According to the JISZ3101 standard, the highest hardness test of the heat affected zone of short bead welding shows that the maximum hardness of the weld heat affected zone is less than HV350 when the weld length exceeds 20 mm, and it has good welding performance that can meet the Japanese building construction standard (JASS6). .

JFE used CO ₂ gas shielded welding to weld the H-beam with a flange thickness of 40 mm for multi-layer surfacing to check the performance of the welded joint. The welding material is MG-56 grade (diameter 1.2 mm). The experimental conditions are no preheating, the maximum temperature between the passes is less than 250 °C, and the welding of 9 layers and 16 passes is performed. The input heat of the welding is 3 kJ/mm. The test results of the welded joint showed that no harmful welding defects such as poor fusion and crack were found in the welded portion. At the same time, the fracture strength is greater than 550 MPa, and the phenomenon that the fracture occurs on the base metal indicates that the H-shaped steel has good weld joint strength. In addition, the Charpy impact test results of welded joints show that the weld metal, weld line and weld heat affected zone have good Charpy absorbed energy values ​​above 100 joules.

At present, low yield ratio SM520 (tensile strength) SHH steel has been applied to high-rise buildings in Japan.

Low temperature toughness H-beam for cold environments

Chemical composition and production process. The typical chemical composition of SM490Y grade (tensile strength) H-beam is shown in Table 2. In order to meet the low temperature toughness including the welded part, JFE applied the heat-affected zone high toughness technology (JFEEWEL) to design its composition. Using the advanced TMCP process, JFE manufactures SHH steel with a maximum size of H918 mm × 303 mm × 19 mm × 37 mm and a maximum flange thickness of H900 mm × 400 mm × 19 mm × 40 mm, and adds it. Conventional H-beams with a microalloying element such as Nb, V and Ni (with a flange thickness of 24 mm) were compared.

Table 2 Typical chemical composition (% by mass) of SM490Y grade H-beam

　　 C Si Mn PS other

　　 Traditional controlled rolling conditions 0.15 0.35 1.45 0.015 0.005 Ni, Nb, V

　　 TMCP condition 0.13 0.27 1.56 0.017 0.003 TiN treatment

Material properties. The strength and toughness test results of traditional H-beam and TMCP steel show that despite the large flange thickness of TMCP steel, the high strength of SM490Y grade (tensile strength) is obtained, and the Charpy impact energy at -40 °C reaches 200 joules. Above, it has excellent low temperature toughness at a low temperature ductile-brittle transition temperature of less than -50 °C.

JFE uses YGW-23 grade (diameter 1.2 mm) welding material, and uses MAG welding (molding active gas shielded arc welding, shielding gas 80% Ar+20% CO2) to carry out 7-layer 13-pass welding of the H-beam. The maximum welding input heat is 3 kJ/mm, and the temperature between passes is less than 350 °C. The final welding head test results show that the fusion line and the welded heat affected zone have a high Charpy absorption energy value of more than 200 J at a low temperature of -40 ° C. The H-shaped steel (including the welded portion) has excellent low temperature toughness.