Cautions for Hot Forming

 

1) For Non-heat Treated Steel Iron must be heated to a sufficient extent （above 900°C, the A3 transformation temperature） so that it may be properly formed. Hence the saying, "Strike when the iron is hot!" But if the iron is heated too long or too much, overheating or burning may occur,

which leads to plate cracking during the forming stage or marked deformation or defects even if forming can be accomplished. Especially Al-killed steel, characterized by its fine-grained structure, has a relatively narrow hot-shortness range, so special caution must be exercised during hot forming so that its steel properties may be well considered and the temperature properly controlled.

 

If low-temperature toughness is strictly required, it may be necessary to adjust the heating temperature to the lower side, or to carry out normalizing after hot forming. Depending on the steel quality and heating period, overheating can occur at temperatures over 1000°C or so. From a structural standpoint, as steel grains become coarse, the steel surface becomes rough or the mechanical properties deteriorate. Thus, the level of fineness after heat treatment should also be considered.

 

Burning produces a kind of steel embrittlement at high temperature. This occurs because soluble compositions present on the grain boundary of the steel dissolve if the metal is heated too much （about 1300°C or more） or for too long a time period. The result of such burning is the plate's tendency to develop fine cracks and a deterioration of its mechanical properties.

 

Aluminum embrittlement is special hot shortness peculiar to finegrained Al-killed steel that occurs in the heating range of 800 to 1000°C. Within this range, the mechanical properties of Al-killed steel deteriorate, so that high-degree forming cannot be achieved. This is the result of the formation of fine-grain aluminum nitrides scattered along the steel's grain boundary, leading to a splinter phenomenon since the nitride deposits are so difficult to deform. The more residual aluminum that is present in the steel, the higher the splintering risk. In contrast, silicon-killed steel and rimmed steel are not associated with such risk. Need to specify delivery conditions in advance to the mill when fine-grained steel is required for high-temperature forming.

 

Moreover, even if the steel does not actually deteriorate, heating to levels of 600 to 700°C can change its mechanical properties from the original condition. Representative examples of local heating test results that clearly showed such changes are given below

 

Example 1: Results of heating tests on steel plates of YP355 N/mm2 class （32mm plate thickness）

 

Results of tensile tests using JIS 4 specimens

Results of V-notch Charpy impact tests (Absorbed energy at 0°C)

 

Example 2: Results of heating tests on WEL-TEN 590 steel plates （13mm plate thickness）

 

Results of tensile tests using JIS 5 specimens

 

Results of V-notch Charpy impact tests (Absorbed energy at 0°C)

 

Example 3: Results of heating tests on WEL-TEN 590 steel plates （32mm plate thickness）

 

Results of tensile tests using JIS 4 specimens

 

 

Results of V-notch Charpy impact tests (Absorbed energy at 0°C)

 

(2) For Heat-treated Steel

The quality of heat-treated steel improves in respect to strength and toughness through quenching & tempering（QT）. The approach used for QT depends on a variety of factors, including the type of steel, alloying elements, thickness, fineness, and intended use.

When steel is heated at too high a temperature or for too long, the tempering effect becomes null, resulting in deterioration of the good properties of steel. As a result, such steel cannot be hot-formed, or cracking and other problems may occur. If strainrelieving annealing is required, or should you have any concerns or questions about heat treatment, please ask you're your steel mill. In general, the appropriate heat treatment condition is 580°C x 2 hours / 25mm thickness.

 

(3) For TMCP Steel

Thermo-Mechanical Controlled Process（TMCP）steel obtains strength and toughness levels that are equal to or better than those of heat-treated steel through controlled rolling and controlled water cooling without off-line reheating. However, if TMCP steel is reheated, its properties may markedly

deteriorate due to structural changes. For this reason, hot forming cannot be conducted on TMCP steel. If strain-relieving annealing is required, please consult your steel mill. Caution is necessary because even if steel batches go through the same TMCP process, the conditions can differ depending on the rolling temperature and whether the water cooling process is conducted or not.

 

 

Source: www.nssmc.com