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Round steel product defects, hot rolling technology

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Round steel product defects, hot rolling technology

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For round steel used for cold heading, scratches are one of the defects that are not allowed to appear. During the cold heading process, the surface damage and stress concentration caused by scratches will directly cause the cold heading to crack and cause waste.

Keyword

Features;Reasons;Elimination method;Causes of scarring

Realed productRolling mill rolls

The main defects and causes of round steel products I

  • Surface scratches

1.1 Features

Surface scratches are also called abrasions, scratches, and scratches. It is a single or multiple parallel linear mechanical damage that appears in the axial direction of the round steel surface. The bottom of the ditch can be seen visually or under a magnifying glass. Due to the temperature of the scratch, the groove marks will show different colors. Therefore, it can be used to determine the approximate process location where scratch defects are generated.

Generally, for scratches formed at high temperature (above 600℃), the entire groove marks are oxidized; for scratches formed in the blue brittle temperature zone, the groove marks are blue; when low temperature scratches, the groove marks appear bright white on the iron matrix. The depth of the iron sheet can be visually seen several millimeters, the length is from several millimeters to several meters or more, and the width varies. Most of the scratches and grooves are of equal width and appear intermittently on the part or the entire length of the steel. Under the microscope, the bottom of the scratched groove is smooth, and there is basically no decarburization at the edges and bottom of the groove.

For general turning round steel, when the depth of the scratch does not exceed half of the tolerance specified by the national standard, it generally does not affect the use.

Obvious scratches have a certain effect on the round steel used for drawing. During the cold drawing process, the scratches on the surface of the round steel will deteriorate the lubrication state between the round steel and the drawing abrasive, and the surface damage of the round steel (steel wire) after drawing will be aggravated, which can lead to serious drawing. Damage to the mold.

 

1.2 Reasons

Scratch locations and procedures that may occur in high temperature areas are: due to poor processing of the guide and guard device, installation deviation, severe wear, or scratches caused by oxide iron; loopers, running troughs, etc., are rough due to installation and processing factors The surface or protrusions will cause the rolling piece to be scratched when it jumps out of the sleeve; the location and process of the scratch that may be formed in the medium temperature or low temperature zone are: the steel is in contact with the rough roller table, floor, cooling bed and other equipment during the production and transportation process. .

 

1.3 Elimination method

Wherever scratches can be formed and processes, sparks or sharp noises often appear, and residual wire-like metal objects can be found. Based on this, the location of the scratch can be found, and then deal with it according to the specific situation.

 

The main defects and causes of round steel products II

  • Scar

A surface defect of round steel. It is formed when excess metal appears on the surface of the raw material or rolled piece and is pressed into the matrix during the next rolling.

2.1 Features

Scars generally appear on the surface of round steel. Larger and thicker scars can generally be seen with the naked eye or a low-power magnifying glass. Some thin and small can only be seen after pickling or even twisting. The scars are generally tongue-shaped or nail-shaped, and the thickness is often inconsistent. The tip is thin and can sometimes be tilted, and the wide and thick end is connected to the steel base. In the case of thick surface oxide scale, only a closed curve can even be seen, embedded in the substrate. The scars caused by roller defects during the rolling process will be periodically distributed on the axis of the surface of the round steel. The distance is approximately equal to the product of the roll circumference and the elongation coefficient.

Unless the section is made along the axis of the round steel, the metallographic examination room is generally performed on the cross section of the round steel with scars, and it is not easy to see the connection between the scarred root and the substrate. When the cross-section only passes through the tongue part, only the excess metal can be seen pressed into the base under the microscope. There is an oxide layer on the substrate and excess metal. There is generally no damage under the substrate. If the longitudinal section passes through the root of the tongue along the rolling direction, the connection between the excess metal and the substrate can be observed under the microscope. There is an oxide layer on the connecting surface.

Scarring is the pressing of excess metal into the matrix. For turning steel, when the depth of the scar does not exceed half of the tolerance specified in the national standard for round steel, it has little effect on the turned parts.

The surface of the round steel used for drawing is not allowed to have scarring defects. During the drawing process, especially when the tip of the tongue and the drawing direction are the same, the excess metal will be up and fall off, blocking the mold, and making the surface of the round steel after drawing. More scratches (commonly known as Daozi) are produced, which can cause severe pull-out fractures.

For round steel for upsetting (cold and hot), when there is a scarring defect in the deformation zone, the defect will warp or fall off. Seriously, it will affect the shape of the upsetting workpiece, resulting in an increase in the amount of waste products.

 

2.2 Causes of scarring

  1. During the rolling process, a certain pass of the rolled piece is damaged or the roll ring is broken in front of the finished hole. When the rolled piece passes through the pass, bumps are formed on the surface. In the subsequent rolling process, the bumps It is pressed into the matrix and extends with the rolled piece, and finally produces periodic rooting and scarring on the surface of the finished product;
  2. During the rolling process, because external metal objects fall on the surface of the rolled piece and are brought into the pass, press into the surface of the rolled piece, forming scars on the surface of the steel. This kind of scarring does not take root and is irregular;
  3. The rolled piece slips in the pass, so that the metal accumulates on the surface around the deformation zone, causing scarring during rolling;
  4. Poor notches in the rolling grooves, which form higher bumps on the surface of the rolled piece, which will produce periodic scars during re-rolling;
  5. The surface of the billet has large defects such as cold splashing, peeling, scarring, etc. When the steel ingot is used to roll into a material, it will produce scars on the surface of the finished product;
  6. Improper surface treatment of raw materials, leaving sharp edges or pits with relatively large depth and width, forming scars after rolling.

Note: The scars formed on the surface of the round steel product due to the billet factors are relatively large or long, and their locations are irregular. This is one of the basic signs that determine that the defect is a steelmaking or steel rolling process.[/vc_column_text][vc_column_text]正文[/vc_column_text][vc_column_text]One stop solution for steel industry[/vc_column_text][/vc_column][/vc_row]

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