Materials Science Forum Vols. 783-786 (2014) pp 1046-1052 © (2014) Trans Tech Publications, Switzerland doi:10.4028//MSF.783-786.1046Research and development of a yield strength 400 MPa class structural steel plate with enhanced weldability Yu Zhang*, Xiaobao Li, Xin Pan(Institute of Research of Iron and Steel, Shasteel, Jinfeng, Zhangjiagang, Jiangsu, 215625, China) *Corresponding author: zhangyu02@ Keywords: Structural steel plate, high heat input welding, heat-affected zone, intra-granular nucleated ferrite, impact property;Abstract: A 400 MPa yield strength structural steel plate with enhanced weldability was produced by using advanced steel making technology and thermo-mechanical controlled processing technique. A microstructure consisting of acicular ferrite (3~8 m) and polygonal ferrite was observed in the rolled plate, which exhibits a yield strength ≥ 420 MPa, tensile strength ≥ 560 MPa, elongation ≥ 26 % and charpy impact toughness ≥ 300 J at -40 °C. Three-wire flux copper backing submerged arc welding with heat input of 230 kJ/cm was applied to butt weld the 36 mm thick plate, and defect-free joint with satisfactory mechanical properties were produced. The coarse grain heat affected zone (CGHAZ) contains mostly intra-granular nucleated ferrite plus a few grain boundary ferrite and ferrite side plate, and shows charpy impact toughness ≥ 90 J at -40 °C. The enhancement impact toughness of CGHAZ resultant from high heat input welding is due to improvement of intra-granular ferrite formation induced by Ca and Ti containing oxides and sulphides. 1. Introduction Steels with yield strength over 400 MPs are getting increased application for shipbuilding and offshore platform construction for increasing capacity [1-3]. Welding heat input for on-site fabrication is strictly controlled below 50 kJ/cm for ensuring low temperature impact property of the weld joint. For conventional steel grades, the impact property of the heat affected zone (HAZ) will deteriorate with increasing heat input due to the formation of brittle bainitic structure [4-7]. Welding methods with high heat input of 80~200 kJ/cm, such as electro-gas welding and multi-wire submerged arc welding which enable one-pass welding of 40 mm thick plate, were employed by the industry for improving construction efficiency and cost reduction [8-10]. It is obvious that the lack of high quality steel plate limits the efficiency improvement of shipbuilding. There are some activities aiming to develop the steel plate with enhanced weldability, and some promising results were reported [11-13]. However most of them are laboratory trial results and lack of verification of mill facilities. In this paper, microstrucrtural characteristics, mechanical property and weldability of a 400 MPa yield strength class steel plate produced by industrial mill facilities were reported. 2. Experimental procedure 2.1 Industrial production of the steel plate The alloy design is basically low carbon and low carbon equivalent type. Steel-making is conducted on a 180t converter-ladle fining-RH, and finally continuous casted into a with a thickness of 220 mm, and the measured composition includes 0.05%C, 0.15%Si, 1.45%Mn, 0.006%P, 0.004%S, 0.001%B, and minor Ti and Ca.All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, . (ID: 112.25.149.196-25/04/14,11:07:47)Materials Science Forum Vols. 783-7861047The hot rolling is carried out on a four-high 5 m width heavy plate mill with accelerated cooling system (MULPIC-AcC). After reheating the slab at 1200 °C for two hours, the slab was subjected to a thermo-mechanically controlled rolling and accelerated cooling process (TMCP), and the key parameters includes: 1) rough rolling above 1000 °C with a thickness reduction ratio ≥50 %; 2) finish rolling below 880 °C with a thickness reduction ratio ≥60 %; and 3) the rolled plates should be cooled to a temperature between 400 and 500 °C with a cooling rate 10~25 °C/s. 2.2 Microstructure and mechanical property characterization Microstructure observation was conducted on samples after polishing, etching in 4 % nital solution by using optical microscopy and field emission scanning electron microscopy (FE-SEM, JSM 7100F). Thin foils for transmission electron microscopy (TEM,JEM 2100F) examination were prepared using standard procedures, and then subjected to twin jet electro-polishing using an elecroyte of 5 % perchloric acid, 35% butoxy ethanol and 60% methanol. The hardness values are in Hv using a load of 49 N and a dwell time of 5 s. Charpy V-notch (CVN) impact tests were performed on standard samples (10×10×55 mm) on a 450-J instrumented pendulum impact tester. Mechanical properties at center thickness (t/2) and quarter thickness (t/4) of the plate were measured. Round tensile samples with 8 mm gage diameter were machined and tested at room temperature at a cross-head speed of 5 mm/min by a 250-kN machine. 2.3 Welding tests Flux copper backing submerged arc welding (FCB-SAW) was applied to butt weld the industrially produced 26 mm thick plate without preheating and post weld heat treatment. Groove of single V of 50 ° and a gap distance of 4 mm was used. Three solid wires with diameters of 4.8 and 6.0mm was selected, which have a nominal composition of 0.08%C. 0.35%Si, 1.40%Mn, 0.15%Mo, 0.011%P, 0.012%S, and other residual elements at resultant all weld metal. The detailed welding parameters were listed in Table 1. The flux for SAW contains a nominal composition (weight percentage) of 15% SiO2, 18% Al2O3, 5% CaO, 22% CaF2, 32% MgO and others. The welding is conducted without preheating and post heat treatment, and linear heat input during welding is calculated to be 230 kJ/cm. Table 1 FCB-SAW ParametersCurrent (A) I1(Φ4.8mm) 1280 I2(Φ4.8mm) I3(Φ6.0mm) 1180 1100 Voltage (V) U1 35 U2 40 U3 42 Wire distance (mm) W1→W2 35 W2→W3 120 Travel speed Heat input (cm/min) (kJ/cm) 37 2303 Results and discussion 3.1 Microstructure and property of the hot rolled plate Fig. 1 shows the optical microstructure of the hot rolled 36 mm thick plate, which contains 50~60% volume fraction of quasi-polygonal ferrite (QPF, 10~25 µm) and acicular ferrite (AF, 3~8 µm). The t/4 shows a finer structure than the t/2, and this is the common feature of TMCP steel plates [14,15]. Table 2 lists the tensile property and CVN impact properties of the hot rolled plate. The plate exhibits yield strength (YS) ≥ 405 MPa, tensile strength (TS) ≥ 528 MPa and elongation (El) ≥ 27 %. The plate shows an absorbed energy ≥ 300 J at -40 °C and ≥ 200 J at -60 °C, and the ductile to brittle transition temperature is below -70 °C. This suggests the excellent low temperature of the rolled plate.1048THERMEC 2013Fig. 1 Optical microstructure of the hot rolled 36 mm thick plate Table 2 Mechanical properties of the hot rolled plate YS./MPa t/4 t/2 415 405 TS/MPa 542 528 El./% 27 28 RA./% 83 82 CVN impact property / J vE0 °C 393, 388, 378 319, 325, 339 vE-20 °C 388, 385, 377 303, 321, 327 vE-40 °C 360, 327, 348 310, 311, 307Fig. 2 shows the typical morphology of the fracture surface after impact testing. Dimples with varied size (2~60 µm) and depths are observed, which suggests the ductile fracture behavior. The excellent match of strength and low temperature impact toughness is contributed from two points: 1) refined PF and QPF grain structure resulted from TMCP process; 2) high fraction of sub-grain boundaries produced from low temperature rolling with high thickness reduction ratio.Fig.2 Typical SEM micrograph of fractured surface of the plateProcedures like cold leveling, rolling bending, press forming is inevitable during fabrication of larger steel structures. Strain aging occurs that results in increased strength, decreased ductility and toughness. The effect of strain aging on the above properties is concerned by fabrication unit and should be examined for insurance of structure safety. The CVN testing was therefore conducted on the plate experienced varied strain aging conditions at -40 °C, and the properties were summarized in Table 3. It is observed that the plate retains sufficient toughness after strain aging.Materials Science Forum Vols. 783-7861049Table 3 CVN Impact property of the plate after strain aging treatment with a soaking time of 1 hour CVN impact toughness at -40 °C Location t/4 t/2 Strain 2.5 % Aging at 250 °C 326, 320, 322 331, 333, 338 Aging at 570 °C 307, 317, 339 350, 340, 360 Strain 5.0 % Aging at 250 °C 233, 315, 288 327, 344, 331 Aging at 570 °C 297, 327, 331 338, 336, 3303.2 Microstructure and impact property of the FCB-SAW weld joint Three-wire FCB-SAW with a heat input of 230 kJ/cm was applied to the 36 mm thick plate, and defect-free weld joint was produced, as shown in Fig. 3. Hardness profile across the weld joint was shown in Fig. 4, and it is clearly observed that heat-affected zone (HAZ) has the lowest hardness value across the joint. Moreover, the lowest hardness value falls in the region adjacent to HAZ boundary line, which shows a value of HV 10 lower than their adjacent BMFig. 3 Cross-section of the FCB-SAW weld jointFig. 4 Hardness profile across the weld jointFig.5 shows microstructures of FL, FL+1mm, FL+3mm and FL+5mm Hardness were observed along the line b at Fig.5, which corresponds to the center thickness of the weld joint. FL contains grain boundary ferrite (GBF), ferrite side plate (FSP) and intra-granular nucleated ferrite (IGF), and the prior austenite grain size was estimated to be 100~120 µm. A much finer gain structure was found in FL+1 mm and FL+3 mm, which consists of PF, IGF and few bainite. FL+5 mm shows a mostly PF grain structure, which corresponds to sub-critical HAZ. The weld joint fails at HAZ boundary region during transverse tensile test, and shows a tensile strength of 516 MPa and elongation of 20%. Both transverse tensile test and hardness profile proves that the HAZ is the weakest part of the weld joint. Table.4 lists the CVN impact property of weld joint, and it is clear that fusion line (FL) has the lowest value than other regions because of its coarse grain structure. The FL exhibits an absorbed energy ≥ 239 J at -20 °C, ≥ 121 J at -40 °C and ≥ 97 J at -60 °C. Fig. 6 lists the typical morphology of the fracture surface of the CVN impact specimen. Cracking along prior austenite grain boundary (indicated by dotted line and arrows), intra-granular dimple fracture and cleavage cracking was observed for FL, which corresponds to its IGF, GBF and FSP structure. While FL+1mm shows more dimples, and no prior austenite grain boundary cracking were observed. As compared to FL, the higher CVN impact value of FL+1 mm is due to the smaller grain structure. Previous studies [16-18] have suggested that IGF grains bounded by crystallographic large angle boundary are effective obstacles to hinder crack propagation, and thus to enhance fracture toughness. Thus the enhanced formation of IGF found in the CGHAZ would be beneficial to1050THERMEC 2013fracture toughness. A large number of inclusions are found at HAZ, and typical morphology and composition were shown in Fig. 7. A dual structure with a core TiOx-Al2O3 oxide and an outer layer of CaS and MnS is found. It is assumed that the complex TiOx-Al2O3 oxide with high-melting point temperature is formed during the ladle refining stage, and the CaS and MnS is formed at cooling stage of slabs or welding process.Fig. 5 Optical microstructure of the HAZ along the line b at Fig. 4 Table 4 CVN impact property of the FCB-SAW weld jointFL FL+1mm FL+3mm FL+5mmvE0 °C 269, 275, 239 219, 225, 259 312, 311, 319 319, 325, 339vE-20 °C 183, 121, 127 213, 181, 237 303, 292, 272 303, 321, 327vE-40 °C 110, 111, 97 185, 211, 207 262, 251, 247 310, 311, 307Fig. 6 SEM micrograph of the fractured surface of CVN impact test sampleThe potency of complex oxide TiOx-Al2O3 to nucleate ferrite during austenite decomposition has also been approved [19]. Sulphides like CaS and MnS are frequently reported [20-23] toMaterials Science Forum Vols. 783-7861051enhance intra-granular nucleation ferrite by acting directly as nuclei. This kind particle has a size range of 0.3~3 mm and a number density of 4.3×104/mm3. This clearly indicates that that the enhanced formation of IAF and IPF at HAZ resulting from a dense distribution of Ti and Mg containing oxide particles dispersed in the plate leads to the enhanced impact toughness of the HAZ.Fig. 7STEM morphology and composition of the particle found in HAZ of the weld joint4 Conclusions (1) 36 mm thick structural steel plate with enhanced weldability was industrially produced by employing advanced steel-making and thermo-mechanically controlled technology, which exhibits an yield strength ≥ 405 MPa, tensile strength ≥ 528 MPa, elongation ≥ 27%, and CVN impact toughness ≥ 300 J at -40 °C. (2) Flux copper backing submerged arc welding with heat input of 230 kJ/cm was applied to butt weld the 36mm thick plate, and defect-free weld joint was produced. The weld joint failed at HAZ during transverse tensile test, and showed a tensile strength ≥ 516 MPa. 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