Cold-rolled High-strength Steel Research Articles

Effect of Niobium Addition on the Texture Formation of High Strength Cold-Rolled Low Carbon Steel Sheets

The effect of niobium (Nb) addition on recrystallization texture formation in cold-rolled low carbon steel sheets containing 2% manganese (Mn) was investigated. The microstructures of hot-bands were significantly refined by Nb addition, which led to the development of the cold-rolling texture in both the γ-fiber (<111>//ND-fiber) and the α-fiber (<110>//RD-fiber). Recrystallization was retarded by Nb addition, in particular, the growth of <110>//ND grains was retarded. However, the γ-fiber and {112}<110> grains developed during annealing even in the intercritical (α-γ) region. Consequently, the r-value increased as the content of Nb was increased due to the development of the intensity ratio of the <111>//ND texture to the <100>//ND texture, which is desirable for deep-drawable high strength steel sheets.

Microstructural and mechanical characterization of C–Mn–Al–Si cold-rolled TRIP-aided steel

Retained austenite characteristics and tensile properties in a 0.21C–1.77Mn–1.2Al–0.283Si, high strength cold-rolled TRIP steel were investigated. The microstructure of this steel is comprised of ferrite, bainite and retained austenite, which is obtained by controlled cooling from the intercritical annealing temperature to the isothermal bainitic holding temperature. The effects of cooling rate from intercritical annealing temperature to isothermal transformation temperature, as well as effect of isothermal transformation time on microstructure and mechanical properties, were studied using optical microscopy, SEM, TEM, XRD, dilatometry and mechanical testing. These studies revealed that the microstructure of the steel mainly consisted of bainitic ferrite lath matrix, blocky martensites and stable retained austenite films of 7–8 vol%. When austempered at temperatures above M S temperature, the steel possessed high tensile strength of 600 MPa, total elongation of 28–32% and n-value 0.2. The present paper deals with the effect of heat treatment on microstructure and mechanical properties of C–Mn–Al–Si cold-rolled TRIP steel.

Ermittlung von Grenzformänderungskurven an laser- und elektronenstrahlgeschweißten Stahlblechplatinen

Abstract In the last few years various groups of high-strength cold-rolled steels have been developed for the car body lightweight design. Apart from this materials development, new concepts of lightweight construction have been worked out, as, e.g., welded blanks (so-called tailored blanks). These sheets, consisting of different materials and/or of different sheet thicknesses, are being evaluated in this paper regarding their formability. To this end, forming limit curves are determined by experiment and calculated by means of a finite element model. It turns out that the failure behaviour is influenced by the shape and structure as well as the position of the weld during the forming test. In this process, the width of the weld is more significant than the local hardness increase.

New Type of IF-High Strength Steel with Superior Anti-secondary Work Embrittlement.

High strength cold-rolled steel sheets (HSS) with sufficient formability have been developed for the IF steel-bases in the last decade, in which the major strengthening method was solid-solution hardening with silicon, manganese and phosphorous. When the IF steel is strengthened with the high amount of solid-solution elements, it becomes susceptible to the secondary work embrittlement because of the lack of grain boundary strength, which is the essential drawback of interstitial free steel. Although the grain refinement is an effective method to improve the toughness of steel, this method has not been taken into consideration in view of press-formability because it leads the steel to higher yield ratio, lower n-value and lower r-value. A new type of IF-HSS was strengthened by hybridizing the grain refinement and the supplemental solid-solution hardening. The grain refinement was achieved by means of the fine distribution of carbide under the appropriate combination of the relatively higher carbon content near 60 ppm with a suitable carbide-forming element. While this steel has the fine grain structure, yield strength hardly increases due to the formation of unique microstructure containing PFZ, and the y fiber texture sufficiently develops. As the result, a new type of grain-refined IF-HSS has been successfully developed to reach a higher r-value and a superior secondary work embrittlement as compared with the conventional IF-HSS.

車体構造部材用高γ値高張力冷延鋼板の開発

車体構造部材用高γ値高張力冷延鋼板の開発

銅の相分解を活用した深絞り加工用高強度冷延鋼板の開発

銅の相分解を活用した深絞り加工用高強度冷延鋼板の開発

Sheared edge extension of high-strength cold-rolled steels

The edge formability of a spectrum of high-strength cold-rolled steels has been evaluated using the hole expansion test. The effects of edge condition, micro structure, and tensile properties on hole expansion performance were determined. Modification of stringer inclusions through rare-earth treatments caused the hole expansion response of these steels to be much less sensitive to edge condition, as-blanked vs de-burred. Independently of edge condition, rare-earth treatment also produced a significant increase in hole expansion performance compared to the same steel untreated. Circle grid analysis showed that deformation modes generated in a hole expansion test are drawing near the hole edge, stretching farther from the edge, and plane strain separating these two modes. Recovery annealed steels with low transverse ductility were observed to fail away from the edge in the plane strain region during hole expansion. The hole expansion performance of the entire range of steels tested was found to vary linearly with the product of the steels’ transverse total elongation and rm values. A phenomenological expression that determines hole expansion performance was derived using linear analysis. It predicts that for steels with total elongations of about 30 pct, each increase in rm of about 0.1 will increase its hole expansion performance by greater than 10 pct.

Strengthening mechanisms in high-strength microalloyed steels

The mechanisms responsible for strengthening a series of high-strength cold-rolled steels with tensile strengths up to 800 MN m−2 have been investigated. The magnitude of precipitation strengthening in the annealed steels is shown to be in agreement with the Orowan-Ashby model for non-deforming particles. Strengthening depends only upon the volume fraction and diameter of the precipitates, and is not influenced by their chemical composition, nor by whether the precipitation-hardening elements are added singly or in combination. Manganese alone is a weak solid-solution-strengthening agent, but has a synergistic effect in combination with titanium or niobium, which is attributed to its depression of the austenite-ferrite transformation and precipitation temperatures. Vanadium is a much less efficient strengthening element than titanium or niobium in annealed steel, owing to the rapid coarsening rate of vanadium carbonitride precipitates and the considerable loss in strength on processing from hot-rolled coil to annealed sheet. Sulphur acts to reduce the strength of the annealed steels and tends to coarsen the grain structure. The physical basis of this effect is not known, but it is suggested that it may be associated with the partial solution of managanese sulphide during slab reheating and its subsequent re-precipitation during hot rolling. Phosphorus and nitrogen are the most efficient strengthening agents up to tensile strengths of 450 MN m−2, but stronger materials require a combination of strengthening modes, depending upon the application.MST/111

Strengthening mechanisms in high-strength microalloyed steels

The mechanisms responsible for strengthening a series of high-strength cold-rolled steels with tensile strengths up to 800 MN m−2 have been investigated. The magnitude of precipitation strengthening in the annealed steels is shown to be in agreement with the Orowan-Ashby model for non-deforming particles. Strengthening depends only upon the volume fraction and diameter of the precipitates, and is not influenced by their chemical composition, nor by whether the precipitation-hardening elements are added singly or in combination. Manganese alone is a weak solid-solution-strengthening agent, but has a synergistic effect in combination with titanium or niobium, which is attributed to its depression of the austenite-ferrite transformation and precipitation temperatures. Vanadium is a much less efficient strengthening element than titanium or niobium in annealed steel, owing to the rapid coarsening rate of vanadium carbonitride precipitates and the considerable loss in strength on processing from hot-rolled coil to annealed sheet. Sulphur acts to reduce the strength of the annealed steels and tends to coarsen the grain structure. The physical basis of this effect is not known, but it is suggested that it may be associated with the partial solution of managanese sulphide during slab reheating and its subsequent re-precipitation during hot rolling. Phosphorus and nitrogen are the most efficient strengthening agents up to tensile strengths of 450 MN m−2, but stronger materials require a combination of strengthening modes, depending upon the application.MST/111

Strain hardening of high-strength steels

AbstractStrain hardening and the loss of ductility with increasing strength have been investigated for a series of cold-rolled high-strength steels. The strain hardening exponent n decreases with increasing strength S according to the relationship nSp=C, where p and C are constants for a given strengthening mechanism. The value of p (and hence the ductility) at a fixed strength level depends on the operative strengthening mode. Solid-solution hardening is the most ductile method of strengthening, followed, in order of decreasing ductility, by grain refining, precipitation hardening, partial annealing, and cold working. It is proposed that the influence of a strengthening mechanism is related to the strain at which a steady-state dislocation-cell structure is developed in the material. The lower this critical strain, the lower the ductility, since further strain must then be accommodated by microband formation. The critical strain is considered to be dependent on the rate of accumulation of geometrically n...

Strain hardening of high-strength steels

Strain hardening and the loss of ductility with increasing strength have been investigated for a series of cold-rolled high-strength steels. The strain hardening exponent n decreases with increasing strength S according to the relationship nSp=C, where p and C are constants for a given strengthening mechanism. The value of p (and hence the ductility) at a fixed strength level depends on the operative strengthening mode. Solid-solution hardening is the most ductile method of strengthening, followed, in order of decreasing ductility, by grain refining, precipitation hardening, partial annealing, and cold working. It is proposed that the influence of a strengthening mechanism is related to the strain at which a steady-state dislocation-cell structure is developed in the material. The lower this critical strain, the lower the ductility, since further strain must then be accommodated by microband formation. The critical strain is considered to be dependent on the rate of accumulation of geometrically necessary dislocations in the structure, which is a function of the material's geometric slip distance. Strengthening mechanisms relying on obstacles in the microstructure - e.g. precipitation hardening - will have relatively short geometric slip distances, and thus low ductilities. Solid-solution hardening is preferable, because it operates only by increasing the lattice friction resistance to dislocation motion, and so does not reduce the geometric slip distance. The cell structure in the cold-worked steels is expected to approach that of the steady state at the start of the tests, a feature which, will result in very low critical strains and ductilities.MST/92

The Influence of High Strength Cold Rolled Steel and Zinc Coated Steel Surface Characteristics on Phosphate Pretreatment

Abstract In order to reduce vehicle weight, high strength cold rolled (HSCR) steels are being used as a replacement for low carbon cold rolled steels in many exposed automobile body panel applications. The use of these thinner sheet materials has increased the need for improved corrosion resistance. That need has led to a growing interest in zinc coated steels for those applications. Electrochemical techniques and Auger electron spectroscopic surface analysis were used to determine the factors which influence zinc phosphate conversion coating formation for the HSCR steels and zinc coated steels. Zinc phosphate coating quality was related to cathodic delamination tendency of the painted steels.

Deep Drawability and Recrystallization Texture of Rephosphorized Al-killed High Strength Cold-rolled Steel

The deep drawability and the recrystallization texture of rephosphorized Al-killed high strength steels (P: -0.07%) were studied as compared with those of a conventional Al-killed steel (P: 0.016%).A rephosphorized Al-killed steel shows the peak r-value at a certain heating rate in annealing as well as a conventional Al-killed steel. However, phosphorus slightly weakens the retardation effect of AlN on recrystallization, which results in a lower r-value and a lower grain elongation ratio. The preferred orientation of a rephosphorized Al-killed steel is {111} <112> though that of a conventional Al-killed steel is {111} <110>.These results are discussed by taking into account the interaction between phosphorus and AIN precipitation behavior.

Metallurgical Factors Affecting the Formability of Cold-rolled High Strength Steel Sheets

The metallurgical factors affecting the press formability of cold rolled high strength steels are discussed and the properties of the following 40 kgf/mm2 class tensile strength steels are examined: dual phase steel consisting of a ferrite-martensite structure, produced by rapid cooling in a continuous annealing line, which has a very low yield to tensile strength ratio and high n-value; rephosphorized steel, which is a batch-annealed aluminum-killed steel hardened with phosphorus and manganese, having a high r-value and a relatively low yield strength; and a recently developed steel of extra deep drawing quality and high strength which is characterized by its extremely high r-value, over 2.0, and low yield strength. This can be produced by continuous annealing of niobium-stabilized, extra low carbon, phosphorus-bearing steel. The formability and brittleness of the steels are also examined.

Automotive Application of High-strength Cold-rolled Steel

Automotive Application of High-strength Cold-rolled Steel

Properties of cold-rolled high-strength steel sheets

For high-strength steel sheets, a new concept has become necessary,viz, the relation between strength and formability. When the relation between tensile strength and elongation is appraised for ranking in terms of the strengthening mechanism, it is found that the substitutional solid-solution hardening type is excellent and the precipitation hardening type is inferior. In batch annealing, the phosphorus-added aluminum-killed steel sheets are representative of the former type having excellent formability as indicated by a highr values despite their tensile strength of 450 N/mm2. The titanium-added aluminum-killed steel sheets, which are representative of the latter type, have a tensile strength of 600 N/mm2 and a relatively highr value. Continuous annealing of the highstrength steel sheets of the same chemical composition causes higher yield stresses and combinations of yield stress and elongation. Alternatively with continuous annealing the same level of strength can be achieved with smaller additions of alloying elements than with batch annealing. An additional advantage of continuous annealing is the uniformity of properties along the length of the coil. The rapid cooling possible after continuous annealing allows production of high strength steel sheets having excellent mechanical properties that are unobtainable in the batched annealed steels. For example, steel sheets of 0.4 pet Si and 1.4 pet Mn after continuous annealing, haven values and Erichsen values as higher than conventional low-carbon rimmed or capped steel sheets even while they have a tensile strengths of 550 N/mm2.