Understanding dual precipitation strengthening in ultra-high strength low carbon steel containing nano-sized copper precipitates and carbides (original) (raw)
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High-Strength Low-Carbon Ferritic Steel Containing Cu-Fe-Ni-Al-Mn Precipitates
Metallurgical and Materials Transactions A, 2008
An investigation of a low-carbon, Fe-Cu-based steel, for Naval ship hull applications, with a yield strength of 965 MPa, Charpy V-notch absorbed impact-energy values as high as 74 J at -40°C, and an elongation-to-failure greater than 15 pct, is presented. The increase in strength is derived from a large number density (approximately 10 23 to 10 24 m -3 ) of copper-iron-nickelaluminum-manganese precipitates. The effect on the mechanical properties of varying the thermal treatment was studied. The nanostructure of the precipitates found within the steel was characterized by atom-probe tomography. Additionally, initial welding studies show that a brittle heat-affected zone is not formed adjacent to the welds.
Low-carbon Cu precipitation-strengthened steel
2011
Our national infrastructure and other structural applications can greatly benefit from steels with increased strength but without sacrificing ductility, toughness and weldability. Most high-strength steels are martensitic. The strength of martensitic steels increases with carbon content. High carbon content, however, leads to poor weldability due to the formation of a brittle heat-affected zone adjacent to the weld. One can overcome this problem by using steels with low carbon content and enhancing the strength by precipitates. This was the basis for the development of HSLA-80 and HSLA-100 Cu-precipitation-strengthened steels 1-6 now used in Naval applications, mining and dredging equipment, heavy duty truck frames and are beginning to be used in bridge applications.
Materials Science and Engineering: A, 2013
A Cu-bearing Nb-microalloyed bainitic steel with high yield strength (4700 MPa), good elongation (435%) and low temperature toughness was developed for heavy plates. An intercritical annealing plus tempering (L-T) heat treatment was applied for combining reversed transformation with the precipitation during the tempering stage. The enrichment of alloying elements during the annealing process leads to the drop of A c1 temperature, which makes the reversed transformation and precipitation occur simultaneously in the subsequent tempering process. The secondary enrichment of Mn and Ni makes the reversed austenite stable enough to be retained at room temperature. About 29% retained austenite introduced by the L-T process contributes to excellent elongation more than 10% by the TRIP effect. The formation of Nb(C,N) and (Nb, Mo)C precipitates during the annealing and cooling stages inhibits the recovery of dislocations, while more Nb precipitates smaller than 10 nm and Cu precipitates with sizes of 20 nm play significant strengthening role on the matrix. The combination effects of retained austenite and nano-sized dual-precipitate increased yield strength, ductility and toughness remarkably.
This article summarizes the state of the art of the comprehensive strengthening mechanism of steel. By using chemical phase analysis, X-ray small-angle scattering (XSAS), room temperature organic (RTO) solution electrolysis and metal embedded sections micron-nano-meter characterization method, and high-resolution transmission electron microscopy (TEM) observation, the properties of nanoscale cementite precipitates in Ti microalloyed high-strength weathering steels produced by the thin slab continuous casting and rolling process were analyzed. Except nanoscale TiC, cementite precipitates with size less than 36 nm and high volume fraction were also found in Ti microalloyed high-strength weathering steels. The volume fraction of cementite with size less than 36 nm is 4.4 times as much as that of TiC of the same size. Cementite with high volume fraction has a stronger precipitation strengthening effect than that of nanoscale TiC, which cannot be ignored. The precipitation strengthening contributions of nanoscale precipitates of different types and sizes should be calculated, respectively, according to the mechanisms of shearing and dislocation bypass, and then be added with the contributions of solid solution strengthening and grain refinement strengthening. A formula for calculating the yield strength of low-carbon steel was proposed; the calculated yield strength considering the precipitation strengthening contributions of nanoscale precipitates and the comprehensive strengthening mechanism of steels matches the experimental results well. The calculated r s = 630 to 676 MPa, while the examined r s = 630 to 680 MPa. The reason that ''ultrafine grain strengthening can not be directly added with dislocation strengthening or precipitation strengthening'' and the influence of the phase transformation on steel strength were discussed. The applications for comprehensive strengthening theory were summarized, and several scientific questions for further study were pointed out.
Nanoscale Cementite Precipitates and Comprehensive Strengthening Mechanism of Steel
Metallurgical and Materials Transactions A, 2011
This article summarizes the state of the art of the comprehensive strengthening mechanism of steel. By using chemical phase analysis, X-ray small-angle scattering (XSAS), room temperature organic (RTO) solution electrolysis and metal embedded sections micron-nano-meter characterization method, and high-resolution transmission electron microscopy (TEM) observation, the properties of nanoscale cementite precipitates in Ti microalloyed high-strength weathering steels produced by the thin slab continuous casting and rolling process were analyzed. Except nanoscale TiC, cementite precipitates with size less than 36 nm and high volume fraction were also found in Ti microalloyed high-strength weathering steels. The volume fraction of cementite with size less than 36 nm is 4.4 times as much as that of TiC of the same size. Cementite with high volume fraction has a stronger precipitation strengthening effect than that of nanoscale TiC, which cannot be ignored. The precipitation strengthening contributions of nanoscale precipitates of different types and sizes should be calculated, respectively, according to the mechanisms of shearing and dislocation bypass, and then be added with the contributions of solid solution strengthening and grain refinement strengthening. A formula for calculating the yield strength of low-carbon steel was proposed; the calculated yield strength considering the precipitation strengthening contributions of nanoscale precipitates and the comprehensive strengthening mechanism of steels matches the experimental results well. The calculated r s = 630 to 676 MPa, while the examined r s = 630 to 680 MPa. The reason that ''ultrafine grain strengthening can not be directly added with dislocation strengthening or precipitation strengthening'' and the influence of the phase transformation on steel strength were discussed. The applications for comprehensive strengthening theory were summarized, and several scientific questions for further study were pointed out.
Compositional Variants of Cu-rich Precipitate in Thermally Aged Ferritic Steel
Acta Metallurgica Sinica (English Letters)
Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy steel isothermally aged at 500°C for 1, 4, 16, and 64 h after water-quenching from 900°C. With prolonged aging time, the Curich precipitates (CRPs) increased in size and decreased in number density, and gradually evolved from spheroidal to elliptical in morphology. The small CRPs were rich in a high amount of Fe and a certain amount of Ni and Mn at their early nucleation stage. The large CRPs with increased size due to extensive aging contained less Fe and more Cu at their later growth stage. Additionally, Ni and Mn were both readily to segregate at the CRP/matrix heterophase interfaces, and Mn was higher in content than Ni in the precipitate interior especially when the CRPs were large in size. KEY WORDS: High-strength low-alloy steel; Thermal aging; Cu-rich precipitate; Atom probe tomography * 400°C [7-13]. Therefore, it is important to tailor the precipitation morphology of Cu-rich precipitates (CRPs) for attaining desired strength and toughness balance. Actually, the characteristics of CRPs in aspect of size, number density, shape, etc., that determine final mechanical properties are largely dependent of the compositional and resultant structural evolution during heat treatment process, and the segregation of Ni and Mn at the precipitate/matrix interface significantly prohibits the growth of CRPs to large size [14-21]. In additional, the compositional evolution of CRPs provides important information for composition modification, processing optimization, and property improvement of Cu-containing steels. However,
Novel ultrafine Fe(C) precipitates strengthen transformation-induced-plasticity steel
A transmission electron microscopy study was conducted on nanoprecipitates formed in Ti microalloyed transformation-induced-plasticity-assisted steels, revealing the presence of Ti(N), Ti 2 CS and a novel type of ultra-fine Fe(C) precipitate. The matrix/precipitate orientation relationships, sizes and shapes were investigated in detail. The ultrafine, disc-shaped Fe(C) precipitates have sizes of 2–5 nm and possess a hexagonal close packed crystal structure with lattice parameters a = 5.73 ± 0.05 A ˚ , c = 12.06 ± 0.05 A ˚. They are in a well-defined Pitsch–Schrader orientation relationship with the basal plane of the precipitate parallel to the [1 1 0] habit plane of the surrounding body-centred-cubic ferritic matrix. Detailed analysis of precipitate distribution, orientation relationship, lattice mismatch and inter-particle spacing suggests that these ultrafine precipitates contribute considerably to the strengthening of these steels.
Influence of Cu Addition on Microstructure and Strength of Low Carbon Steel
2016
This study aim is to determine the influence of Cu addition on microstructure and strength of low carbon steel. 0.1% C steel, which contained Cu were used as specimens. The temperatures for heat treatment were determined using a software. The type of specimens was heat treated at specific temperatures in order to obtain 20% and 80% of martensite. Specimens were austenised at 1000 °C for 30 second and followed by water quenching to obtain martensitic structure. The base steel used as the base metal as specimen. The hardness increases with increasing temperature for both of steels. It is found that the hardness, yield strength and ultimate tensile strength of Cu was higher than Base steel. Changes of hardness of annealed samples almost the same in both steels. On the other hand, it is found that addition of Cu can improve tensile strength, total elongation and strength-ductility balance of the steel although no significant effect on yield stress and uniform elongation. Total elongation for Cu steel is 19%, and base steel the elongation values are 15% respectively. Although the martensite content is the same. Total hardness for Cu steel is 390 Hv and 281.8 Hv. However, the Cu steel has the highest hardness than base steel. Therefore, the addition of Cu will increase the hardness, strength and elongation of steel.