Hot-rolled Low-carbon Steel Research Articles

Effects of inclusion and loading direction on the fatigue behavior of hot rolled low carbon steel

The anisotropic fatigue behaviors of hot rolled low carbon steel were investigated for specimens with different angles between the loading and rolling direction. It was found that with the increase of loading angles crack initiation site changed from the matrix of surface to internal inclusions (either alumina or MnS) and fatigue strength decreased significantly. The large difference of the projected area of inclusions was responsible for this anisotropic fatigue strength. A good linear relationship between inclusion area and fatigue life in log–log plot was found. The fatigue limits can be predicted by empirical equation and a modified Murakami’s model.

Artificial Neural Network: Some Applications in Physical Metallurgy of Steels

Artificial neural networks are parameterized nonlinear models used for empirical regression and classification modelling. Their flexibility enables them to discover very complex relationships among large number of variables with complex interdependencies. Hence it is a very appropriate technique for developing predictive models in the short term. This article discusses the development of neural network models based on a Bayesian framework and some applications of the same. These examples include: (i) prediction of yield and tensile strengths of hot-rolled low-carbon ferrite-pearlite steel plates as a function of composition and rolling parameters, (ii) estimation of bainite plate thickness, (iii) estimation of retained austenite as a function of process parameters in Transformation Induced Plasticity aided (TRIP-aided) steels, and (iv) analysis of strain-induced transformation behavior of retained austenite during uniaxial tensile testing of TRIP-aided steels. While examples (i), (ii), and (iv) provide an overview of the work reported earlier, example (iii) is reported here in open literature for the first time. In all these four cases, it has been shown that the results are consistent with the established physical metallurgy principles.

Aging Behaviour in Hot‐Rolled Low Carbon Steels

Lüders strain and aging index were employed to characterize the aging properties of hot rolled low carbon steels during production. The Lüders strain was found to increase with aging time while the aging index decreased. It is interpreted that the aging index represents the susceptibility of steels to aging, while the Lüders strain measures the effects of carbon and nitrogen atmosphere pinning on dislocations. By comparing accelerated aging with room temperature aging behaviours of low carbon steels for up to two years, an algorithm deriving the activation energy of solute carbon and nitrogen migration from aging property measurements was proposed. The equation derived for accelerated aging of hot‐rolled low carbon steels investigated was in reasonable agreement with that reported in the literature.

Influence of process parameters on scale reduction with H2

A new pickling method that reduces the metal oxide on steel strip using H2 has been studied through an extensive campaign of experimental tests. The process, referred to as 'acid free pickling', is environmentally friendly and does not use acid. It is most suitable for application to hot rolled low carbon steel strip. Experimental studies are in progress to validate the application of hydrogen on different metals.

Precipitate characterisation of an advanced high-strength low-alloy (HSLA) steel using atom probe tomography

Increased fuel economy, combined with the need for the improved safety has generated the development of new hot-rolled high-strength low-alloy (HSLA) and multiphase steels such as dual-phase or transformation-induced plasticity steels with improved ductility without sacrificing strength and crash resistance. However, the modern multiphase steels with good strength-ductility balance showed deteriorated stretch-flangeability due to the stress concentration region between the soft ferrite and hard martensite phases [1]. Ferritic, hot-rolled steels can provide good local elongation and, in turn, good stretch-flangeability [2]. However, conventional HSLA ferritic steels only have a tensile strength of not, vert, similar600 MPa, while steels for the automotive industry are now required to have a high tensile strength of not, vert, similar780 MPa, with excellent elongation and stretch-flangeability [1]. This level of strength and stretch-flangeability can only be achieved by precipitation hardening of the ferrite matrix with very fine precipitates and by ferrite grain refinement. It has been suggested that Mo [3] and Ti [4] should be added to form carbides and decrease the coiling temperature to 650 °C since only a low precipitation temperature can provide the precipitation refinement [4]. These particles appeared to be (Ti, Mo)C, with a cubic lattice and a parameter of 0.433 nm, and they were aligned in rows [4]. It was reported [4] that the formation of these very fine carbides led to an increase in strength of not, vert, similar300 MPa. However, the detailed analysis of these particles has not been performed to date due to their nanoscale size. The aim of this work was to carry out a detailed investigation using atom probe tomography (APT) of precipitates formed in hot-rolled low-carbon steel containing additions Ti and Mo. The investigated low-carbon steel, containing Fe–0.1C–1.24Mn–0.03Si–0.11Cr–0.11Mo–0.09Ti–0.091Al at.%, was produced by hot rolling. The processing route has been described in detail elsewhere [5] European Patent Application, 1616970 A1, 18.01.2006.[5]. The microstructure was characterised by transmission electron microscopy (TEM) on a Philips CM 20, operated at 200 kV using thin foil and carbon replica techniques. Qualitative energy dispersive X-ray spectroscopy (EDXS) was used to analyse the chemical composition of particles. The atomic level of particle characterisation was performed at the University of Sydney using a local electrode atom probe [6]. APT was carried out using a pulse repetition rate of 200 kHz and a 20% pulse fraction on the sample with temperature of 80 K. The extent of solute-enriched regions (radius of gyration) and the local solute concentrations in these regions were estimated using the maximum separation envelope method with a grid spacing of 0.1 nm [7]. A maximum separation distance between the atoms of interest of dmax = 1 nm was used. The microstructure of the steel consisted of two types of fine ferrite grains: (i) small recrystallised grains with an average grain size of 1.4 ± 0.2 μm; and (ii) grains with a high dislocation density (5.8 ± 1.4 × 1014 m−2) and an average grain size of 1.9 ± 0.1 μm in thickness and 2.7 ± 0.1 μm in length (Fig. 1a). Some grains with high dislocation density displayed an elongated shape with Widmanstatten side plates and also the formation of cells and subgrains (Fig. 1a). The volume fraction of recrystallised grains was 34 ± 8%.

Determination of spatial grain size with the area-weighted grain area distribution of the planar sections in polycrystalline materials

The electron backscattering diffraction (EBSD) technique has made it quite easy to obtain the grain area distribution in a planar section of the polycrystalline materials. Usually, area-weighted grain area and number-weighted grain area show completely different distribution profiles. Instead of the nominal average grain size calculated from the grain area distribution results, the spatial grain size and its distribution character reflect the real features of the materials and those parameters are expected through a simple measurement on the planar sections. In the present work, the section area distribution model of a randomly sliced truncated octahedron is used to relate the lognormally distributed spatial grain size to the area-weighted grain size distribution features. Based on the truncated octahedron grain shape model, area-weighted grain area distribution is derived by convoluting the lognormal distribution of the area of the section with the maximum section area (SMSA) in each grain and the area-weighted section area distribution of the sliced truncated octahedron. It is found that area-weighted grain area distribution on a planar section shows also the lognormal distribution character when the spatial grain size has the lognormal distribution character. Meanwhile, the variance parameter, τ(ins), and the mean value of grain area, \(X_s^{mean} \), obtained by fitting the area-weighted grain area distribution, have also been related to the lognormal distribution characters of the spatial grain size, i.e., A and \(\overline \nu _n \). The simulation results are then experimentally checked on a polygonal grain structure of hot-rolled low-carbon steel by applying the point-sampling method to obtain the spatial grain size distribution features.

Development of a Hot-rolled Low Carbon Steel with High Yield Strength

A high strength steel with yield strength on the order of 700 MPa had been developed successfully with the addition of titanium alloy element based on a low-carbon steel. The results show the hot deformation accelerates ferrite and pearlite transformation and retards bainite transformation under continuous cooling condition. The strengthening factors of this steel are attributed to the tiny interlocked distribution of bainite laths and titanium carbide precipitation. When the amount of bainite is more than 70%, the yield strength of steel is higher than 700 MPa, and simultaneously ratio of tensile of yield is more than 1.07, and good mechanical performances are obtained. As a result of the presence of finely dispersed cementite particles which improve the work hardening capacity, the ductility of steel is improved, with the steel's elongation being about 19%. Many tiny TiC precipitates are observed under transmission electron micrograph (TEM), and these particles result in precipitation strengthening.

Low-Cycle Fatigue Strength of Notched Low-Carbon Steel Materials

Large structures are usually constructed using the conventional low-carbon steel and often contain many kinds of discontinuous cross sections or initial defects. The fatigue cracks commonly initiate at these sites under repeated loadings. So, the estimation of low-cycle fatigue life for the notched components is one of the most important technologies for the structural metals. In this study, the low-cycle fatigue tests were carried out using the smooth round bar specimens and the notched plate ones with the stress concentation factors Kt = 2.43, 3.26 and 4.25. The strains at the notch root were measured over the fatigue tests. The materials used are the hot-rolled low-carbon steel sheets, SM400A and SM490A. On the basis of test results, the method to estimate the strain on the notched structure using Neuber method, Stowell ones and the fatigue life were considered

Analytical Electron Microscopy Study of Interfacial Oxides formed on a Hot-rolled Low-Carbon Steel

The detailed microstructure of oxides at the metal–scale interface of a commercial, hot-rolled steel strip has been examined using analytical electron microscopy. A continuous magnetite layer of about 1–2 μm thick was observed at the interface. The magnetite layer often contained cracks propagating in directions parallel to the interface. In the vicinity of the cracks, the magnetite was oxidized to nanosized maghemite (γ-Fe2O3). We also observed evidence indicating the existence of microchannels which were roughly perpendicular to the interface and might allow oxygen to reach the embedded cracks. Oxide above the magnetite layer was a mixture of α-iron and magnetite produced by wustite decomposition.

Prediction of shape defects during cooling of hot rolled low carbon steel strip

Shape defects are found in hot rolled steel strip when unwrapping tightly wound coils. This problem is particularly acute in thin strips that were considered to be defect free while processing. A model developed to predict the occurrence and magnitude of such defects in hot rolled low carbon steel strip is described in the present paper. The model assumes that the strip is free of shape defects as it exits the last stand of a continuous mill, but, as a result of processing conditions, thermal and microstructural gradients are present across the width of the strip. It is considered that the variation of ferrite and austenite mixture is caused by the chemical composition of the steel and the actual temperature of the strip. On cooling to room temperature, the distribution of both temperature and microstructure will cause variation in the local contraction that the steel is subjected to, and will promote shape defects.

Behaviour of copper during the high temperature oxidation of steels produced by the electric furnace route

The surface quality of hot rolled low-carbon steels may be affected by their copper content when it exceeds 0.3% by weight. At high temperature (more than 1200 °C), a large part of the non oxidized copper is occluded into the primary scale and hot shortness phenomenon is prevented. At lower temperature (around 1000 °C), surface cracking may occur because Cu enrichment at the scale/steel interface cannot be avoided. Then the running risk depends on time and respective contents in Cu, Ni and Sn.

Relevance of ROT control for hot rolled low carbon steels

The need for precision in controlling coiling temperature and cooling profiles on the run out table for hot rolled low carbon steel strips has been investigated. It is claimed in literature that a high degree of automation and control of the run out table is required to control the yield strength variation to within 20 MPa. However, calculations based on models available in literature predict that the variation in strip temperature encountered in a typical run out table does not affect the mechanical property significantly. This has been shown to be true even from experimental data available in literature. Hence even a coarse control over the run out table is adequate to achieve the desired yield strength. A detailed report concentrating on the influence of coiling temperature on the mechanical properties of HSLA steels is under preparation. Bedeutung der Steuerung des Auslaufrollgangs für warmgewalzte unlegierte Stähle. Im Zentrum dieser Untersuchung steht die Frage, ob eine Notwendigkeit zur präzisen Steuerung der Haspeltemperatur und der Abkühlprofile im Auslaufrollgang einer Warmwalzstraße besteht, wenn unlegierte Sorten umgeformt werden. Das Schrifttum fordert einen hohen Automationsgrad und eine Steuerung des Auslaufrollgangs zur Einstellung der Streckgrenze mit einer Genauigkeit von 20 MPa. Dennoch beeinflussen solche Mittel die mechanischen Eigenschaften nicht deutlich, wie Rechnungen mit verschiedenen Modellen, die im Schrifttum beschrieben sind, zur Vorhersage der Bandtemperatur in typischen Auslaufrollgängen zeigten. Dies wird im Schrifttum auch mit Versuchsdaten untermauert. Folglich reicht schon eine grobe Steuerung des Auslaufrollganges aus, um die gewünschte Streckgrenze einzustellen. Eine detaillierte Studie zum Einfluß der Haspeltemperatur auf die Festigkeitseigenschaften von HSLA-Stählen ist in Vorbereitung.

Modelling the Austenite to Ferrite Phase Transformation in Low Carbon Steels in Terms of the Interface Mobility.

The kinetics of the phase transformation between the high temperature FCC-phase austenite and the low temperature BCC-phase ferrite as it occurs during controlled cooling of hot rolled low carbon steels is described using a physical model that considers austenite grain size, nucleus density, composition effects, and the austenite/ferrite interface mobility. The model is verified against experimental dilatometry data for three lean carbon-manganese steel grades. The model yields adequate reproductions of the transformation kinetics. Ferrite grain coarsening during the transformation appears to have a significant effect on the final microstructure.

High-temperature surface oxidation of low-carbon rimming steel

Phase composition and macrostructure of the surface oxide films formed during the oxidation of hot-rolled low-carbon rimming steel in air at 600°C have been investigated by standard and grazing incidence x-ray diffraction. The oxidation kinetics have been analysed by the weight change method. It has been revealed that the oxide film consists of layers of FeO, Fe 3 O 4 and Fe 2 O 3 . The latter two phases have been observed after 2 min of oxidation, whereas the first one appears only after 6 min of annealing in air at 600°C. The oxidation kinetics obeys a logarithmic law during the whole investigated time interval of annealing.

Composition design of a new type low-alloy high-strength steel

Taking the Fe-Mn-W-Nb-Si polybasic alloy as the main alloying additive and adding micro amounts of Boron (B), a type of hot-rolled low-carbon low-alloy bainite steel with a 750 MPa yield strength and a 5 ∼ 10-mm diameter after air cooling is designed by means of our own calculated formulas which give the chemical composition and the cooling rate of supercooled austenite on the microstructure and strength in steels. The results show that the experimental steel after smelting and rolling is in accordance with the design idea that in alloying element contents, they should be kept to a minimum level, 0.2% in the case of carbon content, assuring the steel strength requirement and obtaining bainite or bainite and martensite.

Fatigue, Cyclic Deformation and Microstructure. Effects of Texture on Fatigue Crack Propagation of Low-carbon Steel.

Plane bending fatigue tests were carried out on several kinds of hot-rolled low-carbon steel sheets with texture to clarify the effects of texture on fatigue crack propagation. The crack growth rate of each material was determined uniquely by a term σanl, where σa is nominal stress amplitude, l is the crack length, and n is a material constant. The n exhibited different values according to the materials, whereas there is not so much difference in the σB of each material. The different n in such materials may result from the difference in texture. Moreover, the value of n tended to decrease with an increase in r value, plastic anisotropy ratio. The analysis based on the small-crack growth law, dl/dN=C3(σa/σB)nl, in which the effect of a material property was considered, suggested that the effect of texture on the value of C3 is negligible. The reciprocal of C3 represents the resistance to small crack propagation in each material.

溶接構造用圧延鋼材の円孔切欠き部の低サイクル疲労挙動

Large structures are usually constructed using the conventional low-carbon steel and often contain many kinds of discontinuous cross sections or initial defects. The fatigue cracks commonly initiate at these sites under repeated loadings. So, the estimation of low-cycle fatigue life of the notched components is significantly important in the design of structures.In this study, the low-cycle fatigue tests were carried out using the smooth round bar specimens and the notched plate specimens machined from the hot rolled low-carbon steel sheets of SM400A, SM400B and SM490A. After analyzing the experimental data, the estimating method of low-cycle fatigue life of notched specimens was proposed.

Deformation behaviour of hot rolled low carbon steel sheets: the effects of sheet thickness and modes of testing on formability

Deformation behaviour of hot rolled low carbon steel sheets: the effects of sheet thickness and modes of testing on formability

Banding and the nature of large, irregular pearlite nodules in a hot-rolled low-alloy plate steel: A second report

The microstructure of a hot-rolled low-carbon plate steel has been examined using a combination of light microscopy and scanning electron microscopy. It has been found that in the hot-rolled condition, the microstructure consists of alternate bands of ferrite and pearlite, together with relatively large, irregular pearlite nodules. These large nodules were found to be comprised of pearlite, intragranularly nucleated ferrite (both Widmanstatten and idiomorphic), together with carbide-deficient and/or carbide-free regions. It is argued that the carbide-deficient and carbide-free regions form as a result of the premature initiation of the pearlite reaction, i.e. pearlite forms prior to the body of the austenite grains attaining the eutectoid composition. In order to model the formation of the banded structure, specimens were reaustenitized at 1050 °C for 10 min and furnace cooled. This heat-treatment cycle produced an austenite grain size which was less than the chemical banding wavelength. A model for the decomposition of austenite, under these conditions, is presented.

Microstructural features of sliver defects in hot-rolled low-carbon steel sheets

Microstructural features of sliver defects in hot-rolled low-carbon steel sheets