Cold-rolled High-strength Steel Research Articles

980 MPa Grade Low-Alloy Carbide-Free Bainitic Steel Obtained by Dynamic Continuous Cooling Transformation

The addition of high-content alloying elements and the unbefitting process make carbide-free bainite steel difficult in industrial production. Thus, we adopted a dynamic continuous cooling process for developing the high-strength cold-rolled low-alloy carbide-free bainitic steels in this study. The influence of cooling rates on the microstructure and mechanical properties was investigated by dilatometry, scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and tensile tests. The results show that the bainitic ferrite plates were refined by decreasing the cooling rate, and more austenite was retained in the steel with a medium cooling rate. Both the TRIP effect and the refined bainitic ferrite plate contribute to the good strength–ductility match. Consequently, the propitious microstructure adjustment is critical for developing 980 MPa carbide-free bainitic steel.

Creation of an Effective Technology for the Production of Cold-Rolled High-Strength Low-Alloy Steels with High and Stable Properties. Part 2. Cold-rolled Products

Creation of an Effective Technology for the Production of Cold-Rolled High-Strength Low-Alloy Steels with High and Stable Properties. Part 2. Cold-rolled Products

Creation of an Effective Technology for the Production of Cold-Rolled High-Strength Low-Alloy Steels with High and Stable Properties. Part 1. Hot-rolled Products

Creation of an Effective Technology for the Production of Cold-Rolled High-Strength Low-Alloy Steels with High and Stable Properties. Part 1. Hot-rolled Products

Partitioning of Solute Elements and Microstructural Changes during Heat-treatment of Cold-rolled High Strength Steel with Composite Microstructure

Partitioning of Solute Elements and Microstructural Changes during Heat-treatment of Cold-rolled High Strength Steel with Composite Microstructure

Effects of initial microstructure before cold rolling on microstructure evolution and mechanical behaviour of CGLcompatible Q&P steel

Quenching and partitioning (Q&P) steels offer impressive combination of strength and ductility that can reduce the mass and improve the crash worthiness. In order to be able to meet consumer’s durability expectations, corrosion protection and galvanizability become crucial. In this paper, based on a cold-rolled high strength steel (0.225C-0.85Si-2.02Mn-0.91Al, in wt.%) and continuous galvanizing line (CGL) compatible Q&P treatment, the effects of the initial microstructure before cold-rolling were systematically investigated by means of SEM, EPMA, TEM, EBSD and XRD. It is shown that the resulting microstructure and mechanical properties after Q&P treatment are influenced by the initial microstructure. About 10.2-16.1vol.% RA are obtained, which is potential to gain the desired mechanical stability. Compared with “Q&P sample with initial microstructure prepared by quenching and austempering (i.e., Q&P(QAT))”, “Q&P sample with initial microstructure prepared by quenching and austempering followed by tempering (i.e., Q&P(QAT&T))” obtained a higher fraction of retained austenite (16.1%RA) and relatively low carbon content, thus resulted in an excellent ductility because of better mechanical stability and consequently more effective and sustained TRIP effect at high strain. With increasing the tempering time, the tempering degree of martensite increases markedly, while it does not have strong effects on the microstructures, leading to the similar fractions of RA and properties in “Q&P sample with initial microstructure prepared by quenching and tempering for 4min (i.e., Q&P(Q&T-1))” and “Q&P sample with initial microstructure prepared by quenching and tempering for 10min (i.e., Q&P(Q&T-2))”.

Effects of Al addition on tensile properties of partially recrystallized austenitic TRIP/TWIP steels

Abstract Austenitic high-Mn steels are regarded as a promising candidate for high-strength cold-rolled steels because their combination of strength and ductility improves greatly by combined effects of twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP). Although it is well known that Al plays key roles in tensile properties of austenitic TWIP steels, its effects in austenitic TRIP or TRIP/TWIP steel are still unclear yet. In this study, three austenitic steels (composition; 0.4C–15Mn–1Si-(0,0.5,1)Al-0.3Mo-0.5V (wt.%)) were fabricated, and the effect of Al alloying on microstructures and tensile properties were investigated in relation to the deformation behavior with TRIP and TWIP mechanisms. A partial recrystallization was conducted for enhancing the yield strength, which was characterized with electron backscatter diffraction (EBSD) grain orientation spread maps. The present steels showed 1 GPa of yield strength achieved by partial recrystallization with the precipitation of (V + Mo) complex carbides. Particularly in the non-Al-alloyed steel, the e-martensite formed in the early deformation stage, and the martensitic transformation continued until the failure, thereby resulting in the highest tensile strength (1.5 GPa) along with the highest strain hardening.

Relationship between Morphology of Mn Oxides Simulated by Ion Plating and Phosphatability of Mn-added High-strength Cold-rolled Steel Sheets

Relationship between Morphology of Mn Oxides Simulated by Ion Plating and Phosphatability of Mn-added High-strength Cold-rolled Steel Sheets

Delaying Effect of Cementite on Recrystallization Kinetics of a Ti-Nb Microalloyed High-Formability Steel

Annealing of cold-rolled high-strength steels leads to various microstructural changes such as recrystallization, cementite precipitation, microalloying elements precipitation and austenite formation. These transformations are expected to interact with each other. Understanding how and where austenite forms in a microstructure is of prime importance to avoid formation of banded microstructures, which are detrimental to good in-use properties. In this work, a mean-field model is used to simulate concomitant recrystallization, cementite precipitation, microalloying elements precipitation and austenite formation kinetics, as well as their interactions during 1 and 10 °C/s heating. Excellent agreement with experimental data is obtained only if cementite pinning effect on recrystallized grain boundaries is considered. It is shown that cementite exhibits a much stronger delaying effect on recrystallization kinetics than microalloying elements, leading to the formation of banded microstructures. Carbon nominal content of a steel appears to be the most important parameter to acknowledge to understand recrystallization kinetics.

Sub-zero temperature mechanical properties of cold-rolled steel sheets

Cold-formed steels are replacing conventional building materials in many building applications due to the many benefits including lightweight and low cost of construction. Recent research has focused on the fire resistance of cold-formed steel (CFS) construction at material and member levels and advanced the knowledge of their fire performance significantly. However, the performance of CFS members at sub-zero temperatures has not been investigated. Although several studies have been conducted on the mechanical properties of other types of steel at sub-zero temperatures, no studies have been conducted on sub-zero temperature mechanical properties of thin cold-formed steels. In this research, low and high strength cold-rolled steel sheets were tested in the temperature range of 20 to −70 °C to determine their sub-zero temperature mechanical properties. Predictive equations are proposed for yield strength, Young's modulus, upper yield strength, ultimate strength and stress at 2% total strain using the experimental results. Finally, suitable stress-strain models are recommended for the prediction of sub-zero temperature stress-strain curves and their use in numerical studies.

Partitioning of Solute Elements and Microstructural Changes during Heat-treatment of Cold-rolled High Strength Steel with Composite Microstructure

The partitioning of solute elements during intercritical annealing and the effects of partitioning on ferrite transformation during slow cooling after intercritical annealing in a 0.17% C–1.5% Si–1.7% Mn (mass%) steel were investigated by a new FE-EPMA (field emission electron probe microanalysis) technique. This new technique enables highly accurate measurement of the C distribution. During the intercritical annealing, C and Mn concentrated into austenite, while Si concentrated into ferrite. The distribution of Mn in austenite was inhomogeneous, and austenite with small Mn content was transformed into ferrite during slow cooling. This ferrite transformation proceeded in the NPLE (negligible partitioning local equilibrium) mode. Two kinds of ferrite were produced due to slow cooling, one being intercritically-annealed ferrite, and the other transformed ferrite. The transformed ferrite had larger Mn content than the intercritically-annealed ferrite. Furthermore, the transformed ferrite was classified into the ferrite grown epitaxially from the intercritically-annealed ferrite and that nucleated in the austenite with relatively small Mn content. Prior microstructure and distribution of solute elements before cooling are determined by the intercritical annealing conditions, and then control the ferrite transformation. Precise control of the ferrite transformation is effective for stable production of cold-rolled high strength steel with composite microstructure.

Features of the Effect of Microstructure Characteristics on Corrosion Resistance of Cold-Rolled High-Strength Low-Alloy Steels (HSLA) Grade 260–300 for Automobile Building

An experimental study is conducted on the effect of production parameters on the corrosion resistance of cold-rolled high-strength low-alloy steels (HSLA) grade 260–300, treated in continuous annealing lines (CAL). It is been shown that corrosion resistance is mainly determined by development of ageing in overageing sections and during subsequent cooling. A reduction in strip movement and an increase of overageing temperature leads to more complete development of this process. A tendency towards ageing is also affected by hot rolling parameters, i.e., the end of rolling temperature in the roughing group of stands T6, correlating with an increase reheating temperature, and finishing rolling temperature Ter . The favorable effect of the T6 increase is probably connected with more complete dissolution of manganese sulfide during heating and formation of a larger number of submicron precipitates of manganese sulfide during rolling, which are a substrate for precipitation of aluminum nitride and cementite on their surface. With a reduction in finishing rolling temperature Ter , the end of aluminum nitride precipitation is more complete. These factors cause a reduction in nitrogen and carbon concentration ferrite and, consequently the tendency towards aging.

Elevated temperature mechanical properties of cold-rolled steel sheets and cold-formed steel sections

Cold-formed steel (CFS) is increasingly used in building construction in many countries due to its lightweight and fast and easy construction characteristics. However, its fire resistance is not well understood, which may restrict its applications. For fire design purposes, a good knowledge of the elevated temperature mechanical properties of CFS is essential. Although several useful studies have been conducted on the mechanical properties of CFS, the elevated temperature reduction factors vary significantly among them while no predictive equation is available to calculate the proportional limit stress of CFS at elevated temperatures. Moreover, they also show significant variations between the elevated temperature reduction factors of cold-rolled steel sheets and CFS sections. In this research, low and high strength cold-rolled steel sheets and high strength CFS lipped channel sections and floor decks were tested in the temperature range of 20–700 °C under isothermal conditions to determine the reductions in their mechanical properties. Predictive equations given in AS/NZS 4600 for yield strength and Young's modulus were verified, and new predictive equations for ultimate strength, stress at 2% total strain, 0.05% proof stress and proportional limit stress were proposed. Finally, a two-stage stress-strain model was proposed to accurately predict the stress-strain curves of CFS at ambient and elevated temperatures.

Materials and Processes for the Third-generation Advanced High-strength Steels

The third-generation advanced high-strength steels (AHSS 3. Gen.) have been developed in order to meet the requirements of the automotive industry for weight reduction, improved fuel efficiency, and CO2 mitigation. The alloy design of AHSS 3. Gen., their microstructure characteristics, mechanical properties, potential applications as well as the need for thorough process parameter control are discussed in this paper. The cold-rolled AHSS 3. Gen. can be differentiated as quenching and partitioning (Q&P) steels and medium-Mn steels (MMnS). Specific to these materials are the occurrence of different phases with typical sizes on the nm-scale, the elemental partitioning between these phases and the impact of the metastable austenite phase on the mechanical properties by means of transformation-induced-plasticity (TRIP) as well as of twinning-induced-plasticity (TWIP) effects. The requirements for the process design and the new annealing concepts are emphasized. Furthermore, the industrial feasibility and challenges of production of cold-rolled high-strength steels are discussed. In particular, the development of new furnace technologies is introduced for modern steels with complex microstructures.

Relationship between the Morphology of Mn-oxides Simulated by Ion Plating and Phosphatability for Mn-added High Strength Cold-Rolled Steel Sheets

It is well known that Si, Mn and B, the alloying elements for high strength steel sheets, easily form oxides on the steel surface during annealing in a reducing atmosphere, and those oxides have a large influence on the surface performance of steel sheets, such as phosphatability. In this work, we discovered that the oxidation behavior of Mn-added high strength cold-rolled steel sheets could be simulated on mild steel sheets by using an ion plating method and investigated the relationship between the morphology of Mn oxides and phosphatability under the condition that both the amount and kind of Mn oxides were fixed. In a simulated Mn-O layer, fine surface oxides, which covered most of the steel surface, were observed after annealing. On the other hand, in a Mn-B-O layer, large globular surface oxides were observed on the steel surface, and the Fe surface was partially bare. The B-Mn compound oxide is considered to be in a molten phase during annealing because the melting point of the compound oxide is lower than the annealing temperature, and as a result, it is thought that large B-Mn compound oxides coagulate and grow during annealing. In addition, it was found that the large B-Mn compound oxides (about 500 nm) interfere with steel dissolution in the phosphate solution. These results demonstrate the importance of controlling the morphology as well as the amount and kind of surface oxides for obtaining good phosphatability of Mn-added high strength cold-rolled steel sheets.

A Study of the Optimum Quenching Temperature of Steels with Various Hot Rolling Microstructures after Cold Rolling, Quenching and Partitioning Treatment

Quenching and partitioning (Q&P) processes were applied to a cold-rolled high strength steel (0.19C-1.26Si-2.82Mn-0.92Ni, wt %). The effects of the prior hot-rolled microstructure on the optimum quenching temperature of the studied steels were systematically investigated. The microstructure was analyzed by means of transmission electron microscope (TEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). Compared with the ferrite pearlite mixture matrix, the lower martensite start (Ms) temperature and smaller prior austenite grain size in the cold-rolled martensite matrix are the main reasons for the optimum quenching temperature shifting to a lower temperature in the Q&P steels. We found that an empirical formula that only considers the influence of the alloy composition in the calculation of the Ms temperature will cause a certain interference to the pre-determined optimum quenching temperature of the Q&P steel.

High accuracy springback simulation by using material model considering the SD effect

In order to enhance the accuracy of springback simulation, the strength differential (SD) effect, i.e., the difference in flow stress between tension and compression, of a high strength cold-rolled steel sheet with a tensile strength of 980 MPa is measured by means of in-plane tension-compression test apparatus. Bi-axial stress tests are also performed to measure the contour of plastic work of the test material. From those experimental results, the material model which can consider the SD effect is determined. Furthermore, this material model is implemented into commercial FEM code by using user-subroutine function. To check the validity of this model and established FEM analysis system, curvature-hat crush forming experiment is performed. By comparing the experimental result and forming simulation result, the accuracy of the material model which can consider the SD effect is validated. Consequently it is concluded that the use of material model which is capable of reproducing the SD effect is a must to enhance the accuracy of springback simulation.

Combined Effect of Heating Rate and Microalloying Elements on Recrystallization During Annealing of Dual-Phase Steels

Adjusting ferrite recrystallization kinetics during annealing is a way to control the final microstructure and thus the mechanical properties of advanced cold-rolled high-strength steels. Two strategies are commonly used for this purpose: adjusting heating rates and/or adding microalloying elements. The present work investigates the effect of heating rate and microalloying elements Ti, Nb, and Mo on recrystallization kinetics during annealing in various cold-rolled Dual-Phase steel grades. The use of combined experimental and modeling approaches allows a deeper understanding of the separate influence of heating rate and the addition of microalloying elements. The comparative effect of Ti, Nb, and Mo as solute elements and as precipitates on ferrite recrystallization is also clarified. It is shown that solute drag has the largest delaying effect on recrystallization in the present case and that the order of solute drag effectiveness of microalloying elements is Nb > Mo > Ti.

Studies on the Influence of a Counterpart on Fretting Wear of Cold-Rolled High Strength Steel

This article investigates the influence of a counterpart on fretting wear of cold-rolled high strength steel for automotive applications. Fretting wear tests are conducted using a ball-on-flat test apparatus. The friction forces between cold-rolled high strength steel plates and various counterparts are measured. The worn profile of a cold-rolled high strength steel specimen is determined to calculate the wear rate. Experimental results show that the ratio of the maximum tangential to the normal force at a contact surface varies with a counterpart material. Measured worn surface profiles indicate that the cross-sectional area of a wear scar is enlarged as the number of cycles increases. The effects of counterpart hardness and the maximum Hertzian contact pressure are identified on the wear rate of cold-rolled high strength steel. From measurement and analysis results, ZrO2 counterparts are found to be more adequate than the commercial ones for the cold-rolled high strength steel rails.

Flatness evolution of cold-rolled high-strength steel strips during quenching process

Drastic flatness changes in high-strength steel strips during water quenching cause problems for both their subsequent manufacture and final products. In this work, a temperature–microstructure–str...

Study of Weld Integrity and Nugget Development in Seam-Welded Cold-Rolled High-Strength Steels

Production processes like continuous annealing and continuous galvanizing/galvannealing rely on welding of two strips across their width, for continuous operation. The operational stability of these lines depends highly on the consistency of the welding process. Any failure of weld joint during processing is detrimental to production as the recovery times are very high (~1–3 days). Two instances of similar weld failures having identical chemical composition and dimensions of the strips were investigated to identify the root cause and establish adequate countermeasures. A systematic approach to evaluate weld performance using metallographic examination was carried out. The examinations revealed that poor nugget formation was the root cause of failure. The nugget formation, its orientation and size were also evaluated after carrying out a series of trials with different welding process parameters, and an optimum condition was arrived at. In summary, the weld failure occurred due to insufficient and unbalanced nugget formation which could be rectified by modifying the welding parameters such as current, pressure and speed.