Influence of substrate topography on formation of FeZn phases and properties of galvanneal coatings

Abstract

The influence of cold rolled topography on titanium (Ti) and titanium niobium (TiNb) interstitial free steels has been investigated with respect to FeZn intermetallic phase development and the adhesion properties of hot dip galvanised and galvannealed coatings. It is shown that substrate chemistry determined the initial reactivity in the galvanising bath and the consequent development of FeZn intermetallic phases. TiNb steels were found to have a lower susceptibility to coating failure, in particular flaking, during forming operations. The results suggest that this is due to the higher interfacial strength of TiNb material, which in part is determined by the nucleation of orientated FeZn crystals with slow growth rates on {111} αFe grains. this reduces the effective surface area of the brittle Γphase and increases the microroughness of the galvanneal coating and the substrate/coating interface. An interaction between FeZn phase growth and topographical features was found. However, the consequent effect on coating performance remains unclear and is the subject of further research.     

Developments in the Production of Galvannealed Steel for Automotive

The state of knowledge of formation of the galvanneal coating on continuous process lines is reviewed. Influences of materials and process variables on phase makeup that are key to achieving acceptability are reviewed. Current targets for key process variables are then described, followed by a consideration of the most frequent galvanneal coating quality issues and their causes. New developments in roll texturing for temper rolling of galvanneal are then described.     

Computer algorithms for radiometric measurement of temperature during the galvanneal process

A set of computer algorithms has been developed to facilitate the measurement of temperature by radiometry during the galvannealing of steel strip. During the galvanneal process, the emissivity of the surface of the strip increases rapidly from a value of approximately 0.2 for the highly specular surface of the liquid zinc coating to a value as high as 0.8 for the surface of the diffuse intermetallic layer that is formed. Reliable noncontact measurement of temperature during galvannealing requires an approach that can accommodate and compensate for this large and rapid change in emissivity. The galvanneal process was simulated in 63 laboratory tests during which the temperature was measured by a thermocouple, and spectral radiance was measured using a dual-wavelength radiation thermometer (DWRT). The emissivity of the surface was obtained from these data. The tests were conducted at galvannealing temperatures of 753, 793, and 833 K and simulated line speeds of 60, 80, and 100 m/min. The laboratory data were used to develop a linear dual-wavelength emissivity compensation algorithm to infer the temperature of the strip to within ±20 K. The iron content of the galvannealed layers varied in the range 5 to 13 pct, and measurements of surface roughness gave arithmetic roughness values in the range 1 to 3 Μm. Formerly Graduate Research Assistant, School of Mechanical Engineering     

The effect of steel chemistry on the formation of Fe-Zn intermetallic compounds of galvanneal-coated steel sheets

The effects of steel chemistry on the formation of Fe-Zn intermetallic compounds in the galvanneal coatings have been investigated by examining the microstructure of galvanneal coat-ings on extra-low-carbon (ELC) steel, interstitial-free (IF) steel, and interstitial-free rephos-phorized (IFP) steel. The layer structure of the coatings was revealed by chemical etching. Phases present in each layer were then identified using electron diffraction in transmission elec-tron microscopy (TEM). A two-layer structure, one consisting of the δ phase with a small fraction of the ζ, phase dispersed on the surface and Γ phases and another consisting of the δ and Γ₁ phases, was observed in the ELC sample and the IFP sample, respectively. A three-layer structure consisting of the δ, Γ₁ + δ, and Γ phases was observed in the IF sample. The presence of C in the steel substrate retarded the alloying between Fe and Zn; while P in the steel favored the formation of the Γ₁, phase over the Γ phase by its surface segregation in the steel substrate. The orientation relationship between coating and substrate was also studied by electron diffraction. Three α-Fe/Γ orientation relationships were frequently observed.     

Corrosion characteristics of newly developed structural DMR-1700 steel and comparison with different steels for industrial applications

Corrosion characteristics of different steels in industrial environment were studied systematically to understand the protective nature of oxide scales that form on their surfaces with a view to explore the possibility of using a newly developed ultrahigh strength steel, DMR-1700, for fabrication of components to use in industrial applications. Further, the studies related to pitting and crevice corrosion resistance of a variety of steels under industrial environmental conditions have been carried out for comparison purposes. The surface morphologies of corroded steels were observed by scanning electron microscope (SEM) in order to understand the nature of corrosion. The corrosion mechanism of steel components that fail under industrial environmental conditions has been discussed. Based on the results obtained with different techniques, the newly developed DMR-1700 ultrahigh strength low alloy steel in association with successfully developed high-performance protective coating has been recommended for manufacture of components to use in industrial systems. This steel helps in improving the efficiency of the systems significantly by eliminating failures during service.     

Features and control methods of the galvanneal furnace

The recent rapid developments in the automobile industry have demanded the extensive use of galvannealed(GA)steel sheets.In particular,the development of lightweight automobiles is putting increasingly higher requirements on the strength of GA steel sheets.The galvanneal furnace,which is used for processing galvannealed steel sheets,is typically composed of the induction heating section,holding section and fog cooling section.This paper described the structural characteristics of each component of the galvanneal furnace,and analyzed temperature control methods of the galvanneal furnace that are important for the successful production of high-strength GA steel sheets for automotive applications.     

Corrosion-resistant coating for GTE compressor parts made of steels with low tempering temperatures

The corrosion resistance of an Ni–Co–Cr–Al–Si–Y + SPh (SPh is silicophosphate impregnation) alloy coating on 30Kh13, 38Kh2MYuA, VKS5, and VKS7 structural steels with low tempering temperatures has been studied. The steel–coating compositions have been tested to determine the accelerated cyclic corrosion resistance, the corrosion resistance under tropic climate chamber conditions and in salt fog, the stress corrosion resistance, and the corrosion resistance in an industrial atmosphere. The heat stability of coated samples is studied, metallographic studies of the samples before and after the tests are performed, and the influence of the coating on the strength characteristics of the structural steels is studied.     

Application of communication program between atomic absorption spectrometer and electronic balance for iron content determination of the galvanneal coating

At present,iron content in a galvanneal coating is determined by an atomic absorption spectrometer (AAS) in Baosteel.The mass of a sample is recorded by operators two times on paper,then the mass of the coating is manually calculated and input in a computer.With the aid of a communication program between an AAS and an electronic balance (EB),the above process can be modified.First,the mass of a sample is sent to a computer by the EB.Second,the mass of the coating is calculated by the computer automatically.Finally,the iron mass is uploaded to the communication program,and the iron content can also be calculated automatically.As such,the modified process is more efficient.     

Surface Oxidation of the High-Strength Steels Electrodeposited with Cu or Fe and the Resultant Defect Formation in Their Coating during the Following Galvanizing and Galvannealing Processes

This study examined the surface oxidation of high-strength steels electrodeposited with Cu or Fe and the resultant defect formation in their coating during the following galvanizing and galvannealing processes. The high-strength steels were coated with an Cu or Fe layer by the electroplating method. Then, the coated steels were annealed in a reducing atmosphere, dipped in a molten zinc, and finally transformed into galvannealed steels through the galvannealing process. The formation of Si and Mn oxides on the surface of the high-strength steel was effectively suppressed, and the density of surface defects on the galvanized steel was significantly reduced by the pre-electrodeposition of Cu and Fe. This effect was more prominent for the steels electrodeposited at higher cathodic current densities. The finer electrodeposit layer formed at higher cathodic current density on the steels enabled the suppression of partial surface oxidation by Mn or Si and better wetting of Zn on the surface of the steels in the following galvanizing process. Furthermore, the pre-electrodeposited steels exhibited a smoother surface without surface cracks after the galvannealing process compared with the untreated steel. The diffusion of Fe and Zn in the Zn coating layer in the pre-electrodeposited steels appears to occur more uniformly during the galvannealing process due to the low density of surface defects induced by oxides.     

热成型钢及热成型技术

对热成型钢的成分设计、热成型工艺过程以及表面氧化的处理进行了论述。热成型工艺可以通过直接热成型或者间接热成型实现;热成型钢都是含硼钢,硼能抑制成型过程中铁素体形核;通过锰、铬和钼等提高淬透性防止珠光体形成;一般热成型钢中含有0.2%左右的碳。热成型钢可通过喷丸的方法消除表面氧化层或者镀层技术保护其表面不受氧化。     

Selective Oxidation and Surface Segregation in High Strength Steels during Short Term Annealing in H2‐N2 ‐ Influence of B on Surface Chemistry

In order to achieve appropriate mechanical properties, new high strength steels aimed for the car industry have to be alloyed with solution strengthening elements. The annealing treatment undergone on cold rolled sheets induces the selective oxidation of alloying elements such as Al, Mn, Si and Cr. The formed oxides exhibit a poor wetting by the Zn bath during hot dip galvanising, thus deteriorating the properties of the zinc coating. While surface‐segregating elements get oxidised, they interact with each other through the formation of spinels and/or mixed oxides during annealing and oxides which have a deleterious effect on wetting can be formed. The formation of (Mn, B) oxide was observed on alloys containing even very small amounts of B and this oxide is almost not wetted at all by Zn. Boron is added to interstitial‐free steels to improve the cold work embrittlement, by replacing phosphorus at the grain boundaries. In this paper, the selective oxidation of steels with and without B, in 5 vol. % H₂‐N₂ atmosphere at 820°C and different dew points was investigated. We found a very strong effect of segregation and oxidation of B on Si and S segregation and oxidation behaviour.     

Characterization and High-Temperature Oxidation Behavior of Cold-Sprayed Ni-20Cr and Ni-50Cr Coatings on Boiler Steels

Microstructure and mechanical properties of cold-spray coatings are usually required in order to explore the potential industrial application of the latter. This article demonstrates the successful formulation of Ni-20Cr and Ni-50Cr coatings on two boiler steels, namely, SAE 213-T22 and SA 516 steel by cold-spray process. The microstructure, coating thickness, phase formation, and microhardness properties of the coatings were evaluated. The coatings were subjected to cyclic heating and cooling cycles at an elevated temperature of 1173.15 K (900 °C) to ascertain their high-temperature oxidation behavior. Moreover, these cyclic exposures can give useful information regarding the adhesion of the coatings with the substrate steels. Of all the coatings, the Ni-50Cr coating on SA 516 steel had a maximum average hardness value of 469 Hv. As observed from the surface field emission–scanning electron microscopy (FE-SEM) analysis, the coatings were found to have nearly dense microstructure with the sprayed particles in interlocked positions. It was concluded that the cold-spray process is suitable for spraying the preceding powders onto the given boiler steels to produce nearly dense and low oxide coatings. The coatings, in general, were found to follow the parabolic rate of oxidation and were successful in maintaining their surface contact with their respective substrate steels.     

Microcracking of flash coatings and its effect on the Zn-Ni coating adhesion of electrodeposited sheet steel

In light of its excellent corrosion resistance, Zn-Ni alloy coated steels could be one of the major coated sheet products used in the automobile industry, provided that the Zn-Ni coating layer is adherent to the sheet steel after various press-forming processes or during service at subzero temperatures. In this study, a flash-coating preplating treatment, made from a chloride bath using a high-speed flow cell, is performed to enhance the adhesion of Zn-Ni electrodeposited sheet steels. A Ni-rich flash coating was deposited first as under-layer, at a low current density of 12 A/dm², in the same electrolyte used to plate the major overlay at a current density of 70 A/dm². The flash coating was then immersed in the hydrochloride acid for time periods ranging from 10 to 40 seconds. Upon immersing, Zn-preferential dissolution was observed; meanwhile, numerous microcracks formed. The density of microcracks increases with immersion time, reaching a saturated state around an etching time of 40 seconds. A similar tendency was also observed for the increase of the Ni content of the flash coating. The internal stress of the as-deposited flash coating is in compression and changes to tension at the very beginning of acid immersion. Cross-sectional transmission electron microscopy (TEM) revealed that microcracks formed on the surface of the flash coating propagate perpendicularly to the coating/substrate interface and stop at a distance of 0.03 μm ahead of the interface, even after a 40-second immersion. The flash coating, after proper etching, is shown to enhance the interfacial shear strength, formability, and adhesion of the coated sheet steels. The development of microcracks and the reduction of the thickness of the flash coating upon acid immersion account for the existence of an optimal extent of etching. The TEM observations on the morphological change of the microcracks upon acid immersion and the structural morphology of the steel/flash coating/major overlay interfaces further support the present strengthening mechanism.     

Effect of substrate surface polishing on galvannealing characteristics of a mild IF and an HSIF steel

Abstract

Mechanical properties and visual appearance of galvannealed steel sheets are strongly influenced by steel substrate surfaces before dipping into molten Zn. It has been known that oil contamination, iron fragments and surface enrichment of oxides deteriorate the Fe–Zn reaction and result in various harmful surface defects on the galvannealed coating. In order to prepare the optimal galvannealed coating for automotive exposed panels, the steel substrate surfaces were mechanically polished by SiO₂ polishing brushes.

The types of brush and pressure between the brush roll and strip are major factors for removing the appearance of visual defects on the surfaces of galvannealed coatings. The microstructure of the polished substrate, galvannealed coating and whiteness index on the surface of steel substrate were characterised by optical microscopy, scanning electron microscopy and colour difference meter.     

Microstructure and formability of ZnNi alloy electrodeposited sheet steel

ZnNi alloy electrodeposited sheet steels were made from a chloride bath using a high-speed flow cell. A Ni-rich flash coating was deposited first, upon which the ZnNi coating, with Ni contents ranging from 8 to 16 wt pct, was subsequently electrodeposited. It is demonstrated that the Ni content of the coating affects the forming properties and microstructure of the ZnNi coatings. The hardness of the ZnNi coating increased with Ni content, leading to poor formability and inferior adhesion of the coated steels, as evident from the large amount of coating loss during swift cupping and coating peel-off during low-temperature adhesion tests. On the other hand, the friction force between the coated steel and cupping die decreased with increasing Ni content. At low Ni contents of 8 wt pct, the coating had a porous equiaxed grain structure. As the Ni content increased, the coating surface changed to dense faceted morphologies. A pyramid morphology was observed for 16 wt pct ZnNi coatings. An X-ray diffraction (XRD) analysis showed that all coatings containing up to 16 wt pct Ni contained only γ phase. Transmission electron microscopy (TEM) observations showed the 8 wt pct Ni coating to have a fine-grained structure, which changed to a columnar structure at 16 wt pct Ni. The formation of the columnar structure is explained by the smaller amount of hydrogen discharge as the bath Ni ion concentration increased.     

Flame straightening of thermomechanically rolled structural steel

Shape control by local heating is often used in processing of structural steels. Experience has proven that normalised steels are suitable for this operation if it is carried out properly. In the last years thermomechanically controlled rolled (TMCP) steels have been developed. For their application it was necessary to investigate the response of this steel type to flame straightening. Two essential processing conditions must be considered: heating limited to the surface and heating of the full material thickness. The temperatures were varied from 650 to 950°C by different heat inputs. Tensile and Charpy impact tests have proven that within this temperature range the investigated steels (S355ML -EN10113) were not affected. For conditions of wedge heating a drop in the mechanical properties was observed if the temperatures exceeded 650°C. Both limits, 950°C in line heating and 650°C in wedge heating allow an effective shaping of steel constructions. Experienced operators can easily respect these limitations. Hence the TMCP steels are suitable for flame straightening.     

连退炉后带钢表面清洗装置的应用

我厂现有的硅钢连续退火炉后没有清洗装置,导致出炉的带钢表面沾染碳粉和其他粉尘,造成带钢涂层质量缺陷。针对这一问题,在水喷淋处增加一套带钢清洗装置来清洗带钢表面污渍,改善了涂层质量。     

Improvement of Mechanical Properties of Reinforcing Steel Used in the Reinforced Concrete Structures

SAE1010 structural carbon steel, which has a low cost price and wide range of use in the construction industry, has been studied as dual phase (DP) steel subjected to appropriate heat treatment, and its mechanical properties have been investigated under various tempering conditions. Intercritical annealing heat treatment has been applied to the reinforcing steel in order to obtain DP steels with different martensite volume fraction. In addition, these DP steels have been tempered at 200, 300 and 400 °C for 45 min and then cooled to the room temperature. Mechanical properties such as tensile strength, yield strength, reduction in cross-sectional area, total elongation, resilience modulus and toughness have been examined. Furthermore, fractographic examination has been done with scanning electron microscope (SEM) as well as metallographic examination of the steels. As a result of this study, it is found that mechanical properties of DP steel have changed according to the hardness and ratio of martensite phase. In addition, tensile strength, yield strength and resilience modulus of the steels have been reduced. In contrast, the total elongation, reduction of the cross-sectional area and toughness have been increased.     

Coining Test for Evaluation of Tool Performance

The performance of M2 tool steel without any special coating on a set of cold formable steels is evaluated in this work using the coining technique. Cylindrical specimens were prepared from spheroidize annealed SAE 1012, 1020, 5120 and 1050 steels (representatively covering the whole gamut of cold workable steels) with an aspect ratio of d_o/h_o = 1.5 (Where d_o being the original diameter and h_o being the original height). These annealed materials were measured for their initial roughness as well flatness values. They are compressed to true strains of 0.1, 0.2, 0.3 and 0.4 under unlubricated and phosphated conditions of the chosen work materials. For each of the specimen tested, the results of this study are presented and discussed. The surface roughness, crown height and barreled volume were evaluated. The study shows that the lubricated work pieces were found to have lower crown heights, smaller barreled volumes, lesser surface roughness compared to unlubricated samples.     

Structure and mechanical properties of sintered Ni free structural parts

Abstract

Ni, Cu and in some cases Mo are the alloying elements which have traditionally been used in sintered steels. High performance of powder metallurgy (PM) structural parts from Fe powders is reached mainly by alloying of Ni. The use of Mn in Fe base PM structural parts has been avoided because of its high affinity to oxygen. It is difficult to sinter Mn steel, without oxidation, in industrial atmospheres. However, the PM industry follows also possibilities in order to develop Ni free sintered steels which render as high mechanical properties as diffusion alloyed Ni containing sintered steels and further fulfil the requirements of health protection. In recent years Mn have been introduced as alloying element in Fe based structural parts, on laboratory scale and also for pilot scale production. In this paper the factors that contribute to the structure and mechanical properties of sintered Mn steels are summarised.