Deep Drawing and Forming of Steels and Deep Drawing Steels
Deep Drawing and Forming of Steels and Deep Drawing Steels
Deep drawing of steel sheets is used to form parts by a process in which a flat blank is constrained by a blank holder while the central portion of the sheet is pushed into a die opening with a punch to draw the steel into the desired shape without causing wrinkles or splits in the drawn part. This normally needs the use of presses having a double action for hold-down force and punch force.
For deep drawing, steel is required to have good formability which means the steel has the ability of plastic deformation without fracture and loss of stability. The basic parameters which influence the steel property of deep drawing are (i) production methods mainly related to secondary steelmaking and rolling processes, (ii) chemical composition, (iii) micro-structure, (iv) mechanical properties, (v) surface quality, (vi) dimensional tolerances, and (vii) vertical anisotropy.
The property of formability in steels is a result of the interaction of several variables, the main ones being the mechanical properties of the steel, the forming system (tooling) used to manufacture parts, and the lubrication used during forming. Tight control over chemical composition, hot rolling parameters, amount of cold reduction, annealing time and temperature, and the amount of temper rolling allow the production of steels with good formability. To prevent the occurrence of fluting or stretcher strains during forming, deep drawing steels are tempered as a normal step in the mill processing.
The term deep drawing implies that some drawing-in of the flange metal occurs and that the formed parts are deeper than can be obtained by simply stretching the metal over a die. Clearance between the punch and the die is to be closely controlled to minimize the free span so that there is no wrinkling of the sidewall. This clearance is sufficient to prevent ironing of the steel being drawn into the sidewall of the drawn part. If ironing of the walls is to be part of the process, it is done in operations subsequent to deep drawing.
Drawability of steel depends on two factors namely (i) the ability of the material in the flange region to flow easily in the plane of the sheet under shear, and the ability of the sidewall material to resist deformation in the thickness direction. Taking both of these factors into account, it is desirable in drawing operations to maximize material flow in the plane of the sheet and to maximize resistance to material flow in a direction perpendicular to the plane of the sheet. Low flow strength in the plane of the sheet is of little value if the work material also has low flow strength in the thickness direction. The flow strength of steel sheet in the thickness direction is difficult to measure, but the plastic strain ratio ‘r’ compares strengths in the plane and thickness direction by determining true strains in these directions in a tension test.
Formability is the ability of steel sheet to be deformed into a desired shape while maintaining structural integrity without tearing, buckling, and wrinkling, excessive thinning, and so on. Formability depends greatly on the nature of the forming operation, and various criteria of formability have been developed, depending on the nature of the forming operation and the applied forces, which can be tensile, compressive, bending, shearing, or various combinations of these. Forming applications can range from simple bending to stamping, and deep drawing of complex shapes. Appreciable stretching of the steel also can take place in some operations. In general, one key aspect of formability is the distribution of strain during forming and the evaluation of mechanical properties in terms of the (i) strain-hardening exponent (‘n’), (ii) strain-rate sensitivity factor (‘m’), and (iii) plastic-strain ratio, or normal anisotropy factor (‘r’, or ‘rm’).
Deep drawing is a complex process which involves several types of forces and deformation modes, such as tension in the wall and the bottom, compression and friction in the flange, bending at the die radius, and straightening in the die wall. Fig 1 shows the circular cup deep drawing process and its mechanics.
Fig 1 Circular deep drawing and its mechanics
Suitable radii in the punch bottom to side edge, as well as the approach to the die opening, are necessary to allow the steel sheet to be formed without tearing. In the majority of deep-drawing operations, the part has a solid bottom to form a container and a retained flange which is trimmed later in the processing. In some cases, the cup shape is fully drawn into the die cavity, and a straight-wall cup shape is ejected through the die opening. To control the flange area and to prevent wrinkling, a hold-down force is applied to the blank to keep it in contact with the upper surface of the die. Presses used for deep drawing operations can be either hydraulic or mechanical.
Any steel grade which can be processed into sheet form by a cold rolling process is to be sufficiently ductile to be capable of deep drawing. Both hot rolled and cold rolled sheet products are used in the deep drawing processes. The cold work effects introduced during processing of the steel sheet for deep drawing applications is to be removed (by annealing, for example), and the as-delivered coils are to be free of any aging. This implies that the aluminum (Al) killed drawing quality steel, for example, is to be preferred over rimmed quality steel. After the deep drawing operation, ductility can be returned to that of the original sheet by in-process annealing, if necessary. In several cases, however, steel which has been deep drawn in a first operation can be further reduced in cup diameter by additional drawing operations, without the need for intermediate annealing.
Indicative information of deep drawing properties of steels can be obtained from their behaviour during the tensile test. For the pressability criterion of steel, the properties of the yield point, the tensile strength, the strain uniform deformation and the work hardening exponent are used. Also by ensuring the low scatter in the mechanical properties in the deep drawing steels, the optimum productivity in the drawing press operation can be achieved.
The properties considered to be important in sheet products designed for deep drawing include (i) composition, with a minimum amount of inclusions and residual elements contributing to better drawability, (ii) mechanical properties, of which the elongation as measured in a tension test, the plastic strain ratio ‘r’, and the strain hardening exponent ‘n’ are of primary importance and also the strength of the final part as measured by yield strength is to be considered, but this is more a function of the application than forming by deep drawing, (iii) physical properties which include dimensions, die geometry, modulus of elasticity, and any special requirements for maintaining shape after forming.
As noted, a high ‘r’ value is the measure of a material with good deep-drawing characteristics. The ‘r’ value is a measure of the ability of the material to resist thinning while undergoing width reduction. A high ‘r’ value is the measure of a material with good deep-drawing characteristics. High values of ‘n’ and ‘m’ lead to good formability in stretching operations, since they promote uniform strain distribution, but they have little effect on drawability of the steel. Majority of the steel sheet forming operations normally involve stretching and some shallow drawing, in which case the product of the strain-hardening exponent ‘n’ and the normal anisotropy ‘r’ of the sheet has been shown to be a significant parameter.
The drawability of a steel grade depends on two factors namely (i) the ability of the material in the flange region to flow easily in the plane of the sheet under shear, and (ii) the ability of the sidewall material to resist deformation in the thickness direction. The punch prevents sidewall material from changing dimensions in the circumferential direction. Hence, the only way the sidewall material can flow is by elongation and thinning. Thus, the ability of the sidewall material to withstand the load imposed by drawing down the flange is determined by its resistance to thinning, and high flow strength in the thickness direction of the sheet is desirable.
Taking both of these factors into account, it is desirable in drawing operations to maximize material flow in the plane of the sheet and to maximize resistance to material flow in a direction perpendicular to the plane of the sheet. Low flow strength in the plane of the sheet is of little value if the work material also has low flow strength in the thickness direction. The flow strength of steel sheet in the thickness direction is difficult to measure, but the plastic strain ratio ‘r’ compares strengths in the plane and thickness directions by determining true strains in these directions in a tension test.
Effect of different elements on deep drawing properties of steel
Carbon (C) content is particularly significant in steels which are intended for complex forming applications. An increase in the C content of steel increases the strength of the steel and reduces its formability. These effects are caused by the formation of carbide particles in the ferrite matrix and by the resulting small grain size. The amount of C in steel sheet is normally limited to 0.1 % or less to maximize the formability of the sheet.
Manganese (Mn) enhances the hot working characteristics of the steel and facilitates the development of the desired grain size. Some Mn is also necessary to neutralize the detrimental effects of sulphur (S), particularly for hot workability. Typical Mn contents for low C steel sheet range from 0.15 % to 0.35 %. Mn contents upto 2 % can be specified in high strength low-alloy (HSLA) steels. When the S content of the steel is very low, the Mn content also can be low, which allows the steel to be processed to develop high ‘r’ values.
S and P (phosphorus) are considered undesirable in steel sheet intended for forming, drawing, or bending since their presence increases the likelihood of cracking or splitting. Allowable levels of S and P depend on the desired quality level. For example, commercial quality cold rolled sheet is required to contain S less than 0.4 % and P less than 0.035 %. For some applications, P can be added to the steel to increase the strength. S normally appears as manganese sulphide (MnS) stringers in the microstructure. These stringers can promote splitting, particularly whenever an unrestrained edge is deformed.
Silicon (Si) content in low carbon steel varies according to the deoxidation practice employed during production. In rimmed steels, the Si content is normally less than 0.1 %. When Si rather than Al is used to kill the rimming action, the Si content can be as high as 0.4 %. Si can cause silicate inclusions, which increase the likelihood of cracking during bending. Si also increases the strength of the steel and hence decreases its formability.
Cr (chromium), Ni (nickel), Mo (molybdenum), V (vanadium), and other alloying elements are present in the low C steel only as residual elements. With proper scrap selection and control of steelmaking operations, these elements are normally held to minimum amounts. Each of these elements increases the strength and decreases the formability of steel sheet. HSLA steels can contain specified amounts of one or more of these elements.
Cu (copper) is normally considered a harmless residual element in steel sheet. The strengthening effect of Cu is almost negligible in typical residual amounts of less than 0.1 %. However, Cu is added to steel in amounts exceeding 0.2 % to improve resistance to atmospheric corrosion.
Nb (niobium) strengthens HSLA steel through the formation of niobium carbides and nitrides. It can also be used either alone or in combination with Ti (titanium) to develop high ‘r’ values in the ‘interstitial free’ (IF) steels. These alloying elements remove the interstitial elements C and N2 (nitrogen) from solid solution. As a result, the steel shows no yield point elongation.
Ti is a strong carbide and nitride former. It helps develop high ‘r’ values and eliminates yield point elongation and the aging of cold rolled annealed steel sheet. Ti streaks can be a problem in some grades, especially in the form of surface defects in exposed applications.
Al is added to steel to kill the rimming action and hence produce very clean steel known as an aluminum killed, or special killed steel. Al combines with both N2 and O2 (oxygen) to stop the out-gassing of the liquid steel when it is added to the ladle or continuous casting mould. Al also aids the development of preferred grain orientations to attain high ‘r’ values in cold rolled and annealed steel sheet. Elongated grains of an around ASTM 7 size are found in the majority of well processed Al killed steels. Since the Al combines with the N2, the steel is not subject to strain aging.
N2 can considerably strengthen low C steel. It also causes strain aging of the steel. The effects of N2 can be controlled by deoxidizing the liquid steel with Al. Cerium (Ce) and other rare earth elements can be added to steel to change the shape of MnS inclusions from being needle like or ribbon like to being globular. Globular inclusions reduce the likelihood of cracking if the sheet is formed without restraining the edges.
O2 content of liquid steel determines its solidification characteristics in the ingot and continuous casting. Excessive amounts of O2 impede nitride formation and thus negate the effects of alloying elements added to minimize strain aging. Deoxidizers such as Si, Al, and Ti control the O2 content. When O2 combines with these deoxidants, complex non metallic inclusions are formed. Although the majority of the non metallic inclusions dissolve in the slag, some can become trapped in the steel, causing the surface defects of seams and slivers.
Types of steels used for deep drawing
The traditional steels used for the deep drawing have extremely low C content. The deep drawing steels can be either alloyed with special alloying elements or can be unalloyed steels with special rolling strategies in order to meet demands for lowest possible yield strength levels, good cold formability properties, and good ageing resistance. Deep drawing steels are normally specified by yield strength and tensile strength and by minimum elongation values. For forming properties, minimum values for vertical anisotropy (‘r’ value) and work hardening exponent (‘n’ value) are normally specified.
Sheet steels with higher strengths and better formability are now available. Developments such as vacuum degassing and inclusion shape control have been especially beneficial in increasing the drawability of steels.
Low C steels are normally used for the deep drawing because of their low cost and good formability. Drawing quality steels are used mostly in sheet form normally as a cold rolled sheet while for some of the applications hot rolled sheets are also used. Low C steel can be formed by any of several types of dies. Bending dies include V-dies, wiping dies, U-bending dies, rotary bending dies, cam-actuated flanging dies, wing dies, and compound flanging dies. However, presently ordinary low strength, low C sheet steel has been replaced by a number of higher strength sheet steels requiring new process technology. These new steels include the high strength, precipitation strengthened steels, the dual-phase (DP) and tri-phase steels, and the bake hardenable (BH) steels. Also, new coating techniques have been developed to protect these new steels from corrosion.
Low C sheet steel materials such as 1006 and 1008 steel grades have typical yield strengths in the range of 170 MPa to 240 MPa and elongations of 35 % to 45% in 50 mm test sample. These steels have very good formability and are available hot or cold finished in various quality levels and a wide range of thicknesses. Other low C steels which are normally deep drawn are grades 1010 and 1012. These materials are slightly stronger than 1006 and 1008 grades and are slightly less formable. They are frequently specified when drawing is not severe and strength of the finished part is of some concern.
Low C steel sheet and strip are used for low cost production of various products having good dimensional tolerance and appearance. The steels used for these products are supplied over a wide range of chemical compositions. However, the vast majority are unalloyed low C steels are selected for stamping applications, such as automobile bodies and white goods. For these major applications, typical compositions are C – 0.03 % to 0.1 %, Mn – 0.15 % to 0.35 %, P – 0.035 % max (maximum), and S – 0.04 % max.
Low C sheet steels are normally preferred for forming. These steels typically contain less than 0.1 % C and less than 1 % total intentional and residual alloying elements. The quantity of Mn, the principal alloying addition, normally ranges from 0.15 % to 0.35 %. Controlled amounts of Si, Nb, Ti or Al can be added either as deoxidizers or to develop certain properties. Residual elements, such as S, P, Cr, Ni, Mo, Cu, and N2 are normally limited as much as possible. During steelmaking, these amounts are based on the quality of sheet being produced. Alloy sheet steels which include HSLA steel grades, however, contain specified amounts of one or more of these elements.
In the past, rimmed (or capped) ingot cast steel has been used because of its lower price. More recently, however, rimmed steels have been largely replaced by killed steels produced by the continuous casting process. Continuous casting is inherently suited to the production of killed steels, but killed steels are also produced by ingot casting. Regardless of the method of casting or manufacture, killed steels are preferred because they have better formability and are not subject to aging or strain aging which means that the mechanical properties can deteriorate with time
Some material variables affect the deep drawing of steel. Grain size affects the drawability of these steels, and it can affect the selection of a grade. Grain sizes of ASTM 5 or coarser can result in excessive surface roughness as well as reduced drawability. Surface finish also influences drawability. The dull finish normally supplied on drawing steels is designed to hold lubricants and to improve drawability. Brighter finishes can be needed if, for example, parts are to be electroplated.
Anisotropy is another variable which affect the deep drawing of steel. There are two types of anisotropy which are to be considered. These are planar anisotropy, in which properties vary in the plane of the sheet, and normal anisotropy, in which the properties of the material in the thickness direction differ from those in the plane of the sheet. Planar anisotropy (variations in normal anisotropy in the plane of the sheet) causes undesirable earing of the work material during drawing. Between the ears of the cup are valleys in which the material has thickened under compressive hoop-stress rather than elongating under radial tensile stress. This thicker steel sometimes forces the die open against the blank holder pressure, allowing the steel in the relatively thin areas near the ears to wrinkle.
Thickness of steel sheet also has an effect. In deep drawing, the pressure on the dies increases proportionally to the square of sheet thickness. The pressure involved is concentrated on the draw radius, and increasing sheet thickness localizes wear in this area without similar effect on other surfaces of the die. Thick steel has fewer tendencies to wrinkle than thin steel.
In addition to the low C steel sheet and strip, there are numerous additional steel types available which are designed to satisfy specific customer requirements. These steels are frequently produced in low C grades with chemical composition slightly modified. For example, in the structural quality (SQ) steels, alloying additions of Mn and P are used to increase strength by substitutional solid-solution strengthening which is around 3 MPa per 0.1 % Mn, and 7 MPa per 0.0 1 % P. Hot rolled SQ steels contain Mn from 0.9 % to 1.35 % max and P 0.035 % max. Cold rolled SQ steels contain Mn from 0.6 % to 0.9 % max and P from 0.035 % to 0.2 %. C content of SQ steel is normally in the range of 0.2 % to 0.25 %.
Drawing quality steels have a pure ferritic microstructure or consist of a ferritic matrix which can contain isolated grainy carbides. The grain boundaries and fine carbides in these steels are visible if the steel is etched with Nital. Fig 2 shows microstructures of deep drawing steel with contrasting by Nital etching and contrasting by Klemm colour etching.
Fig 2 Microstructure of deep drawing steel
Recent materials developments include the introduction or increased application of IF drawing quality steels, high strength steels (HSS) , and advanced high strength steels (AHSS) for sheet forming. AHSS is a material which is receiving much attention in the automotive industry as a means to reduce vehicle weight and improve fuel economy. Spurred by initiatives such as the Ultra-Light Steel Auto Body program, the implementation of AHSS is aimed at providing material properties which bridge the range between conventional micro-alloyed high strength steel grades (with strengths of 210 MPa to 550 MPa) and ultrahigh-strength steels (with strengths 4,550 MPa. This is being done while maintaining of the adequate sheet formability. This property combination is due to the higher strain-hardening rates of AHSS, which possess multi-phase microstructures of ferrite, martensite, bainite, and / or retained austenite in various amounts. Specific types of AHSS include dual-phase, transformation-induced plasticity, complex-phase, and martensite grades. The strength of these grades is also enhanced during the bake-hardening cycle following painting. The increase in strength increases with prior forming strain, unlike conventional bake hardenable grades for which little additional hardening occurs in regions which have undergone strains higher than around 2 % during forming.
Low C steels, coated and uncoated, are normally produced as commercial quality (CQ), drawing quality (DQ), and drawing quality special killed (DQSK) grades. Some of the specialized steel grades, such as IF drawing steels and enameling steels, are also produced by some plants. Grade designations for the common drawing quality steels include (i) CQ steel, (ii) DQ steel, (iii) deep drawing (DD) steel, (iv) extra deep drawing (EDD) steel, (iv) extra deep drawing plus (EDD+) steel, (v) SQ steel, (vi) high strength low alloy (HSLA) steel, (vii) dent resistant (DR) steel, (viii) bake hardenable (BH) steel, and (ix) inclusion shape controlled steel. General characteristics of some of the steel grades used for the drawing operations are given below.
Commercial quality steel – CQ steel is available in hot rolled steel, cold rolled steel, and coated steel grades. It is the least expensive grade of sheet steel. The steel is subject to aging. The steel is not intended for difficult-to-form shapes.
Drawing quality steel – DQ steel is available in hot rolled steel, cold rolled steel, and coated steel grades. This steel shows better ductility than CQ grade steels, but has low plastic strain ratio (‘r’) values. The steel is subject to aging. It has good base metal surface quality.
Drawing quality special killed steel – DQSK steel is available in hot rolled steel, cold rolled steel, and coated steel grades. This steel has good forming capabilities. It is not subject to aging.
Interstitial free steel – IF steel is available as cold rolled steel, and coated steel grades. This steel has good forming capabilities. This steel is used for deep drawing and can be used as EDD steel, and EDD+ steel. In the IF steel, the elimination of interstitials, C and N2 is accomplished by adding sufficient amounts of carbide / nitride forming elements such as Ti and /or Nb to tie up C and N2 completely, the levels of which can be reduced to less than 50 ppm by the present day steelmaking / casting practices, including vacuum degassing. Steels with very low interstitial content show very good formability with low yield strength (138 MPa to 165 MPa), high elongation (41 % to 45 %), and good deep drawability. With the addition of carbo-nitride forming elements, the deep drawability and the non-aging properties are further improved.
Enameling steel – It is available as cold rolled steel. Different types of processing are used to make a product which is satisfactory for porcelain enameling. All grades of enameling steel have good forming capabilities.
Structural quality steel – SQ quality steel is used when higher strength is needed, although with some sacrifice in ductility. This steel is available in hot rolled, cold rolled, and coated steel grades. Different types of processing are used to obtain the desired strength levels. In general, the formability of these grades decreases as yield strength increases. Spring back can be a problem at lower thicknesses of steel sheets.
High carbon steels – High C steel strip (including spring steel and tool steel) is blanked, pierced, and formed to make a variety of parts. The practices, precautions, presses, and tools used in making high C steel parts are comparable to those used for producing similar parts of low C steel. The key differences in blanking, piercing, and forming of high C steels compared to low steels include(i) higher yield strength and lower ductility results in less bendability and, hence, needs greater bend radii than plain C steels, (ii) higher allowance is needed for spring back, (iii) more force is needed for high C steel because of its higher strength, (iv) higher clearance between the punch and die is necessary in blanking and piercing, (v) a more wear-resistant tool material can be needed before acceptable tool life can be obtained.
In blanking and piercing of high C steels, the most important difference with that of low C steel is the need for higher clearance between punch and die. Mould forming of high C steel in the quenched and tempered (pre-tempered) condition (normally 47 HRC to 55 HRC) is common practice. The severity of forming which can be done without cracking of the work metal depends mainly on the thickness, when metal thickness is no more than around 0.38 mm. It is possible to make relatively severe bends without fracturing the work metal. However, as the metal thickness increases, the amount of forming which can be done on pre-tempered steel decreases rapidly.
Moderately severe forming can be done on the cold rolled steel which has not been quenched and tempered and on high C steel which has been spheroidize annealed. Such materials are normally hardened and tempered after forming to improve spring properties.
High strength low alloy steel – The HSLA steels are normally formed at room temperature using conventional equipment. Cold forming is not to be done at temperatures below 10 deg C. As a class, high strength steels are inherently less formable than low C steels because of their greater strength and lower ductility. This reduces their ability to distribute strain. The higher strength makes it necessary to use greater forming pressure and to allow for more spring back compared to the low C steels. However, high strength steels have good formability, and straight bends can be made to relatively tight bend radii, especially with the grades having lower strengths and greater ductility. Further, high-strength steels can be stamped to relatively severe shapes, such as automotive bumper facings, wheel spiders, and engine mounting brackets.
HSLA steels can be hot formed. However, hot forming normally alters the mechanical properties, and a particular problem which arises in several applications is that some of the more recent thermo-mechanical processing techniques (such as controlled rolling), used for plates in particular, are not suitable where hot forming is used during fabrication. This problem can be circumvented by the use of a rolling finishing temperature which coincides with the hot forming temperature (900 deg C to 930 deg C). Subsequent hot forming hence simply repeats this operation, and deterioration in properties is then small or even absent, provided that grain growth does not occur.
Inclusion shape controlled steels – In this steel, cold formability is considerably improved because of the shape control of the inclusions. This enables the steel to be formed to nearly the same extent in both the longitudinal and transverse directions. Any grade produced with inclusion shape control can be more severely formed than a grade of the same strength level produced without inclusion shape control. Inclusion shape controlled steels are responsible for the moderately good formability of the higher strength HSLA steels, such as the grades having 550 MPa yield strengths.
Bake hardening steel – BH steel is characterized by its ability to show an increase in yield strength due to C strain aging during the paint baking operations at moderate temperature (125 deg C to 180 deg C). BH has little effect on the tensile strength. BH steels are finding increased use in automotive outer-body applications (hoods, doors, and fenders) to achieve an improvement in dent resistance and, in some cases, a sheet thickness reduction as well.
The BH behaviour is dependent on the steel chemistry and processing, in addition to the amount of forming strain and paint-baking conditions (temperature and time). Steels which show BH behaviour include plain low C steels (continuously annealed or batch annealed), IF steels (continuously annealed), and dual phase steels (continuously annealed).
Automotive specifications for BH steels can be categorized according to those which specify a minimum yield strength level or a minimum bake hardening increment, in the formed (strained) plus baked condition. The conventional test for determining bake hardenability characteristics involves a 2 % tensile pre-strain, followed by baking at 175 +/- 5 deg C. The resulting increase in yield strength measures the bake hardenability of the material.
While all the specifications call for a minimum yield strength level in the as-received (that is, prior to forming) condition, some also require a minimum yield strength after baking the as received material in the absence of any tensile pre-strain. The as-received yield strength is in the range of 210 MPa to 310 MPa (compared with around 175 MPa for DQSK), whereas the final yield strength after 2 % pre-strain plus bake, ranges between 280 MPa and 365 MPa.
High strength steels and advanced high strength steels – Applications of HSS and AHSS in the automotive industry revolve around two major areas namely (i) improving crash performance (front, rear, side) through increased strength levels, and (ii) reducing weight through the gauge reduction. Other important applications criteria include stiffness, fatigue life, corrosion resistance, formability, and weldability. Typical applications of HSS and AHSS include panels, chassis, and structural components (door beams, wheels, bumpers, seats, and suspensions, etc.).
The HSS and AHSS are more difficult to bend than plain C steels because of their higher yield strength and lower ductility. This requires more power, greater bend radius, more die clearance, and greater allowance for spring back. It can be necessary to remove shear burrs and to smooth corners in the area of the bend.
Due to the high ratio of yield strength to elastic modulus of HSS and AHSS, a larger quantity of spring back, compared to low C steel, develops in the formed part. Spring back is of great concern to sheet metal forming tool designers, since it can cause serious problems in the assembly of parts and can lead to expensive modifications of the forming tools. Spring back and sidewall curl after unloading from stamping dies is a very important technical barrier to the widespread use of HSS and AHSS. Spring back can be reduced through proper tooling design and by controlling the binder forces acting on the drawn steel.
Major disadvantages of using HSS and AHSS include reduced stiffness due to the thickness reduction, increased spring back due to increase of flow strength, and limited formability due to low ductility. Also included are increased notch sensitivity, reduced dent resistance (when increase in strength cannot compensate for thickness reduction), and increased tool wear on the draw-die binder surfaces due to the high strength of these materials. However, tool wear can be reduced by upgrading tool materials and by applying protective coatings on the surface of the forming tools.
Forming limits of steel are described by a forming-limit diagram which shows the limit strains of the steel when it is formed through a linear strain path. The formability of various grades of HSS, AHSS, and mild steel (for reference) is determined by the forming-limit diagram. The Fig 3 shows that the formability follows the general trend of decreasing with increase in the strength of the steel. Fig 3 shows forming-limit diagram for various grades of HSS and AHSS, and mild steel.
Fig 3 Forming-limit diagram for various grades of HSS and AHSS, and mild steel
AHSS such as dual-phase (DP), complex-phase (CP), and transformation-induced plasticity (TRIP) steels show superior strength compared to the HSLA grades with the same formability. The AHSS derive their superior mechanical and forming properties from their final micro-constituents (ferrite, bainite, martensite, and retained austenite). For example, DP steels derive their strength from the martensite phase and their ductility from the ferrite phase.
Coated steels – Coated steels are formed using the same general equipment, tooling, and lubrication used to form uncoated steels. While the properties of the base steel remain the primary determinants of the formability of a coated product, coatings do have an effect on the forming process and is to be taken into account when designing parts, dies, and forming strategies. Not only do the surface coatings affect the lubricity at the die / part interface, the forming process is to be carried out in such a way as to maintain the coating integrity and preserve the original purpose of the coating.
Coatings can have a different coefficient of friction with common die materials than the steel substrate and can be more ductile or more brittle than the base metal. Various coatings also react differently with different lubricants and die materials than bare steels. Given the friction and formability differences between steel substrates and common coating materials, some adjustments to the forming process are normally necessary to obtain optimal results when switching from bare to coated steels or when changing coatings.
While coated steels can normally be successfully formed on tooling designed for bare steel, part cost and quality improvements can frequently be realized if tooling, lubrication, forming parameters, and part geometry are adjusted to suit the coating / substrate combination being used.
Because most coatings are softer than the steel substrate, steps to protect the coating from damage are sometimes necessary. Coating damage can take place in the die but also can occur during blank or part handling, especially if sharp edges or burrs on blanks or formed parts are allowed to rub or scrape against adjacent pieces during stacking or handling. The brittle nature of some coatings can impose forming path or bend radii limits on coated steels which are much more stringent than is the case with bare steels.
The selection of the DQ steel sheets is to be based on an understanding of available grades of sheet and forming requirements. Other factors which are to be considered when selecting a material for forming into a particular part include (i) purpose of the part and its service requirements, (ii) thickness of the steel sheet and allowable tolerances, (iii) size and shape of blanks for the forming operation, (iv) equipment available for forming, (v) quantities needed, (vi) available handling equipment for sheets or coils, (vii) local availability of sheet products, (viii) surface characteristics of the steel sheet, (ix) special finishes or coatings for appearance or for corrosion resistance, (ix) aging propensity and its relation to time before use, (x) strength of the steel sheets as-delivered, and (xi) strength requirements in the formed part. Some parts need specialized low C steel which has been processed to enhance a given mechanical property and / or reduce production costs and forming problems.
The most frequently used joining methods for deep drawing steels are resistance and shield gas welding. The preferred resistance welding processes are spot welding, projection welding and roll seam welding
Tests for deep drawing of steel sheets
The two popular tests for determining the deep drawing capabilities of steel sheets are (i) Erichsen cupping test, and (ii) deep drawing cup test. Fig 4 shows these tests.
Fig 4 Tests for deep drawing of steel sheets
Erichsen cupping test – It is the worldwide well known deep drawing test method. It was patented as early as 1913 by Mr. Erichsen. It is a ductility test which is employed to evaluate the deep drawing capability of the steel sheet. The test consists of forming an indentation by pressing a punch with a spherical end against a test piece clamped between a blank holder and a die, until a through crack appears. The depth of the cup is measured.
To conduct this test, a steel sheet sample is clamped between a blank holder and a die and then dented (deep drawn) with a hardened spherical punch. This process is continued at a prescribed speed until it results in a fine, continuous crack in the steel sheet. The displacement of the spherical punch till cracking occurs is known as the Erichsen cupping index (IE). The IE values represent a significant deep drawing quality attribute of the tested steel sheet.
Erichsen cupping test is a fast cost effective testing method which does not need any lengthy preparation of the sample. It is used both for the in-process controls as well as for the testing of the in-coming steel sheets.
Deep drawing cup test – The deep-drawing cup test is as per standard ISO 11531. The deep-drawing cup test is a sheet metal testing method in which a circular plate (round blank) is punched out of a sheet metal strip and then formed into a cup by a drawing die. The largest possible ratio between the diameter of the round blank and the diameter of the drawing die, which just allows the perfect production of a cup, is called the limiting drawing ratio ‘Bmax’ and is a quality characteristic for the forming capacity of the sheet metal material. The ears, which form as a result of the flow properties of the material, are undesirable since they necessitate rework on the drawn products when they occur in practice.
The deep-drawing ratio for cups is calculated by dividing the diameter ‘D0’ of the round blank by the diameter ‘Dp’ of the punch (B = D0 / Dp). Since the maximum drawing force increases as the drawing ratio increases, a limit drawing ratio results for a single draw, above which failure due to bottom rupture occurs. For most metals, this limit is B = 2 in the first draw, and B = 1.6 in the second draw. Greater forming depths can therefore only be achieved over several draws with possibly interposed softening anneals. The total draw ratio is obtained by multiplying the individual steps (Bges = B1 x B2 x ——- x Bn = (D0/D1) x (D1/D2) x ——- x (Dn-1/Dn) whereas Bges = D0/Dn which is less than or equal to 6.5.
This value has proven to be the upper limit in practice. Quantitative statements about the structural changes are provided by the ‘r’ and ‘n’ values, which can be used as a measure of the deep drawing suitability of a sheet.
The peaks on the deep-drawn cup resulting from the flow behaviour of the material are undesirable, as in practice they need unnecessary reworking of formed parts. Also in this case, the deep drawing cup test can be used to determine the optimally suitable sheet material for the desired forming process.
The ‘r’ value – The definition for the logarithmic plastic deformation is derived from the law of volume constancy. During deep drawing, the work piece shows various effects. Essentially, the molecules are displaced against each other, which lead to changes in strength and inhomogeneous (direction-dependent) material properties (anisotropy). To determine the anisotropy of the plastic properties of sheet metal, the perpendicular anisotropy, the so-called ‘r’ value, is determined in the tensile test. It is defined as the ratio of the degrees of deformation in the width and thickness direction of the sample. For r=1, the material behaves isotropically and equal deformation occurs in the width and thickness direction. ‘r’ greater than 1 (‘r’ less than 1) occurs when the sheet changes its width (thickness) more than its thickness (width) under tensile stress. Since a change in shape without a decrease in thickness of the sheet is normally desired, ‘r’ values greater than 1 are considered advantageous. Due to the multi-axial loading during deep drawing, experience has shown that averaging the ‘r’ values from different directions with respect to the rolling direction is useful. For this purpose, tensile samples are taken from different sheet layers.
A strong deviation of the ‘r’ values (delta ‘r’) from each other results in an undesired tip formation at the cell in the direction of the largest ‘r’ values.
The ‘n’ value – The strain hardening exponent ‘n’ describes the strain hardening of a material during forming. If the flow curve of a material is plotted as a double logarithmic curve, the result is a straight line with a slope of ‘n’. The hardening exponent is the highest value of the ‘n’ value. The strain hardening exponent for most metals is in the range 0.1 less than ‘n’ which is less than 0.5.
Application
Deep drawing steels have high elongation values and a wide range of yield strengths and hence these steels have outstanding forming properties. Deep drawing steels ensure a minimum performance as regards crash behaviour and fatigue strength and represent the lower limit with regard to loadability (strength, crash resistance and fatigue strength).
Deep drawing is a widely used sheet metal forming process. It is suitable for several types of forming operations. Several products such as visible and structural parts of an automobile, rail coach, aircraft, machine body panels, pressure vessels, gas cylinders, shell containers, white goods, kitchen sinks, cooking ware, medical containers, beverage cans, and vehicle rims are manufactured with the deep drawing process.
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