GLOSSARY OF TERMS RELATING TO STEEL STRIPS
Accelerated Cooling: Cooling a plate with water immediately following the final rolling operation. Generally the plate is water cooled from about 1400F to approximately 1100F.
Ageing: A change in properties that may occur gradually at atmospheric temperatures (natural ageing) and more rapidly at higher at higher temperatures (artificial ageing).
Age Hardening: The hardening of steel induced by ageing.
Alloy Steel: Steel is considered to be an alloy steel when either (1) the maximum of the range given for the content of alloying elements exceeds one or more of the following percentages: manganese 1.65, silicon 0.60, copper 0.60; or (2) a definite range or definite minimum quantity of those elements considered alloys is specified. For example, chromium, molybdenum and nickel.
Annealing: Heating to holding at a suitable temperature, followed by cooling at a suitable rate, for such purposes as:
- 1. inducing softness
- 2. improving machinability
- 3. improving cold working properties
- 4. obtaining a desired structure
- 5. removing stresses
When applicable, the more specific terms, full annealing, isothermal annealing or sub-critical annealing could be used:-
- 1. Full annealing. Heating to and holding at some temperature above the transformation range, followed by cooling slowly by the transformation range.
- 2. Isothermal Annealing. Heating to and holding at some temperature above the transformation range, then cooling to and holding at a suitable temperature until the austenite to pearlite transformation is complete, and finally cooling freely.
- 3. Sub-Critical Annealing. Heating to and holding at some temperature below the transformation range, followed by cooling at a suitable rate.
Austenitizing: The process of forming the austenite phase by heating a ferrous alloy into the transformation range (partial austenitizing above the lower critical temperature) or above this range (complete austenitizing above the upper critical temperature).
Bainite: A decomposition product of austenite consisting of an aggregate of ferrite and carbide. In general, it forms at temperatures lower than those where very fine pearlite forms, and higher than those where martensite begins to form on cooling.
Blue Annealing: Sub-critical annealing of bright steel during which the surface becomes oxidized to a blue temper colour by the controlled admission of air and/or steam.
Blued Edges: Edges of sheet or strip which have become coloured due to slight oxidation during the heat treatment.
Carbon Steel: A steel the properties of which are determined mainly by the percentage of carbon present.
Chamfering: The removal of sharp edges. The term is practically synonymous with bevelling but has a less a restricted application.
Cold Reduction: Reducing the thickness of steel sheet or strip to the finished gauge by heavy cold working between rolls.
Cold Rolling: Passing sheet or strip at room temperature between a pair of rotating rolls. The reduction in thickness may be very bright, as in the finishing process applied to hot rolled sheets, or heavy as in the cold rolling of narrow strip.
Cold Working: The operation of permanently altering the shape or dimensions of the steel, carried out at atmospheric temperature by, for example, cold rolling or cold reduction. Other methods of applying cold work are by drawing, pressing, forming, bending, swaging,etc.
Corrosion Fatigue: Fatigue accelerated by simultaneous corrosion.
Creep: Plastic deformation which proceeds slowly and continuously when stress is applied at elevated temperatures. In steel, creep is negligible below about 300 degree centigrade.
Critical Grian Growth: A drastic enlargement of the grains when certain steels, particularly low carbon steels, are subjected to a certain small amount of cold work and then annealed at a temperature below the upper critical point.
Decarburization: The loss of carbon from the surface of steel as a result of heating in a medium that reacts with the carbon.
Deoxidation: A process used during melting and refining of steel to remove and/or chemically combine oxygen from the molten steel to prevent porosity in the steel when it is solidified.
Ductility: Ability to undergo cold plastic deformation usually as a result of tension.
Elasticity: The property of the material by which it returns to its original dimensions after the removal of a stress.
Elastic Limit: The highest stress that can be applied without producing permanent deformation.
Elongation: The increase in length of a tensile test piece when stressed. The elongation at fracture is usually expressed as a percentage of the original gauze length.
Etching: Treatment of prepared metal surfaces with acid or other reagents which, by differential attack, reveal the structure.
Fatigue: The tendency to fracture by means of a progressive crack under repeated alternating or cyclic stresses considerably below the tensile strength.
Ferrite: The room temperature form of alpha iron, one of the two major constituents of steel (cementite) in which it acts as the solvent to form solid solutions with such elements as manganese, nickel, silicon and, to a small degree, carbon.
Grain Growth: A coarsening of the crystal structure under certain conditions of heating. This should not be confused with critical grain growth.
Hardness: Resistance to deformation, indentation, abrasion, cutting, etc.
Hardness Test: A test of hardness usually by determining the resistance of the material to indentation under standard conditions.
The following are established methods of hardnesss testing :-
Brinell hardness test:A test to determine hardness by pressing a hard steel ball of known diameter under a standard load into the surface of the material and measuring the diameter of the indentation produced. The Brinell hardness number, BHN = Load in kg / Spherical area of impression in sq. mm.
Rockwell hardness test: Determines hardness by indicating on a dial the depth of the impression caused by a loaded indenter in the form of either a diamond cone (Scales A and C) or a hardened steel ball (Scale B). When the load is removed, the dial gauge recordes the depth of impression in terms of Rockwell numbers.
Vickers pyramid hardness test- When the Brinell test is used on very hard materials, low values result owing to the spherical shape of the indenter and flattening of the ball. These are eliminated by using a square based diamond pyramid indenter which does not deform easily and gives geometrically similar impressions under various loads. The diamond pyramid with an angle between opposite faces of 136 degree is pressed under a standard load into the surface of the material, and the diagonal of the indentation produced is measured- The load divided by the contact area of impression gives the Vickers Pyramid Number. VPN = Load in kg / Pyramidal area of impression in sq. mm
Inclusions (Non-metallic inclusions): Particles of oxides, silicates, sulphides, refractory materials, slag, etc., embedded in the metal.
Internal Stress: Stress produced within the metal by transformation, by temperature differences on heating or cooling, or by mechanical working.
Killing: Applying to finally heat treated narrow strip a small amount of cold work to prevent kinks and stretcher strains on further manipulation. Skin passing is the usual method of killing.
Lamination: Separation into two or more layers due to some discontinuity in the steel, usually a layer of non-metallic inclusions.
Malleability: Capacity for undergoing deformation in all directions, usually cold deformation by hammering or squeezing.
Martensite: A microconstituent or structure in hardened steel, characterized by an acicular or needle-like pattern, and having the maximum hardness of any of the decomposition products of an austenite.
Mild Steel: Carbon steel containing approximately 0.12 to 0.25 per cent of carbon.
Mill Edge: The natural edge left when sheet or strip is rolled on the flat surfaces only.
Mill Shearing: Shearing the edges of a mill pack to approximate size.
Pearlite: A microconstituent of iron and steel consisting of a lamellar aggregate of ferrite and cementite (a compound of iron and carbonÜFe3C).
Quenching & Tempering: A thermal process used to increase the hardness and strength of steel. It consists of austenitizing, then cooling at a rate sufficient to achieve partial or complete transformation to martensite. Tempering involves reheating to a temperature below the transformation range and then cooling at any rate desired. Tempering improves ductility and toughness, but reduces the quenched hardness by an amount determined by the tempering temperature and time.
Pickling: The removal of scale by treatment with diluted acids or other chemicals.
Roll Marks: Periodic surface marks due to some imperfection on the surface of a roll.
Seam: A longitudinal surface defect similar to a roak, except that in a cross rolled sheet such a defect would run transversely.
Sheared Edges: Edges resulting when a sheet or strip is either sheared or slit in rotary cutters.
Sperodized Annealing: A prolonged heating of the steel in a controlled-atmosphere furnace at or near the lower critical point, followed by retarded cooling in the furnaces, to produce a lower hardness than can be obtained by regular annealing.
Stress Relieving: A thermal cycle involving heating to a suitable temperature, usually 1000-1200F, holding long enough to reduce residual stresses from either cold deformation or thermal treatment, and then cooling slowly enough to minimize the development of new residual stresses.
Stress Relieving (Stabilising): Heating to and, if necessary, holding at, some temperature generally below the transformation range, usually followed by slow cooling, for the sole purpose of relieving internal stresses.
Note:- Other treatments, eg., annealing, tempering, etc., whilst primarily applied to bring about changes in structure or properties may also relieve internal stresses.
Stretcher Strains (Luder lines): A furrowed roughening of the surface of low carbon sheet or strip due to uneven yielding in the first stages of cold deformation after annealing or, in a less degree, after normalising or hot rolling.
Temper: The mechanical condition of strip as controlled by heat treatment and cold rolling. For example, a strip in the finally annealed condition is 'soft temper', whilst strip subjected to heavy cold rolling as the final treatment is 'hard temper'. Between these are the intermediate tempers of which the common ones are 'skin passed', 'quarter hard' and 'half hard'.
Tempering: The carbon trapped in the martensite transformation can be released by heating the steel below the A1 transformation temperature. This release of carbon from nucleated areas allows the structure to deform plastically and relive some of its internal stresses. This reduces hardness and increases toughness, but it also tends to reduce tensile strength. The degree of tempering is dependant on temperature and time; temperature having the greatest influence.
Tensile Strength: The maximum load reached in a tensile test divided by the cross-sectional area of the gauge length portion of the test piece. Also termed maximum stress or ultimate tensile stress.
Tolerance: The permitted deviation from a specified dimension or weight, usually expressed as 'plus' or 'minus' on that quantity.
Yield Stress: The stress (load divided by original area of cross-section of a test piece) at which, in a tensile test, elongation of the test piece first occurs without increase of load.