Effect of Phosphorus on the Properties of Carbon Steels: Part Two

Texture (R-Value)

Anisotropy of plastic-flow properties has a strong influence on the deep drawability of sheet steels. For good drawability the through-thickness strength in order to avoid thinning during drawing operations. A useful measure of drawability is the R-value, defined as the ratio of width to thickness strain in a uniaxial tensile specimen at fixed extension (typically 17 percent). Since the R-value generally varies with direction in the sheet, a mean R-value is typically reported. A high R-value implies good drawability with values of 2.0 being considered excellent.

Plastic anisotropy is determined by crystallographic texture i.e. the orientation distribution of grains within the sheet, which is closely related to steel composition and processing. In body centered cubic materials (e.g. carbon steels) favorable drawing textures correspond to a high proportion of {1 1 1} planes aligned parallel to the sheet surface. The topic of annealing textures in steels has been thoroughly reviewed by Hutchinson.

Phosphorus affects the texture and R-value of cold rolled steel in a complex manner that is dependent on the composition of the steel and the annealing treatment. Early investigations showed that up to 0.04 percent of phosphorus markedly improves the R-value of low-carbon rimmed steel given a decarburizing open-coil anneal. Hu also noted an improvement in the R-value from about 1,5 to 2,0 with the phosphorus content ranging from %P=0,004÷0,12 in vacuum-melted rimmed steels given a simulated batch anneal.

However, Al-killed steels that were air melted showed a less dramatic improvements in the R-value which appeared to reach a maximum at about %P=0.08. Hu proposed that segregation of phosphorus to grain boundaries and subgrain boundaries influences the nucleation and growth of recrystallized grains. He showed that the recrystallized texture is significantly altered by phosphorus additions to Al-killed steel.

More recent work has demonstrated the importance of the heating rate during annealing, carbide morphology and solute carbon content. Ono et al. showed that both low phosphorus (%P=0.016) cold rolled, low carbon, Al-killed steels exhibit a maximum in R value versus heating rate during annealing. At an optimum heating rate of about 50°C/hour, the high phosphorus steel had a lower R value than the low phosphorus steel, whereas the opposite was observed at heating rates greater than 80°C/hour. These effects were attributed to an interaction between P and AlN precipitation during annealing, which in turn, affected the final grain structure and crystallographic texture.

In a subsequent paper, these authors describe an experiment whereby interference from AlN was eliminated by heat treatment prior to cold rolling. Treatments were varied such that a decarburized steel (20-30 ppm C) and steels with fine and coarse Fe-carbide precipitates were produced. Figure 1 summarizes the results of annealing experiments for these steels.

Figure 1: Effect of the heating rate on the R-value as a function of the phosphorus content and carbide morphology in Al-killed steels. ALN precipitation was complete prior to cold rolling and annealing.

For the decarburized steel, the R-value decreased with both increasing P content and faster heating rate during annealing. This apparently reflects the behavior of ultra-low-carbon, carbide-free matrix. At low heating rates phosphorus improved the R-value for both coarse- and fine-carbide steels. This may indicate that phosphorus reduces the inhibiting effect of carbon which enters solution via carbide dissolution prior to recrystallization. At high heating rates phosphorus degraded the R-value of coarse-carbide steel. In the fine-carbide steel, the R-value first increased slightly and then decreased with increasing P content. This suggests that either phosphorus had insufficient time to segregate to grain or subgrain boundaries or that carbide dissolution had not progressed sufficiently to show a beneficial effect of phosphorus.

Hutchinson concluded that phosphorus is only beneficial in those situations where dissolved carbon would otherwise degrade texture. Furthermore, in order to be effective, phosphorus must be allowed to segregate e.g. during slow heating. However, in the case of Ti stabilized, ULC steels where virtually carbon does not exist in solution during annealing, small phosphorus additions tend to improve the R-value. Figure 2 shows this effect as a function of grain size for samples rapidly heated to 800°C. Note that at the same time grain size, the R-value of the P-bearing steel is up to 0.3 units greater than that of the base steel. Further study is needed to elucidate the effect of phosphorus on texture development in stabilized steels.

Figure 2: R-value as a function of grain size for rapidly annealed, Ti-stabilized, ultra-low carbon steels containing Si, Mn or P.

Although the available data is somewhat confusing, up to 0.1 percent of phosphorus does not appear to seriously impair the drawability (R-value) of carbon steels. Furthermore, with the proper choice of processing parameters, drawability can be significantly improved by additions of phosphorus; any improvement is usually a side benefit to the strengthening effect of phosphorus, however. It is interesting to note that phosphorus is the only known element capable of increasing both the strength and R-value of steel.

Spot Weldability

Sawhill and Baker compared the resistance spot-welding characteristics of plain-carbon steel with those of rephosphorized steel containing 0.04 percent to 0.12 percent of phosphorus. Adequate weld button size, strength and toughness were obtained over a practical range of welding conditions.

To obtain an adequate button size over a large range of weld currents, electrodeface diameters and welding times need to be increased slightly compared with those of plain-carbon steels. Others report excellent weldability of rephosphorized Al-killed steel and galvannealed Ti stabilized, ULC steel, although it has been suggested that the phosphorus content be limited to approximately 0.1 percent.

Magnetic Properties (Core Loss)

Phosphorus is added in amounts up to 0.15 percent to cold rolled, motor-lamination steels in order to reduce AC core loss during service. A significant portion of core loss is due to eddy currents, and this contribution depends inversely on electrical resistivity. Phosphorus is desirable for this purpose because it strongly increases resistivity at low cost. The enhanced punchability of motor laminations due to the hardening effect of P is an additional benefit gained by adding phosphorus to these steels.

Coating Behavior

Phosphorus has a strong effect on the alloying rate of galvannealed Fe-Zn coatings in low-carbon, aluminum killed steels. Phosphorus in the base steel retards alloy layer formation, thus reducing the alloying rate during galvannealing. This is not necessarly detrimental since the galvanneling temperature can be increased to maintain high productivity.

However, very soft steels may encounter shapes problems at galvanneling high temperatures. Nevertheless, tight control of phosphorus is desirable in order to specify optimum operating conditions (temperature, line speed, etc.) for a given steel grade. Variable phosphorus content can lead to difficulties in process control.

Phosphorus exerts a beneficial effect on the galvanneling reaction by inhibiting the so called "outburst reaction" at grain boundaries which contributes to poor powdering performance. The outburst reaction refers to the formation of brittle Fe-Zn intermetallic compound at ferrite grain boundaries. Volume expansion during this process causes the grain boundary to open up, breaking a protective Fe-Al layer. This allows liquid Zn to locally form new intermetalics. Phosphorus segregation to grain boundaries apparently inhibits the local Fe-Zn intermetallic formation.

Figure 3 shows results of investigation the effects of phosphorus in steels on the formation of Fe-Zn compounds in continuously galvanized Nb-B-P-ultra low carbon steel sheets (%C=0.006; %Mn=0.27; %P=0.076; %S=0.005; %Nb=0.016; %B=0.0018; %Al_sol=0.004; %N=0.0023) which suggest that phosphorus inhibits the formation of outburst structure.

Figure 3: The effects of phosphorus on formation of Fe-Zn compounds outburst structure.

Sheet steels coated with Zn or Al-Zn can become embrittled by heating in the temperature range of 335°C to 400°C. This is a result of Zn penetration into the grain boundaries at temperatures less than the melting point of Zn. Embrittlement can be prevented if the P content in the steel exceeds 0.04 percent. Phosphorus segregation to ferrite grain boundaries inhibits intergranular diffusion of Zn, thus preventing Zn embrittlement.

Phosphatability was found to be markedly improved by the addition of 0.07 percent P to a Nb-bearing, ULC steel. In addition, phosphorus markedly improved the corrosion resistance of phosphated steel. The addition of phosphorus to certain enameling steel grades has been reported to enhance fishscaling resistance and after-fire strength. Furthermore, phosphorus accelerates etching and nickel deposition rates during pre-enameling treatments. Although these latter effects may be advantageous, process control difficulties can arise if the phosphorus content is not consistently controlled to a predetermined level.