Low-carbon structural steels

Underlying metallurgical principles

        Structural steels, in the form of plates and sections, are produced in the hot-rolled condition and, in many respects, the underlying metallurgy is essentially similar to that involved in the strip grades. However, structural steels are generally used in thicker sizes and higher strengths, and toughness rather than ductility/formability is the more important requirement. The higher levels of elements such as carbon and manganese in structural steels also impose more detailed consideration of the control of microstructure and maintenance of properties in the welded condition. A vast amount of research has been carried out on factors affecting the strength and toughness of structural steels, particularly those involving ferrite-pearlite microstructures which account for the bulk production of these steels. However, the most important issue is the control of ferdte grain size since refinement of the ferrite grains leads to an increase in both yield strength and toughness. This effect contrasts sharply with other strengthening mechanisms, such as solid solu- tion strengthening and precipitation strengthening, which are accompanied by a reduction in toughness. In conventional hot rolling, structural steels would be reheated to a temperature of around 1250~ and rolling would be completed at temperatures of the order of 1000*C. This will result in a fully recrystaUized, coarse austenitic structure which transforms to a coarse ferrite-pearlite structure on cooling to ambient temperature. In turn, this will yield material with a low level of toughness and the additional process of normalizing is required to refine the microstructure and improve the impact strength. This involves reheating the hot-rolled product to a temperature of about 910"C and, during the heating cycle, A1N is precipitated from solid solution in the ferrite which restricts the growth of the austenite grains at the normalizing temperature and leads to the formation of fine ferrite grains on air cooling to ambient temperature. However, as indi- cated in the Overview, the costly process of normalizing has now been largely superseded by controlled rolling, whereby lower temperatures are employed in the finishing stages of hot rolling in order to produce a fine austenite grain size which transforms subsequently to a fine-grained ferrite microstructure. In general, alloy additions are not employed specifically in structural grades in order to produce solid solution strengthening. However, solid solution strength- ening effects will arise from the presence of carbon and nitrogen in solution and also from silicon and manganese, which are added primarily for deoxidation and sulphide control purposes. On the other hand, structural steels can contain manganese contents up to 1.5% which also result in substantial strengthening due to the depression of the austenite to ferrite transformation and the conse- quent refinement of the ferritic grains. As indicated later in this chapter, precipitation strengthening reactions are of major importance in the production of high-strength structural steels, particularly those involving the carbides or nitrides of elements such as niobium, vanadium and titanium. In this context, these elements are called micro-alloying elements and are taken into solution in the austenite phase during the reheating stage, but form compounds such as Nb(CN), V4C3 and TiC on transformation to ferrite. However, the metallurgy of these high-strength low-alloy (HSLA) steels is complex, requiring detailed consideration of solubility/temperature effects at the reheating stage and precipitate size/cooling rate effects on transformation to ferrite. A further consideration is that these micro-alloying elements are also employed to retard the recrystallization kinetics of steels which are subjected to controlled rolling. This involves the strain-induced precipitation of Nb(CN) or TiC in the austenitic condition at a temperature below 950~ which retards recrystallization, producing an elongated, pancake morphology. On cooling to ambient temperature, the deformation substructure in the austenite grains produces a fine ferritic structure with higher strength and toughness than that achieved by normalizing. There is obviously a limit to the strength that can be developed in ferrite-pearlite microstructures through grain refinement, solid solution and precipitation strengthening and for yield strength levels greater than about 500 N/mm 2, transformation strengthening is employed. Thus the alloy content and cooling rate must be sufficient to produce a martensitic structure on quenching which in turn must be tempered in order to provide an adequate balance of strength and ductility/toughness.

 

Strengthening mechanisms in structural

Steels Major research effort has been devoted to the detailed understanding of factors affecting the properties of low-carbon structural steels. Whereas considerable cost savings accrued from the use of lighter sections in higher strength steels, there was also the need to maintain, or indeed improve upon, other important properties such as toughness and weldability. Therefore detailed attention was given to identifying the strengthening mechanisms which were most cost-effective or that provided the best combination of properties.

The practical options for increasing the strength of steels are:

 1. Refining the ferrite grain size.

2. Solid solution strengthening.

3. Precipitation strengthening.

4. Transformation strengthening.

 5. Dislocation strengthening.

Whereas work hardening or dislocation strengthening can result in very high levels of strength, these are achieved at the expense of toughness and ductility. For this reason, little use is made of this method of strengthening but, as illustrated later in this chapter, work hardening is used in the production of high-strength reinforcing bars.

 

Solid solution strengthening

The solid solution strengthening effects of the common alloying elements are illustrated in Figure 2.2 and work by Picketing and Gladman 4 has provided the strengthening coefficients shown in Table 2.1 for ferrite-pearlite steels containing up to 0.25% C and 1.5% Mn. These data illustrate the very powerful strengthening effects of the interstitial elements, carbon and nitrogen, but it must be borne in mind that these elements have only a very limited solid solubility in ferrite. However, both carbon and nitrogen also have a very adverse effect on toughness. Of the substitutional elements, phosphorus is the most potent and, as indicated in Chapter 1, additions of up to about 0.1% P are incorporated in the higher strength rephosphorized grades that are used in automotive body panels. However, like carbon and nitrogen, phosphorus has a detrimental effect on toughness and therefore it is not used as a strengthening agent per se in structural steels. On the other hand, phosphorus is added to the so-called weathering grades because of its beneficial effect on atmospheric corrosion resistance. These steels will be discussed later in this chapter. Of the remaining elements, only silicon and manganese are cost-effective as solid solution strengtheners but silicon is added to steels primarily as a deoxidizing agent.

 

Transformation strengthening

 

As stated earlier, both alloying elements and faster cooling rates depress the temperature of transformation of austenite to ferrite and, ultimately, the effect will be sufficient to cause transformation to bainite or martensite. The consequence of this progression is illustrated in Figure 2.5, which relates to steels containing 0.05-0.20% C. Thus the strength is increased progressively with the introduction of lower temperature transformation products but, of course, with some sacrifice to toughness and ductility. However, in the context of structural steels, there is a demand for quenched and tempered low-alloy grades with yield strengths up to 700 N/mm 2. Such steels are normally alloyed with molybdenum and boron to promote hardenability but there may also be a need to include elements such as vanadium to improve tempering resistance.

 

Controlled rolling/thermomechanical processing

 

As stated earlier, the traditional route to a fine grain size in ferrite-pearlite structural steels has been to incorporate grain-refining elements, such as aluminium, and to normalize the materials from about 920~ after rolling. However, prior to the introduction of continuous casting, basic carbon steel plate was made from semi-killed (balanced) ingots and the additional costs associated with aluminium grain refinement were very considerable.