Cu-containing Steel Research Articles

A Novel Technology to Develop a Nickel-Enriched Layer on Slab Surface by Utilizing NiO-Containing Synthetic Powder

Cu, as one of the typical tramp elements, is known to cause hot shortness during reheating of slabs followed by hot rolling of sheet products. In order to prevent such harmful aspects, a new idea is proposed by using synthetic powders containing NiO in the mold flux during continuous casting of the slab. During the casting, NiO is reduced and absorbed on initial solidified steel shell, and a Ni-rich layer is developed near the surface region of the slab. According to the proposed idea, it is expected that both the Cu solubility and the melting temperature of Cu-segregated region would increase considerably by virtue of Ni-rich layer, which is believed to play an important role to prevent the Cu hot shortness. A series of laboratory-scale experiments were carried out in order to confirm the reduction and the absorption of Ni into the steel matrix. It was observed by SEM–EDS and FE-EPMA that a Ni-enriched layer, as thick as a few hundred μm, formed near the surface of the slab. Also, a number of laboratory-scale heat treatment tests under oxidizing atmosphere showed that the samples with the Ni-enriched layer had a decreased Cu enrichment at the interface between scale and steel, compared to a case without Ni-rich layer. A pilot-plant-scale steel slab (medium carbon steel containing 0.3 wt pct Cu) was obtained in a continuous casting process with the NiO-containing mold flux, and a Ni-enriched layer was also observed. It was concluded that the use of NiO in the mold flux is a promising new approach for suppressing the hot shortness of Cu-containing steel, without an expensive addition of Ni to the whole steel matrix.

Alloying effect of copper on the corrosion properties of low-alloy steel for flue gas desulfurization system

The alloying effect of Cu for a flue gas desulfurization materials was investigated using the electrochemical methods in the modified green death solution and the surface analyses. The test results demonstrated that the densely formed rust layer with high metallic Cu content improves the corrosion resistance of Cu-containing steel in the flue gas desulfurization (FGD) environment. The rust layer on the surface of the 0.02 wt% Cu steel, which has an insufficient Cu content, was less protective than others. The 0.05 wt% Cu steel represented the highest corrosion resistance due to the formation of the densely formed rust layer with optimum Cu content. Because the free standing Cu2S precipitates had the insoluble characteristic in highly acidic solution, it produced the relatively porous Cu-enriched layer on the 0.08 wt% Cu steel surface. From these phenomena, the corrosion resistance of specimen decreased as the Cu content of specimen increased from 0.05 wt% to 0.08 wt%.

High Temperature Oxidation Resistance of 9Ni Steel

Oxidation rates and scale/steel interface configuration of 9Ni steels were investigated at 1000–1250 °C in air. The results revealed that Cu addition caused high temperature oxidation resistance to deteriorate. High temperature oxidation rates increased and scale/steel interface configuration became complicated due to Cu addition. Scale/steel interface appeared to be network above certain temperature. Temperature required to form network scale/steel interface dropped more than 100 °C for 1.5% Cu-containing steel. (Fe, Ni, Cu)x Oy in inner oxidation layer dissociated to Fe-Ni-Cu phase and released active oxygen which can react with base steel easily. So the inner oxidation layer became the second source of oxidizing agent besides atmosphere. Internal stress at austenite grain boundary caused local oxide to fragment. So the scale/steel interface appeared to be network. Liquid Si-rich phase formed at sufficiently high temperature. Penetration of the liquid Si-rich phase along austenite grain enhanced austenite grain oxidizing.

Wear behavior of copper-containing ferritic iron under a dry sliding condition

The effect of solute Cu and Cu precipitates on the wear behavior of ferritic iron under an unlubricated condition was investigated. The specific wear rate of Cu-containing steel abruptly decreased up to 50 N of load, and then gradually decreased with further increased load. The specific wear rate of the as-quenched specimen, in which Cu was in a solid solution, was the lowest among all the specimens at low loads, and all specimens had almost the same specific wear rate at high loads. Subsurface observation showed that the hardness increments of all specimens decreased with increased depth below the worn surface. The as-quenched specimen had a relatively large depth of deformed region than the other specimens even though the increments in hardness were almost the same for all specimens at low loads. With the same hardness at an unworn state, the as-quenched and over-aged specimens exhibited a substantial increase in hardness and large deformed regions below the worn surfaces. This finding indicated that the enhancement in plastic deformation and work hardening led to the decrease in the specific wear rate of the as-quenched specimen at low loads and the improvement in the wear resistance of all specimens at high loads.

Mechanical and magnetic properties of copper dispersed ferrum films under heat treatment

The Cu-containing steels are widely used for nuclear pressure vessel materials because of their good performance under high pressure and high temperature. In this article, magnetron sputtering was used to prepare iron films with various Cu contents. The samples were annealed at temperature range of 300–500 °C, and the structural, mechanical, and magnetic properties were studied. The results show that both hardness and modulus change along with copper content and annealing temperature. The change in coercivity after annealing is similar to that of hardness. The crystal grain growth in matrix ferrum and Cu precipitation during annealing influences both the mechanical and magnetic properties.

Cu Precipitation Dynamics in an Fe-1.18%Cu Alloy

The precipitation of copper during aging at 600oC in high-purity Fe-Cu alloy was examined by means of transmission electron microscopy (TEM).Nano-scale copper-rich clusters with a B2-like structure were observed during heat treatment. These micro structural features play an important role in precipitation strengthening. In addition, the precipitation process has been analyzed in terms of the evolution of microstructure by a Monte Carlo method. A description of the coherent precipitation of copper in iron, based on a vacancy diffusion mechanism, thermally activated jump frequencies and cohesive energy is discussed in order to deal with simultaneous precipitation of metastable and stable phases in Cu-containing steel during aging. This analysis gives an estimation of the precipitation dynamics, as well as the evolution of Cu precipitates across a wide range of temperatures.

Ageing Precipitate Strengthening of Cu-Containing Structural Steels

Microstructure evolution of Fe-1.18%Cu high purity steels during solution and aging was investigated under high-resolution electron microscope (HREM). In addition, the aging strengthening mechanisms were discussed based on the microstructure observation. The results show that there were lots of Cu atom clusters in ferrite matrix during solid solution and aging initial stages, subsequently, Cu-rich metastable Fe-Cu particles precipitate at the aging strength peak. It is found that the intense strengthening is controlled by the coherency relationship of Fe-Cu metastable phase with matrix that forms the obstacle of the dislocation motion, while the decrease of strength after the peak is attributed to the loss of coherency, which should highly likely be the dominant reason of aging strengthening in Cu bearing high purity steels Thus our TEM observation results are in reasonably agreement with some previous assume.

Surface Hot Shortness of Copper Containing Steel in a Compact Strip Production Process

The compact strip production (CSP) process has received much attention because it is environment friendly and can be used for recycling resources. However, steel scrap, the main material used in the CSP process, causes surface cracks in the hot strip being manufactured as a result of surface hot shortness by Cu and Sn in the steel scrap. This study investigates the influence of heat pattern on the brittleness of Cu-containing steels prior to treatment in a reheating tunnel furnace at 1100°C. The prior austenite grain size of a sample that was reheated from room temperature before being transferred to a tunnel furnace was finer than that obtained by the direct transfer process, and the crack depth was inhibited by 50%. In contrast, the prior austenite grain size of a sample obtained by a process in which austenite is reheated from a cooling stop temperature of 650°C–850°C was almost the same as that obtained by direct transfer to the tunnel furnace. However, the crack depth in the case of reheating from the cooling stop temperature of 650°C–850°C was greater than in the case of direct transfer. This deep cracking was caused by a noninternal oxidation area at the scale/steel interface.

The Corrosion Mechanism of Cu-Containing Weathering Steel in a Cyclic Dry-Wet Condition

The weather resistance of 10CuPRE、10CuP and Q235 steels were studied by dry-wet cyclic immersion test. The corrosion resistance mechanism of rare earth Cu-containing weathering steel was studied through electrochemical polarization test, scanning electron microscope(SEM) and X ray diffraction(XRD). The results show the small and spherical rare earth oxysulfides replace the elongated MnS inclusions in the rare earth weathering steel. Less and fewer rare earth oxysulfides heavily decrease pitting susceptibility and rate of pit propagation. So the electrochemical corrosion of microarea in the steel matrix is weakened after rare earth was added in the Cu-containing weathering steel. The inner rust layer of rare earth weathering steel is more compact and uniform than that of weathering steel without rare earth. The main corrosion product on the rare earth weathering steel is α-FeOOH. The formation of the steady corrosion product is promoted by rare earth, which result in that the protective property of the inner rust layer on weathering steels is enhanced. As a result, the corrosion resistance of Cu-containing weathering steel is improved by rare earth elements.

Effects of Cu and B addition on microstructure and mechanical properties of high-strength bainitic steels

Effects of Cu and B addition on microstructure and mechanical properties of high-strength bainitic steels were investigated in this study. Six kinds of steels were fabricated by controlling the amount of Cu and B addition, and their microstructures and tensile and Charpy impact properties were investigated. Their effective grain sizes were also characterized by the electron back-scatter diffraction analysis. The tensile test results indicated that the B- or Cu-containing steels had the higher yield and tensile strengths than the B- or Cu-free steels because their volume fractions of acicular ferrite and martensite were quite high. The B- or Cu-free steels had the higher upper shelf energy than the B- or Cu-containing steels because of their lower volume fraction of martensite. In the steel containing 10 ppm B without Cu, the best combination of high strengths, high upper shelf energy, and low energy transition temperature could be obtained by the decrease in effective grain size due to the presence of acicular ferrite having fine effective grain size.

The influence of copper addition on microstructure and mechanical properties of thermomechanically processed microalloyed steels

The present study concerns the influence of Cu addition on the microstructural evolution and mechanical properties of directly air-cooled microalloyed thin-gauge steel. For this purpose, 1.5 wt% Cu was added to a Ti–B microalloyed steel. It is known that Ni addition to Cu-containing steel is beneficial to eliminate hot shortness caused by Cu. Therefore, the effect of Ni addition (half that of Cu in wt%) on the microstructure formation and mechanical properties has also been studied. Microstructures and mechanical properties of the directly air-cooled steels have demonstrated that addition of 1.5 wt% Cu along with 0.8 wt% Ni in Ti–B microalloyed steel results in a dual phase-like microstructure, which yielded attractive tensile strength (746 MPa) and ductility (31%). However, Cu addition deteriorated the impact toughness of the directly air-cooled Ti–B microalloyed steels.

Cu precipitation dynamics in Fe-Cu alloy

The precipitation of copper during aging at 600°C in high-purity Fe-Cu alloy was examined by means of transmission electron microscopy, Nano-scale copper-rich clusters with B2-like structure were observed during either solution treatment or aging, which should play important role on precipitation strengthening. In addition, the precipitation process has been analyzed in terms of the evolution of microstructure by Monte Carlo method. An description of the coherent precipitation of copper in iron, based on vacancy diffusion mechanism, thermally activated jump frequencies and cohesive energy was discussed to deal with simultaneous precipitation of metastable and stable phases of Cu-containing steel during aging, which gives an estimation of the precipitation dynamics, as well as the evolution of Cu precipitates in a wide range of temperature.

Effect of Small Amounts of Ni on Liquid-Cu Embrittlement in Hot-Rolled Mild Steel After High-Temperature Oxidation

The effects of additions of as little as 0.02%Nito mild steel on the morphology of the scale-steelinterface and surface cracking were studied duringhigh-temperature oxidation and subsequent hot-ductility tests of 0.5%Cu-bearing steel. Liquid-Cuembrittlement occurs in a steel containing0.5%Cu-0.001%Ni because of concentrated liquid-Cu layerat the scale-steel interface over the temperature rangeof 1100-1300°C. The addition of as little as0.023%Ni to 0.50%Cu-containing steel recovers theductility in the oxidation temperature range of1250-1300°C. This is because liquid-Cu particles areoccluded into the scale. Thus, the amount of enriched Cuper unit surface area decreases at the scale-steelinterface. It is suggested that liquid-Cu particleocclusion, which has not yet been elucidated, results from heterogeneous enrichment of Ni andheterogeneous oxidation at the scale-steelinterface.

Effects of Cu and Other Tramp Elements on Steel Properties. Evaluation of Susceptibility to Surface Hot Shortness in Cu-containing Steels by Tensile Test.

The feasibility of a new method for evaluating the susceptibility of steels to surface hot shortness due to Cu and Sn was examined. Steels used are IF steels containing 0.1-0.2%Cu and 0.1%C-0.5%Cu steels. Chemical compositions of the IF steels were controlled with the purpose of being as similar as possible to those of steels of which the susceptibility was previously examined by a visual method in a practical rolling process at Nippon Steel Corporation. Tensile tests were carried out at 1 100°C after heating a specimen in air or Ar gas. The total elongation and maximum load were measured from a load-elongation curve. By dividing the difference between the total elongations or maximum loads obtained in air and Ar gas by the total elongation or maximum load in Ar gas, the parameters Ee or Ep were calculated. By considering the reduction in specimen diameter due to oxidation, the Ep corrected, Ep', was calculated. The Ee, Ep and Ep' of the IF steels corresponded to the degree of surface cracks observed in the practical rolling process. Penetration of Cu-enriched phase into grain boundaries tends to occur under thermal and external stresses. Surface cracks stop growing along the depth direction at the earlier stage of deformation when the amount of Cu-enriched phase is the smaller at steel/scale interface. Tensile tests using specimens having artificial round cracks clarified that the Ee and Ep' increased with an increase in the crack depth and that, in contrast, the Ep' increased slightly and the Ee decreased with an increase in the number of cracks. The parameter Ee and Ep' showed a maximum value at a strain rate of around 3 x 10 -2 s -1 , and the difference in their values among the steels is largest around this strain rate. Therefore it is possible to know sensitively the effects of any factors like alloying or impurity elements on surface hot shortness at around this strain rate although this strain rate is much slower than those of hot workings.

Irradiation-induced microstructural changes, and hardening mechanisms, in model PWR reactor pressure vessel steels

A series of model steels and commercial alloys from the IAEA Phase 3 irradiation programme was irradiated to a dose of 14 mdpa (∼ 9 × 10 22 n m −2 ( E > 1 MeV)) at 290°C. The steels were examined using small angle neutron scattering and a range of transmission electron microscope techniques. Measured yield stress and hardness changes were interpreted in terms of the irradiation-induced microstructural developments. The irradiation of Cu-containing steels produced Cu-rich precipitates of ∼ 2 nm diameter which were alloyed with Mn and Ni. The Ni content of the precipitates increased markedly for bulk Ni contents > 0.7 wt%. The volume fractions of precipitate increased with increasing bulk Cu content and, also, with increasing bulk Ni content > 0.7 wt%. The precipitate-induced strengthening and hardening varied according to the square root of the volume fraction of precipitate. Yield stress changes in low-Cu steels were consistent with data from the IAEA Phase 2 irrradiation programme.

Influence of Incorporated Copper on the Formation, Solid State Structure and Electrochemical Properties of γ-FeOOH

It is well known, that the rate of the atmospheric corrosion of iron is slowed down considerably, if the metal is alloyed by small amounts of Cu. This effect is specifically pronounced during those times, when the metal surface is covered by thin electrolyte layers. As then the oxygen reduction is rate determining for the corrosion rate, the kinetics of the oxygen reduction is slowed down on Cu-containing steels with respect to pure iron. On pure iron oxygen is reduced within the oxide scale at electron conducting oxide crystals. Such oxides contain high amounts of Fe2+ states and result from an electrochemical reduction of γ-FeOOH. Therefore it is quite likely, that on Cu-containing steel the solid state properties of the oxide scale are changed such, that the electronic conductivity decreases. – In this study model electrodes of Cu-doped γ-FeOOH are investigated. The morphology of the scale is analysed by SEM, the chemical state of Cu and Fe by XPS and the structure of the scale using FT-IR spectroscopy. The results clearly show, that the morphology of the Cu-doped oxide is changed significantly and XPS data suggest, that the reducibility of the doped oxide is much less than the one of undoped γ-FeOOH.

Effect of copper precipitation on the development of recrystallization textures through continuous annealing of low-carbon sheet steel

The development of {111} recrystallization textures in 0.5 pct and 1 pct Cu-containing steels through a simulated continuous annealing cycle has been investigated. A low-carbon content of 0.05 pct compared with extra low-carbon content of 0.005 pct accelerated the precipitation of e-Cu by preaging prior to cold rolling, and a nickel content up to 0.95 pct gave the adverse effect. Pole intensity ratio, I222/I200, of the cold rolled and continuous-annealed sheet depended more strongly upon the distribution density of e-Cu particles than upon the amount of carbon in solid solution. The optimum dispersion of e-Cu precipitates for producing a strong {111} texture was about 300 particles per μm3 in distribution density, and 20 to 30 nm in particle size. Also, it was suggested that through continuous annealing by utilizing the effect of prior precipitates such as e-Cu, a high strength, dual-phase steel with a highr-value could be produced.

Atmospheric corrosion of Fe-Cu alloys and Cu-containing steels

Studies have been made of the atmospheric corrosion of commercial Cu-containing steelsand of pure Fe alloys containing Cu, S and Cu + S. Exposure tests in rural, marine and industrial environments have shown that the effect of Cu is small in S-free alloys, but a marked decrease in the corrosion rate is observed when the alloy contains S. These results have been explained by assuming that the Cu dissolved from the alloy reacts with S 2− and HS −, which are known to stimulate the corrosion of Fe, with the formation of insoluble Cu sulphides. Thus the deleterious action of the sulphide ions, which may be formed from sulphides in the alloy or from atmospheric pollutants, is nullified with a consequent decrease in the corrosion rate. Studies of rust structure indicate that differences in initial corrosion rates have a marked effect on the structure of the rust formed; a view that is in opposition to the usual concept of the formation of “protective rusts”he mechanism proposed, which has been developed from studies of pure Fe alloys, should be applicable to Cu-containing commercial steels, although the behaviour of these two types of alloys hds shown certain differences.