Seams in Steel Rebars 1

[CCQS2014]

Good day for all,
I want to ask about seam defect in steel rebar , its appearing in higher rolling sizes only more than 25 mm , but its disconitnuous.do you know how its generating from Rolling Mill or Caster.

 

[remetaper]

Rebars have basically same kinds of seams of other hot rolled round bars.
Depending on kind of seam ( as crack ,if you like ), it's more easy to detect by visual testing on large diameters than in small ones.
Typical surface defects of carbon steel depending on poor surface quality of continous casting billets or poor grinding process of billets that didn't completely remove cracks,pinholes ,slivers ,ecc. Therefore these defects are generated during the melting/casting process. In addition, hot rolling process could cause laps ,overfill and slivers ( hot tears ) in case of oveheating or poor hot plasticity of steel. However ,this last failure is very unlikely on carbon and low alloyed grades.
In case of stainless steels rebars, same seams ( as cracks )if billets are not well grinding or if surface of continous casting had some cracks ,pinhole , blowholes and so on.
During hot rolling ,stainless steel ( usually austenitic and duplex ) could have poor hot plasticity due to unbalanced chemical analysis, overheating or, on the contrary, lowest finishing temperature ( mainly duplex grades). The typical morphology of defect is slivers ( hot tears on the side of rebars. Dimension and morphology of slivers emphasize poor plasticity of steel while longitudinal cracks ( or seams )are typical of poor care during casting and grinding processes.
Obviously, even austenitic stainless steels and duplex could have laps or slivers in case rolling temperature getting wrong.
Finally, a visual testing can immediately allow to identify the origin of failure (steelmaker technicians know very well the subject )but in case of doubt, a microscope examination solves the question.

 

[FennLane]

One of the problems with Rebar is that it is commonly manufactured in a Mini-Mill environment and uses a high percentage of scrap.

The residuals in this type of steel - particularly Grade 460 - are tending to increase and the high levels of copper and tin that are being found can present a problem which is known as 'hot short' where grain boundaries separate during hot rolling.

It may be worthwhile looking at the detailed chemistry of the rebar you have and checking the levels of copper and tin to eliminate this potential cause.

 

[gtaw]

To determine if the seams or any other discontinuity is acceptable or unacceptable, one must refer to the appropriate material specification.

Concrete reinforcing steel is typically loaded in tension or shear. As such, laminations, seams, and laps have little influence on the load capacity. The acceptance limits are rather generous as long as the applied stresses are in tension or shear.

Most hot rolled/wrought materials contain discontinuities of one type or another. The steel, once melted, are poured in air from the furnace to the lad, from the ladle to the tundish (continuous casting) or in to the ingot mold. During each pour, the material is exposed to air where oxides are introduced. Whether processed in a continuous casting machine or an ingot, discontinuities are introduced. then there is the rolling process. Additional discontinuities can be introduced as a result of the rolling process. The various types of discontinuities and the acceptance limits for each are provided by the material specifications.

Best regards - Al