Hot-rolled Steel Sheets Research Articles

熱間圧延の組織材質予測制御技術とホットスタンピング技術への貢献

熱間圧延の組織材質予測制御技術とホットスタンピング技術への貢献

Effect of Material Anisotropies of Hot-Rolled High-Strength Steel Sheet on Localized Deformation Behavior in Hole Expansion - Part-I Experimental Consideration to Circular and Oval Hole

In this study, the effect of the material anisotropies of hot-rolled high-strength steel sheet on localized deformation behavior in hole expansion test has been investigated experimentally. First, the hole expansion test with the circular hole has been conducted to investigate the effect of anisotropies of material properties on the localized deformation behavior around the hole edge. Next, the hole expansion test with the oval hole has been conducted to investigate the effect of the major axis direction of the oval hole on the localized deformation behavior around the hole edge. As a result, it was clarified that the effect of anisotropies of r-value and n-value on the localized deformation behavior is strong, especially the anisotropy of n-value.

Effect of Material Anisotropies of Hot-Rolled High-Strength Steel Sheet on Localized Deformation Behavior in Hole Expansion - Part-II Simulation and Evaluation of Anisotropic Yield Function

In this study, the circular hole expansion process of high-strength steel sheet is numerically simulated using FE analysis with Hill48 quadratic, Gotoh’s fourth order, Yld2000-2d and Yoshida’s sixth order polynomial yield function. The effects of anisotropic yield functions on local reduction of thickness are evaluated. The thickness distribution around the circular hole edge at just before necking depends on the initial hole diameter. When the initial hole diameter is relative large, the simulation results give almost same thickness distribution among different yield functions. While the initial hole is relative small, individual characteristics of yield function becomes clear and the sixth order yield function gives the best prediction.

Micro-computed tomographic imaging of void damage in a hot-rolled complex phase sheet steel

Interrupted hole tension tests were carried out on four samples of hot-rolled complex phase Advanced High Strength Steel (AHSS) sheet (UTS=780MPa) to investigate the evolution of void damage at the edge of an expanding hole. Optical microscopy and micro-computed tomographic (microCT) imaging of plastically deformed hole tension specimens indicated that voids nucleate at TiN particles and grow primarily in the direction of tensile loading, developing ellipsoidal shapes as plastic strain accumulates. Larger, cubic particles are associated with larger voids in one of the four sheets under study, culminating in a single, large coalescence event at high plastic strains. Several smaller coalescence events are observed in the other three sheets containing small, irregularly-shaped TiN particles. Coalescence of closely spaced voids was found to occur through necking of the inter-void ligament while void sheet formation was the primary mode of coalescence for large voids spaced farther apart and at an angle of ~45° between the loading and through-thickness directions. A peak in the void volume fraction was observed approximately 0.6mm from the hole edge as a result of the competing influences of plastic strain and triaxiality.

Effect of dual beam laser welding on microstructure–property relationships of hot-rolled complex phase steel sheets

The work addresses microstructure–property relationships of hot-rolled complex phase steel sheets subjected to various conditions of dual beam laser welding. The article also contains a comparison between single-spot and twin-spot laser welding. Test-related joints were made using a Yb:YAG disc laser having a maximum power of 12kW and a welding head enabling the focusing of a laser beam on two spots. The tests involved investigating the effect of power distribution on the macrostructure, microstructure and mechanical properties of joints. Microscopic investigations revealed the stabilisation of some fraction of retained austenite in the intercritical heat affected zone (ICHAZ) of joints. The application of the second beam resulted in tempering-like effects in the martensite of the fusion zone and in the HAZ, favourably reducing the hardness of the joint. The use of bifocal welding enabled the obtainment of a 10% hardness reduction in the fusion zone as against single-spot laser beam welding.

Evaluation of the Residual Stress State of 42crmo4 Steel Sheets in a Production Line

The residual stress state of a mechanical component is an important factor in its production planning and in estimates of its lifecycle since it can be responsible for geometric distortions and degradation of fatigue properties. Therefore, the development of reliable methods for non-destructively quantifying these stresses remains in the interest of most manufacturing industries; Barkhausen magnetic noise measurements have been investigated in several applications and remains a viable option. However, its effective implementation has occurred mostly in components with simple geometries and insignificant microstructural gradients; even in these cases, successful industrial adoption of the method depends on previous calibration with samples that are often difficult and costly to prepare and validate. This work aims at investigating the capability of the method of characterizing the residual stress state in a simple but generally useful application: samples of hot-rolled steel sheets collected at two different stages of processing in an industrial mechanical conformation and heat treatment plant. In this analysis Barkhausen noise measurements were compared to X-ray diffraction results, and statistical analysis tools were used to correlate the results.

Influence of Thermal Cyclic Deformation and Hardening Heat Treatment on the Structure and Properties of Steel 10

The results of the influence of preliminary thermal cyclic deformation and subsequent hardening heat treatment on the microstructure and mechanical properties of hot-rolled sheet steel 10 are presented. It is shown that the use of preliminary thermal cyclic deformation of the steel 10 stock material results in a fine-grained structure of a hot-rolled sheet (3 mm thick) produced by an industrial technology. Deformation occurred at a temperature above AC3 (1250 °C), with cooling to 200-300 °C during 10 cycles and the deformation ratio per cycle being 6-8 %. Such a treatment before sheet hot-rolling allows increasing the strength characteristics (tensile strength, yield strength) by almost 30 %. It has been established that the use of subsequent heat treatment (quenching, 900 °C, water and tempering 1 h, 600 °C) leads to a further increase in strength characteristics by 15-20% while maintaining a sufficient level of ductility of sheet steel.

Quantitative prediction of deformed austenite and transformed ferrite texture in hot-rolled steel sheet

A model to quantitatively predict ferrite (α) textures in hot-rolled steel sheets has been developed. In this model, the crystal plasticity model, called “Grain Interaction model (GIA)”, and the transformation texture model, called “Double K-S relation (DKS)”, are linked together. The deformed austenite (γ) texture is predicted by GIA with taking not only the standard {111}<110> slip system but also non-octahedral slip systems into account. Then the transformed a texture is calculated by DKS, in which a nucleated α prefers to have orientation relationship near the Kurdjumov-Sachs relation with both of two neighboring γ grains. For validation, single pass hot-rolling tests on a C-Si-Mn steel were carried out. The comparison between the predicted and the experimental textures shows that the linked model (GIA & DKS) can lead to a remarkable reproduction of the texture of hot-rolled steel sheets.

Quantitative prediction of transformation texture in steel by Double Kurdjumov-Sachs relation

The transformation texture prediction by so-called double K-S relation is described. Various types of transformation textures such as the one in hot-rolled steel sheets and those of texture memory in hot-rolled steel and cold-rolled pure iron have been reported to be able to be quantitatively predicted by this variant selection rule. Recently such an experimental investigation has been extended to the texture memory in ECAP-processed pure iron. Although the type of texture as well as the symmetry of samples in ECAP iron is very different from the previous materials, the investigation has clearly indicated that the double K-S relation should indeed be the mechanism governing variant selection on the phase transformation in iron and steel.

Development of Ti–V–Mo Complex Microalloyed Hot-Rolled 900-MPa-Grade High-Strength Steel

A new Ti–V–Mo complex microalloyed hot-rolled high-strength steel sheet was developed by controlling a thermo-mechanical controlled processing (TMCP) schedule, in particular with variants in coiling temperature. The effects of coiling temperature (CT) on various hardening mechanisms and mechanical properties of Ti–V–Mo complex microalloyed high-strength low-alloy steels were investigated. The results revealed that the steels are mainly strengthened by a combined effect of ferrite grain refinement hardening and precipitation hardening. The variation in simulated coiling temperature causes a significant difference in strength, which is mainly attributed to different precipitation hardening increment contributions. When the CT is 600 °C, the experimental steel has the best mechanical properties: ultimate tensile strength (UTS) 1000 MPa, yield strength (YS) 955 MPa and elongation (EL) 17%. Moreover, about 82 wt% of the total precipitates are nano-sized carbide particles with diameter of 1–10 nm, which is randomly dispersed in the ferrite matrix. The nano-sized carbide particles led to a strong precipitation hardening increment up to 310 MPa.

Microstructures and Textures of Hot Rolled and Hot Rolled-Normalized 2.9 % Silicon Steel Sheets

Effect of hot rolling and effect of hot rolled-normalizing on microstructures and textures of silicon steel sheets were investigated. In hot rolled steel sheets, fine elongated grains and etch pits were observed. \( \left( {001} \right)[1\bar{1}0] \), \( \left( {112} \right)[1\bar{1}0] \) and \( \left( {223} \right)[1\bar{1}0] \) were strong orientations at the core of steel sheet. At surface and subsurface, \( \left( {110} \right)[001] \) and \( \left( {110} \right)[1\bar{1}2] \) orientations were dominant orientations. In hot rolled-normalized steel sheets, coarse elongated grains and randomly distributed etch pits were observed. Volume fraction (%) of \( \left( {001} \right)[1\bar{1}0] \) orientation was found to be decreasing from core to surface, similar trend was found for \( \left( {112} \right)[1\bar{1}0] \) and \( \left( {223} \right)[1\bar{1}0] \) orientations. \( \left( {110} \right)[1\bar{1}2] \) & \( \left( {110} \right)[001] \) orientations are the dominant orientations at the surface. Volume fraction (%) of \( \left( {110} \right)[1\bar{1}2] \) is more than Goss at the surface.

Microstructure and Composition of Grain Boundary- and Intragranular Oxides in Low Alloy Mn-Cr, Mn-Al-Si and Si-Al-P Hot-rolled Steel Sheets

Abstract During hot rolling, coiling and subsequent cooling of low-alloy steel sheets, internal oxides are formed at the grain boundaries (intergranular) and within the grains (intragranular) as a consequence of oxygen diffusion into the steel and segregating alloy elements. The assembly/microstructure and composition of the oxides found were investigated via transmission electron microscopy (TEM) and electron energy loss (EELS)- or energy-dispersive X-ray (EDX) spectroscopy therein. Our spectroscopic data reveal various simple oxide species, located separately or side by side. Those findings apply to grain boundary oxides and intragranular oxides, for all the steel compositions investigated (Fe-Mn-Cr, Fe-Mn-Al-Si, Fe-Si-Al-P). For example, within the Mn-Cr-alloyed steel we find Mn oxide at all oxide locations (inter- and intragranular), but Cr oxide is not present always. We also discuss the sequences of oxide formation, which follow the thermodynamic rules of oxide formation.

Metallurgical studies with the HIPPO diffractometer at LANSCE

Advantages of neutron diffraction for metallurgical studies are highlighted with research examples performed on the High Pressure - Preferred Orientation diffractometer at the Los Alamos Neutron Science Center. The previously inconclusive crystallographic structure of the Fe–Al ϵ-phase, stable between 1095 and 1231°C, was determined by in situ neutron diffraction to have the formula Fe5Al8 with a body centred cubic structure of the Hume–Rothery Cu5Zn8 type at 1120°C. As a second example, the effect of variations in accumulative roll bonding processing parameters on interfacial structure, hardness, and thermal stability are explored for lamellar Cu–Nb multilayers with individual layer thicknesses of 20 nm. Furthermore, we derive an explanation for observed texture memory effects during the α→γ→α transformation cycle in a 0·1%C–1%Mn hot rolled steel sheet from the double Kurdjumov–Sachs relationship. Finally, we present in situ results of a Zr–2·5Nb sample heated into the β-field and subsequently compressed by 20% via our recently developed creep furnace which uniquely allows deconvolution of the effects of heating, phase transformations, deformation and cooling on the texture evolution.

Recent developments in steel products for automotive applications

This paper reviews recent developments in steels for motor car manufacture and the applications of these steels in the automotive industry. The following items are discussed: hot and cold rolled steel sheets including high strength steel sheets, vibration damping steel sheets, laminated steel sheets, various coated steel sheets and structural specialty steels such as free cutting steel and wire rods for suspension springs.

Surface Defect on Hot Rolled Steel Sheet with Cu Content

Structural steel grades S235JRCCu+N and S355JRCCu+N with 0.25 – 0.35% copper content are intended for products with increased corrosion resistance. They are used for railcar and container bodies. The low copper alloying strongly increases the material life-time in the conditions of atmospheric corrosion. Sheets with fine sharp needle-type bodies on both sheet surfaces were delivered for the analysis. As per the customer information, they were present on almost whole length of on cast strips. Considering the steel copper content, it was assumed that the improper steel alloying was the reason of imperfection. Chemical composition results didn’t confirm this expectation. Metallographic analysis should determine the imperfection origin and decide if copper presence was the primary factor of it. The analyses using SEM coupled with EDS analyzer have shown that the sliver-type defects, few millimeters long bands of ductile iron oxides, were present in very small material depth of about 50 – 150 μm in the analyzed material The bands were formed in more layers parallel to the sheet surface; their thickness was similar to the thickness of surface scales. Oxidic material under the sliver had the same micro-morphology and chemical composition like the scales on the sheet surface. Very fine globular copper particles were randomly and very sporadically present in the layer of ductile oxides in slivers and also in surface scale. Besides this, the material in defect area was chemically homogeneous and no particles were found in the layer boundaries. Copper particles were not detected in the steel basic material; the copper was dissolved in the matrix. The nature of imperfection was typical for V-shaped slivers, caused by overheating sheet surface in reheat furnace. After revising technical conditions of reheat furnace operation, this imperfection type was suppressed.

Metallography of Low Alloy Cr-Mn Hot-Rolled Steel and Quantitative Evaluation of Grain Boundary- and Internal Oxides by TEM

Alloy elements undergo complex diffusion and segregation processes during steel production. Oxygen diffuses into the steel matrix and forms oxides at the surface (called scale), at the grain boundaries and within the grains. In our work we present assemblies and composition of various oxides found at the grain boundaries and within the grains of a low alloy Mn-Cr hot-rolled steel sheet. We utilise metallographic means (surface etching) and transmission electron microscopy (TEM). After etching we observe four zones of different composition: (i) the scale, followed by a decarburised metallic layer with (ii) oxidised grain boundaries, (iii) non-oxidised grain boundaries and last, (iv) the bulk. Via TEM, we analyse alloy element oxides at grain boundaries and within the grains with high spatial resolution. At the grain boundaries and within the grains we find oxides, such as Mn- or Cr oxides, rather than binary oxides (e.g. Fe-Mn- and Fe-Cr oxides) or even more complex oxides, such as Fe-Mn-Cr oxides. The oxide species found lie next to- or embedded amongst one another.

Effects of manganese addition on microstructures and corrosion behavior of hot-dip zinc coatings of hot-rolled steels

In the present work, the Zn coatings of hot-rolled steel sheets with different Mn additions were prepared by hot dipping process. The effects of Mn addition on the microstructures and corrosion resistances of the hot-dip Zn coatings were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffractometry (XRD). The Mn distribution in δ-layer is lower than that of ζ-layer. Mn addition could promote the growth of δ-layer of the coating and inhibit the growth of the columnar ζ phase which morphology was changed to discontinuous phase. It is also found that the surface color of Zn–Mn coating varies with the dipping temperatures. Such phenomenon is supposed to be attributed to the optical interference effect of the Mn oxide film formed on the coating surface. For the corrosion behavior, the as-dipped and ζ layer-exposed Zn coatings were subjected to both Tafel polarization measurements and electrochemical impedance spectroscopy (EIS) tests in 3.5% NaCl solution. The results indicate that Mn addition enhances the corrosion resistance of the as-dipped coatings. The dense corrosion products formed on the top η layer of the Zn–Mn coatings after Tafel polarization corrosion are assumed to inhibit the further corrosion of the coating. However, minor effects of Mn addition to the anti-corrosion performance of ζ layer were noticed.

Hot-dip Galvanizing of Carbon Steel after Cold Rolling with Oxide Scale and Hydrogen Descaling

A new processing method for producing hot-dip galvanized steel is designed and tested, in which pickling is skipped. Hot-rolled low carbon steel sheets are rolled with oxide scale in an experimental mill at room temperature, prior to annealing under a 20% hydrogen reducing atmosphere and galvanizing on a hot-dip galvanizing simulator. Micro-cracks formed in the oxide scale during cold rolling roughen the steel surface and enlarge the specific surface. Through-thickness cracks provide transport channels for hydrogen, and hence the reduction of oxide scale is enhanced. When the sheet is dipped in the zinc bath, cracks are submerged by liquated zinc and the defects are not distinct after hot-dip galvanizing. The overlay coating occludes with rough surface of the sheet, whereby a superior coating adherence is realized.

High Strength Hot Rolled Steel Sheet for Automobile – A Review

Large weight reduction by application of high strength steels to underbody parts can be expected since relatively thick sheets are now adopted. In several different approaches to improve press formability of high strength hot rolled steels are explained. The basic concept of mechanical property control and the performances of a couple of high strength hot rolled steels. The fatigue strength of the steel sheet with scale increases with the decrease in the roughness of the steel sheet surface. The fatigue strength of the sheared edge deteriorates with the increase in the fractured plane roughness of the sheared edge. In case of the microstructure consisting of pearlite and coarse carbides, these phases increase the fractured plane roughness through the connection of the micro-voids arising around them. Accordingly, the reduction of these phases is significant for improving the fatigue strength. Based on these findings, three types of the 780MPa grade high strength hot rolled steel sheet for truck frame use have been developed. Keywords - Hot rolled steel sheet, microstructures, and mechanical tests. The thickness of hot rolled steel usually greater than 1.83 mm for unexposed surface. Hot rolled steel are used in making chassis and underbody component that are about 25% of the total weight of the car. For some component, crashworthiness is required. Since relatively thick sheets are used for these components, considerable weight reductions are expected. Safety is a major concern for the chassis. High durability and reliability are required as well as good press formability to their complicated shapes. The application of hot rolled high-strength steel sheets to the chassis components, which leads to considerable weight reduction, is expected to expand rapidly in the coming years. It is, therefore, demanded to be developed as hot rolled high-

Investigation into the Incidence of Severe Rusting and Pitting Corrosion in Imported Hot-Rolled AISI 430 Ferritic Stainless Steel Coils

Metallurgical investigations were directed to probe into the incidence of inordinate rusting and pitting in imported AISI 430 grade hot-rolled ferritic stainless steel sheet coils. Visual examination, electron microprobe analyses (EPMA), scanning electron microscopy (SEM) and electrochemical potentiokinetic reactivation (EPR) were concomitantly employed to investigate the problem. Studies revealed that the unprecedented degree of corrosion in ferritic stainless steel coils, during the short span of shipment time, was attributable to the ingress of sea water and its retention within the tight folds/ wraps of the steel coils during their shipment. The abundance of moisture and chloride (from the entrapped saline electrolyte) on the steel surface together with depleted O2 supply within the tight folds are presumed to have created conditions akin to an actively-corroding crevice, by way of passive film instability and its eventual breakdown on the stainless steel surface. As a consequence, the coils are believed to have suffered an accelerated and intensified chloride-induced corrosion attack and damage within the short span of shipment time. The investigations also revealed that the corrosive conditions were further exacerbated by the vulnerability and susceptibility of ferritic stainless steel to intergranular corrosion (IGC) due to its inherent sensitized condition. The paper thus throws light on an unusual precedent of chloride-induced corrosion in ferritic stainless steel and highlights the investigative metallographic work and corrosion failure analysis that led to above revelations.