Low-carbon strip steels

General processing considerations

          Developments in steelmaking, the use of vacuum degassing and other secondary steel making techniques enable the steel to contain much more controlled levels of the important elements carbon, nitrogen, sulphur, aluminium and manganese than were available previously. Very low levels of the first three of these elements, down to below 0.003%, are now also easily obtained if required. In many cases, however, higher levels must be used to give the properties needed. The most common of the existing process routes for the conversion of liquid steel into a usable sheet form are summarized . Prior to about 1970, all strip steel was cast into ingots about 500 mm thick. After cooling and removal from the moulds the ingots were reheated to about 1250"C and rolled to slabs about 200-250 mm thick and allowed to cool. The surface was then scarfed to remove surface defects. With the development of continuous casting, mainly in the 1970s, the ingot- rolling process was eliminated and the slabs were cast directly to a thickness of 200-250 mm as a continuous process and allowed to cool. The first hot-rolling stage then consisted of reheating to temperatures up to about 12500C and rolling in two linked stages. The first stage, called roughing, reduced the thickness to an intermediate gauge, usually in the range 30-45 mm, and the second stage, called finishing, reduced the thickness to the final hot-rolled gauge required, often in the range 1-2 mm up to 5-12 mm, depending on details of the mill employed. The roughing section may consist of a single reversing stand through which the steel passes backwards and forwards, usually five or seven times, or it may consist of several non-reversing stands through which the steel passes once. It may, however, consist of a combination of both reversing and non-reversing stands. Whichever combination is used, the steel completely leaves one stand before it enters the next one. Critical matching of rolling speeds is not, therefore, required. A finishing train usually contains seven stands which are positioned close together. The front end of the strip exits the last stand well before the back end of the strip enters the first stand. Exact matching of the speeds of each stand is required, therefore, depending on the reductions in gauge in each stand. The steel usually exits the last stand at a finishing temperature up to and above 900~ depending on grade, which ensures that all the deformation takes place in the single-phase austenite region of the phase diagram. It is then cooled with water on a run-out table before coiling at a coiling temperature that is usually close to 600~ but almost always in the range 200-7500C, depending on the metallur- gical needs of the grade. Not all hot mills would, however, have the capability of covering this entire range. The finishing temperature may be controlled by using a suitable slab reheat temperature, usually in the range 1100-12500C, by adjusting the speed of rolling through the mill and, if necessary, by introducing delays between roughing and finishing to increase temperature loss or by inter- stand cooling using water sprays. The temperature drop between roughing and finishing may, however, on certain mills be reduced by the use of radiation shields or by forming the steel into a coil to reduce the surface area for heat loss. Some of the hot-rolled coil, coveting the complete gauge range, is sold for direct use and some, usually in the gauge range up to about 5 mm, is cold rolled to thinner gauges. An oxide-free surface is required for cold reduction and this is achieved by passing the steel through a pickle line. Traditionally, the acid used was sulphuric acid, but most pickle lines now use hydrochloric acid. The cold rolling is usually carried out using a tandem mill containing five stands, with controlled front and back tension. Each stand usually contains four rolls consisting of two work rolls and two back-up rolls, but six high stands, each containing four back-up rolls, may be used in one or more positions if a particularly high cold reduction is required.

            Cold-rolled gauges are usually in the range 0.4-3.0 mm and generally involve the use of 50-80% cold reduction, though tinplate mills provide up to 90% cold reduction to give thinner gauges. Clearly the thinner cold-rolled gauges are usually rolled from the thinner hot band gauges in order to limit the amount of work required from the cold-rolling mill. When the application requires a gauge that is common to both hot- and cold-rolled gauges, a cold-rolled and annealed material would only be used if a good surface and a high degree of formability were required. Otherwise, a hot-rolled material would be used for cost reasons. Tinplate and certain other packaging applications require gauges down to less than 0.2 mm and are rolled from the thinner hot-rolled gauges. Certain grades of tinplate, however, involve two stages of cold reduction.

          Cold rolling causes the steel to become hard and strong and with very little ductility. For almost all applications, therefore, an annealing treatment is required to reduce the strength and to give the formability that is needed for the final application. Various metallurgical changes take place during annealing including recovery, recrystallization and grain growth, and the formation, growth or disso- lution of precipitates or transformation products. It is the complex controlled interaction of these changes which provides the steel with its final properties.

          Both batch or continuous annealing may be used for many types of steel, but certain steels may only be processed by continuous annealing. The traditional method was, however, batch annealing and this method in still in common use. With this method, several tightly wound coils are stacked on top of each other with their axes vertical. They are enclosed by a furnace cover which contains a protective atmosphere which is recirculated to promote heat transfer between the cover and the steel during both heating and cooling. Traditionally, ther protec- tive atmosphere has been HNX gas, which is nitrogen with up to 5% hydrogen, but furnaces designed for the use of 100% hydrogen were introduced during the 1980s and now provide an appreciable proportion of the batch-annealing capacity world-wide. Rapidly recirculated hydrogen has a higher heat transfer coefficient than the HNX gas and this enables faster heating and cooling rates to be achieved. The complete cycle may still, however, take up to several days. Hydrogen annealing gives improved control over mechanical properties and improved surface cleanliness.

 

Underlying metallurgical principles

        Low-carbon strip steels are based primarily on ferrite microstructures and are almost invariably subjected to cold-forming operations in order to achieve specific shapes, e.g. automotive body panels. They are produced in the hot-rolled condition in thicknesses down to about 2 mm but they are used primarily in the cold-reduced and annealed state where the gauges can extend down to 0.16 mm for tinplate grades, e.g. for packaging applications. The basic consideration in the development of microstructure in strip and low-alloy steels is the Fe-C phase diagram, a portion of which is shown in Figure 1.3. At temperatures above about 900"C, a steel containing about 0.05% C will consist of a single phase called austenite or y iron which has a face-centred cubic crystal structure, as illustrated in Figure 1.4(a), and all the carbon will be in solid solution. On cooling slowly, the material reaches a phase boundary at which a separate iron phase, called ferrite or ot iron, begins to form. This phase contains a low carbon content and has a body-centred cubic structure, as illustrated in Figure 1.4(b). As cooling continues, the formation of further low-carbon ferrite leads to a carbon enrichment of the untransformed austenite. This process continues on cooling down to a temperature of 723~ the eutectoid temperature, at which stage the remaining austenite will have been enriched in carbon to a level of about 0.8%.

            As the carbon content of a steel is increased, the y to 7' + ct transformation temperature is depressed and the ratio of pearlite to ferrite in the microstructure is progressively increased, reaching 100% pearlite at about 0.8% carbon, the eutectic composition. Whereas the above sequence of events relates to slow cooling and an approach to equilibrium, faster cooling rates usually produce ferrite with globular carbides within the grains. Lower-temperature transformation products such as bainite or martensite may, however, be produced particularly when a suitable alloy addition has been made. These will be discussed later.

          The ferrite grain size has a very important effect on the properties of low- carbon strip steels, as indicated by the Hall-Petch equation.

Thus the yield stress increases with decreasing ferrite grain size. Pickering 9 has indicated that the value of ky often lies in the range 15-18 MPa mm 1/2.

          The strength is also influenced by other factors such as solid solution and precipitation strengthening. In the former, the strengthening is often related to the square root of the atomic concentration of the solute atoms, but at low concentrations, the strengthening effect may be regarded as linearly dependent. The magnitude of the effect depends on the atomic size difference between the iron and the solute element, the largest effects being produced by small elements such as carbon and nitrogen which go into interstitial solid solution. Elements such as phosphorus, manganese and silicon are often added to low-carbon strip to provide solid solution strengthening.

Cold forming behaviour

          Cold formability and strength, as indicated above, represent the two most impor- tant requirements for low-carbon strip grade steels. For many applications, the main requirement is to be able to form the part without splitting, necking or wrinkling. The most suitable steel, therefore, is one with a low strength and high formability. For structural applications, the strength of the steel is more important and must be above a given minimum value. It is found, however, that there is a general tendency for the cold formability of any type of steel to reduce as the strength increases. The reduced formability of higher-strength steels tends, there- fore, to limit their use to those applications which do not require the very highest formability. Many of the developments of higher-strength steels, therefore, have been specifically aimed at providing higher strength while minimizing the loss in formability that would otherwise have taken place.

 

Work-hardening coefficients and normal anisotropy

            The formability of sheet steels may also be assessed using parameters that may be measured directly from a conventional tensile test, provided suitable length and width extensometers are available. The first parameter is the strain ratio which was originally devised by Lankford and others 38 and is usually called the r value. It gives a measure of the drawability of the steel but also gives a measure of the resistance to thinning resulting from the orientation of the slip systems that are active during drawing. The second parameter is the work-hardening coefficient, designated the n value which is closely related to the stretchability of the steel. The uniform and total elongation measured in a tensile test are also related to the stretchability of a steel, but it is necessary to introduce the concepts of true stress and true strain before the r value and the n value may be defined precisely.

True stress and true strain

            The stress used in a conventional tensile test, often called the engineering stress, is defined as the load at any moment during the test divided by the original cross-sectional area of the test piece. During the test, the load increases up to the point of maximum load which defines the tensile strength of the material and then decreases as the specimen undergoes local necking. The true stress, however, is defined as the load at any moment during the test divided by the current cross- sectional area at the same moment.

Other forming effects

          The above remarks have concentrated primarily on the factors that influence the possibility of local necking or splitting during the formation of any pressing. It should be remembered, however, that there are other factors that may lead to a pressing being unsatisfactory. These include wrinkling, hole expansion effects and constancy of shape, sometimes known as shape fixability which is related to springback.

Buckling, distortion and wrinkling

    Sometimes, when there are large strain gradients across a pressing or when there is some other critical condition, the transfer of stress across a pressing may cause buckling, or the relaxation of the elastic stresses introduced during forming may cause distortion. The extent of these effects is very dependent on the nature of the pressing. Wrinkling consists of corrugations produced mainly near the flanges of a pressing when the material is subject to in-plane compression. Wrinkling would make a pressing unsuitable for any exposed applications, but the existence of wrinkling on internal flanges makes the process of spot welding difficult.

          Wrin- kling may be controlled by adjusting press conditions but the main material parameters that affect the tendency are yield stress and gauge. The tendency for wrinlding increases with increasing yield stress and decreasing gauge.